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Admixtures in Concrete
Admixtures in Concrete F. Massazza Italcementi, Laboratories Chimico Centrale, Bergamo, Italy
CONTENTS 1 2
3
4
Introduction Grinding Aids 2.1 General 2.2 Effect of Grinding Aids 2.2.1 Influence on the specific surface and particle size distribution 2.2.2 Influence on agglomeration 2.2.3 Grinding efficiency 2.2.4 Influence on the cement properties 2.3 Mechanism of Action Accelerators 3.1 General 3.2 Set-accelerating Admixtures 3.2.1 Inorganic products 3.2.2 Organic products 3.3 Hardening-accelerating Admixtures 3.3.1 Inorganic products 3*. 3.2 Organic products 3.4 Collateral Effects 3.4.1 Rheology of the mixes 3.4.2 Durability 3.4.3 Dimensional changes 3.4.4 Heat of hydration 3.4.5 Antifreezing properties 3.5 Composition and Structure of the Hydrated Phases 3.6 Mechanism of Action 3.6.1 Action on setting times 3.6.2 Action on hardening Retarders 4.1 General 4.2 Inorganic and Organic Retarders 4.3 Factors Affecting the Action of Retarders 4.4 Other Effects 4.5 Mechanism of Action
569
571 57 2 572 573 573 576 576 577 578 580 580 580 580 582 584 584 586 586 586 587 588 588 588 588 589 589 590 590 590 591 592 593 596
570 5
F. Massazza
Plasticizers 601 5.1 General 601 5.2 Effects on Mortars and Fresh Concretes 602 5.2.1 Mixing water reduction 602 5.2.2 Workability increase 602 5.2.3 Secondary effects 607 5.3 Effects on hardened concrete 608 5.3.1 Mechanical strength 608 5.3.2 Drying shrinkage 608 5.3.3 Creep 609 5.3.4 Durability 610 5.3.5 Changes in the hydration products 610 5.4 Mechanism of Action of Plasticizers 611 6 Superplasticizers 611 6.1 General 611 6.2 Effects on the Properties of Fresh Concrete 612 6.2.1 Fluidizing action and water reduction 612 6.2.2 Slump loss 615 6.2.3 Setting times 618 6.3 Effects on the Properties of Hardened Concretes 619 6.3.1 Mechanical strengths 619 6.3.1.1 Ordinary curing 619 6.3.1.2 Steam curing and autoclaving 619 6.3.2 Shrinkage and creep 620 6.3.3 Durability 621 6.3.3.1 Impermeability 621 6.3.3.2 Frost resistance 621 6.3.3.3 Salt resistance 622 6.4 Effects on the morphology and composition of the hydrated phases 622 6.5 Mechanism of Action 622 6.5.1 Viscosity of cement pastes 622 6.5.2 Viscosity of concretes 626 6.5.3 Adsorption and paste rheology 627 6.5.3.1 Adsorption 627 6.5.3.2 ζ-potential 628 6.5.3.3 Steric obstacle 629 6.5.3.4 Solid-liquid affinity 629 6.5.3.5 Morphological changes 629 6.5.3.6 Changes in the hydration kinetics 629 6.5.4 Final remarks 629 7 Air-entraining Admixtures 631 7.1 General 631 7.2 Entrained Air 632 7.3 Effects on Concrete 633 7.3.1 Effects on fresh concrete 633 7.3.2 Effects on hardened concrete 635 7.3.2.1 Effects on mechanical strengths 635 7.3.2.2 Effects on durability 636 References 638
Admixtures in Concrete 1
571
INTRODUCTION
Much recent progress occurring in cement and concrete technology is related to the use of admixtures. Admixtures are inorganic or organic products which, when added in small amounts, modify some properties of the comminuted, dry or dispersed-in water materials (raw materials, cements, mortars and concretes). The great number of products (especially organic) proposed as admixtures and the various effects that each of them can produce make their classification difficult. These products are generally subdivided into admixtures for cement and admixtures for concrete. The former are used to improve the technological process of the cement production; the latter modify particular properties of concrete, by reducing or enhancing them. However, this subdivision is not strict since the same admixture can be used in cement and concrete for different purposes. A second criterion of classification subdivides admixtures according to the prinary function they have: also in this case, the boundaries between groups are not sharp since a product can perform more than one function. The case of plasticizers which are also retarders is typical. Based on these criteria, the more generally used admixtures can be subdivided into: A. Al A2 A3
plasticizers and water-reducing admixtures for raw cement slurries admixtures for granulation admixtures for dry grinding of raw meals. B.
Bl B2
B3
B4
B5 B6
Admixtures for Cement:
Admixtures for Concrete
admixtures modifying set and hardening: accelerators retarders admixtures modifying the mix rheology and the air content: , , ._ (superplasticizers water-reducing admixtures , Ji · b I plasticizers water-retaining admixtures thickening admixtures admixtures entraining air into the mixes: air-entraining and air-detraining admixtures gas-forming admixtures foam-forming admixtures admixtures modifying the resistance to physical and chemical actions: frost-resisting and anti-freezing admixtures water-repelling admixtures permeability-reducing admixtures corrosion-inhibiting admixtures products improving the resistance to chemical actions expansion-producing agents miscellaneous admixtures colouring admixtures agents improving the resistance to biological actions a.s.o.
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F. Massazza
From this list it is possible to notice the great variety of effects obtainable with the admixtures and to deduce the complexity of the chemical-physical phenomena which must be faced in order to explain the action of these materials. For brevity's sake it is impossible to examine all the above-mentioned admixtures. Therefore this chapter will consider in detail only the most largely used products, namely: (a) (b) (c) (d) (e) (f)
grinding aids set- and hardening accelerators set- and hardening retarders plasticizers superplasticizers air-entraining admixtures.
Information about other products and more particular effects can be found in reviews and proceedings of many symposia and congresses devoted to admixtures, in books and specialized reviews [1-10J. Again for brevity's sake the number of bibliographic references was limited. Nevertheless those who are interested can find an exhaustive bibliography in Cement Research Progress in the chapters devoted to admixtures [11,12,13,14 15,16]. The references relevant to patents are not considered in this chapter not for want of confidence towards the obtained results but because they concern a restricted experimentation. 2
GRINDING AIDS 2.1
General
Grinding aids are products that improve the efficiency of the grinding process when added to cement in small amounts. These admixtures allow: the mill production to increase, at the same energy consumption and product fineness or grinding duration, or the cement fineness to increase, at the same energy consumption and mill production. In modern mills, whose diameter attains and often exceeds 4 m, the use of grinding aids sometimes becomes indispensable when very fine grindings are required, since the high kinetic energy of the grinding media favours the reagglomeration of the ground material [17]. The more widely used products are surfactants, such as: amines and relative salts, polyoils, alcohols, lignosulphonates, fatty acids and relative salts. Moreover, judging from the great number of patents found in the literature, many and different other products have been used including pure compounds as well as complex mixes [10].
573
Admixtures in Concrete
All admixtures are usually employed in very small amounts, generally included between 0.01 and 0.1 wt% of cement [18], although coke gives the best results at the dosage of 0.50% [19]. 2.2
2.2.1
Influence
Effect of Grinding Aids
on the specific
surface
and particle
size
distribution.
Figures 2.1 and 2.2 show that the effect of grinding aids depends considerably on the nature of the admixture and its dosage [20]. ^- 90
50
Fig. 2.1
100
150 200 Grinding mid
250 content
Undersize of a clinker versus grinding aid content [20]. A - Phenylpolyglycol ether; B - Heptan-phosphonic acid; C - Triethanolamine; D - Sodium Stéarate; E - Sodium salt of hydroxamic acid; F - Decylic Alcohol; G - Potassium butylxanthate; H - Sulphite liquor; I - Sodium cetylsulphate; L - I^HPO^; M - Phosphoric acid ester; N - Methylsilicon oil.
574
F. Massazza 3600
2900 2 800
150
200
Grinding aid
Fig. 2.2.
250 content
Blaine specific surface area of a clinker versus grinding aid content [20]. A - Phenylpolyglycol ether; B - Heptanphosphonic acid; C - Triethanolamine; D - Sodium Stéarate; E - Sodium salt of hydroxamic acid; F - Decylic alcohol; G - Potassium butylxanthate; H - Sulphite liquor; I - Sodium cetylsulphate; L - ^ΗΡΟ^; M - Phosphoric acid ester; N - Methylsilicon oil.
In the former figure the grinding fineness is expressed by the amount of clinker passing through the 40 urn mesh sieve and in the latter by the Blaine specific surface area. These tests showed that the best effects are obtained by using surfactants and, in particular, cationic reagents (primary amines and ethanolamines). However phosphonic acid, glycols, stearic acid, sodium stéarate, hydroxamic acid and its sodium salt are little less effective. In other laboratory tests polyvalent alcohols and amines were more effective than fatty acids and sulphonates [18]. The different effectiveness of the grinding aids of homologous series depends on their molecular weight. In fact it was indicated that the action of saturated fatty acids [21] and alcohols increases as the chain length shortens and that unsaturated acids are more active than the saturated ones [21 ]. At equal grinding times the specific surface increase on average by about 20% [21,22,23], but increases up to 29% were obtained [24]. Admixture fineness increases with grinding duration up to certain limits (see Fig. 2.3) [25].
Admixtures in Concrete
575
10.000-
8.000
10.000 ■
2.000 i
Fig. 2.3.
50
100
150
200 Hours
Grinding curves of the Type I clinker with different grinding aids [25]. (a) A - Oleic acid; B - Diethyl carbonate; C - Reax 70; D - AR-100; E - TMN; F - NP-33; G - NP-40. (b) A - Emcol HB; B - Emcol H-20; C - Emcol H-2A; D - CPH-12J-N; E - Emcol HC (0.5%); F - Emcol HC (0.25%); G - F-90.
Grinding aids modify the particle size distribution of cement. For instance, the addition of surfactants was found to increase the amount of medium size (3-30 urn) particles by 10-20% [26]. Figures 2.1 and 2.2 show that fineness, expressed as Blaine specific surface and residue, rises with increasing amounts of admixture up to certain limits [20,27]. Indeed, certain authors [28,29] found that specific surface decreases with certain admixtures and when some optimum contents (0.025-0.5%) are exceeded.
576
F. Massazza
Parallel tests performed in the laboratory and the factory [18] showed that, when the amount of grinding aid rises, fineness first increases gradually but, beyond a certain value, it remains constant in the laboratory tests whereas it decreases in the industrial ones (Fig. 2.4). The behaviour observed in the industrial tests would be explained by the higher flow of the cement containing grinding aids. Therefore, when flow exceeds a given value, the coarse particles come out quickly together with the fines.
* 10
Industrial
Laboratory Optimum
0.01
tests
content
0.02
-L
0.03
Triethanolamine
Fig. 2.4.
tests
J.
0.04 content
«L
0.05 (%)
Fineness of a cement versus grinding aid content [18].
2.2.2 Influence on agglomeration. The most effective grinding aids also prevent agglomeration since they reduce the adhesion between particles [30]. Consequently, very high grinding finenesses (9000-9600 cm2/g) can be obtained [31,32]. 2.2.3 Grinding efficiency. The economic advantages obtainable in the industrial processes by using suitable admixtures appropriately are by now largely ascertained and estimated by the following values. The grinding time [33] required to obtain a certain fineness can be reduced by 10-50% [34,35,36] and the production be increased by 17-34% [21,37]. The energy consumption can decrease from 15 [38] to 25% [21] and the grinding efficiency can rise by 30-40% [39]. A synthesis of the collected results is shown in Table 2.1.
577
Admixtures in Concrete Table 2.1.
Some Results Obtained in Industrial Grindings by Using Grinding Aids
07
Basic product present in the admixture Diethylene glycol Ethylene oxide Propylene glycol fethylene glycol 'Combustible wastes from butanol manufacture Lignin compound Urea Diethanolamine salt MAVEKLIN grinding aid Dodecylbenzenesulphonic acid amine salt Monostearate
of addition
Increase in productivity
Shortening in grinding time
Decrease in energy consumption
Ref.
10-20% 0.01-0.05 0.05 0.08-0.22 0.08-0.22 0.01-0.02 0.1 0.03 0.1
10% -10%
-10% up to 20% up to 20% >13.2% 30-40% -13% 15-17%
0.05 0.05
~16% -15%
40 41 42 23 23 43 44 45 46 47 48
According to some research [17], grinding aids allow fineness to increase in the open-circuit mills, whereas in the closed-circuit ones considerable production increases can be reached, at the same fineness. This difference is due to that in the latter the fineness of the product is established by the parameters of the air separator. On the contrary, in the open-circuit mills, the overall length of the mill is actually utilized thanks to the anti-agglomerating action of the grinding aid and this causes a fineness increase. The use of grinding aids modifies the technological cycle. Therefore an increase occurs in the dust amount [17,49,50] and in the difficulty of separating it by electrostatic precipitators owing to its higher electric resistance (from 10s to 10 11 Ci.cm) [36,51]. Nevertheless the process can be brought again to its oy.timum conditions by modifying the parameters that affect the mill operation (steel/cement ratio, ciculating load, humidity a.s.o.).
2.2.4
Influence
on the cement properties.
Grinding aids generally have
little influence on the properties of cement pastes and concretes. This is because of the low concentrations of these admixtures with respect to the cement weight. The most important effects can be summarized as follows: the arriount of water required to obtain a paste of normal consistency is slightly increased by glycols and ethanolamines [34,52]; the setting times show slight increases with glycols and ethanolamines [52] and remain unchanged with diethylene glycol [34]. However, according to other authors, they can also be reduced [43]. This means that the effects of grinding aids on setting times are rather changeable; the early mechanical strengths remain generally unaltered [52,53,54] but some increases have been found [22,34,37,55,56].
578
F. Massazza
Moreover, it must be remembered that some surfactants and, partly, the soap of sunflower oil, cause strength to decrease during the first 3 days [21,22]. Anyhow, the differences between samples containing admixtures and plain ones tend to disappear at about 28 days, even if strength increases were found at 28 days [22,57]. Some tests pointed out that the percent strength increase obtained by certain admixtures is higher for blast-furnace cements than for the corresponding Portland ones [56]. Grinding aids seem to exert a slight or negligible effect on the shrinkage cement pastes, at least within the first 28 days [32,58].
of
Grinding aids, depending on their nature, can have a certain influence also
on the pore volume and the distribution
ratio
and the spécifia
surface
2.3
of yore surfaces
[59], the CaO/SiOi
of C-S-H gel [60]. Mechanism of Action
The mechanism of action of grinding aids is not known with certainty and still today these products can be said to be used in a widely empiric way. Their mechanism of action is assumed to be essentially based on the decrease in the resistance to comminution (Rehbinder effect) and on the prevention of agglomeration phenomena. The former effect is based on Griffith's theory [61] according to which brittle materials contain microcracks or, more generally, flaws creating discontinuities in the crystal structure of the material. Under rather strong impacts microcracks propagate through the material as far as to cause its fracture. The same action can create other microcracks where internal imperfections exist. During grinding, however, mechanical stresses act discontinuously. Consequently in the active periods the existing cracks propagate and new ones form, whereas in the inactive ones microcracks can rejoin owing to the attraction of the unsaturated valency forces occurring on the walls of the cracks themselves. When an equilibrium between the two actions establishes, fineness no longer increases. According to Rehbinder's hypothesis [62], shared by other authors [63], grinding aids are adsorbed on the microcracks, so eliminating or reducing their valency forces and therefore preventing microcracks from rejoining. After all, according to this mechanism, the grains show a lower resistance to comminution. After the latter hypothesis, based on the agglomeration prevention, when the ground grains have reached a given fineness, Wan der Waals' forces or electrostatic forces reach such high values that the finest grains are forced to adhere to one another. As a consequence, even if the grinding process is prolonged, the specific surface no longer increases or begins decreasing beyond a certain time. This phenomenon of adhesiveness is also represented by the formation of coatings on the mill walls and the grinding media.
579
Admixtures in Concrete If, however, suitable substances able to neutralize the valencies of the surface charges of the grains are added to the materials, the tendency to agglomeration will decrease.
However, the two effects, that is grain mechanical strength decrease and dust agglomeration reduction, must not be considered as antithetic, even if industrial and laboratory grindings performed by using different admixtures lead to think that grinding aids mainly act on the tendency to agglomeration [30]. In fact, the same molecule of the adsorbed admixture could act first on the fracture strength by preventing microcracks from joining. Then, after the fragment detaching, it could act on agglomeration by preventing the fragments from joining to one another under the compression of the grinding media. These theories are proved by the fact that all the compounds able to affect the grinding process favourably, water included, have a more or less strongly polar character. Since the optimum content of grinding aids is generally included between 0.01 and 0.1 wt%, it can be assumed that within these limits of concentration, depending on the cement fineness and the product nature, the admixture forms a monomolecular layer on the surface of the comminuted particles [27,52j. Since the adhesion is considered to take place between the polar part of the molecule and the surface of the solid, different degrees of coating of the cement grain surface can be obtained if equal amounts of grinding aids are added [20]. For this reason the effectiveness of the saturated fatty acids perhaps decreases, at the same weight amount, as the chain length increases (see Table 2.2) [21 ]. Table 2.2.
Effect of Saturated Fatty Acids on the Grinding Process of White Portland Cement [21]
Acid
Chemical formula
Without grinding aid Formic acid Acetic acid Propanoic acid Butanoic acid Hexanoic acid Decanoic acid Tetradecanoic acid Hexadecanoic acid Octadecanoic acid
CH 2 0 2 C2Hi+0£ C3H6O2 Ci+Hs02 C H 6 12°2 Cj0^20^2 Cli+H2802 C 16H32°2 C 18 H 3 6°2
_
Content
(%) „
0.1 0.1 0.1 0. 1 0.1 0.1 0.1 0.1 0.1
Cumulative residue on N° 0085 sieve
(%)
Specific surface (cm2/g)
8.9 0.8 1.1 1.3 1.5 4.4 4.4 5.5 6.2 6.4
2100 2616 2600 2580 2556 2350 2350 2300 2280 2270
Chemical reactions can occur in addition to the physical phenomenon of adsorption since, by using formic and acetic acid, the corresponding calcium salts form [21]. However, the chemical reaction between admixture and cement grain does not affect grinding. Therefore searching for the boundaries between chemical reaction and adsorption is useless. After all
580
F. Massazza
the tests performed so far seem to confirm that the grinding aid action develops through the neutralization of the free charges or valencies which cause microcracks to rejoin and fine grains to agglomerate. The specific electric resistance of cement, which generally is about 108 Ω.αη, gradually rises to 10 11 Ω.αη when certain amounts of admixtures are added [36,51 ]. 3
ACCELERATORS
3.1
General
These products are added to concrete to reduce setting times and/or to accelerate hardening, Accelerators temperature, slowing down times allows formwork.
allow a rapid concrete hardening to be obtained already at room which therefore avoids thermal treatments, or the hardening at low temperature to be offset. The reduction in the curing an earlier demoulding and therefore a more rapid reuse of
Accelerating admixtures are used in the placing of rapid hardening concrete pavements, groutings, quick repair works, especially at low temperature, and finally as antifreezing agents. Accelerators can be single and composite, inorganic or organic, or more or less complex mixes of different products. They are usually subdivided into set-accelerating and hardening-accelerating admixtures. 3.2
Set-accelerating Admixtures
A too-marked set acceleration can constitute an inconvenience for the corresponding too rapid loss of workability. Nevertheless a mix which sets immediately is required in the placing of the so-called "gunite" (mix sprayed as coating for vertical walls or tunnels) or in other particular cases. Set accelerating admixtures are often used to offset the set delay caused by other admixtures, such as for example water-reducing admixtures [64,65]. 3.2.1 Inorganic products. The most common inorganic set-accelerating admixtures are salts (chlorides, fluorides, carbonates, silicates, fluosilicates, aluminates, borates, nitrates, nitrites, thiosulphates etc.) or alkaline bases (sodium, potassium and ammonium hydroxides) [66,67,68]. A synthesis of their effects is given in Table 3.1 [64], Among these the most known is calcium chloride. As Fig. 3.1 [69] shows, this salt has the double effect to advance the initial set and to reduce the interval between initial and final set. It can be generally said that, if a concrete at room temperature shows an initial setting time of 3 hours, the addition of 2% of CaCl2 reduces this time to 1 hour. Similarly if another concrete, under the same environmental conditions, sets at 6 hours, the addition of the same amount of CaCl2 reduces this time to about 2 hours.
581
Admixtures in Concrete
0,5% 1,0 % 1,5^/ 2,0 %
n
1
i
2 3 4 SETTING TIME
Fig. 3 1.
5 6 HOURS
Initial and final setting times of a cement paste containing different amounts of calcium chloride [from 69].
The effect of calcium chloride is more marked at low temperature and it results to be also more efficacious on slow hardening Portland cements than on more rapid hardening ones [64]. Also the other chlorides have analogous behaviour. The accelerating action becomes stronger by passing from monovalent to bivalent and trivalent chlorides and as the cationic ray increases (see Figs. 3.2 and 3.3) [70].
0.16 Q32
Fig. 3.2
0.96 Molality [-]
Influence of the content in alkaline and alkaline-earthy chlorides in the mixing water on the initial set of Portland cement [70].
582
F. Massazza
QI6
0.32
0.96 Molality C-]
Fig. 3.3.
Influence of the content in alkaline and alkaline-earthy chlorides in the mixing water on the final set of Portland cement [70].
The effect of alkaline carbonates is more marked for pozzolanic cements than Portland ones [71 ]. The accelerating effect of carbonates is considerable not only at 20°C but also at lower temperatures [72]. Nevertheless they reduce strengths [73], even if after 7 days [74]. Sodium aluminate is an effective setting time reducer and therefore it is successfully used in guniting. As Fig. 3.4 [4] shows, its action is stronger, at constant weight, than the one of calcium chloride. A way to transform an ordinary Portland cement into a rapid hardening cement consists in adding high-alumina cement to it. Fig. 3.5 [4] shows that the setting time reduction is mutual for both cements. However, the ultimate strength of these mixes is much lower than the one of the cements. The effectiveness of the admixtures depends on their dosage and therefore it must be remembered that an excessive addition of accelerator to cement causes rapid setting [69,71]. Analogous effect has a temperature rise [69]. 3.2.2 Organic products. The most common organic accelerator is triethanolamine (TEA) used alone or mixed with other organic and inorganic products.
Admixtures in Concrete
583
time (h)
Fig. 3.4.
Influence of sodium aluminate on the setting times of a CPA 325 1:3 mortar versus w/c ratio at 20°C [4]. I «= initial set, F * final set.
As Table 3.1 175] shows, this organic base produces no appreciable effect when the dosage is lower than a certain critical value. Beyond this, there is a sharp reduction in the initial setting time which causes a sort of rapid set. Table 3.1.
No. 1 2 3 4 5 6
Initial and Final Setting Characteristics of Cement Mortars with Added TEA [75]
Per cent TEA
0 0.01 0.025 0.05
0.1 0.5
Initial sett ing time
Final setting time
4.3 hr 4.7 hr 4.9 hr 4.8 hr ~2 min ~6 min
8.3 hr 8.1 hr 8.1 hr 8.4 hr 24 hr
-
The existence of a critical value can explain the different results obtainable by adding equal amounts of admixture to different cements. For example triethanolamine, added in amount of 0.1-1%, caused a set acceleration in a high-alite cement and a delay in a sulphate-resistant cement [76]. However, TEA considerably retards final set at dosages over 0.1% [75]. This action can be related to the fact that TEA retards the hydration of silicates [75,77].
F. Massazza
584
Setting Times I
6 5 4 3 2 1
Alumina Cement^00 Portland Cement 0 Fig. 3.5.
80 20
60 40
40 60
20 80
0 100
Influence of the mixture of Portland and alumina cements on setting times.
Triethanolamine is often used to oppose the retarding effect of other admixtures, such as lignosulphonates, used to reduce the mixing water. One of the advantages of TEA compared with CaCl2 is that it does not create fears of corroding the reinforcement. Diethanolamine [78], sodium formate [76] and saccharose [79] behave as triethanolamine. Many other products were proposed to accelerate setting but there are not exhaustive experiences of them, being mainly mentioned in patents [10]. Organic accelerators are sometimes used together with inorganic products but, of course, in these cases it is not easy to separate and analyse the effects of the different constituents. 3.3
Hardening-accelerating Admixtures
3,3.1 Inorganic products. The most known and used inorganic accelerator is calcium chloride, even if some authors [80] consider that nitrite, nitrate and calcium formate are better. The highest strength increases are reached between the first and third day of curing; of course, the development of these increases depends on the curing conditions of the specimens, the w/c ratio, the type of cement and the percentages of admixtures [69]. As a rule the strength gain, compared with a plain concrete (without CaCl 2 ), can vary between 30% and 100% in the first 3 days [69]. The gain is particularly significant at lower temperatures (Fig. 3.6)[69].
585
Admixtures in Concrete d) Ό 1_
O O
t3 E
80
O
40
D
D O
■
100
7 3 ° F (22 8°C)
eo 20
O H
0
+* Ξ u_
100
ro N
60
0
-K
D
r. +* σ> c
><
σ
'Λ)
0
?0
o c 0) a
■+-'
0)
40°F (4 5°C)
HO 40
0)
νΤλ
80
00 o
t
40
-D
C\J
5 5 ° F (I2.8°C)
80
i
m.
0 60
25°F (-3.9°C)
4(
•>o 0
2% 0% I day
I
P77i
2% 0 %
3 days
1 Υ7Ά
2 % 0%
2 % 0 % Calcium
7 days
28 days
chlor,de
Curing period Fig. 3.6.
Effect of calcium chloride on strength development in concrete at different temperatures.
Table 3.2 shows the influence of some admixtures on the strengths of a Portland cement mortar [81], The table shows, and experience confirms, that CaCl2 has a positive accelerating effect during the first stages of hardening but it tends to lower the ultimate strength. Calcium nitrate does not improve the initial hardening and calcium thiosulphate acts as chloride only with much higher dosages. In both cases ultimate strengths are lower in the plain sample. The strength decrease caused by CaCl2 after 90-180 days can be eliminated by adding also CaO [82] and Al 2 (SOO3 [83]. The strength increase obtained in the cements containing CaCl2 follows the order blast-furnace> Portland> pozzolanic. In the first ones also the slag hydration is activated, in the last ones the pozzolanic reaction is not affected [84].
586
F. Massazza Table 3.2.
%
Effects of Calcium Chloride, Calcium Nitrate and Calcium Thiosulphate on the Strength Development of Portland Cement Mortar [81]
1 dav B C
7 days B C
3 days B C
28 days B C
0
15.8
39.0
34.3
120 52.8
221
79.0
379
CaCl2
1.0 2.0 3.0
20.8 24.2 26.7
69.5 82.5 85.3
40.0 46.9 45.5
150 172 177
54.9 51.5 55.5
291 243 265
76.1 77.1 74.9
381 352 364
Ca(N0 3 ) 2
2.0 5.0
7 15.1
18.7 42.3
34.2 41.0
113 173
47.3 63.1
193 261
74.6 77.3
316 351
1.0 17.1 2.0 18.1 3.0 18.8 5.0 20.1 6.5 21.1
44.7 46.7 55.8 59.3 69.3
34.6 35.6 37.8 40.6 37.6
117 127 130 150 143
51.9 53.2 53.1 55.7 50.9
210 203 237 205 207
73.6 76.2 75.2 66.1 63.4
341 328 366 310 268
B: Bending strength, C: Compressive strength, unit of strength: kg/cm2. 3,3,2 Organic products. The cement hardening is accelerated by many organic products of which calcium formate is a typical representative (see Fig. 3.7). This compound is less effective than calcium chloride but it does not decrease 28-day strengths and presents no risk of reinforcement corrosion [86]. The literature gives many other examples of organic accelerators of varied compositions: hydroxylated carboxylic acids, formaldehyde and paraformaldehyde, cationic polyesters, phenolic and epoxy resins etc. [10]. For example, calcium sulphonate increases the compressive strength from 7 to 71 kg/cm2 after 6 hours and from 560 to 663 kg/cm2 after 28 days, when added in significant amounts (4-10 wt%) [87]. The technological advantages obtained by using these products are important but, in most cases, they require verification. As regards their possible applications, their cost has been prohibitive. 3.4
Collateral Effects
All the accelerators have, besides their main action, sometimes important collateral effects that must not be ignored.
3,4,1
Rheology of the mixes.
Some accelerating compounds show a
fluidizing
effect on the cement paste such as, for example, alkali-metal chlorides and nitrates, alkaline-earth-metal chlorides and nitrates, aluminium chloride and trivalent ferric chloride [70], The plasticity of the cement paste containing alkaline-earth-metal chlorides is higher than that of the plain cement, till about 1 hour1s hydration. On the contrary alkaline carbonates reduce the cement plasticity and, as alkaline silicates, they increase the water demand [67,70,73]. A favourable effect of these admixtures, related to their accelerating action, is the reduction in the segregation phenomena occurring in fresh concrete.
Admixtures in C o n c r e t e £ 700 c
587
^
v.
* 600
4% y | 500 o O
200
100
0
/
'A
h
/
400
300
/
1 1 /
/
f
Without
/ /
accelerating
W/C = 0..35 C = 'Χ2Ό kg/rr>3
/
/ 4
admixture
6 8
12
16
24
.
48
Curing - hours ( Y - scale)
Fig. 3.7.
Influence of calcium formate on the early strengths of a concrete [85].
3.4.2 Durability. The accelerating action of calcium chloride has also some negative aspects that must not be ignored. In fact calcium chloride favours the reinforcement corrosion, even if it must be remembered that the steel attack depends on its characteristics [88], The reinforcement corrosion is infrequent since the alkaline environment of the cement paste passivates the steel by the formation of a protective oxide surface film which is stable until the pH is high enough. Since chlorides decrease the pH [89], the film stability lowers and therefore the corrosion process is favoured. Corrosion can be prevented by adding ΝΗι*Ν03 [90] or CaO to CaCl2. CaO rises the solution pH which increases from 7.5-8 to 12-13 [82]. Also Ca nitrate is an inhibitor of the reinforcement corrosion. Calcium chloride, besides decreasing ultimate mechanical strengths, may also lower the resistance to sulphates of concretes [91], but this conclusion is not shared by all [92]. Some authors found that some accelerators increased the resistance to frost of concretes. This is not surprising since the frost resistance of a concrete is the higher the more advanced the degree of hydration when freezing occurs.
588
F. Massazza
3.4.3 Dimensional changes. Calcium chloride increases shrinkage by 10-50% [93], An analogous effect is obtained by high contents of triethanolamine [94], This admixture also enhances the shrinkage caused by lignosulphonates [95,96,97]. Shrinkage increases were also obtained on C3S pastes containing CaCl2[98,99 ]. Generally, the concrete shrinkage seems to increase in the presence of set-accelerating admixtures [100], The strong shrinkage caused by CaCl2 can be attributed to the higher degree of hydration and the higher specific surface of the hydrated cement pastes [84]. Nevertheless the CaCl2 addition decreases the wideness of the crack caused by drying [101]. The effect of CaCl2 on shrinkage is lower for pozzolanic cements [84]. Creep increases by adding CaCl£ [95,102], calcium nitrite and nitrate [103]. This effect is assumed to be due to an intensified contribution of the changes in the pore distributions [99]. Triethanolamine causes an increase if the concrete specimens are loaded after 7 days' curing, whereas no difference is noticed for specimens loaded after 28 days [95]. This result is likely to be the consequence of the hardening delay caused by the admixture. Finally triethanolamine intensifies the creep increase of concrete caused by lignosulphonate [96,97]. 3.4.4 Heat of hydration. CaCl2 increases the heat of hydration evolved during the first phases of the hydration [64], The second peak characterizing the heat evolution curves of cement pastes is advanced and intensified in the presence of CaCl2· The effect is observed with both Portland [81] and blastfurnace [104] cements. 3.4.5 Antifreezing properties. Antifreezing properties, which favour winter concretings, are attributed to calcium chloride and also to other inorganic and organic accelerators [10]. However, it must be noted that the decrease in the melting point of the mixing water, caused by CaCl2 employed at the usual contents (<2%), is small (<0.9 C ) . Therefore, the antifreezing action of the admixture is essentially due to its capacity of increasing the rates of the hydration reactions and, thus, of the heat evolution [105].
3.5
Composition and Structure of the Hydrated Phases
The addition of accelerators to cements generally modifies the composition and the structure of the hydrated phases to a different, but small, extent depending on the admixture nature. Some authors found that, at the same degree of hydration, the cements containing CaCl2 have a higher specific surface than the plain ones [106], [84], whereas others found no difference [107]. This discrepancy probably depends on the precision in determining the degree of hydration of pastes. The surface increase of cements after 7 days was observed in Portland, pozzolanic and blast-furnace cements [84], but this decreased (after 28 days) for the last ones [108], The C3S pastes containing CaCl2 have a higher specific surface area than the plain ones [109,110], even if some authors [118] found the opposite. Also TEA enhances the specific surface of hydrated C3S [77], CaCl2 increases the specific surface as well of the C2S hydration products [142],
Admixtures in Concrete
589
The presence of CaCl2 modifies the pore size distribution of the hydrated pastes of C3S [107,111] and C2S [112]. The admixture nature modifies the morphology of ettringite [111,113], C-S-H [114] and Ca(0H)2 [115]. For example, by hydrating C3S in the presence of CaCl2, it was possible to notice a "honeycomb" instead of "sponge-like" gel [116] or a "foil-like" C-S-H(I) phase and a fibrous C-S-H(II) phase 1117]. It was confirmed that CaCl2 modifies the chemical composition of C-S-H since it increases the CaO/Si02 ratio of cement [106], C3S 1117,118] and C2S [119] pastes. The strength rise caused by CaCl2 is attributed to an enhanced degree of polymerization of the silicate hydrates [120]. Nevertheless the validity of these results also for cements must be still established with certainty [121 ]. As regards Ca(0H)2 no hexagonal plate appears in the presence of CaCl2 [116], or at least the hexagonal appearance becomes less marked as the CaCl2 concentration increases [117], Of course, admixtures can cause the formation of other phases, such as nitroaluminate hydrates [122], chlorohydrates [123] and chromate-derivatives analogous to ettringite [124]. Anyhow these compounds can form when admixtures are present in high concentrations. There are also signs that during the C3S hydration in the presence of CaCl2 a chlorinated complex with silicate hydrates forms [117,125]. However, the amount of combined chloride is relatively small (~0.40 g/100 g C3S) and depends on the Cl~ concentration in the solution contained in the pores [126]. In the presence of triethanolamine there is a reduction in the Ca(0H)2 content compared with the expected one and a higher Ca0/Si02 ratio in C-S-H [77,127]. This fact*was explained by assuming that a CaO accommodation occurs in the C-S-H lattice [128] or the formation of not crystalline Ca(0H)2 is favoured [77]. Anyhow triethanolamine causes only small changes in the morphology and microstructure of the hydration products of C3S and C2S [127,129,130,131]. A complete review on the structure alterations induced by the other accelerators would be too long. In any case it must be pointed out that the changes relevant to the cement gel, if any, are small whereas the ones relative to Ca(0H)2 seem to be more important [10]. 3.6
Mechanism of Action
At present there are no theories explaining the mechanism of action of the different accelerators in an exhaustive and controllable way. Therefore only the particular effects related to their presence in the systems concerning cements can be mentioned. 3,6,1 Action on setting times. It is a consolidated opinion that the set acceleration is generally due to an intensification of the hydration of aluminates. Triethanolamine [75,132,133,134,135,136,137], sugar, formate [113], chloride [100,104,113,138,139], calcium nitrite and nitrate [122,140] intensify the formation of ettringite, either pure C3A or the one present in cements is considered. For example, by adding 0.3wt% of TEA, all calcium sulphate has reacted after 20-30 minutes [79].
590
F. Massazza
The more rapid set of the cement paste would be also attributed to the crystalline shape of ettringite which, during precipitation, would produce a felt of very fine needles. During the hydration of Portland cement in the presence of calcium chloride, ettringite forms initially as far as the complete disappearance of gypsum. Subsequently, if free tricalcium aluminate still exists, Friedel salt (3CaO.Al203.CaCl2.10H20) forms [126,141]. At higher temperatures (40-80°C) the formation of chloroderivative prevails on sulphoaluminate [126]. 3. 6.2 Action on hardening, CaCl2 accelerates the set and hardening of cement, not only because it accelerates the ettringite formation but also because it increases the reaction rate of C3S [109,118] and S-C2S [84,119]. Also calcium formate acts positively on hardening since it accelerates the C-S-H formation, even if with lower effectiveness than CaCl2 [113]· The effect on C3S is mainly attributed to the fact that the diffusion rate of the Cl" ions is much higher than the one of the cations accompanying them. Since the Cl~ ion penetrates into the hydrated layers covering the C3S grains more quickly than the cations, a counterdiffusion of OH" ions must occur to maintain the electric equilibrium in the solution. As a consequence, the precipitation of calcium hydroxide is accelerated as well as the decomposition of calcium silicates [143]. The accelerating action of CaCl2 was also attributed to other causes, such as the alkalinity reduction of the liquid phase, and consequent increase in the dissolution rate of hydrolysis lime [144], a salt adsorption on the cement grains which would facilitate the attack from H2O [145] and, finally, an increase in the concentration of calcium ions in the solution caused by the presence of salt [146]. In any case, CaCl2 would not modify the mechanism of hydration of C3S but only some kinetic parameters governing the formation of crystalline germs [147]. Very little is known of the mechanism of action of the organic hardening accelerators. 4 4.1
RETARDERS General
Cements usually begin setting 2-5 hours after mixing and begin hardening after about 3-7 hours. Nevertheless, in some cases these limits can be unsatisfactory, for example when it is necessary for the concrete to remain pasty for 24-48 hours. This occurs when the construction joints, which are a weak point in structures, need to be eliminated. The problem can be solved by using suitable admixtures retarding the hydration of the last placed concrete until (after 16, 24 and 36 hours) the recasting with fresh concrete is started again [148]. Likewise, the surfaces of the concrete roads can be roughened by covering the fresh concrete surface with an emulsion containing a retarder or with paper or fabric soaked with admixtures and by removing the not yet hardened surface layer by brushing or water jets after 6-48 hours [149]. The use of retarders has a great importance also in the most common practice, such as for example when setting times need to be prolonged in order to cast in warm climates, to transport fresh concrete for long distances (especially in summer), for groutings etc.
591
Admixtures in Concrete
The problem of retarding the cement set can become even essential in some special cements, such as the "regulated set" or "jet" cements and expansive cements. These products are indispensable also to retard set during the cementing of oil and geothermal wells or under conditions of high temperature. Retarding admixtures can consist of both inorganic and organic products. Besides substances especially used to retard setting, there are others whose retarding action is secondary. This is the case of air-entraining admixtures, plasticizers and grinding aids. 4.2
Inorganic and Organic Retarders
Among the inorganic retarders there are: some acids (boric, phosphoric, hydrofluoric, chromic), the respective salts, and some oxides (of zinc and lead). A typical retarder of this group is zinc oxide that, as Fig. 4.1 shows, is very effective even if added to cement in small amounts [150]. An analogous action, even if less marked, is shown by phosphoric acid [4],
50-
n 4o-\ Φ
I 30. Z 20H Φ
10
0r10
0,15
r— 0,20
ZnO V· Fig. 4.1.
Influence of ZnO addition on the initial set of a 1:3 Portland cement mortar, w/c = 0.35, curing at 20°C [150].
Among the numerous organic compounds used as retarders there are: lignosulphonates, hydroxycarboxylic and carboxylic acids and their salts, amines and aminoacids, carbohydrates and their oxidation products, such as the acids and their respective salts. The effect of sodium gluconate is shown in Fig. 4.2 [4J. Some of these agents are plasticizers and air-entraining admixtures.
592
F. Massazza
% based
Fig. 4.2.
on cement
weight
Effect of varying contents of a set-retarder (gluconate-based admixture) on the initial and final setting times (Mortar 1/3, W/C = 0.50, 20°C) [4].
Many other organic substances were proposed as set- and hardening-retarders but, for brevity's sake, they cannot be considered here. 4.3
Factors Affecting the Action of Retarders
A great number of factors affect the action of retarders. The dosage of the admixture is of great importance and in general it can be said that the retarding effect increases as the admixture addition increases (see Figs. 4.1, 4.2). The effect of retarders is more marked at low than high temperatures. This behaviour is shown in Fig. 4.3 where it can be seen that the delays are considerably reduced by passing from 10 to 30 C [148]. Moreover, many organic admixtures (mono- and dicarboxylic acids, sulphoacids, amines, mono-, di- and trihydric aliphatic acids were found to lose their effectiveness at 60°C [151 ]. Also the way of adding the admixture affects the retarding properties since calcium lignosulphonate, alone [152] or with K2CO3 [153], was found to retard setting even more when added to the mixing water.
593
Admixtures in Concrete
AB C
D
+
G30
| 201
V. v \
I I 2 io 1 1
\
2 S- 0
\ 'h
■V\
\
X
10
20 Initial
Fig. 4.3.
1
30 setting
time
40 (hours)
Initial setting time of concrete versus temperature and retarder content [148].
Of course, the effect of the retarder not only depends on its nature but also on that of cement. As admixtures mainly act on clinker, the optimum amount of retarder decreases in blast-furnace cements with increasing slag content [86]. Other parameters affecting the delay are the fineness of cement, the [78,154,155]. composition of concrete and the water/cement ratio 4.4
Other Effects
Retarding admixtures improve the workability of mixes at both low and high temperatures. This fact is particularly advantageous in works demanding a prolonged time of hot-workability of mixes. As Fig. 4.4 shows, the rapid loss of workability undergone by concretes at 55-60 C can be efficaciously opposed by the addition of a retarder-plasticizer [156]. There is no strict correlation between set and development of strengths in mortars and concretes containing retarders. As an early set does not necessarily result in a high early strength, so a delayed set does not always cause a strength reduction [157,158], In fact, both organic (see Table 4.1) and inorganic (see Fig. 4.5) retarders can increase strengths at different ages [159,160]. The effects can be higher on concretes since many organic retarders have a fluidizing action enabling a decrease in the w/c ratio [148]. The effect on strengths depends on the nature and the amount of admixture added to cement. Figure 4.6 shows that, up to 0.5%, triethanolamine does not considerably affect the strength development but, with an addition of J%, the strength recovery only occurs after 28 days [79].
594
F. Massazza Table 4.1.
Influence of Some Set-retarders on Mechanical Strengths. Portland Cement 1/3 Mortar [4]
2d
1d
90 d
28 d
C
TF
C
TF
C
TF
C
36
120
49
220
77
385
88
0.5%o l%o
30 4
102 13
51 29
220 120
80 77
480 440
83 81
1%0
2%o
20 1
72 10
50 26
242 85
68 56
375 285
76 75
.4 υ
A
Plain Sucrose
7d
TF
Glucose
TF
C
462
90
550
610 550
84 96
545 465
81 81
640 615 600 525
?
\
-£ >
<
v
V
^**"* **»
\"^^ \
10
Fig. 4.4.
^
20
^
\
1 > 1
30 40 Minutes after mixing
Effect of the amount of a lignosulphonatebased retarder on time-dependent change in slump in a hot-mixed normal weight concrete (NWC) and a lightweight aggregate concrete (LWC). Mixing temperature: 55-60 C; Cement content: 530 kg/m3 [156]. A - NWC 55°C (131°F) Retarder 0.25 wt% of cement; B - NWC 55 C Retarder O wt% of cement; C - LWC 60°C (140°F) Retarder 0.25 wt% of cement; D - LWC 60°C Retarder O wt% of cement.
595
Admixtures in Concrete
700h
600
650
_
^
con
—
" -^
0660
—»··'■'
598^<.^606
572, < £
£^2 4 ϋÜ ^- - o - 700 ^ — ,ο676 ■—
[i5h]
370 [0h]
>620 Plain
500
9
400
·?
300
ΖηΟ 0 %
Plain
ΖηΟ 0 15 % Immediate placing [01 o
200
o
Ζη0 0 Ι 5 % Placing after 15th [15] ZnO 0 15 % Placing after 24 th [24]
14
25
28
Days
Fig. 4.5.
Effect of ZnO addition on the compressive strength of a Portland cement paste. w/c « 0.36. Water curing at 20°C [150].
Some retarders can show an inversion of the effect. In these cases concrete sets rapidly and workability decreases [148,161]. This means that the dosage and the use of retarders should be preceded by careful preliminary tests. The setting delay is a consequence of the retarded beginning of the cement hydration process. For this reason these admixtures also retard the development of the heat of hydration (see Figs. 4.7 [4] and 4.8 [162]) and cause a decrease in the chemically combined water in the cement paste [163] (see Fig. 4.9). Also the specific surface of the hydration products is simultaneously decreased [163]. As already mentioned, the delay is generally accompanied by a fluidizing effect which modifies the rheological properties of pastes. This can be attributed, at least partly, to the enhanced air entrainment [164]. As regards drying shrinkage of concrete, it can be said that set-retarding admixtures have generally rather limited effects [100,165]. Some retarders caused volume instability with certain types of cement, however, no explanation of the phenomenon has been given so far [161].
596
F. Massazza 600 PZ 450 F S 500
. ' ^ < ^ f \
-400 ^ ^l
S 300
A'
* 200
1
C4
1
2iII
0}
*
'
7y
'
-'
'
\
1
/ / / f
\
ι (
2
Ί
1 28
7 Curing
Fig. 4.6.
\
1
/
Br
v.
α 100 o
/
{days)
Influence of triethanolamine (TEA) on compressive strength of a Portland cement PZ 450 F according to DIN 1164. A = without admixture; B, C, D = 0.2%, 0.5%, 1.0% TEA respectively [79]. 4.5
Mechanism of Action
The mechanism of action according to which retarders act on cement has not yet been explained completely. Several theories were proposed about the matter but none is able to give a valid explanation in all cases. Therefore the delay is likely to be the result of several different causes acting simultaneously. Owing to the composite nature of cement, the same admixtures can be expected to act differently with the various phases of clinker. In fact, some admixtures were found to have a different effect on aluminates and silicates. A typical case is triethanolamine and sugar that accelerate the ettringite formation but retard the Ca(0H)2 one. Therefore they accelerate setting but retard hardening. The main theories proposed to explain the action of retarders can be summarized as follows: Admixtures are adsorbed on the surface of the anhydrous products which are, therefore, protected from the water attack. Admixtures react with the anhydrous compound and the reaction products cover the grains with an insoluble and waterproof layer. Admixtures hamper the nucleation and growth of the hydrated crystals since they form complexes with the hydrated phases and are incorporated in the lattices of the hydrated crystals.
Admixtures in Concrete
597
Sugar
Fig. 4.7.
Influence of sugar and zinc oxide on the heat of hydration of cement. Thermos flask method. 1:3 AFNOR Mortar [4].
In any case, once the admixture has been combined and removed by the solution, the hydration resumes its normal course. The major remarks that can be made about these theories are: 1
The amount of calcium lignosulphonate, typical retarder-plasticizer, which is adsorbed on both C3A [166] and C3S [167] is very limited. The adsorption on the anhydrous compounds is also very low with other admixtures [168], In any case the adsorption on the anhydrous phases seems to cause an initial acceleration of the hydration, whereas the interaction with the hydrated products brings about a delay [169,170]. 2 No correlation was found between insolubility of calcium salts of carboxylic acids and delay, however it cannot be excluded that some retarders (such as zinc) act via the formation of an insoluble protective layer. 3
The influence of retarders on the growth and morphology of crystals is an ascertained fact. A slowing down of the ettringite formation [164,171] and changes in its morphology [79] were found in the cements containing admixtures, whereas a considerable alteration in the sizes and morphology of crystals from Ca(0H)2 was noticed in C3S pastes with admixture [115]. As Figs. 4.10 and 4.11 show, the hydration products can adsorb appreciable amounts of admixtures [168].
598
F. Massazza
Days Fig. 4.8.
20|
0
Retarding effect of different sodium gluconate amounts on the heat of hydration [162], (1) Plain CPA; (2) 0.5%o; (3) 1%0; (4) 2%0; I
I
I
1
I
20
40
60
80
100
Hardening time Fig. 4.9.
I
(days)
Chemically combined water in cement pastes containing 0.1, 0.2 and 0.3% sodium gluconate [163].
Admixtures in Concrete
599
005 0 10 015 Initial Concentration (wt%) Fig. 4.10.
Apparent adsorption of salicylic acid on C3S and 3-C2S [168].
005 0 10 0.15 I n i t i o l C o n c e n t r a t i o n < wt % )
Fig. 4.11.
0 20
020
Apparent adsorption of salicylic acid on pasteand bottle-hydrated C3S and on bottle-hydrated S-C2S [168].
F. Massazza
600
In these cases the action that retarders have on the hydration of cements could be explained as follows. When the admixture is employed in the usual amounts, the initial hydration of aluminates and, mainly, silicates is delayed since the retarder strongly affects the formation and growth of the crystals of the hydrated phases. When the retarder is removed from the solution, since it is preferentially adsorbed on the hydrated phases containing alumina, the hydration of silicates begins again vigorously. As a consequence, the strength of the samples containing admixtures decreases at short ages but is equal to plain cements at 28 days. If, on the contrary, the initial content of admixtures is high, the aluminates are insufficient to remove it completely from the solution and the hydration is stopped or, at least, strongly retarded. This mechanism is justified by different experimental results: (a) The addition of C3A eliminates the strong delay caused by lignosulphonate (LSA) on the hydration of calcium silicates [129]. Table 4.2 shows that C3S and alite hydration, as measured by the loss on ignition, is strongly accelerated by the presence of tricalcium aluminate [172]. (b) Setting is retarded if the admixture is added after some minutes of mixing [152,153]. In this case the initial hydration products of C3A formed in the absence of admixture are assumed to have a lower adsorption capacity [173,174,175]. Consequently a higher amount of * retarder will remain available in the solution and the hydration rate of C3S will be considerably reduced [172]. Table 4.2.
Silicate paste C3S only C3S + 5% Alite(M) Alite(M) Alite(J) Alite(J)
Ignition Loss (% at 1100°C) of Hydrated Silicate Pastes [172] No additive 7 days 14 days
C3A only + 5% C3A only + 2% C3A
17.2 17.8 13.2 16.9
19.5 21.8 14.8 18.6 16.7 15.9
7 days
3.9 4.1 2.6
With LSA 90 days 14 days 1.4 21.5 1.4 16.1 2.0 19.2
4.0 22.2 1.9 19.8 21.6
180 days 2.5 25.4 2.1 22.0
The retarding effect of lignosulphonates was not attributed directly to the lignosulphonic acid salts but to the simple sugars in the commercial products. However, it was pointed out that these compounds, owing to their weak content have a slight retarding effect and therefore it is not possible to eliminate them from the commercial products [176]. Assuming that pure lignosulphonic acid has a little retarding action, it must be sugar acids with the respective salts (always present in commercial lignosulphonates) which strongly retard the C3A hydration [177].
Admixtures in Concrete
601
Of course, the effectiveness of admixtures depends on a great deal of factors, only some of which are recognized with certainty. For example, the retarding action is related to the nature and number of functional hydroxyl groups contained in the molecule [178,179,180,181]. The retarding action seems to be attributed not only to hydroxyl groups but also to carboxylic and carbonylic groups as well as to the number and arrangement of functional groups in the molecule [182,183]. The example given by diphenols is typical: pyrocatechol accelerates setting, resorcinol is practically inactive and hydroquinone retards it [151 ]. The different intensity of action found in the gluconates of Na, K, Mg and Ca seems to be related to the ionic radius and electronegativity of the cation [184]. Also the strong alkalinity of the cement paste plays an important role, as shown by the fact that glucose is less effective than its oxidation products, since it is less stable in basic medium [185]. To conclude these short notes it can be said that, although much progress in interpreting the retarding action of the different compounds has been made, the diversity of their nature and the complexity of the cement hydration process make additional studies necessary for the complete understanding of the phenomenon. 5
PLASTICIZERS 5.1
General
The organic substances or the combinations of organic and inorganic substances which allow a reduction in the water demand of concrete, at the same consistency, or give a better workability to the mix, at the same water content, are classified as plasticizing admixtures. The advantages are considerable in both cases: in the former, concretes are stronger, in the latter they are more workable. This advantage concerns all concretes but it is particularly important for thin and vary reinforced concretes or for the conveyance of concrete in pipelines. The basic products constituting plasticizers can be classified as follows: anionic surfactants, such as lignosulphonates and their modifications and derivatives, resin and alkaline abietate soaps, salts of sulphonated hydrocarbons, alkyl aryl sulphonates; nonionic surfactants, such as polyglycol esters; acid of hydroxylated carboxylic acids and their modifications and derivatives; other products, such as carbohydrates, polysaccharides, etc. Among these calcium, sodium and ammonium lignosulphonates are the most used. As a matter of fact the compounds that exhibited more or less marked plasticizing properties are so numerous and diverse that at present it is practically impossible to list them completely. The base products used to prepare plasticizers have often other undesirable properties (setting retards, a ir-entr ai riment etc.) and therefore they are frequently mixed with other products which reduce or eliminate the negative effects and also improve the positive ones [186,187],
602
F. Massazza
Plasticizers are used in the amount of 0.1-0.4% of the cement weight. Owing to their small content, their dosage and their homogeneous distribution in concrete must be checked carefully. For this reason the products dissolved in water are preferred to the dry ones. Plasticizers can be employed with all the usual types of cement. Some polymeric compounds, such as polyethylene oxides, improve the concrete flow even if they do not increase slump. They are used successfully in pumping of ordinary and lightweight concretes [188,189,190]. 5.2
Effects on Mortars and Fresh Concretes
5.2.1 Mixing water reduction. By reducing the mixing water, at constant workability, the mechanical strength of concrete increases appreciable. At the same time the risk of segregation is reduced and the homogeneity and compactness of mixes are improved. The water reduction obtainable with these admixtures, provided negative effects do not occur, is generally included between 5 and 15% [4,191,192,193,194,195,196,197], even if different patents guarantee higher reductions. Obviously the effects caused by an admixture depend on its dosage and nature (see Table 5.1) [198]. 5.2.2 Workability increase. At the same w/c ratio, admixtures improve the mix workability with an effectiveness depending on their dosage (see Fig. 5.1) [199].
J
O
\
I
L
0,2 0,4 Admixture - %
0,6
Fig. 5.1. Workability of mortars containing increasing amounts of citric acid [199].
603
Admixtures in Concrete
Table 5.1.
Effect of Some Admixtures on Concrete Properties [198]
Cement content (kg/m3)
Water content (kg/m3)
Slump (cm)
Vebe value (sec.)
300
195
5.5
13.2
1.4
159
292
AEA - 3
0.04 0.08 0.12
300 300 300
190 183 174
4.5 4.5 5.8
13.4 8.9 6.4
2.6 4.0 6.8
167 168 142
305 300 249
WR(A) - 3
0.25 0.50 0.75
300 300 300
177 169 158
4.3 4.6 6.0
9.6 8.2 7.5
1.9 4.2 5.6
21 1 228 256
349 349 373
WR(B) - 3
0.25 0.50 0.75
300 300 300
184 177 169
4.7 4.7 6.1
16.6 8.8 7 .4
2.3 3.3 5.7
230 231 247
365 367 384
WR(C) - 3
0.25 0.50 0.75
300 300 300
176 174 170
5.1 5.2 4.8
1 1 .4 16.3 17.0
2.2 3.2 3.3
214 238 244
343 355 382
WR(D) - 3
3.0 6.0 9.0
300 300 300
171 161 151
5.5 5.6 8.0
7.2 11.6 19.6
2.1 1 .1 1.4
232 271 212
351 377 296
WR(E) - 3
0.40 0.80 1.2
300 300 300
185 177 172
5.0 5.2 4.5
15.4 17.4 19.0
2.0 2.3 2*4
214 228 238
351 372 411
Admixture content
(%) -
Mix. No. Plain - 3
Air content
(%)
Compressiv«> strength (kg/cm 2 ) 7-day 28-day
-
500
189
5.5
10. 1
1.1
415
550
AEA - 5
0.04 0.08 0.12
500 500 500
182 182 179
4.6 4.8 4.8
12.8 1 1 .4 10.5
3.1 4.1 4.3
387 379 366
544 539 531
WR(A) - 5
0.25 0.50 0.75
500 500 500
185 177 168
4.5 4.6 4.6
21.4 17.8 9.8
2.2 3.3 5.2
448 436 377
576 638 590
WR(B) - 5
0.25 0.50 0.75
500 500 500
179 176 166
4. 1 4.6 5.2
31.0 21.4 20.6
2.4 2.7 4.5
440 460 466
621 653 651
WR(C) - 5
0.25 0.50 0.75
500 500 500
182 177 175
4.5 5.6 4.0
27.0 22.1 16.0
2.0 2.0 1.9
443 475 468
616 703 702
3.0 6.0 9.0 0.40 0.80 1.20
500 500 500
167 158 144 181 174 163
5.9 9.6 9.8
13.4 6.1 3.9
1.4 0.8 1.1
714 725 733
4.5 5.8 9.2
22.6 7.0 4.0
1.5 1.8 1.7
537 544 586 484 534 578
Plain - 5
WR(D) - 5
WR(E) - 5
•
AEA WR(A) WR(B) WR(C) WR(D) WR(E)
500 500 500 Air-entraining Water-reducing Water-reducing Water-reducing Water-reducing Water-reducing
agent agent agent agent agent agent
(A) (B) (C) (D) (E)
Salts of wood resins Calcium ligninsulfonate Derivatives of ligninsulfonate acid Polyol complexes Highly condensed triazine compounds Polyalkyl aryl sulfonates
675 715 770
604
F. Massazza
The action of the admixture and the optimum content which generally act simultaneously.
depend on many factors
Among these the mineralogiaal composition of clinker [200,201 ] plays an important role since the optimum content of the admixture increases as aluminates increase [202]. In fact, as Fig. 5.2 shows, the flow of mortars containing 0.2% calcium lignosulphonate decreases as the C3A content increases [201]. Nevertheless the fluidizing effect of some phenol-formaldehyde resins was found to increase as the C3A percentage in clinker rises [203].
46
r
■S» 4 2 | 38
si 1*
34
Addition
of 2%0 calcium
lignosulphonate
30 26 22 18
O O
££
14
o o 10
II
6 2
-L 8
9
J10
I II
1 12
-L 13 14 %C3A
C3S/C2S = Const Fig. 5.2.
Reduction in the workability of a cement mortar containing calcium lignosulphonate vs. percent content in tricalcium aluminate [201 ].
Plasticizers do not modify [204], or at most slightly retard, the workability loss that concrete undergoes with time. However, increases in the workability loss, which can only partly be offset by overdosages, were indicated [206]. The addition of water to recover the initial workability of concretes mixed for some hours (retempering) can be indispensable, for example in ready-mixed concrete, to allow concrete to be discharged and placed. Nevertheless this practice lowers strengths considerably. On the contrary, retempering on concretes containing plasticizers-retarders at dosages above the usual ones has a favourable effect since it allows the total water content to be reduced, resulting in a mechanical strength increase [206,207].
605
Admixtures in Concrete
In ordinary concretes workability generally decreases as temperature rises, but the effect can be different in the presence of plasticizers. In fact, as Fig. 5.3 [208] shows, three results can occur: workability loss (A-type admixtures based on lignosulphonates and air-entraining admixtures), no variation (B-type admixtures such as sodium salt of a poly alkyl aryl sulphonic acid),and workability increase (C-type admixtures, that is of the improved B-type).
| 360 W/C*0,35
I
g
320
280 240 200 160 120 20
Fig.
5.3.
30
40
50
Temperature - °C
Relationships between temperature and flow value of mortars containing different types of water-reducing admixtures [208].
At constant slump, mixes demand increasing water quantities with increased temperature [208], as is shown in Table 5.2. It is also noticed that, by using A- and B-admixtures, the water reduction does not change from 20 C to 50 C but it increases considerably (from 25 to 34%) with C-admixtures. Also the type
of aggregate
affects the water reduction [198].
Plasticizers modify the viscosity of the cement paste. Figure 5.4 [209] shows the changes in the apparent viscosity of a cement paste containing increasing percentages of sodium lignosulphonate (LS) and polyvinyl alcohol (PVA) at different degrees of polymerization. Viscosity shows a sharp decrease on using lignosulphonate beyond a certain concentration whereas it increases with polyvinyl alcohol as the content and the polymerization degree of the admixture increase.
F. Massazza
606 Table 5.2.
Type of admixture plain A B C
Influence of Temperature on the Reduction in Unit Water Content of a Portland Cement-based Concrete Added with Three Water-reducing Admixtures [208]
20 c >C W/C Slump (cm) (%) 7.8 8.0 7.7 8.3
45.0 40.6 38.0 33.6
40C >C W/C Slump (cm) (%)
50C >c W/C Slump (cm) (%)
47.8 42.2 40.0 31.7 .
48.3 43.4 41.1 32.2
8.2 8.4 8.2 8.0
Reduction i n the u n i t water coicitent 20°C
8.2 8.3 8.5 8.2
40°C
.
.
.
-10% -15% -25%
-12% -16% -33%
-10% -15% -34%
A = lignosulphonate B s poly-alkyl aryl sulphonate C = improved poly-alkyl aryl sulphonate
.8*
L PVA 2390
5 /
PVA1750
§40 •2
i
1 /l^
^30
20
^5*-
^PVA 610
•
V ^ _ L S 10
0
•
1
0.1
0.2
'
0.3 Amount
Fig. 5.4.
■
0.4
'
0.5
of LS & PVA
50°C
1 (%)
Influence of sodium lignosulphonate (LS) and polyvinyl alcohol (PVA), at different degrees of polymerization, on the fluidity of cement pastes [209].
607
Admixtures in Concrete
5,2.5 Secondary effects. The use of plasticizers involves not only the primary effect of fluidization but also secondary undesired effects, such as the delay of set and the air-entrainment in concretes. Products retarding setting times are mainly lignosulphonates [170,191,210, 211] and salts of hydroxycarboxylie acids [157,191]; delay is generally limited to 1-2 hours at ordinary dosages but it becomes excessive at overdosages (see Table 5.3) [194], Table 5.3.
Effect of Overdose of a Standard Plasticizer on the Initial Setting Time of Portland (A) and Slag (B) Cements [194]
Nr.
% Admixture
1 2 3 4 5 6
«» 0.2 0.6
-
0.2 0.6
Cement
A A A B B B
Initial set 210 250 450 275 325 550
min. min. min. min. min. min.
A = Portiand Cement 350 F B ■ Slag Cement 350 L Also the impurities in the admixtures play an important role: at dosage >0.2% of the cement weight commercial calcium lignosulphonate causes rapid set whereas purified CaLS and NaLS retard set. To obviate the delay other products such as sodium carbonate, able to accelerate set, are added to plasticizers - retarders [65]. The action of sodium carbonate is complex since, for example, it stops the C^AF hydration for a certain time, when mixed with lignosulphonate [212], In some cases, lignosulphonates and other plasticizers-retarders can give false set when used with high or moderately high content in C3A and alkalis [213,214]. The phenomenon can be prevented by adding the admixture in the concrete mixer after about 2 minutes' stirring [213,214] or by using high-SÛ3 cements [214]. A frequent collateral effect is the air entrainment in concrete. This phenomenon is related to the surfacting nature of many plasticizers and depends, of course, on the nature of the used additive. In fact anionic surfactants entrain more air than the nonionic ones[215]. The air entrained in concrete contributes to the fluidizing action. Nevertheless it. must not reach too high levels since it reduces mechanical strength. Table 5.4 shows that, when admixtures without a retarding action are used, the overdosages accompanied by an air entrainment lower mechanical strengths. This fact necessitates a careful check of the admixture dosages. However, the air entrainment is an inconvenience at standard concentrations. In fact Table 5.1 shows that, by using air-entraining admixtures, compressive strength does not increase as the corresponding water reduction would suggest [198].
608
F. Massazza Table 5.4.
Water-reducing admixture Sulfonated lignin agent
Over-doses and Compressive Strength [216]
Overdoses Slump Air (times ) (cm) (%) 1 5 ^ 1
7.4 19.4 [^ 7.5 ^
bugar acid agent
10
22.5
Sugar acid agent (without airentrained agent)
1 5 10
7.9 15.6 17.9
5
19
4.1 10.5 J^ 3.9
9<2
19.0 2.3 2.05 1.8
Compressive strength 7-days 14-days 28-days (kg/cm2) (%) (kg/cm2) (%) (kg/cm2) (%) 269 278
J53
100
100
55
332 64 346
2J5
62
100 19
388 102
100
404
253
100 26 ^
100
85
31
143
41
183
45
226 239 181
100 106 80
343 368 286
100 107 83
407 434 363
100 107 89
Anti-foaming agents can be added to reduce the entrained air. Among these tributyl phosphate is the most used [191], even though the literature mentions others. 5.3
Effects on Hardened Concrete
5.3.1 Mechanical strength. The water reduction caused by plasticizers is accompanied by a 28 day-strength increase of both mortars and concretes. At constant workability, strength increases generally ranging between 2 and 20% [157,170,191,194,196,198], which may also rise to 40% [193,195], can be obtained. The 24 hour-strength can be reduced owing to the retarding action of certain admixtures [191] but, in other cases, a set and strength acceleration is observed [157]. Figure 3.7 refers to the action of calcium formate. The strength gains obtainable with calcium lignosulphonates are appreciable for Portland cement mortars but seem negligible for blast-furnace and pozzolanic cements [170]. However, the subject has not been studied exhaustively. On the contrary, at constant w/c ratio, mechanical strengths of concretes are not modified [195,217,224] or, at the most, they are slightly lowered [218]. However, at temperatures below the usual ones (15° and 5°C) the concretes with admixture were found to show a higher strength than that of plain sample after 24 hours as well as 28 days. The over-dosages of plasticizers-retarders generally involve not only a set delay but also a considerable decrease in the concrete strength, mainly at short ages [193,216] (see Fig. 5.5 and Table 5.4). 5.3.2 Drying shrinkage. Plasticizers can increase drying shrinkage [95,96, 100,170,193,216]. This depends on the admixture nature and dosage. For example, if the usual amount of lignosulphonate is tripled, shrinkage nearly doubles [94] whereas calcium saccharate causes a much lower shrinkage [216]. Anyhow at ordinary dosages the shrinkage increase is not considerable since it ranges between 5 and 15% on pastes [94,96] and is around a few per cent units on concrete [96]. However, the shrinkage increase is higher when plasticizets contain accelerating compounds [96] (see Fig. 5.6). Also the type of cement affects shrinkage that, in the presence of calcium lignosulphonate, is higher for Portland cements than for blast-furnace and pozzolanic ones [170]. The shrinkage increase caused by overdosages is lower when Portland cements have a high alkali content [94].
Admixtures i n C o n c r e t e
Amount of admixture
(*) Fig. 5.5.
( a = standard
609
dose)
Lignosulphonate-type water-reducing admixture. Effect of overdose of a water-reducing admixture on the compressive strengths of concretes [193 ].
5.3.3 Creep. There is not a great deal of available data on creep and moreover they refer to different experimental conditions. Therefore it is not possible to draw conclusions of general validity. Admixtures modify total creep with variations ranging between +30% and -30% depending on the loading moment, the type of cement "and other unidentified factors [219]. Lignosulphonate-based admixtures cause a creep increase in concrete stored in a dry room [96,195 ]. On the contrary, the basic creep of ordinary and plasticizer-containing concretes (lignosulphonate and hydroxycarboxylic acids) is practically the same when the stress/strength ratio is equal on loading [96,195]. When the samples are stored in a fog room (this condition approaches the basic creep), no considerable differences between samples with and without admixtures occur. Moreover, it seems to be important that mixes should be loaded at the same degree of hydration [220] and have a given hydration increase under load. After 6 months the specific creep of admixture concretes having a lower w/c ratio is, however, only 80% of that of plain concrete, since on loading (28 days) they have a higher strength and a greater applied stress [195]. The addition of accelerating compounds to plasticizers increases creep [96, 195].
610
F. Massazza 4000
3500
c
3000
σ
*-»
§
2500
ε I
g* 2000 .c σ» c
Admixture Nil Lignosulphonate P 2 Lignos. + CaCl2 P 3 Lignos. + Trieth
1500
Û
1000
500
0
20
40
60
80
100
Drying time in days Fig. 5.6.
Drying shrinkage of cement pastes [96].
5.3.4 Durability. There are not many studies about this subject. However, as a rule, it can be stated that the water reduction caused by the admixture gives a higher impermeability to concretes. This, of course, exerts a favourable influence on the resistance to the chemical attack, the action of freeze-thaw cycles [217,221,222] and the sea-water action [223].
5.3.5
Changes in the hydration
products.
The reaction course, the hydration
products and the morphology changes obtained with cements containing lignosulphonates are the same as those obtained with the synthetic minerals of cement [131]. Nevertheless, it was noticed that the ettringite crystals are much smaller when C3A + gypsum hydrates in the presence of lignosulphonates. All the examinations performed with the electron microscope have never showed appreciable differences in morphology between the products present in the plain cement pastes and in those containing lignosulphonates [131,170,217, 225,226] or other admixtures [187]. However, it was observed that admixtures have a certain influence on the Ca(0H)2 morphology from the silicate hydration since they would favour the formation of smaller crystals [227]. Moreover, they would increase the level of Ca(0H)2 supersaturation before crystallization begins [228]. This modification is likely to be due to the fact that admixtures are adsorbed on the Ca(0H)2 crystals in a largely irreversible manner [167].
Admixtures in Concrete
611
Lignosulphonate adsorbed on the other hydrated phases of cement not only covers the particle surface but also occupies interlayer positions in the crystals [167,229]. This fact could be related to the higher shrinkage shown by the cements containing it. The presence of calcium lignosulphonate determines a slight increase in the specific surface of the hydrated products, attributable to the increase in the hydration degree and the number of larger pores (>70 A) [170], The greater shrinkage shown by the mixes with admixture could be attributed just to this higher content in macropores through which water can pass more easily
[170].
5.A
Mechanism of Action of Plasticizers
Interpreting the mechanism of action of plasticizers is rather complex owing to the variety of compounds forming the family of these admixtures and to the composite nature of cements and concrete. The fluidizing effect is, at least partly, the consequence of retarding phenomena caused by the admixture on the cement hydration. In fact, the mixing water reacts more slowly and therefore it remains available longer to fluidize the mix. Refer to par. 4.5 as regards the retarding mechanism. However this reason, together with eventual morphological changes in the hydration products, is likely of minor importance, whereas the physical actions resulting from the presence of the admixtures are more meaningful. The dispersing action of plasticizers is also attributed to the adsorption of the plasticizer molecules on the cement grains as well as to the resulting changes in the surface charge and ζ potential of the solid particles. Charges of the same sign cause repulsive forces which favour the solid dispersion and therefore increase the mix plasticity. As regards polymeric admixtures, it must be remembered that the best performances are obtained with certain values of the molecular weight and the number of functional groups {230]. These notes are too short to focalize and define the mechanism of -action of plasticizers. Nevertheless, they point out the need of studying the particleadmixture-water interlayer thoroughly in order to understand the mechanism through which plasticizers act. Anyhow the subject is dealt with more exhaustively in the next paragraph. 6
SUPERPLASTICIZERS 6.1
General
Superplasticizers constitute a relatively new category of admixtures for concrete, the use of which developed particularly in Japan and Germany from the sixties. Superplasticizers can produce: (a) (b)
at the same w/c ratio, much more workable concretes than the plain ones; at the same workability, a considerable decrease in the w/c ratio and therefore concretes having higher strengths.
612
F. Massazza
In the former case it is even possible to obtain the so-called "flowing concretes" which are pumpable and self-levelling and demand a little work for compaction. In the fluidized concretes the phenomena of aggregate segregation or water separation are practically absent and, anyhow, much more reduced than in the presence of normal plasticizers [231 ]. In the latter case, it is possible to make in yard concretes having strengths of 60 MPa [232] under ordinary curing conditions and even above 150 MPa, if they are autoclaved [233], Used with rapid hardening cements, superplasticizers allow steam curing to be eliminated or reduced [234], Superplasticizers have been provisionally grouped in the following categories [235]: A B C D
sulphonated melamine-formaldehyde condensate (MSF) naphtalene sulphonate-formaldehyde condensates (NSF) modified lignosulphonates (LS) other types.
Each category includes products differing from one another because either the base component can have different molecular weights or can have undergone some chemical changes or other chemical substances can be present. As a consequence, each commercial product will have a different action on cements. Whilst the dosage of the conventional plasticizers do not exceed 0.25 wt% in the case of lignosulphonates or 0.1 wt% in the case of carboxylic acids, tha products of groups A and B are used in considerably higher dosages (0.5-1.0%), since they do not entrain air [236]. Group C includes admixtures which have an effective fluidizing action but, at relatively high dosages, they can produce undesired effects, such as accelerations or delays in setting times [237,238]. Moreover, they increase the air-entrainment in concrete. Generally cohesiveness and workability increase on passing from groups A and B to C [239]. Category D consists of many products, usually of polymeric nature, which have not yet been tested thoroughly and of which only the results given by the manufacturers are known. Many products belonging to this group are quoted in [10]. 6.2
6.2.1
Fluidizing
Effects on the Properties of Fresh Concrete·
action
and water reduction.
By adding superplasticizers to
stiff concretes, slump (according to ASTM Standards) attains and exceeds 200 mm, flowtable spread (according to DIN Standards) takes a value ranging between 51 and 62 cm and the compacting factor (according to BS Standards) rises to 0.96-0.98 [235]. The workability of concrete increases as the superplasticizer dosage rises (see Fig. 6.1) [240]. Nevertheless there is no improvement beyond certain limits; on the contrary unfavourable consequences can occur, such as segregation and strength drop.
Admixtures in Concrete
613
24 | |22| 5 (0
20 18
16 l·14
10«
J
0
I
L
0.2
1 0.4
I
I 0.6
0.8
1.0
% of superplasticizer by weight of cement Fig. 6.1.
Effect of additions of superplasticizer (alkyl aryl sulphonate-formaldehyde condensate) on the workability of a concrete cement content 300 kg/m3, w/c = 0.6 [240].
Plasticity of concrete increases with increasing w/c ratios but, at least with mortars, the flowtable curves take a peculiar S-trend when superplasticizers are used (see Fig. 6.2) [241], This figure shows that, within certain values of the w/c ratio, a slight change in the water content causes a considerable variation in the flowtable spread. In concretes the curves of plain and admixture-containing samples are less differentiated [242]. Nevertheless, it is obvious that the determination of the exact water dosage in mixes is of great importance. When the purpose is not to improve the mix workability but to increase the concrete strength, superplasticizers allow the mixing water to be reduced by keeping workability unchanged. At ordinary dosages the reduction in the water content generally reaches 20% [10]. Also stronger reductions are possible but, as Fig. 6.3 shows, the gains tend to disappear beyond certain dosages [243]. The effects of superplasticizers depend not only on their nature but also on a great deal of other factors.
and
dosage
As Fig. 6.3 shows, the type of cerrient affects the admixture effectiveness and this fact must be taken into account when particular construction problems are to be faced. This influence depends not only on the clinker nature since, by increasing the gypsùm content in cement, the effect of melamine resins enhances [245]. The water reduction is also more marked in concretes containing fly ashes than in the plain ones [246]. On the contrary these admixtures do not produce good results in aluminous cements [247].
614
F. Massazza
0.40 Fig. 6.2.
0.45
0.50
0.55
0.60
0.65
0.70
0.75 W/C
F l o w t a b l e of 350 L P o r t l a n d cement m o r t a r s v e r s u s W/C r a t i o and dosage of Melment (Melment F 10 and Melment F 300 1:1) [241]
1
2
3
4
5
6
7
8
9
% Me/ment L 10 referred to the cement weight Fig. 6.3.
Water r e d u c t i o n o b t a i n a b l e w i t h a d d i t i o n of Melment L 10 i n c o n c r e t e s made w i t h CPA 3 2 5 , CLK 325 and CPA 325 HTS r e s p e c t i v e l y f 2 4 3 ] .
Admixtures in Concrete
615
Very low or very high cement contents, as we'll as very discontinuous gradings are not advisable in concretes containing superplasticizers. To obtain the most favourable results with aggregates having a maximum diameter of 32 mm, cement dosages of 300-350 kg/m~ are suggested [248]. The concretes too rich in cement, 450-550 kg/m 3 , become extremely adhesive [249]. 6.2.2 Slump loss. A negative property of superplasticizers, which sometimes conditions their use, is the workability loss with time (slump loss). Generally, it can be calculated that the effectiveness of the fluidizing effect ends after 30-60 minutes [237,248]. This workability decrease is particularly grave for flowing concrete (slump > 200 m m ) , the most important properties of which are pumpability and self-levelling. In these cases the consequences of the rapid increase in consistency are obvious. As Fig. 6.4 shows, slump loss is the content. Of course, it also depends 252]. The workability loss of plain nevertheless the overall time during unchanged is longer for the concrete
more retarded the higher the admixture on the nature of the admixture [250,251, concretes (Fig. 6.5) [253] is slower, which the concrete keeps its workability with higher initial slump [254].
Superplasticizer= Melment lio
300 250 200
150
100 50 0
10
20
30
40
Elapsed time,
Fig. 6.4.
50
60
70
mins
Slump loss with time
[250].
80
E E
616
F. Massazza 200 ■
■ 0.3 % SMF (added 3 min after mixing with water)
A——▲ 0 . 3 % SMF(with mixing water)
•
OVoSMF
60
90
150
Time, min Fig. 6.5.
Effect of the delayed addition of sulphonated melamine formaldehyde condensate on slump loss [253 ].
The workability loss is accelerated by the temperature rise [248,255] and therefore, if at 15 C a concrete must be placed within 60-120 minutes, this time is halved at 28°C [256]. The mixes subject to remixing set more quickly than the ones kept still [237]. The slump loss rate also depends on the cement properties and generally increases with finer cements, richer in C3A or fly ashes [257], Nevertheless, it seems that the relationship between workability loss and cement properties cannot be determined easily. In fact four Type I cements (according to ASTM) containing 7 to 13% C3A showed no significant differences in behaviour whereas a Type V cement (S03 <5%) showed a slower workability loss than two Type I and III cements [255], The decrease in the flowtable spread as a function of time seems to be much more rapid in concretes having a low cement dosage [248,255,258], Different devices were proposed to obviate the inconvenience of the workability loss with time, such as, for example, adding the admixture to concrete immediately before its placing or at small successive doses or even by overdosing it. (a)
The delayed addition of the admixture improves its effectiveness [255, 259], even if the delay is only 1-5 minutes [253,257,260,261] (see Fig. 6.5). For this reason, it is suggested that the concrete is first mixed with most of the water and the admixture is then added with the remaining water.
Admixtures in Concrete
617
(b)
The workability lost with time can be recovered by adding a further dose of admixture before placing (retempering) [238,262]. A third dosage produces still positive but less lasting effects [239,254] (see Fig. 6.6). (c) Admixture overdoses prolong the workability of the mixes but expose them to the risk of segregation [254] and decrease in mechanical strengths [251,261 ].
Superplasticizer -C W/C(by weight) «0.42 « ASTM type K C S A type 10) Cement Max. agg. size «3/4 in. (19 m m ) F.A. « Natural sand A.E.A. * Sulphonated hydrocarbon
10 \
After first dosage # ««^After second dosage
_
After third dosage
8 c *~ 6
250 200 50
GL
E 3 4
H100
(f)
50
2
-168 mm 0
I
1
I
2
•-j?Ç-· m m
3
4
5
6
7~
Elapsed time, hr Fig. 6.6.
Effect of repeated dosages of a sulphonated naphthalene formaldehyde condensate on slump [239].
Since resorting to these remedies is often difficult in practice [264], other devices have been suggested such as, for example, adding retarders together with superplasticizers [253,255,265,266,267]. As Table 6.1 shows, a mix of NSF and sodium heptonate in the 3:1 ratio considerably prolong the workability times without prolonging setting times excessively or reducing mechanical strengths [268]. Another trick which gives good results is to increase the gypsum content of the NSF-containing mix [269]. This method demands a very exact dosage and opposes the limits fixed by the standards on the SO3 content of cements. Finally another solution is to use admixtures of particular formulation. In this way a concrete can maintain its slump (-180 mm) for about 3 hours [264]. [264].
618
F. Massazza Table 6.1.
Workability, Setting Time and Compressive Strength of Flowing Concrete Containing a Retarder and a Superplasticizer [268]
OPC
Cement type Mix No. % Retarder in admixture (*) Admixture dos;age (ml/100 Kg)
SRPC
1
2
3
4
5
6
0 980
25 860
50 730
100 730
0 730
25 814
200 180 105 70 55
200 190 180 170 160 150
200 190 170 150
200 190 180 160 110
200 150 105 75 50
210 180 150 120 85 80
550 490 420 400 380
610 570 540 480
-
610 560 520 500 480 450
-
620 590 540 520 470
540 470 410 385 350
—
570 530 500 425 390 370
500 630
650 820
780 960
1200
920
415 540
575 680
15.8 21 .3
15.4 23.5
14.3 22.9
16.7 24.4
16.4 28.1
18.4 27.7
Workability Slump (mm) after time
15 45 75 105 135 165
min min min min min min
DIN Flow (mm) after time
15 45 75 105 135 165
min min min min min min
-
-
65
420
-
-
-
Setting time Time (min) to develop penetration iresistance of 0.5 N/mm z 3.5 N/mm 2 Compressive strength UCS (N/mm 2 ) at 7 days 28 days
(*) Admixture based on sodium heptonate (retarder) and sulphonated naphthalene-formaldehyde condensate (superplasticizer). 6.2.3 Setting times. At ordinary dosages melamine resins slightly retard the setting times of mortars and concretes, whereas naphthalenesulphonates and lignosulphonates act more vigorously [250,259]. In concretes, delays are of the order of 3% in the presence of melamine resins and 20% with naphthalenesulphonates [270,271], Very irregular behaviours with unforeseeable and often excessive accelerations or retardations were sometimes indicated for lignosulphonates [237,238], whereas the variations are much smaller with the admixtures of groups A and B [237].
Admixtures in Concrete
619
This behaviour can be attributed to impurities in lignosulphonates and, in particular, to the sugar content [300], Delay depends not only on the molecular weight of the admixture [303] or its degree of sulphonation [303] but also on the length of its hydrophobic groups [230], In any case excessive admixture dosages considerably retard set without essentially increasing the plasticity of mixes [250,303,308], Of course, much longer delays are observed when retarders are added to superplasticizers to keep the concrete fluid for a longer time [255]. The delayed addition [259], as well as repeated dosages, retard set, especially with lignosulphonate-based superplasticizers [239J. 6.3
6,3.1
Meohanioal
Effects on the Properties of Hardened Concretes
strengths
6.3.1.1 Ordinary curing. It is a shared opinion that, by working at constant w/c ratios, the concrete strengths are only slightly modified by superplasticizers [248,264,272,273,274]. However, overdoses decrease strengths because they bring about phenomena of segregation [251,275] and air entrainment [274], Strengths are favoured by high admixture dosages only if the cement content is high (~500 kg/m3) [263], Also repeated dosages of admixture cause strength to increase [239]. If admixtures are used to reduce the w/c ratio, by keeping the slump equal to that of plain concrete, much higher strengths are generally obtained 1217,235, 236,256,259,276,277] (see Fig. 6.7). This effect is found also on mortars [241,278,279]. Within certain limits the effect is proportional to the amount of added admixture but beyond certain dosages strengths tend to decrease, without considering the danger of the concrete segregation [280]. It is therefore important to avoid excessive dosages of superplasticizers, especially when particularly high long-term strengths are required for concretes [281 ]. Reducing the w/c ratio and maintaining a good workability and cohesion by using superplasticizers are particularly important factors in reaching high early strengths. The advantages are considerable, especially for medium — high cement dosages as Fig. 6.8 shows. After 7 days the strength gains tend to level out, although they always remain relevant. It must also be noticed that the admixture effect is more differentiated at low dosages [271]. Of course, the results also depend on the type of cement and the w/c ratio. It is generally thought [235] that a reduction in the water content by 25-35% involves a 24 hour-strength increase by 50-75%. As regards the long-term strength of concretes containing superplasticizers, there are not yet any exhaustive case histories. At least unsatisfactory results have not been pointed out. 6.3.1.2 Steam curing and autoclaving. The steam cured concretes containing admixtures give higher ultimate strengths than those of the control samples having the same slump. On the other hand, they have lower ultimate strengths than identical samples cured at 20 C. In any case early strengths are much higher for steam cured samples [234,270,282].
F. Massazza
620
Double
Normal
Fig. 6.7.
dose W/C - 0.46
dosage
W/C - 0.55
Strength versus time of a water-reduced concrete containing an A-type superplasticizer [235].
Melamine resins are much more effective than naphthalenesulphonates in increasing early and ultimate strengths of concretes. By passing from short cycles (4 h heating) to long cycles (11 h) the effectiveness of naphthalenesulphonates considerably enhance, but always remains lower than that of melamine resins [272]. Superplasticizers cause no inconveniences even when concretes are autoclaved but, at the same workability, they give higher strengths to the plain concrete [316]. The considerable water reduction allowed by naphthalenesulphonates and a timely choice of the steam curing and autoclaving cycles enable very high strength concretes (above 150 N/mnr) to be obtained [233]. 6,2.2 Shrinkage and creep. At the same w/c ratio, A- and B-superplasticizers do not considerably modify drying shrinkage of concrete [235,264,273] whereas, at the same consistency, they sometimes reduce it appreciably [217,235]. Moreover, the concretes with admixtures, when steam cured for 28 days, seem to shrink less than plain ones, whereas the opposite occurs when they are cured for 7 days [277].
621
Admixtures in Concrete 200
r0 Δ • ■
Mighty-150 Melment Lomar-D FX-32C
Cement lot 21734 type Cement content 517 Ib/cu yd
E o u
Fig. 6.8.
Relative compressive strength of mixes containing different admixtures [271].
Total creep (basic creep plus drying creep) is higher naphthalenesulphonates, at least for the mixes having (~0.64). On the contrary, when the w/c ratio is low, creep between samples with and without admixtures are the admixture effect on basic creep is slight [273],
6.2,3
when concretes contain a high w/c ratio the differences in insignificant. However,
Durability
6.3.3.1 Impermeability. Impermeability plays a primary role on the durability of concrete and since this depends on the w/c ratio, superplasticizers should exert a favourable effect. In fact, permeability decreases if the mixing water is reduced by both melamine [279,283] and naphthalenesulphonate resins [217]. On the other hand, plain and fluidized concretes made with the same w/c ratio have the same permeability [248]. 6.3.3.2 Frost resistance. As is known, concrete with a good frost resistance is obtained by introducing suitable air-entraining substances into the mix. As opposed to lignosulphonate-based admixtures [239], MSF- and NSF-type superplasticizers do not entrain air in the mixes [270,284], but rather, they favour its loss with time, especially when dosages are repeated [258]. In the presence of superplasticizers the air voids and also the spacing factor L are larger [(L = 0.254-0.432 mm [251,275])] than those necessary to guarantee a good frost resistance of concrete (L < 0.2 mm).
622
F. Massazza
Therefore, in the presence of superplasticizers, it is necessary to increase the dosage of the air entraining admixture if an established air amount needs to be introduced into the mix [264] and L needs to be improved [285]. As a matter of fact superplasticizers, even if they decrease the air content and increase the spacing factor, improve the concrete resistance to freezethaw cycles [248,250,258,259,271,284,285,286,287,288]. Moreover, the reduction in the w/c ratio can even cause an increase in the frost resistance [256]. In any case, it is advisable to introduce air-entraining substances into the concrete and to reduce the w/c ratio in order to obtain a good frost resistance [284]. 6.3.3.3 Salt resistance. Superplasticizers, owing to the reduction in the w/c ratio they allow, decrease the penetration of chlorides into concretes [288] and therefore improve their resistance to the deicing salts [258,259 ]. For the same reason the resistance to the sulphate attack increases [264]. If, however, the w/c ratio is the same, the behaviour of the concretes with admixture do not differ from that of the corresponding plain concretes [273]. 6.4
Effects on the Morphology and Composition of the Hydrated Phases
According to some authors the morphology of the hydrated phases from the hydration of C3S and cement does not change in the presence of admixtures [217]. However, it is more likely to think that some modifications, even if limited, do occur. In fact, it was seen that the habi-tus and size of the hydrated crystals formed during the hydration of the systems C3S-C1 ^Αγ .CaF2-CaS0i+ are modified [289,290], The ettringite crystals which form by hydrating C3A in the presence of gypsum and superplasticizers are much smaller [224,291]. Moreover, admixtures considerably increase the C/S ratio of CSH formed during the C3S hydration and, also, maybe, of that resulting from cement. The H/S ratio slightly decreases [292], Differences in the BET specific surface and the micropore sizes were also found in the pastes with and without admixtures, but these differences were reduced by 28 days [293]. 6.5
Mechanism of Action
6.5.1 Viscosity of cement pastes. given by the simple equation
In Newtonian fluids, viscosity was
n-4where τ = shear stress and γ
a
(i)
shear rate.
However, the flow of many fluids deviates from the ideal Newtonian flow and gives curves such as those of 2 and 3 in Fig. 6.9 [296], The former curve relates to fluids having viscous or pseudoplastic behaviour, the latter to fluids with an expanding behaviour. In these two cases viscosity varies with shear stress and is called apparent viscosity.
Admixtures in Concrete
Shear rate y
1
623
(1/s) 3
1
2
q = cot ax q' = cot ax'
\<*'
Shear stress τ Fig. 6.9.
(dyn/cm^)
Flow curves of liquids without yield stress. Behaviour: Newtonian (1), pseudoplastic (2), dilatant (3) [296].
Other fluids, and in particular suspensions, behave at first like solids and begin flowing only if shear stress exceeds a certain threshold value. If, beyond this value, the fluid behaves as a Newtonian liquid, flow obeys the equation of a Bingham1s* body τ « τ 0 + μγ
(2)
where τ s shear stress, TQ β yield stress, γ · shear rate, u * plastic viscosity (curve 4 of Fig. 6.10) [296], In many cases flow does not obey this linear relation and Bingham1s bodies with plastic or expanding behaviour can form (curves 5 and 6 of Fig. 6.10). In these two cases an apparent viscosity η « Σ depending on the shear rate value is defined again. Ύ Finally reversible, thixotropic and antithixotropic curves can be obtained by subjecting the fluids to increasing and decreasing cycles of shear stress. The yield stress is the force that has to be applied to a material before a movement occurs and, therefore, the force necessary to break the interactive bounds between particles is produced. In contrast, plastic viscosity measures the friction of the fluid or the shear shear stress/shear rate ratio [294] and it controls the rate at which flow occurs [295]. The flow curves of cement pastes are of the pseudoplastic type with [296] or without [297] yield stress. Finally, the curves can show a thixotropic, antithixotropic or reversible behaviour [297],
624
F Massazza
Shear r a t e y
(1/s)
η
=cot oc pi q p [ =cotöc' l
T
Fig.
6.10.
f—-
Shear stress r
(dyn/cm 2 )
Flow curves of liquids with yield stress. Behaviour: ideal plastic (4), pseudoplastic (5) and dilatant (6) [296].
The addition of superplasticizers to cement pastes causes the lowering of the ^app apparent viscosity [248,292,293,294,298], However, the effect is not proportional to the addition, since the apparent viscosity remains stable enough with increasing sodium naphthalenesulphonate .confent until this reaches a certain concentration. Beyond this value viscosity decreases considerably and then remains relatively stable [209,294] (see Fig. 6.11). Viscosity depends on the polymerization degree of the admixture as well as, of course, the w/c ratio [209 ].
5
Ci.
oc
1200 1000 800 600
•
400
\ \
200 4C
I
o o
0.2
0.6
% of superplasticizer Fig. 6.11.
JL 1-0
_L 1.4
J.
_L 1.8
by weight of cement
Reduction in the viscosity of a Type I Portland cement containing sodium salt of condensed naphthalene sulphonic acid [294].
Admixtures in Concrete
625
The superplasticizer addition mainly affects T C which decreases with increasing admixture contents [299,300] and disappears completely beyond certain values [301 ] (Fig. 6.12). scole units 10
20
30
40
50
6(
no odd 0 25%
70 60 50 40 30 20 C cement 04
50
100 150 Sheor stress (Po)
10
200
Fig. 6.12. Changes in shear stress-shear rate relationship with admixture concentration [301 ]. However, the yield stress decrease is not gradual but shows a sharp drop at certain admixture contents (Fig. 6.13). The phenomenon is particularly marked at low w/c ratio [301 ]. The yield stress of pastes containing admixtures increases with time [300] but the change is extremely small during the first hours if the admixture content exceeds certain values [302]. Of course, the yield stress depends on the admixture nature and, as regards the polymeric substances, on their molecular weight [300], The polymerization degree of these substances is important since its increase causes the paste minislump to rise rapidly up to a constant value [303]. The considerable lowering of the yield stress caused by superplasticizers is not shown by the conventional air-entraining and water-reducing admixtures [299]. Plastic viscosity slightly decreases with increasing admixture contents [300, 301,302] and, up to 3 hours, it rises, slightly with time [300,302].
626
F. Massazza
V
200
A
Admixture A4 o
150h
2
100
Q.
A5w/c s 0.30° H
2
w/c=0.25 *, , rt,ew/c«0.35aA5, A3w/c«0.35 w/c«0.40 a A5
0.5 0.75 wt % admixture Fig. 6.13.
i.o
2.0
Changes in yield stress with admixture concentration, increasing shear rate, time 6.5 m. A3 ■ 3-naphthalene formaline condensate; Ai+ - id with retarder; A5 = sulphonated melamine formaline condensate [301 ].
That the cement pastes have a thixotropic behaviour is shown by the hysteresis loop in the flow curves [297,304] (Fig. 6.12). Thixotropy reveals the existence of bonds between particles which are broken by shear [304]. It is decreased by adding superplasticizers. With suitable admixture contents the cement pastes take on a nearly Newtonian ideal behaviour which is lost only after some hours, when viscosity begins increasing [293]. 6.5.2 Viscosity of concretes. The slump test and flowtable are not sensitive enough to detect changes in the workability of flowing concrete. In fact, these methods are insufficient to characterize a material which behaves as a Bingham plastic and which therefore demands the knowledge of two constants, τ 0 (yield stress) and μ (plastic viscosity), as indicated in eq. (2). The usual tests of consistency can be misleading since two concretes having different τ 0 and μ can be judged identical only because their Bingham curves cross at the shear stress of the test [305]. A two-point workability apparatus was developed to characterize flowing concretes [306]. It is used to measure the torque T or an impeller rotating inside a cylinder full with concrete at different N speeds. On a graph T(N) it is possible to determine the constants g and h of the equation g + hN constants which are proportional to τ 0 and y respectively.
(3)
627
Admixtures in Concrete
The tests on concretes carried out by this system show that h decreases to a minimum as the admixture concentration increases, whereas plastic viscosity initially rises up to 0.7 wt% of the admixture and then decreases [295],
6.5.3
Adsorption
and paste
Theology.
6.5.3.1 Adsorption. Considering that all solid particles dispersed in water tend to attract with one another more or less markedly, the mechanism of action of superplasticizers can be attributed to phenomena capable of eliminating or overcoming the attractive forces between particles. These phenomena are related to the adsorption of the admixture molecules on the cement particles. This adsorption causes changes in the water-cement system and therefore in its rheology. By dispersing the cement in aqueous solutions of superplasticizers, the latter are adsorbed on the surface of the solid particles. The adsorption attains values beyond which it no longer increases [307,308] (Fig. 6.14) or decreases slowly [307]. This lowering or, in general, the apparent irregularity sometimes found in the adsorption curves, is due to particular equilibria forming between individual molecules and micelles in the superplasticizer solutions [309],
0 8 h A Polymer delayed addition (w/c «0.3) A Polymer delayed addition (w/c«2 ) — I · Polymer immediate addition (w/c «0.3) | o Polymer immediate addition (w/c «2 ) g | «Monomer delayed addition (w/c «0.3) ° o.6h-o Monomer immediate addition
0
0.5
1.0
15
Addition ( % by weight of cement) Fig. 6.14.
Polymer or monomer adsorption on cement as a function of addition [308].
628
F. Massazza
The cement compounds which truly adsorb superplasticizers are still being investigated. Anhydrous compounds react with water and therefore the adsorption cannot be measured in aqueous dispersions. Measurements can be carried out by dissolving the admixture in an unreactive solvent, such as dimethylsulphoxide (DMSO), but it is obvious that with this method the real conditions of the cement use are not complied with. Anyhow these tests show a very small adsorption for C3A, being about 1/10 of that showed by monosulphate hydrate [310]. As regards the hydrated compounds, both aluminates (C3AH5 and Ci+AH^) and sulphoaluminates (C3A.3CS. 32^0 and C3A.CS. 12H2O) were found to adsorb appreciable amounts of the most used superplasticizers [291,310,311]. Of course, adsorption depends on the type of admixture as well as its degree of polymerization [312]. The expectable consequences caused by adsorption are as follows: —Formation of repulsive electrostatic forces which move the particles away from each other (ζ-potential). —Formation of layers of admixture molecules on the solid particles, which create a steric obstacle to their adhesion. —Increase in the solid-liquid affinity to the detriment of the solid-solid affinity. — Changes in the morphology and the crystalline sizes of the hydration products. —Changes in the hydration kinetics of the cement constituents. Since these phenomena affect the rheology of the cement pastes to a different extent, it is advisable to examine and evaluate them separately. 6.5.3.2 ζ-potential. By dispersing the cement in water containing the dissolved superplasticisers, a negative zeta potential forms on the particle surface. By increasing the admixture content, the absolute value of the ζ-potential rises up to reach, asymptotically, values ranging between -35 and -45 mV after 15 min (Fig. 6.15 [313]). The values of ζ-potential depend on the type of cement, the quality of the water used, the nature of the admixture [313,314] and its degree of polymerization [303]. High ζ-potentials are also measured by dispersing C3S, alite, C3A,Ca(0H)2 and mixed cements in water [314]. ζ-potentials of the anhydrous compounds are not known because these compounds react with the mixing water more or less immediately. Some tests carried out by dispersing the pure cement compounds in dimethylsulphoxide (DMSO) containing some dissolved superplasticizers gave values ranging between -6 and -11 mV for C3A [310]. The ζ-potential of fly ashes decreases even more sharply and reaches values around -60 mV with only small admixture contents [314],
629
Admixtures in Concrete
More precise data were obtained on ζ-potentials relevant to some hydrated phases dispersed in aqueous solutions of the admixtures and having a suitable composition to avoid the compound decomposition. As Table 6.2 shows, all aluminates and calcium sulphoaluminate hydrates have appreciable negative ζ-potential. Table 6.2.
ζ-potential in mV of Some Hydrated Compounds Dispersed in Lime Water.
Without admixture C 3 AH 6 C U AH 13 C3A.3CS.H32 C3 A.CS.H 12
-9 -9 -5 -5
With admixture
Adsorbed admixture
NSF
MSF
LSS
(%)
Ref.
-15 -17 -34 -28
-12 -20 -31 -18
-12 -18 -30 -23
2 4 2 2
311 311 291 310
6.5.3.3 Steric obstacle. Considering the sizes of the monomeric units of melamine resins and the specific surface of cement, it is found that, after the saturation has been reached, the cement grains are covered on average by about 40 layers of polymeric molecules. Assuming the thickness of an individual layer is about 10 A, each cement particle is therefore covered by a resin layer of about 400 A. This sheath is thick enough to cause steric stabilization of the particles [304]. 6.5.3.4 Solid-liquid affinity. Superplasticizers only slightly modify the surface tension of water and therefore the solid-liqui.d affinity [310] consequently. This factor should not appreciably affect the mechanism of the fluidizing action [307]. 6.5.3.5 Morphological changes. To a certain extent superplasticizers modify the morphology and the sizes of the newly formed crystals. In particular, it was seen that the ettringite crystals forming in the presence of the most used superplasticizers have sizes about 1/10 of those obtained in the absence of admixtures [224,291]. The small sizes of the crystals can hamper the formation of bridges and links among the particles. 6.5.3.6 Changes in the hydration kinetics. Superplasticizers have a retarding action on the hydration of cement and its pure compounds, which varies depending on different factors such as nature of the admixture, type of cement etc. (see 6.2.3). This delay contributes to the plasticizing effect of the admixtures since it slows down the water combination [315]. 6.5.4 Final remarks. Thus, it can be seen that the action of superplasticizers results from several factors, each of different importance. The fluidification of mortars and concretes is mainly attributed to the changes in ζ-potential caused by adsorption [313], In fact ζ-potential, mortar flow and paste minislump all increase as adsorption increases [302,313], The strict correlation between adsorption, ζ-potential and fluidizing action is verified by the fact that, if using NSF monomer as an admixture instead of the polymer, there is neither adsorption, nor change in ζ-potential or fluidizing action [308 ].
630
F. Massazza
50r > E 40 i
c 30 <« o • de-ionized water
S 201
o 5 0 % tap water J 1.0 concentration of
IQ
L
2.0 # - 2 (a/w wt %)
Fig. 6.15. Change in zeta potential of Type I Portland cement with increasing concentration of sulphonated naphthalene formaldehyde condensate. Electrophoretic mobility was determined 15 min after the cement was immersed in the solution [313 ].
Shear rate«-40s* w/s-1.6 0
NSF
D
LSS
Δ
MSF
3 2
4
6
8
SUPERPLASTICIZER ADDITION, ( Fig. 6.16. Apparent viscosity of ^ΑΗχ3 pastes with lime water vs. superplasticizer addition.
10
Admixtures in Concrete
631
Nevertheless, a decrease in ζ-potential does not necessarily involve an improvement of the suspension plasticity. In fact, the apparent viscosity of Ci+AH^ and C3AH5 pastes can greatly increase owing to the presence of admixtures (Fig. 6.16), whereas under the same conditions, that of sulphoaliminate hydrates decreases [311], These results indicate that apparent viscosity is affected not only by ζ-potential but also by other factors. One of these is the bridging between solid particles, caused by the adsorbed molecules. Bridging opposes the electric repulsion forces caused by ζ-potential. Therefore if the latter is relatively low, as in the case of aluminate hydrates (Table 6.2), the agglomeration phenomena prevail and the paste viscosity increases [311]. However, Fig. 6.16 shows that the phenomenon depends on the admixture/solid ratio. The bridging effect was also observed in cements containing low percentages of an admixture composed of a sodium salt of polystyrene sulphonate [312], Another important factor is the steric hinderance caused by the multilayer adsorption of the admixture on the cement particles [304]. The decrease in the sizes of the newly formed crystals and the mixing water probably also contribute to the fluidizing action but to a lesser extent. Another factor that could affect the action of superplasticizers is the mineralogical composition of cement and the composition of the mixing water. In fact it was found that these admixtures do not modify the viscosity of alumina-cement pastes or calcareous filler suspensions. If, however, acetic acid is added to the suspensions, viscosity is considerably reduced [228] since Ca** ions are now present in solution. 7
AIR-ENTRAINING ADMIXTURES 7.1
General
The accidental discovery of air-entraining admixtures, which occurred in the thirties, can be considered one of the fundamental stages of concrete technology since it proved that small amounts of organic substances could improve the concrete properties and in particular its frost resistance. The mechanism of the frost attack of concrete is now known: whenever the concrete temperature drops below 0 C, the water in the capillary pores of the saturated hardened cement paste freezes. Since ice has a higher volume than the liquid water (~9%), an expansion of concrete takes place. The occurring stresses cause cracks and crumblings if they exceed the tensile strength of concrete. The harmful action of frost can appear only if freezable water is present in concrete. In fact, dry concrete or concrete having a water content below a certain limit does not undergo deterioration. The presence of air bubbles improves the frost resistance of concrete since each bubble acts as an expansion tank which accommodates the water forced out of the pores by the forming ice, so eliminating the stresses produced by frost.
APC - U
F. Massazza
632
Nevertheless, the air accidentally entrapped in the concrete is not able to give an effective protection since it is distributed in relatively large and spaced cavities. Better results are obtained by adding the so-called airentraining admixtures to mixes. They are surfactants capable of forming a very fine and uniform system of non-communicating discrete cavities inside the cement paste. Also plasticizers are generally surfactants and therefore they introduce air into the mixes. In fact there is no clear difference between these admixtures and the air-entraining ones since workability increase and air entrainment are properties common to both products. A distinction could be made by considering the stability of the air bubbles [317J. In plasticizers a good portion of air disappears before the concrete sets whereas, in the case of entraining admixtures, the entrained air remains in the hardened concrete. The most widely used air-entraining admixtures consist of: abietic and pimeric acid salts fatty acid salts alkyl-aryl sulphonates alkyl sulphonates phenol ethoxylates 7.2
Entrained Air
There are a great many factors affecting the amount of entrained"air and they*have been well studied. Among the major reference works are [318, 319,320,321,332]. Of course, the main factor is the nature content.
of the admixture itself [215] and its
The entrained air increases with increasing content of air-entraining admixture [323,324] (see Fig. 7.1), but, beyond certain dosages, the air content tends to reach asymptotic values. The higher the w/a ratio [323], the more effective the admixture. The admixture content being unchanged, the entrained air decreases as the cement dosage and its fineness decrease [325]. The low-alkali [326], high early strength and pozzolana-containing [327] cements demand a higher airentraining admixture content, at the same entrained air content. In the two last cases this is the consequence of the higher fineness of the cements. The amount of air-entraining required to obtain a given air content slightly varies also depending on shape, grading [327] and maximum size of the aggregate (see Fig. 7.2) [4]. A temperature increase of concrete during mixing causes a decrease in the air content [155,328]. This occurs because the water surface tension is lower at high temperatures. Moreover, the air content of a concrete decreases as the mixing [155,328] and the vibration duration [320] increase.
633
Admixtures in Concrete 35 r
0
0 010
0 020
0 030
0 040
0 050
0060
0 070
0 080
CONCENTRATION OF SURFACE-ACTIVE AGENT (PERCENT BY WT OF CEMENT)
Fig. 7.1.
Air entrained in cement pastes by surface active agents. W/C = 0.45 by. wt, 25°C. Surface-active agents: 1. sodium dodecyl sulphate, 2. neutralized "Vinsol" resin, 3. sodium abietate, 4. tetradecyl trimethyl ammonium bromide, 5. "Triton EN", 6. "Lissapol N 300", 7. Saponin [324].
7.3
Effects on Concrete
7.Z,1 Effects on fresh concrete. The entrained air improves the concrete workability, makes it more homogeneous and flowing and avoids segregation phenomena. It must be said that air-entraining admixtures are not used to improve the concrete workability as this is better enhanced by using specific products which entrain air slightly or not at all and, therefore, do not impair mechanical strengths [6,324,329,330]. Roughly speaking, an air increase of 1% corresponds to the addition of 2% water. The air voids replace a portion of the fine sand because of their size distribution and they give the concrete a higher flow owing to their elastic properties [4J. Figure 7.3 shows the relationship between slump and air content, at different H20/cement ratios, by using a soap as an air-entraining admixture [323]. Of course, the fluidizing effect depends not only on the content of surfactants but also on their nature [215].
F. Massazza
634
% air
Slump: 5-8 mm
_ 225
k k
Sr
k g / * 3'
__ 300 k g / m 1
1
^ ^
•^ΞΤ^
_440
^ " - -
_440
kg/m1
^ 2 2 S kg/m' kg/m'
^» 10
12,5
20
25
<0
Maximum size of grains Fig. 7.2.
50
63
(mm)
Effect of cement content and maximum size of aggregate on the concrete air content [4].
4 12 20 Air content, per cent Fig. 7.3.
Relationship between slump and air content for various w/c ratios [323].
635
Admixtures in Concrete
7.3,2
Effects
on hardened
concrete
7.3.2.1 Effects on mechanical strengths. The air entrainment in concrete reduces mechanical strengths unavoidably [6,317,327,329,330J. Figure 7.4 shows the effect caused by a triethanolamine salt of alkylbenzenesulphonic acid [215]. As can be seen, strengths decrease rapidly with increasing admixture content, and then they tend to reach very low asymptotic values. It can be observed that, with an addition of 0.01%, compressive strength decreases by about 3 0 % and 3 5 % at 3 and 28 days respectively.
0.05 %
Fig. 7.4.
0.075
Surfactant
Strength of Portland cement specimens versus content in surfactant (triethanolamine salt with an alkybenzenesulphonic acid) [215] 1 - Flexural-tensile strength at 3 days; 2 - Flexural-tensile strength at 28 days; 3 - Compressive strength at 3 days; 4 - Compressive strength at 28 days.
At the usual dosages ( < 0 . 0 1 % ) , the mechanical strength decreases linearly as the entrained air [331 J and admixture [215] contents increase (Fig. 7 . 4 ) . It is calculated that each percentage increase in the total air causes the compressive strength to decrease by 3-5%, even if higher reductions were pointed out [332]. This strength decrease could also depend on the decrease in the degree of hydration observed in (slag) cement pastes containing air-entraining admixtures, hydrated for 28 days [333]. Nevertheless, it must be remembered that, at*the same workability, the airentrained concretes have a lower w/c ratio. Consequently the air entrainment causes a lower strength loss than that which could be expected from the air content.
636
F. Massazza
7.3.2.2 Effects on durability. The air bubbles affect the frost resistance of concrete favourably since they accommodate the water forced out of the capillary pores because of the gradual ice formation. In this way they relieve the hydraulic pressure and prevent the formation of stresses. Figure 7.5 [334] shows that air entrainment appreciably improves the resistance to freezing and thawing of concrete, but it illustrates also that too high w/c ratios cancel the advantages obtained by using the admixtures.
4000
0-35
Fig. 7.5.
0-45 0-55 0-65 075 Wöter/Cement Ratio
085
Influence of w/c ratio on the frost resistance of concrete moist cured for 28 days [334].
The air accidentally entrapped in concrete does not give an effective protection since it is distributed in relatively large and spaced cavities. On the other hand, the air entrained by the admixtures creates a uniform system of very fine dispersed bubbles in the cement paste. It is essential for the distribution of the bubbles to be uniform and their size be small. Practically, their diameter varies between 0.01 and 1.00 mm according to a continuous size distribution. In order to relieve the stresses occurring when water freezes, the air bubbles must not be very spaced. This means that the air content must not be too low (Fig. 7.6) [335]. Experience shows (Fig. 7.7) [336] that the spacing between bubbles must not exceed 0.25 mm in order to assure a good protection against frost. For a given entrained air content, the average spacing between bubbles depends on the w/c ratio [334]. It is important to notice that the microscopic bubbles entrained into the cement paste are not filled with water either during mixing or during the cement hydration.
637
Admixtures in Concrete
4 8 12 Air Content - per cent Fig. 7.6.
16
Influence of air content on expansion of concrete after 300 freeze-thaw cycles [335].
10° in.
100
8 80
o 60
o
Û
ö
^*"·*
I
I I
S 40 σ
20
16
12
120
I
°\
\
\ °
20
\c 100
Fig. 7.7
200 300 400 Spacing of Bubbles - um
Q.
500
600
Relationship between spacing of entrained air bubbles and durability of concrete [336],
638
F. Massazza REFERENCES
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287. 288.
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the
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(Tokyo, May, 1969), 1970, 244-248.
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of Japan
- Review
of
the
Twenty-Eighth Gen. Meet, (Tokyo, May 1974), 1974, 173-174. Roberts, L. R., Scheiner, P., Developments in the Use of Superplasticizers, ACI Publication SP-68, 189-213. American Concrete Institute, 1981. Lukas, W., Developments
in the
Use of Superplasticizers,
ACI Publication
SP-68, 253-267. American Concrete Institute 1981. Uchikawa, H., Uchida, S., Cem, Concr. Res. 1972, 3_ (5), 607-624. Uchikawa, H., Uchida, S., Onoda Kenkyu Hokoku 1974, 2β_ (91), 1-3. Massazza, F., Costa, U., Barrila, A., XIII. Conference on Silicate Industry and Silicate Science (Budapest, June 1981), 1981, 177-186. Odler, I., Becker, Th., Cem. Concr, Res, 1980, J_0 (3), 321-331. Wittmann, F. H., Silicates Industriels 1979, (1), 5-12. Pétrie, E. M., Ind,
Eng,
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â Dev.
1976,
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Banfill, P. F. G., Magazine of Concrete Research 1980, 32. (110), 17-27. Wesche, K., Vom Berg, W., Beton 1973 (1), 21-27. Odler, I., Becker, T., Weiss, B., Il Cemento 1978 (3), 303-310.
Admixtures in Concrete 298. 299. 300. 301. 302. 303. 304. 305. 306. 307. 308. 309. 310. 311. 312. 313. 314. 315. 316. 317. 318. 319. 320. 321. 322. 323. 324. 325. 326. 327. 328. 329. 330. 331. 332.
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CONTENTS 1 2
3
4
Introduction Grinding Aids 2.1 General 2.2 Effect of Grinding Aids 2.2.1 Influence on the specific surface and particle size distribution 2.2.2 Influence on agglomeration 2.2.3 Grinding efficiency 2.2.4 Influence on the cement properties 2.3 Mechanism of Action Accelerators 3.1 General 3.2 Set-accelerating Admixtures 3.2.1 Inorganic products 3.2.2 Organic products 3.3 Hardening-accelerating Admixtures 3.3.1 Inorganic products 3*. 3.2 Organic products 3.4 Collateral Effects 3.4.1 Rheology of the mixes 3.4.2 Durability 3.4.3 Dimensional changes 3.4.4 Heat of hydration 3.4.5 Antifreezing properties 3.5 Composition and Structure of the Hydrated Phases 3.6 Mechanism of Action 3.6.1 Action on setting times 3.6.2 Action on hardening Retarders 4.1 General 4.2 Inorganic and Organic Retarders 4.3 Factors Affecting the Action of Retarders 4.4 Other Effects 4.5 Mechanism of Action
569
571 57 2 572 573 573 576 576 577 578 580 580 580 580 582 584 584 586 586 586 587 588 588 588 588 589 589 590 590 590 591 592 593 596
570 5
F. Massazza
Plasticizers 601 5.1 General 601 5.2 Effects on Mortars and Fresh Concretes 602 5.2.1 Mixing water reduction 602 5.2.2 Workability increase 602 5.2.3 Secondary effects 607 5.3 Effects on hardened concrete 608 5.3.1 Mechanical strength 608 5.3.2 Drying shrinkage 608 5.3.3 Creep 609 5.3.4 Durability 610 5.3.5 Changes in the hydration products 610 5.4 Mechanism of Action of Plasticizers 611 6 Superplasticizers 611 6.1 General 611 6.2 Effects on the Properties of Fresh Concrete 612 6.2.1 Fluidizing action and water reduction 612 6.2.2 Slump loss 615 6.2.3 Setting times 618 6.3 Effects on the Properties of Hardened Concretes 619 6.3.1 Mechanical strengths 619 6.3.1.1 Ordinary curing 619 6.3.1.2 Steam curing and autoclaving 619 6.3.2 Shrinkage and creep 620 6.3.3 Durability 621 6.3.3.1 Impermeability 621 6.3.3.2 Frost resistance 621 6.3.3.3 Salt resistance 622 6.4 Effects on the morphology and composition of the hydrated phases 622 6.5 Mechanism of Action 622 6.5.1 Viscosity of cement pastes 622 6.5.2 Viscosity of concretes 626 6.5.3 Adsorption and paste rheology 627 6.5.3.1 Adsorption 627 6.5.3.2 ζ-potential 628 6.5.3.3 Steric obstacle 629 6.5.3.4 Solid-liquid affinity 629 6.5.3.5 Morphological changes 629 6.5.3.6 Changes in the hydration kinetics 629 6.5.4 Final remarks 629 7 Air-entraining Admixtures 631 7.1 General 631 7.2 Entrained Air 632 7.3 Effects on Concrete 633 7.3.1 Effects on fresh concrete 633 7.3.2 Effects on hardened concrete 635 7.3.2.1 Effects on mechanical strengths 635 7.3.2.2 Effects on durability 636 References 638
Admixtures in Concrete 1
571
INTRODUCTION
Much recent progress occurring in cement and concrete technology is related to the use of admixtures. Admixtures are inorganic or organic products which, when added in small amounts, modify some properties of the comminuted, dry or dispersed-in water materials (raw materials, cements, mortars and concretes). The great number of products (especially organic) proposed as admixtures and the various effects that each of them can produce make their classification difficult. These products are generally subdivided into admixtures for cement and admixtures for concrete. The former are used to improve the technological process of the cement production; the latter modify particular properties of concrete, by reducing or enhancing them. However, this subdivision is not strict since the same admixture can be used in cement and concrete for different purposes. A second criterion of classification subdivides admixtures according to the prinary function they have: also in this case, the boundaries between groups are not sharp since a product can perform more than one function. The case of plasticizers which are also retarders is typical. Based on these criteria, the more generally used admixtures can be subdivided into: A. Al A2 A3
plasticizers and water-reducing admixtures for raw cement slurries admixtures for granulation admixtures for dry grinding of raw meals. B.
Bl B2
B3
B4
B5 B6
Admixtures for Cement:
Admixtures for Concrete
admixtures modifying set and hardening: accelerators retarders admixtures modifying the mix rheology and the air content: , , ._ (superplasticizers water-reducing admixtures , Ji · b I plasticizers water-retaining admixtures thickening admixtures admixtures entraining air into the mixes: air-entraining and air-detraining admixtures gas-forming admixtures foam-forming admixtures admixtures modifying the resistance to physical and chemical actions: frost-resisting and anti-freezing admixtures water-repelling admixtures permeability-reducing admixtures corrosion-inhibiting admixtures products improving the resistance to chemical actions expansion-producing agents miscellaneous admixtures colouring admixtures agents improving the resistance to biological actions a.s.o.
572
F. Massazza
From this list it is possible to notice the great variety of effects obtainable with the admixtures and to deduce the complexity of the chemical-physical phenomena which must be faced in order to explain the action of these materials. For brevity's sake it is impossible to examine all the above-mentioned admixtures. Therefore this chapter will consider in detail only the most largely used products, namely: (a) (b) (c) (d) (e) (f)
grinding aids set- and hardening accelerators set- and hardening retarders plasticizers superplasticizers air-entraining admixtures.
Information about other products and more particular effects can be found in reviews and proceedings of many symposia and congresses devoted to admixtures, in books and specialized reviews [1-10J. Again for brevity's sake the number of bibliographic references was limited. Nevertheless those who are interested can find an exhaustive bibliography in Cement Research Progress in the chapters devoted to admixtures [11,12,13,14 15,16]. The references relevant to patents are not considered in this chapter not for want of confidence towards the obtained results but because they concern a restricted experimentation. 2
GRINDING AIDS 2.1
General
Grinding aids are products that improve the efficiency of the grinding process when added to cement in small amounts. These admixtures allow: the mill production to increase, at the same energy consumption and product fineness or grinding duration, or the cement fineness to increase, at the same energy consumption and mill production. In modern mills, whose diameter attains and often exceeds 4 m, the use of grinding aids sometimes becomes indispensable when very fine grindings are required, since the high kinetic energy of the grinding media favours the reagglomeration of the ground material [17]. The more widely used products are surfactants, such as: amines and relative salts, polyoils, alcohols, lignosulphonates, fatty acids and relative salts. Moreover, judging from the great number of patents found in the literature, many and different other products have been used including pure compounds as well as complex mixes [10].
573
Admixtures in Concrete
All admixtures are usually employed in very small amounts, generally included between 0.01 and 0.1 wt% of cement [18], although coke gives the best results at the dosage of 0.50% [19]. 2.2
2.2.1
Influence
Effect of Grinding Aids
on the specific
surface
and particle
size
distribution.
Figures 2.1 and 2.2 show that the effect of grinding aids depends considerably on the nature of the admixture and its dosage [20]. ^- 90
50
Fig. 2.1
100
150 200 Grinding mid
250 content
Undersize of a clinker versus grinding aid content [20]. A - Phenylpolyglycol ether; B - Heptan-phosphonic acid; C - Triethanolamine; D - Sodium Stéarate; E - Sodium salt of hydroxamic acid; F - Decylic Alcohol; G - Potassium butylxanthate; H - Sulphite liquor; I - Sodium cetylsulphate; L - I^HPO^; M - Phosphoric acid ester; N - Methylsilicon oil.
574
F. Massazza 3600
2900 2 800
150
200
Grinding aid
Fig. 2.2.
250 content
Blaine specific surface area of a clinker versus grinding aid content [20]. A - Phenylpolyglycol ether; B - Heptanphosphonic acid; C - Triethanolamine; D - Sodium Stéarate; E - Sodium salt of hydroxamic acid; F - Decylic alcohol; G - Potassium butylxanthate; H - Sulphite liquor; I - Sodium cetylsulphate; L - ^ΗΡΟ^; M - Phosphoric acid ester; N - Methylsilicon oil.
In the former figure the grinding fineness is expressed by the amount of clinker passing through the 40 urn mesh sieve and in the latter by the Blaine specific surface area. These tests showed that the best effects are obtained by using surfactants and, in particular, cationic reagents (primary amines and ethanolamines). However phosphonic acid, glycols, stearic acid, sodium stéarate, hydroxamic acid and its sodium salt are little less effective. In other laboratory tests polyvalent alcohols and amines were more effective than fatty acids and sulphonates [18]. The different effectiveness of the grinding aids of homologous series depends on their molecular weight. In fact it was indicated that the action of saturated fatty acids [21] and alcohols increases as the chain length shortens and that unsaturated acids are more active than the saturated ones [21 ]. At equal grinding times the specific surface increase on average by about 20% [21,22,23], but increases up to 29% were obtained [24]. Admixture fineness increases with grinding duration up to certain limits (see Fig. 2.3) [25].
Admixtures in Concrete
575
10.000-
8.000
10.000 ■
2.000 i
Fig. 2.3.
50
100
150
200 Hours
Grinding curves of the Type I clinker with different grinding aids [25]. (a) A - Oleic acid; B - Diethyl carbonate; C - Reax 70; D - AR-100; E - TMN; F - NP-33; G - NP-40. (b) A - Emcol HB; B - Emcol H-20; C - Emcol H-2A; D - CPH-12J-N; E - Emcol HC (0.5%); F - Emcol HC (0.25%); G - F-90.
Grinding aids modify the particle size distribution of cement. For instance, the addition of surfactants was found to increase the amount of medium size (3-30 urn) particles by 10-20% [26]. Figures 2.1 and 2.2 show that fineness, expressed as Blaine specific surface and residue, rises with increasing amounts of admixture up to certain limits [20,27]. Indeed, certain authors [28,29] found that specific surface decreases with certain admixtures and when some optimum contents (0.025-0.5%) are exceeded.
576
F. Massazza
Parallel tests performed in the laboratory and the factory [18] showed that, when the amount of grinding aid rises, fineness first increases gradually but, beyond a certain value, it remains constant in the laboratory tests whereas it decreases in the industrial ones (Fig. 2.4). The behaviour observed in the industrial tests would be explained by the higher flow of the cement containing grinding aids. Therefore, when flow exceeds a given value, the coarse particles come out quickly together with the fines.
* 10
Industrial
Laboratory Optimum
0.01
tests
content
0.02
-L
0.03
Triethanolamine
Fig. 2.4.
tests
J.
0.04 content
«L
0.05 (%)
Fineness of a cement versus grinding aid content [18].
2.2.2 Influence on agglomeration. The most effective grinding aids also prevent agglomeration since they reduce the adhesion between particles [30]. Consequently, very high grinding finenesses (9000-9600 cm2/g) can be obtained [31,32]. 2.2.3 Grinding efficiency. The economic advantages obtainable in the industrial processes by using suitable admixtures appropriately are by now largely ascertained and estimated by the following values. The grinding time [33] required to obtain a certain fineness can be reduced by 10-50% [34,35,36] and the production be increased by 17-34% [21,37]. The energy consumption can decrease from 15 [38] to 25% [21] and the grinding efficiency can rise by 30-40% [39]. A synthesis of the collected results is shown in Table 2.1.
577
Admixtures in Concrete Table 2.1.
Some Results Obtained in Industrial Grindings by Using Grinding Aids
07
Basic product present in the admixture Diethylene glycol Ethylene oxide Propylene glycol fethylene glycol 'Combustible wastes from butanol manufacture Lignin compound Urea Diethanolamine salt MAVEKLIN grinding aid Dodecylbenzenesulphonic acid amine salt Monostearate
of addition
Increase in productivity
Shortening in grinding time
Decrease in energy consumption
Ref.
10-20% 0.01-0.05 0.05 0.08-0.22 0.08-0.22 0.01-0.02 0.1 0.03 0.1
10% -10%
-10% up to 20% up to 20% >13.2% 30-40% -13% 15-17%
0.05 0.05
~16% -15%
40 41 42 23 23 43 44 45 46 47 48
According to some research [17], grinding aids allow fineness to increase in the open-circuit mills, whereas in the closed-circuit ones considerable production increases can be reached, at the same fineness. This difference is due to that in the latter the fineness of the product is established by the parameters of the air separator. On the contrary, in the open-circuit mills, the overall length of the mill is actually utilized thanks to the anti-agglomerating action of the grinding aid and this causes a fineness increase. The use of grinding aids modifies the technological cycle. Therefore an increase occurs in the dust amount [17,49,50] and in the difficulty of separating it by electrostatic precipitators owing to its higher electric resistance (from 10s to 10 11 Ci.cm) [36,51]. Nevertheless the process can be brought again to its oy.timum conditions by modifying the parameters that affect the mill operation (steel/cement ratio, ciculating load, humidity a.s.o.).
2.2.4
Influence
on the cement properties.
Grinding aids generally have
little influence on the properties of cement pastes and concretes. This is because of the low concentrations of these admixtures with respect to the cement weight. The most important effects can be summarized as follows: the arriount of water required to obtain a paste of normal consistency is slightly increased by glycols and ethanolamines [34,52]; the setting times show slight increases with glycols and ethanolamines [52] and remain unchanged with diethylene glycol [34]. However, according to other authors, they can also be reduced [43]. This means that the effects of grinding aids on setting times are rather changeable; the early mechanical strengths remain generally unaltered [52,53,54] but some increases have been found [22,34,37,55,56].
578
F. Massazza
Moreover, it must be remembered that some surfactants and, partly, the soap of sunflower oil, cause strength to decrease during the first 3 days [21,22]. Anyhow, the differences between samples containing admixtures and plain ones tend to disappear at about 28 days, even if strength increases were found at 28 days [22,57]. Some tests pointed out that the percent strength increase obtained by certain admixtures is higher for blast-furnace cements than for the corresponding Portland ones [56]. Grinding aids seem to exert a slight or negligible effect on the shrinkage cement pastes, at least within the first 28 days [32,58].
of
Grinding aids, depending on their nature, can have a certain influence also
on the pore volume and the distribution
ratio
and the spécifia
surface
2.3
of yore surfaces
[59], the CaO/SiOi
of C-S-H gel [60]. Mechanism of Action
The mechanism of action of grinding aids is not known with certainty and still today these products can be said to be used in a widely empiric way. Their mechanism of action is assumed to be essentially based on the decrease in the resistance to comminution (Rehbinder effect) and on the prevention of agglomeration phenomena. The former effect is based on Griffith's theory [61] according to which brittle materials contain microcracks or, more generally, flaws creating discontinuities in the crystal structure of the material. Under rather strong impacts microcracks propagate through the material as far as to cause its fracture. The same action can create other microcracks where internal imperfections exist. During grinding, however, mechanical stresses act discontinuously. Consequently in the active periods the existing cracks propagate and new ones form, whereas in the inactive ones microcracks can rejoin owing to the attraction of the unsaturated valency forces occurring on the walls of the cracks themselves. When an equilibrium between the two actions establishes, fineness no longer increases. According to Rehbinder's hypothesis [62], shared by other authors [63], grinding aids are adsorbed on the microcracks, so eliminating or reducing their valency forces and therefore preventing microcracks from rejoining. After all, according to this mechanism, the grains show a lower resistance to comminution. After the latter hypothesis, based on the agglomeration prevention, when the ground grains have reached a given fineness, Wan der Waals' forces or electrostatic forces reach such high values that the finest grains are forced to adhere to one another. As a consequence, even if the grinding process is prolonged, the specific surface no longer increases or begins decreasing beyond a certain time. This phenomenon of adhesiveness is also represented by the formation of coatings on the mill walls and the grinding media.
579
Admixtures in Concrete If, however, suitable substances able to neutralize the valencies of the surface charges of the grains are added to the materials, the tendency to agglomeration will decrease.
However, the two effects, that is grain mechanical strength decrease and dust agglomeration reduction, must not be considered as antithetic, even if industrial and laboratory grindings performed by using different admixtures lead to think that grinding aids mainly act on the tendency to agglomeration [30]. In fact, the same molecule of the adsorbed admixture could act first on the fracture strength by preventing microcracks from joining. Then, after the fragment detaching, it could act on agglomeration by preventing the fragments from joining to one another under the compression of the grinding media. These theories are proved by the fact that all the compounds able to affect the grinding process favourably, water included, have a more or less strongly polar character. Since the optimum content of grinding aids is generally included between 0.01 and 0.1 wt%, it can be assumed that within these limits of concentration, depending on the cement fineness and the product nature, the admixture forms a monomolecular layer on the surface of the comminuted particles [27,52j. Since the adhesion is considered to take place between the polar part of the molecule and the surface of the solid, different degrees of coating of the cement grain surface can be obtained if equal amounts of grinding aids are added [20]. For this reason the effectiveness of the saturated fatty acids perhaps decreases, at the same weight amount, as the chain length increases (see Table 2.2) [21 ]. Table 2.2.
Effect of Saturated Fatty Acids on the Grinding Process of White Portland Cement [21]
Acid
Chemical formula
Without grinding aid Formic acid Acetic acid Propanoic acid Butanoic acid Hexanoic acid Decanoic acid Tetradecanoic acid Hexadecanoic acid Octadecanoic acid
CH 2 0 2 C2Hi+0£ C3H6O2 Ci+Hs02 C H 6 12°2 Cj0^20^2 Cli+H2802 C 16H32°2 C 18 H 3 6°2
_
Content
(%) „
0.1 0.1 0.1 0. 1 0.1 0.1 0.1 0.1 0.1
Cumulative residue on N° 0085 sieve
(%)
Specific surface (cm2/g)
8.9 0.8 1.1 1.3 1.5 4.4 4.4 5.5 6.2 6.4
2100 2616 2600 2580 2556 2350 2350 2300 2280 2270
Chemical reactions can occur in addition to the physical phenomenon of adsorption since, by using formic and acetic acid, the corresponding calcium salts form [21]. However, the chemical reaction between admixture and cement grain does not affect grinding. Therefore searching for the boundaries between chemical reaction and adsorption is useless. After all
580
F. Massazza
the tests performed so far seem to confirm that the grinding aid action develops through the neutralization of the free charges or valencies which cause microcracks to rejoin and fine grains to agglomerate. The specific electric resistance of cement, which generally is about 108 Ω.αη, gradually rises to 10 11 Ω.αη when certain amounts of admixtures are added [36,51 ]. 3
ACCELERATORS
3.1
General
These products are added to concrete to reduce setting times and/or to accelerate hardening, Accelerators temperature, slowing down times allows formwork.
allow a rapid concrete hardening to be obtained already at room which therefore avoids thermal treatments, or the hardening at low temperature to be offset. The reduction in the curing an earlier demoulding and therefore a more rapid reuse of
Accelerating admixtures are used in the placing of rapid hardening concrete pavements, groutings, quick repair works, especially at low temperature, and finally as antifreezing agents. Accelerators can be single and composite, inorganic or organic, or more or less complex mixes of different products. They are usually subdivided into set-accelerating and hardening-accelerating admixtures. 3.2
Set-accelerating Admixtures
A too-marked set acceleration can constitute an inconvenience for the corresponding too rapid loss of workability. Nevertheless a mix which sets immediately is required in the placing of the so-called "gunite" (mix sprayed as coating for vertical walls or tunnels) or in other particular cases. Set accelerating admixtures are often used to offset the set delay caused by other admixtures, such as for example water-reducing admixtures [64,65]. 3.2.1 Inorganic products. The most common inorganic set-accelerating admixtures are salts (chlorides, fluorides, carbonates, silicates, fluosilicates, aluminates, borates, nitrates, nitrites, thiosulphates etc.) or alkaline bases (sodium, potassium and ammonium hydroxides) [66,67,68]. A synthesis of their effects is given in Table 3.1 [64], Among these the most known is calcium chloride. As Fig. 3.1 [69] shows, this salt has the double effect to advance the initial set and to reduce the interval between initial and final set. It can be generally said that, if a concrete at room temperature shows an initial setting time of 3 hours, the addition of 2% of CaCl2 reduces this time to 1 hour. Similarly if another concrete, under the same environmental conditions, sets at 6 hours, the addition of the same amount of CaCl2 reduces this time to about 2 hours.
581
Admixtures in Concrete
0,5% 1,0 % 1,5^/ 2,0 %
n
1
i
2 3 4 SETTING TIME
Fig. 3 1.
5 6 HOURS
Initial and final setting times of a cement paste containing different amounts of calcium chloride [from 69].
The effect of calcium chloride is more marked at low temperature and it results to be also more efficacious on slow hardening Portland cements than on more rapid hardening ones [64]. Also the other chlorides have analogous behaviour. The accelerating action becomes stronger by passing from monovalent to bivalent and trivalent chlorides and as the cationic ray increases (see Figs. 3.2 and 3.3) [70].
0.16 Q32
Fig. 3.2
0.96 Molality [-]
Influence of the content in alkaline and alkaline-earthy chlorides in the mixing water on the initial set of Portland cement [70].
582
F. Massazza
QI6
0.32
0.96 Molality C-]
Fig. 3.3.
Influence of the content in alkaline and alkaline-earthy chlorides in the mixing water on the final set of Portland cement [70].
The effect of alkaline carbonates is more marked for pozzolanic cements than Portland ones [71 ]. The accelerating effect of carbonates is considerable not only at 20°C but also at lower temperatures [72]. Nevertheless they reduce strengths [73], even if after 7 days [74]. Sodium aluminate is an effective setting time reducer and therefore it is successfully used in guniting. As Fig. 3.4 [4] shows, its action is stronger, at constant weight, than the one of calcium chloride. A way to transform an ordinary Portland cement into a rapid hardening cement consists in adding high-alumina cement to it. Fig. 3.5 [4] shows that the setting time reduction is mutual for both cements. However, the ultimate strength of these mixes is much lower than the one of the cements. The effectiveness of the admixtures depends on their dosage and therefore it must be remembered that an excessive addition of accelerator to cement causes rapid setting [69,71]. Analogous effect has a temperature rise [69]. 3.2.2 Organic products. The most common organic accelerator is triethanolamine (TEA) used alone or mixed with other organic and inorganic products.
Admixtures in Concrete
583
time (h)
Fig. 3.4.
Influence of sodium aluminate on the setting times of a CPA 325 1:3 mortar versus w/c ratio at 20°C [4]. I «= initial set, F * final set.
As Table 3.1 175] shows, this organic base produces no appreciable effect when the dosage is lower than a certain critical value. Beyond this, there is a sharp reduction in the initial setting time which causes a sort of rapid set. Table 3.1.
No. 1 2 3 4 5 6
Initial and Final Setting Characteristics of Cement Mortars with Added TEA [75]
Per cent TEA
0 0.01 0.025 0.05
0.1 0.5
Initial sett ing time
Final setting time
4.3 hr 4.7 hr 4.9 hr 4.8 hr ~2 min ~6 min
8.3 hr 8.1 hr 8.1 hr 8.4 hr 24 hr
-
The existence of a critical value can explain the different results obtainable by adding equal amounts of admixture to different cements. For example triethanolamine, added in amount of 0.1-1%, caused a set acceleration in a high-alite cement and a delay in a sulphate-resistant cement [76]. However, TEA considerably retards final set at dosages over 0.1% [75]. This action can be related to the fact that TEA retards the hydration of silicates [75,77].
F. Massazza
584
Setting Times I
6 5 4 3 2 1
Alumina Cement^00 Portland Cement 0 Fig. 3.5.
80 20
60 40
40 60
20 80
0 100
Influence of the mixture of Portland and alumina cements on setting times.
Triethanolamine is often used to oppose the retarding effect of other admixtures, such as lignosulphonates, used to reduce the mixing water. One of the advantages of TEA compared with CaCl2 is that it does not create fears of corroding the reinforcement. Diethanolamine [78], sodium formate [76] and saccharose [79] behave as triethanolamine. Many other products were proposed to accelerate setting but there are not exhaustive experiences of them, being mainly mentioned in patents [10]. Organic accelerators are sometimes used together with inorganic products but, of course, in these cases it is not easy to separate and analyse the effects of the different constituents. 3.3
Hardening-accelerating Admixtures
3,3.1 Inorganic products. The most known and used inorganic accelerator is calcium chloride, even if some authors [80] consider that nitrite, nitrate and calcium formate are better. The highest strength increases are reached between the first and third day of curing; of course, the development of these increases depends on the curing conditions of the specimens, the w/c ratio, the type of cement and the percentages of admixtures [69]. As a rule the strength gain, compared with a plain concrete (without CaCl 2 ), can vary between 30% and 100% in the first 3 days [69]. The gain is particularly significant at lower temperatures (Fig. 3.6)[69].
585
Admixtures in Concrete d) Ό 1_
O O
t3 E
80
O
40
D
D O
■
100
7 3 ° F (22 8°C)
eo 20
O H
0
+* Ξ u_
100
ro N
60
0
-K
D
r. +* σ> c
><
σ
'Λ)
0
?0
o c 0) a
■+-'
0)
40°F (4 5°C)
HO 40
0)
νΤλ
80
00 o
t
40
-D
C\J
5 5 ° F (I2.8°C)
80
i
m.
0 60
25°F (-3.9°C)
4(
•>o 0
2% 0% I day
I
P77i
2% 0 %
3 days
1 Υ7Ά
2 % 0%
2 % 0 % Calcium
7 days
28 days
chlor,de
Curing period Fig. 3.6.
Effect of calcium chloride on strength development in concrete at different temperatures.
Table 3.2 shows the influence of some admixtures on the strengths of a Portland cement mortar [81], The table shows, and experience confirms, that CaCl2 has a positive accelerating effect during the first stages of hardening but it tends to lower the ultimate strength. Calcium nitrate does not improve the initial hardening and calcium thiosulphate acts as chloride only with much higher dosages. In both cases ultimate strengths are lower in the plain sample. The strength decrease caused by CaCl2 after 90-180 days can be eliminated by adding also CaO [82] and Al 2 (SOO3 [83]. The strength increase obtained in the cements containing CaCl2 follows the order blast-furnace> Portland> pozzolanic. In the first ones also the slag hydration is activated, in the last ones the pozzolanic reaction is not affected [84].
586
F. Massazza Table 3.2.
%
Effects of Calcium Chloride, Calcium Nitrate and Calcium Thiosulphate on the Strength Development of Portland Cement Mortar [81]
1 dav B C
7 days B C
3 days B C
28 days B C
0
15.8
39.0
34.3
120 52.8
221
79.0
379
CaCl2
1.0 2.0 3.0
20.8 24.2 26.7
69.5 82.5 85.3
40.0 46.9 45.5
150 172 177
54.9 51.5 55.5
291 243 265
76.1 77.1 74.9
381 352 364
Ca(N0 3 ) 2
2.0 5.0
7 15.1
18.7 42.3
34.2 41.0
113 173
47.3 63.1
193 261
74.6 77.3
316 351
1.0 17.1 2.0 18.1 3.0 18.8 5.0 20.1 6.5 21.1
44.7 46.7 55.8 59.3 69.3
34.6 35.6 37.8 40.6 37.6
117 127 130 150 143
51.9 53.2 53.1 55.7 50.9
210 203 237 205 207
73.6 76.2 75.2 66.1 63.4
341 328 366 310 268
B: Bending strength, C: Compressive strength, unit of strength: kg/cm2. 3,3,2 Organic products. The cement hardening is accelerated by many organic products of which calcium formate is a typical representative (see Fig. 3.7). This compound is less effective than calcium chloride but it does not decrease 28-day strengths and presents no risk of reinforcement corrosion [86]. The literature gives many other examples of organic accelerators of varied compositions: hydroxylated carboxylic acids, formaldehyde and paraformaldehyde, cationic polyesters, phenolic and epoxy resins etc. [10]. For example, calcium sulphonate increases the compressive strength from 7 to 71 kg/cm2 after 6 hours and from 560 to 663 kg/cm2 after 28 days, when added in significant amounts (4-10 wt%) [87]. The technological advantages obtained by using these products are important but, in most cases, they require verification. As regards their possible applications, their cost has been prohibitive. 3.4
Collateral Effects
All the accelerators have, besides their main action, sometimes important collateral effects that must not be ignored.
3,4,1
Rheology of the mixes.
Some accelerating compounds show a
fluidizing
effect on the cement paste such as, for example, alkali-metal chlorides and nitrates, alkaline-earth-metal chlorides and nitrates, aluminium chloride and trivalent ferric chloride [70], The plasticity of the cement paste containing alkaline-earth-metal chlorides is higher than that of the plain cement, till about 1 hour1s hydration. On the contrary alkaline carbonates reduce the cement plasticity and, as alkaline silicates, they increase the water demand [67,70,73]. A favourable effect of these admixtures, related to their accelerating action, is the reduction in the segregation phenomena occurring in fresh concrete.
Admixtures in C o n c r e t e £ 700 c
587
^
v.
* 600
4% y | 500 o O
200
100
0
/
'A
h
/
400
300
/
1 1 /
/
f
Without
/ /
accelerating
W/C = 0..35 C = 'Χ2Ό kg/rr>3
/
/ 4
admixture
6 8
12
16
24
.
48
Curing - hours ( Y - scale)
Fig. 3.7.
Influence of calcium formate on the early strengths of a concrete [85].
3.4.2 Durability. The accelerating action of calcium chloride has also some negative aspects that must not be ignored. In fact calcium chloride favours the reinforcement corrosion, even if it must be remembered that the steel attack depends on its characteristics [88], The reinforcement corrosion is infrequent since the alkaline environment of the cement paste passivates the steel by the formation of a protective oxide surface film which is stable until the pH is high enough. Since chlorides decrease the pH [89], the film stability lowers and therefore the corrosion process is favoured. Corrosion can be prevented by adding ΝΗι*Ν03 [90] or CaO to CaCl2. CaO rises the solution pH which increases from 7.5-8 to 12-13 [82]. Also Ca nitrate is an inhibitor of the reinforcement corrosion. Calcium chloride, besides decreasing ultimate mechanical strengths, may also lower the resistance to sulphates of concretes [91], but this conclusion is not shared by all [92]. Some authors found that some accelerators increased the resistance to frost of concretes. This is not surprising since the frost resistance of a concrete is the higher the more advanced the degree of hydration when freezing occurs.
588
F. Massazza
3.4.3 Dimensional changes. Calcium chloride increases shrinkage by 10-50% [93], An analogous effect is obtained by high contents of triethanolamine [94], This admixture also enhances the shrinkage caused by lignosulphonates [95,96,97]. Shrinkage increases were also obtained on C3S pastes containing CaCl2[98,99 ]. Generally, the concrete shrinkage seems to increase in the presence of set-accelerating admixtures [100], The strong shrinkage caused by CaCl2 can be attributed to the higher degree of hydration and the higher specific surface of the hydrated cement pastes [84]. Nevertheless the CaCl2 addition decreases the wideness of the crack caused by drying [101]. The effect of CaCl2 on shrinkage is lower for pozzolanic cements [84]. Creep increases by adding CaCl£ [95,102], calcium nitrite and nitrate [103]. This effect is assumed to be due to an intensified contribution of the changes in the pore distributions [99]. Triethanolamine causes an increase if the concrete specimens are loaded after 7 days' curing, whereas no difference is noticed for specimens loaded after 28 days [95]. This result is likely to be the consequence of the hardening delay caused by the admixture. Finally triethanolamine intensifies the creep increase of concrete caused by lignosulphonate [96,97]. 3.4.4 Heat of hydration. CaCl2 increases the heat of hydration evolved during the first phases of the hydration [64], The second peak characterizing the heat evolution curves of cement pastes is advanced and intensified in the presence of CaCl2· The effect is observed with both Portland [81] and blastfurnace [104] cements. 3.4.5 Antifreezing properties. Antifreezing properties, which favour winter concretings, are attributed to calcium chloride and also to other inorganic and organic accelerators [10]. However, it must be noted that the decrease in the melting point of the mixing water, caused by CaCl2 employed at the usual contents (<2%), is small (<0.9 C ) . Therefore, the antifreezing action of the admixture is essentially due to its capacity of increasing the rates of the hydration reactions and, thus, of the heat evolution [105].
3.5
Composition and Structure of the Hydrated Phases
The addition of accelerators to cements generally modifies the composition and the structure of the hydrated phases to a different, but small, extent depending on the admixture nature. Some authors found that, at the same degree of hydration, the cements containing CaCl2 have a higher specific surface than the plain ones [106], [84], whereas others found no difference [107]. This discrepancy probably depends on the precision in determining the degree of hydration of pastes. The surface increase of cements after 7 days was observed in Portland, pozzolanic and blast-furnace cements [84], but this decreased (after 28 days) for the last ones [108], The C3S pastes containing CaCl2 have a higher specific surface area than the plain ones [109,110], even if some authors [118] found the opposite. Also TEA enhances the specific surface of hydrated C3S [77], CaCl2 increases the specific surface as well of the C2S hydration products [142],
Admixtures in Concrete
589
The presence of CaCl2 modifies the pore size distribution of the hydrated pastes of C3S [107,111] and C2S [112]. The admixture nature modifies the morphology of ettringite [111,113], C-S-H [114] and Ca(0H)2 [115]. For example, by hydrating C3S in the presence of CaCl2, it was possible to notice a "honeycomb" instead of "sponge-like" gel [116] or a "foil-like" C-S-H(I) phase and a fibrous C-S-H(II) phase 1117]. It was confirmed that CaCl2 modifies the chemical composition of C-S-H since it increases the CaO/Si02 ratio of cement [106], C3S 1117,118] and C2S [119] pastes. The strength rise caused by CaCl2 is attributed to an enhanced degree of polymerization of the silicate hydrates [120]. Nevertheless the validity of these results also for cements must be still established with certainty [121 ]. As regards Ca(0H)2 no hexagonal plate appears in the presence of CaCl2 [116], or at least the hexagonal appearance becomes less marked as the CaCl2 concentration increases [117], Of course, admixtures can cause the formation of other phases, such as nitroaluminate hydrates [122], chlorohydrates [123] and chromate-derivatives analogous to ettringite [124]. Anyhow these compounds can form when admixtures are present in high concentrations. There are also signs that during the C3S hydration in the presence of CaCl2 a chlorinated complex with silicate hydrates forms [117,125]. However, the amount of combined chloride is relatively small (~0.40 g/100 g C3S) and depends on the Cl~ concentration in the solution contained in the pores [126]. In the presence of triethanolamine there is a reduction in the Ca(0H)2 content compared with the expected one and a higher Ca0/Si02 ratio in C-S-H [77,127]. This fact*was explained by assuming that a CaO accommodation occurs in the C-S-H lattice [128] or the formation of not crystalline Ca(0H)2 is favoured [77]. Anyhow triethanolamine causes only small changes in the morphology and microstructure of the hydration products of C3S and C2S [127,129,130,131]. A complete review on the structure alterations induced by the other accelerators would be too long. In any case it must be pointed out that the changes relevant to the cement gel, if any, are small whereas the ones relative to Ca(0H)2 seem to be more important [10]. 3.6
Mechanism of Action
At present there are no theories explaining the mechanism of action of the different accelerators in an exhaustive and controllable way. Therefore only the particular effects related to their presence in the systems concerning cements can be mentioned. 3,6,1 Action on setting times. It is a consolidated opinion that the set acceleration is generally due to an intensification of the hydration of aluminates. Triethanolamine [75,132,133,134,135,136,137], sugar, formate [113], chloride [100,104,113,138,139], calcium nitrite and nitrate [122,140] intensify the formation of ettringite, either pure C3A or the one present in cements is considered. For example, by adding 0.3wt% of TEA, all calcium sulphate has reacted after 20-30 minutes [79].
590
F. Massazza
The more rapid set of the cement paste would be also attributed to the crystalline shape of ettringite which, during precipitation, would produce a felt of very fine needles. During the hydration of Portland cement in the presence of calcium chloride, ettringite forms initially as far as the complete disappearance of gypsum. Subsequently, if free tricalcium aluminate still exists, Friedel salt (3CaO.Al203.CaCl2.10H20) forms [126,141]. At higher temperatures (40-80°C) the formation of chloroderivative prevails on sulphoaluminate [126]. 3. 6.2 Action on hardening, CaCl2 accelerates the set and hardening of cement, not only because it accelerates the ettringite formation but also because it increases the reaction rate of C3S [109,118] and S-C2S [84,119]. Also calcium formate acts positively on hardening since it accelerates the C-S-H formation, even if with lower effectiveness than CaCl2 [113]· The effect on C3S is mainly attributed to the fact that the diffusion rate of the Cl" ions is much higher than the one of the cations accompanying them. Since the Cl~ ion penetrates into the hydrated layers covering the C3S grains more quickly than the cations, a counterdiffusion of OH" ions must occur to maintain the electric equilibrium in the solution. As a consequence, the precipitation of calcium hydroxide is accelerated as well as the decomposition of calcium silicates [143]. The accelerating action of CaCl2 was also attributed to other causes, such as the alkalinity reduction of the liquid phase, and consequent increase in the dissolution rate of hydrolysis lime [144], a salt adsorption on the cement grains which would facilitate the attack from H2O [145] and, finally, an increase in the concentration of calcium ions in the solution caused by the presence of salt [146]. In any case, CaCl2 would not modify the mechanism of hydration of C3S but only some kinetic parameters governing the formation of crystalline germs [147]. Very little is known of the mechanism of action of the organic hardening accelerators. 4 4.1
RETARDERS General
Cements usually begin setting 2-5 hours after mixing and begin hardening after about 3-7 hours. Nevertheless, in some cases these limits can be unsatisfactory, for example when it is necessary for the concrete to remain pasty for 24-48 hours. This occurs when the construction joints, which are a weak point in structures, need to be eliminated. The problem can be solved by using suitable admixtures retarding the hydration of the last placed concrete until (after 16, 24 and 36 hours) the recasting with fresh concrete is started again [148]. Likewise, the surfaces of the concrete roads can be roughened by covering the fresh concrete surface with an emulsion containing a retarder or with paper or fabric soaked with admixtures and by removing the not yet hardened surface layer by brushing or water jets after 6-48 hours [149]. The use of retarders has a great importance also in the most common practice, such as for example when setting times need to be prolonged in order to cast in warm climates, to transport fresh concrete for long distances (especially in summer), for groutings etc.
591
Admixtures in Concrete
The problem of retarding the cement set can become even essential in some special cements, such as the "regulated set" or "jet" cements and expansive cements. These products are indispensable also to retard set during the cementing of oil and geothermal wells or under conditions of high temperature. Retarding admixtures can consist of both inorganic and organic products. Besides substances especially used to retard setting, there are others whose retarding action is secondary. This is the case of air-entraining admixtures, plasticizers and grinding aids. 4.2
Inorganic and Organic Retarders
Among the inorganic retarders there are: some acids (boric, phosphoric, hydrofluoric, chromic), the respective salts, and some oxides (of zinc and lead). A typical retarder of this group is zinc oxide that, as Fig. 4.1 shows, is very effective even if added to cement in small amounts [150]. An analogous action, even if less marked, is shown by phosphoric acid [4],
50-
n 4o-\ Φ
I 30. Z 20H Φ
10
0r10
0,15
r— 0,20
ZnO V· Fig. 4.1.
Influence of ZnO addition on the initial set of a 1:3 Portland cement mortar, w/c = 0.35, curing at 20°C [150].
Among the numerous organic compounds used as retarders there are: lignosulphonates, hydroxycarboxylic and carboxylic acids and their salts, amines and aminoacids, carbohydrates and their oxidation products, such as the acids and their respective salts. The effect of sodium gluconate is shown in Fig. 4.2 [4J. Some of these agents are plasticizers and air-entraining admixtures.
592
F. Massazza
% based
Fig. 4.2.
on cement
weight
Effect of varying contents of a set-retarder (gluconate-based admixture) on the initial and final setting times (Mortar 1/3, W/C = 0.50, 20°C) [4].
Many other organic substances were proposed as set- and hardening-retarders but, for brevity's sake, they cannot be considered here. 4.3
Factors Affecting the Action of Retarders
A great number of factors affect the action of retarders. The dosage of the admixture is of great importance and in general it can be said that the retarding effect increases as the admixture addition increases (see Figs. 4.1, 4.2). The effect of retarders is more marked at low than high temperatures. This behaviour is shown in Fig. 4.3 where it can be seen that the delays are considerably reduced by passing from 10 to 30 C [148]. Moreover, many organic admixtures (mono- and dicarboxylic acids, sulphoacids, amines, mono-, di- and trihydric aliphatic acids were found to lose their effectiveness at 60°C [151 ]. Also the way of adding the admixture affects the retarding properties since calcium lignosulphonate, alone [152] or with K2CO3 [153], was found to retard setting even more when added to the mixing water.
593
Admixtures in Concrete
AB C
D
+
G30
| 201
V. v \
I I 2 io 1 1
\
2 S- 0
\ 'h
■V\
\
X
10
20 Initial
Fig. 4.3.
1
30 setting
time
40 (hours)
Initial setting time of concrete versus temperature and retarder content [148].
Of course, the effect of the retarder not only depends on its nature but also on that of cement. As admixtures mainly act on clinker, the optimum amount of retarder decreases in blast-furnace cements with increasing slag content [86]. Other parameters affecting the delay are the fineness of cement, the [78,154,155]. composition of concrete and the water/cement ratio 4.4
Other Effects
Retarding admixtures improve the workability of mixes at both low and high temperatures. This fact is particularly advantageous in works demanding a prolonged time of hot-workability of mixes. As Fig. 4.4 shows, the rapid loss of workability undergone by concretes at 55-60 C can be efficaciously opposed by the addition of a retarder-plasticizer [156]. There is no strict correlation between set and development of strengths in mortars and concretes containing retarders. As an early set does not necessarily result in a high early strength, so a delayed set does not always cause a strength reduction [157,158], In fact, both organic (see Table 4.1) and inorganic (see Fig. 4.5) retarders can increase strengths at different ages [159,160]. The effects can be higher on concretes since many organic retarders have a fluidizing action enabling a decrease in the w/c ratio [148]. The effect on strengths depends on the nature and the amount of admixture added to cement. Figure 4.6 shows that, up to 0.5%, triethanolamine does not considerably affect the strength development but, with an addition of J%, the strength recovery only occurs after 28 days [79].
594
F. Massazza Table 4.1.
Influence of Some Set-retarders on Mechanical Strengths. Portland Cement 1/3 Mortar [4]
2d
1d
90 d
28 d
C
TF
C
TF
C
TF
C
36
120
49
220
77
385
88
0.5%o l%o
30 4
102 13
51 29
220 120
80 77
480 440
83 81
1%0
2%o
20 1
72 10
50 26
242 85
68 56
375 285
76 75
.4 υ
A
Plain Sucrose
7d
TF
Glucose
TF
C
462
90
550
610 550
84 96
545 465
81 81
640 615 600 525
?
\
-£ >
<
v
V
^**"* **»
\"^^ \
10
Fig. 4.4.
^
20
^
\
1 > 1
30 40 Minutes after mixing
Effect of the amount of a lignosulphonatebased retarder on time-dependent change in slump in a hot-mixed normal weight concrete (NWC) and a lightweight aggregate concrete (LWC). Mixing temperature: 55-60 C; Cement content: 530 kg/m3 [156]. A - NWC 55°C (131°F) Retarder 0.25 wt% of cement; B - NWC 55 C Retarder O wt% of cement; C - LWC 60°C (140°F) Retarder 0.25 wt% of cement; D - LWC 60°C Retarder O wt% of cement.
595
Admixtures in Concrete
700h
600
650
_
^
con
—
" -^
0660
—»··'■'
598^<.^606
572, < £
£^2 4 ϋÜ ^- - o - 700 ^ — ,ο676 ■—
[i5h]
370 [0h]
>620 Plain
500
9
400
·?
300
ΖηΟ 0 %
Plain
ΖηΟ 0 15 % Immediate placing [01 o
200
o
Ζη0 0 Ι 5 % Placing after 15th [15] ZnO 0 15 % Placing after 24 th [24]
14
25
28
Days
Fig. 4.5.
Effect of ZnO addition on the compressive strength of a Portland cement paste. w/c « 0.36. Water curing at 20°C [150].
Some retarders can show an inversion of the effect. In these cases concrete sets rapidly and workability decreases [148,161]. This means that the dosage and the use of retarders should be preceded by careful preliminary tests. The setting delay is a consequence of the retarded beginning of the cement hydration process. For this reason these admixtures also retard the development of the heat of hydration (see Figs. 4.7 [4] and 4.8 [162]) and cause a decrease in the chemically combined water in the cement paste [163] (see Fig. 4.9). Also the specific surface of the hydration products is simultaneously decreased [163]. As already mentioned, the delay is generally accompanied by a fluidizing effect which modifies the rheological properties of pastes. This can be attributed, at least partly, to the enhanced air entrainment [164]. As regards drying shrinkage of concrete, it can be said that set-retarding admixtures have generally rather limited effects [100,165]. Some retarders caused volume instability with certain types of cement, however, no explanation of the phenomenon has been given so far [161].
596
F. Massazza 600 PZ 450 F S 500
. ' ^ < ^ f \
-400 ^ ^l
S 300
A'
* 200
1
C4
1
2iII
0}
*
'
7y
'
-'
'
\
1
/ / / f
\
ι (
2
Ί
1 28
7 Curing
Fig. 4.6.
\
1
/
Br
v.
α 100 o
/
{days)
Influence of triethanolamine (TEA) on compressive strength of a Portland cement PZ 450 F according to DIN 1164. A = without admixture; B, C, D = 0.2%, 0.5%, 1.0% TEA respectively [79]. 4.5
Mechanism of Action
The mechanism of action according to which retarders act on cement has not yet been explained completely. Several theories were proposed about the matter but none is able to give a valid explanation in all cases. Therefore the delay is likely to be the result of several different causes acting simultaneously. Owing to the composite nature of cement, the same admixtures can be expected to act differently with the various phases of clinker. In fact, some admixtures were found to have a different effect on aluminates and silicates. A typical case is triethanolamine and sugar that accelerate the ettringite formation but retard the Ca(0H)2 one. Therefore they accelerate setting but retard hardening. The main theories proposed to explain the action of retarders can be summarized as follows: Admixtures are adsorbed on the surface of the anhydrous products which are, therefore, protected from the water attack. Admixtures react with the anhydrous compound and the reaction products cover the grains with an insoluble and waterproof layer. Admixtures hamper the nucleation and growth of the hydrated crystals since they form complexes with the hydrated phases and are incorporated in the lattices of the hydrated crystals.
Admixtures in Concrete
597
Sugar
Fig. 4.7.
Influence of sugar and zinc oxide on the heat of hydration of cement. Thermos flask method. 1:3 AFNOR Mortar [4].
In any case, once the admixture has been combined and removed by the solution, the hydration resumes its normal course. The major remarks that can be made about these theories are: 1
The amount of calcium lignosulphonate, typical retarder-plasticizer, which is adsorbed on both C3A [166] and C3S [167] is very limited. The adsorption on the anhydrous compounds is also very low with other admixtures [168], In any case the adsorption on the anhydrous phases seems to cause an initial acceleration of the hydration, whereas the interaction with the hydrated products brings about a delay [169,170]. 2 No correlation was found between insolubility of calcium salts of carboxylic acids and delay, however it cannot be excluded that some retarders (such as zinc) act via the formation of an insoluble protective layer. 3
The influence of retarders on the growth and morphology of crystals is an ascertained fact. A slowing down of the ettringite formation [164,171] and changes in its morphology [79] were found in the cements containing admixtures, whereas a considerable alteration in the sizes and morphology of crystals from Ca(0H)2 was noticed in C3S pastes with admixture [115]. As Figs. 4.10 and 4.11 show, the hydration products can adsorb appreciable amounts of admixtures [168].
598
F. Massazza
Days Fig. 4.8.
20|
0
Retarding effect of different sodium gluconate amounts on the heat of hydration [162], (1) Plain CPA; (2) 0.5%o; (3) 1%0; (4) 2%0; I
I
I
1
I
20
40
60
80
100
Hardening time Fig. 4.9.
I
(days)
Chemically combined water in cement pastes containing 0.1, 0.2 and 0.3% sodium gluconate [163].
Admixtures in Concrete
599
005 0 10 015 Initial Concentration (wt%) Fig. 4.10.
Apparent adsorption of salicylic acid on C3S and 3-C2S [168].
005 0 10 0.15 I n i t i o l C o n c e n t r a t i o n < wt % )
Fig. 4.11.
0 20
020
Apparent adsorption of salicylic acid on pasteand bottle-hydrated C3S and on bottle-hydrated S-C2S [168].
F. Massazza
600
In these cases the action that retarders have on the hydration of cements could be explained as follows. When the admixture is employed in the usual amounts, the initial hydration of aluminates and, mainly, silicates is delayed since the retarder strongly affects the formation and growth of the crystals of the hydrated phases. When the retarder is removed from the solution, since it is preferentially adsorbed on the hydrated phases containing alumina, the hydration of silicates begins again vigorously. As a consequence, the strength of the samples containing admixtures decreases at short ages but is equal to plain cements at 28 days. If, on the contrary, the initial content of admixtures is high, the aluminates are insufficient to remove it completely from the solution and the hydration is stopped or, at least, strongly retarded. This mechanism is justified by different experimental results: (a) The addition of C3A eliminates the strong delay caused by lignosulphonate (LSA) on the hydration of calcium silicates [129]. Table 4.2 shows that C3S and alite hydration, as measured by the loss on ignition, is strongly accelerated by the presence of tricalcium aluminate [172]. (b) Setting is retarded if the admixture is added after some minutes of mixing [152,153]. In this case the initial hydration products of C3A formed in the absence of admixture are assumed to have a lower adsorption capacity [173,174,175]. Consequently a higher amount of * retarder will remain available in the solution and the hydration rate of C3S will be considerably reduced [172]. Table 4.2.
Silicate paste C3S only C3S + 5% Alite(M) Alite(M) Alite(J) Alite(J)
Ignition Loss (% at 1100°C) of Hydrated Silicate Pastes [172] No additive 7 days 14 days
C3A only + 5% C3A only + 2% C3A
17.2 17.8 13.2 16.9
19.5 21.8 14.8 18.6 16.7 15.9
7 days
3.9 4.1 2.6
With LSA 90 days 14 days 1.4 21.5 1.4 16.1 2.0 19.2
4.0 22.2 1.9 19.8 21.6
180 days 2.5 25.4 2.1 22.0
The retarding effect of lignosulphonates was not attributed directly to the lignosulphonic acid salts but to the simple sugars in the commercial products. However, it was pointed out that these compounds, owing to their weak content have a slight retarding effect and therefore it is not possible to eliminate them from the commercial products [176]. Assuming that pure lignosulphonic acid has a little retarding action, it must be sugar acids with the respective salts (always present in commercial lignosulphonates) which strongly retard the C3A hydration [177].
Admixtures in Concrete
601
Of course, the effectiveness of admixtures depends on a great deal of factors, only some of which are recognized with certainty. For example, the retarding action is related to the nature and number of functional hydroxyl groups contained in the molecule [178,179,180,181]. The retarding action seems to be attributed not only to hydroxyl groups but also to carboxylic and carbonylic groups as well as to the number and arrangement of functional groups in the molecule [182,183]. The example given by diphenols is typical: pyrocatechol accelerates setting, resorcinol is practically inactive and hydroquinone retards it [151 ]. The different intensity of action found in the gluconates of Na, K, Mg and Ca seems to be related to the ionic radius and electronegativity of the cation [184]. Also the strong alkalinity of the cement paste plays an important role, as shown by the fact that glucose is less effective than its oxidation products, since it is less stable in basic medium [185]. To conclude these short notes it can be said that, although much progress in interpreting the retarding action of the different compounds has been made, the diversity of their nature and the complexity of the cement hydration process make additional studies necessary for the complete understanding of the phenomenon. 5
PLASTICIZERS 5.1
General
The organic substances or the combinations of organic and inorganic substances which allow a reduction in the water demand of concrete, at the same consistency, or give a better workability to the mix, at the same water content, are classified as plasticizing admixtures. The advantages are considerable in both cases: in the former, concretes are stronger, in the latter they are more workable. This advantage concerns all concretes but it is particularly important for thin and vary reinforced concretes or for the conveyance of concrete in pipelines. The basic products constituting plasticizers can be classified as follows: anionic surfactants, such as lignosulphonates and their modifications and derivatives, resin and alkaline abietate soaps, salts of sulphonated hydrocarbons, alkyl aryl sulphonates; nonionic surfactants, such as polyglycol esters; acid of hydroxylated carboxylic acids and their modifications and derivatives; other products, such as carbohydrates, polysaccharides, etc. Among these calcium, sodium and ammonium lignosulphonates are the most used. As a matter of fact the compounds that exhibited more or less marked plasticizing properties are so numerous and diverse that at present it is practically impossible to list them completely. The base products used to prepare plasticizers have often other undesirable properties (setting retards, a ir-entr ai riment etc.) and therefore they are frequently mixed with other products which reduce or eliminate the negative effects and also improve the positive ones [186,187],
602
F. Massazza
Plasticizers are used in the amount of 0.1-0.4% of the cement weight. Owing to their small content, their dosage and their homogeneous distribution in concrete must be checked carefully. For this reason the products dissolved in water are preferred to the dry ones. Plasticizers can be employed with all the usual types of cement. Some polymeric compounds, such as polyethylene oxides, improve the concrete flow even if they do not increase slump. They are used successfully in pumping of ordinary and lightweight concretes [188,189,190]. 5.2
Effects on Mortars and Fresh Concretes
5.2.1 Mixing water reduction. By reducing the mixing water, at constant workability, the mechanical strength of concrete increases appreciable. At the same time the risk of segregation is reduced and the homogeneity and compactness of mixes are improved. The water reduction obtainable with these admixtures, provided negative effects do not occur, is generally included between 5 and 15% [4,191,192,193,194,195,196,197], even if different patents guarantee higher reductions. Obviously the effects caused by an admixture depend on its dosage and nature (see Table 5.1) [198]. 5.2.2 Workability increase. At the same w/c ratio, admixtures improve the mix workability with an effectiveness depending on their dosage (see Fig. 5.1) [199].
J
O
\
I
L
0,2 0,4 Admixture - %
0,6
Fig. 5.1. Workability of mortars containing increasing amounts of citric acid [199].
603
Admixtures in Concrete
Table 5.1.
Effect of Some Admixtures on Concrete Properties [198]
Cement content (kg/m3)
Water content (kg/m3)
Slump (cm)
Vebe value (sec.)
300
195
5.5
13.2
1.4
159
292
AEA - 3
0.04 0.08 0.12
300 300 300
190 183 174
4.5 4.5 5.8
13.4 8.9 6.4
2.6 4.0 6.8
167 168 142
305 300 249
WR(A) - 3
0.25 0.50 0.75
300 300 300
177 169 158
4.3 4.6 6.0
9.6 8.2 7.5
1.9 4.2 5.6
21 1 228 256
349 349 373
WR(B) - 3
0.25 0.50 0.75
300 300 300
184 177 169
4.7 4.7 6.1
16.6 8.8 7 .4
2.3 3.3 5.7
230 231 247
365 367 384
WR(C) - 3
0.25 0.50 0.75
300 300 300
176 174 170
5.1 5.2 4.8
1 1 .4 16.3 17.0
2.2 3.2 3.3
214 238 244
343 355 382
WR(D) - 3
3.0 6.0 9.0
300 300 300
171 161 151
5.5 5.6 8.0
7.2 11.6 19.6
2.1 1 .1 1.4
232 271 212
351 377 296
WR(E) - 3
0.40 0.80 1.2
300 300 300
185 177 172
5.0 5.2 4.5
15.4 17.4 19.0
2.0 2.3 2*4
214 228 238
351 372 411
Admixture content
(%) -
Mix. No. Plain - 3
Air content
(%)
Compressiv«> strength (kg/cm 2 ) 7-day 28-day
-
500
189
5.5
10. 1
1.1
415
550
AEA - 5
0.04 0.08 0.12
500 500 500
182 182 179
4.6 4.8 4.8
12.8 1 1 .4 10.5
3.1 4.1 4.3
387 379 366
544 539 531
WR(A) - 5
0.25 0.50 0.75
500 500 500
185 177 168
4.5 4.6 4.6
21.4 17.8 9.8
2.2 3.3 5.2
448 436 377
576 638 590
WR(B) - 5
0.25 0.50 0.75
500 500 500
179 176 166
4. 1 4.6 5.2
31.0 21.4 20.6
2.4 2.7 4.5
440 460 466
621 653 651
WR(C) - 5
0.25 0.50 0.75
500 500 500
182 177 175
4.5 5.6 4.0
27.0 22.1 16.0
2.0 2.0 1.9
443 475 468
616 703 702
3.0 6.0 9.0 0.40 0.80 1.20
500 500 500
167 158 144 181 174 163
5.9 9.6 9.8
13.4 6.1 3.9
1.4 0.8 1.1
714 725 733
4.5 5.8 9.2
22.6 7.0 4.0
1.5 1.8 1.7
537 544 586 484 534 578
Plain - 5
WR(D) - 5
WR(E) - 5
•
AEA WR(A) WR(B) WR(C) WR(D) WR(E)
500 500 500 Air-entraining Water-reducing Water-reducing Water-reducing Water-reducing Water-reducing
agent agent agent agent agent agent
(A) (B) (C) (D) (E)
Salts of wood resins Calcium ligninsulfonate Derivatives of ligninsulfonate acid Polyol complexes Highly condensed triazine compounds Polyalkyl aryl sulfonates
675 715 770
604
F. Massazza
The action of the admixture and the optimum content which generally act simultaneously.
depend on many factors
Among these the mineralogiaal composition of clinker [200,201 ] plays an important role since the optimum content of the admixture increases as aluminates increase [202]. In fact, as Fig. 5.2 shows, the flow of mortars containing 0.2% calcium lignosulphonate decreases as the C3A content increases [201]. Nevertheless the fluidizing effect of some phenol-formaldehyde resins was found to increase as the C3A percentage in clinker rises [203].
46
r
■S» 4 2 | 38
si 1*
34
Addition
of 2%0 calcium
lignosulphonate
30 26 22 18
O O
££
14
o o 10
II
6 2
-L 8
9
J10
I II
1 12
-L 13 14 %C3A
C3S/C2S = Const Fig. 5.2.
Reduction in the workability of a cement mortar containing calcium lignosulphonate vs. percent content in tricalcium aluminate [201 ].
Plasticizers do not modify [204], or at most slightly retard, the workability loss that concrete undergoes with time. However, increases in the workability loss, which can only partly be offset by overdosages, were indicated [206]. The addition of water to recover the initial workability of concretes mixed for some hours (retempering) can be indispensable, for example in ready-mixed concrete, to allow concrete to be discharged and placed. Nevertheless this practice lowers strengths considerably. On the contrary, retempering on concretes containing plasticizers-retarders at dosages above the usual ones has a favourable effect since it allows the total water content to be reduced, resulting in a mechanical strength increase [206,207].
605
Admixtures in Concrete
In ordinary concretes workability generally decreases as temperature rises, but the effect can be different in the presence of plasticizers. In fact, as Fig. 5.3 [208] shows, three results can occur: workability loss (A-type admixtures based on lignosulphonates and air-entraining admixtures), no variation (B-type admixtures such as sodium salt of a poly alkyl aryl sulphonic acid),and workability increase (C-type admixtures, that is of the improved B-type).
| 360 W/C*0,35
I
g
320
280 240 200 160 120 20
Fig.
5.3.
30
40
50
Temperature - °C
Relationships between temperature and flow value of mortars containing different types of water-reducing admixtures [208].
At constant slump, mixes demand increasing water quantities with increased temperature [208], as is shown in Table 5.2. It is also noticed that, by using A- and B-admixtures, the water reduction does not change from 20 C to 50 C but it increases considerably (from 25 to 34%) with C-admixtures. Also the type
of aggregate
affects the water reduction [198].
Plasticizers modify the viscosity of the cement paste. Figure 5.4 [209] shows the changes in the apparent viscosity of a cement paste containing increasing percentages of sodium lignosulphonate (LS) and polyvinyl alcohol (PVA) at different degrees of polymerization. Viscosity shows a sharp decrease on using lignosulphonate beyond a certain concentration whereas it increases with polyvinyl alcohol as the content and the polymerization degree of the admixture increase.
F. Massazza
606 Table 5.2.
Type of admixture plain A B C
Influence of Temperature on the Reduction in Unit Water Content of a Portland Cement-based Concrete Added with Three Water-reducing Admixtures [208]
20 c >C W/C Slump (cm) (%) 7.8 8.0 7.7 8.3
45.0 40.6 38.0 33.6
40C >C W/C Slump (cm) (%)
50C >c W/C Slump (cm) (%)
47.8 42.2 40.0 31.7 .
48.3 43.4 41.1 32.2
8.2 8.4 8.2 8.0
Reduction i n the u n i t water coicitent 20°C
8.2 8.3 8.5 8.2
40°C
.
.
.
-10% -15% -25%
-12% -16% -33%
-10% -15% -34%
A = lignosulphonate B s poly-alkyl aryl sulphonate C = improved poly-alkyl aryl sulphonate
.8*
L PVA 2390
5 /
PVA1750
§40 •2
i
1 /l^
^30
20
^5*-
^PVA 610
•
V ^ _ L S 10
0
•
1
0.1
0.2
'
0.3 Amount
Fig. 5.4.
■
0.4
'
0.5
of LS & PVA
50°C
1 (%)
Influence of sodium lignosulphonate (LS) and polyvinyl alcohol (PVA), at different degrees of polymerization, on the fluidity of cement pastes [209].
607
Admixtures in Concrete
5,2.5 Secondary effects. The use of plasticizers involves not only the primary effect of fluidization but also secondary undesired effects, such as the delay of set and the air-entrainment in concretes. Products retarding setting times are mainly lignosulphonates [170,191,210, 211] and salts of hydroxycarboxylie acids [157,191]; delay is generally limited to 1-2 hours at ordinary dosages but it becomes excessive at overdosages (see Table 5.3) [194], Table 5.3.
Effect of Overdose of a Standard Plasticizer on the Initial Setting Time of Portland (A) and Slag (B) Cements [194]
Nr.
% Admixture
1 2 3 4 5 6
«» 0.2 0.6
-
0.2 0.6
Cement
A A A B B B
Initial set 210 250 450 275 325 550
min. min. min. min. min. min.
A = Portiand Cement 350 F B ■ Slag Cement 350 L Also the impurities in the admixtures play an important role: at dosage >0.2% of the cement weight commercial calcium lignosulphonate causes rapid set whereas purified CaLS and NaLS retard set. To obviate the delay other products such as sodium carbonate, able to accelerate set, are added to plasticizers - retarders [65]. The action of sodium carbonate is complex since, for example, it stops the C^AF hydration for a certain time, when mixed with lignosulphonate [212], In some cases, lignosulphonates and other plasticizers-retarders can give false set when used with high or moderately high content in C3A and alkalis [213,214]. The phenomenon can be prevented by adding the admixture in the concrete mixer after about 2 minutes' stirring [213,214] or by using high-SÛ3 cements [214]. A frequent collateral effect is the air entrainment in concrete. This phenomenon is related to the surfacting nature of many plasticizers and depends, of course, on the nature of the used additive. In fact anionic surfactants entrain more air than the nonionic ones[215]. The air entrained in concrete contributes to the fluidizing action. Nevertheless it. must not reach too high levels since it reduces mechanical strength. Table 5.4 shows that, when admixtures without a retarding action are used, the overdosages accompanied by an air entrainment lower mechanical strengths. This fact necessitates a careful check of the admixture dosages. However, the air entrainment is an inconvenience at standard concentrations. In fact Table 5.1 shows that, by using air-entraining admixtures, compressive strength does not increase as the corresponding water reduction would suggest [198].
608
F. Massazza Table 5.4.
Water-reducing admixture Sulfonated lignin agent
Over-doses and Compressive Strength [216]
Overdoses Slump Air (times ) (cm) (%) 1 5 ^ 1
7.4 19.4 [^ 7.5 ^
bugar acid agent
10
22.5
Sugar acid agent (without airentrained agent)
1 5 10
7.9 15.6 17.9
5
19
4.1 10.5 J^ 3.9
9<2
19.0 2.3 2.05 1.8
Compressive strength 7-days 14-days 28-days (kg/cm2) (%) (kg/cm2) (%) (kg/cm2) (%) 269 278
J53
100
100
55
332 64 346
2J5
62
100 19
388 102
100
404
253
100 26 ^
100
85
31
143
41
183
45
226 239 181
100 106 80
343 368 286
100 107 83
407 434 363
100 107 89
Anti-foaming agents can be added to reduce the entrained air. Among these tributyl phosphate is the most used [191], even though the literature mentions others. 5.3
Effects on Hardened Concrete
5.3.1 Mechanical strength. The water reduction caused by plasticizers is accompanied by a 28 day-strength increase of both mortars and concretes. At constant workability, strength increases generally ranging between 2 and 20% [157,170,191,194,196,198], which may also rise to 40% [193,195], can be obtained. The 24 hour-strength can be reduced owing to the retarding action of certain admixtures [191] but, in other cases, a set and strength acceleration is observed [157]. Figure 3.7 refers to the action of calcium formate. The strength gains obtainable with calcium lignosulphonates are appreciable for Portland cement mortars but seem negligible for blast-furnace and pozzolanic cements [170]. However, the subject has not been studied exhaustively. On the contrary, at constant w/c ratio, mechanical strengths of concretes are not modified [195,217,224] or, at the most, they are slightly lowered [218]. However, at temperatures below the usual ones (15° and 5°C) the concretes with admixture were found to show a higher strength than that of plain sample after 24 hours as well as 28 days. The over-dosages of plasticizers-retarders generally involve not only a set delay but also a considerable decrease in the concrete strength, mainly at short ages [193,216] (see Fig. 5.5 and Table 5.4). 5.3.2 Drying shrinkage. Plasticizers can increase drying shrinkage [95,96, 100,170,193,216]. This depends on the admixture nature and dosage. For example, if the usual amount of lignosulphonate is tripled, shrinkage nearly doubles [94] whereas calcium saccharate causes a much lower shrinkage [216]. Anyhow at ordinary dosages the shrinkage increase is not considerable since it ranges between 5 and 15% on pastes [94,96] and is around a few per cent units on concrete [96]. However, the shrinkage increase is higher when plasticizets contain accelerating compounds [96] (see Fig. 5.6). Also the type of cement affects shrinkage that, in the presence of calcium lignosulphonate, is higher for Portland cements than for blast-furnace and pozzolanic ones [170]. The shrinkage increase caused by overdosages is lower when Portland cements have a high alkali content [94].
Admixtures i n C o n c r e t e
Amount of admixture
(*) Fig. 5.5.
( a = standard
609
dose)
Lignosulphonate-type water-reducing admixture. Effect of overdose of a water-reducing admixture on the compressive strengths of concretes [193 ].
5.3.3 Creep. There is not a great deal of available data on creep and moreover they refer to different experimental conditions. Therefore it is not possible to draw conclusions of general validity. Admixtures modify total creep with variations ranging between +30% and -30% depending on the loading moment, the type of cement "and other unidentified factors [219]. Lignosulphonate-based admixtures cause a creep increase in concrete stored in a dry room [96,195 ]. On the contrary, the basic creep of ordinary and plasticizer-containing concretes (lignosulphonate and hydroxycarboxylic acids) is practically the same when the stress/strength ratio is equal on loading [96,195]. When the samples are stored in a fog room (this condition approaches the basic creep), no considerable differences between samples with and without admixtures occur. Moreover, it seems to be important that mixes should be loaded at the same degree of hydration [220] and have a given hydration increase under load. After 6 months the specific creep of admixture concretes having a lower w/c ratio is, however, only 80% of that of plain concrete, since on loading (28 days) they have a higher strength and a greater applied stress [195]. The addition of accelerating compounds to plasticizers increases creep [96, 195].
610
F. Massazza 4000
3500
c
3000
σ
*-»
§
2500
ε I
g* 2000 .c σ» c
Admixture Nil Lignosulphonate P 2 Lignos. + CaCl2 P 3 Lignos. + Trieth
1500
Û
1000
500
0
20
40
60
80
100
Drying time in days Fig. 5.6.
Drying shrinkage of cement pastes [96].
5.3.4 Durability. There are not many studies about this subject. However, as a rule, it can be stated that the water reduction caused by the admixture gives a higher impermeability to concretes. This, of course, exerts a favourable influence on the resistance to the chemical attack, the action of freeze-thaw cycles [217,221,222] and the sea-water action [223].
5.3.5
Changes in the hydration
products.
The reaction course, the hydration
products and the morphology changes obtained with cements containing lignosulphonates are the same as those obtained with the synthetic minerals of cement [131]. Nevertheless, it was noticed that the ettringite crystals are much smaller when C3A + gypsum hydrates in the presence of lignosulphonates. All the examinations performed with the electron microscope have never showed appreciable differences in morphology between the products present in the plain cement pastes and in those containing lignosulphonates [131,170,217, 225,226] or other admixtures [187]. However, it was observed that admixtures have a certain influence on the Ca(0H)2 morphology from the silicate hydration since they would favour the formation of smaller crystals [227]. Moreover, they would increase the level of Ca(0H)2 supersaturation before crystallization begins [228]. This modification is likely to be due to the fact that admixtures are adsorbed on the Ca(0H)2 crystals in a largely irreversible manner [167].
Admixtures in Concrete
611
Lignosulphonate adsorbed on the other hydrated phases of cement not only covers the particle surface but also occupies interlayer positions in the crystals [167,229]. This fact could be related to the higher shrinkage shown by the cements containing it. The presence of calcium lignosulphonate determines a slight increase in the specific surface of the hydrated products, attributable to the increase in the hydration degree and the number of larger pores (>70 A) [170], The greater shrinkage shown by the mixes with admixture could be attributed just to this higher content in macropores through which water can pass more easily
[170].
5.A
Mechanism of Action of Plasticizers
Interpreting the mechanism of action of plasticizers is rather complex owing to the variety of compounds forming the family of these admixtures and to the composite nature of cements and concrete. The fluidizing effect is, at least partly, the consequence of retarding phenomena caused by the admixture on the cement hydration. In fact, the mixing water reacts more slowly and therefore it remains available longer to fluidize the mix. Refer to par. 4.5 as regards the retarding mechanism. However this reason, together with eventual morphological changes in the hydration products, is likely of minor importance, whereas the physical actions resulting from the presence of the admixtures are more meaningful. The dispersing action of plasticizers is also attributed to the adsorption of the plasticizer molecules on the cement grains as well as to the resulting changes in the surface charge and ζ potential of the solid particles. Charges of the same sign cause repulsive forces which favour the solid dispersion and therefore increase the mix plasticity. As regards polymeric admixtures, it must be remembered that the best performances are obtained with certain values of the molecular weight and the number of functional groups {230]. These notes are too short to focalize and define the mechanism of -action of plasticizers. Nevertheless, they point out the need of studying the particleadmixture-water interlayer thoroughly in order to understand the mechanism through which plasticizers act. Anyhow the subject is dealt with more exhaustively in the next paragraph. 6
SUPERPLASTICIZERS 6.1
General
Superplasticizers constitute a relatively new category of admixtures for concrete, the use of which developed particularly in Japan and Germany from the sixties. Superplasticizers can produce: (a) (b)
at the same w/c ratio, much more workable concretes than the plain ones; at the same workability, a considerable decrease in the w/c ratio and therefore concretes having higher strengths.
612
F. Massazza
In the former case it is even possible to obtain the so-called "flowing concretes" which are pumpable and self-levelling and demand a little work for compaction. In the fluidized concretes the phenomena of aggregate segregation or water separation are practically absent and, anyhow, much more reduced than in the presence of normal plasticizers [231 ]. In the latter case, it is possible to make in yard concretes having strengths of 60 MPa [232] under ordinary curing conditions and even above 150 MPa, if they are autoclaved [233], Used with rapid hardening cements, superplasticizers allow steam curing to be eliminated or reduced [234], Superplasticizers have been provisionally grouped in the following categories [235]: A B C D
sulphonated melamine-formaldehyde condensate (MSF) naphtalene sulphonate-formaldehyde condensates (NSF) modified lignosulphonates (LS) other types.
Each category includes products differing from one another because either the base component can have different molecular weights or can have undergone some chemical changes or other chemical substances can be present. As a consequence, each commercial product will have a different action on cements. Whilst the dosage of the conventional plasticizers do not exceed 0.25 wt% in the case of lignosulphonates or 0.1 wt% in the case of carboxylic acids, tha products of groups A and B are used in considerably higher dosages (0.5-1.0%), since they do not entrain air [236]. Group C includes admixtures which have an effective fluidizing action but, at relatively high dosages, they can produce undesired effects, such as accelerations or delays in setting times [237,238]. Moreover, they increase the air-entrainment in concrete. Generally cohesiveness and workability increase on passing from groups A and B to C [239]. Category D consists of many products, usually of polymeric nature, which have not yet been tested thoroughly and of which only the results given by the manufacturers are known. Many products belonging to this group are quoted in [10]. 6.2
6.2.1
Fluidizing
Effects on the Properties of Fresh Concrete·
action
and water reduction.
By adding superplasticizers to
stiff concretes, slump (according to ASTM Standards) attains and exceeds 200 mm, flowtable spread (according to DIN Standards) takes a value ranging between 51 and 62 cm and the compacting factor (according to BS Standards) rises to 0.96-0.98 [235]. The workability of concrete increases as the superplasticizer dosage rises (see Fig. 6.1) [240]. Nevertheless there is no improvement beyond certain limits; on the contrary unfavourable consequences can occur, such as segregation and strength drop.
Admixtures in Concrete
613
24 | |22| 5 (0
20 18
16 l·14
10«
J
0
I
L
0.2
1 0.4
I
I 0.6
0.8
1.0
% of superplasticizer by weight of cement Fig. 6.1.
Effect of additions of superplasticizer (alkyl aryl sulphonate-formaldehyde condensate) on the workability of a concrete cement content 300 kg/m3, w/c = 0.6 [240].
Plasticity of concrete increases with increasing w/c ratios but, at least with mortars, the flowtable curves take a peculiar S-trend when superplasticizers are used (see Fig. 6.2) [241], This figure shows that, within certain values of the w/c ratio, a slight change in the water content causes a considerable variation in the flowtable spread. In concretes the curves of plain and admixture-containing samples are less differentiated [242]. Nevertheless, it is obvious that the determination of the exact water dosage in mixes is of great importance. When the purpose is not to improve the mix workability but to increase the concrete strength, superplasticizers allow the mixing water to be reduced by keeping workability unchanged. At ordinary dosages the reduction in the water content generally reaches 20% [10]. Also stronger reductions are possible but, as Fig. 6.3 shows, the gains tend to disappear beyond certain dosages [243]. The effects of superplasticizers depend not only on their nature but also on a great deal of other factors.
and
dosage
As Fig. 6.3 shows, the type of cerrient affects the admixture effectiveness and this fact must be taken into account when particular construction problems are to be faced. This influence depends not only on the clinker nature since, by increasing the gypsùm content in cement, the effect of melamine resins enhances [245]. The water reduction is also more marked in concretes containing fly ashes than in the plain ones [246]. On the contrary these admixtures do not produce good results in aluminous cements [247].
614
F. Massazza
0.40 Fig. 6.2.
0.45
0.50
0.55
0.60
0.65
0.70
0.75 W/C
F l o w t a b l e of 350 L P o r t l a n d cement m o r t a r s v e r s u s W/C r a t i o and dosage of Melment (Melment F 10 and Melment F 300 1:1) [241]
1
2
3
4
5
6
7
8
9
% Me/ment L 10 referred to the cement weight Fig. 6.3.
Water r e d u c t i o n o b t a i n a b l e w i t h a d d i t i o n of Melment L 10 i n c o n c r e t e s made w i t h CPA 3 2 5 , CLK 325 and CPA 325 HTS r e s p e c t i v e l y f 2 4 3 ] .
Admixtures in Concrete
615
Very low or very high cement contents, as we'll as very discontinuous gradings are not advisable in concretes containing superplasticizers. To obtain the most favourable results with aggregates having a maximum diameter of 32 mm, cement dosages of 300-350 kg/m~ are suggested [248]. The concretes too rich in cement, 450-550 kg/m 3 , become extremely adhesive [249]. 6.2.2 Slump loss. A negative property of superplasticizers, which sometimes conditions their use, is the workability loss with time (slump loss). Generally, it can be calculated that the effectiveness of the fluidizing effect ends after 30-60 minutes [237,248]. This workability decrease is particularly grave for flowing concrete (slump > 200 m m ) , the most important properties of which are pumpability and self-levelling. In these cases the consequences of the rapid increase in consistency are obvious. As Fig. 6.4 shows, slump loss is the content. Of course, it also depends 252]. The workability loss of plain nevertheless the overall time during unchanged is longer for the concrete
more retarded the higher the admixture on the nature of the admixture [250,251, concretes (Fig. 6.5) [253] is slower, which the concrete keeps its workability with higher initial slump [254].
Superplasticizer= Melment lio
300 250 200
150
100 50 0
10
20
30
40
Elapsed time,
Fig. 6.4.
50
60
70
mins
Slump loss with time
[250].
80
E E
616
F. Massazza 200 ■
■ 0.3 % SMF (added 3 min after mixing with water)
A——▲ 0 . 3 % SMF(with mixing water)
•
OVoSMF
60
90
150
Time, min Fig. 6.5.
Effect of the delayed addition of sulphonated melamine formaldehyde condensate on slump loss [253 ].
The workability loss is accelerated by the temperature rise [248,255] and therefore, if at 15 C a concrete must be placed within 60-120 minutes, this time is halved at 28°C [256]. The mixes subject to remixing set more quickly than the ones kept still [237]. The slump loss rate also depends on the cement properties and generally increases with finer cements, richer in C3A or fly ashes [257], Nevertheless, it seems that the relationship between workability loss and cement properties cannot be determined easily. In fact four Type I cements (according to ASTM) containing 7 to 13% C3A showed no significant differences in behaviour whereas a Type V cement (S03 <5%) showed a slower workability loss than two Type I and III cements [255], The decrease in the flowtable spread as a function of time seems to be much more rapid in concretes having a low cement dosage [248,255,258], Different devices were proposed to obviate the inconvenience of the workability loss with time, such as, for example, adding the admixture to concrete immediately before its placing or at small successive doses or even by overdosing it. (a)
The delayed addition of the admixture improves its effectiveness [255, 259], even if the delay is only 1-5 minutes [253,257,260,261] (see Fig. 6.5). For this reason, it is suggested that the concrete is first mixed with most of the water and the admixture is then added with the remaining water.
Admixtures in Concrete
617
(b)
The workability lost with time can be recovered by adding a further dose of admixture before placing (retempering) [238,262]. A third dosage produces still positive but less lasting effects [239,254] (see Fig. 6.6). (c) Admixture overdoses prolong the workability of the mixes but expose them to the risk of segregation [254] and decrease in mechanical strengths [251,261 ].
Superplasticizer -C W/C(by weight) «0.42 « ASTM type K C S A type 10) Cement Max. agg. size «3/4 in. (19 m m ) F.A. « Natural sand A.E.A. * Sulphonated hydrocarbon
10 \
After first dosage # ««^After second dosage
_
After third dosage
8 c *~ 6
250 200 50
GL
E 3 4
H100
(f)
50
2
-168 mm 0
I
1
I
2
•-j?Ç-· m m
3
4
5
6
7~
Elapsed time, hr Fig. 6.6.
Effect of repeated dosages of a sulphonated naphthalene formaldehyde condensate on slump [239].
Since resorting to these remedies is often difficult in practice [264], other devices have been suggested such as, for example, adding retarders together with superplasticizers [253,255,265,266,267]. As Table 6.1 shows, a mix of NSF and sodium heptonate in the 3:1 ratio considerably prolong the workability times without prolonging setting times excessively or reducing mechanical strengths [268]. Another trick which gives good results is to increase the gypsum content of the NSF-containing mix [269]. This method demands a very exact dosage and opposes the limits fixed by the standards on the SO3 content of cements. Finally another solution is to use admixtures of particular formulation. In this way a concrete can maintain its slump (-180 mm) for about 3 hours [264]. [264].
618
F. Massazza Table 6.1.
Workability, Setting Time and Compressive Strength of Flowing Concrete Containing a Retarder and a Superplasticizer [268]
OPC
Cement type Mix No. % Retarder in admixture (*) Admixture dos;age (ml/100 Kg)
SRPC
1
2
3
4
5
6
0 980
25 860
50 730
100 730
0 730
25 814
200 180 105 70 55
200 190 180 170 160 150
200 190 170 150
200 190 180 160 110
200 150 105 75 50
210 180 150 120 85 80
550 490 420 400 380
610 570 540 480
-
610 560 520 500 480 450
-
620 590 540 520 470
540 470 410 385 350
—
570 530 500 425 390 370
500 630
650 820
780 960
1200
920
415 540
575 680
15.8 21 .3
15.4 23.5
14.3 22.9
16.7 24.4
16.4 28.1
18.4 27.7
Workability Slump (mm) after time
15 45 75 105 135 165
min min min min min min
DIN Flow (mm) after time
15 45 75 105 135 165
min min min min min min
-
-
65
420
-
-
-
Setting time Time (min) to develop penetration iresistance of 0.5 N/mm z 3.5 N/mm 2 Compressive strength UCS (N/mm 2 ) at 7 days 28 days
(*) Admixture based on sodium heptonate (retarder) and sulphonated naphthalene-formaldehyde condensate (superplasticizer). 6.2.3 Setting times. At ordinary dosages melamine resins slightly retard the setting times of mortars and concretes, whereas naphthalenesulphonates and lignosulphonates act more vigorously [250,259]. In concretes, delays are of the order of 3% in the presence of melamine resins and 20% with naphthalenesulphonates [270,271], Very irregular behaviours with unforeseeable and often excessive accelerations or retardations were sometimes indicated for lignosulphonates [237,238], whereas the variations are much smaller with the admixtures of groups A and B [237].
Admixtures in Concrete
619
This behaviour can be attributed to impurities in lignosulphonates and, in particular, to the sugar content [300], Delay depends not only on the molecular weight of the admixture [303] or its degree of sulphonation [303] but also on the length of its hydrophobic groups [230], In any case excessive admixture dosages considerably retard set without essentially increasing the plasticity of mixes [250,303,308], Of course, much longer delays are observed when retarders are added to superplasticizers to keep the concrete fluid for a longer time [255]. The delayed addition [259], as well as repeated dosages, retard set, especially with lignosulphonate-based superplasticizers [239J. 6.3
6,3.1
Meohanioal
Effects on the Properties of Hardened Concretes
strengths
6.3.1.1 Ordinary curing. It is a shared opinion that, by working at constant w/c ratios, the concrete strengths are only slightly modified by superplasticizers [248,264,272,273,274]. However, overdoses decrease strengths because they bring about phenomena of segregation [251,275] and air entrainment [274], Strengths are favoured by high admixture dosages only if the cement content is high (~500 kg/m3) [263], Also repeated dosages of admixture cause strength to increase [239]. If admixtures are used to reduce the w/c ratio, by keeping the slump equal to that of plain concrete, much higher strengths are generally obtained 1217,235, 236,256,259,276,277] (see Fig. 6.7). This effect is found also on mortars [241,278,279]. Within certain limits the effect is proportional to the amount of added admixture but beyond certain dosages strengths tend to decrease, without considering the danger of the concrete segregation [280]. It is therefore important to avoid excessive dosages of superplasticizers, especially when particularly high long-term strengths are required for concretes [281 ]. Reducing the w/c ratio and maintaining a good workability and cohesion by using superplasticizers are particularly important factors in reaching high early strengths. The advantages are considerable, especially for medium — high cement dosages as Fig. 6.8 shows. After 7 days the strength gains tend to level out, although they always remain relevant. It must also be noticed that the admixture effect is more differentiated at low dosages [271]. Of course, the results also depend on the type of cement and the w/c ratio. It is generally thought [235] that a reduction in the water content by 25-35% involves a 24 hour-strength increase by 50-75%. As regards the long-term strength of concretes containing superplasticizers, there are not yet any exhaustive case histories. At least unsatisfactory results have not been pointed out. 6.3.1.2 Steam curing and autoclaving. The steam cured concretes containing admixtures give higher ultimate strengths than those of the control samples having the same slump. On the other hand, they have lower ultimate strengths than identical samples cured at 20 C. In any case early strengths are much higher for steam cured samples [234,270,282].
F. Massazza
620
Double
Normal
Fig. 6.7.
dose W/C - 0.46
dosage
W/C - 0.55
Strength versus time of a water-reduced concrete containing an A-type superplasticizer [235].
Melamine resins are much more effective than naphthalenesulphonates in increasing early and ultimate strengths of concretes. By passing from short cycles (4 h heating) to long cycles (11 h) the effectiveness of naphthalenesulphonates considerably enhance, but always remains lower than that of melamine resins [272]. Superplasticizers cause no inconveniences even when concretes are autoclaved but, at the same workability, they give higher strengths to the plain concrete [316]. The considerable water reduction allowed by naphthalenesulphonates and a timely choice of the steam curing and autoclaving cycles enable very high strength concretes (above 150 N/mnr) to be obtained [233]. 6,2.2 Shrinkage and creep. At the same w/c ratio, A- and B-superplasticizers do not considerably modify drying shrinkage of concrete [235,264,273] whereas, at the same consistency, they sometimes reduce it appreciably [217,235]. Moreover, the concretes with admixtures, when steam cured for 28 days, seem to shrink less than plain ones, whereas the opposite occurs when they are cured for 7 days [277].
621
Admixtures in Concrete 200
r0 Δ • ■
Mighty-150 Melment Lomar-D FX-32C
Cement lot 21734 type Cement content 517 Ib/cu yd
E o u
Fig. 6.8.
Relative compressive strength of mixes containing different admixtures [271].
Total creep (basic creep plus drying creep) is higher naphthalenesulphonates, at least for the mixes having (~0.64). On the contrary, when the w/c ratio is low, creep between samples with and without admixtures are the admixture effect on basic creep is slight [273],
6.2,3
when concretes contain a high w/c ratio the differences in insignificant. However,
Durability
6.3.3.1 Impermeability. Impermeability plays a primary role on the durability of concrete and since this depends on the w/c ratio, superplasticizers should exert a favourable effect. In fact, permeability decreases if the mixing water is reduced by both melamine [279,283] and naphthalenesulphonate resins [217]. On the other hand, plain and fluidized concretes made with the same w/c ratio have the same permeability [248]. 6.3.3.2 Frost resistance. As is known, concrete with a good frost resistance is obtained by introducing suitable air-entraining substances into the mix. As opposed to lignosulphonate-based admixtures [239], MSF- and NSF-type superplasticizers do not entrain air in the mixes [270,284], but rather, they favour its loss with time, especially when dosages are repeated [258]. In the presence of superplasticizers the air voids and also the spacing factor L are larger [(L = 0.254-0.432 mm [251,275])] than those necessary to guarantee a good frost resistance of concrete (L < 0.2 mm).
622
F. Massazza
Therefore, in the presence of superplasticizers, it is necessary to increase the dosage of the air entraining admixture if an established air amount needs to be introduced into the mix [264] and L needs to be improved [285]. As a matter of fact superplasticizers, even if they decrease the air content and increase the spacing factor, improve the concrete resistance to freezethaw cycles [248,250,258,259,271,284,285,286,287,288]. Moreover, the reduction in the w/c ratio can even cause an increase in the frost resistance [256]. In any case, it is advisable to introduce air-entraining substances into the concrete and to reduce the w/c ratio in order to obtain a good frost resistance [284]. 6.3.3.3 Salt resistance. Superplasticizers, owing to the reduction in the w/c ratio they allow, decrease the penetration of chlorides into concretes [288] and therefore improve their resistance to the deicing salts [258,259 ]. For the same reason the resistance to the sulphate attack increases [264]. If, however, the w/c ratio is the same, the behaviour of the concretes with admixture do not differ from that of the corresponding plain concretes [273]. 6.4
Effects on the Morphology and Composition of the Hydrated Phases
According to some authors the morphology of the hydrated phases from the hydration of C3S and cement does not change in the presence of admixtures [217]. However, it is more likely to think that some modifications, even if limited, do occur. In fact, it was seen that the habi-tus and size of the hydrated crystals formed during the hydration of the systems C3S-C1 ^Αγ .CaF2-CaS0i+ are modified [289,290], The ettringite crystals which form by hydrating C3A in the presence of gypsum and superplasticizers are much smaller [224,291]. Moreover, admixtures considerably increase the C/S ratio of CSH formed during the C3S hydration and, also, maybe, of that resulting from cement. The H/S ratio slightly decreases [292], Differences in the BET specific surface and the micropore sizes were also found in the pastes with and without admixtures, but these differences were reduced by 28 days [293]. 6.5
Mechanism of Action
6.5.1 Viscosity of cement pastes. given by the simple equation
In Newtonian fluids, viscosity was
n-4where τ = shear stress and γ
a
(i)
shear rate.
However, the flow of many fluids deviates from the ideal Newtonian flow and gives curves such as those of 2 and 3 in Fig. 6.9 [296], The former curve relates to fluids having viscous or pseudoplastic behaviour, the latter to fluids with an expanding behaviour. In these two cases viscosity varies with shear stress and is called apparent viscosity.
Admixtures in Concrete
Shear rate y
1
623
(1/s) 3
1
2
q = cot ax q' = cot ax'
\<*'
Shear stress τ Fig. 6.9.
(dyn/cm^)
Flow curves of liquids without yield stress. Behaviour: Newtonian (1), pseudoplastic (2), dilatant (3) [296].
Other fluids, and in particular suspensions, behave at first like solids and begin flowing only if shear stress exceeds a certain threshold value. If, beyond this value, the fluid behaves as a Newtonian liquid, flow obeys the equation of a Bingham1s* body τ « τ 0 + μγ
(2)
where τ s shear stress, TQ β yield stress, γ · shear rate, u * plastic viscosity (curve 4 of Fig. 6.10) [296], In many cases flow does not obey this linear relation and Bingham1s bodies with plastic or expanding behaviour can form (curves 5 and 6 of Fig. 6.10). In these two cases an apparent viscosity η « Σ depending on the shear rate value is defined again. Ύ Finally reversible, thixotropic and antithixotropic curves can be obtained by subjecting the fluids to increasing and decreasing cycles of shear stress. The yield stress is the force that has to be applied to a material before a movement occurs and, therefore, the force necessary to break the interactive bounds between particles is produced. In contrast, plastic viscosity measures the friction of the fluid or the shear shear stress/shear rate ratio [294] and it controls the rate at which flow occurs [295]. The flow curves of cement pastes are of the pseudoplastic type with [296] or without [297] yield stress. Finally, the curves can show a thixotropic, antithixotropic or reversible behaviour [297],
624
F Massazza
Shear r a t e y
(1/s)
η
=cot oc pi q p [ =cotöc' l
T
Fig.
6.10.
f—-
Shear stress r
(dyn/cm 2 )
Flow curves of liquids with yield stress. Behaviour: ideal plastic (4), pseudoplastic (5) and dilatant (6) [296].
The addition of superplasticizers to cement pastes causes the lowering of the ^app apparent viscosity [248,292,293,294,298], However, the effect is not proportional to the addition, since the apparent viscosity remains stable enough with increasing sodium naphthalenesulphonate .confent until this reaches a certain concentration. Beyond this value viscosity decreases considerably and then remains relatively stable [209,294] (see Fig. 6.11). Viscosity depends on the polymerization degree of the admixture as well as, of course, the w/c ratio [209 ].
5
Ci.
oc
1200 1000 800 600
•
400
\ \
200 4C
I
o o
0.2
0.6
% of superplasticizer Fig. 6.11.
JL 1-0
_L 1.4
J.
_L 1.8
by weight of cement
Reduction in the viscosity of a Type I Portland cement containing sodium salt of condensed naphthalene sulphonic acid [294].
Admixtures in Concrete
625
The superplasticizer addition mainly affects T C which decreases with increasing admixture contents [299,300] and disappears completely beyond certain values [301 ] (Fig. 6.12). scole units 10
20
30
40
50
6(
no odd 0 25%
70 60 50 40 30 20 C cement 04
50
100 150 Sheor stress (Po)
10
200
Fig. 6.12. Changes in shear stress-shear rate relationship with admixture concentration [301 ]. However, the yield stress decrease is not gradual but shows a sharp drop at certain admixture contents (Fig. 6.13). The phenomenon is particularly marked at low w/c ratio [301 ]. The yield stress of pastes containing admixtures increases with time [300] but the change is extremely small during the first hours if the admixture content exceeds certain values [302]. Of course, the yield stress depends on the admixture nature and, as regards the polymeric substances, on their molecular weight [300], The polymerization degree of these substances is important since its increase causes the paste minislump to rise rapidly up to a constant value [303]. The considerable lowering of the yield stress caused by superplasticizers is not shown by the conventional air-entraining and water-reducing admixtures [299]. Plastic viscosity slightly decreases with increasing admixture contents [300, 301,302] and, up to 3 hours, it rises, slightly with time [300,302].
626
F. Massazza
V
200
A
Admixture A4 o
150h
2
100
Q.
A5w/c s 0.30° H
2
w/c=0.25 *, , rt,ew/c«0.35aA5, A3w/c«0.35 w/c«0.40 a A5
0.5 0.75 wt % admixture Fig. 6.13.
i.o
2.0
Changes in yield stress with admixture concentration, increasing shear rate, time 6.5 m. A3 ■ 3-naphthalene formaline condensate; Ai+ - id with retarder; A5 = sulphonated melamine formaline condensate [301 ].
That the cement pastes have a thixotropic behaviour is shown by the hysteresis loop in the flow curves [297,304] (Fig. 6.12). Thixotropy reveals the existence of bonds between particles which are broken by shear [304]. It is decreased by adding superplasticizers. With suitable admixture contents the cement pastes take on a nearly Newtonian ideal behaviour which is lost only after some hours, when viscosity begins increasing [293]. 6.5.2 Viscosity of concretes. The slump test and flowtable are not sensitive enough to detect changes in the workability of flowing concrete. In fact, these methods are insufficient to characterize a material which behaves as a Bingham plastic and which therefore demands the knowledge of two constants, τ 0 (yield stress) and μ (plastic viscosity), as indicated in eq. (2). The usual tests of consistency can be misleading since two concretes having different τ 0 and μ can be judged identical only because their Bingham curves cross at the shear stress of the test [305]. A two-point workability apparatus was developed to characterize flowing concretes [306]. It is used to measure the torque T or an impeller rotating inside a cylinder full with concrete at different N speeds. On a graph T(N) it is possible to determine the constants g and h of the equation g + hN constants which are proportional to τ 0 and y respectively.
(3)
627
Admixtures in Concrete
The tests on concretes carried out by this system show that h decreases to a minimum as the admixture concentration increases, whereas plastic viscosity initially rises up to 0.7 wt% of the admixture and then decreases [295],
6.5.3
Adsorption
and paste
Theology.
6.5.3.1 Adsorption. Considering that all solid particles dispersed in water tend to attract with one another more or less markedly, the mechanism of action of superplasticizers can be attributed to phenomena capable of eliminating or overcoming the attractive forces between particles. These phenomena are related to the adsorption of the admixture molecules on the cement particles. This adsorption causes changes in the water-cement system and therefore in its rheology. By dispersing the cement in aqueous solutions of superplasticizers, the latter are adsorbed on the surface of the solid particles. The adsorption attains values beyond which it no longer increases [307,308] (Fig. 6.14) or decreases slowly [307]. This lowering or, in general, the apparent irregularity sometimes found in the adsorption curves, is due to particular equilibria forming between individual molecules and micelles in the superplasticizer solutions [309],
0 8 h A Polymer delayed addition (w/c «0.3) A Polymer delayed addition (w/c«2 ) — I · Polymer immediate addition (w/c «0.3) | o Polymer immediate addition (w/c «2 ) g | «Monomer delayed addition (w/c «0.3) ° o.6h-o Monomer immediate addition
0
0.5
1.0
15
Addition ( % by weight of cement) Fig. 6.14.
Polymer or monomer adsorption on cement as a function of addition [308].
628
F. Massazza
The cement compounds which truly adsorb superplasticizers are still being investigated. Anhydrous compounds react with water and therefore the adsorption cannot be measured in aqueous dispersions. Measurements can be carried out by dissolving the admixture in an unreactive solvent, such as dimethylsulphoxide (DMSO), but it is obvious that with this method the real conditions of the cement use are not complied with. Anyhow these tests show a very small adsorption for C3A, being about 1/10 of that showed by monosulphate hydrate [310]. As regards the hydrated compounds, both aluminates (C3AH5 and Ci+AH^) and sulphoaluminates (C3A.3CS. 32^0 and C3A.CS. 12H2O) were found to adsorb appreciable amounts of the most used superplasticizers [291,310,311]. Of course, adsorption depends on the type of admixture as well as its degree of polymerization [312]. The expectable consequences caused by adsorption are as follows: —Formation of repulsive electrostatic forces which move the particles away from each other (ζ-potential). —Formation of layers of admixture molecules on the solid particles, which create a steric obstacle to their adhesion. —Increase in the solid-liquid affinity to the detriment of the solid-solid affinity. — Changes in the morphology and the crystalline sizes of the hydration products. —Changes in the hydration kinetics of the cement constituents. Since these phenomena affect the rheology of the cement pastes to a different extent, it is advisable to examine and evaluate them separately. 6.5.3.2 ζ-potential. By dispersing the cement in water containing the dissolved superplasticisers, a negative zeta potential forms on the particle surface. By increasing the admixture content, the absolute value of the ζ-potential rises up to reach, asymptotically, values ranging between -35 and -45 mV after 15 min (Fig. 6.15 [313]). The values of ζ-potential depend on the type of cement, the quality of the water used, the nature of the admixture [313,314] and its degree of polymerization [303]. High ζ-potentials are also measured by dispersing C3S, alite, C3A,Ca(0H)2 and mixed cements in water [314]. ζ-potentials of the anhydrous compounds are not known because these compounds react with the mixing water more or less immediately. Some tests carried out by dispersing the pure cement compounds in dimethylsulphoxide (DMSO) containing some dissolved superplasticizers gave values ranging between -6 and -11 mV for C3A [310]. The ζ-potential of fly ashes decreases even more sharply and reaches values around -60 mV with only small admixture contents [314],
629
Admixtures in Concrete
More precise data were obtained on ζ-potentials relevant to some hydrated phases dispersed in aqueous solutions of the admixtures and having a suitable composition to avoid the compound decomposition. As Table 6.2 shows, all aluminates and calcium sulphoaluminate hydrates have appreciable negative ζ-potential. Table 6.2.
ζ-potential in mV of Some Hydrated Compounds Dispersed in Lime Water.
Without admixture C 3 AH 6 C U AH 13 C3A.3CS.H32 C3 A.CS.H 12
-9 -9 -5 -5
With admixture
Adsorbed admixture
NSF
MSF
LSS
(%)
Ref.
-15 -17 -34 -28
-12 -20 -31 -18
-12 -18 -30 -23
2 4 2 2
311 311 291 310
6.5.3.3 Steric obstacle. Considering the sizes of the monomeric units of melamine resins and the specific surface of cement, it is found that, after the saturation has been reached, the cement grains are covered on average by about 40 layers of polymeric molecules. Assuming the thickness of an individual layer is about 10 A, each cement particle is therefore covered by a resin layer of about 400 A. This sheath is thick enough to cause steric stabilization of the particles [304]. 6.5.3.4 Solid-liquid affinity. Superplasticizers only slightly modify the surface tension of water and therefore the solid-liqui.d affinity [310] consequently. This factor should not appreciably affect the mechanism of the fluidizing action [307]. 6.5.3.5 Morphological changes. To a certain extent superplasticizers modify the morphology and the sizes of the newly formed crystals. In particular, it was seen that the ettringite crystals forming in the presence of the most used superplasticizers have sizes about 1/10 of those obtained in the absence of admixtures [224,291]. The small sizes of the crystals can hamper the formation of bridges and links among the particles. 6.5.3.6 Changes in the hydration kinetics. Superplasticizers have a retarding action on the hydration of cement and its pure compounds, which varies depending on different factors such as nature of the admixture, type of cement etc. (see 6.2.3). This delay contributes to the plasticizing effect of the admixtures since it slows down the water combination [315]. 6.5.4 Final remarks. Thus, it can be seen that the action of superplasticizers results from several factors, each of different importance. The fluidification of mortars and concretes is mainly attributed to the changes in ζ-potential caused by adsorption [313], In fact ζ-potential, mortar flow and paste minislump all increase as adsorption increases [302,313], The strict correlation between adsorption, ζ-potential and fluidizing action is verified by the fact that, if using NSF monomer as an admixture instead of the polymer, there is neither adsorption, nor change in ζ-potential or fluidizing action [308 ].
630
F. Massazza
50r > E 40 i
c 30 <« o • de-ionized water
S 201
o 5 0 % tap water J 1.0 concentration of
IQ
L
2.0 # - 2 (a/w wt %)
Fig. 6.15. Change in zeta potential of Type I Portland cement with increasing concentration of sulphonated naphthalene formaldehyde condensate. Electrophoretic mobility was determined 15 min after the cement was immersed in the solution [313 ].
Shear rate«-40s* w/s-1.6 0
NSF
D
LSS
Δ
MSF
3 2
4
6
8
SUPERPLASTICIZER ADDITION, ( Fig. 6.16. Apparent viscosity of ^ΑΗχ3 pastes with lime water vs. superplasticizer addition.
10
Admixtures in Concrete
631
Nevertheless, a decrease in ζ-potential does not necessarily involve an improvement of the suspension plasticity. In fact, the apparent viscosity of Ci+AH^ and C3AH5 pastes can greatly increase owing to the presence of admixtures (Fig. 6.16), whereas under the same conditions, that of sulphoaliminate hydrates decreases [311], These results indicate that apparent viscosity is affected not only by ζ-potential but also by other factors. One of these is the bridging between solid particles, caused by the adsorbed molecules. Bridging opposes the electric repulsion forces caused by ζ-potential. Therefore if the latter is relatively low, as in the case of aluminate hydrates (Table 6.2), the agglomeration phenomena prevail and the paste viscosity increases [311]. However, Fig. 6.16 shows that the phenomenon depends on the admixture/solid ratio. The bridging effect was also observed in cements containing low percentages of an admixture composed of a sodium salt of polystyrene sulphonate [312], Another important factor is the steric hinderance caused by the multilayer adsorption of the admixture on the cement particles [304]. The decrease in the sizes of the newly formed crystals and the mixing water probably also contribute to the fluidizing action but to a lesser extent. Another factor that could affect the action of superplasticizers is the mineralogical composition of cement and the composition of the mixing water. In fact it was found that these admixtures do not modify the viscosity of alumina-cement pastes or calcareous filler suspensions. If, however, acetic acid is added to the suspensions, viscosity is considerably reduced [228] since Ca** ions are now present in solution. 7
AIR-ENTRAINING ADMIXTURES 7.1
General
The accidental discovery of air-entraining admixtures, which occurred in the thirties, can be considered one of the fundamental stages of concrete technology since it proved that small amounts of organic substances could improve the concrete properties and in particular its frost resistance. The mechanism of the frost attack of concrete is now known: whenever the concrete temperature drops below 0 C, the water in the capillary pores of the saturated hardened cement paste freezes. Since ice has a higher volume than the liquid water (~9%), an expansion of concrete takes place. The occurring stresses cause cracks and crumblings if they exceed the tensile strength of concrete. The harmful action of frost can appear only if freezable water is present in concrete. In fact, dry concrete or concrete having a water content below a certain limit does not undergo deterioration. The presence of air bubbles improves the frost resistance of concrete since each bubble acts as an expansion tank which accommodates the water forced out of the pores by the forming ice, so eliminating the stresses produced by frost.
APC - U
F. Massazza
632
Nevertheless, the air accidentally entrapped in the concrete is not able to give an effective protection since it is distributed in relatively large and spaced cavities. Better results are obtained by adding the so-called airentraining admixtures to mixes. They are surfactants capable of forming a very fine and uniform system of non-communicating discrete cavities inside the cement paste. Also plasticizers are generally surfactants and therefore they introduce air into the mixes. In fact there is no clear difference between these admixtures and the air-entraining ones since workability increase and air entrainment are properties common to both products. A distinction could be made by considering the stability of the air bubbles [317J. In plasticizers a good portion of air disappears before the concrete sets whereas, in the case of entraining admixtures, the entrained air remains in the hardened concrete. The most widely used air-entraining admixtures consist of: abietic and pimeric acid salts fatty acid salts alkyl-aryl sulphonates alkyl sulphonates phenol ethoxylates 7.2
Entrained Air
There are a great many factors affecting the amount of entrained"air and they*have been well studied. Among the major reference works are [318, 319,320,321,332]. Of course, the main factor is the nature content.
of the admixture itself [215] and its
The entrained air increases with increasing content of air-entraining admixture [323,324] (see Fig. 7.1), but, beyond certain dosages, the air content tends to reach asymptotic values. The higher the w/a ratio [323], the more effective the admixture. The admixture content being unchanged, the entrained air decreases as the cement dosage and its fineness decrease [325]. The low-alkali [326], high early strength and pozzolana-containing [327] cements demand a higher airentraining admixture content, at the same entrained air content. In the two last cases this is the consequence of the higher fineness of the cements. The amount of air-entraining required to obtain a given air content slightly varies also depending on shape, grading [327] and maximum size of the aggregate (see Fig. 7.2) [4]. A temperature increase of concrete during mixing causes a decrease in the air content [155,328]. This occurs because the water surface tension is lower at high temperatures. Moreover, the air content of a concrete decreases as the mixing [155,328] and the vibration duration [320] increase.
633
Admixtures in Concrete 35 r
0
0 010
0 020
0 030
0 040
0 050
0060
0 070
0 080
CONCENTRATION OF SURFACE-ACTIVE AGENT (PERCENT BY WT OF CEMENT)
Fig. 7.1.
Air entrained in cement pastes by surface active agents. W/C = 0.45 by. wt, 25°C. Surface-active agents: 1. sodium dodecyl sulphate, 2. neutralized "Vinsol" resin, 3. sodium abietate, 4. tetradecyl trimethyl ammonium bromide, 5. "Triton EN", 6. "Lissapol N 300", 7. Saponin [324].
7.3
Effects on Concrete
7.Z,1 Effects on fresh concrete. The entrained air improves the concrete workability, makes it more homogeneous and flowing and avoids segregation phenomena. It must be said that air-entraining admixtures are not used to improve the concrete workability as this is better enhanced by using specific products which entrain air slightly or not at all and, therefore, do not impair mechanical strengths [6,324,329,330]. Roughly speaking, an air increase of 1% corresponds to the addition of 2% water. The air voids replace a portion of the fine sand because of their size distribution and they give the concrete a higher flow owing to their elastic properties [4J. Figure 7.3 shows the relationship between slump and air content, at different H20/cement ratios, by using a soap as an air-entraining admixture [323]. Of course, the fluidizing effect depends not only on the content of surfactants but also on their nature [215].
F. Massazza
634
% air
Slump: 5-8 mm
_ 225
k k
Sr
k g / * 3'
__ 300 k g / m 1
1
^ ^
•^ΞΤ^
_440
^ " - -
_440
kg/m1
^ 2 2 S kg/m' kg/m'
^» 10
12,5
20
25
<0
Maximum size of grains Fig. 7.2.
50
63
(mm)
Effect of cement content and maximum size of aggregate on the concrete air content [4].
4 12 20 Air content, per cent Fig. 7.3.
Relationship between slump and air content for various w/c ratios [323].
635
Admixtures in Concrete
7.3,2
Effects
on hardened
concrete
7.3.2.1 Effects on mechanical strengths. The air entrainment in concrete reduces mechanical strengths unavoidably [6,317,327,329,330J. Figure 7.4 shows the effect caused by a triethanolamine salt of alkylbenzenesulphonic acid [215]. As can be seen, strengths decrease rapidly with increasing admixture content, and then they tend to reach very low asymptotic values. It can be observed that, with an addition of 0.01%, compressive strength decreases by about 3 0 % and 3 5 % at 3 and 28 days respectively.
0.05 %
Fig. 7.4.
0.075
Surfactant
Strength of Portland cement specimens versus content in surfactant (triethanolamine salt with an alkybenzenesulphonic acid) [215] 1 - Flexural-tensile strength at 3 days; 2 - Flexural-tensile strength at 28 days; 3 - Compressive strength at 3 days; 4 - Compressive strength at 28 days.
At the usual dosages ( < 0 . 0 1 % ) , the mechanical strength decreases linearly as the entrained air [331 J and admixture [215] contents increase (Fig. 7 . 4 ) . It is calculated that each percentage increase in the total air causes the compressive strength to decrease by 3-5%, even if higher reductions were pointed out [332]. This strength decrease could also depend on the decrease in the degree of hydration observed in (slag) cement pastes containing air-entraining admixtures, hydrated for 28 days [333]. Nevertheless, it must be remembered that, at*the same workability, the airentrained concretes have a lower w/c ratio. Consequently the air entrainment causes a lower strength loss than that which could be expected from the air content.
636
F. Massazza
7.3.2.2 Effects on durability. The air bubbles affect the frost resistance of concrete favourably since they accommodate the water forced out of the capillary pores because of the gradual ice formation. In this way they relieve the hydraulic pressure and prevent the formation of stresses. Figure 7.5 [334] shows that air entrainment appreciably improves the resistance to freezing and thawing of concrete, but it illustrates also that too high w/c ratios cancel the advantages obtained by using the admixtures.
4000
0-35
Fig. 7.5.
0-45 0-55 0-65 075 Wöter/Cement Ratio
085
Influence of w/c ratio on the frost resistance of concrete moist cured for 28 days [334].
The air accidentally entrapped in concrete does not give an effective protection since it is distributed in relatively large and spaced cavities. On the other hand, the air entrained by the admixtures creates a uniform system of very fine dispersed bubbles in the cement paste. It is essential for the distribution of the bubbles to be uniform and their size be small. Practically, their diameter varies between 0.01 and 1.00 mm according to a continuous size distribution. In order to relieve the stresses occurring when water freezes, the air bubbles must not be very spaced. This means that the air content must not be too low (Fig. 7.6) [335]. Experience shows (Fig. 7.7) [336] that the spacing between bubbles must not exceed 0.25 mm in order to assure a good protection against frost. For a given entrained air content, the average spacing between bubbles depends on the w/c ratio [334]. It is important to notice that the microscopic bubbles entrained into the cement paste are not filled with water either during mixing or during the cement hydration.
637
Admixtures in Concrete
4 8 12 Air Content - per cent Fig. 7.6.
16
Influence of air content on expansion of concrete after 300 freeze-thaw cycles [335].
10° in.
100
8 80
o 60
o
Û
ö
^*"·*
I
I I
S 40 σ
20
16
12
120
I
°\
\
\ °
20
\c 100
Fig. 7.7
200 300 400 Spacing of Bubbles - um
Q.
500
600
Relationship between spacing of entrained air bubbles and durability of concrete [336],
638
F. Massazza REFERENCES
1.
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