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Interactions of polymers and organic admixtures on portland cement hydration
CEMENT and CONCRETE RESEARCH. Vol. 17, pp. 875-890, 1987. Printed in the U S A 0008-8846/87 $3.00+00. Copyright (c) 1987 Pergamon Journals, Ltd.
INTERACTIONS OF POLYMERS AND ORGANIC ADMIXTURES ON PORTLAND CEMENT HYDRATION
Satish Chandra Div of Building Materials Per Flodin Dep of Polymer Technology Chalmers University of Technology, GOTEBORG, Sweden
(Refereed) (Received Oct. 21, 1986; in final form July 21, 1987) ABSTRACT Interaction of polymers and other organic admixtures on Portland cement hydration is reviewed. This has been compiled in a systematic way. First hydration of Portland cement is described in short. Later, interaction with 4 important components of Portland cement is discussed. Finally interphase effects in polymer modified hydraulic cement are discussed. It is concluded that polymers and organic admixtures interact with the components of Portland cement when they come in contact with water. This interaction is due to ionic bonding, causing cross-llnks which inhibit the film formation property of polymers and influence considerably the crystallisation process during the hardening of concrete. Some low molecular weight organic substances also have a considerable influence on Portland cement during its reaction with water.
Introduction The concept of the use of admixtures in concrete is not new. The available literature of interest dates back to 1909 /1,2/. More work has been done in this field in the 1920s /3,4,5/. In the 1940s products were developed based on poly vinyl chloride which were used for improving the bond strength between old and new concrete and for making concrete pipes /6/. Air entraining admixtures and water reducers are other examples of admixtures widely used in concrete. There are two theories for the mechanism of action of polymers in concrete. According to the first theory there is no interaction between the polymer and concrete. During hydration the hydrophillc part of the polymer is oriented towards the water phase whereas the hydrophobic part heads towards the air phase (pores and capillaries not filled with water). On drying, the water is taken away, the hydrophobic particles coalesce together and form film. The other theory is that the polymer interacts with the components of the Portland cement hydration products and makes com875
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plexes. This creates a type of reinforcement in the concrete and produces semipermeable membranes. The present paper discusses the interactions of different admixtures with concrete. Different types of polymers have been used in concrete. These polymers can be broadly divided into three groups: latices, liquid resins and water soluble polymers. Emphasis will be given here on the latices as they are mostly used. Latices are dispersed suspensions of solid polymer micropartlcles in water. The polymer in dispersion is usually glassy amorphous (Tg approximately above 50°C) or viscous rubbary (Tg below room temperature). The dispersion contains 30-50% of solid polymers by weight. Latices are produced by emulsion polymerization of liquid monomers in water. Thus sometimes they are named as emulsions. Elastomers used in latices are natural rubber, poly(styrene-co-butadlene), poly(acrylonitrile-co-butadlene) and polychloroprene. The major polymer dispersions used in concrete are following /7/: i. Poly(vinyl acetate) or copolymers of vinyl acetate with monomers such as vinyl chloride, vinylidene chloride, dlbutyl maleate and vinyl propionare. 2. Polystyrene or copolymers of styrene with different acrylate monomers. 3. Poly(vinyl chloride) or copolymers of vinyl chloride with monomers such as vinylidene chloride and vinyl propionate. 4. Polyacrylates and their copolymers.
Hydration of Portland cement When Portland cement comes into contact with water, its components react and different hydration products are formed. Out of these, calcium silicate hydrates (consist mainly of fibrous crystals which have a very small degree of structural order /8/) are the most important constituents. The hydration process can be derided into several successive stages: In stage i, there is an initial hydrolysis of C3S and saturation with respect to calcium hydroxide is quickly achieved or surpassed. The first formation of calcium silicate hydrates starts as very fine crystals. At this time the volume of the cement particles has not appreciably decreased since only a thin surface layer has reacted and the interspace between the cement grains still has nearly the same size as at the beginning of the hydration. Consequently the calcium silicate hydrates grow to be long interlocking fibres. Stage 2 is a dormant period, when the reaction becomes very slow. During this period, hydrolysis slows down and super-saturatlon is gradually reached. Poorly crystalline C-S-H is the major hydration product which is low in bound water and has a little silica polymerization /9/. This hydration product is considered to be a diffusion barrier to water and hence causes the induction period. An alternative explanation is that the rate of hydrolysis is controlled by the calcium ion concentration in the solution. Calcium ions released from the solid have to move into a solution of increasing chemical potential. The hypothesis is that the soluble silica is adsorbed onto the calcium hydroxide nuclei /I0/ hindering their growth. Supersaturation is needed to overcome this poisoning effect and to continue crystal growth.
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877 POLYMERS, ADMIXTURES,
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In stage 3 (2-6 hours after cement is mixed with water), the calcium ion concentration drops and the hydration reaction proceeds rapidly again. The decrease of the calcium ion concentration in the solution and the renewed hydration coincide with the crystallization of calcium hydroxide. At the same time the C-S-H gel changes its composition and properties, If forms the characteristic circular morphology /ll/. Calcium hydroxide behaves as a calcium sink which helps the hydrolysis of C3S relatively rapidly during stage 3 until diffusion through stage 4. Large crystals of CH are also formed. Calcium aluminate, C3A , hydrates according to the following equation when no sulphate ions are present C3A + 21H ~ C4AHI3 + C2AH 8 hexagonal hydrates
2C3AH 6 + 9H cubic
The hexagonal hydrates are thermodynamically unstable with respect to C3AH 6 and slowly convert to the cubic hydrate at room temperature. The crystal structure of the hexagonal hydrates (C3A • CX • HX) can be regarded as a negative clay with exchangable anions X in the interlayer region. Here relevant to Portland cement hydration is the interaction of C3A with the sulphate ions present as gysum whlch is added to control the flash set potential of C3A. The reaction under these circumstances proceeds as follows C A3+ 3CS H
~ 26H ~ C A 3. 3CS • H32 ettrlnglte
At this time the structure mainly consists of long interlocked fibrous calcium silicate hydrates which bridge a considerable part of the orglnally water filled space. During the next stage of hydration from about 24 hours up to the end of the reaction, the pore (still empty) is filled by new hydration products. During this stage of hydration, tetra calcium alumlnoferrlte C4AF ~ HI3 is formed due to the hydration of alumlno-ferrlte phase. Alumlno-ferrlte phases form hydration phases similar to those of the alumlnates in which F replaces part of A. Further the formation of ettrlnglte is almost complete at this point. This means that all the sulphate ions are consumed and thus ettringite in combination of more calcium aluminates converts to monosulphate. C3A • 3CS • H32 + C3A ~ C3A • CS • HI2 monosulphate Interaction between Ca(OH)2 and styrene methacrylate polTmer dispersion It is seen in the previous section that Ca(OH)2 is produced immediately after the addition of water to Portland cement. This emphasizes the importance to study the reaction of CA(OH) 2 with the polymer dispersion. It has been shown /12,13/ that there is a tendency of gelation in a mixture of Ca(OH)2 and styrene methacrylate polymer dispersion containing a minor amount of acrylic acid residues when a little solid powder of Ca(OH)2 is added. The dispersion settles down along with Ca(OH)2 leaving the supernatant liquid transparent. It is further noticed that 5 g of Ca(OH)2 powder has the capacity to digest i lltre of dispersion containing i0 g of the solid polymer leaving the supernatant liquid transparent. When this limit is exceeded the supernatant liquid becomes turbid again.
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The polymer dispersion used is composed of soft microparticles and has film forming ability. It was observed that with small addition of Ca(OH)2 the polymer film became translucent. With increasing amount of Ca(OH)2 , the film formation gradually reduces and finally no film is formed. This can be explained by ionic bonding between Ca and carboxylate ions of the polymer, causing cross-llnks which inhibit film formation. Furthermore, when some polymer dispersion was treated with NaOH in the similar manner, neither gelatlon tendency could be noticed, nor the polymer had lost the film forming ability. This shows that there is interaction between Ca 2+ ions and the carboxylate groups of polymer dispersion and n o t the destabilization of polymer due to the alkalinity of the solution. With solid Ca(OH)2, the carboxylate groups of these polymer particles interact with the free valencies of the calcium atoms on the surface of Ca(OH)2 solid particles. This is illustrated in Figure i.
I Ca (OH) 2~ ~ . ~ C a 2
- Ca2+---~--Ca2+---~ / (a)
+
+l s o l i d / ~ Ca
I
/+ /
ca {-ooc
\
I
(b)
Figure i. Schematic illustration of cross-llnklng of polymer particles by a) divalent Ca-ions and b) Ca(OH) 2 /12/. Since carboxyllc groups are present all over the surface of the polymer particles, another crystal may be bonded to the particles which then act as glue between solid Ca(OH)2 particles. As long as there are free bonding sites available on the Ca(OH)2 crystals, the concentration of polymer particles in the supernatant liquid will be low, which is in agreement with the transparency of the supernatant liquid and large increase in the rate of precipitation. X-ray diffraction analysis of different mixes of Ca(OH)2 and polymer dispersion showed the diminishing peak of Ca(OH)2 /14/. Thus the complex is not highly crystalline and confirms that the polymer has considerable influence on the crystallization of calcium hydroxide. A decrease in the free calcium ion concentration was noted /15/ when tensides and a number of polymer dispersions, made in the laboratory /16,17/, were mixed with Ca(OH)2 saturated solution. Similar results were obtained with the polymers having no tensides. This shows that there is an interaction between the divalent Ca 2+ ions and the anionic tensides (Figure 2). The decrease in the Ca 2+ ion concentration with polymer dispersion without tenside indicate that there is an interaction between the carboxylate ions of the polymer and the divalent ca 2+ ions (Figure 2). The results obtained were not very precise and systematic as the membrane of the calcium ion selective electrode used reacted with the polymers and it was difficult to clean it properly. It has been shown that the divalent calcium ions do interact with the anionic tensides present in the polymer dispersions as well as with the carboxylate group of acrylate based polymer dispersions without tensides. A calcium complex is formed by ionic interactions and the polymer looses its film forming ability.
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879 POLYMERS, ADMIXTURES,
PMMA covered by surfactant
INTERACTION,
CEMENT HYDRATION
(anionic)
Insoluble Ca-slat of surfactant covers surface.
Figure 2. Schematic diagram showing the interaction of Ca 2+ with polymer dispersions containing tensldes (SO 3 group).
Interaction between calcium silicates and polymer dispersions Sugama and Kukacka /18,19,20/ studied the effect of dicalcium silicate (~C2S) and tri calcium silicate (C3S) on the thermal stability of vinyl type polymer concrete. A cross-linklng structure of calcium ions from C-S compounds with carboxylate anions of trimethylopropane trimethylacrylate (TMPTMA), polystyrene (PST) and polyacrylonitrile (PAN) was discussed. DSC thermograms (Figure 3) showed a peak which was attributed to the decomposition of -CH 2- groups of the main chain in PST.
a, BULK PMMA; b, 34wt% PMMA-66wt% CaO.Si02; o b
c, 34wt% PMMA-66wt%
leO. 193
3CaO.2SiO2;
d, 34wt% PMMA-66wt% 2CaO.SiO2; 386
i
°
\
3~
e, 34wt% PMMA-66wt%
3CaO.SiO2;
//.__
2~
d
i
Figure 3. Differential scanning calorimeter (DSC) thermograms of P~A-CaO-SiO 2 systems /18/.
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The second peak was attributed to the decomposition of single -CH 2- linkages with the aromatic ring. Therefore it was considered that the thermal decomposition between -CH 2- main chain in PST was prevented because of the presence of ~C2S and C3S. In order to obtain further information about the interaction between calcium compounds and the -CH 2- group of polymethyl methacrylate (PMMA) and PST, a qualitative analysis was performed by using IR absorbance ratio. The absorbance of the -CH 2- group in PMMA and PST illustrates a decreasing tendency with increasing amounts of calcium oxides in the C-S system. The results indicate that an apparent interaction between the -CH 2- group of the vinyl type polymer and the calcium oxide of the C-S system may take place. The degree of bonding between Ca 2+ ions of C-S compounds and COO- anions of TMPTMA occurring within styrene (ST), ACN-TMPTMA copolymer examined by an atomic absorption spectrophotometer, showed that the calcium ion concentration decreased with an increase in the mole fraction of TMPTMA in the copolymer. This fact is associated with a higher degree of reactivity of Ca z+ ion with the carboxylate anions formed by the thermal decomposition of an ester group in TMPTMA with regards to the hydrolysis of TMPTMA and the nitrile groups of PAN. It was stated that the nltrile groups undergo thermal decomposition to form amino groups and do not hydrolyse. Regarding the ester group very little hydrolysis could be evidenced. The reaction mechanism
0~
~C
"O-cMz'
~x._//~ ~'c"
'O -- CH 3 BRINE | H2O)
MOT
ANHTD*OU5
641
~5
COR[
~ "
C.-D.L,:'r",, ,.-,,.~ /"-
°
""~" "
0
JJ.
'o~'~2.
c, ~'. ,~.,-~c,.~o~ o • "
P R ~ ' I P ~rATlON OF H Y D R , T F D ~.35
" "
1-~- ": '.. , : - / ~
C~
-
CROSS-LINKING
/ - -
, O
Figure 4. Reaction mechanisms between C3S and PMMA layer during exposure to brine /20/.
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881 POLYMERS, ADMIXTURES,
INTERACTION,
CEMENT HYDRATION
occurring at the interface of PMMA with C3S grains is hypothetical as represented by a reaction process (Figure 4). It is indicated in the figure above that Ca 2+ ions are released readily by the attack of the hot brine on the anhydrous C3S grains. Ca 2+ ions produced in an aqueous medium develop an electrostatic interaction with the carbonyl group (- C = 0), which becomes more strongly electro-positlve at the elevated temperature. The effect of ionic polymers such as I) polyvinyl sulfonic acid and 2) polystyrene sulfonlc acid on hydration of C3S is studied by Ben-Dot et al /21/. A decrease in the degree of hydration of C3S sample made by the addition of these polymers was noticed. The retardation in the formation of Ca(OH)2 by the polymers was explalned as follows: 1) An interaction of the Ca 2+ ions or Ca(OH)2, released during the hydration of C3S with sulfonate groups of the polymer. 2) The sulfonate group of the polymer being hygroscopic absorb water, diminishing the amount of water available for the hydration of the silicate. For both C3S - PESA and (C3S - ESA) polysystems, the DTA analysis showed two endothermlc peaks in the Ca(OH)2 dehydration range; one at a comparatively higher temperature, which becomes more intense with age and simultaneously shifts to higher temperatures (451-496°C) and a second rather weak one appearing on somewhat lower temperatures (392-431°C). The first endotherm is of neat-hydrated C3S , vlz Ca(OH)2, and the second one is probably associated with Ca(OH)2 bonded to the polymer through its sulfonate groups. It did not grow with age owing to the limited, fixed amount of polymer present. Similar results are also reported by Ramachandran /22/. IR spectroscopy results showed shifts in symmetric and antlsymmetrlc vibrations. These agree with the work of Zundel /23/. The shifts of Ca 2+ by about i00 cm -I in the symmetric as well as antlsym~etrlc vibration of S-O confirm that an interaction between Ca 2+ ions and the sulfonate groups of the polymer occur. Diminishing of the peak of Ca(OH)2 in these systems as observed by X-ray diffraction analysis further confirms the interaction. This also confirms the results obtained by Chandra et al /13/. To summarize, the work on C3S and polymers at high temperatures with DSC and IR spectroscopy indicates that there are interactions. However, much remains to be done at normal room temperature. It is shown /24,25/ that many types of organic compounds can form interlayer compounds with C4AHI3, at hydration product of C3A. It was considered that incorporation of organic additives into the hydrate latices could be responsible for preventing the transformation to C3AH 6. Accordingly this aspect of C 3 A h y d r a t l o n was investigated more in detail /26/. The organic compounds studied were divided into two classes. Class A comprises sugars and their derivatives and class B is a series of aliphatic 3-carbon compounds with increasing numbers of hydroxyl and carboxyllc acid groups. It was observed /27/ that the organic compounds retard the hydration of C3A by forming a more impermeable barrier which is prevented from converting to C3AH 6. The nature and number of oxy functional groups indicate the ability of the compound to act as a retarder. However, the fact that mandellc acid accelerates the hydration of C3A whereas lactic acid retards
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it /28/ indicates that the chemical structure has a substantial effect on curing. Nevertheless, it is clear that sugars and their derivatives are strong retarders. Compounds such as malic acid, tartaric acid and citric acid would also be expected to be effective. The factors which determine the retarding properties of a compound towards C3A may not be the same for cement. The reaction of C3A In cement occurs in the presence of gypsum and a large quantity of calcium silicates. Surveys by Suzuki and Nishl /29/ and by Taplin /30/ indicate that other factors also must be involved. For example glycol aldehyde, glyceraldehyde and glycollc acid strongly retard cement setting. Previous work /31/ on the C3A gypsum reaction showed that sugars have a retarding effect but not enough data were available to determine if the conversation from the trlsulphate sulpho-alumlnate to the monosulphate sulpho-alumlnate was affected. The influence of glucose, lignosulphonate and gluconate on C3A hydration will be discussed below in combination with C4AF and gypsum. Interaction between cement minerals and hydroxycarboxyllc acid retarders (mainly salicylic acid) was studied by Diamond /32,33,34/. It has been shown that trl calcium alumlnate, hydrated calcium alumlnates and calcium aluminate sulphates appear to adsorb large amounts of salicylic acid from aqueous solution and that for C3A at least, the "absorption" is due to the precipitation of an amorphous sallcylate containing reaction product. Their experiments indicate that the amorphous complex is composed of sallcylate and A1 and does not contain Ca 2+. Residual Ca 2+ ions precipitate as Ca(OH)2 when the solution is dried. The formation of alumlnosallcylate complexes in aqueous solution was reported by Babko and Rychkova /35/. The complexes were synthesized by mixing sodium salicylate and Al-salts. The primary complex was characterized by transference experiments as a positively charged ion A(Sal). (Sal denotes the double sallcylate group.) In alkaline solutions containing high ratios of sallcylate to alumlnium (AI), a second less stable complex in the form of a single negatively charged ion AI (Sal)3 was also detected. The i:i complex ion was confirmed by Das & Adltya /36/ and a considerable work on the nature and stabilities of all three complex ions was done by Onlclu and co-workers /37,38/. The complexes are said to be chelate structures presumably implying the structure of the primary i:i complex and represented by the following formula:
~
__C~
0
~O~A1 o
The AI probably occurs in 6 fold coordination with water molecules occupying the remaining sites. A solid crystalline Al-sallsylic acid compound, indentlfied as "alumlno-salisyllc acid" was prepared by Burrows and Wast /39/ in the form of pink needle shaped crystals. The ratio of sallcylate to AI was 2 and these workers assigned a structure of OH n 2 [AI (Sal)< ] OH
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883 POLYMERS, ADMIXTURES,
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CEMENT HYDRATION
to the compound. Subsequently Illarl /40/ considered the alumlno sallcylate as equlmolar addition compund by sallcylic acid with a 1:1 compound. He designated it as H(Sal) AI(OH)2. Kalyanarlman et al /41/ prepared aluminosalicylic acid and asslgnated it a hydroxylated chelate structure indicated by
~0~ A I - OH
0~ 2
Diamond /33/ has confirmed that the amorphous salicylate-bearing phase responsible for the extensive "apparent adsorption" of salicylic acid on C3A and related compounds has the structure given by Kalyanarlman /41/. Glucose, gluconate and sodium lignosulfonate are widely used in the production of plasticizing admixtures. The effect of these admixtures on the reaction of C3A and C4AF as well as on the hydration of C3A-C4AF-CaSO 4 • 2H20 has been studied by some researchers /42,43,44/. The effect of glucose is quite similar to that of llgnosulfonate as to the kinetics of formation of the ettringite and to the absence of conversion into monosulphate. On contrary, the retardation in the hydration of C3S is more marked and takes up to 7 days. Production of Ca(OH) 2 appears to be much slower than in the presence of sodium lignosulfonate (NLS). Glucose and lignosulfonate accelerate the early formation of ettringlte, obtained through the hydration of the system C3A-C4AF-CSH 2. The effect is similar to the system C4AF-CH-CSH 2. The transformation of ettrlnglte into monosulphate occurs at least after 7 days even in the system without admixtures. Therefore, this confirms that the ettrlnglte obtained through the hydration of C4AF is more stable than ettringite obtained through the hydration of C3A. The retardation that the admixtures cause on the hydration of C3A alone is much higher than that of the system C3S-C4AF-CSH 2. It is probable that the adsorption of admixtures on ettringlte and the consequent lower concentration of admixtures in the liquid phase, may be the cause of different retarding effect on the hydration of C3S. Analogous conclusions can be drawn comparing the retarding effect of admixtures on the hydration of C3S alone and on the hydration of C3S in the system C3S-C3A-CSH 2 /43/ or in Portland cement. The higher retardation caused by glucose and above all, by gluconate, with respect to llgnosulfonate, on the hydration of C3A in the system G3S • C3A • CSH 2 is due to lower adsorption of the first two admixtures on ettrlnglte. In addition, with respect to llgnosulfonate gluconate retards the early production of ettrlnglte. Therefore, even supposing that admixtures are adsorbed to the same extent, the lowering of concentration in aqueous phase occurs more slowly with gluconate than with llgnosulfonate. Sodium llgnosulfonate, natphatalene sulphate formaldehyde and melamine sulfonate formaldehyde condensates are known as superplastlclzers (Figure 5) (low molecular weight polymer). They are used in concentrate to improve the workability by entraining extra air. Thus they act as water reducing admixtures. It has been shown /45/ that these superplastlclzers dissolved in
884
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llme water are adsorbed on C4AHI3 and C3AH 6. When dissolved in dlmethylsufoxldes the same admixtures are adsorbed on C4AHI3 but apparently not on C3AH 6. The adsorption isotherms of the two polycondensates are very slmilar but different from those of llgnosulfonates. This fact can be attributed to the considerable structural difference between the synthetic admixtures and llgnlno derivative. The particle zeta potential is modified by the presence of the admixtures, minimum addition of which is enough to bring the zeta potential to the negative constant values. Nevertheless, the values of the potential cannot be correlated with the viscosity of the aluminate hydrate pastes since the viscosity first increases and then decreases as the admixture increases. No references were found on the interaction between polymer dispersions and C3A. However, some low molecular weight organic substances (superplasticlzers) have considerable influence on C3A during its reaction with water. The detailed mechanisms are still not known but complex and chelate formation seem to play an important role.
iH20H HC - 0 -
C-C-C-
I
HC - SO~
~OCH 0i
3
Figure 5. Major structural element in llgnosulfonates /54/. Influence of polymer dispersions on the hydration of cement Interphase effects in polymer modified hydraulic cement was studied by Wagner and Greenley /47/. In initial experiments the vlnylldene chloride vinyl chloride - ethyl acrylate, 75:20:5, latlces were mixed with sand and cement and the extracted with methyl ethyl ketone at various time intervals up to 24 hours. They showed that somewhat less than half of the polymer was thus extracted, even though the starting polymer was completely soluble in this solvent. In a similar manner, an ethyl acrylate - methyl methacrylate copolymer latex was found to be completely converted from a methyl ethyl ketone - soluble to an insoluble form. Calcium ions were found to be contained in the polymeric residue to the extent of 0.013 g calcium per 1 g polymer. It appears likely that here the initial reaction, in the alkaline environment, involves saponification of the ethyl acrylate portion of the copolymer and an "ionomer" type bonding of the carboxylate groups obtained which crossllnked the polymer. Furthermore, it was found that when this latex is allowed to react with aqueous sodium hydroxide solution at the pH
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885 POLYMERS, ADMIXTURES,
INTERACTION,
CEMENT HYDRATION
value of 12.3, this isolubilization does not occur. This confirms the hypothesis of Chandra et al /13/ that the divalent calcium ions are responsible for the interaction and not tha alkalinity of the solution. Four latices were treated in similar way for 7 days in contact with the CA(OH) 2 slurry. The four latices are i. 2. 3. 4.
Polystyrene homopolymer latex Poly (ethylene-co-vlnyl chloride) copolymer latex Poly (butadienr o-styrene) copolymer latex Neoprene latex
A styrene homopolymer latex and an ethylene - vinyl chloride copolymer latex showed no insolubilization indicating that there is no interaction; a butadiene styrene copolymer latex showed partial insolubilizatlon (partial interaction and a neoprene latex showed nearly complete insolubilization. In the pH range 12.3-ii.7 at ambient temperatures vinylidiene chloride hompolymer and copolymers in latex form undergo substantial dehydrohalogenation, where such polymer exhibit solubility in organic solvents prior to such reaction, a large weight fraction becomes insoluble after reaction. Infrared spectra obtained on the polymer fractions resulting from these reactions show that significant production of carbonyl groups has occurred. The development of new chromophoric groups as reaction progressed was evident. This being almost entirely associated with the insoluble polymer fraction. This latter effect can probably be explained on the basis of development of conjugated double bonds. With the ethyl acrylate - methyl methacrylate copolymer latex, the insolubillzing power in the presence of Ca 2+ is believed to result from hydrolysis of the ethyl acrylate, followed by cross-llnking of the carboxylate ions by Ca 2+. As could be expected no insolubilizing power was found for Na +. In the initiated reaction between these latex particles and the aqueous phase there is a reaction between OH- from the aqeous phase and Cl- from the polymer phase. Perhaps a more realistic picture of this process is not penetration by the OH- ion but sufficient polymer segment mobility in amorphous regions near the particle surface, so that access to Gl- atoms is offered to OHions. As formation of carbonyl- and perhaps also hydroxyl groups progresses at these sites, the hydrophilic character of these polymer segments would be increased and this process is facilitated. It is also possible that disruption of crystalline regions of the polymer structure might accompany reaction and thus a gradual "unraveling" of the initial polymer structure be brought about. On the basis of the foregoing and earlier reported observations, the following is the present picture of the process occurring during hardening of polymer latices modified cements. When cement, sand, water and latices are mixed, several processes occur simultaneously: a) formations of the "cement gel" at the cement grain surfaces and within the water phase; b) development of an adherent layer of C-sillcate over the sand particles surfaces and c) with reactive polymers substantial modification of the chemical particle surfaces. With certain polymer types studied, such as polyethylene and polystyrene, no evidence has been found of the latter reaction and from chemical consideration none could be expected. With others, such as poly (vinylldene chlorlde-co-vlnyl chloride) neoprene and poly (ethyl acrylate-co-methylmethacrylate), substantial extents of reaction occurs. Crossllnking and insolubility frequently accompany these reactions and the nature of the polymer is substantially altered.
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During the next period, attachment of latex particles to the cement gel and to the silicate layer at the sand surface interface occurs with the reactive polymers. This may involve chemical reaction and bonding between the now modified polymer particle surfaces and the silicate surfaces. The nature of these reactions seems specific to the particular latex. During the final period, namely during withdrawal of water by cement hydration, coalescence of polymer latex particle occurs. It is during this last period that the polymer "net-work" structure is developed. Polymers which are not "film forming" initially and which undergo no modification in the cement environment, show no coalescence. However, even polymers which are initally not film forming may nevertheless exhibit coalescence after such a chemical alteration. Besides polymer, llgnosulfonates are also used very much in concrete. This is discussed in the paper of Hansen /48/. Blank, Rosslngton and Weinland /49/ studied the absorption of salisylic acid and calcium lignosulfonate by C3S, C2S, C3A and C4AF and a commercial cement. It has been shown by then that the adsorption of calcium lignosulphonate and salicylic acid on the hydration products of the four principal components of Portland cement is a function of the hydration time allowed. In the case of C3A the adsorption of both admixtures decreased with development of hydration products. On the other hand adsorption of salicylic acid by C4AF hydration products was proportional to the hydration time. But in the case of calcium lignosulphonate it was opposite. Further it was shown that C2S and C3S hydration products adsorb calcium llgnosulphonate more readily than salicyllc acid without any significant dependence on hydration time. Hansen et al /48/ concluded that the van der Waal forces play little, if any, part in the adsorptions of SA and CLS by either the anhydrous cement compounds or by the hydrated reaction products in an aqeuous medium. It has been pointed out /50/ that the crystals of the anhydrous compounds must contain a relatively large number of oxygen ions. Hence the formula
HOC6H4C / O H
for SA, they visualized that the leave in the surface of the crystal of the cement compound to form an adsorbed layer with the COOH group of the acid exposed to the liquid phase. The C-O group could then form a hydrogen bond with the OH group of another molecule of the acid. In this way several layers of this acid could be adsorbed by hydrogen bonding with oxygen ions in the surface of the crystals. At high pH prevailing in Portland cement during hydration, SA is dissociated and is present in carboxylate form HOC6H4COO-. Since the crystals of cement components are positively charged due to excess of partial valencies from predominently divalent calcium ions. These ions adsorb the negative ions of this acid (chemisorption). Ca 2+ might react as follows: Ca 2+ + 2(HOC6H4CO0) ~ Ca(OOC6H4OH)2 This adsorption mechanism might be the reason why the crystallization process is influence by SA. The surfaces of the crystals of hydrated reaction products such as C4AHI3, C3AH 6 and C3S2H 3 are also positively charged and thus can adsorb SA by the mechanism outlined above. Seligmann and Greening /51/ reported that some commercial'lignosulphonate
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887 POLYMERS, ADMIXTURES,
INTERACTION,
CEMENT HYDRATION
admixtures retarded the reaction of C3S with water indefinitely. However, in the presence of alkalies a delayed set ultimately occurred. This led them to suggest that the alkalies reacted chemically with CLS and destroyed its retarding ability. Manabe et al /52/ reported that cements containing low C3A contents and high soluble alkali contents are sometimes retarded strongly by ordinary additions of CLS. The work by Sellgmann and Greening appears to show that the retarding power of CLS can be destroyed by alkalies in cement pastes, but the work by Manabe et al supports the conclusion that a chemical reaction of the alumina-bearlng phases with CLS and other organic retarding admixtures is the mechanism by which the retarding effect of such admixtures is destroyed in commerlclal cement pastes. The work by Danlelsson /53/ supports the conclusion that hydrogen bonding is not the only mechanism by which organic compounds are adsorbed by calcium silicates in cement pastes. CLS are almost spherical molecules of varying size containing sulphonate groups /54/. Some of these are pendent on the surface and may react with surface of the cement components. These CLS molecules are so large that similar reaction can take place independently on the opposite side of the molecule. This mechanism might explain the influence of CLS on the crystallization process. To summarize, with reactive polymers, substantial modification of the crystallization process occurs by the interaction of latex particles with the surfaces of the growing crystals. Cross-linking and insolubility of the polymer frequently accompany these reactions and the behavlour is substantially changed. There is a bonding between the modified polymer partlcles and the silicate surfaces. Besides, the alkalies (produced during the hydration of Portland cement) react chemically with CLS and destroy its retarding ability in the reaction of C3S with water, but in the presence of low C3A content the reaction becomes different. Conclusions Polymers and organic admixtures interact with the components of Portland cement when they come in contact with water. This interaction is due to the ionic bonding, causing cross-llnks which inhibit the film formation property of polymers and influence considerably the crystallization process during the hardening of concrete. Some low molecular weight organic substances also have a considerable influence on Portland cement during their reaction with water. The behavlour of the pure Portland cement components with water is very much different than when they are present together in Portland cement. Consequently the interactions of polymers as well as other low molecular weight organic compounds will behave differently with Portland cement than with its pure components. The mechanisms of interaction are not clear with these additives and the pure components. In this situation it is very difficult to explain the mechanism of interaction of these additives in Portland cement during hydration. Besides in the references available no study on the interaction between the components of Portland cement and one polymer and then the same polymer and cement. It will be very interesting to do a comprehensive study of such a system to enable answers to many questions in this important field. Acknowledgements The authors are thankful to Mr. T. Sugama and Mr. L.E. Kukacka for their permission to reproduce their line sketch diagram, to the Swedish Institute
888
Vol. 17, No. 6 S. Chandra and P. Slodin
for Building Research for their grant and to Mrs A. Palmdin for preparing the manuscript. References i.
L.H. Baekland, US Patent No 939.966, 1909.
2.
M.E. Varegyas, French Patent No 436.061, 1911.
3.
L. Cresson, British Patent No 191474, 1923.
4.
V. Lefebure, British Patent No 217279, 1924.
5.
S.H. Kirkpatrick, British Patent No 242345, 1925
6.
M. Itakura, Japan Soe for Testing Materials, Vol 2, No i0, Dec 1983.
7.
V.R. Riley & I. Razl, Composites, Jan 1974, p 27.
8.
A. Jolsel, Proc. 5th Int. Symp. Chem. of Cement, Tokyo 1968.
9.
H.N. Stein, J.G,M. de Jong & J.N. Stevels, J. Appl. Chem. Vol 17, 1967.
I0.
N.R. Greening, Proe. 5th Int. Symp. Chem. of Cement, Tokyo 1968.
ii.
F.V.Lawrenee & J.F. Joung, Cem. Cone. Res., Vol 3, 1973.
12.
S. Chandra, P. Flodln & L. Berntsson, 3rd Int. Cong. on Polymers in Concrete. Koriyama, Japan. May 1985, p 125.
13.
S. Chandra, L. Berntsson & P. Flodin, Cem. Cone. Res., Vol ii, No i. 1981.
14.
S. Chandra & P. Flodln (To be published).
15.
S. Chandra & M. Arvidsson (Unpublished work).
16.
S. Chandra & M. Avidsson (Nord. Cone. Res., Publ. I, 1982.
17.
S. Chandra, Report 83:5, Chalmers Univ. of Technology, 1983.
18.
T. Sugama & L.E. Kukaeka, Cem. Cone. Res, Vol 9, No I, 1979, p 69.
19.
T. Sugama & L.E. Kukaeka, Cem. Conc. Res, Vol 9, No 4, 1979, p 461.
20.
T. Sugama & L.E. Kukaeka, Cem. Conc. Res, Vol i0, No 2, 1980.
21.
L. Ben-Dor, H.C. Wirguln & H. Diab, Cem. Conc, Res. Vol 15, 1985, p 681.
22.
V.S. Ramanehandran, Cem. Cone. Res, Vol 6, 1976, p 623.
23.
G. Zundel, Hydration and intermolecular interaction, Academic Press, NewYork, London 1969.
24.
W. Dosch, Neues Jahrb. Mineral Abs. 106 (2) 200 39, 1967.
Vol. 17, No. 6
889 POLYMERS, ADMIXTURES,
INTERACTION,
CEMENT HYDRATION
25.
W. Dosch, Clay and clay minerals, Proc. 5th Conf. Pittsburgh Pa, Oct 1966. Edited by S.W. Baily, Pergamon Press Inc. N.Y.
26.
Young, J . F . J .
27.
K.E. Daughtery, M.J. Kowalewski Jr., Proc. 5th Int. Symp. Chem. of Cem. Part IV, Tokyo 1968, pp 42.
28.
Young, J.F. Mag. Conc. Res. 14 /42/, 1962, p 137.
29.
S. Suzuki & S. Nishi, Semento GiJutso Nembo 13, 1959, p 160.
30.
J.H. Taplin, 4th Int. Symp. Chem. of Cem., Washington 1960, p 924, Nat. Bur. Stand. (U.S.) Monogr. 1962, No 43, p 548.
31.
J.F. Young, Abstr. Part II of ref 27.
32.
S. Diamond,J, Am. Ceram. Soc. 54 /6/, 1971, p 273.
33.
S. Diamond,
Ibid 55 /4/, 1972, p 177.
34.
S. Diamond,
Ibid 55 /8/, 1972, p 405.
35.
H. Babko & T.N. Rychkova, Zh. Obshch. Khim. 18, 1948, p 1617.
36.
B. Das & S. Aditya, J. Indian Chem. Soc. 36 /7/, 1958, p U73.
37.
L. Oniciu & R. Hales, Sled Univ. Babes Bolyai, Ser. Chem. 7 /2/, 1962, p 15.
38.
L. Oniciu, E. Schmidt & Ladarin, Slad. Cercet Chem. 13 /12/, 1964, p 893.
39.
G.J: Burrows & I.W, Wast, J. Chem. Soc., London 1928, p 222.
40.
G. Illari, Ann, Chim.
41.
R. Kalyanariman, S. Shasranaman, M. Sundaresan & D.B. Trivedi, J. Chem. 9 /1/, 1971, p 76.
42.
S. Monosi, G. Morlconi, M. Pauri & M. Collepardi, 1983, p 508.
43.
M. Collepardi, 1984, p 105.
44.
S. Monosi, G. Moriconi, M. Pauri & M. Collepardi, Proc. llth National Meeting ASMI, Engg. Mat. 437-444, Milano 26-27, 1983.
45.
F. Massazza, U. Costa & R. Barrila, J. Am. Ceram. Soc., Vol 65, No i, April 1982.
46.
V.H. Malhotra & D. Molanka, Superplasticizers Conc. Inst., Detroit, 1979, pp 21.
47.
H.B. Wagner & D.g. Greenley, J. Appl. Pc1. Sc., Vol 22, 1978, p 813.
Am. Ceramic Soc., Vol 53, No 2, Feb 1970.
(Rome), 42, 1952, p 32. Indian
Cem. Conc. Res.,
S. Monosi, G. Moriconi & M. Pauri, Cem. Conc. Res.,14,
in Concrete, SP 62, Am.
890
Vol. 17, No. 6 S. Chandra and P. Flodin
48.
W.C. Hansen,
49.
B. Blank, D.R. Rossington & Weinland, 1963, p 395.
50.
D.R. Rossington & E.J. Runk, J. Am. Cer. Soc., JACTA,Vol 46.
51.
J. Materials,
Vol 5, No 4, 1970, p 842.
P. Seligmann & N.R. Greening,
J. Am. Cer. Soc. JACTA, Vol 46,
51, 1968, p
Highway Res. Record, HIRRA, No 62, 1964,
p 80.
52.
Manabe, Toshio, Kawada, Naoya & Nishiyama, Nembo, Vol 14, 1960, 1960, p 14.
53.
U. Danielsson, "Studies of the effects of some simple organic admixtures on the properties of cement pastes". International Symposium on Admixtures for Mortar and Concrete, Brussels, Belgium 1967; Reprint 50; Forsknlngsinstutet fSr Cement och Betong rid Kungl. Tekniska HSgskolan, Stockholm, Sweden, 1968.
54.
K. Lundquist,
personal communication.
Masoya,
Semento Gijutso
INTERACTIONS OF POLYMERS AND ORGANIC ADMIXTURES ON PORTLAND CEMENT HYDRATION
Satish Chandra Div of Building Materials Per Flodin Dep of Polymer Technology Chalmers University of Technology, GOTEBORG, Sweden
(Refereed) (Received Oct. 21, 1986; in final form July 21, 1987) ABSTRACT Interaction of polymers and other organic admixtures on Portland cement hydration is reviewed. This has been compiled in a systematic way. First hydration of Portland cement is described in short. Later, interaction with 4 important components of Portland cement is discussed. Finally interphase effects in polymer modified hydraulic cement are discussed. It is concluded that polymers and organic admixtures interact with the components of Portland cement when they come in contact with water. This interaction is due to ionic bonding, causing cross-llnks which inhibit the film formation property of polymers and influence considerably the crystallisation process during the hardening of concrete. Some low molecular weight organic substances also have a considerable influence on Portland cement during its reaction with water.
Introduction The concept of the use of admixtures in concrete is not new. The available literature of interest dates back to 1909 /1,2/. More work has been done in this field in the 1920s /3,4,5/. In the 1940s products were developed based on poly vinyl chloride which were used for improving the bond strength between old and new concrete and for making concrete pipes /6/. Air entraining admixtures and water reducers are other examples of admixtures widely used in concrete. There are two theories for the mechanism of action of polymers in concrete. According to the first theory there is no interaction between the polymer and concrete. During hydration the hydrophillc part of the polymer is oriented towards the water phase whereas the hydrophobic part heads towards the air phase (pores and capillaries not filled with water). On drying, the water is taken away, the hydrophobic particles coalesce together and form film. The other theory is that the polymer interacts with the components of the Portland cement hydration products and makes com875
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Vol. 17, No. 6 S. Chandra and P. Flodin
plexes. This creates a type of reinforcement in the concrete and produces semipermeable membranes. The present paper discusses the interactions of different admixtures with concrete. Different types of polymers have been used in concrete. These polymers can be broadly divided into three groups: latices, liquid resins and water soluble polymers. Emphasis will be given here on the latices as they are mostly used. Latices are dispersed suspensions of solid polymer micropartlcles in water. The polymer in dispersion is usually glassy amorphous (Tg approximately above 50°C) or viscous rubbary (Tg below room temperature). The dispersion contains 30-50% of solid polymers by weight. Latices are produced by emulsion polymerization of liquid monomers in water. Thus sometimes they are named as emulsions. Elastomers used in latices are natural rubber, poly(styrene-co-butadlene), poly(acrylonitrile-co-butadlene) and polychloroprene. The major polymer dispersions used in concrete are following /7/: i. Poly(vinyl acetate) or copolymers of vinyl acetate with monomers such as vinyl chloride, vinylidene chloride, dlbutyl maleate and vinyl propionare. 2. Polystyrene or copolymers of styrene with different acrylate monomers. 3. Poly(vinyl chloride) or copolymers of vinyl chloride with monomers such as vinylidene chloride and vinyl propionate. 4. Polyacrylates and their copolymers.
Hydration of Portland cement When Portland cement comes into contact with water, its components react and different hydration products are formed. Out of these, calcium silicate hydrates (consist mainly of fibrous crystals which have a very small degree of structural order /8/) are the most important constituents. The hydration process can be derided into several successive stages: In stage i, there is an initial hydrolysis of C3S and saturation with respect to calcium hydroxide is quickly achieved or surpassed. The first formation of calcium silicate hydrates starts as very fine crystals. At this time the volume of the cement particles has not appreciably decreased since only a thin surface layer has reacted and the interspace between the cement grains still has nearly the same size as at the beginning of the hydration. Consequently the calcium silicate hydrates grow to be long interlocking fibres. Stage 2 is a dormant period, when the reaction becomes very slow. During this period, hydrolysis slows down and super-saturatlon is gradually reached. Poorly crystalline C-S-H is the major hydration product which is low in bound water and has a little silica polymerization /9/. This hydration product is considered to be a diffusion barrier to water and hence causes the induction period. An alternative explanation is that the rate of hydrolysis is controlled by the calcium ion concentration in the solution. Calcium ions released from the solid have to move into a solution of increasing chemical potential. The hypothesis is that the soluble silica is adsorbed onto the calcium hydroxide nuclei /I0/ hindering their growth. Supersaturation is needed to overcome this poisoning effect and to continue crystal growth.
Vol. 17, No. 6
877 POLYMERS, ADMIXTURES,
INTERACTION,
CEMENT HYDRATION
In stage 3 (2-6 hours after cement is mixed with water), the calcium ion concentration drops and the hydration reaction proceeds rapidly again. The decrease of the calcium ion concentration in the solution and the renewed hydration coincide with the crystallization of calcium hydroxide. At the same time the C-S-H gel changes its composition and properties, If forms the characteristic circular morphology /ll/. Calcium hydroxide behaves as a calcium sink which helps the hydrolysis of C3S relatively rapidly during stage 3 until diffusion through stage 4. Large crystals of CH are also formed. Calcium aluminate, C3A , hydrates according to the following equation when no sulphate ions are present C3A + 21H ~ C4AHI3 + C2AH 8 hexagonal hydrates
2C3AH 6 + 9H cubic
The hexagonal hydrates are thermodynamically unstable with respect to C3AH 6 and slowly convert to the cubic hydrate at room temperature. The crystal structure of the hexagonal hydrates (C3A • CX • HX) can be regarded as a negative clay with exchangable anions X in the interlayer region. Here relevant to Portland cement hydration is the interaction of C3A with the sulphate ions present as gysum whlch is added to control the flash set potential of C3A. The reaction under these circumstances proceeds as follows C A3+ 3CS H
~ 26H ~ C A 3. 3CS • H32 ettrlnglte
At this time the structure mainly consists of long interlocked fibrous calcium silicate hydrates which bridge a considerable part of the orglnally water filled space. During the next stage of hydration from about 24 hours up to the end of the reaction, the pore (still empty) is filled by new hydration products. During this stage of hydration, tetra calcium alumlnoferrlte C4AF ~ HI3 is formed due to the hydration of alumlno-ferrlte phase. Alumlno-ferrlte phases form hydration phases similar to those of the alumlnates in which F replaces part of A. Further the formation of ettrlnglte is almost complete at this point. This means that all the sulphate ions are consumed and thus ettringite in combination of more calcium aluminates converts to monosulphate. C3A • 3CS • H32 + C3A ~ C3A • CS • HI2 monosulphate Interaction between Ca(OH)2 and styrene methacrylate polTmer dispersion It is seen in the previous section that Ca(OH)2 is produced immediately after the addition of water to Portland cement. This emphasizes the importance to study the reaction of CA(OH) 2 with the polymer dispersion. It has been shown /12,13/ that there is a tendency of gelation in a mixture of Ca(OH)2 and styrene methacrylate polymer dispersion containing a minor amount of acrylic acid residues when a little solid powder of Ca(OH)2 is added. The dispersion settles down along with Ca(OH)2 leaving the supernatant liquid transparent. It is further noticed that 5 g of Ca(OH)2 powder has the capacity to digest i lltre of dispersion containing i0 g of the solid polymer leaving the supernatant liquid transparent. When this limit is exceeded the supernatant liquid becomes turbid again.
878
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The polymer dispersion used is composed of soft microparticles and has film forming ability. It was observed that with small addition of Ca(OH)2 the polymer film became translucent. With increasing amount of Ca(OH)2 , the film formation gradually reduces and finally no film is formed. This can be explained by ionic bonding between Ca and carboxylate ions of the polymer, causing cross-llnks which inhibit film formation. Furthermore, when some polymer dispersion was treated with NaOH in the similar manner, neither gelatlon tendency could be noticed, nor the polymer had lost the film forming ability. This shows that there is interaction between Ca 2+ ions and the carboxylate groups of polymer dispersion and n o t the destabilization of polymer due to the alkalinity of the solution. With solid Ca(OH)2, the carboxylate groups of these polymer particles interact with the free valencies of the calcium atoms on the surface of Ca(OH)2 solid particles. This is illustrated in Figure i.
I Ca (OH) 2~ ~ . ~ C a 2
- Ca2+---~--Ca2+---~ / (a)
+
+l s o l i d / ~ Ca
I
/+ /
ca {-ooc
\
I
(b)
Figure i. Schematic illustration of cross-llnklng of polymer particles by a) divalent Ca-ions and b) Ca(OH) 2 /12/. Since carboxyllc groups are present all over the surface of the polymer particles, another crystal may be bonded to the particles which then act as glue between solid Ca(OH)2 particles. As long as there are free bonding sites available on the Ca(OH)2 crystals, the concentration of polymer particles in the supernatant liquid will be low, which is in agreement with the transparency of the supernatant liquid and large increase in the rate of precipitation. X-ray diffraction analysis of different mixes of Ca(OH)2 and polymer dispersion showed the diminishing peak of Ca(OH)2 /14/. Thus the complex is not highly crystalline and confirms that the polymer has considerable influence on the crystallization of calcium hydroxide. A decrease in the free calcium ion concentration was noted /15/ when tensides and a number of polymer dispersions, made in the laboratory /16,17/, were mixed with Ca(OH)2 saturated solution. Similar results were obtained with the polymers having no tensides. This shows that there is an interaction between the divalent Ca 2+ ions and the anionic tensides (Figure 2). The decrease in the Ca 2+ ion concentration with polymer dispersion without tenside indicate that there is an interaction between the carboxylate ions of the polymer and the divalent ca 2+ ions (Figure 2). The results obtained were not very precise and systematic as the membrane of the calcium ion selective electrode used reacted with the polymers and it was difficult to clean it properly. It has been shown that the divalent calcium ions do interact with the anionic tensides present in the polymer dispersions as well as with the carboxylate group of acrylate based polymer dispersions without tensides. A calcium complex is formed by ionic interactions and the polymer looses its film forming ability.
Vol. 17, No. 6
879 POLYMERS, ADMIXTURES,
PMMA covered by surfactant
INTERACTION,
CEMENT HYDRATION
(anionic)
Insoluble Ca-slat of surfactant covers surface.
Figure 2. Schematic diagram showing the interaction of Ca 2+ with polymer dispersions containing tensldes (SO 3 group).
Interaction between calcium silicates and polymer dispersions Sugama and Kukacka /18,19,20/ studied the effect of dicalcium silicate (~C2S) and tri calcium silicate (C3S) on the thermal stability of vinyl type polymer concrete. A cross-linklng structure of calcium ions from C-S compounds with carboxylate anions of trimethylopropane trimethylacrylate (TMPTMA), polystyrene (PST) and polyacrylonitrile (PAN) was discussed. DSC thermograms (Figure 3) showed a peak which was attributed to the decomposition of -CH 2- groups of the main chain in PST.
a, BULK PMMA; b, 34wt% PMMA-66wt% CaO.Si02; o b
c, 34wt% PMMA-66wt%
leO. 193
3CaO.2SiO2;
d, 34wt% PMMA-66wt% 2CaO.SiO2; 386
i
°
\
3~
e, 34wt% PMMA-66wt%
3CaO.SiO2;
//.__
2~
d
i
Figure 3. Differential scanning calorimeter (DSC) thermograms of P~A-CaO-SiO 2 systems /18/.
880
Vol. 17, No. 6 S. Chandra and P. Flodin
The second peak was attributed to the decomposition of single -CH 2- linkages with the aromatic ring. Therefore it was considered that the thermal decomposition between -CH 2- main chain in PST was prevented because of the presence of ~C2S and C3S. In order to obtain further information about the interaction between calcium compounds and the -CH 2- group of polymethyl methacrylate (PMMA) and PST, a qualitative analysis was performed by using IR absorbance ratio. The absorbance of the -CH 2- group in PMMA and PST illustrates a decreasing tendency with increasing amounts of calcium oxides in the C-S system. The results indicate that an apparent interaction between the -CH 2- group of the vinyl type polymer and the calcium oxide of the C-S system may take place. The degree of bonding between Ca 2+ ions of C-S compounds and COO- anions of TMPTMA occurring within styrene (ST), ACN-TMPTMA copolymer examined by an atomic absorption spectrophotometer, showed that the calcium ion concentration decreased with an increase in the mole fraction of TMPTMA in the copolymer. This fact is associated with a higher degree of reactivity of Ca z+ ion with the carboxylate anions formed by the thermal decomposition of an ester group in TMPTMA with regards to the hydrolysis of TMPTMA and the nitrile groups of PAN. It was stated that the nltrile groups undergo thermal decomposition to form amino groups and do not hydrolyse. Regarding the ester group very little hydrolysis could be evidenced. The reaction mechanism
0~
~C
"O-cMz'
~x._//~ ~'c"
'O -- CH 3 BRINE | H2O)
MOT
ANHTD*OU5
641
~5
COR[
~ "
C.-D.L,:'r",, ,.-,,.~ /"-
°
""~" "
0
JJ.
'o~'~2.
c, ~'. ,~.,-~c,.~o~ o • "
P R ~ ' I P ~rATlON OF H Y D R , T F D ~.35
" "
1-~- ": '.. , : - / ~
C~
-
CROSS-LINKING
/ - -
, O
Figure 4. Reaction mechanisms between C3S and PMMA layer during exposure to brine /20/.
Vol. 17, No. 6
881 POLYMERS, ADMIXTURES,
INTERACTION,
CEMENT HYDRATION
occurring at the interface of PMMA with C3S grains is hypothetical as represented by a reaction process (Figure 4). It is indicated in the figure above that Ca 2+ ions are released readily by the attack of the hot brine on the anhydrous C3S grains. Ca 2+ ions produced in an aqueous medium develop an electrostatic interaction with the carbonyl group (- C = 0), which becomes more strongly electro-positlve at the elevated temperature. The effect of ionic polymers such as I) polyvinyl sulfonic acid and 2) polystyrene sulfonlc acid on hydration of C3S is studied by Ben-Dot et al /21/. A decrease in the degree of hydration of C3S sample made by the addition of these polymers was noticed. The retardation in the formation of Ca(OH)2 by the polymers was explalned as follows: 1) An interaction of the Ca 2+ ions or Ca(OH)2, released during the hydration of C3S with sulfonate groups of the polymer. 2) The sulfonate group of the polymer being hygroscopic absorb water, diminishing the amount of water available for the hydration of the silicate. For both C3S - PESA and (C3S - ESA) polysystems, the DTA analysis showed two endothermlc peaks in the Ca(OH)2 dehydration range; one at a comparatively higher temperature, which becomes more intense with age and simultaneously shifts to higher temperatures (451-496°C) and a second rather weak one appearing on somewhat lower temperatures (392-431°C). The first endotherm is of neat-hydrated C3S , vlz Ca(OH)2, and the second one is probably associated with Ca(OH)2 bonded to the polymer through its sulfonate groups. It did not grow with age owing to the limited, fixed amount of polymer present. Similar results are also reported by Ramachandran /22/. IR spectroscopy results showed shifts in symmetric and antlsymmetrlc vibrations. These agree with the work of Zundel /23/. The shifts of Ca 2+ by about i00 cm -I in the symmetric as well as antlsym~etrlc vibration of S-O confirm that an interaction between Ca 2+ ions and the sulfonate groups of the polymer occur. Diminishing of the peak of Ca(OH)2 in these systems as observed by X-ray diffraction analysis further confirms the interaction. This also confirms the results obtained by Chandra et al /13/. To summarize, the work on C3S and polymers at high temperatures with DSC and IR spectroscopy indicates that there are interactions. However, much remains to be done at normal room temperature. It is shown /24,25/ that many types of organic compounds can form interlayer compounds with C4AHI3, at hydration product of C3A. It was considered that incorporation of organic additives into the hydrate latices could be responsible for preventing the transformation to C3AH 6. Accordingly this aspect of C 3 A h y d r a t l o n was investigated more in detail /26/. The organic compounds studied were divided into two classes. Class A comprises sugars and their derivatives and class B is a series of aliphatic 3-carbon compounds with increasing numbers of hydroxyl and carboxyllc acid groups. It was observed /27/ that the organic compounds retard the hydration of C3A by forming a more impermeable barrier which is prevented from converting to C3AH 6. The nature and number of oxy functional groups indicate the ability of the compound to act as a retarder. However, the fact that mandellc acid accelerates the hydration of C3A whereas lactic acid retards
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Vol. 17, No. 6 S. Chandra and P. Flodin
it /28/ indicates that the chemical structure has a substantial effect on curing. Nevertheless, it is clear that sugars and their derivatives are strong retarders. Compounds such as malic acid, tartaric acid and citric acid would also be expected to be effective. The factors which determine the retarding properties of a compound towards C3A may not be the same for cement. The reaction of C3A In cement occurs in the presence of gypsum and a large quantity of calcium silicates. Surveys by Suzuki and Nishl /29/ and by Taplin /30/ indicate that other factors also must be involved. For example glycol aldehyde, glyceraldehyde and glycollc acid strongly retard cement setting. Previous work /31/ on the C3A gypsum reaction showed that sugars have a retarding effect but not enough data were available to determine if the conversation from the trlsulphate sulpho-alumlnate to the monosulphate sulpho-alumlnate was affected. The influence of glucose, lignosulphonate and gluconate on C3A hydration will be discussed below in combination with C4AF and gypsum. Interaction between cement minerals and hydroxycarboxyllc acid retarders (mainly salicylic acid) was studied by Diamond /32,33,34/. It has been shown that trl calcium alumlnate, hydrated calcium alumlnates and calcium aluminate sulphates appear to adsorb large amounts of salicylic acid from aqueous solution and that for C3A at least, the "absorption" is due to the precipitation of an amorphous sallcylate containing reaction product. Their experiments indicate that the amorphous complex is composed of sallcylate and A1 and does not contain Ca 2+. Residual Ca 2+ ions precipitate as Ca(OH)2 when the solution is dried. The formation of alumlnosallcylate complexes in aqueous solution was reported by Babko and Rychkova /35/. The complexes were synthesized by mixing sodium salicylate and Al-salts. The primary complex was characterized by transference experiments as a positively charged ion A(Sal). (Sal denotes the double sallcylate group.) In alkaline solutions containing high ratios of sallcylate to alumlnium (AI), a second less stable complex in the form of a single negatively charged ion AI (Sal)3 was also detected. The i:i complex ion was confirmed by Das & Adltya /36/ and a considerable work on the nature and stabilities of all three complex ions was done by Onlclu and co-workers /37,38/. The complexes are said to be chelate structures presumably implying the structure of the primary i:i complex and represented by the following formula:
~
__C~
0
~O~A1 o
The AI probably occurs in 6 fold coordination with water molecules occupying the remaining sites. A solid crystalline Al-sallsylic acid compound, indentlfied as "alumlno-salisyllc acid" was prepared by Burrows and Wast /39/ in the form of pink needle shaped crystals. The ratio of sallcylate to AI was 2 and these workers assigned a structure of OH n 2 [AI (Sal)< ] OH
Vol. 17, No. 6
883 POLYMERS, ADMIXTURES,
INTERACTION,
CEMENT HYDRATION
to the compound. Subsequently Illarl /40/ considered the alumlno sallcylate as equlmolar addition compund by sallcylic acid with a 1:1 compound. He designated it as H(Sal) AI(OH)2. Kalyanarlman et al /41/ prepared aluminosalicylic acid and asslgnated it a hydroxylated chelate structure indicated by
~0~ A I - OH
0~ 2
Diamond /33/ has confirmed that the amorphous salicylate-bearing phase responsible for the extensive "apparent adsorption" of salicylic acid on C3A and related compounds has the structure given by Kalyanarlman /41/. Glucose, gluconate and sodium lignosulfonate are widely used in the production of plasticizing admixtures. The effect of these admixtures on the reaction of C3A and C4AF as well as on the hydration of C3A-C4AF-CaSO 4 • 2H20 has been studied by some researchers /42,43,44/. The effect of glucose is quite similar to that of llgnosulfonate as to the kinetics of formation of the ettringite and to the absence of conversion into monosulphate. On contrary, the retardation in the hydration of C3S is more marked and takes up to 7 days. Production of Ca(OH) 2 appears to be much slower than in the presence of sodium lignosulfonate (NLS). Glucose and lignosulfonate accelerate the early formation of ettringlte, obtained through the hydration of the system C3A-C4AF-CSH 2. The effect is similar to the system C4AF-CH-CSH 2. The transformation of ettrlnglte into monosulphate occurs at least after 7 days even in the system without admixtures. Therefore, this confirms that the ettrlnglte obtained through the hydration of C4AF is more stable than ettringite obtained through the hydration of C3A. The retardation that the admixtures cause on the hydration of C3A alone is much higher than that of the system C3S-C4AF-CSH 2. It is probable that the adsorption of admixtures on ettringlte and the consequent lower concentration of admixtures in the liquid phase, may be the cause of different retarding effect on the hydration of C3S. Analogous conclusions can be drawn comparing the retarding effect of admixtures on the hydration of C3S alone and on the hydration of C3S in the system C3S-C3A-CSH 2 /43/ or in Portland cement. The higher retardation caused by glucose and above all, by gluconate, with respect to llgnosulfonate, on the hydration of C3A in the system G3S • C3A • CSH 2 is due to lower adsorption of the first two admixtures on ettrlnglte. In addition, with respect to llgnosulfonate gluconate retards the early production of ettrlnglte. Therefore, even supposing that admixtures are adsorbed to the same extent, the lowering of concentration in aqueous phase occurs more slowly with gluconate than with llgnosulfonate. Sodium llgnosulfonate, natphatalene sulphate formaldehyde and melamine sulfonate formaldehyde condensates are known as superplastlclzers (Figure 5) (low molecular weight polymer). They are used in concentrate to improve the workability by entraining extra air. Thus they act as water reducing admixtures. It has been shown /45/ that these superplastlclzers dissolved in
884
Vol. 17, No. 6 S. Chandra and P. Flodin
llme water are adsorbed on C4AHI3 and C3AH 6. When dissolved in dlmethylsufoxldes the same admixtures are adsorbed on C4AHI3 but apparently not on C3AH 6. The adsorption isotherms of the two polycondensates are very slmilar but different from those of llgnosulfonates. This fact can be attributed to the considerable structural difference between the synthetic admixtures and llgnlno derivative. The particle zeta potential is modified by the presence of the admixtures, minimum addition of which is enough to bring the zeta potential to the negative constant values. Nevertheless, the values of the potential cannot be correlated with the viscosity of the aluminate hydrate pastes since the viscosity first increases and then decreases as the admixture increases. No references were found on the interaction between polymer dispersions and C3A. However, some low molecular weight organic substances (superplasticlzers) have considerable influence on C3A during its reaction with water. The detailed mechanisms are still not known but complex and chelate formation seem to play an important role.
iH20H HC - 0 -
C-C-C-
I
HC - SO~
~OCH 0i
3
Figure 5. Major structural element in llgnosulfonates /54/. Influence of polymer dispersions on the hydration of cement Interphase effects in polymer modified hydraulic cement was studied by Wagner and Greenley /47/. In initial experiments the vlnylldene chloride vinyl chloride - ethyl acrylate, 75:20:5, latlces were mixed with sand and cement and the extracted with methyl ethyl ketone at various time intervals up to 24 hours. They showed that somewhat less than half of the polymer was thus extracted, even though the starting polymer was completely soluble in this solvent. In a similar manner, an ethyl acrylate - methyl methacrylate copolymer latex was found to be completely converted from a methyl ethyl ketone - soluble to an insoluble form. Calcium ions were found to be contained in the polymeric residue to the extent of 0.013 g calcium per 1 g polymer. It appears likely that here the initial reaction, in the alkaline environment, involves saponification of the ethyl acrylate portion of the copolymer and an "ionomer" type bonding of the carboxylate groups obtained which crossllnked the polymer. Furthermore, it was found that when this latex is allowed to react with aqueous sodium hydroxide solution at the pH
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885 POLYMERS, ADMIXTURES,
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CEMENT HYDRATION
value of 12.3, this isolubilization does not occur. This confirms the hypothesis of Chandra et al /13/ that the divalent calcium ions are responsible for the interaction and not tha alkalinity of the solution. Four latices were treated in similar way for 7 days in contact with the CA(OH) 2 slurry. The four latices are i. 2. 3. 4.
Polystyrene homopolymer latex Poly (ethylene-co-vlnyl chloride) copolymer latex Poly (butadienr o-styrene) copolymer latex Neoprene latex
A styrene homopolymer latex and an ethylene - vinyl chloride copolymer latex showed no insolubilization indicating that there is no interaction; a butadiene styrene copolymer latex showed partial insolubilizatlon (partial interaction and a neoprene latex showed nearly complete insolubilization. In the pH range 12.3-ii.7 at ambient temperatures vinylidiene chloride hompolymer and copolymers in latex form undergo substantial dehydrohalogenation, where such polymer exhibit solubility in organic solvents prior to such reaction, a large weight fraction becomes insoluble after reaction. Infrared spectra obtained on the polymer fractions resulting from these reactions show that significant production of carbonyl groups has occurred. The development of new chromophoric groups as reaction progressed was evident. This being almost entirely associated with the insoluble polymer fraction. This latter effect can probably be explained on the basis of development of conjugated double bonds. With the ethyl acrylate - methyl methacrylate copolymer latex, the insolubillzing power in the presence of Ca 2+ is believed to result from hydrolysis of the ethyl acrylate, followed by cross-llnking of the carboxylate ions by Ca 2+. As could be expected no insolubilizing power was found for Na +. In the initiated reaction between these latex particles and the aqueous phase there is a reaction between OH- from the aqeous phase and Cl- from the polymer phase. Perhaps a more realistic picture of this process is not penetration by the OH- ion but sufficient polymer segment mobility in amorphous regions near the particle surface, so that access to Gl- atoms is offered to OHions. As formation of carbonyl- and perhaps also hydroxyl groups progresses at these sites, the hydrophilic character of these polymer segments would be increased and this process is facilitated. It is also possible that disruption of crystalline regions of the polymer structure might accompany reaction and thus a gradual "unraveling" of the initial polymer structure be brought about. On the basis of the foregoing and earlier reported observations, the following is the present picture of the process occurring during hardening of polymer latices modified cements. When cement, sand, water and latices are mixed, several processes occur simultaneously: a) formations of the "cement gel" at the cement grain surfaces and within the water phase; b) development of an adherent layer of C-sillcate over the sand particles surfaces and c) with reactive polymers substantial modification of the chemical particle surfaces. With certain polymer types studied, such as polyethylene and polystyrene, no evidence has been found of the latter reaction and from chemical consideration none could be expected. With others, such as poly (vinylldene chlorlde-co-vlnyl chloride) neoprene and poly (ethyl acrylate-co-methylmethacrylate), substantial extents of reaction occurs. Crossllnking and insolubility frequently accompany these reactions and the nature of the polymer is substantially altered.
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During the next period, attachment of latex particles to the cement gel and to the silicate layer at the sand surface interface occurs with the reactive polymers. This may involve chemical reaction and bonding between the now modified polymer particle surfaces and the silicate surfaces. The nature of these reactions seems specific to the particular latex. During the final period, namely during withdrawal of water by cement hydration, coalescence of polymer latex particle occurs. It is during this last period that the polymer "net-work" structure is developed. Polymers which are not "film forming" initially and which undergo no modification in the cement environment, show no coalescence. However, even polymers which are initally not film forming may nevertheless exhibit coalescence after such a chemical alteration. Besides polymer, llgnosulfonates are also used very much in concrete. This is discussed in the paper of Hansen /48/. Blank, Rosslngton and Weinland /49/ studied the absorption of salisylic acid and calcium lignosulfonate by C3S, C2S, C3A and C4AF and a commercial cement. It has been shown by then that the adsorption of calcium lignosulphonate and salicylic acid on the hydration products of the four principal components of Portland cement is a function of the hydration time allowed. In the case of C3A the adsorption of both admixtures decreased with development of hydration products. On the other hand adsorption of salicylic acid by C4AF hydration products was proportional to the hydration time. But in the case of calcium lignosulphonate it was opposite. Further it was shown that C2S and C3S hydration products adsorb calcium llgnosulphonate more readily than salicyllc acid without any significant dependence on hydration time. Hansen et al /48/ concluded that the van der Waal forces play little, if any, part in the adsorptions of SA and CLS by either the anhydrous cement compounds or by the hydrated reaction products in an aqeuous medium. It has been pointed out /50/ that the crystals of the anhydrous compounds must contain a relatively large number of oxygen ions. Hence the formula
HOC6H4C / O H
for SA, they visualized that the leave in the surface of the crystal of the cement compound to form an adsorbed layer with the COOH group of the acid exposed to the liquid phase. The C-O group could then form a hydrogen bond with the OH group of another molecule of the acid. In this way several layers of this acid could be adsorbed by hydrogen bonding with oxygen ions in the surface of the crystals. At high pH prevailing in Portland cement during hydration, SA is dissociated and is present in carboxylate form HOC6H4COO-. Since the crystals of cement components are positively charged due to excess of partial valencies from predominently divalent calcium ions. These ions adsorb the negative ions of this acid (chemisorption). Ca 2+ might react as follows: Ca 2+ + 2(HOC6H4CO0) ~ Ca(OOC6H4OH)2 This adsorption mechanism might be the reason why the crystallization process is influence by SA. The surfaces of the crystals of hydrated reaction products such as C4AHI3, C3AH 6 and C3S2H 3 are also positively charged and thus can adsorb SA by the mechanism outlined above. Seligmann and Greening /51/ reported that some commercial'lignosulphonate
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887 POLYMERS, ADMIXTURES,
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CEMENT HYDRATION
admixtures retarded the reaction of C3S with water indefinitely. However, in the presence of alkalies a delayed set ultimately occurred. This led them to suggest that the alkalies reacted chemically with CLS and destroyed its retarding ability. Manabe et al /52/ reported that cements containing low C3A contents and high soluble alkali contents are sometimes retarded strongly by ordinary additions of CLS. The work by Sellgmann and Greening appears to show that the retarding power of CLS can be destroyed by alkalies in cement pastes, but the work by Manabe et al supports the conclusion that a chemical reaction of the alumina-bearlng phases with CLS and other organic retarding admixtures is the mechanism by which the retarding effect of such admixtures is destroyed in commerlclal cement pastes. The work by Danlelsson /53/ supports the conclusion that hydrogen bonding is not the only mechanism by which organic compounds are adsorbed by calcium silicates in cement pastes. CLS are almost spherical molecules of varying size containing sulphonate groups /54/. Some of these are pendent on the surface and may react with surface of the cement components. These CLS molecules are so large that similar reaction can take place independently on the opposite side of the molecule. This mechanism might explain the influence of CLS on the crystallization process. To summarize, with reactive polymers, substantial modification of the crystallization process occurs by the interaction of latex particles with the surfaces of the growing crystals. Cross-linking and insolubility of the polymer frequently accompany these reactions and the behavlour is substantially changed. There is a bonding between the modified polymer partlcles and the silicate surfaces. Besides, the alkalies (produced during the hydration of Portland cement) react chemically with CLS and destroy its retarding ability in the reaction of C3S with water, but in the presence of low C3A content the reaction becomes different. Conclusions Polymers and organic admixtures interact with the components of Portland cement when they come in contact with water. This interaction is due to the ionic bonding, causing cross-llnks which inhibit the film formation property of polymers and influence considerably the crystallization process during the hardening of concrete. Some low molecular weight organic substances also have a considerable influence on Portland cement during their reaction with water. The behavlour of the pure Portland cement components with water is very much different than when they are present together in Portland cement. Consequently the interactions of polymers as well as other low molecular weight organic compounds will behave differently with Portland cement than with its pure components. The mechanisms of interaction are not clear with these additives and the pure components. In this situation it is very difficult to explain the mechanism of interaction of these additives in Portland cement during hydration. Besides in the references available no study on the interaction between the components of Portland cement and one polymer and then the same polymer and cement. It will be very interesting to do a comprehensive study of such a system to enable answers to many questions in this important field. Acknowledgements The authors are thankful to Mr. T. Sugama and Mr. L.E. Kukacka for their permission to reproduce their line sketch diagram, to the Swedish Institute
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for Building Research for their grant and to Mrs A. Palmdin for preparing the manuscript. References i.
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