A review of the mechanisms of set-retardation in portland cement pastes containing organic admixtures

CEMENT and CONCRETERESEARCH. Vol. 2, pp. 415-433, 1972. Pergamon Press, Inc. Printed in the United States.

A REVIEWOF THE MECHANISMSOF SET-RETARDATION IN PORTLANDCEMENTPASTES CONTAINING ORGANIC ADMIXTURES J. F. Young Assistant Professor of Civil Engineering University of I l l i n o i s at Urbana-Champaign Urbana, I l l i n o i s 61801 USA

(Communicated by D. M. Roy)

ABSTRACT

Various mechanisms have been proposed to explain how organic admixtures affect the hydration of cement clinker compounds. These are reviewed and discussed c r i t i c a l l y . Complex formation between the organic compounds and aluminate or silicate ions may enhance the i n i t i a l reactivity of the anhydrous compounds. Set-retardation may be primarily due to retarding of the hydration of tricalcium s i l i cate through the adsorption of organic admixtures onto calcium hydroxide nuclei. Adsorption onto the i n i t i a l hydration products of tricalcium aluminate can also retard further hydration.

RESUM~ Diff6rents m6canismes ont 6te proposes pour expliquer l ' e f f e t des adjuvants organiques sur l'hydratation des composants du clinker de ciment. Dans le present article, ils sont passes en revue et discut6s. La formation de complexes entre compos~s or~aniques et ions aluminate ou silicate peut accroltre la r~activite initiale des compos~s anhydres. Le retardement de la prise est peut-~tre dO principalement au ralentissement de l'hydratation du silicate tricalcique par adsorption des produits organiques sur les noyaux de chaux hydrat6e. L'adsorption sur les produits de l'hydratation i n i t i a l e de l'aluminate tricalcique peut ~galement retarder la poursuite de l'hydratation.

415

416

Vol. 2, No. 4 SETTING, RETARDATION, ADMIXTURES, CEMENTPASTES Introduction Organic admixtures are now widely used in the cement and concrete

industry to prolong setting times, reduce water requirements, and entrain air. The two latter effects do not necessarily imply that the hydration reactions are significantly affected; but set-retarding admixtures must be profoundly influencing the early reactions of cement. Evaluation of admixtures has relied traditionally on measuring their effects on physical properties of concretes and mortars, and comparatively l i t t l e attention has been paid to the chemical changes that might be taking place in the cement pastes. However, cement chemists are becoming increasingly interested in the way admixtures influence the chemical reactions taking place in hydrating cement ( l - l l ) .

The most common

properties measured are the composition of the liquid phase, reaction kinetics (using conduction calorimetry or x-ray diffraction) and physical and chemical changes in the hydration products. The chemical and physical composition of cement is sufficiently complicated to make the investigation of its hydration reactions d i f f i c u l t enough without introducing other factors.

Thus a common approach has been to consider

the effect of admixtures on individual cement compounds, or simple mixtures. Many investigations have been concerned with the influence of admixtures on tricalcium aluminate (C3A*) which, as the fastest reacting component, will have a strong influence on the i n i t i a l setting processes. Investigations with sugars (12,13), lignosulfonate admixtures (14,15), and other organic species (16) have been carried out in recent years. However, increasing attention is being paid to the effects of both organic and inorganic admixtures on the hydration of C3S (3,4,6,7,18-21) since this is the major cementitious component present in cement and dominates the early strength development.

I t has been shown that

the retarders and accelerators act predominantly through their effects on the kinetics of C3S hydration (3-5,17). In the presence of C3S the role of C3A is primarily to remove the admixture from solution (22) and thereby prevent a strong effect on C3S. In spite of the lack of experimental data, several theories of the mechanism of retarders have, nevertheless, been formulated. These can be conveniently summarized as: and 4) Nucleation control.

l) Absorption; 2) Precipitation; 3) Complexation; Although such speculations might be considered

Cement chemistry notation is used throughout the paper. C = CaO; S = Si02; A = A1203; H = H20.

Vol. 2, No. 4

417 SETTING, RETARDATION, ADMIXTURES, CEMENTPASTES

premature, they serve as useful reference points from which to examine existing data and to plan further experiments.

In the remainder of this a r t i c l e , the

supporting experimental data and the consequences of each theory w i l l be discussed.

A more general review of the l i t e r a t u r e of admixtures has been publish-

ed recently (23). Hydration of C3S and C3A A preliminary discussion of the ways in which C3S and C3A react with water is pertinent to the consideration of the reaction of admixtures on portland cement, since these compounds are primarily responsible for the setting and early hardening mechanisms. Hydration of C3S

workers.

The hydration of C3S has been the subject of intensive study by many Comprehensive reviews of our current knowledge were presented at the

Fifth International Symposium on the Chemistry of Cements (24,25). The b r i e f discussion of the mechanism of hydration can be presented in terms of the conduction calorimetric curve of C3S (Fig. I ) .

• /-Stage

. _ _ _ St__~_~Tr

Stage TIT

W

Time

FIG. 1 Heat liberation curve of hydrating C3S as determined by isothermal conduction calorimetry. After Kondo and Daimon (23)).

418

Vol. 2, No. 4 SETTING, RETARDATION, ADMIXTURES, CEMENTPASTES Stage I represents the i n i t i a l hydrolysis of C3S. A rapid release

of calcium hydroxide into solution occurs, leaving an outer layer of "hydrated" o

C3S about lO0 A thick. This layer may well be a pseudo C3S structure deficient in lime, with only partially hydrolyzed ortho-silicate groups. In Stage II the reaction becomes self-retarding but a continual slow release of lime continues into the surrounding water, giving a highly supersaturated solution. Thin foils of C-S-H material appear at the particle surface, which are probably due to the polymerization of the hydrolyzed silicate groups and a general rearrangement of the "hydrated" layer, giving lower C/S ratios and higher H/S ratios. Stage I I I is marked by the i n i t i a l crystallization of solid calcium hydroxide and an acceleration in the hydration of C3S. These phenomena are accompanied by a change in the nature of C-S-H gel, and this transformation is considered to expose fresh surfaces of the anhydrous particle. As the new hydration product builds up, the reaction becomes diffusion controlled and starts the deceleration period, Stage IV. In the final Stage V, the system continues to react slowly with a very low rate of heat liberation. Hydration of C3A The C3A shows a rapid and strongly exothermic reaction with water. The heat generated by pure C3A pastes is sufficient to ensure that the sole reaction product is thermodynamically stable C3AH6. I f the temperature of the system is kept below 30°C, the metastable hydrates, C4AHI3, and C2AH8 are formed i n i t i a l l y , but at ambient temperatures transformation to C3AH6 eventually occurs. In the presence of gypsum, the reaction of C3A is retarded, due to the formation of a coating of ettringite (C3A-3CaSO4.32H20). Once the supply of sulfate is exhausted, the ettringite layer breaks down by converting to the monosulfoaluminate (C3A-CaSO4-12H20), and ultimately, to a solid solution with C4AHI9. The final hydration products are stable with respect to C3AH6. Theories of Retardation Absorption Hansen (27,28) has suggested that retardation is due to absorption of organic compounds onto the surface of cement compounds, thereby preventing attack by water.

He noted that there were two widely used classes of admixtures,

lignosulfonic acid derivatives and hydroxylated carboxylic acids, which contain

Vol. 2, No. 4

419

SETTING, RETARDATION, ADMIXTURES, CEMENTPASTES both carboxylic acid groups and hydroxyl groups. The former would be absorbed at surface Ca2+ ions, while the latter could bond simultaneously to adjacent 02- ions (Fig. 2a). Hansen's original idea of the importance of the H-C-OH groupings was extended by Steinour (29,30), who considered that any unionized hydroxylgroup would be able to hydrogen bond with oxygen ions on the surface, O~C

^/R

J jR

\

z'.

o

o

co

c/R

0/

0/

o

S u r l o c e ~A ~nh ~ydoro(us

(o)

(b)

Comp

(c)

FIG. 2 Possible adsorption mechanisms of organic admixtures onto cement surfaces. thereby inducing chemisorption. More recently, Taplin's (31) survey of retarders indicated that organic compounds with HO-C-C=Oentity were particularly effective. This would imply that Hansen's views on chemisorption are correct, but Taplin also found retarding action with compounds that contained aromatic HO-C-C-OH groupings (for example, catechol), or with compounds in which oxygen atoms would approach closely although not on adjacent carbon atoms (e.g., maleic acid). Furthermore, In alkaline solution, compounds such maleic and pyruvic acids have no hydroxyl groups and thus hydrogen bonding need not be postulated in every case. Therefore, chelation to surface (Figs. 2b or 2c) may be the most important mechanism of absorption. Calcium, aluminum, iron or silicon ions are all potentially capable of chelating with organic compounds. The foregoing discussion indicates that an absorption theory of retardation is feasible, but has not demonstrated that this is the actual cause. Quantitative absorption of admixtures onto cement clinker compounds have been measured in only a few instances (32-38). I t has been shown that lignosulfonates are strongly adsorbed onto portland cement (32-34) and that this is primarily due to strong adsorption onto C3A (34,35). The apparent adsorption of calcium ]ignosulfonate onto C3A (>150 mg/g) leads to an abnormal thickness of adsorbed molecules (see Table I).

Figures for otheradmixtures also correspond

420

Vol. 2, No. 4 SETTING, RETARDATION, ADMIXTURES, CEMENTPASTES

TABLE l Adsorption Data for Organic Admixtures on C3A and Portland Cement (aqueous solutions)

On C3A Admixture

Calcium lignosulfonate

On Cement

Molecular Maximum No. of" Area Adsorption Molecular Layers* (A°) 2 mg/g

Dosage Adsorption on C3At wt % mg/g

Refs

- 6250

> 150

> 300

0.2

20

Sucrose Glucose

70 40

> 50 5

> 200 20

0.05 0.05

5 5

17 39

Salicylic acid

40

60

0.2

20

38

lO0**

Specific surface: *0.3 m2/g **l.O m2/g. to unusually thick layers.

t

34,37

Assuming I0% C3A content.

I t appears that some chemical reaction between C3A

and organic admixtures is occurring.

Most adsorption studies have been carried

out in aqueous solutions where C3A will be hydrating simultaneously.

In non-

aqueous solvents the adsorptions of lignosulfonates (from dimethylsulfoxide) (36) and salicylic acid (from ethanol)(37) onto C3A are quite low and less than the adsorption onto the calcium silicates. ly on the hydration products of C3A (36,38).

These admixtures adsorb quite strongEvidence was obtained (36) for a

complex between C4AHI3 and calcium lignosulfonate, as has been reported earlier by Kawada and Nishiyama (35). Other organic complexes of C4AHI9 have been detected (4) in cement pastes containing large amounts of admixtures. Recent studies (16) have shown that the retarding effect of organic compounds on the hydration of C3A is closely linked with their adsorption into the structures of the metastable hexagonal calcium aluminate hydrates that are f i r s t formed. In paste hydration sorption has the effect both to inhibit crystal growth of the hexagonal hydrates and to inhibit their conversion to C3AH6. Consequently, very stable and impermeable sheaths of hydration products are considered to form around the anhydrous material and prevent further access of water. The a b i l i t y of organic compounds to do this roughly correlates with the total number of hydroxyl, carboxylic acid and carbonyl groups in the molecule, and therefore, sugars are particularly effective.

Vol. 2, No. 4

421 SETTING, RETARDATION, ADMIXTURES, CEMENTPASTES

Suzuki and Nishi (2) have observed that organic retarders can accelerate the i n i t i a l hydration of a cement paste before retardation begins. This behavior has been confirmed for C3A hydration in the presence of sugars (12, 15,17,39) and also for the hydration of C3S with lignosulfonates in the absence of l~me (17).

Small additions of sucrose act as an accelerator towards

C3A hydration without subsequent retardation (12).

Therefore, i t appears that

at least in the case of C3A, i n i t i a l absorption of sugars onto the anhydrous component accelerates hydration, while subsequent sorption into the hydration products retards i t .

I t must be noted that the behavior of admixtures towards

C3A does not completely parallel the behavior toward cement, where sulfate ions are present. The effect of admixtures on the interrelation of hydrate products in the C3A-gypsum-C3S system have not yet been investigated in detail. Adsorption of calcium lignosulfonate or sugar onto C3A is much reduced in the presence of gypsum (36) and ettringite morphology can be modified by sugars (13). The C3S and C2S adsorb organic molecules from aqueous solution much less strongly. The adsorption of calcium lignosulfonates follow a BET-type adsorption isotherm (36) indicating physical adsorption.

Even the amount ad-

sorbed (3-5 mg/g on C3S and I-2 mg/g on C2S) are sufficient to cover the surfaces with several molecular layers.

Yet, analyses of the liquid phase in

C3S suspensions containing other organic admixtures do not indicate a significant decrease in reactivity of C3S during the f i r s t few minutes of contact (39)(Fig. 3).

Subsequent depression of the calcium ion concentration by strong

retarders (sucrose and tartaric acid) indicate that the hydration kinetics are being affected.

Calcium lignosulfonate is more strongly adsorbed on prehy-

drated C3S (37,38,40) due to irreversible adsorption on both CSH gel and calcium hydroxide. Possibly adsorption of admixtures by the f i r s t hydration products forms an impermeable sheath as postulated for C3A. High concentrations of strong retarders can retard the hydration of C3S indefinitely unless either hydrated C3S or pure calcium hydroxide is added subsequently (40,41). The figures in Table l indicate that the adsorption of admixtures by cement, could be considered as being due wholly to adsorption by C3A. Nevertheless, i t is an established fact that set-retarding admixtures act primarily by retarding the hydration of C3S. Thus, although the strong adsorption properties of C3A will remove most of the admixture from solution (17,22) some must remain associated with C3S to provide retardation. The differential adsorptive properties are considered to be the cause of enhanced

422

Vol.

2, No. 4

SETTING, RETARDATION, ADMIXTURES, CEMENTPASTES 35.

I

1

I

1

I

I

I

I I

I

I

I

I

I

I

1

]

I I

I

No Admixture Sucrose Tarton'c Acid Succinic Acid

30 - -

D,,J~" I

!

25

I

0

o c.) %

IO o

/

20

E E

15 /

i0~

/

/

/

-- 1 1

/

/

/

/

/

J

__

ff I . . , i . - ~

®

I-----

08

"',,,

" ~ _

~--..........

J'

__.~'=,,,"~ . . . .

,

.,......,,,,">",,.

--....,.._

I

I

,o Time in Hours

FIG. 3 Influence of some organic admixtures on soluble CaO and SiO2 during early hydration of C3S. (l wt % additions) retardation observed for the delayed addition of an admixture (42-44).

Ad-

sorption of admixtures onto C3A is decreased, and probably increased onto C3S when a prehydration period is allowed. Adsorption onto C3A is decreased whether or not sulfate is present, so that, clearly the C3A-water-admixture system is very complex. There is thus no strong case for adsorption onto anhydrous surfaces being the mechanism of set retardation. Indeed, as has been shown for C3A, there is evidence for acceleration of i n i t i a l reaction. Surface absorption of organic molecules is possible without retardation being observed. Daxad-15 is a water-reducing admixture which only shows a slight retarding property (43). Purified lignosulfonates can lose much of their retarding power (15) and their strong adsorptive properties must be primarily responsible for the

Vol. 2, No. 4

423

SETTING, RETARDATION, ADMIXTURES, CEMENTPASTES

well-known plasticizing action.

Low molecular weight carbohydrates (not nec-

essarily sugars) could be responsible for the retarding action of commercial lignosulfonates.

The addition of sucrose to a purified lignosulfonate gave a

better set-retarding admixture than either component alone (15).

I t has also

been noted (43) that a long delay in the addition of lignosulfonate admixtures to mortars can restore workability without significantly affecting its setting time. Although adsorption by the anhydrous compounds, as originally proposed, is not considered to be the primary cause of retardation, i t is clearly an important parameter in the behavior of admixtures. Present data indicated that adsorption has an important bearing on the interaction between cement compounds during their hydration. Complexing Taplin (31) pointed out that effective retarders contained one or more oxy-functional groups in which the oxygen atoms are able to approach each other.

This fact implies that chelation to metal ions may be occurring and i t

has been suggested (12,13) that complexing might be an important factor in the mechanism of retardation. I t is well known that calcium ions can chelate with hydroxy acids, such as tartaric, c i t r i c and glyceric acids, and with dibasic acids such as succinic and malonic acids.

These complexes are not strong, having s t a b i l i t y

constants of the order of lO0 (45), and no correlation can be made between experimental s t a b i l i t y constants and the a b i l i t y to retard cement hydration (Table 2).

Sugars are also considered to form complexes with metal hydroxides

or salts (49), hence the high solubility of lime in sugar solutions.

However,

recent nuclear magnetic resonance measurements (50) have indicated that the stability of these complexes is very low, being of the order 0.2.

The low

stability constants and the low concentrations of retarders in solution mean that complexing reactions will not affect the calcium ion equilibria significantly. The high levels of calcium ions which are maintained in solution by the presence of carboxylic acids, such as formic or acetic, provide an accelerating rather than a retarding effect, and therefore complexing of calcium ions in solution is judged not to be an important factor. I t is also possible that aluminate ions can complex with sugars. is well known that the ionization of weak boric acid is increased by sugars and other polyol complexes. Thus, complexing can be followed by acid-base

It

424

Vol. 2, No. 4 SETTING, RETARDATION, ADMIXTURES, CEMENT PASTES

TABLE 2 Comparison of S o l u b i l i t i e s and S t a b i l i t y Constants for Calcium Salts of Organic Acids and t h e i r Retarding A b i l i t y in Portland Cement Hydration

Acid Radical [A]

S o l u b i l i t y in water at 20°C (46) g/lO0 g

mM/l

Stability Constants (Kl) for CaA Ref. 47

16.6

127

.

Acetate

34.7

220

3.4

~ 4

Propionate

39.85

214

3.2

.

6.2

28

ll.7

-

.

Ref. 48

Formate

Lactate

.

.

.

.

13.1

.

Glycollate

.

.

.

.

12.9

--

Fumarate

~

0.005

.

Ref. 31

None

None

None

None

.

.

.

" 6

.

~ 0.0006

Ref. 2

.

Glycerate Oxalate

Retardation of Portland Cement

.

.

None None

.

.

.

.

None Weak

Strong

Strong

None

None

1.5 (30°C)

~ lO (30°C)

--

3.2

--

None

Maleate

2.49(25°C)

16 (25°C)

--

12.9

--

Weak

Malonate

0.37

2.6

29

23

--

None

Succinate

1.28

8.2

16

lO

Weak

Weak

Malate

0.82

4.8

63

Tartrate

0.35

1.9

63

60

V.Strong Strong

0.2

--

1415

V.Strong Strong

Citrate Gluconate

~ 0.088 3.5

8

16.2

ll5

V.Strong Strong

16.6

--

Strong

t i t r a t i o n s (51) and formation constants for borate ion-sugar complexes are in the range Kl = 50 and K2 = 150. The interaction with the borate ion occurs through condensation with cis-hydroxylgroups (Fig. 4).

IHO OHTrHO O\ /R-~\.i I + ~o..~ __. P \,I o, I --HO /

OHJ

Borate Ion (Alkaline Solution)

The aluminate ion,

[R\c/O\B/O\cl/R ]\oA,,j

Ho/C'R ' Polyol

I:1 Complex

1:2 Complex

FIG. 4 Complexation between borate ion and polyols.

Vol. 2, No. 4

425 SETTING, RETARDATION, ADMIXTURES, CEMENTPASTES

AI(OH)4-, is known to have a tetrahedral structure analogous to the borate ion and therefore, one might expect complexes of comparable s t a b i l i t y to be formed with polyols.

There are only isolated reports (52,53) in the literature con-

cerning complexing with the aluminate ion because the insolubility and low basicity of its conjugate acid precludes measurements by the pH t i t r a t i o n technique used for the borate ion.

However, recent nuclear magnetic resonance ob-

servations of both IH and 27AI resonances have indicated (54) that there is appreciable complexing between the aluminate ion and sugars which is of a simi l a r nature to borate complexes. There is no reason why ~-hydroxy acids might not chelate similarly. The effects of complexing could be more marked in the case of the aluminate and ferrite ions because of their much lower concentrations in solution.

Several workers have shown (1,2,8) that additions of sucrose increased

the amounts of iron and alumina found in the liquid phase of a cement paste, although this increase is only marked when comparatively large amounts of sucrose are used. Roberts (8) also found calcium ion concentrations were increased above normal levels.

Sucrose has been shown to inhibit precipitation of cal-

cium aluminate hydrates from mixtures of calcium hydroxide and sodium aluminate (52).

Other admixtures also increase the amounts of alumina and calcium in

solution at early ages. Someexperiments with C3A (39) show that I wt % additions of sucrose, succinic acid and tartaric acid tend to increase the amounts of calcium and alumina (Fig. 5) in solution on f i r s t contact with water, but by ten minutes, concentrations decreased to normal, or below normal, levels. However, with tartaric acid, concentrations of calcium and alumina increased sharply between 15 and 45 minutes. This could be attributed to the formation of a very stable chelate with alumina, perhaps even Al (tartrate)33-.

Very

high concentrations of iron and alumina are found in solutions in the presence of triethanolamine, which is also a good chelating ligand. Increases of soluble silica have also been noted with additives that affect alumina, although the changes are not as great.

L i t t l e work has been

done with pure C3S hydrated in the presence of admixtures. Preliminary experiments (39) have shown that tartaric additions to C3S suspensions (Fig. 3) affect calcium and silica concentrations in a similar manner to C3A. Sucrose also gave an increase in the i n i t i a l solubilities of these elements, whereas succinic acid, which has no retarding action, showed very l i t t l e change from suspensions free of admixtures.

426

Vol. 2, No. 4

SETTING, RETARDATION,ADMIXTURES,CEMENTPASTES 40

I

I

I

I

I

N o Admixture __ESucrose ........ Tartoric Acid - ~ S u c c i n i c Acid ~

~----

:50 / I

_=

20

E

%%~%

| I

%% %%

!

\/

%,

IO

I

t

I

1

%%

iI i I

O.E

I "~

"N

O
N

\

%, % %

0.2

I

I

I

i

I

IO

20

30

40

50

60

Time in Minutes

FIG. 5 Influence of some organic admixtures on soluble CaO and Al203 during early hydration of C3A. (l wt % additions) Conditions in a cement paste are therefore favorable for complexing to occur between admixtures and the aluminate, ferrite and silicate ions, and this could well be the cause of the i n i t i a l increase in reactivity observed for cement and C3A hydration.

By preventing early precipitation of hydration

products, more of the cement compounds will be dissolved before hydration barriers are set up and admixtures removed from solution.

Although complexing of

the calcium ion can occur, most complexes are not strong and would not contribute appreciably to the supersaturation of calcium hydroxide during Stage II of the C3S hydration process. Precipitation Inasmuch as the formation of insoluble hydration products retards the

Vol. 2, No. 4

427 SETTING, RETARDATION, ADMIXTURES, CEMENTPASTES

subsequent rate of hydration of cement compounds by forming a barrier to water transport, solubility and precipitation are important factors in the setting and hardening of cement paste. Suzuki and Nishi (2) concluded on the basis of their experimental data, that organic retarders act by virtue of forming insoluble compounds when in contact with the strongly alkaline environment of a cement paste. They suggested that sugars and related compounds, and high molecular weight acids, (e.g., lignosulfonic acids) form insoluble complexes with alkalis, and also that those carboxylic acid admixtures which are retarders form insoluble calcium salts. The f i r s t explanation is in direct contrast with generally recognized fact that the solubility of metal hydroxides is increased in the presence of carbohydrates, and the subsequent precipitation is very slight.

Their conclu-

sion was based on the observations that suspensions of calcium hydroxide remove most of the carbohydrate admixtures from solution in the same way cement pastes do.

The large amount of solid calcium hydroxide, which is not present

in the early stages of cement hydration, could be leading to premature removal of the organic compounds, perhaps by adsorption (40).

Both Suzuki and Nishi

(2) and Dodson and Farkas (44) have shown that some precipitation of lignosulfonates with calcium hydroxide can occur, but Greening (41) has found that when lignosulfonate preparations are precipitated by lime, they do not show strong retarding properties. Secondly, Suzuki and Nishi (2) noted a correlation between solubility of the calcium salts of carboxylic acids and their retarding power. These data are represented in Table 2, together with additional data from the literature, and i t can be seen that the correlation often breaks down. The ineffectiveness of oxalic acid as a retarder, for example, is no doubt due to the insolubility of the calcium salt which completely removes the organic acid from the cement paste by harmless precipitation. Taplin's (31) correlation of retarding power with structure is a better guide. On i n i t i a l mixing, adsorption will be a very competitive process. Although some precipitation may occur, i t is probable that i t would be very d i f f i c u l t , i f not impossible, to distinguish between the two processes in cement pastes or suspensions.

I f the precipitation of admixtures were the main

cause of their retarding action, the process would be relatively nonselective, since both C3S and C3A release calcium ions rapidly into solution, whereas the effect of a retarding admixture on a cement is known to be a function of its C3A content.

Postulating selective precipitation onto the more reactive C3A

428

Vol. 2, No. 4 SETTING, RETARDATION, ADMIXTURES, CEMENTPASTES

surfaces would essentially be an adsorption theory.

Retardation by precipita-

tion cannot adequately explain the i n i t i a l increase in the activity that occurs in the presence of organic retarders.

Finally, precipitation is likely

to lead to higher water requirements rather than to the reduction which is a feature of lignosulfonate admixtures and which can be observed, in some degree, with most retarders. Nucleation The self-retarding feature of C3S hydration has been explained (41) as due to the inhibition of nucleation of crystalline calcium hydroxide by soluble silica which is present in small, but not insignificant, quantities.

If

the silicate ions adsorb onto the calcium hydroxide nuclei, growth will not proceed until some level of supersaturation is reached. This level is reached during the induction period (Stage I I ) , but only slowly, since further calcium hydroxide must be leached from C3S into a solution of increasing chemical potential.

Also, the buildup of C-S-H layer produces a diffusion barrier in the

system. Once the c r i t i c a l supersaturation level is reached, crystal growth recommences and the hydration rate increases. The adsorbed layer of silicate ions is trapped within the crystal by new crystal growth in the highly supersaturated solutions. Small quantities of silica are invariably found in calcium hydroxide formed in cement or C3S pastes (55,56).

The calcium hydroxide

grows in the form of short prisms since the silicate ions adsorb preferentially on the OOl faces of the crystal and growth rates are much slower than for other faces. Three observations are noted in the presence of organic retarder: l) lengthening of the Stage II induction period; 2) an increase in the level of calcium hydroxide supersaturation before crystallization begins; and 3) a higher rate of heat liberation in the Stage I l l acceleration period.

I t is con-

sidered (41) that organic compounds adsorb on the calcium hydroxide nuclei and poison their future growth in the same way as silicate ions are postulated to do. Organic admixtures have been shown (57) to influence the morphology and number of calcium hydroxide crystals formed in C3S pastes. Becausemore retarder molecules can be put in solution than silicate ions, and perhaps also due to better adsorption energetics, higher levels of calcium hydroxide supersaturation are required to overcome the effects of organic molecules. However, in the meantime, further nuclei are formed and are available for subsequent growth. The rapid hydration of C3S in Stage I I I . This acceleration of C3S hydration following the retardation period has been observed (21) in pastes

Vol. 2, No. 4

429 SETTING, RETARDATION, ADMIXTURES, CEMENTPASTES

containing calcium maleate. This theory envisages an adsorption step upon the hydration products rather than on the anhydrous surface and, thus, the molecular considerations of the molecular geometries that have been discussed earlier would be valid here. Such a hypothesis also conveniently explains the action of accelerating compounds, e.g., calcium chloride.

The addition of a soluble calcium salt pro-

duces a very rapid supersaturation with respect to calcium hydroxide and thus, advances the crystallization of solid calcium hydroxide without the need for calcium ions to move from the C3S structure into solution.

Crystal growth

occurs preferentially on the OOl face so that calcium hydroxide crystals init i a l l y form long prisms. Silicate ions will s t i l l be adsorbed on the OOl face but no longer affect growth kinetics under the prevailing conditions of very high supersaturation.

Soluble calcium salts having anions that can be adsorbed

on the calcium hydroxide surface should have a retarding action that counterbalances the acceleratory effects of high calcium concentrations.

Thus, cal-

cium salts, such as perchlorate, sulfate, or propionate, are weak accelerators (21), and there is evidence (7) that calcium nitrate becomes a retarder at high concentrations. This hypothesis is an interesting explanation of the effect that admixtures have on C3S. Besides the logical explanation of the features of C3S hydration, i t explains some related observations. The induction period lengthens in proportion to the amount of retarder added. Eventually the hydration of C3S is completely inhibited when about l percent of a strong retarding compound is added. However, the addition of prehydrated C3S to an inhibited paste can overcome the effect of the organic admixture. Discussion I t has been the purpose of this paper to try to delineate the validity and shortcomings of the various theories in the light of existing experimental data. Probably no one mechanism is sufficient to explain all aspects of retardation. At the present time, i t would seem that the most useful description would be one that allows the different processes to be occurring simultaneously. Therefore, one can visualize a mechanism along the following lines. The organic admixture may be f i r s t concentrated at the surface of the aluminate components by preferential adsorption. Rather than slowing down the i n i t i a l rate of attack by water, an increased reactivity is observed due to complexing between the organic molecules and the aluminate ions.

This increases the concentrations

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Vol. 2, No. 4 SETTING, RETARDATION, ADMIXTURES, CEMENTPASTES

of ions in solution, thereby allowing a higher solubility of the anhydrous compounds before hydration products precipitate, and may also increase the rate of dissolution of surface material.

Whenprecipitation of insoluble hy-

drate occurs, the crystal growth is modified by the admixture and this results in a more efficient barrier to further hydration than is the case without admixtures.

By being incorporated into the structure of the hydrated material

the admixtures are f i n a l l y removed from solution.

I t is not necessary to vis-

ualize the formation of new complex hydrates. Although i t is probable that the early hydration of C3A is most affected, the silicates and ferrites are probably influenced to a lesser extent in the same manner. Subsequent to this period of i n i t i a l reactivity, the retardation of C3S hydration and the length of the dormant period w i l l be determined primarily by the effects of admixtures on the nucleation of calcium hydroxide. Conclusion There is a clear need for further experimental work to obtain both chemical and physical data. Much valuable information is to be gained from an analysis of the aqueous phase during the early hydration reactions.

The inter-

relation between the cement components during hydration are s t i l l imperfectly understood. The effect of low C3A contents on retarder action is now well extablished (42,44,58) and other factors which influence retarder action are f e r r i t e (59), and alkali (58) contents.

In field experience of over-retarded

concrete (60) low sulfate contents were considered to be the major cause. Therefore, i t is important to gain an insight into how cement composition affects retarder action.

The fact that one is dealing with a material of variable com-

position undergoing a dynamic process makes i t d i f f i c u l t to get consistent data from a laboratory.

Nevertheless, by starting with simple systems and

working up to more complicated admixtures approximating to cement, i t should be possible to substantially increase our knowledge in this field. Acknowledgements The author wishes to thank Dr. R. L. Berger, Department of Civil Engineering, University of I l l i n o i s , and Mr. N. R. Greening of Portland Cement Association for their critical reading of the manuscript. Mr. Greening has kindly allowed publication of the author's interpretation of nucleation control based on their many interesting discussions.

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431 SETTING, RETARDATION, ADMIXTURES, CEMENTPASTES

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