Mechanism and kinetics of hydration of C3A and C4AF. Extracted from cement

CEMENT and CONCRETE RESEARCH. Vol. 14, pp. 238-248, 1984. Printed in the USA 0008-8846/84 $3.00+00. Copyright (c) 1984 Pergamon Press, Ltd.

OF

MECHANISM C3A AND C.

Department

of

AND C4AF.

KINETICS EXTRACTED

OF

and

Cabrera

Plowman

Civil

J.G.

Engineering,

Leeds

LS2

9JT,

The

HYDRATION FROM CEMENT.

University

of

Leeds,

England.

(Communicated by F.H. Wittmann) (Received July 21, 1983)

ABStRACt

The early hydration of C3A + ~AF extracted from cement and mixes with quartz, gypstzn and pulverised fuel ash (PFA) has been studied by x-ray diffraction. The investigation has shown that the hydration of both aluminates is essentially a mechanism which obeys a modified diffusion equation. The values obtained for the reaction rates show that the hydration of C3A takes place at seven times the rate of hydration of C~AF. PFA was shown to be a very effective retarder. A mechanism to explain retardation is also proposed. Introduction

Addition of gypsum during the grinding process for the production of cement is the industrial aeeepted method for controlling flash setting which arises from the rapid hydration of cement. It is accepted that the main constituent of cement responsible for flash setting is C~, and that gyps~n retards the hydration of this phase. What is still a matter of controversy is the manner in which gyps~n retards hydration; also there is very little information regarding the mechanism and kinetics of hydration of C3A and C4AF. The theories proposed to explain retardation of hydration can be broadly divided in two groups: (a)

The protective ]ayer theory: researchers s~oporting this theory disagree mainly on the composition of the protective layer. Earlier work in the 1950's (I) advanced the idea that the protective layer consisted of a thin layer of ettringite formed around the aluminate particles. Later work (2,3,4) including the investigations of Collepardi et al (5) have supported this theory. Other investigators, notably Gupta, Chatterji and Jeffery (6) came to the conclusion that the ir~0ervious layer consists of C4AH n which is formed on the al~rninate grains in the presence of CH and CSH 2 . They indicated that the thin C4AH n layer is overlaid with ettringite; when the ettringite cover becomes sufficiently thick, the mobility of the sulphate ion is restricted and thus the tetra-al~ninate will be converted to monosulphate instead of ettringite.

238

Vol. 14, No. 2

239

C3A , C4AF , HYDRATION, MECHANISM,

KINETICS,

FLY ASH

(b)

Incongruent dissolution of CBA and C4AF: Skalny and Tadros (7) studied the electrokinetic behaviour of C3A and came to the conclusion that retardation of the hydration process is due to the formation of positively charged C3A particles by adsorption of calcium, which reduces the active dissolution sites. They also suggested that C~H2 does not change the reaction path of the chemical processes which occur during hydration, but that adsorption of sulphate ions on the positively charged particles reduces still further the number of active sites. Forsen (8) postulated retardation to he due solely to sulphate, and his view was later supported by Felcknan and Ramachandran (9). They suggested that it is the adsorption of sulphate ions on C3A which is responsible for reduction of the rate of reaction. In the incongruent dissolution theories, the formation of ettringite is not related to the retardation effect. More recently Birchall et al (I0) have presented a mechanism based on the formation of an amorphous and gelatinous solid which acts as a continuous membrane around C3A grains. This os~notic mem~0rane allows water to diffuse inwards but stops the reaction proceeding until the intra-membrane pressure is sufficient to rupture the membrane. The manner in which the amorphous membrane is formed can then he explained either in terms of incongruent dissolution or in terms of a reaction of the type supported by Gupta et al (6). Boikova et al (ii) have recently studied the mechanism of hydration of C3A. They have suggested that hydration of C3A can be described by three distlnct stages: the first stage is characterized by a rapid intense reaction lasting i0 to 12 minutes, the second stage distinguished by a retardation of the hydration process lasting up to i00 minutes and finally a long term third stage with very little hydration. Boikova et al applied to their studies the kinetic empirical equation developed by Kondo and Ueda (12) for the study of C3S hydration. They limited their study to the second stage of the hydration of C3A and, in the light of their results, suggested that the seoond stage of the hydration of C3A is a diffusion controlled process. A problem of practical inportance is the effect of pozzolanic additives on the rate of hydration of C3A and C4AF since it is well known that the addition of pulverized fuel ash (PFA) to the constituents of concrete reduces the heat of hydration. Collepardi et al (13), investigating the effects of natural pozzolanas on the hydration of C3A , confirmed the known effect of reduction of heat evolution during the hydration process but also suggested that in addition to the reaction between lime and pozzolanas, some other reaction between C3A or its hydration products and the pozzolanas can occur. Cabrera and Plowman (14) have reported that pulverized fuel ash is an effective retarder of the hydration of C3A and C4AF. They have suggested that the mechanism of retardation of the hydration of the al~ainate is probably similar to the mechanism which operates when gypsun is used as a retarding agent. The object of the present paper is to present information obtained from an experimental study of hydration of C3A and C4AF using as retarders gypsum, quartz and PFA. The laboratory data is also used to study the mechanism of hydration and thus to determine quantitatively the rates of hydration of C3A and C4AF. Materials Most of the studies on the hydration of C3A and C4AF reported in the literature have used laboratory produced materials. However there are methods which can be used to isolate C3A and C4AF from ground ce~rent clinker. The calcium silicate phases and calcium oxide can be extracted by using either a

240

Vol. 14, No. 2 C. Plowman and J.G. Cabrera

TABLE Chemical

Composition

Oxide

1

of the ~aterials

Pulverised ~klei Ash

Used

Ordinary Port land cement

in the

Investigation

C 3A/C 4AF

SiO2

48.96

20.90

3.O5

AI20B

27.18

5.42

28.70

Fe20 3

iO. 85

2.70

Ii. 33

i. 57

64.90

48.90

CaO M@O

I. 30

2.23

3.52

K20

3.81

O. 86

O. 71

Na20

O. 78

O. 13

O. 16

Ti02

O. 93

O. 25

O. 62

SO 3

I. 25

2.68

O. 15

maleic acid/methanol solution (15,16) or a salicylic acid/methanol solution (17,18). Anhydrite told other calcit~n sulphates can be removed with ammonium chloride solution (19). During this investigation the altm~inates were obtained using the maleic acid/ methanol solution followed by the smmonium chloride solution. The products obtained at this stage cannot be hydrated. Both C3A and QAF are passivated by the maleic acid, but they can be reactivated by ignition at a t~nperature high enough to destroy the maleic acid. This method will not remove all of the SiO 2 content of the cement, since some of this is in solid solution with the C3A. Table 1 shows the oxide conposition of the final mixture of C3A and C4AF, and also the oxide composition of the original cement and that of the PFA used, The specific surface of the extracted material measured by the nitrogen adsorption method gave a value of 4.6m2/g with a mean particle diameter of approximately 2urn. The specific surface of the PFA was i.I m2/g with a mean particle diameter of approximately 8urn. Ground quartz, acid-washed, used as a diluting material had a maximum particle diameter of approximately 50~m. The hydration process was studied with five mixes, their ~mposition is shown in Table 2. The mixes were hydrated using a constant water/solid ratio of O. 7. Mix 3 contained an amount of gypst~n equivalent to a mininman addition of 2% to the original cement clinker which had approximately 25% content of C3A and QAF. Mix 4 contains an amount of gypstm~ equivalent to the amount of sulphate present in the PFA.

Vo I. 14, NO. 2

C3A, C4AF, HYDRATION,

241 MECHANISM, KINETICS, FLY ASH

TABLE 2 Composition of the Dry Mixes

Mix No.

C3A + C,AF mg

1

i000

2

Quartz mg

PFA mg

Gypsum mg

-

-

-

700

300

-

-

3

700

309

-

4

700

300

-

8.1

5

700

-

300

-

56

E ~ e r i m e . n t a.1 P.roce.dure s The course of hydrations was followed with a Philips APD-I~ ccmouter controlled diffractometer, with Cu Ka radiation at 45 kV, 55mA, 1 divergence and receiving slits, and a proportional detector fitted with a graphite monoc h r o m a t o r . T h i s was progranmed t o s c a n f r o m 5 u 2e t o 40 U 2e a t 16 28 p e r minute, r e c y c l i n g a f t e r each s c a n . One s c a n was c a r r i e d o u t e v e r y 2 . 5 minutes. Samples were p r e p a r e d by m i x i n g r a p i d l y b u t c a r e f u l l y t h e m a t e r i a l t o be h y d r a t e d w i t h t h e a p p r o p r i a t e anDunt o f d i s t i l l e d , d e i o n i s e d and f r e s h l y b o i l e d w a t e r . The r e s u l t i n g p a s t e was mounted i n a s a b l e h o l d e r o f an a u t o m a t i c sample c h a n g e r and i n t r o d u c e d t o t h e n i t r o g e n - f l u s h e d x - r a y diffractemeter. I f t h e sample was n o t i n t h e d i f f r a c t o m e t e r w i t h i n one m i n u t e o f t h e commencement o f m i x i n g , t h e sarq01e was d i s c a r d e d and t h e p r o cedure was repeated. The sample was maintained in a nitrogen atmosphere during the period of x-ray scanning. An initial investigation carried out prior to the present experiments showed that unless carbon dioxide is excluded, carbonation is very rapid. The ~ u n d C3A.CaCo 3.12H20 was observed after 5 minutes, and this almost completely suppressed the formation of hydration products for several hours. Comparison with external control samples showed that the rate of hydration was not affected by the san~le remaining in the x-ray beam continuously. As indicated before, the scanning speed necessary to follow the hydration process was very high. This precluded the possibility of quantitative analysis mainly due to relatively poor angular resolution, and to the reduced peak intensity (about 10%) resulting from the relatively long response time of the chart recorder. No attempt was made, therefore, to n~Lke an absolute measurement of the amount of a particular phase, but it was found to be perfectly feasible to measure the change in the intensity of a peak, even when quite a severe overlap exists. Results and Discussion Retardation Mechanism It is assumed that peak intensity of the x-ray diffraction trace is proportional to the volume of the crystalline compound being studied, thus the

242

Vol. 14, No. 2 C. Plowman and J.G. Cabrera

course of hydration can be followed by relating graphically the change in peak intensity with time of hydration. Figures I, 2, 3 and 4 show the hydration-time relations for mixes i, 2, 3 and 5. In general the amount of the altmdnates decreases with time, thi.s decrease is, as can be expected, very large for mix 1 and very shall for mix 3. The detection of C2AHs/C~AH n and later on C3AH 6 confirms the accepted view that the initial crystalline product of hydration is the hexagonal hydrate ~hich converts to the stable cubic C3AH 6 . Breval (20) in a scanning electron microscope and x-ray diffraction study on the hydration of laboratory s~n]thesised C 3A detscted irregular flakes of C2AH8 and C4AHI 3 after 6 minutes of hydration at 20 C. Hexagonal aggregates of C2AH 8 and C4AH 1 3 as well as crystals of C3~}I 6 were only seen after i00 hours of hydration. The develorrnent of hexagonal hydrates at 15 minutes can be seen for both the C3A/C4AF (mix I), and the C3A/C~AF with PFA (mix 5), in figures 5 and 6.

'

{,.

~.

C~AF C4AF

100

ZOO Time (minutes)

FIG.

300

100

1

Hydration-Time Relation + C4AF (mix I)

]00

2(10

Time (mmuhrs)

FIG 2 for C3A

Hydration-Time Relation for C3A + C4AF + 30~ quartz (mix 2)

1

1200 "~C~AF IOO~ B00. c

CpA C~,AF

600'

m ,m 400" C#A

50

100

1S0 260 11N (minutes)

2~0

300

FIG 3 Hydration-Time Relation for C3A + C AF + ~ quartz + 56 mg gypsum

(mi~ 3)

3~

100

Time (~.ufes|

FIG 4 Hydration-Time Relation for C3A + C4AF + PFA (mix 5)

Vol. 14, No. 2

243 C3A , C4AF , HYDRATION, MECHANISM, KINETICS, FLY ASH

The most noticeable difference bet~een the hydration-time relations is the time at which the stable cubic hydrates appear. For the pure C3A + C~AF the time is less than 15 minutes. This is interpreted as rapid conversion of the metastable lamellar form to the stable cubic fore. It is thought that in the presence of water the conversion takes place at a rate which is reflected by the decrease in the volume of the alt~ninates. The reaction can then be written as: •C2A~I s } + 2C 3AH6 2C3A + H ÷ IC%~in The C~,%H ohase is unstable in the presence of water but can be stabilised with decreasenin the quantity of available water. Therefore the lamellar C. AH which is formed around the aluminate grains can be viewed as a protective n coating which, if not converted to the more permeable cubic packing will retard the hydration of the aluninates. In the case of mix i, figure 5 shows that C3AH e is detected very early, while in mix 5 (fig. 6) it appears later. In the case of mix 3 which contains 56rag of gypsun, C3AH6 is not detected for the duration of the experiment. In the presence of gypsum or in the presence of sulphate and calcium ions (mix 5) the path of the reactions taking place can be sinplified as follows:

~{3C4AHn(~) ÷

3C4AHn_x (~)

+ 4C3A + H + S

AH 3 C3A3CS3]-H ÷

C3AC~I2H

The hydrates C,AH (6) and C4AH (S) are non-stoichicmetric, and are considered to incorporate variable anoints of sulphate in the hexagonal structure. They are analogous to the confound described as "C4AH13 containing essential carbonate" which has been reported in the literature ( 5 ). The degree of disorder introduced into the crystalline lattice by the incorporation of sulphates, inhibits recrystallisation to C3AH~. The mechanism of retardation can be explained by the action of the protective coating created by the amorphous AH 3 and the lamellar C.AH (S) and not, as been proposed by many (2,3,4,5), by a protective layer 8 c m ~ e d of ettringite. It seems reasonable to suggest that the formation of ettringite results in a reduction of the amount of water availshle in the vicinity of the hydrating compounds and thus it reduces or retards the formation of the cubic C3AH e . This mechanism of retardation is similar to the mechanism proposed by Gupta et al (6) but it clarifies further the actual role of ettringite. The i~portance of postulating that ettringite acts as an inhibitor of water availability is that the mechanism of retardation can be generalized for inorganic and organic compounds. Young for exanple (2.]) has explained the effect of retardation of C3A hydration by organic cc~0ounds, postulating that the conversion from C4AH to C^AH 6 is hindered by the organic coa~0ounds; Sakai et al (22) indicated ~hat o~ganic ~ u n d s retard C3A hydration by preventing contact between water and C3A, and showed that the effectiveness of the organic molecules in retarding hydration was related to their ability to repel water from the vicinity of the hydrating materials.

244

Vol.

14, No. 2

C. Plowman and J.G. Cabrera

j

1000 Hours

Hours

~

~

~

4

~

~

~u 200 Hours

~

.J

~

.J

20 Hours

C~

C AF C,AH, j ~

~

CsAH~

I

20

,

,

,

.

,

I lS

,

Degrees.

,

,

~

B

Hydration

~

~

hydrotes

2 Hours

,

Hexogonol

•

~ %~ , , ~ J

10

,

'

'

~

•

0 Z5 Hours

o.,5 H....

'

I

20

,

,

,

,

J

15

2e

,

h

Oegr'ees,

J

,

I

10

J

2~

FIG 6

FIG 5 XRD traces C3A + C4AF

B

Hexogonol

~ I~~ . ~ . , ~

~

~

20 Hours

for the hydration (mix i)

of

XP~ traces C3A + C~AF

for the hydration + PFA (mix 5)

of

Kinetics

The investigation in this paper w~s limited to the study of the mechanism and kinetics of hydration during the early part of hydration, i.e. during the initial 30 minutes. The authors of this paper agree with Boikova et al (II) with reference to the three distinct stages which may be observed during the hydration process, thus this study deals only with the first stage for which there has been no quantitative dnta available. Various models for solid state reactions have been proposed to quantify the hydration processes in c~nent or its constituents (12,23). During these studies two possible mechanisms were investigated, the dissolution model (24) and the diffusion model originally suggested by Jander (25). The experimental results did not fit the dissolution model but gave very close results with the values obtained using a modified diffusion equation, 1 i.e. (I-

(l-x)~)

2 = Kt - C

whe re: K = 2k/ (0.5d) 2

k = rate constant d = diameter

of reacting

t = time of hydration

sphere

Vol. 14, No. 2

245 C3A , C4AF , HYDRATION, MECHANISM, x = fraction C = empirical

which

has

KINETICS, FLY ASH

reacted

constant

The value of K can be taken as proportional to the rate constant and therefore when plotting time against the first tenn of the modified diffusion equation expressed aS f(x), the slope of the straight line corresponds to the rate constant. The lines obtained are shown in Figures statistical analysis. The corresponding equation are shown in Table 3.

7 and 8. These lines were fitted by constants for the modified diffusion

The values of K show clearly the extent of the differences in rate of hydration of the different systems (mix 3 is not shown here since the hydration is completely suppressed (Figure 3) ). The influence of PFA on the rate of reaction of C3 A is far more effective than the one shown for a greater quantity of gypsum. In fact, the rate constant decreases approximately 7 times and hydration is practically suppressed during the initial 7minutes. This finding is very significant from the point of view of using PFA materials for the production of blended cements. TABLE Numerical

Values

3

for the Constants of the Modified Diffusion Obtained by Statistical Analysis

C3A

Equation

C~AF

Mix No. K

C

*r 2

K

1

4.945 x lo- -i.198 x i0-

2 4 5

3.o0o x lO-~ 6.c~0 x lO-~ o . e o 1.000 x lO-~. 2.632 x IO-~. 1.fIX)x IO-~ 0.94 8.405 x IO-~ 6.705 x IO -~ 4.680 x iO -~ 0.78 1.230 x 10 -2

* r = coefficient The significance

0.95 8.072 × i0-

of correlation level was for all cases

C

-l.5n

r2

i0-

0.91

3.000 x lO-!~ 0.85 1.000 x IO-~ 0.95 1.O30 x IO -~ 0.87

< 0.001

The effect of quartz used as a diluent is one of reducing the rate and the conTnencement of hydration of C3A , these changes are modest when ccmpared with changes occasioned by PFA. It should be pointed out that this finding does not agree with the findings reported by Holten and Stein (26). The rate constants for the hydration of C4AF are, as can be seen in Figure 8, far smaller than for C3A; this agrees with other findings but quantifies the extent of the differences, i.e., for pure C3A the rate constant is approximately six times higher than for C4AF. On the strength of the statistical analysis which clearly shows the relation between the modified diffusion equation and the experimental data, it is proposed that the early stages of the hydration of C3A + C4AF for the different mixes investigated follows a mechanism which can be described as diffusion. The study is continuing, to ascertain whether the other stages of the reaction follow the same meehani~n using the same sir~01e expressions or if they follow other models.

246

Vol. 14, No. 2 C. Plowman and J.G. Cabrera

15-

Hix 1

15-

10-

I@

5'

5

0 X X

/

S

~

~Hix5

1~

is

2'o

2's

3'0

Mix 2 Hix 1 Hix 4

3's

0

t. minutes

FIG 7 Relation Betv~en the First Term of the Diffusion Equation f(x) and the Time of Reaction for C3A

5

10

15 20 f. minutes

25

30

3'5

FIG 8 Relation Between the First Term of the Diffusion Equation f(x) and the Time of Reaction for C4AF

The effectiveness of PFA as a retarder of the hydration of the aluminates can be explained by the extremely rapid release of both SO~- and Ca 2+ ions into solution. This has been reported elsewhere (27). Furthermore, the shape and surface properties of PFA are suitable to produce dispersion of the alou+m+inates during the mixing process so that the distribution of SO~- and Ca 2 ions is far more efficient than when using only equivalent quantities of gypstm in the solution as has been shown for mix 4 (Table 3). Conclusions 1.

This investigation has shown that the study of hydration during the initial stage can be carried out by an x-ray diffraction technique with an acceptable degree of accuracy. The use of C3A + C4AF directly extracted from ordinary Portland cement is preferable to the use of synthetic materials. 2.

The results obtained during this study indicate that the retardation of hydration occurs due to the formation of a protective coating consisting mainly of C 4 A H x(~) which is stabilized by the incorporation of sulphate and the reduction Gf available water when ettringite is formed in the system. Stabilization of the hexagonal hydrates by deficiency of water in the vicinity of the hydrating system is a mechanism which has been invoked to explain the effect of organic retarders; thus it can be generalised to explain the retardation process. 3.

The early stages of the hydration process, with a duration of approximately 30 minutes, obey a diffusion mechanism which can be quantified by a modified diffusion equation.

Vol. 14, No. 2

247 C3A, C4AF, HYDRATION, MECHANISM, KINETICS, FLY ASH

4.

The rate constant for C3A hydration is approximately six times larger than the rate constant for C4AF.

5.

PFA is shown to be a very effective retarder, in fact the rate constant for the mix containing PFA was approximately seven times smaller than the rate constant for the pure altmlinates.

References

i.

H.H. Steinour, Res. Dev. Labs. Portland Cement Assoc. Res. Dept. Bull.

98, 124, (1958) 2,

N.N. Stein, Silicate

3.

P. Seligmann

4.

H.E. Schwiete, U. Ludwig and P. J a e g e r , Highway Research Board SR 90, 353 (1966)

5,

M. Collepardi,

6.

P. Gupta, (1973)

7.

J . Skalny and M.E. Tadros, Jour. Am. Cer. Soc. 60, 3-4, 174 (1977)

8.

L. Forsen, Proc. of the Symposit~n on the Chemistry of Cement, Stockholm, Sweden 298 (1938)

9.

R.F. Feldman

I0.

Industry 28, 3, 41 (1963)

and N.R. Greening,

HRR. 62, 83 (1964)

G. Baldwin and M. Pauri,

S. Chatterji

and J.W. Jeffery,

and V.S. Ramachandran,

Jour. Am. Cer. Soc. 62, 33 (1979) Cement Technology

4, 4, 146

Mag. of Conc. Res. 18, 57. 186 (1966)

J.D. Birchall, A.J. Howard and D.D. Double, Cem. Con. Res. iO, 2, 145

(1980) ii.

A.I. Boikova, I. Daw~nsky, V.A. Paramonova, G.P. S t a v i t s k a j a and V.M. Nikushchenco, Cem. Con. Res. 7, 247, (1977)

12.

R. Kondo and S. Ueda, Proc. Vth Syn~p. on the Chemistry o f Cements, p. 203 Tokyo (1969)

13.

M. Collepardi, S. Monozi, G. Moriooni and M. Corradi, Cem. Con. Res. 9, 431, (1979)

14.

J.G. Cabrera and C. Plovamn, Proc. V I I t h I n t . Congr. Chem. Cem. 3, IV-85 P a r i s (1980)

15.

J . E . Mandar, L.D. Adams and E.E. Larkin, Cem. Con. Res. 4, 533 (1974)

16.

R. Tabikh and R. Weht, Cem. Con. Res. i, 317 (1971)

17.

S. Takashima, Rev. 12th Gen. Meeting, Cem. Ass. Japan, 12-13, Tokyo (1958)

18.

W.A. Gutteridge, Cem. Con. Res. 9, 319 (1979)

19.

K. Mather, Adv. X-ray Anal., 20, 41 (1977)

248

Vol. C. Plowman and J.G.

14, No. 2

Cabrera

20.

E. Breval,

Cem. Con. Res. 6, 129 (1976)

21.

J.F. Young, Jour. Amer. Cer. Soc. 53, 65 (1976)

22.

E. Sakai, K. Raina, iO, 311 (1980)

23.

J.M. Ponlnersheim

24.

E. Raask and M.C. Bhaskar,

25.

J.G. Cabrera and C.A. Nwakanma,

26.

C.L.M. Holten

27.

J.G. Cabrera and C. Pl~mn, iii, Leeds, U.K. (1982)

D. Asaga,

S. Goto and R. Kondo,

and J. Clifton, Cem.

and H.N. Stein,

Cem. and Con. Res.,

Cem. and Con. Res., 9 (1979) and Con. Res. 5, 363, (1975) Transp.

Res. Rec.

Cem. and Con. Res., Proc.

702, 199 (1979) 7, 291 (1977)

Int. Syrup. Use of PFA in Concrete,

I,