Hydration of tetracalcium aluminoferrite in presence of lime and sulfates

CEMENT and CONCRETERESEARCH. Vol. 6, pp. 441-454, 1976. Pergamon Press, Inc Printed in the United States•

HYDRATION OF TETRACALCIUM ALUMINOFERRITE IN PRESENCE OF LIME AND SULFATES

Inam Jawed*, Seishi Goto and Renichi Kondo Tokyo I n s t i t u t e o f Technology Tokyo, Japan

(Refereed) (Received Dec. 2, 1975; in final form March 16, 1976)

ABSTRACT The f i r s t p a r t o f the paper d e s c r i b e s conduction c a l o r i m e t r i c and SEN s t u d i e s o f the i n i t i a l f i r s t hour h y d r a t i o n o f C4AF and C3A with water and saturated solutions of lime, gypsum and gypsum with lime. Lime accelerates while gypsum and gypsum with lime strongly retard the hydration of C4AF. In case of C3A , the effect is less pronounced. The second part deals with the hydration of C4AF at later stages in presence of various additives and the same results as above are obtained. Anhydrite has very little influence while the presence of C3A reduces the effect of gypsum and hemihydrate on hydration of C4AF. A detailed investigation of the hydration process by means of X-RD, DTA, SEM and calorimetry has also been made. Der erste T e l l des Papiers b e s c h r e i b t k o n d u k t i o n s - k a l o r i m e t r i s c h e und SEM-Studxen zn der i n i t x a l e n e r s t s t u n d x g e n Hydratxon yon C4AF und C3A mzt Wasser und gesattxgten Kalk-, Gxps- und Gxps-KalklSsunge . Kalk beschleunigt die Hydration yon C4AF , w~hrend Gips und Gips mit Kalk diese stark verzogern. Im Palle yon C3A ist der Effekt weniger ausgepr~gt. Der zweite Tell behandelt die Hydration yon C4AF in sp~teren Stadien in Gegenwart verschiedener Additiva, mit denselben Ergebnissen wie oben. Anhydrit hat sehr wenig Einfluss w~hrend die Genenwart yon CsA den Effekt yon Gips und Hemihydrat bei der Hydration yon C4AF reduziert. Eine detaillierte Untersuchung des Hydrations-prozesses mittels verschiedener Techniken wurdeebenfalls durchge f4]hrt. •

•

•

•

Cf

¢!

"

.

•

"

"

*UNESCO p o s t - d o c t o r a t e f e l l o w . Present a d d r e s s : I n s t i t u t e of Physical Chemistry, Peshawar University, Pakistan.

441

n

442

Vol. 6, No. 4 I. Jawed, S. Goto, R. Kondo Introduction

Ordinary portland cement contains 8 to 12% of the ferrite phase. This phase of approximate composition C4AF , is known as tetracalcium aluminiferrite. In portland cements it has not been considered to contribute significantly to the strength. However, new types of cements, produced by the use of iron and steel slags and containing much higher percentage of ferrite phase, have shown strength development not inferior to ordinary portland cement (I]. This kind of cement reduces the fuel consumption to about half with production of a kiln doubled. Hence understanding of mechanism of hydration of the ferrite phase deserves more attention than hitherto has been given. There seem to be fewer reports on the hydration of the ferrite phase than other cement phases, as summarized by Cooeland and Kantro (2), Ludwig (3) and Kalousek (4), and some of them seem to be at variance with each other. It was, therefore, considered useful to make a detailed and systematic study of the hydration of C4AF so as to arrive at a better understanding of the mechanism of the processes involved. This study is comprised of two parts. The first part is concerned with the early hydration of C4AF in the initial first hour in saturated solutions of various additives as studied by means of conduction calorimetry and scanning electron microscopy (SEM). CsA was also included in this study to serve as a reference. The second part deals with the hydration process at later stages and has been studied by means of conduction calorimetry, powder X-ray diffratometry (X-RD), differential thermal analysis (DTA) and scanning electron microscopy (SEM).

Experimental C4AF was synthesized from CaCO 3, AI203 and Fe203 of high purity in the proper molar ratio at a temperature of about 1520°C for three firings each of about three hours with intermediate grinding. The purity of the product was checked by X-RD and free lime determination (5]. The final product was ground in a ball mill to a Blaine surface of 3200 cm2/gm. C3A was similarly prepared by heating the proper molar mixture of CaCO 5 and AI203 to a temperature of about 1380-1400°C and then ground to a Blaine surface of 3000 cm27gm. Gypsum, hemihydrate and quick lime were commercial products of high purity and anhydrite was prepared by heating gypsum to a temperature of 600°C for about four hours. In the first part of our investigations, the first hour hydration was studied by means of conduction calorimetry at a temperature of 20°C with W/S = 1.0. Three grams of C3A or C4AF was mixed with water and saturated solutions of gypsum, lime and gypsum and lime. For SEM studies, sintered pieces of C4AF or CsA were treated with water or saturated solutions of the additives, mentioned above, for 50 seconds, five minutes and 30 minutes. The reaction was stopped by acetone and the samples were D-dried for about 48 hours. In the second part of our investigations, hydration of C4AF was studied using solid gypsum, hemihydrate, anhydrite, lime and CsA. Calorimeteric studies were made at 20°C with W/S = 1.0. X-RD measurements were made on wet pastes. For DTA and SEM studies, the samples were D-dried for about 48 hours after stopping the hydration by acetone. Samples were collected after the following periods of hydration: 1 hr., 4 hrs., 8 hrs., 12 hrs., 16 hrs., 20 hrs., 24 hrs., 28 hrs., 52 hrs., 3 days and 7 days. Following systems were studied:

Vol. 6, No. 4

443 C4AF, HYDRATION, LIME, GYPSUM C4AF - CaSO4.2H20 - H20 C4AF - CaSO4.1/2H20 - H20 C4AF

-

CaSO4 - H20

C4AF

-

CaO

-

H20

C4AF - CaSO4.2H20 - CaO - H20 C4AF - CaSO4.2H20 - CsA - H20 C4AF

-

CaSO4.1/2H20

-

C4AF - CaSO4.2H20 -

C3A - H20

CaSO4.1/2H20

-

C3A - H20

Results and Discussion Hydration during the First

Hour

Table 1 and figures 1 and 2 show the results of calorimetric studies of hydration of C4AF and C3A under the influence of various additives in the first hour. Gypsum a l o n e i s seen t o have a marked r e t a r d i n g e f f e c t on t h e h y d r a t i o n o f C3A as i s a l r e a d y w e l l e s t a b l i s h e d . However, lime o r lime and gypsum a r e seen to have no marked r e t a r d i n g e t f e c t whereas it is g e n e r a l l y c o n s i d e r e d t h a t C3A h y d r a t i o n i s r e t a r d e d by lime o r gypsum and lime ( 5 ) ( 6 ) . Present data correspond to saturated solutions of the various additives whereas i n l i t e r a t u r e t h e s e a d d i t i v e s a r e g e n e r a l l y r e v o r t e d t o have been used as s o l i d s and, t h e r e f o r e , formed s u p e r s a t u r a t e d s o l u t i o n s . We used s a t u r a t e d s o l u t i o n s t o e n a b l e us t o compare and c o r r e l a t e c a l o r i m e t r i c r e s u l t s with the SEN data. Perhaps the supply of Ca 2+ and S042- ions was limited in our experiment and, therefore, the retarding effect is not so strong as cited in literature.

For C4AF the results are in contrast to those for C3A. Here lime is found to act as an accelerator while gypsum or gypsum and lime solutions retard the hydration very strongly. This results is in disagreement with the suggestion of Lea (7) that C4AF hydration in saturated lime solution proceeds slower than in water. Bobrov and Shikiryanski (8) have concluded that lime does not change the rate of interaction of C4AF with water. Schwiete (9), however, has noted that C4AF hydration proceeds much faster in saturated lime solution forming hexagonal hydrates. Sudo, Akiba and Nomi (i0) have noted a TABLE 1 Heat l i b e r a t i o n

in the i n i t i a l

first

hour

C a l o r i e s p e r gram Phase

Saturated solutions

of

H20

Gypsum

Lime

Gypsum + Lime

C3A

75.3

56.3

7"7.7

71.0

C4AF

59.7

2.7

64.3

0.3

444

Vol. 6, No. 4 I. Jawed, S. Goto, R. Kondo

150

1S0

,j

~100

~ o

100

$0

~

CaSO4

50

sol. SO4 s o l .

0

-

0

I~

0

-

20

30

~0

SO

60

Time(min)

FIG. 1 Conduction calorimetric curves of C4AF hydrated with water and saturated solutions of gypsum, lime and gypsum with lime.

i

0

10

i

I

*

20 30 40 Time (min)

FIG.

i

50

60

2

Conduction calorimetric curves of C3A hydrated with water and saturated solutions of gypsum, lime and g3rnsum with lime.

trend similar to our results although not as strong. In their studies, lime initially shows a retarding effect, but, after about 40 minutes, begins to accelerate the hydration. They, however, used these additives as solids. We attempt to correlate our calorimetric data with our SEM results. Fig. 3(a-t) shows the scanning electron micrographs of our samples. It may be noted that these samples were sintered C4AF and C3A immersed in saturated solutions of various additives. C3A is known to form hexagonal hydrates immediately on contact with water as previously deseribed 66) although not the entire surface is covered with hydration products after 30 seconds as shown by fig. 3a. After 5 minutes the entire surface can be seen to be covered [fig. 3b), and the hydrates become large to about 2-3 ~. C4AF , on the other hand, does not show any hydration product on the surface even after 30 minutes (fig. 3c-e) although calorimetric data [fig. 1 and table I) show that C4AF hydration is comparable to that of C3A. Ono, Suzuki and Goto (II) have reported that after 15 minutes of hydration C4AF becomes coated with CAFH gel which after 30 minutes turned to hexagonal plates. Chatterji and Jeffery [12), however, noticed the Formation of thin but well-formed hexagonal hydrates after 4 minutes hydration. With lime solution, hexagonal hydrates can be readily seen on the surface of C4AF [fig. 3h-j). After 5 minutes the surface gets entirely covered and the hydrates increase from about 5 ~ to about I0 ~ after 30 minutes. In case of C3A [fig. 3f-g), the surface gets densely covered with very small hexagonal hydrates of less than 1 ~, which grow to about 2 ~ after 5 minutes. As can be seen the hydrated surface on the C4AF crystals is rather loosely packed and has rather larger Dotes which can account for the less retarding effect of lime on the hydration of C4AF as compared to C3A.

Vol. 6, No. 4

445 C4AF, HYDRATION, LIME, GYPSUM

a, b~ c,

d,

C3A e: C4AF

+ water + water

LJ, a.

30 sec.

b.

5 min.

c.

30

d.

5 min.

f.

h.

sec.

30 s e c .

g.

30 s e c .

i.

Fig. 3

e.

30 m i n .

f,

g ~

h,

i,

C3A + CH s o l n .

j:

C4AF + CH s o l n

5 min.

5 min.

j.

SEM of early hydration Droducts.

30 min.

O~

~f

o

~q

~n

Dq

o~

0

b~

0

0

0~

c~

'1:1

~

C3

0 ~a

C3

e~

0 W.a

C3

C3 031 ÷

C%

~e b4

pao

0

O~

~n

0

t~ C~

0

h3

0

C3

0 e~

.

0 ~-~

÷

C3

0

0

0

O

O~

C.~ OJ :E el)

z 0

O~

0 __J

Vol. 6, No. 4

447 C4AF, HYDRATION,LIME, GYPSUM

In gypsum s o l u t i o n , t he C4AF s u r f a c e becomes 9 a r t l y covered with small e t t r i n g i t e c r y s t a l s a f t e r 30 secounds, and a f t e r 30 minutes t h e amount and s i z e o f e t t r i n g i t e i n c r e a s e s ( f i g . 3m-o). S t i l l , t h e pores are not f i l l ed by h y d r a t e s i n d i c a t i n g a l e s s e r e x t e n t o f h y d r a t i o n . In case o f C3A, l a r g e amount o f hexagonal h y d r a t e s can be n o t i c e d a f t e r 30 secounds and a f t e r 5 minutes e t t r i n g i t e i s a l s o found t o avpear ( f i g . 3 k - l ) . This may be exp l a i n e d as due t o t he high s o l u b i l i t y o f C3A. On c o n t a c t with s o l u t i o n l a r g e amount o f C3A goes i n t o s o l u t i o n producing hexagonal h y d r a t e s which form a c o a t i n g on t he unhydrated C3A and c o n s i d e r a b l y slow down the d i f f u s i o n o f ions from C3A i n t o s o l u t i o n . The h y d r a t e d l a v e r then r e a c t s with SO42- ions in s o l u t i o n t o form e t t r i n g i t e . The s o l u b i l i t y o f C4AF i s v e r y low and hence e t t r i n g i t e i s t h e f i r s t p r o d u c t formed immediately on c o n t a c t with s o l u t i o n as t h e r e i s l i t t l e amount o f C4AF p r e s e n t in the s o l u t i o n to r e a c t with SO42i o n s . The h y d r a t e d l a y e r o f e t t r i n g i t e , being more dense, i s more e f f e c t i v e in r e t a r d i n g f u r t h e r h y d r a t i o n than t he l a y e r o f hexagonal h y d r a t e s .

With solution of gypsum and lime, C4AF is found, from calorimetric data, to be strongly retarded. However SEM data (fig. 3r-t) shows very little amount of hexagonal hydrates even after 5 minutes, although, after 30 minutes, the surface is covered with the hydration products. Chatterji and J e f f e r y (12) r e p o r t t he appearance o f some e t t r i n g i t e a f t e r 4 minutes h y d r a tion. S t u d i e s o f J ~ g e r , Ludwig and Schwiete (13) i n d i c a t e e t t r i n g i t e as main h y d r a t i o n p r o d u c t t o ~ e t h e r with small amounts o f monosulphate h y d r a t e and i t s mixed c r y s t a l s with C4AH19 as soon as water i s added. Our d a t a , however, do not show t h i s . In case o f C3A, our r e s u l t s show t h a t t h e s u r f a c e g ets almost e n t i r e l y covered with r a t h e r d e n s e l y packed hexagonal h y d r a t e s w i t h i n 30 seconds which grow l a r g e r in s i z e a f t e r S minutes ( f i g . 3p-q). Hydration at L a t e r Stages In a n o t h e r s e t o f e x p e r i m e n t s , h y d r a t i o n o f C4AF in t h e p r e s e n c e o f s o l i d gypsum, h e m i h y d r a t e , a n h y d r i t e , lime and C3A was s t u d i e d . R e s u l t s o f t h e c a l o r i m e t r i c s t u d i e s a r e shown i n f i g s . 4-6. Both gypsum and hemihydrate a c t as r e t a r d e r s and t h e time f o r t h e appearance o f main h y d r a t i o n peak i n c r e a s e s with t h e amount o f t he a d d i t i v e . However a comparison o f our r e s u l t s (temp. 20°C) with t h o s e o f de Keyser and T e n o u t a s s e (temp. 25°C) (14) and Sudo, Akiba and Nomi (temp. 27°C) suggest s a v e r y marked i n f l u e n c e o f t e m p e r a t u r e on t he r a t e o f h y d r a t i o n . We a l s o n o t e from our r e s u l t s t h a t hem i hydr at e has a s l i g h l y l e s s r e t a r d i n g e f f e c t than gypsum. An h y d r ite seems, t o have e i t h e r no or v e r y l i t t l e r e t a r d i n g e f f e c t even when p r e s e n t in a p p r e c i a b l e q u a n t i t i e s . J ~ g e r , Ludwig and Schwiete (13) a l s o n o t e a l e s s r e t a r d i n g e f f e c t o f a n h y d r i t e as compared t o gyvsum. This has been a t t r i b u t e d t o t h e lower r a t e o f s o l u t i o n o f a n h y d r i t e (15). Lime i s found t o a c c e l e r a t e t he h y d r a t i o n o f C4AF while lime and gypsum have s t r o n g r e t a r d i n g e f f e c t as has a l r e a d y been concluded from our f i r s t hour h y d r a t i o n s t u d i e s . When h y d r a t e d in p r e s e n c e o f m i xt ure o f gypsum and C3A o r hemihyd r a t e and C3A, t h e main h y d r a t i o n peak appears e a r l i e r than with gypsum o r hemihydrate alone and i t s time o f appearance d e c r e a s e s with i n c r e a s i n g amount o f C~A. The o b s e r v a t i o n t h a t i n c r e a s i n g c o n t e n t o f C3A a c c e l e r a t e s t he hyd r a t i o n o f r~4ar A- i n a- i c a t e s t h a t SO4 2 - ions a r e p r e f e r e n t i a l l y taken up by CsA r a t h e r than C4AF l e a v i n g t he s o l u t i o n poor in S u l f a t e c o n c e n t r a t i o n and, t h e r e f o r e , l e s s e f f e c t i v e in r e t a r d i n g t he h y d r a t i o n o f C4AF. This however, c o n t r a s t s with t h e c o n c l u s i o n s o f Seligamann and Greening (16). A c a r e f u l o b s e r v a t i o n o f our r e s u l t s a l s o shows t h a t t h e main h y d r a t i o n peak i n case o f

448

Vol. 6, No. 4

I. Jawed, S. Goto, R. Kondo

hemihydrate and C3A appears a little later than in case of gypsum and C3A. This may be due to the higher rate of dissolution of hemihydrate which reacts not only with C3A but, owing to high concentration of SO42- ions present, can also react with C4AF slowing down its further hydration. In case of gypsum comparatively less SO42- ions may he available to react with C4AF in the presence C3A.

0 °~ 4-J

To i n v e s t i R a t e the hydrat i o n mechanism f u r t h e r , some f u r t h e r s t u d i e s o f the h y d r a t i o n products at various intervals of time during the course of hydration were carried out for certain C4AF systems containing various additives by means of X-RD, DTA and SEM. The results are presented in figs. 7-9. For the estimation of various substances during hydration, the ~ollowin~ X-RD

4~ .C O

m

*

8

4

12

16

20

24

28

32

36

40

Time (hrs.) FIG. 4

E f f e c t o f gypsum and hemihydrate on the h y d r a t i o n o f C4AF.

C3A

+ 10 ÷ 10

0 m

C4AF C~H 2 CH I00 I00

-

2 3

I00

-

I

4

85

20

c~ .e-4

+ 20

2

-

10 4~ o~

I0

IS

2

10

o~ 0

+5 t~

+ S

4

8

12 16 Time (hrs)

20

24

I 28

FIG. 5 Effect of lime, lime with gypsum and anhydrite on the hydration of C4AF.

32

0

4

8

~2

16

20

24

28

32

36

Time (hrs.) FIG. 6 Effect of gypsum or hemihydrate on the h y d r a t i o n o f C4AF in the presence of C3A.

Vol. 6, No. 4

449

C4AF, HYDRATION, LIME, GYPSUM

l i n e s (d v a l u e s in A) were used: 15 C4AF (2.77, 2 . 6 3 ) ; gypsum (7.56, l,. ~ C4AF G 4 . 2 7 , 3 . 0 6 ) ; e t t r i n g i t e (9.73, I0 ] ~ 85 1S 5.61, 3 . 8 8 ) ; monosulphate (8.92, 2 . 8 7 ) . To o b t a i n a d d i t i o n a l i n ",, / \ $ f o r m a t i o n , we coupled t h e s e s t u d i e s •,, / \ with c a l o r i m e t r i c measurements o f 0 'N. "%. C4AF / the same samples. Following t he ....../ co u r s e o f h y d r a t i o n by conduction o ....... . . . . c a l o r i m e t r y , t he r e a c t i o n was ~0 stopped a t v a r i o u s p o i n t s on the h e a t l i b e r a t i o n curve and t he p r o d u c t s were a n a l y s e d . R es ul t s o f X-RD a n a l y s i s ( f i g . 7) , p e r C4AF G H CsA io g haps, d e s e r v e some e l a b o r a t i o n . 12 3 S Although t he d a t a a r e semi-quant i t a t i v e , some features deserve mention. In all the systems ino=. vestigated, the C4AF-curve has a flat portion or a plateau in the G,.,_~F_ A \"x--'-.,--~ . . . . . . region where appreciable amount of < " ' ~ ~-:~.~__ ettringite being produced (ascend1 ing part of the ettringite-curve), indicating that the consumption of 15 C4AF is greatly reduced due to G 10 formation of ettringite which 70 30 S apparently forms a dense coating around the C4AF surface. The ettringite curve continues to [\ ~:~ ; . ~ ' " .............. increase in this region indicating \~¢._\E j-" . that sufficient amount of S042-~s -A'>,_'Cion is available to form ettringite with very little amount of C4AF which presumable diffuses through 6 ,2 Is 2a 5o 36 (3d.) CTd.) the ettringite coating around the Time (hrs,) unhydrated grain. The form of ....... X-zaydiffraction Seat liberation curves also indicates that during the time the hydration of C4AF is FIG. 7 very slow, the rate of formation of Combination o f conduct i on ettringite is quite rapid. The c a l o r i m e t r i c curves with X-ray ettringite curve reaches a maximum a n a l y t i c a l data. and then starts to decrease while monosulphate begins to appear and t h e gypsum curve d i s a p p e a r s . The f l a t p a r t o f C4AF-curve i s followed by a s t e e p p a r t i n d i c a t i n g a c c e l e r a t i o n o f h y d r a t i o n due to exposure o f unhydrated C4AF which accompanies t he break-up o f t he e t t r i n g i t e l a y e r and i t s c o n v e r s i o n to monosulphate. This can a l s o be seen from t he h e a t l i b e r a t i o n curve where the rapid rate of heat liberation almost coincides with the rapid consumption of C4AF. It may also be noted that gypsum disappears almost at the point where the peak of the heat liberation curve occurs. This confirms the results of Tenoutasse (21). Afterwards, ettringite continues to decrease and finally all is converted to monosulphate which continues to increase as seen in fig. 7. After three days, only monosulphate and very little unhydrated C4AF can be detected. All these results are in accord with the established views of formation of a retarding coating of ettringite around the unhydrated grain, followed by a break-up of this coating and exposing the unhydrated surface to solution and thus accelerating the hydration together ~

s

A

b

.

.

c4u c~

450

Vol. 6, No. 4 I. Jawed, S. Goto, R. Kondo

117 4h.

8h.

12h.

16h.

2Oh.

~--

I~0

200

24h.

300

400

C4AF ÷ Gypsum (85) (15) FIG. 8

°C

I00

200

300

400

oC

C 4 A F ~ G y p s u m + Hem1 + C3A (8%)

Thermo~rams

(12)

(3)

(5)

i00

200

300

400

C4AF + G y p s u ~ + C A (70) (30) (~)

of C4AF + various additives

°C

Vol. 6, No. 4

a.

C4AF, HYDRATION, LIME, GYPSUM

lhr

e.

lhr

i.

lhr

b. 8hrs

f. 8hrs

j . 8hrs

c. 16hrs

g. 16hrs

k. 16hrs

d. 28hrs C4AF(8S) +G(IS)

h. 28hrs C4AF(8S)+G(12)+H(3)

1. 28hrs C4AFC70)+G(30)÷C3A(5 )

+C3A(5) Fig. 9

SEM of C4AF ÷ various additives.

451

452

Vol. 6, No. 4 I. Jawed, S. Goto, R. Kondo

with conversion of ettringite to monosulphate due to lack o9 supply of S042ions in solution (2)(3)(14)(17-21). With systems containing large amount of gypsum, the time for the appearance of maximum of ettringite curve, the steep part o f the C4AF-curve and the disappearance of gypsum increases as can been noticed from fig. 7. The results of fig. 7 are also supported by our DTA and SEM studies of the same systems. From the thermograms (fig. 8), the variation in the amounts of ettringite (90-I00°C), gypsum (136°C) and monosulphate (186°C) are readily noticed and support the X-RD analytical data discussed above. Similarly SEM studies provide further support (fig. 9). Presence of gypsum (thick rod-like), ettringite (needle-like) and monosulphate (hexagonal plates) can be clearly detected at various stages of hydration in complete agreement with the results of X-RD and DTA. Conclusion From the present study, the following conclusions may be drawn regarding the hydration of C4AF. C4AF hydrates slower than C3A although the difference is not too

large. In the saturated solution of lime, C4AF hydration is accelerated. CSA hydrates similarly though less strongly. Both produce hexagonal hydrates which in the case of C4AF are much larger in size. In the saturated solution of gypsum, both C4AF and CsA are retarded, the effect being stronger for the former where ettringite is first product formed. In case of C3A, hexagonal products are first produced. In the saturated solution of gypsum and lime, the hydration of both C4AF and C3A is retarded, the former being much more strongly effected. For CBA , however, the retarding effect of gypsum and lime is less as compared to that of gypsum alone. When added as solids, both gypsum and hemihydrate retard the hydration of C4AF. Anhydrite is found to have negligible effect while lime seems to accelerate.

In the presence o f C3A, hemihydrate r e t a r d s the h y d r a t i o n o f C4AF more s t r o n g l y than gypsum, presumably due to p r e f e r e n t i a l t a k i n g uD o f s u l f a t e by C3A t o g e t h e r with h i g h e r r a t e of s o l u t i o n o f hemihydrate. When t h e r e i s s u f f i c i e n t supply o f S042- ions in s o l u t i o n , e t t r i n g i t e is f i r s t formed on the C4AF s u r f a c e . L a t e r , when the S042- concentrat i o n is low, monosulphate i s produced r e s u l t i n g in breaking the e t t r i n g i t e l a y e r and a c c e l e r a t i n g the h y d r a t i o n o f C4AF. Acknowledgements One of us (I. J.) thanks the UNESCO and the Government of Japan for the award of a research fellowship. This work was also supported by a grant from the lwatani Naoji Foundation which is also very gratefully aknowledged.

Vol. 6, No. 4

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