Mechanism of ettringite and monosulphate formation

CEMENT and CONCRETE RESEARCH. Vol. 22, pp. 671-677, 1992. Printed in the USA. 0008-8846/92. $5.00+00. Copyright © 1992 Pergamon Press Ltd.

MECHANISM OF ETTRINGITE AND MONOSULPHATE FORMATION

J. Havlica Institute of Inorganic Chemistry Slovak Academy of Sciences 842 36 Bratislava, Czechoslovakia S. Sahu Department of Ceramics, Glass and Cement Slovak Technical University 812 37 Bratislava, Czechoslovakia (Communicated by J.P. Skalny) (Received July 18, 1991; in final form Jan. 17, 1992)

ABSTRACT The kinetics of the hydration process of the sulphoaluminate phase Ca4(A16012)(SO 4) in the presence of calcium hydroxide and gypsum were studied by differential calorimetry, optical microscope and X-ray phase analysis. The f'trst period of reaction is characterized by the presence of ettringite even in the case of stoichiometric composition of monosulphate. The liquid interlayer on the surface of the sulphoaluminate phase plays an important role in the kinetics of hydration. The solubility of the sulphoaluminate phase is higher in the lower pH values of the surrounding liquid phase which causes acceleration of the hydration process. Ettringite was created at a certain distance (8i) from the surface of the sulphoaluminate phase. This thickness 8i depends on the content of a dissoluting Ca4(AI6012)(SO4) phase.

Introduction The kinetics and the mechanism of the formation of sulphoaluminate hydrates influence the development of mechanical properties of concrete on the basis of sulphoaluminate belite cements and the characteristics of expansive cements. The mechanism of ettringite formation is not a well resolved question. There are two different schools of thought existing on the mechanism of ettringite formation. They are the topotactical and the through solution mechanism.(1) But under which conditions one mechanism follows the other is not well understood. Most of the studies of the mechanism of ettringite formation were focused to explain the expansion characteristics of expansive cements and lack a common opinion about the mechanism. In the system Ca4(AI6012)(SO4)-CaSO4.2H20, in the presence of free lime the rate of ettringite formation is more retarded than the system without free lime,(2) but the exact role of free lime is not clear. The formation of solid hydrated phases in the quarternary system Ca4(AI6012)SO4-CaSO4.2H20-Ca(OH)2-H20, their stability in the four phase assemblage and the phase relation were established.(3) The quarternary hydrates formed in this system are monosulphate Ca4(A1206)(SO4). 12H20 and ettringite Ca6(AI206)(SO4)3.32H20 and they coexist 671

672

J. Havlica and S. Sahu

Vol. 22, No. 4

with the other phases: portlandite Ca(OH) 2, gypsum CaSO4.2H20, gibbsite AI(OH) 3, and hydrogarnet Ca3A12(OH)I 2. A study by Ghorab et al.(4) shows that in higher alkaline conditions ettringite decomposes to form monosulphate. In our previous work,(5") we determined the stability of ettringite and monosulphate in water solutions with various pH values. Measurements performed in non-equilibrium conditions show that the boundary for the disappearance of monosulphate is pH = 11.6 and ettringite pH = 10.7. At lower values of pH only gypsum and aluminum sulphate were the stable phases present in the studied system. The aim of this work is the evaluation of the kinetics of the hydration process in the system Ca4(A16012)(SO4)-CaSO4.2H20-Ca(OH)2-H20 by calorimetry to develop a model to demonstrate the formation of a liquid interlayer on the surface of sulphoaluminate phase and its role. Experimental Calcium sulphoaluminate phase Ca4(A16012)(SO4) was prepared according to Ref. 3. The measurement of the particle size distribution shows that mean radius r was about 14/~m. Ca(OH) 2 was prepared from CaCO 3 by annealing and further hydration in a closed system. These phases were mixed with analytical grade CaSO4.2H20 in the following molar ratios Ca4(AI6012)(SO4) : CaSO4.2H20 : Ca(OH) 2 1 : 2 : 6 and 1 : 8 : 6 to get monosulphate in the fLrStcase and ettringite in the second case as the final equilibrium hydrated products, according to the schemes given below in Equations 1 and 2. Ca4(A16012)(SO 4) + 2CaSO4.2H20 + 6Ca(OH) 2 + 26H20 --> 3Ca4(AI206)(SO4).12H20

(1)

Ca4(A16012)(SO 4) + 8CaSO4.2H20 + 6Ca(OH) 2 + 74H20 ~ 3Ca6(A1206)(SO4)3.32H20

(2)

Calorimetric experiments were performed at room temperature keeping water to solid ratio one (w/s = 1) by a differential calorimeter ZIAC.(6,7) The changes in microstructure during the hydration process were observed by an optical microscope (Carl Zeiss Jena NU 2 type) at various time intervals. For microscopic study the samples were prepared in a more dispersed system keeping water to solid ratio about ten (w/s -_-_ 10). Special care was taken to avoid contamination by CO 2 and the samples were sealed hermetically. The whole scheme of microscopic study is given in Fig. 1. Results and Discussions The kinetics of reactions (1) and (2) expressed by dependence of heat flow dQ/dx, are represented in Fig. 2. This diagram illustrates the difference in reactivity of both systems. After

I

[

......

FIG. 1 Scheme of microscopy observation .......,,.

......

v / / / ,b ) ,w /

tttlf o I

i

.4"

1 - suspension 2 - cover glass 3 - Al-foil (thickness 0.5mm) 4.- glass slide 5 - objective 6 - light

Vol. 22, No. 4

EITRING1TE, MONOSULPHATE,SOLUBILITY

673

dQ W.g 4 0.2

1

0.1

2

|

1

2

3

r/h

FIG. 2 The dependence of heat flow dQ/dx on the time x in the studied system with molar ratios Ca4(A16012)(SO4) : CaSO4.2H20 : Ca(OH) 2 1) 1 : 2 : 6 ; 2) 1 : 8 : 6 the first period of hydration (up to 1 hour), X-ray phase analysis indicates the presence of ettringite in both cases. Monosulphate was formed in the first system after dissolution of gypsum after three hours. A comparison of heat evolution curves shows that the rate of reaction of the first system (Eq. 1) at 0.1 to 0.2 hour is 21.,'8 times greater than the rate in the second system (Eq. 2). The formation of hydrated phases and changes in microstructure at different time intervals are shown in Fig. 3. The series A of microphotographs shows the more intensive formation of the hydrated products. The detailed photograph, Fig. 4, which is the magnification of part "A" of Fig. 3A at x = 24 h, indicates that ettringite was formed at a certain distance away from the original sulphoaluminate phase. Whereas in the series B of photographs with higher content of gypsum, we can notice the big corn of gypsum, which dissolves very slowly. After 24 hours the gypsum corn is not dissolved completely and the hydrated products are formed near the sulphoaluminate phase. Hydration may produce in the first reaction monosulphate and in the second ettringite as the final product. On the other hand, from tables (8) the composition of the equilibrium solution in contact with the solid system Ca(OH)2-CaSO4.2H20 at 20 °C contains 0.0125 mol CaSO4.2H20 (2.15 g) and 0.0186 mol Ca(OH) 2 (1.37 g) in one liter of saturated solution. That means the molar ratio of CaSO4.2H20 to Ca(OH) 2 at eqilibrium condition is 0.6. If we consider the present systems, the mole ratio in the first case (Eq.1) is 0.33 where as in the second (Eq. 2) it is 1.33. The mole ratios in both cases are quite different from the equilibrium conditions in the system Ca(OH)2-CaSO4.2H20. From the present observations and results from work,(5) a model of hydration of sulphoaluminate phase has been developed to explain the whole phenomena. Depending upon the pH condition of the liquid phase surrounding the aluminate phases, the mechanism of ettringite formation is changed. If we consider the mechanism of ettringite formation from wicalcium aluminate and calcium sulphoaluminate phase, we can expect lower pH values of the liquid phase surrounding the Ca4(AI6012)(SO 4) due to the presence of sulphate rather than that of Ca3AI206. It is expected that the pH value of the liquid phase surrounding tricalcium aluminate is more than 10.7 and ettringite is formed on the surface of Ca3A1206 particles (1) at higher pH

674

J. Havlica and S. Sahu

Vol. 22, No, 4

B

A

1 : 2 : 6

1 : 8 : 6

T=0.3h

"r = 0 . 3

x=

"r=

1 h

h

1 h

x=3h

T=3h

T = 24 h

x = 24 h

FIG. 3 Development of microstructure in system with molar ratios Ca4(A16OI2)(SO4):CaSO4.2H20:Ca(OH) 2 A) 1:2:6; B) 1"8:6 at v a r i o u s t i m e s

Vol. 22, No. 4

ING1TE,

MONOSULPHATE, SOLUBILITY

675

FIG. 4 Magnification of part "A" from Fig. 3A at x = 24 h

values. This may be the reason that for identical mixes, the needles of ettringite produced by wicalcium aluminate are shorter than those produced by the sulphoaluminate phase.(2) From the pH of the liquid phase surrounding the surface of the Ca4(A16012)(SO 4) particle, two different cases are possible. In the first case the presence of sulphate ions causes a drop in pH value to higher than 10.7 and ettringite can form on the surface of the Ca4(A16012)(SO 4) particle. This mechanism would be characterized by the formation of a solid layer of the hydrated product, which causes retardation of further crystallization. In the second case the pH value of the solution on the surface of the Ca4(A16OI2)(SO4) particle is even lower than 10.7, as shown in Fig. 5. This fact causes the formation of a liquid interlayer between the Ca4(AI6OI2)(SO 4) particle and hydrated product (as shown in Fig. 5) and increases the rate of transfer of ions into the liquid phase. This is due to the fact that the solubility of aluminium containing phases in these conditions is higher. Increasing the content of the Ca4(A16012)(SO4) phase in the system also causes an increase of the surface of this phase with respect to the total surface of the solid phases, and in this way a higher total rate of dissolution. The total volume of the liquid interlayer depends on the quantity of the Ca4(AI6012)(SO 4) phase, which influences the kinetics of the hydration process. We can presume that an increase in the volume of this liquid interlayer in the system leads to an acceleration of the reaction. Formation of an interlayer can be characterized by thickness 8i, which would be dependent on the concentration gradient (Fig. 6). Considering the Ca4(AI6012)(SO4! particles are spherical and surrounded by spherical liquid interlayer, the total volume in layers m the system containing n particles with various radii r i can be expressed by Eq. 3

Vtot, n=

+ a.

wn ~. [ 3 4- ~t ( r i , n

1,n

)3

4

- ~ r( r i , n

3 ]

(3)

1

where w n represent the weight ratio of sulphoaluminate phase to total solid phase in the studied system. , So,;

I pH

0

pHs

p H : 11;6_

"1

>2 "l'J

~

~ C/') "

"OAU

~

~* z

L.) ( J p,U~ u U

Z G'~ --

r-0 :E

oooo

---I m

.....

pH= 10.7

IN_.. /

o o ×

FIG. 5 The formation of ettringite at pH > 10.7

(/1' C)~,

)Hca4(A16012)S04 X

I,-

FIG. 6 Dependence of thickness of liquid interlayer ~i with respect to the shape of concentration gradient

J. Havlica and S. Sahu

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Vol. 22, No. 4

This total volume of interlayers in the system represents a part of the total solution with higher solubility having higher capacity of dissoluted ions which can be used for the formation of ettringite in the liquid phase on the tentative boundary, as defined by pH = 10.7. The rate of formation of the hydrated product is proportional to the volume Vtot, n. Applying this approach to the two different systems, their ratio can be expressed as d~

( -dT )I dot

(

-dT

Vtot, 1

)2

(4)

Vtot,2

Comparing systems 1 and 2, from Eq. (3), we can derive cubic equation (5). 3 ~, 1, 2 i

i ~1,

i(R

zi -

1)

+ 3 ~, 1, i 6 2

i

~

whe1,e

(R z 2 -

1,i

1"i = ~

1

1)

+ ~, (~3

i

t,i

(R

z3._ 1

1)

= 0

(5)

1"i, 1 = ~ 1"i, 2

p2

2

i = T, ~ i , 1

= F,

12

i,2

w2 V t o t , 1

R-

Wl V t o t , 2

Z

.

1

=

52, i 31, i

If 81 i is approximately constant in all reaction systems, then d 1 i = 51, similarly ~2,i = 52 and z i = z and the radius ri << 81 then only the third member in equation'(5) is acceptable and the ratio 61/82 can be expressed as

w2 V tot, 1

c31

62

-

[ Wl V t o t , 2

)

1/3

(6)

and if ri >> 51, then then exponent is equal to 1, and if ri >> 8 i then 1/3

w2 Vtot, I)

~1

,

(7)

Wl Vtot, 2 Considering -

-

(8)

do~

dQ d'~

N

-

-

d'c

and if ratio (Eq. g) is equal to 21.48 and w2/w 1 = 0.575, we can get from Eq. (7) 2.3<

52

< 12.33

(9)

Vol. 22, No. 4

ETFRINGITE, MONOSULPHATE,SOLUBILITY

solution

new sotid sol id

phase

677

s o l u t,"i o n ~

"~d

~)I; 0

P"°"

ew phclse solid

I'

FIG. 7 The estimation of thickness of liquid interlayer in the studied system with mole ratio Ca4(A16012)(SO4) : CaSO4.2H20 : Ca(OH) 2. 1 : 2 : 6 (~i = 10 mm) We consider that the mean value of variable r, r = 1/n 7-, r i is equal to 0.14/lm and from microscopic observations we can estimate the thickness of al in the system with the ratio 1:2:6 is about 10/~m as is shown in Fig. 7. The solution of cubic equation (5) indicates that 1/z = 7.213 and the thickness a2 is about 1.4 /~m. Results of this work enable us to predict the distance at which the reaction is occuring. If the reaction takes place very near to the surface of the Ca4(A16012)(SO4) phase, then ai is about 1.4 /~m, the process can be characterised as topotactical. In the system with 6i, about 10 Ima ettringite is formed by the solution mechanism.

Conclusions The formation of liquid interlayer on the surface of Ca4(A16012)(SO 4) with pH < 10.7 causes an increase in the rate of dissolution and acceleration of hydration process. Results indicate that if the thickness of this interlayer is about 1.4 ~tm, the process runs by topotactical mechanism. In the case where the thickness of liquid interlayer is about 10 ktrn, then the reaction is run in the solution.

References 1. 2. 3. 4. 5. 6. 7. 8.

Cohen, M.D., Cem. Concr. Res. 13, 809 (1983). Mehta, P.K., J. Amer. Cer. Soc. 56, 6, 315 (1973). Kaprgtlik, F. Hanic and A. Gabrisov~i, Cem. Concr. Res. 19, 671 (1989). Ghorab, H.Y. and E.A. Kishar, Proceedings of the 8th International Congress on the Chemistry of Cement, Vol. IV, Rio de Janiero, 1986, p. 231. Gabrisov~i, J. Havlica and S. Sahu, Cem. Contr. Res. 21, 1023 (1991). Oliew, G. and W. Wieker, Silikattechnik 32, 152 (1981). Oliew, G. and W. Wieker, Silikattechnik 31,333 (1980). Linke, W.F., Solubilities. Inorganic and Metal-Organic Compounds A-Ir. D. Van Nostrand Co., Inc., Princeton, Toronto, London, New York, 1958.