Study of Supersaturation Kinetics in Connection with the Development of Crystal Structure in the Solidification of Gypsum

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Study of Supersaturation Kinetics in Connection with the Development of Crystal Structure in the Solidification of Gypsum

Segalova, E. E.; Izmailova, V. N.; Rebinder, P. A.

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PREFACE

The mechanism of setting of plaster of paris constitutes one of the projects of the plaster =oup of the Materials' Section of the Division of Building Research, An essential part of that study is a con-

tinuing review of the relevant literature.

Such study has shown a controversy between those who propose a purely crystallizational hardening of plaster and those who invoke adsorption as an essential mechanism in the process,

The present paper, while primarily concerned with investigating the origin of the strength of a set plaster mass, favours a process of setting involving only

crystallization.

The Division of Building Research wishes to record its indebtedness to Mr. DoAo Sinclair of the Translations Section of the National Research Council Library for pre- paring this translation,

Ottawa

March 1958

R o F . Legget Director

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Title:

NATIONAL RESEARCH COUNCIL OF CANADA

Technical Translation 730

Study of the supersaturation kinetics in

connection with the development of crystal

structure in the solidification of gypsum

(I ssledovanie kinetiki peresyshcheniia v

sviazi c razvitiem kristallizat sionnykh

struktur pri tverdenii gipsa)

Authors : E.E. Segalova, V o N o Izmailova and P.A. Rebindar

Reference: Doklady Akad, Nauk SSSR, 114 (6) : 594-597, 1957

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STUDY OF SUPERSATURATION KINETICS IN CONNECTION WITH

TH3 DEVELOPMENT OF C3YSTAL STRUCTURE IN THE

SOLIDIFICATION OF GYPSUM

In disperse systems there are two possible types of structure which may form, namely coagulation structures or crystal struc-

tures''). One of the commonest of the latter types is the

structure resulting from the solidification by hydration of mineral bonding materials. The strength of the solidification

structures results from the formation, between separate hydrate crystals precipitating from supersaturated solutions of strong

crystallizatlon contacts, i.e., places of direct coalescence.

These crystallization contacts constitute thermodynamically

unstable formations ( 2 ) and under ordinary conditions their

occurrence is possible only if the degree of supersaturation is suf f iclent.

The mechanical destruction of the crystal structure during its formation is irreversible even long before the hydratlon is

completed(1). Continued hydratlon and the crystallization of dl-

hydrate associated with it in this case does not lead to the

formation of a crystal structure. T h i ~ can only be because

conditions are not favourable for the formation of crystallization contacts between lndivldual crystals of gypsum, Apparently

this is due to the large amount of dihydrate accumulating in

suspension.

The value of the supersaturation and the kinetics of its transformation in suspensions of calcium sulphate hemihydrate can conveniently be observed conductometrically. Hemihydrate

with a nominal specific area of S = 3060 sq. cm. per gm. was

used for the tests. Fig. 1 shows the variation in time of the

specific electric conductivity showing the concentration of

calcium sulphate or the degree of supersaturation in suspensions having different contents of calcium sulphate hemihydrate at a

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temperature of 25OC. The electrical conductivity was measured by the usual method employing alternating current.

First of all it should be noted that in all suspensions of

calcium sulphate hemihydrate made with more than 8 gm. CaSO,

per litre, the same maximum supersaturation (3s4) is observed,

corresponding to a concentration of 8.0 gm. CaSO, per litre in

the liquid phase of the suspension, which in turn corresponds to

the value accepted conventionally as the "~Olubility" of the hemi-

hydrate. This steady, high supersaturation is rnaintalned in the aqueous medium of the suspension until such time as the rate at

++

--

which Ca and SO, enter into solution is equal to the rate at

which they come out of solution owing to crystallization of the

dihydrate. As Fig. 1 shows, the hi&er the concentration of the

suspension, the sooner the drop in supersaturation will begin

and the more rapidly it will proceed. Fig. 2 shows the change

with time of the specific electrical conductivity in suspensions of calcium sulphate hemihydrate containing different amounts of dihydrate during consolidation. Crystals of dihydrated gypsum which are ready centres of crystallization, accelerate the

crystallization of sypswn. As a consequence, dihydrate contents up to 30% in the solid phase of the suspension decrease the tlme during which the supersaturation, after forming rapidly, remains

constant. If more than 30% dihydrate is introduced Into the

solid phase the rate of crystallization of the new mixture be- comes so great that the maximum supersaturation value is not reached and a steep decline of the supersaturation begins im- mediately. The gypsum content at which maximum supersaturation will no longer occur must increase with decreasing dihydrate

dispersion and with an increase in the dispersion of the original calcium sulphate hemihydrate.

The dependence of the degree of supersaturation on the amount of gypsum in the suspension explains the effect of additions of complete newly formed hydrate on the strength of the solidification

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by t h e growth of p l a s t i c s t r e n g t h ( 5 , 1 s 2 ) i n a p a s t e comprising

30% calcium sulphate hemihydrate and 70% f i l l e r (mixture of ground q u a r t z sand and gypsum). Curve 1 corresponds t o the o r i g i n a l suspension without the a d d i t i o n of any d i h y d r a t e , and curve 2 t o t h e suspension c o n t a i n i n g a small a d d i t i o n of gypswn. The l a t t e r shortens t h e induction period of s t r u c t u r e forma-

tion(5s1)

,

i . e . , a c c e l e r a t e s s o l i d i f i c a t i o n but has no e f f e c t on t h e f i n a l s t r e n g t h of t h e s t r u c t u r e .

The remaining curves i n Fig. 3 f o r i n c r e a s i n g d i h y d r a t e con- t e n t s i n d i c a t e continuously i n c r e a s i n g r a t e s of s o l i d i f i c a t i o n , but always accompanied by decreasing maximum s t r e n g t h values.

On r e p l a c i n g h a l f o r a l l of t h e sand by d i h y d r a t e (curves 6 , 7 ) , t h e s t r e n g t h of t h e c r y s t a l l i n e s t r u c t u r e drops 3 0 t o 60 times compared with t h e standard s t r e n g t h of t h e gypswn rock with

sand but no admixture of dihydrate. I t should be noted t h a t i n a l l cases the hydration of t h e calcium s u l p h a t e hemihydrate was complete and the k i n e t i c s of h y d r a t i o n corresponded completely t o

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t h e k i n e t i c s of s t r u c t u r e formation

.

The decrease i n t h e maximum s t r e n g t h of t h e gypsum c r y s t a l s t r u c t u r e observed i n t h e s e t e s t s i s explained by t h e decrease i n t h e maximum value of t h e s u p e r s a t u r a t i o n a t t a i n e d i n t h e presence of t h e d i h y d r a t e admixtures (Pig. 2 ) . The decrease of super-

s a t u r a t i o n ( s h o r t e r d u r a t i o n of i t s e x i s t e n c e ) reduces t h e p r o b a b i l i t y of t h e growth of c r y s t a l s , i . e . , t h e formation of c r y s t a l l i z a t i o n contacts. A t a high d i h y d r a t e content i n t h e suspension only i n s i g n i f i c a n t s u p e r s a t u r a t i o n s occur, which a r e maintained only f o r a s h o r t time. Under t h e s e c o n d i t i o n s p r a c t i -

c a l l y no c r y s t a l l i z a t i o n c o n t a c t s a t a l l occur and hence no s o l i d i f i c a t i o n s t r u c t u r e takes form.

I n o t h e r words, i f a s u f f i c i e n t q u a n t i t y of newly formed d l -

hydrate accumulates i n the suspension, f u r t h e r hydration harden- i n g i s prevented a f t e r t h e d e s t r u c t i o n of t h e s t i l l incompletely formed c r y s t a l l i z a t i o n s t r u c t u r e .

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From this point of view it is clear that in suspensions of highly dispersed gypsum suggested by some authors as an

unusual bonding under ordinary conditions crystal-

lization hardening is impossible. Supersaturations, which might occur in suspensions at the expense of the colloidal fraction formed because of fine grinding, cannot be realized because of the large quantity of gypsm and the recrystalllzation of particles of colloidal dimensions does not lead to the coalescence of crystals but only to their free development, Strength in such systems, where their density is sufficiently high, as in the case of clay, is due to the removal of water during drying and the consequent strengthening of coagulation contacts between the

crystals at their boundary approach. As is known, such structures,

unlike crystal structures, are entirely non-water-resistant, i.e., when wetted they completely lose their strength because of the reversibility of the strength of the coagulation contacts when the thickness of the layer of aqueous medium between the particles

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is changed

,

Peferences

1, Izrnailova, V . N , , Segalova, E.E. and Rebinder, P.A.

Doklady Akad. Nauk SSSR, 107: 425, 1956.

2, Segalova, E.E., Izrnailova, V.N. and Rebinder, P.A.

3, Zaberzhinskii, Ia,L,, Ratinov, V.B. and Rozenberg, T.1,

4. Ratinov, V.B,, Zaberzhinskii, Ia,L,, Rozenberg, T.I.

5. Segalova, E.E., Rebinder, P.A. and Luk'ianova, 0,I.

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6, Gulinova, L.G. and I p a t leva, V.A. Gipsovyi bezobzhigovyi tsement i i z d e l i i a iz nego (Unburnt gypsum cement and a r t i c l e s t k e r e f o r m ) . Izd. Akad. Arkhit Ukr. SSR, 1954; R i b l i n , 1.1. and Papkov, L.P. Trud. Kharkovsk. P o l i t e k h n .

I n s t . s e r . khim.-tekhnol, 8 ( 3 ) : 219, 1956.

7. Rebinder, P. Disc. Faraday Soc. No, 18, 1954;

Rebinder, P. and Segalova, E. R e p r i n t s Proc, 2nd. I n t . Congr. of S u r f a c e A c t i v i t y , London, A p r i l 1957.

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Fig. 1

Kinetics of the specific electric conductivity of various

quantities of CaSO, .. 0.5 &O suspended In 500 ml. water:

1

-

6

e m ,

2

-

10 @ a , 3

-

25 @ o r 4

-

50

@I.,

5 - 150 @ a

Kinetics of the specif ic electrical conductivi ty of suspensions

containing 25 gm. CaSO, 0.5 &O in 500 ml. of water for vari-

ous additions of dihydrate: 1

-

0 _gm., ₂

-

₁_gm., 3

-

3 gm.,

4

-

10 p a , 5

-

20 ma, 6

-

25 g ~ n * , 7

-

50 @ a , 8

-

75 gm., 9

-

100 gm. CaSO,

.

2H,O

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Fig* 3

Kinetics of structure formation in systems containing 30$ CaSO, 0 - 5 H ,O and 70% flller (various quantities

of sand and dihydrate in the solid mixture), Ratio water/solids = 0.5