Study of structure formation in water suspensions of gypsum

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Study of structure formation in water suspensions of gypsum

Izmailova, V. N.; Segalova, E. E.; Rebinder, P. A.; National Research Council of Canada. Division of Building Research

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NATIONAL RESEARCH COUNCIL OF CANADA Technical Translation TT-672

Title: A study of structure formation in water sus- pensions of gypsum.

(~ssledovanie strukturoobrazovaniia v vodnykh suspenziiakh gipsa),

Author: V.N. Izmailova, E. Em Segalova and P.A, Rebinder. Reference: Doklady Akad. Nauk SSSR,

1.07 (3)

: 425-427,

1956.

Translator: (3, Belkov, Translations Section, N. R. C, Library.

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Whether or not it is necessary to postulate the development of gel structure to explain the phenomenon of "initial set1' in plaster of paris has constituted a

point of controversy for almost a hundred years.

Modern work indicates the presence, during initial set, of a complex owing its existance to the phenomenon of adsorption. However the marked disparity between the results obtained by different workers using supposedly identical techniques makes further reliable data most welcome.

The present paper presents data which are readily interpretable, and are interpreted, to show that an adsorption complex plays a large, but not an exlusive, part in explaining the phenomenon of initial set in plaster of paris.

This Division includes among its projects the study sf the mechanism of set of plaster of paris and is conse- quently grateful to Mr. G. Belkov of the National Research Council's translation staff for this translation, which

makes this valuable contribution to plaster research gener-

ally available for other North American workers in this field.

O t t a w a , M w o

1957.

R.F. Legget, Director.

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A STUDY OF STRUCTURE FORMATION IN WATER SUSPENSIONS OF GYPSUM

Calcium sulphate hemihydrate ( C ~ S O ~ a 3120) is a unique bonding su3stance and is therefore of particulale interest fop investigating the laws governing setting an8

hardening, The aim of this work was to investigate the mechanism sf structure formation in water suspensions of calcium sulphate during setting,

For the investigations we used suspensions of

calcium sulphate hemihydrate containing a large quantity of inert microfiller

-

finely ground quartz sand or eal-

cite, The introduction of" an inert filler into a suspension of calcium sulphate makes it possible to get very high values sf the water

-

calcium sulphate ratio without producing layer formation in the suspension and thus re- tard the process of structure formation. This facilitates the investigation of %he mechanism of structure fosmation,

The kinetic^ of structure formation, as is usual in our

laboratory, w a s studied by measuring the increase in

plastic strength of structure (specific shear stress), in the suspension, For this purpose we used a conic plasto- meter (1'2) which is a simple and very convenient instru- ment for this purpose(3). The substance under investigation was 30% calcium sulphate hemihydrate, 70% _{ground quartz sand}

with 50% water of the dry weight, After- mixing the samples were kept in a desiccator over a saturated solution of calcium sulpha tea

As seen from Fig. 4 structure formation takes

place in two stages: in the first stage the plastic strength of the system is very low (about 1,5 gm./sq. cm, ) and in-

creases rather slowly, Four to five minutes after mixing, the second stage begins. Here there is a very rapid increase

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in strength for about

30

min. reaching a maximum (1

5

kern./ sq. cm. )

,

which is about 1 0,000 times greater than the initial value, At the first stage the system is a fairly mobile suspension containing particles of calcium sulphate hemihydrate on the surface of which highly dispersed microcrystals of dihydrate are formed. This is newly formed CaS04 2H20. In this system coagulation structure a pears

$294) like the usual type of thixotropic structural network 9

where the particles are bonded by van der Walls forces

weakened by the remaining thin layers of the water medium ( 5 ~ 6 ) ~

Thixotropy in this system does not become established owing to the rapid changes in structure due to hydration and the accumulation of dihydrate. The increase in strength of this structure is due to the continuous bonding of water and the increase in number of solid phase particles. The coagulation property of the structure at this stage is confirmed by its complete reversibility after mechanical disturbance: continuous shearing* of the suspension for the first

3

to

4

minutes after mixing not only gives no changes in its plastic

strength but also has no effect on its further structure format ion.

During hydration the suspension accumulates an increasing quantity of microcrystals of calciurn sulphate dihydrate and finally there is enough of them to form a continuous coagulation network which traps the larger crystals of calcium sulphate hemihydrate which are Con- tinuing to hydrate, Rith this the first stage of structure formation ends. The crystallization of the dihydrate leads to growth chiefly of crystals formed around surface

F,ditorts note:

THE LITERAL TRANSLRTIOIv OF THE RUSSIAN GIVES t'REPEATED RUBBING" BUT, IN THE CONTEXT, TI-IE INTENT OF THE PHRASE WOULD APPEAR ~ TO BE "CONTINUOUS SHEARING",

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centres and to strengthening, as if it were molecular welding, of the structural network with the formation

of strong crystallized contacts. Thus a continuous crystalline structure of calcium sulphate dihydrate develops in the system. Its strengthening as the re-

sult of recrystallization of the dihydrate through a supersaturated solution, taking into account the increase in the solubility of the highly dispersed crystals throughout the already existing framework, is evident from the rapid increase in strength,

In contrast to the initial coagulational structure the strong crystallized structures are not reversible when mechanically disturbed. This can be seen from Fig. I which shows the increase in the plastic strength when the cryetalline structure is disrupted at various stages of its formation. With continuous shear-

ing of the suspension five and seven minutes after mixing, crystalline structure continues to develop in

the system but its strength decreases as the time after mixing increases. Here the increase in strength is con- nected with the presence of non-hydrated calcium sulphate hemihydrate which again forms acrystalline structure.

However, repeated shearing I 0 to 12 minutes after mixing results in the complete rupture of the crystalline structure and to the formation of a coagulation network of dihydrate crystals which cannot harden any further, ob- viously owing to the absence of conditions for the re- establishment of crystalline contacts, The strength of this Itsecondary" coagulation structure

-

dihydrate (about

30 _gm./sq, _cm.

-

_curve

4

_{in Fig,}_I_{) is higher than that of}

the initial suspension which is partially due to the ex- tensive bonding of water during complete hydration,

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crystallization of calcium sulphate prevents the prolonged retention of microcrystals of colloidal size and therefore this secondary coagulation does not show signs of

thixo tropy.

As mentioned above, the maxirmun strength of the crystallization structure is attained in approximately

30

minutes (Fig. 1). By this time hydration should also have finished which can be expected in calcium sulphate and lime ( 7 ) owing to the high solubility. Independent parallel measurements of the kinetics of the strength in-

crease (crystallizational setting) and the kinetics of the hydration of calcium sulphate by the rate of heat liberation

(by the method developed by 0. I. Luk'ianov in our laboratory) in the same suspensions (the same composition) confirmed

this hypothesis (see Fig. 2), in full agreement with the work of O.V. Kuntsevich, P,E, Aleksandrov, V.B. Ratinov,

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T e I . Rozenberg and (3.G. Bogautdinov

.

The usual prolonged setting of calcium sulphate which led A.A. Baikov ( 9 ) to the notion that the end of hydration did not coincide with the end of setting, is in fact explained by the increase in strength of the cry-

stallized structure of the gypsum on drying (as is known (10,~ 1 ) the strength of gypsum rock, like that of monocrystalline

dihydrate, falls abruptly with increase in humidity). After the first

30

minutes a decrease in strength is ob-

served (when kept under moist conditions) owing to recrystallization and the gradual disappearance of some of the non-equilibrated crystallisation contacts* Later it was shown that with increase in strength of the crystalline structure of the dihydrate when there was an increase in the content of the calcium sulphate hemihydrate in the mixture with an inert filler, just as when the quantity of water is reduced on mixing, the duration of the first

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stage of structure formation is reduced and in fairly con- centrated suspensions of pure calcium sulphate this stage scarcely appears and cannot be investigated.

The strength of porous crystalline structure, in contrast to coagulation structure and to compact polycry- stalline aggregates, does not increase continuously with the size of the crystals but reaches an optimum at a medium size, as follows from general theoretical considerations. This was confirmed by our experiments which showed that the strength of gypsum rock, within a specific size range of crystals, decreases as the size of the cr stals decrease in agreement with the data of B.P. Budnikov

I .

Here the size varied owing to the changes in temperature of the crystallizing system (i.e., the extent of supersaturation) and the modification by small additions of adsorbing sUb- stances. It is possible to show that the strength of these structures is not determined in a well-defined way by the sizes of the crystals but depends on the conditions of their fusion.

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R e f e r e n c e s

Rebinder, P, A, and Semenenko, N. A. D o k l ~ d y Akad. Nauk SSSR, 64 ( 6 ) : 835, 1949.

I a m p o l T s k i i , B. Ia. and Re'binder, P.A. K o l l o i d

zhur. 1 0 ( 6 ) : 466, 19118.

Rebinder, P.A. and Segalova, E. E. P r i r o d a , (1 2 ) : 45, 1952;

Segalova, E. E.

,

Rebinder, P. A. and ~ u k ' ianova, 0. I.

Vestn. MGY, . ( 2 ) : 17, 1954; ~

Rebinder, P.A, 1n book by B u t t , 1u.M. a.nd

Berkovich, T.M. Bonding m a t e r i a l s w i t h s u r f a c e - a c t i v e a d d i t i v e s . hloscow, 1953. p. 19.

Serb-Serbina N. N. and Rebinder, P. A, K o l l o i d 2 3 ~ ~ . 9 ( 5 ) : 381, 1947.

Rebinder, P.A. Discuss. Faraday Soc. C o a g u l a t i o n and f l o c c u l a t i o n of c o l l o i d s , ( l b ) , S e p t , 1954, Abduragirnova, L. A.

,.

Rebinder, P.A. and Serb-Serbina,

N.N, K o l l o i d Zhur. I 7 (3): 18Lc, 1955,

Logginov, G.I., Rebinder, P.A. and Sukhova, V.P. Doklady Akad. Nauk SSSR, 99

( 4 ) :

569, 1954.

Kuntsevicli, O.V.

,

Aleksandrov, P.E. e t a l . Doklady Akad. Nauk SSSR, 104 ( 4 ) : 1955.

Baikov, A. A. Sobr. tr. 5: I 14. hToscon-Leningrad, I 948. Rebinder, P.A. Sborn. Akad. Nauk SSSR k 3 0 - l e t i i u

o k t i a b r t s k o i r e v o l i u t s i i ( c o l l e c t e d w o r k s . o f Academy of S c i e n c e s USSR corrrnemorating; t h e t h i r t y - y e a r

a n n i v e r s a r y of t h e October r e v o l u t i o n , I : 123, 1947. Logginov, G.I. and E l i n z o n , M,P. St. v. sborn.

M a t e r i a l y i k o n s t r u k t s i i v sovremennoi a r k h i t e k t u r e (A p a p e r p u b l i s h e d i n t h e c o l l e c t e d work, m a t e r i a l s and c o n s t r u c t i o n i n p r e s e n t day a r c h i t e c t u r e . Moscow, 1948. p. 95.

Budnikov, POP, Gips ego i s s l e d o v a n n i i a i p r i m e n i n i i a (~ypsurn, i t s i n v e s t i g a t i o n and u s e ) . Moscow, 1950.

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Fig. 1,

The kinetics of increase in plastic strength Pm of structure in a suspension of 30% CaSOk l )H*o,

7046 ground quartz sand and

50%

water, of ' the dry

weight, (W/G =

0.5%)

with rupture of the structure at various stages of formation, 1

-

initial suspension was continuously sheared 1 to

3

min. after mixing (continuous shearing within this time inter- val did not affect the stren th increase); 2

-

structure was again sheared ?for 2 mine) after a lapse of

5

min. (marked by the arrow 2);

3 -

the same but after a lapse of

7

min.;

4 -

the same after 10 to I 2 min. The maximum levels of plastic strength are shown on the curves.

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Fig. 2

Parallel measurements of the kinetics of heat lib- eration Q(T) in percentages of the heat effect of full hydration of CaSOk

.

$H*O to dihydpate and

the kinetics of crystal formation by the increase in plastic strength P

(z)

made on suspensions of