Thermoluminescence and reactivity of calcium sulphate hemihydrate

CEMENTand CONCRETERESEARCH. Vol. 8, pp. 311-318, 1978. Pergamon Press, Inc. Printed in the United States.

THERMOLUMINESCENCE AND REACTIVITYOF CALCIUM SULPHATE HEMIHYDRATE

M. T r i o l l i e r and B. Guilhot Ecole Nationale Sup~rieure des Mines Centre de Chimie Physique 158, cours Fauriel 42023 - Saint-Etienne Cedex (France)

(Communicated by B. Cottin) (Received Feb. 21, 1978) ABSTRACT The influence of methods of preparation of calcium sulphate hemihydrate on its rehydration kinetics is studied using thermoluminescence. The different treatments modify the number of traps. There exists a relation between the concentration of a defect in the calcium sulphate hemihydrate and the parameters of the hydration kinetics, measured by calorimetry. The o6tained differences cannot be related to the variation of specific surface.

L'influence du mode d'obtention du sulfate de calcium "h~mihydrat~" sur sa cin~tique d'hydratation est ~tudi~e par thermoluminescence. Les diff~rents traitements modifient le nombre de pi~ges. II existe une relation entre la concentration d'un d~faut dens le sulfate de calcium h~mihydrat~ et les param~tres cin~tiques de l'hydratation mesur~s par calorim~trie. Les differences obtenues ne proviennent pas de la variation d'aire sp~cifique.

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M. T r i o l l i e r , B. Guilhot

Introduction Temperature (1,2) and water vapour pressure (3,4,5) have a great influence on the dehydration of gypsum and strongly modify the rehydration mechanism of the hemihydrate obtained (6). The reactivity of a solid is often found to be related to the presence of defects, which may be detected in certain cases by thermoluminescence techniques (7,8). Thus, in the present work, a relation is sought between the concentration of a defect in the calcium sulphate hemihydrate measured by thermoluminescence and the hydration kinetics measured by isothermal calorimetry. Experimental Results 1. Material The calcium sulphate hemihydrate, CaSO~, ~H20 (9), is prepared by decomposition of pure calcium sulphate dihydrate supplied by MERCK, at constant temperature and water vapour pressure. 2. Hydration by water vapour The influence of the methods of preparation of calcium sulphate hemihydrate on its rehydration by water vapour has been studied by thermogravimetry (5) (6). 2.1. Influence of the temperature of dehydration The calcium sulphate dihydrate was decomposed in a vacuum corresponding to a residual partial water vapour pressure of 10-~ t o r t at 90°C, 110°C et 130°C. Each product was rehydrated under the same conditions (P/Po = 0,93 ; T = 22°C). An increase in the temperature of dehydration has three consequences (fig. 1 ) : a lower i n i t i a l absorption of water a lengthening of the induction period a decrease in hydration rate. -

-

-

2.2. Influence of the water vapour pressure The experiments were carried out in water vapour at 130°C. The thermogravimetric curves are shown in figure 2. I t is possible to inhibit the hydration of the hemihydrate by water vapour. For instance, i t is sufficient to dehydrate the gypsum at a temperature of 130°C, in a water vapour pressure of 5 torrs (curve 3 of the figure 2). 3. Hydration by liquid water An adequate study of the hydration mechanism of calcium sulphate by liquid water requires the use of many techniques, such as isothermal calorimetry (10), adiabatic calorimetry (11) (12), scanning electron microscopy (13), chemical analysis (14). The isothermal calorimetry seems to be the most s u i t f u l l technique for obtaining useful results on the overall kinetics of the reaction.

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313 THERMOLUMINESCENCE, REACTIVITY, CaSO4.1/2H20

mole H20 /mole CaSO~ -Z

nole H"2'O/mole C,~,

®

®

r

•

13o'c

-,,s

0

PH,O = l O "

torr ""

CT) Q "" o o,s to~ :

~O"H:°

" 2 t°~"~

•

I

0

T * 130°C T - IIO°C

O,S

Q

lO I

T * 90°C

20 I

30 I

(hours)

t

®

,S

FIG. 1 Influence of temperature of dehydration

r

20

40

60

I

m

!

80 I t(hours)

FIG. 2 Influence of the water vapour pressure of dehydration

3.1. Influence.of the temperature of dehydration An increase of the dehydration temperature at constant water vapour pressure has two consequences (figure 3) : - a .lengthening of the induction period - a decrease in hydration rate. 3.2. Influence of the water .yapour pressure The length o f the induction period and the hydration rate vary monotonically with the water vapour pressure for a decomposition at a temperature of 130°C (figure 4). ~t (cal/nm'~)

Oenydration conditions PH=O = 10"7 tort

~t (ca]'mn'Z)

Dehydration conditions T - I~°C

IIO°C 0 4

-4

! FIG. 3 Hydration by liquid water (E/P = 1 ; T = 25°C) Influence of dehydration temperature

Q

@

10 -7 torr 0,S torr 2 torts (~) S torts (~) I0 torts

*")

FIG. 4 Hydration by liquid water (E/P = 1 ; T = 25°C) Influence of water vapour pressure

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Vol. 8, No. 3 M. T r i o l l i e r , B. Guilhot

4. Specific areas The specific surface of the samples were measured by the B.E.T. method. The calcium sulphate hemihydrate has a specific surface of 14 m2/g*,irres pective of heat treatment conditions. 5. Thermoluminescence Classical chemical analysis does not differentiate between the samples obtained from a given gypsum under different conditions. Their reactivity can be related to their concentration of structural defects. Thermoluminescence being an e f f i c i e n t means for the detection of defects in t r i c a l cium s i l i c a t e (15), recommends i t s e l f to this study. The samples are irradiated for 10 minutes with ultra violet radiation at a temperature of - 180°C. They are then heated to 400°C (linear heating rate) under continuous vacuum pumping (P = I0 -s t o r t ) . Measurement of the l i g h t emission as a function of temperature shows the existence of a peak near - 100°C. Thus calcium sulphate hemihydrate presents an a r t i f i c i a l thermoluminescence. Having undergone the same treatment, gypsum emits no luminous radiation. The dehydration conditions of calcium sulphate dihydrate strongly modify the intensity of the thermoluminescence peak (figures 5 a~d 6). This parameter determines the concentration of defects in the hemihydrate. I t is thus concluded that a decrease of the temperature or of the water vapour pressure involves an increase of the number of traps. Characteristics of traps The parameters of the traps can be calculated according to several methods (16,17,18,19). The formalism of RANDALL and WILKINS (20) is prefered. Experimentally, the sample is heated according to a linear heating rate B. The temperature Tm of the peak maximum is noted in each case. The model implies that the curve Log ~FT = f (~-) is a straight line. m m In order to obtain the maximum precision, the hemihydrate prepared in a vacuum is chosen on account of the great intensity of its thermoluminescence peak. The straight line obtained in coordinates of RANDALL and WILKINS (figure 7) enables us to calculate E (depth of the traps), s (frequency of attempt to escape) and p (probability of escape of trapped electrons at 20°C). The values obtained are as follows : E = 0,22 eV s = 9,2. 10s sec-~ p (20°C) = 7,8. 10-3 sec-l No strong modification of the traps characteristics is noted according * The values of specific surface recently published (5) are in error. They are to be remplaced by 14 m2/g.

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315

THERMOLUMINESCENCE, REACTIVITY, CaSO4.1/2H20 I (arbitral

I (arbitrary units)

units)

Oeh},dration conditions T - 130°C Dehydrmtion conditions

0

( ~ I0-' tort

_

(2)0,5 t o r t ~ ) 2 torts ~K) s t°rrs

A I

PH=O= IO'T tort

~

90oc llO°C

130°C

T(K)

FIG. 5

FIG. 6 Thermoluminescence - excitation wavelength 2537 heating rate : 2,8°C/S Influence of the temperature Influence of the water vapour pressure to the method of preparation of calcium sulphate hemihydrate. Thus the different thermal treatments of gypsum have mainly an influence on the number of trapped electrons. Discussion The thermogravimetric study indicates monotonic kinetics of calcium sulphate hemihydrate hydration by water vapour for the variables, temperature on the one hand and water vapour pressure on the other hand, characterising its method of preparation (6). Similar results are obtained for the hydration by liquid water. The thermoluminescence indicates the existence of defects in the hemihydrate, which may be considered as potential nuclei. Actually, the intensity of the thermoluminescence peak is related to the kinetics parameters of hydration, in particular to the position of the second calorimetric peak corresponding to the maximum rate of reaction. S is the thermoluminescence peak surface and t is the time corresponding to the maximum rate. The curve S = f (tm~v) c~Xbe considered as a straight line under the chosen experimental ~fiditions (figure 8). An increase of thermoluminescence, indicating a modification of the number of traps, corresponds to an increase of the hydration rate. A rise in temperature or a chemisorption of a polar gas on a solid promotes the mobility of defects. They group and disappear on the surface, which could explain the observed surface state in the case of a dehydration

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M. T r i o l l i e r , B. Guilhot

Ln

-

S(a~ounits)

Tz

I0

9

.....

~,o

6.2

{

lo' ~ (oK-,) I

0

FIG. 7 Relation of RANDALL and WILKINS

2~

~0

6ptFmn

FIG. 8 Relationship between thermoluminescence and maximum rate of hydration S = f (tmax)

under a water vapour pressure (5) and the decrease of the thermoluminescence peak. The nuclei appear at points where the activation energy is minimal. The number of nuclei formed in a given time depends on the number of potential nuclei and on the average value of the activation energy of germination. The thermoluminescence experiments show that the conditions of dehydration of gypsum modify the number of potential nuclei in the hemihydrate obtained, without appreciable change of the characteristics of the traps. The specific surface is not the dominating parameter of the hydration reaction. Indeed, the method of preparation of the hemihydrate has no effect upon the specific area, but on the other hand strongly modifies the hydration kinetics. Conclusion Defects in calcium sulphate hemihydrate are detected by thermoluminescence. The aptitude to hydration of hemihydrate, related to its concentration in structural defects, is characterised by a thermoluminescence signal. Measurement of the peak surface can be envisaged as a method of determining the product reactivity with respect to water vapour or liquid water. Acknowledgment The authors thank Professor FIERENS for his help in developing the thermoluminescence appliance.

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1.

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2.

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3.

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4.

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5.

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6.

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7.

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8.

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9.

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10. Jeandot, Th~se Lyon (1972). 11. R. Magnan, Amer. Ceram. Soc. Bull., 4_99,314 (1970). 12. E. Karmazsin and M. Murat, Bull. Soc. Chim. Fr., ~, 17 (1974). 13. G. Neuhauser, Zement. Kalk. Gips., Dtsh, 2_99(5), 227 (1976). 14. F.W. Birss and T. Thorvaldson, Can. J. Chem., 6 (7), 292 (1955). 15. P. Fierens, J. Tirlocq and J.P. Verhaegen, Cement and Concrete Research, 3, 549 (1973). 16. G.F.J. Garlick and A.F. Gibson, Proc. Roy. Soc. A60, 574 (1948). 17. C.B. Lushchik, Dokl. Acad. Nauk SSSR, 10_~1,641 (1955). 18. R. Chen, J. Appl. Phys., 40, 570 (1969). 19. C. Pedrini, Th~se Lyon (1971). 20. J.T. Randall and M.H.F. Wilkins, Phosphorescence and Electron Traps. Proc. Roy. Soc., A184, 347, 346, 390 (1945).