Mechanism of dehydration of calcium suphate dihydrate : application of the helium pyknometric technique

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Publisher’s version / Version de l'éditeur:

Journal of the Chemical Society Faraday Transactions, 79, pp. 2071-2075, 1983

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Mechanism of dehydration of calcium suphate dihydrate : application of

the helium pyknometric technique

Beaudoin, J. J.; Feldman, R. F.

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MECHANISM OF DEHYDRATION OF CALCIUM SULPHATE DIHYDRATE

by J.J. Beaudoin and R.F. Feldman

Reprinted from

Journal Chemical Society Faraday Trans. 1, 1983, 79 p. 2071

-

2073

BLDG.

RES.

L t B R A R Y

DBR Paper No. 1154

Division of Building Research

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R ~ S U M ~

Une t e c h n i q u e f a i s a n t i n t e r v e n i r l a d i f f u s i o n d'hglium a 6t'e a p p l i q u ' e e B 1 1 6 t u d e d e l a d d s h y d r a t a t i o n s o u s v i d e du

CaS0,;2H20. D e n o u v e l l e s donn'ees S t a y e n t les h y p o t h b e s s u i v a n t l e s q u e l l e s , d ' u n e p a r t , l ' e a u e s t 'elimin'ee du 8-hgmihydrate comme s ' i l s ' a g i s s a i t d ' u n e z d o l i t h e e t , d ' a u t r e p a r t , d e p e t i t e s q u a n t i t ' e s d ' e a u s o l i d e m e n t r e t e n u e s b l o q u e n t Z ' a c c S s a u x m i c r o - e s p a c e s l i b r e s d e l a s t r u c t u r e du B-hsmihydrate. Une B v a l u a t i o n d e s mesures d e masse volumique a l n s i q u e d e s changements d e masse volumique d u s aux p r o c e s s u s

de d 6 s h y d r a t a t i o n e s t f o u r n i e . - - - . . - - - - P. - - - - L - - -

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J. Chem. Soc., Faraday Trans. 1, 1983,79,2071-2075

Mechanism of Dehydration of Calcium Sulphate

Dihydrate

Application of the Helium Pyknometric Technique

National Research Council of Canada, Division of Building Research, Ottawa, Canada KIA OR6

Received 18th October, 1982

A technique involving helium diffusion has been applied to a study of the dehydration of CaSO, 2H,O in vacuo. New evidence supports arguments that removal of water from the B-hemihydrate is 'zeolite-like' and that small amounts of tightly held water block entrances to microspace in the B-hemihydrate structure. An appraisal is given of density measurements and changes due to dehydration processes.

Numerous studies have been conducted with the object of defining the processes, mechanisms and structural changes that occur during the decomposition of CaS0,-2H,O (dihydrate) and CaSO,.iH,O (hemihydrate).' Various mechanisms of dehydration have been proposed.Vt has been inferred from single-crystal X-ray diffraction studies that pseudomorphous hemihydrate crystals form when water is removed from the d i h ~ d r a t e . ~ It has also been suggested that water removed from hemihydrate is zeolitic., The existence of microspace (channels 5-50

A

in diameter) in the B-hemihydrate structure (one of a possible series of hemihydrates) was also inferred from surface-chemical experiment^.^ Several workers2, have argued that the P-hemihydrate is crystallographically disordered, and that restoration of order, including formation of microspace, results on formation of the so-called orthorhombic form of anhydrite. A helium pyknometric technique developed earlier to study processes involved in the removal of water from various microporous calcium silicate hydrates6 has now been applied in an attempt to resolve some of the uncertainties described in the literature.

EXPERIMENTAL

Reagent-grade CaSO4.2H,O powder was pressed into compacts. Increments of water were removed from samples initially conditioned to 11 % relative humidity by vacuum drying and heating in steps; the majority of the steps involved heating in vacuo for 1 h at temperatures between 50 and 200 OC. After each dehydration step, measurements were made with a helium comparison pykn~meter.~ Helium intake against time curves and values of instantaneous solid volume were obtained upon removal of each increment of water from the samples. The length change of companion samples was monitored continuously throughout the dehydration process. For this purpose compacted samples were mounted on modified Tuckerman extensometers placed in vacuum cells suitable for optical reading of the extensometers. Differential scanning calorimetric (d.s.c.) traces were obtained at various stages of CaS0;2HZO dehydration, using a Dupont 900 thermal-analysis system.

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2072 DEHYDRATION OF CaSO,. 2H,O RESULTS AND DISCUSSION

Helium intake against time curves (not presented) were obtained after each dehydration step, each curve corresponding to a particular increment of weight loss from the weight at 11% relative humidity. After the final step 0.58% water still remained in the samples. In general the rate of helium intake approached a constant value at 40 h. The amount at 40 h obtained from each condition is plotted against weight change in fig. 1. There are maxima at weight change values of 8.63 and 14.60%,

2 0

0 1 2 3 4 5 6 7 8 9 10 1 1 12 13 14 15 16 17 18 19 20 2 1 weight change (%)

Fig 1. Helium intake at 40 h plotted against weight change of CaSO, -2H,O dehydrated in increments from the 1 1 % relativity humidity condition.

corresponding to 0.82 and 1.40 H,O molecules, respectively. The decrease in He intake between 8.63 and 12.79% may be due to structural collapse of CaS04.2H,0 as holes due to dehydration increase. At 20.33% weight change there is a large increase in helium intake, with a small weight change. Space has suddenly become accessible to helium, although there is ca. 0.58% water remaining in the sample, possibly because water blocking some of the micropores has been removed. Length-change data provide additional support for this explanation.

Fig. 2 is a plot of 3Al/l (corresponding to apparent volume change) against solid- volume change. The abscissa was obtained from instantaneous solid-volume measurements in the helium pyknometer. Values for the ordinate are similar to those for volume change up to a weight change of 12.79%. The data point at 8.53% lies above the line, possibly a result of the decrease in He intake after this weight loss and structural collapse of CaS04.2H,0. At 12.79% weight change the d.s.c. curves (not presented) indicate that the system is essentially 8-hemihydrate. The weight loss corresponding to 1.5 H,O molecules commonly assigned to 8-hemihydrate is 15.15

%,

although this stoichiometry has been debated in the literature.'ps

At weight changes 2 12.79% the length change remains constant in spite of large changes in the solid volume. This suggests that water simply vacates the micropores without affecting the structure and that the phase change to anhydrite (similar in crystal structure) causes only slight volume changes. The effect on the length change of removal of the remaining 0.58% water was not determined, but may be large. When there is no length change, changes in solid volume are detected because helium enters

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J. J. BEAUDOIN AND R. F. FELDMAN 2073 14 ₁ ₁ ₁ ₁ ₁ ₁ ₁ ₁ ₁ ₁ ₁ ₁ ₁ ₁ ₁ ₁ ₁ ₁ 13

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12

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_-

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0 -0 11

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0----7)---0

-

E I G H T C H A N G E . 1 2 . 7 9 2 Q 2 9 -

-

a m 8 -

-

1 5 1 1 1 1 0 1 2 2 2 25 solid-volume change (%)

Fig. 2. Length change plotted against solid-volume change during dehydration of CaS0,. 2H,O.

weight change (%)

Fig. 3. Density of CaS0;2H20 at various stages of dehydration. (-) Calculated using

solid-volume measurements corrected for helium intake; (-

-

-) calculated assuming the density of water to be 1.26 x loS g dm-3.

the space vacated by water, giving the appearance of a solid-volume decrease. The ability of helium to penetrate microspace in this system, which apparently is inaccessible to other gases, e.g. N, and Ar, suggests this is the reason for the paucity of density measurements (and inconsistency of surface area) reported in the literature.

A plot of density of CaSO, .2H,O against weight change is given in fig. 3. The solid curve represents density values obtained by directly measuring weight and solid

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2074 DEHYDRATION OF CaSO,. 2 H 2 0

volume; the apparent solid volume is reduced by the volume of helium that has diffused into the microspace. The dashed curve is calculated by correcting the initial solid volume by the volume assigned to the water removed. The value used for the density of water is 1.26 x lo3 g dm-3. This is referred to later. Initially the solid and dashed curves give similar density values that increase to 2.68 x lo3 g dm-3 at a weight change of 14.60% (ca. 1.40 H 2 0 molecules removed).

A decrease was observed in He intake after this weight loss (fig. 1). Beyond 14.60% weight loss (solid curve) it is apparent that entry of helium into the microstructure was restricted, resulting in density values lower than expected. At 20.33% weight loss there was a sudden large increase in the amount of helium entering the system and the 'measured' density values approach the 'calculated' values. This is additional support for the view that water is trapped in microspaces during dehydration of hemihydrate, i.e. occupying and blocking space in channels, preventing entry and affecting estimation of the true volume of the solid. Removal of some of this water allows access to additional microspace.

An additional composite term, A V- AD, was calculated. A V is solid-volume change (with a negative sign for shrinkage) and A D is the total amount of helium intake at any particular stage of the dehydration process. In systems where there is no effective volume change due to phase change (in this context a change in the lattice or in the orientation of Ca2+ and SO:-) the term is a measure of the volume occupied by water prior to its removal from the structure.

In a plot of A V- A D against weight change (not presented) two linear regions were observed, the first ending at a weight change of ca. 13.86% and the second terminating at a weight change of 20.25%. At a weight change incrementally greater than 20.25% there was a large change in AV-AD. The inverse slope of the first linear curve is 1.26 x lo3 g dm-s, a density value comparable to other published values for adsorbed water.s This suggests that the volume change associated with the phase change due to 8-hemihydrate formation may not be as significant as shrinkage of solid particles due to water-removal itself. The inverse slope of the second linear region is much greater than 1.26 x lo3 g dm-3, indicating that helium cannot penetrate all the space vacated by water. The subsequent large change in A V - A D may be due, as previously indicated, to removal of a small amount of water that blocked the access of helium to the micropores.

It may be argued that if a true value for the density of water is determined from the inverse slope of the first curve (up to 12.79% weight change) then the influence of a crystal lattice change from CaSO,

.

2 H 2 0 to 8-hemihydrate is possibly minor. The major piece of evidence for the applicability of the term 'zeolitic behaviour' to the dehydration of P-hemihydrate is considered to be the similarity of X-ray diffraction lines for the 8-hemihydrate and the soluble anhydrite. Length and solid-volume measurements in this study appear to corroborate this view. As indicated, significant changes in the slopes of the solid curve (fig. 3) and plots of A V - A D against weight change occur at weight losses between 13.86 and 14.60%, corresponding to 1.32-1.40 H 2 0 molecules. In stoichiometric terms this corresponds to formation of CaSO,.nH,O where n varies from 0.60 to 0.68. It supports the opinion of Dunn7 and B ~ n n , ~ both of whom have suggested a series of subhydrates.

CONCLUSION

A new helium-diffusion technique has been applied to the study of CaS0,-2H20 dehydration. The experimental results appear to support arguments that removal of water on dehydration of 8-hemihydrate is zeolite-like. Removal of the final 0-1

%

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J. J. BEAUDOIN AND R. F. FELDMAN 2075

water from the dehydrated system is more difficult. Small amounts of water effectively block the entrances to micropore space during dehydration.

We thank J. J. Wood for his fine work in performing the experiments. This paper is a contribution from the Division of Building Research, National Research Council Canada, and is published with the approval of the Director of the Division.

P. J. Sereda and V. S. Ramachandran, J. Am. Ceram. Soc., 1975,58, 94. H. G. McAdie, Can. J. Chem., 1964, 42, 792.

0. W. Florke, Neues Jahrb. Mineral., 1952,84, 189.

S . Eld. Hamada, Trans. J. Br. Ceram. Soc., 1981, 80, 56.

M. C. Ball and L. S. Norwood, J. Chem. Soc., Faraday Trans. 1, 1978,74, 1477. R. F. Feldman, Cem. Concr. Res., 1971, 1, 285.

'

J. S. Dunn, Chem. Znd. (London), 1938, 144.

C. W. Bunn, Chemical Crystallography (Oxford University Press, Oxford, 1946), pp. 184 and 334.

@ S. Brunauer, The Adsorption of Gases and Vapors (Princeton University Press, Princeton, 1943), p.

420.

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T h i s publication is being d i s t r i b u t e d by t h e Division of Building R e s e a r c h of t h e N a t i o n a l R e s e a r c h Council of Canada. I t should not b e r e p r o d u c e d i n whole o r i n p a r t without p e r m i s s i o n of t h e o r i g i n a l p u b l i s h e r . T h e Di- v i s i o n would b e glad t o b e of a s s i s t a n c e i n obtaining s u c h p e r m i s s i o n .

P u b l i c a t i o n s of t h e Division m a y b e obtained by m a i l - ing t h e a p p r o p r i a t e r e m i t t a n c e ( a Bank, E x p r e s s , o r P o s t Office Money O r d e r , o r a cheque, m a d e p a y a b l e t o t h e R e c e i v e r G e n e r a l of Canada, c r e d i t NRC) t o t h e National R e s e a r c h Council of Canada. Ottawa. K I A OR6.

S t a m p s a r e n o t a c c e p t a b l e .

A l i s t of a l l p u b l i c a t i o n s of the Division i s a v a i l a b l e and m a y be obtained f r o m t h e P u b l i c a t i o n s Section, Division of Building R e s e a r c h , National R e s e a r c h C o u n c i l of Canada, Ottawa. KIA OR 6.