THE EFFECT OF WATER VAPOR UPON THE DEHYDRATION OF CaSO_4•2H₂O

Abstract

Kinetics of the two-stage dehydration of CaS04.2HzO have been examined under controlled water vapor pressures up to one atmosphere. For both stages water vapor initially accelerated the rate of dehydration and subsequently retarded it. Separate, temperature-dependent water vapor pressures were noted above which each stage could be suppressed.The hemihydrate was clearly defined either as a change in the rate of weight loss during dehydration or, a t higher water vapor pressures, as a fixed composition. The heat of solution of the hemihydrate increased linearly with the partial water vapor pressure present during its formation, but was independent of the formation temperature over the range studied. *\ctivation energy and pre-exponential factor for the dihydrate -+ hemihydrate process also increased linearly with water vapor pressure. Hemihydrates produced a t the extremes of water vapor pressure corresponded to the a-and 6-modifications, as defined thermodynamically, and the production of a hemihydrate series with properties varying linearly from one extreme t o the other is discussed.The dehydration of calcium sulphate dihydrate (gypsum) has been studied for decades by many investigators, few of whoin have made provision for controlling the partial water vapor pressure during dehydration. No systematic study of its kinetic influence has been reported, although Weiser and co-workers (1, 2) clearly demonstrated the importance of such isobaric studies to a proper understanding of the system. Despite earlier controversy (3) the combination of evidence now available (1, 2 , 4-8) has established the existence of the hemihydrate as the lower hydrate in the CaS04-H20 system. However, the composition in terms of water of hydration can be variable over rather wide liinits and has led to speculation that there inay exist a zeolitic type of structure rather than that of a true hydrate. Clarification has recently been obtained from nuclear magnetic resonance results (5), showing the hemihydrate to contain definite crystal water as well as a variable quantity of relatively free water "occluded" within the crystal lattice. Two modifications of the hemihydrate are known, differing in thermodynamic properties (4), although being structurally quite similar (9, 10). Speculation that these differences reflect the degree of crystal lattice perfection has recently received support (6). This paper will describe a syste~natic study of the influence of water vapor pressure upon the kinetics of the dehydration process and upon the thermodynamic properties of the heinihydrates obtained. Previous studies of these properties involved hemihydrates produced a t the extreme conditions, i.e. under essentially anhydrous conditions or in an atmosphere saturated with water vapor, plus some commercial plasters produced under undefined partial water vapor pressures (-1, 11). A later paper ~vill discuss the influence of partial water vapor pressure upon such practical properties of the hemihydrate as gauging water requirement. E X P E R I M E...

Influence of Alkyl Trimethyl Ammonium Bromides on Hydrothermal Formation of α-CaSO4·0.5H2O Whiskers with High Aspect Ratios

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Thermal Behavior of the Gypsum Binder in Dental Casting Investments

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