THE EFFECT OF WATER VAPOR UPON THE DEHYDRATION OF CaSO4•2H2O

Kinetics of the two-stage dehydration of CaSO4•2H2O 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, at 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. Activation energy and pre-exponential factor for the dihydrate → hemihydrate process also increased linearly with water vapor pressure. Hemihydrates produced at the extremes of water vapor pressure corresponded to the α- and β-modifications, as defined thermodynamically, and the production of a hemihydrate series with properties varying linearly from one extreme to the other is discussed.