Studies in the system calcium sulphate–water. Part III. Kinetics of dehydration of α-calcium sulphate hemihydrate

Abstract

The kinetics of dehydration of α-calcium sulphate hemihydrate have been studied between 68 and 217° at constant partial pressures of water vapour between 10^–5 and 360 mm. The results obtained at temperatures greater than 111° are consistent with a diffusion-controlled mechanism having an activation energy of 15·6 kcal. at 10^–5 mm. of water vapour and 36 kcal. At 44 mm. At lower temperatures the dehydration is more complex, with some diffusion control and some phase boundary control depending on conditions. Some information on the γ-CaSO₄→β-CaSO₄ inversion has been obtained, which indicates a minimum temperature of ca. 140° for the transformation. Variation in external partial pressure of water vapour has a negligible effect on this inversion temperature.