The α- and β-Forms of Calcium Sulphate Hemihydrate

Abstract

Kelley, Southard and Anderson¹ have shown that the thermodynamic properties of calcium sulphate hemihydrate depend on whether the hemihydrate is produced from gypsum calcined in a steam-saturated atmosphere (α-hemihydrate), or under conditions where the partial pressure of water vapour is low (β-hemihydrate), but the nature of a hemihydrate prepared under conditions intermediate between these two extremes (for example, the manufacture of gypsum plaster by the kettle process) is not known for certain. These authors considered that such a hemihydrate would consist of a mixture of the α- and β-forms, but Eipeltauer² considered that there would also be some hemihydrate crystals having properties intermediate between these two forms. Work that I have done shows that only the α- and β-hemihydrates can be detected by differential thermal analysis in a sample of hemihydrate prepared by calcining gypsum under conditions simulating the manufacture of gypsum plaster in a kettle.