Transformation of the α- and β-forms of calcium sulphate hemihydrate to insoluble anhydrite

Abstract

ALTHOUGH there is no difference between the X-ray diffraction patterns of α- and β-calcium sulphate hemihydrate1, differences have been reported for density, specific heat and the heat of formation2. I have shown that the dehydration of each form to soluble anhydrite proceeds at about the same rate, but that the rate of transformation of soluble to insoluble anhydrite is much greater for the α-form. This is of importance in the examination of gypsum plasters, which may contain one or both of these forms according to the method of manufacture. The difference between the α- and β-forms is demonstrated here by simultaneous differential thermal and thermo-gravimetric analyses3, and by determining the rate of the phase transformation at a static temperature. © 1958 Nature Publishing Group.