Investigation of the Calcium Sulphate-Water System by Infrared Spectroscopy

Abstract

THERE has been a great deal of uncertainty in the chemical literature concerning the precise forms of the various products formed when gypsum (CaSO₄,2H₂O) is dehydrated thermally. The system CaSO₄-H₂O has been studied by various workers^1–4; the products were chiefly studied by X-ray diffraction and differential thermal analysis (DTA) techniques. Weiser et al.¹ have shown that a definite hydrate, CaSO₄,½H₂O, is formed, which in turn dehydrates to dehydrated hemihydrate (soluble anhydrite). The X-ray diffraction patterns for these are similar, but on the basis of minor differences between the two patterns, hemihydrate and dehydrated hemihydrate have been established as different chemical entities. Two forms of hemihydrate (α and β) and soluble anhydrite (α and β) have been defined², depending on the method of preparation. Gilliland³ again showed very minor differences between the X-ray diffraction patterns of hemihydrate and soluble anhydrite. It thus seems that X-ray methods are unsuitable for differentiating between the different dehydration products present. Also, DTA, though giving promising results for gypsum and hemihydrate³, could not be used to distinguish between the soluble and insoluble forms of anhydrite.