Chemistry of by-product gypsum and plaster. I. Identity of an important solid solution impurity

An investigation has been made into why calcium sulphate β-hemihydrate, derived from the by-product gypsum of wet-process phosphoric acid manufacture, can, when derived from certain types of phosphate rock, be insensitive to hydration retarders. Aluminium and fluorine, which are common impurities in phosphate rock, form complex aluminofluorides in the phosphoric acid reaction liquor; and gypsum crystallising from this system has its crystal habit and physical properties modified by these complexes. The hemihydrate calcined from the gypsum has a setting time that is less amenable to control by retarder addition than the setting time of pure hemihydrate. The gypsum crystals exhibiting this effect incorporate a solid solution impurity which has been identified as AlF₅^2? substituting for SO₄^2? ions in the crystal lattice.