Process for the conversion of anhydrite into gypsum

Gypsum and gypsum plaster products can be made from an anhydrite or mixed anhydrite and gypsum starting material, of natural origin or derived chemically as a byproduct, by mixing the powdered anhydrite with part or all (but not more) of the water necessary for complete hydration to gypsum, compressing the mixture into compacted blocks or other bodies, and allowing the anhydrite to hydrate. If insufficient water is included in the mixture, the compacted bodies can be treated with water, for example by immersion, until sufficient water has been taken up. Accelerator can be mixed with the anhydrite or with the water. The product is a dry gypsum material in block or extruded form which can be used as such as calcined to form a satisfactory plaster.