Calcium Sulfate → Calcium Sulfate, Hemihydrate

Participants

Reaction Steps

Gypsum, for example, is a very soft mineral composed of calcium sulfate dihydrate, CaSO4.2H2O. This form is referred to in the art as calcium sulfate. Heating gypsum at above approximately 150° C. partially dehydrates it to obtain calcium sulfate hemihydrate, CaSO4.½H2O, is a substance in which the molecular ratio of water molecules to anhydrous calcium sulfate is 1:2, commonly known as “calcined gypsum” or “Plaster of Paris”. When calcium hemihydrate is mixed with water at ambient temperatures, it crystallizes into a strong gypsum crystal lattice in an exothermic reaction. Gypsum also has an anhydrous form, termed anhydrous calcium sulfate or calcium sulfate anhydrate, CaSO4, which is produced by further heating of the calcium sulfate hemihydrate to above approximately 180° C. Anhydrous calcium sulfate reacts slowly with water to return to the dihydrate form.

Provenance & Evidence