Gypsum is hydrated sulphate of calcium (CaSO4 · 2H2O). It is not available in nature in pure form but contains impurities such as alumina, calcium carbonate, magnesium carbonate and silica. It is a white crystalline substance less soluble in water but soluble in hydrochloric acid. As a binding material, it sets and hardens quickly.
Gypsum heated between 120 to 180°C loses about 14.7% of the water of crystallisation in the form steam and forms calcium sulphate hemihydrate, known as plaster of Paris. Further heating leads to removal of all water and forms calcium sulphate anhydrate. This plaster is mixed with a small of accelerator (i.e., alum potassium sulphate, raw gypsum) to make anhydrous or Keene’s gypsum plaster.
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