GYPSUM PLASTER 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 of steam and forms calcium sulphate hemihydrate, known
