Civil Engineering Today

  Gypsum Gypsum is a soft sulfate mineral composed of calcium sulfate dihydrate, with the chemical formula CaSO 4 ·2H 2 O . It is widely mined and is used as a fertilizer and as the main constituent in many forms of plaster, blackboard/ sidewalk chalk, and drywall. It contains 79.1 % calcium sulfate and 20.9% water. Its specific gravity is 2.3. It is a white crystalline substance. 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. Its initial setting time after addition of water is about 4 to 6 minutes and its final setting time is about 30 minutes. Effect of heat and moisture on gypsum - 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 P...