Plaster of Paris

Calcium sulphate hemihydrate, CaSO₄·½H₂O, is plaster of Paris. It is obtained by heating gypsum, CaSO₄·2H₂O, to 393 K: 2CaSO₄·2H₂O(s) → 2CaSO₄·½H₂O + 3H₂O. If heated further, above 393 K, all the water of crystallisation is driven off and anhydrous calcium sulphate, CaSO₄, is formed -- this fully dehydrated form is known as "dead burnt plaster."

Plaster of Paris has the remarkable property of setting with water: mixed with an adequate quantity of water, it forms a plastic mass that hardens into a solid within 5 to 15 minutes. …