When gypsum or plaster of Paris is heated above 393 K , it loses all the water of crystallization and anhydrous calcium sulphate is obtained .
`Caunderset("Gypsum")(SO_(4).2H_(2)) O overset( gt 393K)(to) underset("Dead burnt plaster")(CaSO_(4)) + 2H_(2)O`
`2 underset("Plaster of Paris")(CaSO_(4).2H_(2)) O overset(gt 393 K)(to) 2 CaSO_(4) + H_(2)O`
Anhydrous calcium sulphate does not absorb water to form gypsum and hence loses the properties of setting with water . That is why anhydrous calcium sulphate is called dead burnt plaster .
