Correct Answer - D
`2CaSO_(4).2H_(2)Ooverset(393K)to2CaSO_(4).(1)/(2)H_(2)O+3H_(2)O`
Above 393 K no water of crystallization is left and anhydrous calcium sulphate called head burnt plaster is formed. It is so called because it loses the properties of setting with water. Gypsum should not be allowed to come in contact with C containing fuel otherwise some of it will be reduced to calcium sulphite.
