Dead burnt plaster is-

1. CaSO4

2. CaSO4·12H2O

3. CaSO4·H2O

4. CaSO4·2H2O

Hint: Dead burnt plaster is anhydrous calcium sulphate CaSO4

Plaster of Paris is prepared by heating gypsum at 120°C
2CaSO4·2H2OGypsum(CaSO4)2·12H2OPlaster of Paris+3H2O
On heating plaster of Paris at 200°C, if forms anhydrous calcium sulphate i.e., dead plaster which has no setting property as it absorbs water very slowly.