Question

Why caso4 is called dead burnt plaster?

Answer 1

When the Gypsum is heated at 393 K, at any temperature above it will cause the water to get liberated to form anhydrous calcium sulphate which is known as dead burnt plaster

Answer 2

Plaster of Paris is prepared by heating gypsum at 120° C.
2CaSO4.2H2O → (CaSO4)2.H2O + 3H2O 
  Gypsum        Plaster of Paris   

On heating plaster of Paris at 200°C, it forms anhydrous calcium sulphate, i.e., dead plaster which has no setting property as it absorbs water very slowly.