What is dead burnt plaster?
Plaster of Paris is obtained by heating gypsum. Following is the chemical equation for the reaction
CaSO4.2H2O → CaSO4.1/2H2O + 11/2 H2O
However, when heated beyond 393 K, all the water of crystallisation is lost resulting in the formation of anhydrous calcium sulphate. This anhydrous calcium sulphate obtained by the over-heating of gypsum is known as dead burnt plaster. It does not possess any properties of setting with water.
Plaster of Paris is formed by heating gypsum at 100 degree celsius.At this temperature Gypsum looses 3/4 of its water of crystallization forming plaster of paris.If the temperature is allowed beyond 100 degree celsius,it will become anhydrous calcium sulphate which is called Dead burnt blaster unlike the plaster of paris it does not have the property to set after mixing with water.