Explanation

Plaster of Paris changes into gypsum on mixing with water 

It is commonly called P.O.P. is calcium sulfate hemihydrate.

The chemical formula of plaster of Paris  is CaSO4.1/2 H2O

Crystals of gypsum are produced when Plaster of Paris is mixed with water 

Gypsum is calcium sulfate dihydrate and a mineral used in cement manufacturing.

It helps in controlling the rate of hardening of the cement.