Calcium Sulphate, CaSO4.1/2H2O

Calcium Sulphate, CaSO4.1/2H2O

Calcium Sulphate or Plaster of Paris is hemihydrates of calcium sulphate. It is obtained when gypsum, CaSO4.2H2O, is heatd to 393 K.

2(CaSO4.2H2O)    →    2(CaSO4).H2O   +   3H2O

Above 393 K. no water of crystallization is left and anhydrous calcium sulphate, CaSO4 is formed. T his is known as ‘dead burnt plaster.

It has a remarkable property of setting with water. On mixing with an adequate quantity of water it forms a plastic mass that gets into a hard solid in 5 to 15 minutes.


The largest use of plaster of Paris is in the building industry as well as plasters. It is used for immoblising the affected part of organ where there is a bone fracture or sprain. It is also employed in dentistry, in ornamental work and for making casts of statues and busts.