Calcium sulphate with half a molecule of water per molecule of the salt (hemi-hydrate) is called plaster of paris (plaster of paris).
It is prepared by heating gypsum (CaSO4.2H2O) at 120°C in rotary kilns, where it gets partially dehydrated.
Plaster of paris is a white powder, which when mixed with water sets into a hard mass as crystals of gypsum are produced with evolution of heat. Hence it is used in setting of plaster for fixing broken bones. It is also used in making several decorative items, toys and chemical plugging