Which chemical compound is the main ingredient in plaster of Paris?

Calcium sulfate hemihydrate is the main ingredient in plaster of Paris.

Its formula is commonly written as CaSO₄·½H₂O, meaning each calcium sulfate unit is associated with half a water molecule in the crystal structure. It is made by partially heating gypsum, which is calcium sulfate dihydrate, CaSO₄·2H₂O.

When plaster of Paris is mixed with water, it rehydrates and hardens back into gypsum. The setting process produces interlocking crystals, which is why the material becomes firm. The reaction also releases heat, although the amount depends on the mixture and conditions.

Plaster of Paris is used for casts, molds, model-making, wall finishes, and some medical orthopedic casts. A common mix-up is calling gypsum itself plaster of Paris: gypsum is the dihydrate, while plaster of Paris is the hemihydrate made from it.