Model plaster (white) used to cast study models before mixing with water, is largely composed of:
CaO
CaCO3
CaSO4  • ½H2O
CaSO4 • 2H2O
When plaster is mixed with water, it takes up one and a half molecules of water, i.e, it regains its water of crystallization and becomes calcium sulphate dihydrate.
(CaSO4)2.H2O + 3H2O → 2CaSO4.2H2O+ unreacted (CaSO4)2.½H2O + Heat
Hemihydrate + Water → Dihydrate + Unreacted hemihydrate + Heat
The reaction is exothermic and is the same for all gypsum products. The amount of water required to produce a workable mix varies between the products. As evident from the above reaction, not all of the hemihydrate converts to dihydrate. The amount of conversion is dependent on the type of stone. The highest conversion rate is seen in plaster (90%). In Type 4 and 5 stone, the dihydrate content is about 50%. 
Key Concept
This corresponds to the first step in the reaction. The principal constituent of gypsum-based products is calcium sulfate hemihydrate .
Phillips Science of Dental Materials ed 12 pg183