Gypsum plaster is a white powder made from the mineral gypsum. You mix it with water and it sets hard on the wall in minutes.
The powder is calcium sulphate hemihydrate (CaSO₄·½H₂O). A factory makes it when it heats gypsum rock and grinds it. On site the powder takes up water again and turns back into gypsum (calcium sulphate dihydrate). This reaction makes the plaster hard.
Gypsum plaster goes on the inside walls and ceilings of a building, in one coat, with a smooth white finish.