plasterofParis

Plaster of Paris is a building material used for coating walls and ceilings. It is a fine white powder, a mixture of calcium sulfate hemihydrate (CaSO4·½H2O). When mixed with water, it forms a paste that hardens rapidly into a solid form. This hardening process is a chemical reaction called hydration, where the hemihydrate absorbs water and reverts to gypsum (CaSO4·2H2O).

The name "Plaster of Paris" originates from the large deposits of gypsum found near Paris, France, which were historically quarried and calcined (heated) to produce the hemihydrate. The production process involves heating gypsum in a kiln to drive off about three-quarters of its water of crystallization. The resulting hemihydrate is then ground into a fine powder.

Plaster of Paris is known for its smooth finish and quick setting time, making it popular for decorative elements, casting, and as a base coat for paints and wallpapers. It is also used in medical casts to immobilize broken bones due to its strength and ability to set quickly. When used in construction, it is typically mixed with sand and lime to control its setting time and improve its durability. However, it is susceptible to water damage once hardened, as it can rehydrate and revert to gypsum.