When gypsum rock (CaSO4 2H2O) is heated to a relatively low temperature, about 130C, three-fourths of the water of crystallization is driven off. The resulting product is known by various names such as hemihydrate, calcined gypsum, and firstsettle stucco. Its common name, however, is plaster of paris. It is a fine powder, usually white. While it will set under water, it does not gain strength and ultimately, on continued water exposure, will disintegrate.
Plaster of paris, with set retarded or unretarded, is used as a molding plaster or as a gaging plaster. The molding plaster is used for preparing ornamental plaster objects. The gaging plaster is used for finishing hydrated lime to form the smooth white-coat finish on plaster walls. The unretarded plaster of paris is used by manufacturers to make gypsum block, tile, and gypsumboard (wallboard, lath, backerboard, coreboard, etc.).
When plaster of paris is retarded and mixed with fiber such as sisal, it is marketed under the name of hardwall plaster or cement plaster. (The latter name is misleading, since it does not contain any portland cement.) Hardwall plaster, mixed with water and with from two to three parts of sand by weight, is widely used for base-coat plastering. In some cases wood fiber is used in place of sand, making a wood-fibered plaster.
Special effects are obtained by combining hardwall plaster with the correct type of aggregate. With perlite or vermiculite aggregate, a lightweight plaster is obtained.
Gypsum plasters, in general, have a strong set, gain their full strength when dry, do not have abnormal volume changes, and have excellent fire-resistance characteristics.
They are not well adapted, however, for use under continued damp conditions or intermittent wet conditions. See also Arts. 4.26 to 4.30.