ABSTRACT
Gypsum is a non-hydraulic binder occurring naturally as a soft crystalline rock or sand. There are two commercial varieties of crude gypsum, rock gypsum and gypsum earth or gypsite used for the manufacture of gypsum binding material. Gypsum items have a number of valuable properties like relatively small bulk density, incombustibility, good sound absorbing capacity, and good fire resistance with negligible shrinkage, superior surface finish, resistance to insects and rodents and low energy input during burning to produce gypsum plaster. According to the crystallisation theory proposed by Le-chatelier when water is added to gypsum, the latter dissolves forming a saturated solution of dehydrate gypsum. According to colloidal theory when water is added to gypsum, the semihydrate gypsum goes into solution until the latter is saturated. Gypsum binders are classified as low and high burning varieties. The setting of Plaster of Paris is attributed to the formation of gypsum crystals from a supersaturated aqueous solution.
