Which mineral makes plaster harden

Answer 1

First you need to recognize that "plaster" is not a singularly defined material; it is a family of quite different materials that harden by different mechanisms. The three most common plasters are (1) Plaster of Paris, (2) Lime Plaster, and (3) Cement Plaster.

(1) Plaster of Paris is CaSO4.H2O, a partially-dehydrated form of Gypsum, which is a hydrated Calcium Sulfate crystal CaSO4.2H2O. When Plaster of Paris is mixed with water, it re-hydrates and turns back into Gypsum, which then hardens into a material similar to mortar or concrete but considerably softer. Water is the material that makes Plaster of Paris harden.

(2) Limestone When exposed to atmosphere the Calcium hydroxide turns back to limestone, causing the plaster to set.

(3) Cement Plaster is a mixture of Portland Cement powder, sand or other filler, water, and either Plaster of Paris or Lime Plaster. Due to the great variety of minerals that makeup Portland Cement and Cement Plaster, the hardening is a complex process that cannot be ascribed to the action of any one of the ingredients.