It is thought that the setting of cement is due to the crystallizing of the silicate and aluminate of lime, which, in their dry and anhydrous form after burning, are soluble in water, but which pass into the hydrated state when sufficiently wet, in which state they are insoluble.

The hardening after setting is due to a continued crystallization of salts from solution, accompanied by other chemical and physical changes which occur very slowly and which are not well understood.