Step-by-step explanation:
Calcium sulfate is slightly soluble in water. The small amount of ions given by calcium sulfate on dissolving in water does not interact with water acting as a spectator ions. Due to presence of covalent bonds in calcium sulfate it doesn't dissolves in water which polar solvent.
Hardening of water occurs when solid calcium sulfate is added to water.This hardening of water is due to presence of calcium ions.