Thermotropia, barotropia and hygrotropia of the Ca-SO4-H2O system is reviewed and discussed and a three-dimensional representation of the phase diagram for system at atmospheric pressure is derived. It is concluded that the behaviour of the system is a coupled thermo-hydro-chemical phenomenon in which water activity is the key factor for existence of high concentration of Ca2+ and SO2-4 and confinement pressure plays a secondary role.