The paper summarizes first a set of field observations in connection with the severe heave experienced by Lilla tunnel excavated in a Tertiary anhydritic claystone. Heave and swelling pressures against the tunnel rigid lining were a consequence of gypsum crystal growth on discontinuities. This process requires water because gypsum crystals precipitate from supersaturated solutions of calcium sulfate. The paper concentrates on the precipitation of crystal in pores of varying geometry with the purpose of deriving theoretical expressions for the crystal pressure against the (assumed rigid) pore walls. The problem is solved by two complementary approaches: the thermodynamic requirements for the equilibrium of the chemical reaction of precipitation and the consideration of the surface energy at the crystal-solution interface. The phenomenon has strong similarities with other interfaces of interest in unsaturated soil mechanics which are highlighted. The expressions found for the crystal pressure are considered an upper bound for pressures likely to develop