The swelling behavior observed in anhydritic rocks is explained by the precipitation of gypsum crystals in open discontinuities. Gypsum crystals growing in discontinuities contribute to open them and induce deformations or pressures (if the displacements are restrained). A model has been formulated to reproduce the expansive behavior in sulphated rocks. The formulation describes the kinetics of dissolution and precipitation of sulphated minerals and transforms the mass of precipitated gypsum to imposed swelling strains due to gypsum crystal growth. The model also keeps track of the solute transport and takes into account the effect of the existing load on crystal growth. A simulation of the heave observed in Pont de Candí Bridge is performed. The calculations reproduce the long term vertical displacements measured at the ground surface and the distribution of vertical strains measured in depth. A sensitivity analysis is also reported.