In Germany, radioactive and many types of chemotoxic waste must be disposed in deep mines. Among others, a suitable host rock for the ultimate disposal of the hazardous waste is salt. With respect to safety analyses a contact of the building materials used for backfilling and sealing measures with solutions must be considered. However, many conventional materials deteriorate in the saline milieu and/or their long-term behavior is difficult to describe. Moreover, questions exist concerning the reaction of these materials with gases. For this reason, a new family of building materials was developed. All recipes have in common the mixing of Mg-bearing salts with salt solutions. They harden as a result of salt hydrate crystallisation. After an initial contraction during the liquid phase several systems show a considerable swelling. Measurements have been carried out to quantify the increase in volume (open system) and the resulting crystallisation pressures in closed vessels. A constitutive law describing Young’s modulus and creeping in the early age of the material enables the calculation of crystallisation pressure from swelling strain. The parameters of the constitutive equation for creeping were fit to results of the laboratory experiments.