The dimensional stability of a hydrate/water/water vapor-system is governed by a critical linear relationship between log RH and the temperature T. Porous salt formations can fix water in the form of condensate, water of hydration, suspension or solvent, resulting in deformation. If this fixation of water in porous salts is combined with (partial) hydration, the corresponding forces are enlarged due, inter alia, to adhesion of the salt film to the substrate. The resulting shear stresses in the substrate may cause spalling and material degradation. At low water-salt ratios, dehydrated salt of fine porous structure produces salt cement of high (compressive, tensile, shear) strength; this occurs without theoretical complete hydration.