WATER AND SOLVENT EFFECTS ON THE STRENGTH OF SET PLASTER

Set plaster, commonly used as a modelling material for rock mechanics studies, under goes important variations of its mechanical properties during drying and wetting. It loses about 50% of its mechanical strength when it absorbs 2% of water, but it recovers its properties as soon as water evaporates. In the present study, the effect of water is compared to that of other polar and non-polar liquids. Hardness and three-point bending flexure strength are measured on dry samples and compared to that of samples soaked in these liquids. It is shown that the sole liquids which decrease the mechanical resistance of plaster are polar liquids with a high dielectric constant having some ability to dissolve gypsum. However, dissolution of gypsum crystallites during wetting and recrystallization during drying are not the cause of plaster weakening and strengthening respectively. Actually, very small amounts of water decrease the interfacial free energy of gypsum crystals, which probably induces the decrease of the mechanical strength. A parallel can be drawn between this phenomenon and the drastic decrease of the surface free energy of mineral single crystals when they are cleaved in water instead of being cleaved tinder an inert atmosphere.