RELATION BETWEEN YOUNG'S MODULUS OF SET PLASTER AND COMPLETE WETTING OF GRAIN BOUNDARIES BY WATER

Abstract The mechanical behavior of set plaster has been studied in humid atmospheres. This material is made of entangled needle-shaped gypsum crystals. At room temperature, Young’s modulus of set plaster decreases as the coexistence of gypsum and water is approached by increasing the humidity of the surrounding air. Simultaneously, the thickness of the adsorbed water layer at the grain boundaries between the gypsum crystals is shown to diverge. It is suggested that the gypsum crystals are able to slide along certain grain boundaries because the viscosity of the water film approaches that of bulk liquid water. Small additions of d , l -tartaric acid prevent both the adsorption of water at grain boundaries of gypsum and the decrease of the Young’s modulus of set plaster.