Relation between Young's Modulus of set plaster and complete wetting of grain boundaries by water

The mechanical behavior of set plaster has been studied in humid atmosphere s. This material is made of entangled needle-shaped gypsum crystals. At roo m temperature, Young's modulus of set plaster decreases as the coexistence of gypsum and water is approached by increasing the humidity of the surroun ding air. Simultaneously, the thickness of the adsorbed water layer at the grain boundaries between the gypsum crystals is shown to diverge. It is sug gested that the gypsum crystals are able to slide along certain grain bound aries because the viscosity of the water film approaches that of bulk liqui d water. Small additions of D,L-tartaric acid prevent both the adsorption o f water at grain boundaries of gypsum and the decrease of the Young's modul us of set plaster. (C) 1999 Elsevier Science B.V. All rights reserved.