ADSORBED MOISTURE FEATURE AND SUCTION VARIATION CHARAC￣TERISTICS OF EARTHEN PLASTER IN MOGAO GROTTOES UNDER INFLUENCE OF HUMIDITY

The climate of the Mogao Grottoes is dry,so the earthen plaster of murals in the caves are in the state of low moisture content and high suction. This state of soil is greatly affected by the humidity in the cave environment. To further understand the characteristics of suction variation and the property of adsorbed moisture in earthen plaster under the influence of relative humidity in cave environment,we tested the soil-water characteristic curves at high suction range of different kinds of earthen plasters in Mogao Grottoes through vapor adsorption process,then calculated and analyzed the variation characteristics of different types of suction caused by Vander Waals force and capillary condensation respectively according to the test results. Meanwhile,we also tested the soil-water characteristic curves of plaster samples with different contents of NaCl and discuss the osmotic suction during moisture adsorption process. In addition,we utilized thermogravimetric to test and analyze the features of the moisture which adsorbed by earthen plaster under the action of Vander Waals force and capillary condensation. The results show the follows. Most matric suction in earthen plaster still comes from capillary condensation during the moisture adsorption process. The van der Waals forces between soil particles can provide obvious suction only in the case of very low moisture content. Significant osmotic suction can be present during the moisture adsorption process of earthen plaster only when environmental humidity is greater than the deliquescence relative humidity of the soluble salt in earthen plaster. Most moisture adsorbed by plaster is in the form of loose bound water. The critical moisture content of loose bound water increases with the Dengban soil content in earthen plaster. These findings can offer data and theoretical basis for the research of mechanism of mural damage with the participation of moisture and the water vapor transport in plaster.