Salt Efflorescence In Earth Plaster Generated By Unsaturated Water Migration

Efflorescent disaster of wall-paintings is that soluble salts both in supporting layer and within wall rocks, participating with water movement, transport and accumulate in wall-paintings, leading to the increase of the distance and the decrease of bonding force between soil particles, causing the swelling disintegration of wall-paintings. At present, the endangerment of efflorescent disaster is recognized, and a great deal of work was carried out. However, the mechanisms of efflorescent disaster are not understood deeply.Based on the law of water (liquid and vapor) transport and salts crystallization, capillary water rise tests in laboratory were conducted on compacted silt soil samples from ancient Jiaohe City Ruins in Xinjiang, China, and Mogao grottoes' Dengban soil was used in vapor adsorption tests. It is revealed the law of water and salts transport in wall-paintings is analyzed based on the distribution of water contents, pH, electricity and soluble salts of soil liquid.The test results of soluble salts migration by capillary water, show that NaCl accumulation is predominant at the capillary front while Na₂SO₄ at the other capillary area. Within capillary rise area, less soluble, moderately soluble and most soluble components crystallized out along the water migration direction, resulting in such a damage tendency as salt crust, salt efflorescence and dark appearance.Based on the measured data of temperature and relative humidity about Mogao Grottoes, paper is a physical model about moisture adsorption and desorption of grotto established. When the internal grotto's relative humidity is larger, the flow of adsorption will be greater than desorption, at that time overall layer is in drying state. In contrary, when the internal grotto's relative humidity is smaller, the flow of adsorption will be less than desorption, at that time overall layer is in wetting state.The test of vapor adsorption, under the different relative humidity, show that pure and mixed-sand Dengban soil's moisture adsorption isotherms is in line with the exponential growth. Moreover, on the per unit area, the mixed-sand Dengban soil's flow of adsorption and desorption is smaller than pure Dengban soil, thus it decreases salts accumulation on the surface. However, the mixed-sand soil remains a large part of water and salts, leading to salts transport and recrastallization.