Research On The Mechanism Of Capillary Conveying Of Soluble Salt In Mural Plaster

Benefiting from the rare rain in the northwest of China, a lot of murals have been preserved. The plaster, as the carrier of mural, is a porous material. When salt solution migrates in the plaster, salt will react with the soil particles, physically and chemically, leading to the salt damage of mural by, such as flaking, disruption and efflorescence. Salt damage seriously threatens the long-term preservation of murals.Last dacades, many researches have been conducted to investigate the formation of salt damage of murals. However, there are still some questions unknown. For example, which kind of salts, originally contained in the plaster or transported from the outside, is mainly responsible for the salt damage of mural? What is the relationship between ion concentration, water content, and thermal conductivity of the plaster? In this thesis, laboratory tests were conducted to simulate salt migration in the plaster, expecting to get a better understanding of capillary conveying of soluble salt contributing to the salt damage of mural. Soil samples used include the naturally deposited silt of Daquan River in front of Mogao Grottoes (popularly named as Dengban soil) and the Dengban soil with a treatment of desalination by distilled water. Pore fluids used include potassium chloride (KCl), sodium sulfate (Na2SO4), as well as the mixture of potassium chloride (KC1) and sodium sulfate (Na2SO4) solutions. Based on test study, this study utilize unsaturated soil mechanics and chemistry theory to analyze the distribution of water content, conductivity, ion concentration and thermal conductivity in plaster after salt migration, and to establish the relationship between these factors. The research results show as followings.(1) The migration rate follows the order that the mixed KC1and Na2SO4> KC1> Na2SO4> H2O; the capillary rise speed of solution gradually slows down with the height of specimen increasing; the capillary rise rate of the desalination Dengban soil is greater than the pure Dengban soil. When the soluble salt solution conduct the capillary migration in the mural plaster, chloride crystallization is enriched mainly in the capillary finger, and sodium crystallization is enriched in the sub-finger area.(2) When chloride solution is used as salt migration test, the concentration of Cl-occurs inflection point at the height of about5cm in the sample, and increase slowly below the height of 5cm, while rapidly above the height of5cm; and the inflection point occurs at the height of about7cm in the sample when solution is distilled water, sodium solution or mixed solution. The increasing rate of Cl-concentration in the desalination Dengban soil is greater than that in pure Dengban soil. The SO42-concentration occurs inflection point in the height of about7cm in the sample, and increase slowly below the height of7cm, while decreases rapidly above the height of7cm. The regularity of Na+is similar to the SO42-.(3) The electric conductivity of the samples increases with the rising of the height of sample, and increase slowly below the height of7cm, while rapidly above the height of7cm, and the conductivity of the pure Dengban soil is greater than that of the desalination Dengban soil. The high conductivities of the samples are all in the top of the samples, which implies that, generally, the concentration and crystal of the salt appears in the edge region (forward area) of the capillary migration.(4) The thermal conductivity of the samples firstly increases and then decreases with the rising of the height of sample. Plaster will be more sensitive response to the ambient temperature changes when the thermal conductivity increases. Ambient temperature changes repeatedly, the temperature inside the plaster is also constantly changing, which leads to different expansion in the plaster, therefore, accelerate the formation of salt damage of mural.(5) Compared with the pure Dengban soil, the desalination Dengban soil’s pore size is smaller; when the migration solution is the mixed salt solution, its distribution of pore size is more concentrated than the single salt.(6) Sample destruction by recrystallization of the external solution after the capillary migration is more obvious. Therefore, more attention should be paid to control the rainfall infiltration of the top cave, in order to protect the mural.