Experimental Study On The Moisture Adsorption Characteristics And Mechanism Of The Earthen Plaster In The Mogao Grottoes Of Dunhuang

Dunhuang Mogao Grottoes,located in 25 kilometers southeast of Dunhuang city,gansu province,China,it is the richest Buddhist treasure trove in China and even the world and Mogao Grottoes joined the<World Heritage List>in 1987.Mogao Grottoes involves a large quantity of exquisite murals,painted plastic,architecture and other fine art,which is of great significance to the study of the spread of buddhist art and the development of history and culture along the silk road.The murals are the largest and richest part of Mogao Grottoes art,and they are not only with unique style,exquisite picture and extremely high artistic value,but also contain relevant information about scene of social life,clothing,appliance use,etc.Therefore,the murals in Mogao are with extremely high historical value and scientific value and they are also known as"the library on the wall".The water vapour in cave environment is one of the reasons which can induce or accelerate mural diseases,the salt diseases in murals caused by the interaction between moisture and salt are the most widespread in Mogao Grottoes and cause a great threaten to the protection of ancient murals.The earthen plaster is a mud layer under paint layer of mural to provide a flat surface for the painting of murals,and the thickness of earthen plaster is much larger than the paint layer.The water vapour in cave environment,as one of moisture sources which could induce mural deteriorations,can go into the pore structure of earthen plaster mainly depend on adsorption.Therefore the study of the characteristics and mechanism of water vapor adsorption in earthen plaster could provide theoretical foundation for the research of the water vapor transport and disease mechanism in earthen plaster and the formulation of preventive protection and restoration measures for murals.This dissertation combined the moisture adsorption theory in soil and utilized methods of laboratory testing to studied the characteristics and mechanism of water vapor adsorption in plaster,variation characteristics of suction in and adsorption water features in plaster under the influence of environmental humidity on the basis of analyzing the structure composition of the earthen plaster and the temperature and humidity conditions in cave environment.In addition,this dissertation also analyzed the moisture adsorption process and its impact factors of the plaster with soluble salts and discussed the salt crystal change and salt damage mechanism under the influence of the cave environmental humidity.The main research contents include the following aspects:?1?The structural features and main components in earthen plaster of Mogao Grottoes were analyzed and summarized and the fundamental characteristics of plaster soil were also discussed.The atmospheric environment in Mogao Grottoes area and the variation of temperature and humidity inside or outside the grottoes were summarized and the reasons which could probably cause the increasing of humidity in cave were also analyzed and discussed.?2?Plaster samples were made according to traditional techniques to test the isothermal hygroscopic curves and moisture adsorption rate,the test results showed that the equilibrium moisture adsorption of plaster samples under any humidity condition increase with the Dengban soil content in plaster sample and moisture adsorption rate at certain humidity also increase with the Dengban soil content in plaster sample;?3?The adsorption water characteristics in earthen plaster were studied using the method of thermogravimetric analysis,and analyzed according to the thermogravimetric analysis results combined with isothermal hygroscopic curve of different plaster samples.The results showed that the most moisture adsorbed by plaster soil structure is existing as loosely bound water,and the critical moisture content of loosely bound water and the critical relative humidity condition which loosely bound water start to be formed in plaster are all increase with the Dengban soil content in plaster.?4?It is conclude that the suction in plaster during the moisture adsorption process mainly comes from the suction which caused by Vander Waals force and capillary condensation for the earthen plaster with no soluble salts by analyzing the suction characteristic during the moisture adsorption process.The moisture adsorption process also divided in different stages according to the different water vapor adsorption driving forces and the critical humidity for different stages was also discussed,the analysis results suggest that the capillary condensation which influenced by the pore structure of earthen plaster is the most important driving force during the moisture adsorption process of earthen plaster.?5?The pore size distribution of different earthen plaster samples were tested by nitrogen adsorption method because the capillary condensation is the most important driving force in earthen plaster during the moisture adsorption process,and the pore size distribution results were used to discuss the moisture absorption characteristics of the earthen plaster under the influence of capillary condensation.It is concluded that the pore size distribution which could impact the moisture adsorption process is mainly determined by the properties of Dengban soil particles themselves,so the Dengban soil content in the major factor for moisture adsorption process of earthen plaster.?6?The moisture adsorption process of earthen plaster samples with different contents of NaCl or Na₂SO₄ were tested on the basis of the research results of the moisture adsorption mechanisms of the soil structure in earthen plaster,and the moisture adsorption process of earthen plaster samples with different contents of NaCl or Na₂SO₄at certain humidity were divided to three basic stages:The moisture adsorbed in Stage1 is mainly come from the moisture adsorption performance of the soil structure in earthen plaster,the type and content of soluble salts in earthen plaster have little impact on moisture adsorption process,and the Stage 1 finishes when the soil structure of earthen plaster achieve hygroscopic equilibrium;The Stage 2 is the deliquescence process of the soluble salts which occurs in earthen plaster,the soluble salt could adsorb moisture and form saturated solution continually,and this stage finishes at the moment when all the soluble salts in earthen plaster are dissolved and form saturated solution.The Stage 3 is the moisture adsorption process which driven by the osmotic suction of pore solution in earthen plaster.The moisture adsorption mechanisms and its influence factors were also analyzed detailedly.?7?Salt crystal change characteristics in earthen plaster during moisture adsorption process were discussed combine with the research results of moisture adsorption mechanisms of the earthen plaster with soluble salts.In addition,the moisture adsorption process and salt crystal change characteristics during moisture adsorption process of the earthen plaster with complex soluble salts NaCl-Na₂SO₄ were further analyzed and it is suggested that the moisture adsorbed by soil structure and the interaction effect between different types of soluble salts could promote the development of salt deteriorations in earthen plaster or murals.