Experimental Study On Repairing The Salt Efflorescence Of Red Sandstone Wall Based On MICP Technology

Aerzhai grotto,northwest of Etuoke Banner,Ordos City,Inner Mongolia Autonomous Region,is a grotto excavated on the red sandstone rock wall.The grotto temple contains rich and colorful historical relics,such as Buddhist murals,stone carving art and Uighur Mongolian titles.It is a cultural treasure with high historical value and is known as’Dunhuang on the grassland’.Because it is located in the typical arid area of the northwest region,the geological environment is poor and the climate environment is harsh,so there are serious diseases on the grottoes and rock walls of the grottoes,among which the phenomenon of salt efflorescence pan-salt is one of the representatives.In the long-term natural environment such as sudden change of temperature,fluctuation of groundwater level and freezing and thawing in winter and spring,the inorganic salts represented by Na₂SO₄and NaCl are affected by the changes of temperature,humidity and water content.When they encounter water,they cool down and generate crystals,which release the crystallization pressure and lead to the occurrence of crisp salt efflorescence diseases.At the same time,the inorganic salts transported by capillary water lead to the occurrence of pan-salt diseases.Therefore,the repair of crisp salt efflorescence pan-salt diseases is very urgent.In order to prevent the deterioration of the grotto wall and improve its durability and stability,physical reinforcement and chemical grouting are often used in engineering applications.However,physical reinforcement often leads to unsightly,and chemical reinforcement will cause secondary damage to the wall.In recent years,the microbial induced calcium carbonate precipitation（MICP）technology has been successfully applied to a number of cultural relics restoration technologies by using the mineralization behavior of microorganisms in their physiological activities or metabolic processes and the surrounding calcium source environment to rapidly induce the precipitation of calcium carbonate crystals with cementation ability.As a simple,green and environmentally friendly restoration technology,it has broad prospects in the wall restoration of red sandstone matrix.The main research contents of this paper are as follows:（1）By measuring the concentration of bacteria,the unit enzyme activity,the unconfined compressive strength of the specimen and the amount of calcium carbonate produced,the microbial mineralization ability of Oceanobacillus and Bacillus pasteurii was compared.At the same time,a new product process was proposed and compared with the traditional process.（2）By consulting the literature,the mechanism of the salt efflorescence salt disease is explained,and the salt efflorescence salt disease is simulated through the laboratory test design,and the disease similar to the Aerzhai Grottoes is obtained.At the same time,the physical quantitative indexes for evaluating the development of diseases were proposed,and MICP technology was used to repair them.At the same time,a large-scale direct shear test was carried out to explore the shear performance and mineralization ability of MICP technology coating under different substrates and different bacterial concentrations.（3）Through the preliminary test,it was verified that the interface of shear failure existed between the contact interface of the substrate material-MICP technology coating.The results showed that:（1）The unit enzyme activity of the two mineralized bacteria was the highest at 12 h,but the bacteria cultured for 24 h should be used in combination with factors such as bacterial concentration.In the normal curing environment,the mineralization ability of the specimens formed by Bacillus pasteurii was better than that of Bacillus,but in the saline-salt efflorescence environment,especially when the bacterial concentration was greater than 1.5,Oceanobacillus showed stronger mineralization ability.Therefore,it was considered that Oceanobacillus had stronger salt and salt efflorescence resistance,and with the passage of time（14 d-28 d）,the strength of Oceanobacillus was significantly higher than that of Bacillus pasteurii.（2）The improved MICP process has a good effect,which has the advantages of simple parts,convenient disassembly and reusable materials.At the same time,the strength is 3.37%～9.95%higher than that of the traditional MICP process.（3）Through ion chromatography analysis and mathematical calculation,the mass ratio of NaCl and Na₂SO₄of the red sandstone weathered soil was determined to be 1:1.52,and it was found that the pit with high salt content（15%,12%）was gyro-shaped,and the pit with medium and low salt content（9%,6%,3%）was shallow cylindrical.Under the recommended bacterial concentration of OD₆₀₀=1.5,the salt efflorescence height,powder volume and salt shell thickness were reduced by29.1%,45.0%and 36.4%,respectively.（4）The pulley friction test proves that the shear interface exists between the matrix material and the MICP coating.By comparing the shear strength parameters of the coated and uncoated contact interfaces,the cohesion c of the red sandstone matrix increased by 54.48%,and the internal friction angleφincreased by6.30%.And the coating layer is divided into regions to obtain the conclusion that the amount of calcium carbonate produced is’less in the middle and more outside.