Study On Surface Weathering Mechanism Of Sandstone At Inner Mognolia Aerzhai Grottes

Aerzhai Grottoes is the oldest Buddhist grottoes in the grassland area north of the Great Wall in China.It is a grotto architecture integrating grassland culture,Buddhist esoteric culture and Mongolian culture.After thousands of years of weathering,the preservation and protection of Aerzhai Grottoes is facing severe challenges.In view of the main research direction of the weathering mechanism of grotto cultural relics at present,based on the results of field investigation and indoor test and analysis,this study sets up three groups of indoor single factor weathering simulation tests of salt cycle,dry wet cycle and freeze-thaw cycle respectively to study the weathering mechanism of Sandstone on the surface of grottoes from the perspectives of macro deformation,physical properties,meso and microstructure changes and mineral composition.The main conclusions are as follows:(1)Aerzhai Grottoes in Inner Mongolia is located in the central and Western Ordos Basin.The grottoes area belongs to the hilly landform with low tectonic denudation.It mainly exposes gravelly sandstone in Cretaceous,residual slope soil and aeolian deposits in late Pleistocene.The Aerzhai grottoes are mainly excavated in Cretaceous sandstone.The grottoes area belongs to temperate continental monsoon climate,which is dry all year round,with concentrated rainfall in summer and cold and long winter.The cation with the highest content of groundwater in this area is Na+,and the anion with the highest content is SO₄^2-.(2)The main diseases developed in Aerzhai Grottoes include collapse,slope rock mass instability,fissure,wind sand burial,caustic soda,pulverization and spalling,pore like weathering,etc.Among them,fissures,pan salt crispy alkali and pore weathering are extremely developed and pose a great threat.The field investigation found that the fissures of the grottoes are developed,and the deep and thick weathered layer is closely related to the soluble salt ions,groundwater and seasonal rainfall in the grottoes area;At the same time,the temperature difference between day and night,seasonal dry and wet and freeze-thaw played a significant role in the development of the disease.(3)The existing sandstone in Aerzhai Grottoes is relatively stable in nature,and weathering is mainly due to salt enrichment and caustic soda;The stratum has obvious sedimentary rhythm and obvious differential weathering.They lead to different response mechanism and deterioration degree of sandstone affected by weathering factors.It is also the reason for the obvious difference of weathering between and within the layers of the grotto temple.(4)The indoor degradation simulation test shows that the weathering rate of sandstone samples is salt cycle>freeze-thaw cycle>dry wet cycle,and the apparent weathering characteristics of the samples are consistent.The degradation mode of group A sandstone(With horizontal bedding)is mainly the development of cracks along the horizontal bedding direction,which expands rapidly after the cracks appear.The test block is cut into thin sheets,and then the thin sheets are powdered and peeled from the outside and inside,and the sandstone is cut into more thin pieces;The main failure modes of group B(No bedding)and group C(With sedimentary rhythm)sandstones are fracture development along sedimentary rhythm and siltation and spalling from outside to inside.(5)After 60 dry wet cycles,the mass loss rate of the three groups of sandstones is about 1%,showing strong water resistance;After 60 freeze-thaw cycles,the mass loss rate of sandstone is 1%-3%,which is slightly higher than that of dry wet cycle,the mass loss rate is lower,and the overall frost resistance of sandstone is better;The sand weathering rate is the largest in the salt circulation process,and the higher the salt solution concentration is,the steeper the mass loss curve is.When the salt solution concentration is lower than 50.0g/l,the mass loss of sandstone approaches a straight line.(6)In the process of salt cycle,freeze-thaw cycle and dry wet cycle,physical damage is still the main cause of sandstone deterioration.Under the influence of salt crystallization,water freezing and dissolution and water migration,mineral particles produce tensile and compressive stress,which breaks and loses the cement between pores,and finally leads to the destruction of sandstone microstructure.In the process of salt circulation,the minerals show the loss of argillaceous and calcareous cements and interstitial materials,and the enrichment of quartz and feldspar.At the same time,the pore diameter of sandstone is from mesopore(1-10)?m).small hole(0.1-1)?m).micropore(0.01?0.1)?m)Large hole(10-100)?m)And oversized holes(>100?m)Expansion.