Water-Rock Interaction Characteristics And Rock Mass Degradation Mechanism Of Granite In High Temperature And Acid Environment

The groundwater environment in southwestern China is complex,and hot springs are widely distributed and locally developed acidic hot springs.Due to the coupling effect of temperature field and chemical field different from the surface geological environment,the water-rock interaction process becomes more complex,which has a significant impact on the physical and mechanical properties of deep rock.With the expansion of human science and technology,the exploitation of underground deposits and the excavation of deep rock mass,a series of rock mass strength degradation problems caused by water-rock interaction under high temperature and acid environment will be encountered.Therefore,this study has certain theoretical significance for the study of the evolution mechanism of rock mass engineering characteristics under the water-rock chemistry effect in the high-temperature acidic environment of deep granite.This paper takes granite in Gongjue Basin as the research object,and analyzes the mineral composition and structural characteristics of granite.Through the simulation experiment of water-rock interaction process in granite high-temperature acidic environment under the coupling effect of temperature field and chemical field,the variation characteristics of water quality and mineral microstructure in the reaction process of granite with acidic hot water were obtained.The degradation mechanism of mechanical strength of granite after high temperature acid etching was discussed by conventional triaxial compressive strength test.Finally,the saturation index and dissolution of the main silicate minerals in the process of high temperature water-rock interaction of granite under different conditions are calculated and analyzed by using the water chemistry simulation software PHREEQC,and the following conclusions are obtained :1.The rock slice identification and XRD diffraction analysis of granite rock mass were carried out.The main mineral components were potash feldspar(39.5 %),plagioclase(26.4 %),quartz(13.4 %),clay minerals(7.6 %),siderite(1.7 %),and a small part of calcite(5.3 %)and dolomite(6.1 %)formed in the later stage.2.The simulation results of water-rock interaction process in high temperature and acid environment of granite show that : The high temperature acid etching process of granite is greatly affected by the solution p H value and reaction temperature,mainly by the dissolution of silicate minerals such as anorthite,albite and potash feldspar,and the reaction generates kaolinite and illite.The angle of mineral particles disappears,and the grinding roundness at the edge of particles is high in a bay shape.The pores and microcracks on the surface of particles increase,and the overall structure becomes loose.The lower the p H value of the aqueous solution involved in the reaction,the more favorable the dissolution of the three feldspars and the decomposition of carbonate minerals.Thermodynamic analysis shows that the dissolution of albite and potash feldspar is promoted and the dissolution of anorthite is inhibited by increasing temperature under the same acid condition.3.Through the conventional triaxial compressive strength of confining pressure of 20 MPa was carried out on the granite after high temperature water-rock interaction under different reaction conditions.The results show that : With the decrease of p H value of reaction solution and the increase of reaction temperature,the dissolution of minerals is intensified,and the dissolution of micro-soluble minerals is promoted,which increases the surface cracks of minerals and increases the porosity,resulting in the decrease of elastic modulus and peak compressive strength of granite.The maximum decrease of elastic modulus is about 13.5 %,and the maximum decrease of peak compressive strength is about15.4 %,with significant deterioration of mechanical strength.4.PHREEQC simulation software was used to calculate the saturation index and dissolution amount of main silicate minerals in the water-rock reaction process of granite at high temperature.The results show that : The saturation index of feldspar and clay minerals is negative after 12 hours of high temperature water-rock interaction between granite and distilled water at 60 °C.The saturation index of calcium feldspar is greater than that of albite and potassium feldspar,the solubility is relatively small,and the aqueous solution is alkaline.At the same temperature,the stronger the acidity of aqueous solution,the smaller the saturation index of feldspar,the more conducive to the dissolution of feldspar minerals.The greater the saturation index of quartz,is not conducive to the dissolution of quartz.In acid solution with p H < 5,the stability order of the three feldspars is as follows : anorthite <albite < potash feldspar.At the same p H value,the higher the reaction temperature,the smaller the dissolution of anorthite and potash feldspar.It is consistent with the simulation results of water-rock interaction process in granite high-temperature acidic environment under the coupling effect of temperature field and chemical field.