Structural Degradation And Mechanical Damage Characteristics Of Black Shale Under Oxidative Acid Reosion

Black shale is a low permeability rock mass rich in organic matter and pyrite,which is often formed in anoxic reducing environment.Its weathering behavior is relatively special,and the oxidation behavior represented by pyrite is mainly manifested in water-rock interaction.The main rock in Xujiaping landslide area is medium thick limestone intermingled with black shale soft rock.Due to the chemical weathering of pyrite in the black shale exposed in the study area,erosive acidic water is formed,which can degrade the internal structure of rock mass and then affect the physical and mechanical properties of the black shale.The slope of Xujiaping landslide was subjected to differential weathering due to oxidizing acid erosion of black shale interlayer,and finally lost stability under the induction of the 2008 earthquake.In xujiaping landslide area,oxygen is abundant and the structure of old sliding rock is broken,so the water-rock interaction process of black shale rock exposed in this area becomes more complicated than that of pyrite.Therefore,the study of the degradation mechanism of black shale under the action of oxidative acid erosion has a certain guiding significance for the engineering geological activities of black shale in nature.In this paper,the soft intercalated rock mass of black shale in Xujiaping landslide,Mianyang city,Sichuan Province is taken as the research object.Firstly,XRD test and XRF spectrometer are used to comprehensively analyze the mineral content and chemical composition of the rock,and the geochemical characteristics of black shale in the study area are obtained.Subsequently,the experimental group of black shale samples under the environmental effect of oxidative acid erosion of water rock experiment,by analyzing the p H value,EC value of solution and the change of each ion concentration with the reaction time under different conditions,to explore the reaction mechanism of water-black shale.Furthermore,ultrasonic testing,Furthermore,ultrasonic testing,scanning electron microscopy,EDS and other methods were used to compare and analyze the changes of the microstructure of the rock samples before and after the reaction,and to study the deterioration of the internal structure of the black shale samples by water-rock interaction.By comparing the hardness changes of rock samples before and after water-rock interaction under different conditions,as well as the difference of the axial force of uniaxial compressive peak value,the degradation law of the physical and mechanical characteristics of black shale under the action of water-rock interaction under oxidizing acid erosion environment was analyzed.Finally,the hydrogeochemical software PHREEQC was used to simulate the water-rock interaction process of black shale under different conditions,and the saturation index and kinetic analysis of pyrite and main minerals were analyzed to further elaborate the water-rock interaction mechanism of black shale,and the following conclusions were drawn:1.According to X-ray diffraction test and X-ray fluorescence spectrometer analysis,the main mineral components of black shale in Xujiaping area are quartz（60%）,potassium feldspar（9%）,albite（5%）,pyroxene（6%）,pyrite（6%）,illite（13%）and calcite（1%）.2.The chemical weathering process of water rock of black shale is mainly the oxidative acid erosion of pyrite and the dissolution of feldspar minerals in rock mass,which is greatly affected by the concentration of H⁺ions in solution and the content of oxygen in sealed caldron.H+in sulfuric acid solution displaces other alkali metal ions in feldspar minerals,resulting in the p H value of solution and the concentration of each ion in the solution increase gradually with time.Compared with H⁺ions,the electromobility of the substituted alkali metal ions is lower,so the conductivity of the reaction solution decreases greatly.The higher the oxygen content in the sealed kettle,the higher the oxidation degree of pyrite is.As H⁺and SO₄^2-plasma are formed during the oxidation of pyrite,the p H value of the solution decreases,and the EC value and ion concentration of the solution increase gradually with time.The change of p H value,EC value and ion concentration becomes more obvious with the increase of H⁺content in solution and oxygen content in sealed kettle body.3.The internal structure of black shale degrades to a certain extent after water rock action.Due to the dissolution of soluble minerals and acid erosion of pyrite in the rock mass,secondary pores appear in the black shale and the cavity area increases,so the axial longitudinal wave velocity decreases by about 10-15%.Under the same pressure field,with the change of H⁺concentration in solution,the pores and micro-cracks on the mineral surface inside the rock sample increase,and the overall structure becomes looser.Therefore,the lower the p H value of the solution,the larger the average diameter and average area of the secondary pores in the rock sample.With the same initial chemical field,with the pressure of100%N₂ increasing from 0.1MPa to 3MPa,the degree of oxidizing acid erosion of pyrite gradually decreases,and the average diameter and average area of secondary pores in black shale samples show a decreasing trend.The degradation degree of the internal structure of black shale increases when the reaction medium changes from 100%N₂ to 100%O₂.4.After the water-rock interaction,the secondary pores and fissures in black shale increase,and the physical and mechanical characteristics of black shale inevitably deteriorate to a certain extent.With the increase of H⁺ions concentration in the reaction solution,the dissolution degree of pyrite and feldspar minerals in the rock is intensified,and the secondary pores and fissures in the rock sample are increased,and the hardness of the rock sample is more damaged.With the decrease of the p H value of the reaction solution,the pore compaction stage of black shale becomes more obvious,and the peak axial force of black shale decreases to different degrees compared with the blank group,with a maximum decrease of about 13.6%.Under the condition that the reaction solution is deionized water,the oxygen content in the sealed kettle can be adjusted by adjusting the pressure of 100%N₂and 100%O₂,thus affecting the degree of oxidation acid erosion of pyrite.The lower the pressure of N₂ and the higher the pressure of O₂,the acid erosion degree of pyrite increases,the secondary pore fissure area of rock samples increases gradually,and the axial compaction stage becomes more obvious,and the peak axial force of rock samples decreases to different degrees.5.PHREEQC hydrogeochemical software was used to calculate the saturation index and dissolution amount of pyrite and main minerals in the process of black shale multi-field coupling water rock interaction.The solubility and saturation index of metal cations,sulfate ions and pyrite in black shale are greatly affected by p H value.The degree of oxidation decomposition of pyrite increases with the increase of H⁺ions concentration in solution.