Study On Macro-Meso Evolution Law On Damage Of Sandstone Under Acidic Environment

With the development of modern industry,deterioration of the acidic environment has intensified,causing rock and concrete materials to suffer varying degrees of corrosion damage within their service period,which ultimately leads to poor engineering effects.Research on the physical and mechanical degradation characteristics of rocks subject to acid corrosion has become one of the hot issues in the field of environmental geotechnical engineering.At present,most studies focus on the corrosion characteristics of rocks under normal immersion conditions,few studies on the damage evolution of rocks under different pH values and different acid environments under the accelerated immersion conditions.Considering problems like this,the paper here used the indoor long-term accelerated corrosion experiments to study the physical,chemical and mechanical degradation characteristics of sandstone under the corrosion of hydrochloric acid and sulfuric acid(HCl and H₂SO₄)with pH=1,3,5 and distilled water(H₂O)with pH=7.SEM-EDS and CT are used to analyze the microscopic structure characteristics of the corroded sandstone.The coupled chemical-mechanical damage variable was introduced to establish the statistical damage constitutive model of corroded sandstone,in which the compression phase of the stress-strain curves was considered.A finite element model of the corroded sandstone was established based on the CT images,and the uniaxial compression test was numerically simulated.The main research contents were as follows:(1)The physical,chemical and mechanical degradation laws of the sandstone samples attacked by hydrochloric acid,sulfuric acid and distilled water with different pH were obtained.By monitoring the mass of the samples,the pH and cation concentration of the soaking solution and the uniaxial compression mechanics test,the macroscopic corrosion damage of the samples under acid attack was studied.The results showed that with increasing the acidity and the soaking time,the mass of the samples decreased,the porosity increased,and the pH and the concentration of Na⁺,Mg²⁺,K⁺and Ca²⁺increased.The compaction phase of the stress-strain curves became longer,the elastic phase became shorter,the axial peak stress,elastic modulus,and Poisson's ratio decreased,and the axial and radial peak point strains increased.Rock samples are increasingly characterized by flexible materials.The hydrochloric acid had a stronger corrosive effect on sandstone than sulfuric acid under the same test conditions.(2)The microstructure variation law,damage characteristics and corrosion mechanism of sandstone under acidic environment were studied.Based on scanning electron microscopy and energy spectrum,the microscopic morphology,mineral composition and pore structure of the corroded sandstone were qualitatively and quantitatively studied.The results showed that the pores and secondary pores of the acid-corroded sandstone increased,the structure tended to be loose,and the connection between mineral particles changed to face-to-face,face-to-edge,and edge-to-angle.The average diameter,the number of pores,circularity and fractal dimension of the samples increased.The corrosion mechanism of sandstone in the acidic environment mainly includes hydrolysis,hydration,dissolution,ion exchange,water swelling and disintegration of clay minerals.In the early stage of acid-rock reaction,the cement samples of the rock samples were mainly dissolved.In the middle and later stages,the dissolution of mineral particles was dominant.Stronger the acidity of the soaking solution,more obvious the dissolution.As the acidity weakened,the dissolution slowed down,and the hydration,ion exchange,water swelling and disintegration of the clay minerals were enhanced.(3)The mesoscopic damage evolution law of the acid-corroded sandstone samples was studied.The corroded sandstone samples were scanned by a medical and industrial CT scanner.The structural features of sandstone samples were analyzed through CT images with different precision.The artifact information of medical CT images was analyzed,an effective CT image artifact processing program was compiled by C++.The damage evolution process of acid-corroded sandstone was analyzed based on the CT number histogram technique.The porosity of the corroded sandstone,as well as the mesoscopic feature,in the uncorroded zone and the corrosion zone were calculated based on industrial CT images.(4)A statistical damage constitutive model of the acid-corroded sandstone was established.Based on the continuous damage mechanics,statistical theory and the damage evolution analysis by test results of CT scan,the coupled chemical-mechanical damage variable was introduced,and the compaction section of the stress-strain curves was considered to establish the segmentation statistical damage constitutive model.The results showed that the model can better reflect the deformation characteristics of the acid-corroded sandstone under the uniaxial compression.(5)A zonal finite element model reflecting the characteristics of the corroded and uncorroded areas of the sandstone sample was established,and the uniaxial compression simulation of the acid-corroded sandstone was realized.Based on the CT scan images,the three-dimensional reconstruction and mesh re-division of the corroded and uncorroded rock samples were carried out.The ABAQUS software was used to simulate the uniaxial compression test of the corroded sandstone.The simulation results were compared with the experimental results.The results showed that the simulation results are in good agreement with the experimental results,which can reveal the deformation and failure law during the compression process of the acid-corroded sandstone.