Study On The Deterioration Mechanism Of Limestone Under Dry-wet Cycle And Chemical Dissolution

The continuous chemical dissolution and dry wet cycle will aggravate the weathering degree of rock mass in the"fluctuation zone"of the reservoir area,and make its macro mechanical properties deteriorate,which seriously threatens the long-term stability and durability of rock mass engineering in the reservoir area.Limestone is one of the main foundation rock mass of many hydropower projects in Southwest China.Due to the complex diagenesis and the active calcite property in limestone minerals,it is easy to occur chemical dissolution or hydrolysis ionization reaction,resulting in a wide range of micro cracks in the internal structure of limestone,which also provides a good prerequisite for the entry of hydrochemical solution.In view of the above problems,taking limestone as the research object,the dry wet cycle test and continuous soaking test were carried out under the conditions of HCl solution(p H=3.0),deionized water(p H=7.0)and Na OH solution(p H=11.0).The changes of macro properties,micro morphology and dissolution mechanism of limestone in various solution environments were studied.The main research work and achievements are as follows(1)The test results show that the water absorption of limestone samples in different chemical solution environments increases in different degrees with the increase of cycling times.The increase range of water absorption of limestone samples in acid solution environment is the largest,the second in neutral environment and the smallest in alkaline environment.The water absorption of limestone sample is much lower than that of basalt under the same test conditions,which indicates that the compactness of limestone sample is better than that of basalt sample,and the homogeneity of its internal structure is also higher than that of basalt.(2)The stress-strain curves,uniaxial compressive strength and failure modes of the specimens in different chemical solution environments were obtained by uniaxial compression tests.It is found that the main failure mode of limestone and basalt is brittle tensile failure.With the passage of test period,the ductility of samples in each solution environment is enhanced,and the uniaxial compressive strength is generally on the decline.(3)Through the detection of p H value and main ion concentration of soaking solution in each test period,the corresponding relationship between p H value and main ion concentration of each solution and cycle times or continuous soaking time is obtained.It is found that p H value of acidic solution has approximately reached a relatively stable value at the end of the first test period,while p H value of alkaline solution changes most slowly,and it appears obvious change in the middle and later stages of the test Similarly,the concentration of Ca²⁺dissolved in acidic solution is the largest,and that in alkaline solution is the smallest.It can be concluded that the degradation effect of acidic solution on limestone sample is the strongest,that of neutral solution is the second,and that of alkaline solution is the weakest.Compared with the values of Ca²⁺and Mg²⁺concentration under the corresponding cycle times and continuous soaking time,it is found that there is no difference in the dissolution caused by dry wet cycle and continuous soaking in the preset test period.(4)Based on the results of mineral composition detection,major ion release rate and digital image processing technology quantitative scanning electron microscope observation,the chemical dissolution mechanism of limestone in acidic,neutral and alkaline solution environment is analyzed.It is concluded that the deterioration of limestone macro mechanical strength is mainly controlled by Ca²⁺,and H+and calcite in acidic solution environment will undergo severe chemical corrosion in a short time The reaction results in a large amount of Ca²⁺dissolution,while OH^-in alkaline solution and dolomite form Ca CO₃ precipitation at the reaction interface,which hinders the dissolution of Ca²⁺,so the dissolution of Ca²⁺in alkaline solution is the least.