Study On The Mechanical Properties And Microscopic Mechanism Of Activated Clay-modified Cement Soil Under Freeze-thaw Cycles And Cu²⁺

Cement soil has the characteristics of excellent compression resistance after soft soil reinforcement.However,if only cement is added,OH-generated by cement hydration cannot be used as a component,so adding some volcanic ash active materials to cement soil makes the OH-produced by cement hydration effectively utilized,which has become a hot spot in current research.In this paper,the mechanical properties of activated clay-modified cement soil and the mechanical properties under the influence of freeze-thaw and Cu2+ are systematically studied.The effect and mechanism of activated clay on improving the mechanical properties of cement soil were elucidated,and the engineering mechanical properties of activated clay modified cement soil under freeze-thaw and heavy metal Cu2+ erosion were explored.Taking the amount and age of activated clay as the research object,through the unconfined compressive test and the triaxial unconsolidated and non-draining shear test,the unconfined compressive strength,residual stress,failure strain,elastic modulus,internal friction angle and cohesion index were analyzed,and it was found that the unconfined compressive strength of the sample with 3% activated clay increased by 66% compared with the reference group after the sample was cured for 7d after being mixed with activated clay.At 28 days of curing,the sample with 5% activated clay was 41% higher than the reference group.Moreover,the residual stress,failure strain and elastic modulus of the sample after the addition of activated clay were better than those of the reference group,and the influence of activated clay on the shear strength of the specimen was mainly the effect on the cohesion.At the same time,the optimal content of active clay was determined to be 3%.Taking the 3% activated clay content as the benchmark,1,3,5 and 7 freeze-thaw cycles were designed,and it was found that the addition of activated clay could reduce the expansion of the sample,and the addition of activated clay could effectively reduce the strength loss rate of the sample,and the unconfined compressive strength of the specimen was also higher than that of the unactivated clay,but the final strength loss rate was not much different.After freezethawing,the addition of activated clay can still maintain high residual stress and failure strain,and the cohesion of activated clay is increased by 26%,0.6%,10% and 12% compared with the benchmark group without activated clay.That is,with the increase of the number of freezethaw cycles,the internal friction angle φ values of the specimen with activated clay added and the reference group showed a gradual upward trend.Taking the 3% activated clay content as the benchmark,when cured for 7d,the unconfined compressive strength of Cu2+ samples mixed with different concentrations decreased by 3%,8%,23% and 35%,respectively,compared with that without Cu2+.At 28 d,the unconfined compressive strength of 3% activated clay was reduced by 17%,34%,32% and 44% compared with the control group cured for 28 d,respectively,and the residual stress of the sample was significantly increased by adding activated clay.At different Cu2+ concentrations after the addition of activated clay,the failure strain of the sample was higher than that of the reference group without reactive clay.During 7 days of curing,the high concentration of Cu2+ can inhibit the E50 of the sample added to the active clay,making the ability of the sample to resist deformation worse.When curing for 28 days,the addition of 3% activated clay can increase the E50 of the sample at a certain Cu2+ concentration,but at high concentrations(2500mg/kg),its effect is lower than that of the benchmark group only incorporated with cement.In terms of cohesion and internal friction angle: the incorporation of activated clay was greatly affected by the Cu2+ concentration when curing for 7d,but when the specimen was cured for 28 d,the cohesion of CCSA3 was significantly improved compared with CCSA0,and Cu2+ had little effect on the internal friction angle of the sample.Through XRD characterization analysis,it was found that the active SiO2 in the activatedclay could react with the volcanic ash produced by cement hydration.It can be seen from FTIR that Cu2+ is found to interact mainly with C-S-H gels,resulting in a decrease in the strengthof the specimen.Through SEM analysis and gray-white binary quantitative analysis,it is proved that the improvement of the strength of activated white soil on cement soil is mainly through volcanic ash reaction and pore filling.The effect of the freeze-thaw cycle on the specimen is toexpand the porosity of the specimen due to the freeze-thaw cycle freeze-swelling,resulting ina decrease in the strength of the specimen.