Study On Degradation Of Engineering Characteristics And Mechanisms Of Compound Solidified Lead Contaminated Soil Characteristics Under Freeze-thaw Cycles

With the development of society and the enhancement of people's awareness of environmental protection,a large number of industrial manufacturing enterprises with pollution risk are closed down or relocated with the upgrading of industrial structure and the adjustment of urban layout,thus leaving behind a large number of heavy metal pollution sites.Solidification/stabilization technology is the mainstream technology for the treatment of heavy metal contaminated soil.However,the effect of solidification and remediation is affected by many factors.As an important external force,it is worth further study on the influence of freeze-thaw cycles on the engineering mechanical properties of solidification/stabilization remediation of heavy metal contaminated soils.There is a large area of frozen soil in China.Systematic study on the engineering characteristics of solidified heavy metal contaminated soil under freeze-thaw alternation is of great significance to reuse heavy metal contaminated sites in frozen soil area and ensure the long-term stability of solidification/stabilization effect.This paper relies on the the National Natural Science Foundation of P.R.China,named "Study on the mechanism of Engineering degradation effect of solidified and modified high concentration precious metal contaminated soil under long-term freezethaw cycles(41772306)".The effect of freeze-thaw cycles on the engineering properties of solidified heavy metal lead(Pb)contaminated soil is the main research content.Through laboratory geotechnical test,CT scanning test,scanning electron microscopy test and Fourier transform infrared spectroscopy test,the deterioration law and micromechanism of engineering properties of solidified lead contaminated soil were preliminarily explored.The main research results are as follows:(1)Under short-term freeze-thaw cycles,the unconfined compressive strength,modulus of deformation,angle of internal friction and cohesion of lead-contaminated soil(Pb content is 1%)decreased with the increasing of freeze-thaw cycles and the permeability coefficient fluctuates.The unconfined compressive strength,modulus of deformation,internal friction angle,cohesion and permeability coefficient can not be better if only cement,lime and fly ash(2.5%)are used to solidify and repair the lead contaminated soil.(2)Through orthogonal test,three better curing ratios of C2.5S5F5,C5S2.5F2.5 and C5S5 under short-term freeze-thaw cycles were obtained taking unconfined compressive strength,deformation modulus,internal friction angle,cohesion and permeability coefficient as the evaluation indexes,(3)Three solidifying agents,C2.5S5F5,C5S2.5F2.5 and C5S5,were used to solidify and repair lead-contaminated soils with different degrees of pollution,respectively.After long-term freeze-thaw cycles,the indoor geotechnical test results show that the deterioration rules of engineering properties of the three solidified lead contaminated soils are different with the increase of freeze-thaw cycles due to different ions.The ionic concentration ranges for no significant difference in curing effect of three curing agents were obtained are discussed by non-parametric test,(4)CT scanning test shows that the self-defect of soil sample can be obviously improved by adding cement,lime and fly ash,and the "large volume" pore is obviously reduced.Macropores of C2.5S5F5,C5S2.5F2.5 and C5S5 solidified lead-contaminated soils increase with the increasing of freeze-thaw cycles(0d?30d?90d),which shows that the engineering is especially reduced.(5)SEM analysis shows that there are obvious pores in lead-contaminated soil(Pb1)which has not been solidified and repaired;when cement(2.5%),lime(2.5%)and fly ash(2.5%)are added to lead-contaminated soil,the soil particles are closely bound by hydration products,which improves the integrity and strength of lead-contaminated soil;the micro-particles(< 1um)and micro-pore(< 2um)increases for freeze-thaw cycles increasing.As a result,the unconfined compressive strength decreases.(6)FTIR spectra of C2.5Pb1 and S2.5Pb1 showed that the transmittance of all characteristic peaks in FTIR spectra of C2.5Pb1 was lower than that of Pb1,indicating that hydration of curing agent resulted in corresponding hydration products.Because of the low hydration degree of fly ash,F2.5Pb1 is supposed to produce intermediate products.The characteristic peak transmittance is higher than that of Pb1-contaminated soil functional groups.The failure of functional groups with the increasing of freeze-thaw cycles is one of the reasons for the decreasing of engineering properties of solidified lead contaminated soils.