Study On Strength And Leaching Characteristics Of Pb Contaminated Clay Stabilized By MgO And MgO-Fly Ash In Complex Environment

Solidification/stabilization technology has shown excellent performance in remediation of heavy metal contaminated soils.However,the exploration of new curing agents is still under studying.At the same time,the frequent occurrence of extreme climate poses great challenges to the field of remediation of contaminated soils.In this paper,mechanical properties and leaching characteristics are taken as evaluation indexes,and a series of experime nts and theoretical analysis are made on the solidification performance and long-term durability of new green solidifying agent MFA（active magnesium oxide-fly ash）and active magnesium oxide for stabilizing and repairing lead-contaminated soil under extreme climate conditions.The main results are as follows:（1）Study on the selection of solidifying agent:Three different binders,i.e.MgO,MFA5-5 and MFA3-7,were used to solidify/stabilize Pb-contaminated soils taking into account the effect of binder amount,initial Pb concentration and curing time,and to characterize the unconfined compressive strength and leaching toxicity of Pb-contaminated soils.The test results demonstrate that the compressive strength of stabilized/solidified Pb-contaminated soils increases with curing time,and the order of strength gain for stabilized soils is MgO>MFA5-5>MFA3-7>PC.The compressive strength of MgO,MFA5-5 and MFA3-7-stabilized/solidified soils is respectively 4～17times,2～8 times and1.1～4.5 times higher than PC-treated soils.According to the concentration of leached Pb²⁺,the order of chosen binders for stabilizing Pb²⁺in contaminated soils is MgO<MFA5-5<MFA3-7<PC.This is to say,MgO is the optimum binder to stabilize Pb²⁺,followed by MFA5-5 and MFA3-7,and PC is the relatively worst one.（2）Temperature effect test:Based on unconfined compression test and TCLP test,the results show that high temperature promotes the strength formation of MFA and MgO stabilized lead contaminated soil,and low temperature inhibits its strength growth.The working performance of active MgO curing agent at various temperatures is better than that of MFA curing agent.Reducing curing temperature can damage the strength of MFA and MgO solidified soils in varying degrees.The strength of the solidified body and the pH value of leaching solution increased with the increase of MFA and MgO content in high and low temperature environments.With the increase of initial Pb concentration,the strength of the two solidified soli ds increases first and then decreases with the pH value of the leaching solution.The pH value of leaching solution of active MgO curing body is higher than that of MFA curing body.The critical concentration increases with increasing temperature.With the increase of the initial lead concentration,the leaching amount of Pb²⁺decreases slowly and then increases sharply.The leaching solution Pb²⁺concentration in the temperature effect test range meets the requirements of the specification.The compressive strength,the leaching solution pH value and the leaching solution Pb²⁺concentration of the stabilized soil samples show consistency.At the same time,the active MgO solidified body has better freeze-thaw resistance and solidified Pb²⁺performance than the MFA solidified body.（3）Durability test of freeze-thaw cycles:Using active MgO and MgO-fly ash to stabilize contaminated soils with different initial Pb²⁺concentration,the durability of freeze-thaw cycles of solidified lead contaminated soils was evaluated by freeze-thaw cycles,unconfined compressive tests and toxic leaching tests from unconfined compressive strength,pH value of leaching solution and Pb²⁺leaching concentration.The results show that active MgO and MgO-fly ash can significantly improve the compressive strength,stability of Pb²⁺and freeze-thaw resistance of soil.With the increase of freeze-thaw cycles,the compressive strength and strength loss rate increased first to the peak value and then decreased;with the decrease of freezing temperature,the compressive strength of solidified soil decreased;after 11 freeze-thaw cycles,the residual strength of solidified soil was higher than 75%,meeting the requirements of the code;under freeze-thaw cycles,the compressive strength of solidified soil increased more than two times from 5%to 10%,and the optimum concentration of solidified lead contaminated soil was 0.5%.The leaching amount of Pb²⁺from contaminated soil solidified by MgO and MgO-fly ash meets the requirements of the code.Active MgO and MgO-fly ash have good durability in stabilizing lead contaminated soil,and the former has better solidification effect than the latter.（4）Dry-wet cycle durability test:Based on the unconfined compressive strength and quality characteristics,the results show that The strength of MFA and MgO solidified bodies increased with the number of wet and dry cycles.The strength of MFA and MgO solidified bodies first strengthened（DW<4）and then deteriorated（DW>4）.The optimum concentration of MgO solidified body is0.5%in 12 cycles,and MFA is more suitable for remediation of low lead contaminated soil.Both of them have poor resistance to wet and dry environment at the condition of Pb=2%.After 12 cycles,the final residual strength ratio of MgO solidified body meets the specification requirements,while that of MFA solidified body slightly does not meet the requirements.Increasing the content of MgO and MFA can increase the strength of solidified soil under dry-wet cycling conditions.The cumulative mass loss rate of MgO cured body is lower than that of MFA cured body,and the cumulative mass loss rate of both cured bodies is less than 30%.Initial Pb concentration has a great influence on the mass characteristics of MFA and MgO cured bodies.Compared with MFA cured body,MgO cured body has stronger resistance to wet and dry damage.