Study On Solidification Effect Of Geopolymer On Lead And Cadmium Contaminated Soil

Cement is a common binder for remediation of lead and cadmium contaminated sites.However,due to its high energy consumption and unfriendly environment,it has become a research hotspot in the field of environmental geotechnical engineering to find a substitute for cement.Geopolymer,as a kind of green and environment-friendly cementitious material,has attracted wide attention because of its high strength and strong ability to solidify heavy metals.Therefore,in this paper,the artificially prepared high concentration lead and cadmium contaminated soil is taken as the research object.Based on laboratory test and micro analysis,the optimal ratio of fly ash+ground granulated blast-furnace slag（FA+GGBS）based geopolymer was determined,the solidification effect of geopolymer and cement on lead and cadmium contaminated soil was compared,and the influence of freeze-thaw cycle on the properties of solidified contaminated soil was discussed.The main results are as follows:（1）The effects of liquid-solid ratio and GGBS content on the strength,leached Cd²⁺concentration and p H of solidified contaminated soil were investigated,and the raw material composition of geopolymer was optimized.The influence of liquid-solid ratio on the strength and the leached Cd²⁺concentration was significant,while the influence of GGBS content was slight.With the increase of liquid-solid ratio,the strength of solidified contaminated soil decreased sharply at first and then remained stable.The leached Cd²⁺concentration decreased first and then increased,and reached the lowest value when the liquid-solid ratio was 0.3.Taking the leached Cd²⁺concentration as the screening standard,the optimal ratio of geopolymer was that the mass ratio of FA and GGBS was 7:3,and the liquid-solid ratio was0.3.（2）The effect of geopolymer on solidification of lead and cadmium contaminated soil was evaluated,and the effect of leachate p H on leached Pb²⁺and Cd²⁺concentration was studied.The strength of solidified contaminated soil was about 1.35～1.50 times of that of untreated contaminated soil.And the leached concentration of Pb²⁺and Cd²⁺decreased significantly.Under the same dosage of binder,the unconfined compressive strength of cement solidified contaminated soil was 2～2.5 times of geopolymer.The leached Pb²⁺and Cd²⁺concentration was lower,but the p H of cement solidified contaminated soil was higher,which was easy to cause alkaline pollution.When the p H of leaching solution was less than 4,the leached Pb²⁺and Cd²⁺concentration increased with the increase of p H.When the p H was between 4 and 10,the leached Pb²⁺and Cd²⁺concentration did not change significantly.However,when the p H was greater than 10,the leached Pb²⁺concentration increased sharply,while the leached Cd²⁺concentration first decreased and then increased slightly.（3）The mass loss rate,unconfined compressive strength,stress-strain curve and leached heavy metal concentration of geopolymer solidified contaminated soil under freeze-thaw cycles were studied.With the increase of freeze-thaw cycles,the mass loss rate curve went through slow growth stage,rapid rise stage and stable change stage.Compared with the samples without freeze-thaw cycle,the strength of geopolymer solidified contaminated soil decreased by 19%～32%after freeze-thaw cycle.The deformation modulus E₅₀ decreased by8%～10%.And the leached Pb²⁺and Cd²⁺concentration increased significantly.（4）The effects of lead and cadmium on the morphology,structure and mineral composition of geopolymer were studied by microscopic experiments,and the change of pore structure in solidified contaminated soil was quantitatively analyzed.The product of alkali activated FA and GGBS was C（N）-A-S-H gel.The main mechanism of Cd²⁺immobilization by geopolymer was the formation of Cd（OH）₂.Pb²⁺could be solidified in geopolymer by replacing Ca of C（N）-A-S-H gel.The addition of Pb²⁺and Cd²⁺would reduce the alkalinity of the system and reduce the gel production,making the microstructure of geopolymer looser.The increase of liquid-solid ratio mainly increased the volume of pores between1-10μm and more than 10μm.With the increase of FA+GGBS content,the volume of pores between0.01～1μm increased.