| The development of new closure cover materials is an effective way to solve the service performance defects of traditional landfill barrier systems.In this study,a landfill closure cover material was prepared using industrial solid waste（desulfurization gypsum,slag）and construction waste for solidification of river and lake bottom sediment.The mechanical properties of cured substrates during indoor maintenance,wet and dry cycles,and freeze-thaw cycles were systematically investigated by volume shrinkage,unconfined compressive strength,direct shear,flexural and penetration tests.X-ray diffraction（XRD）,Fourier infrared spectroscopy（FTIR）,swept surface electron microscopy（SEM+EDS）and inductively coupled plasma emission（ICP-OES）were used to characterize the mineral composition,functional groups and microscopic morphology of cured substrates.The research results can provide basic data support for the application of solidified substrate in landfill closure cover system.The main research results are as follows:（1）Industrial solid waste（desulfurization gypsum,slag）and construction waste powder are used to solidify the substrate,a new type of landfill closure cover material was prepared.The optimal ratio of curing agent was selected by using extreme difference and analysis of variance（ANOVA）,the response result of lateral unconfined compressive strength.（2）The mechanical characteristics parameters such as volume shrinkage,unconfined compressive strength,flexural strength and shear strength of cured substrates under different curing age conditions were obtained.The maintenance 28 d cured substrate permeability coefficient was 1.22×10-8-55.4×10-8 cm/s.After 180 d of distilled water immersion,the hazardous heavy metal elements in the leachate of the specimen were lower than the national hazardous waste leaching toxicity standard.（3）The volume shrinkage and permeability coefficients of cured substrates under the action of wet and dry cycles showed an overall increasing trend,the unconfined compressive strength shows a decreasing trend.After 20 wet and dry cycles,the volume shrinkage of the cured substrate increased by 1.10-1.22 times and the unconfined compressive strength decreased by 29.05%-51.63%.The permeability coefficients of GH1,GH3 and GH7 specimens are still less than 1×10-7 cm/s.The wet and dry cycles destroyed the internal cementation products of the specimen,resulting in a loose structure,but the cementation products were still present.（4）As the number of freeze-thaw cycles increases and the temperature decreases the volume shrinkage and permeability coefficient of the cured substrate tends to increase and the unconfined compressive strength tends to decrease.The volume shrinkage of the cured substrate increased by 1.04-1.60 times after 20 freeze-thaw cycles,and the unconfined compressive strength decreased by 21.31%-64.53%.The permeability coefficient of the specimen after 20 cycles at-35°C ranges from 9.77×10-8 to 13.45×10-8 cm/s.During the freeze-thaw cycle,the transformation of the water molecule morphology leads to the disruption of the internal reticulated skeletal structure of soil particles and hydration products.（5）The lake bottom sediment under the condition of calcium hydroxide alkali excitation,and slag,desulfurization gypsum,construction waste powder undergoes synergistic hydration reaction,calcium alumina,C-S-H and other hydration cementation products and layered bimetallic hydroxides（LDHs）such as green rust and water talc were generated.The cementation product and the subsoil particles associate with each other,fill,wrap and bond to form a dense structured crystalline mass.This is the key reason for the high strength and low permeability of the cured substrate. |
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