| In recent years,the rapid development of industrialization and urbanization in China also brings serious underground environmental pollution problems,hindering the development and utilization of sites,threatening the health and safety of the public.Sodium bentonite is widely used as antifouling barrier material in landfills and contaminated sites due to its strong hydration swelling ability and low permeability.The anti-pollution barrier uses low permeability soil or artificial materials to enclose the pollutants or polluted soil water,and isolates the underground pollution sources from the surrounding water and soil,which can effectively prevent the pollutants from spreading to the surrounding groundwater or soil.However,the leachate in the landfill,the contaminated groundwater in the contaminated site and the groundwater in the coastal areas contain a large number of ions,and the hydraulic conductivity of bentonite in the antifouling barrier increases greatly after chemical reaction with it,which weakens the blocking function of pollutants and even fails to meet the requirements of relevant standards.In view of the above situation,NaCl solution and CaCl2 solution with different concentrations were used as test solutions in this paper.Based on testing the basic characteristics of Wyoming bentonite,the influences of cation concentration and valence on the engineering characteristics of sodium bentonite were systematically studied.(1)The effects of concentration and valence of cation on the swelling property and water ratio limit of sodium bentonite were studied by using free swell test and water ratio limit test.The results showed that,with the increase of concentration of NaCI solution and CaCl2 solution,the swell index of sodium bentonite first increased and then decreased,reaching the peak value(42.5ml/2g,38mL/2g)in 20mM NaCl solution and 5mM CaCl2 solution,and the plastic limit increased slightly and the liquid limit decreased significantly.The change of plasticity index was mainly affected by the liquid limit:the plasticity index decreased from 237.8 to 54.6 with the increase of NaCl concentration,and from 229.7 to 63.2 with the increase of CaCl2 concentration.At the same concentration,the swell index,liquid limit and plasticity index of sodium bentonite in NaCl solution were higher than those in CaCl2 solution.(2)The effects of concentration and valence of cation on the basic engineering properties of sodium bentonite slurry were studied by using specific gravity test,viscosity test,pH and conductivity test and precipitation test.The results showed that as the concentration of NaCl solution and CaCl2 solution increased,the slurry density increased slightly and the markov viscosity decreased from 43s to 29s、the precipitation characteristics of the slurry are significantly affected by the cation concentration:when the solution concentration is low(C(Na+)≤100mM,C(Ca2+)≤5mM),the bentonite slurry will not precipitate.When the solution concentration exceeds the above critical concentration,the final precipitation volume decreases from 65.4ml to 18.2ml with the increase of NaCl solution concentration,and from 41.1ml to 20.9ml with the increase of CaCl2 solution concentration.At the same concentration,the markov viscosity and conductivity of the slurry prepared with CaCl2 solution are smaller,and the valence of cation has little effect on the pH and density of the slurry.Therefore,the increase of concentration or valence of cation will lead to the poor workability of slurry properties.When the solution concentration is higher(C(Na+)≥100mM,C(Ca2+)≥20mM),the density,viscosity and precipitation characteristics of the slurry do not meet the anti-pollution barrier engineering requirements,so it is not suitable for the mud protection.(3)Modified fuild loss test and flexible wall permeameter test were used to study the effects of concentration and valence of cation on the permeability of bentonite cake under different pressures.The results show that the hydraulic conductivity of bentonite cake decreases with the increase of filtration pressure/consolidation pressure.The hydraulic conductivity of bentonite cake increased by 18-38 times(Modified fuild loss test)/3-8 times(flexible wall permeameter)with the increase of concentration of NaCl solution,and by 31～5 8 times(Modified fuild loss test)/4-10 times(flexible wall permeameter)with the increase of CaCl2 solution concentration.The hydraulic conductivity of bentonite cake increases with the increase of cation valence.The hydraulic conductivity of bentonite cake prepared by CaCl2 solution was higher than that of bentonite cake prepared by NaCl solution with the same concentration(kcacl2/kNaCl=4-22.9(Modified fuild loss test)/kcaCl2/kNacl=2～7.7(flexible wall permeameter)).It is feasible to replace the flexible wall permeameter test with the modifiled fluid loss test to test the hydraulic conductivity.However,under the medium and high concentration(C(Na+)≥200mM and C(Ca2+)≥20mM),the slurry prepared by salt solution shows precipitation phenomenon,and the hydraulic conductivity tested by modifiled fluid loss test is nearly an order of magnitude higher than the actual hydraulic conductivity,it is not suitable to test the hydraulic conductivity of bentonite cake by modifiled fluid loss test.(4)The effect of concentration and valence of cation on the consolidation characteristics of sodium bentonite cake was studied by standard consolidation test.The results show that with the increase of consolidation pressure,the porosity ratio of bentonite samples decreases by 3～5 times,the volume compression coefficient decreases by 46～120 times,and the compression index decreases on the whole.Under the same consolidation pressure,the higher the cation concentration is,the smaller the compression index of bentonite sample is.The compression index of bentonite sample made from NaCl solution and CaCl2 solution(0.7～3.6)is smaller than that made from deionized water(1.8～5.1).The change of solution concentration has little effect on volume compressibility. |
PDF Full Text Request