Hydraulic Performance And Micro Mechanisms Of Backfills Consisted Of Reactive MgO-GGBS-bentonite In Vertical Cutoff Walls

With the financial support of National Natural Science Foundation of China（Grant Nos.41877248 and 42177133）,National Key Research and Development Program of China（Grant Nos.2018YFC1803100 and 2018YFC1802300）,this study investigated the containment performance of reactive MgO-Ground Granulated Blast-furnace Slag（GGBS）-bentonite slurry-wall（MSB）backfill via workability,unconfined compression strength,hydraulic conductivity test and microscopic analyses.The following conclusions were obtained:（1）The workability of MSB was investigated using slump test.The water content meeting the slump requirement increases with the increase of bentonite content.For example,the water content increases from 33%to 49%when the bentonite content increases from 5%to 10%.The strength characteristics of the material were studied by the unconfined compressive strength test.The results show that increasing the MgO content can significantly improve the unconfined compressive strength.The unconfined compressive strength reaches its highest value at a GGBS:MgO of 4:1.Increasing slag content can improve the unconfined compressive strength,and all the unconfined compressive strength values are hgiher than 100 k Pa after 28 days of curing.（2）The swelling behavior of MSB material was studied by free swell test.The results show that the free swell index of the sample can be improved by increasing the bentonite content and the MgO content,but the free swell index is inhibited by increasing the GGBS content.When the bentonite content is 10%and the GGBS:MgO is 4:1,the maximum free swell index reaches46.2 ml/2g.The hydraulic conductivity of the material was studied using flexible wall permeability experiment.The results show that the hydraulic conductivity of the material is reduced by adding bentonite and MgO.When the bentonite content is 10%and GGBS:MgO is4:1,the hydraulic conductivitiy reaches its minimum value of 3×10^-10m/s.（3）The containment performances of MSB were studied by swelling test and flexible wall permeability experiment under the exposure of simulated landfill leachate.Compared with the results in tap water,the results show that the contaminated liquid significantly inhibites the free swell index and impermeability of MSB.The free swell index decreases by about 50%and the hydraulic conductivity increases by a factor of 10 to 100.The addition of bentonite and MgO can effectively improve the impermeability of MSB.（4）The hydration products of MSB barrier were evaluated by scanning electron microscopy（SEM）and X-ray diffraction（XRD）.The functional groups of MSB components were studied by Fourier transform infrared spectroscopy（FTIR）.The pore characteristics of MSB barrier material were studied by mercury intrusion porosimetry（MIP）.The results show that MgO can stimulate the hydration of GGBS to generate the main hydration product of hydrotalcite C-S-H（CaO·SiO·H₂O）.Compared with the traditional cement-based backfills,the MSB has larger specific surface area,which can effectively block the migration path of pollutants and improve the adsorption of pollutants.