Experimental Study On The Properties Of Tailings-Based Backfill Materials With CaCl₂·6H₂O/Expanded Vermiculite

With the exhaustion of shallow resources and the increasing emphasis on the green mines in China,mining technology has been extended to deep mines continuously,and deep mining faces more solid wastes（tailings,waste rocks,etc.）and underground heat harm problems,which makes it the most important research direction to deal with a large amount of solid wastes,alleviate heat damage and utilize geothermal resources.Based on the concept of heat storage functional backfill,this study takes tailings as backfill aggregate,and CaCl₂·6H₂O/expanded vermiculite shape stabilized phase change material（CEVC）with different particle sizes is preferably added into backfill material,in order to form backfill body with high heat storage capacity.The fluidity of backfill slurry,the mechanical and thermal properties of backfill body under different cement-tailing ratios（1:4,1:6 and 1:8）and different CEVC addition ratios（3%,6%,9%and 12%）were tested and analyzed.During the preparation of shape stabilized phase change material,the thermal performance of CEVC with 6-20 mesh,20-40 mesh and 40-60 mesh were tested.It was found the phase change enthalpy of CEVC with 20-40 mesh reaches 97.81 J/kg,the maximum encapsulation capacity of CaCl₂·6H₂O reaches 60%,and the heat loss rate is only 3.93%after 300 thermal cycles.Therefore,CEVC with 20-40 mesh can be added into the backfill material as the best choice to exert its heat storage effect.The backfill slurry with CEVC obeys the Hereshel-Bulkley model in non-Newtonian fluid.The decrease of cement-tailings ratio enhances the fluidity of slurry,but the addition of CEVC makes the fluidity of slurry worse.The slump can be reduced by up to 5.96%With the addition ratio of CEVC in this study.The critical shear rate increases with the increase of CEVC addition ratio.The critical shear rates of slurry without CEVC,with 3%,6%,9%and 12%CEVC addition are 34.8 s（^-1）,37.9 s（^-1）,40.8 s（^-1）,44.1 s（^-1） and 50.1s（^-1）,respectively.The uniaxial compressive strength and density of the backfill decrease with the decrease of the cement-tailings ratio.However,with the increase of CEVC addition ratio from 3%to 12%,the uniaxial compressive strength and density decrease by 53.42%and 7.37%respectively,which is mainly caused by the increase of porosity with the decrease of the cement-tailings ratio and the increase of CEVC addition ratio.The specific heat capacity of backfill body with different cement-tailings ratios increases with the increase of temperature,and the specific heat capacity is positively correlated with the cement-tailings ratio and the addition ratio of CEVC.With the addition ratio of CEVC in this study,its sensible heat storage capacity and latent heat storage capacity can be increased by at least 10.74%and 218.97%respectively.The thermal conductivity of backfill body decreases with the decrease of cement-tailings ratio and the increase of CEVC addition ratio,but the degree of decrease has no obvious regularity,and the thermal conductivity increases with the increase of temperature.The thermal properties test shows that the temperature cycle range of backfill body with CEVC is recommended to be 20℃ to 40℃.Based on the AHP-TOPSIS method,the backfill scheme（cement-tailings ratio of 1:6 and 12%of CEVC）with CEVC was recommended.On the premise of meeting the fluidity and mechanical properties,this backfill scheme has strong heat storage performance,which can provide basic data for the research on application of shape stabilized phase change materials into the backfill body.