Experimental Study On Deterioration Mechanism Of Filling Paste In Goaf Under Sulphate Erosion

Filling mining can effectively solve the problems of ground subsidence and "three down" coal compaction.However,the filling body in the mine water environment for a long time will inevitably be eroded by salt ions.As a result,the strength of the backfill is reduced,and the service life of the backfill and the safety of the relevant engineering are affected.Considering the influence of solution concentration,erosion time and cationic species,the sulfate erosion paste experiment was carried out,and the damage mechanism of filling paste was studied by macroscopic and microscopic tests.The macroscopical changes of sulfate erosion filling paste were analyzed by three macroscopic methods of measuring quality and uniaxial compressive strength.X-ray diffraction(XRD),scanning electron microscope(SEM)and comprehensive thermal analysis(TG-DSC)were used.Qualitative and quantitative analysis of sulfate erosion paste erosion products were carried out.The research results are as follows:(1)The optimum proportion of paste filling material is obtained by orthogonal test,the ratio of cement to material is 1:2,the mass concentration is 76,the proportion of fine aggregate is 45,and the collapse degree of filling material slurry with the best proportion is 21 cm,which meets the requirement of pumping in construction and the requirement of strength.(2)The hydration process and hydration products of the filling body under the action of SO42-were studied by means of macroscopic mechanics and microstructure tests,and the erosion mechanism was analyzed,and the reasons for the change of macroscopic mechanical properties of the filling body in erosion environment were discussed.(3)The strength of the paste after curing was more than that of the specimen soaked in clear water during the initial period of soaking in sulfate solution,mainly within 90 days.This was because the hydration reaction continued and the residual pores in the paste specimen were continuously filled.The compactness is enhanced,so the compressive strength is increased.(4)At the later stage of soaking the paste in sulphate solution,mainly 90 days later,the strength of the specimen began to decrease,and the inner void of the specimen was filled with gypsum,ettringite and other insoluble substances.After that,the insoluble materials continue to form expansion to cause cracks in the specimen,thus reducing the compactness and compressive strength of the specimen,and the compressive strength of the specimen decreases greatly with the increase of the concentration of the solution,and the speed of the specimen decreases rapidly.When the final paste was soaked in sulfate solution for 180 days,the strength of the specimen was lower than that of water immersion.(5)The influence factors of long-term stability of filling paste were evaluated by grey correlation method.