| As a new type of cementing material,the high water backfill material is widely used in the fields of goaf filling and goaf retaining roadway side filling due to its advantages of good fluidity,short initial setting time and high early strength.However,its further application in the field of mining engineering backfill mining is restricted by its high cost of filling,poor anti-weathering performance and poor shrinkage adaptability to roof rotation subsidence in the gob retaining roadway side filling under hard roof.At the same time,in order to realize the efficient and green development of the coal industry,it has become a new trend to introduce mining and industrial solid wastes such as fly ash and mineral powder to replace cement consumption in the field of filling materials under the dual carbon goal of China.Therefore,this study aims to improve the utilization rate and efficiency of solid waste in the field of high water backfill material,and improve the engineering characteristics of high water backfill material by combining the electrochemical treatment technology while reducing the filling cost.Therefore,in this work,the high water backfill material were used as the research object to research the effect of the doping and electrochemical treatment on the mechanical properties of the high water backfill material.Then the cement hydration principle,fly ash pozolanic activity and its activation mechanism,and electrochemical principle are applied to analyze the modification mechanism of the high water backfill material.Meanwhile,the cement hydration heat test,X-ray diffraction,thermogravimetric analysis,scanning electron microscopy and other technical means were used to test the hydration thermodynamic parameters,hydration products and microstructure of high water backfill material modified by different methods.The main achievements of this work are as follows:(1)By testing the basic properties,mechanical properties and deformation and failure characteristics,hydration heat release characteristics,the hydration products and microstructure of the high water backfill material with different water solid ratio,it was be found that the properties and the hydration process of the high water backfill material are greatly influenced by water solid ratio,within the scope of the 2:1 to 3:1,when the water cement ratio to 3:1,its mechanical properties decreased obviously.This is because with the increase of water content,relative concentration of raw materials decreased,which caused the hydration reaction rate and degree of significantly reduced,the main hydration products of Ettringite formation rate is restrained,Ettringite crystal by short columnar to develop in the long and thin needle,worsen and micro pore structure density increases,eventually results in the decrease of mechanical properties.However,although the strength of the sample is higher with the decreasing of the water solid ratio,the compressibility of the high water backfill materials became worse,which is not conducive to the relief pressure of the filling body when the hard roof rotates and sinks in engineering practice.(2)The method of fly ash incorporation into high water backfill material is divided into three kinds of equal doping,partial substitution of a material and non-equal doping respectively.The influence law of different doping methods and dosage on mechanical properties of the high water backfill material under different curing conditions is studied.It is found that the mechanical properties of the high water backfill material with non-equal amount of fly ash are significantly improved under the same content level,compared with the traditional equal doping method,while the content of fly ash in the high water backfill material is increased to 30% without a significant decrease.It is found that the incorporation of fly ash can effectively improve the weathering resistance of high water backfill materials and inhibit the carbonization of Ettringite.By comparing the influence of different dosage and doping methods on hydration products and microstructure of high-water backfill materials,the influence mechanism of non-equal amount of doping fly ash on mechanical properties and hydration reaction of high-water backfill materials was revealed.(3)By testing the early mechanical properties of the high water backfill material mixed with different proportions of ore powder and fly ash,it is found that the peak strength,elastic modulus and deformation modulus of high-water filling materials after mixed with ore powder show a trend of first increasing and then decreasing with the increase of ore powder dosage.And at the same content level,the early strength of the high water backfill materials mixed with 14% fly ash and 6% ore powder increased by42.86% than that of the high water backfill material mixed with 20% fly ash alone,which is close to that of the pure high water backfill material.By comparing the hydration exothermic characteristics,hydration products,crystallinity and microstructure of the modified high water backfill materials with different ore powder dosages,the activation mechanism of fly ash induced by ore powder addition and the influence mechanism of fly ash and ore powder addition on mechanical properties of high water backfill materials were revealed.(4)The electrochemical modification test of the high water backfill material were carried out to study the effects of the electrochemical treatment on the mechanic properties of the high water backfill material.It is found that mechanical properties of high water backfill materials first increased and then decreased with the increasing of the potential gradient and the polarization time,then the potential gradient of 0.2 V/cm high water filling material of modified peak strength,elastic modulus and deformation modulus are peak,The peak strength of the sample after electrochemical modification is the highest,while the peak value of deformation modulus and elastic modulus is 3h,which indicates that the strength of the high water backfill material can be effectively improved and its variability can be improved through reasonable control of potential gradient and electrification time.It is of great significance to improve the yield characteristics of high water bacfill body in engineering practice.(5)The effects of different electrochemical parameters on mechanical properties of high water backfill materials,it is found that the electrochemical modification has little influence on the peak strength of high water backfill materials,but has obvious influence on the elastic modulus and deformation modulus.From the perspective of electrochemical parameters,the potential gradient has the greatest influence on the mechanical properties of high-water filling materials,while the electrification time is relatively small.By comparing the hydration products and microstructure of high water filling materials before and after electrochemical treatment with different potential gradients,the electrochemical modification mechanism of high water filling materials was revealed.(6)The changes of electric current through the high water backfill materials under the electrochemical treatment process were tested,and the electrical resistance of the high water backfill materials before and after the electrochemical treatment was analyzed.It is found that the variation trend of the electrical resistance is consistent with the uniaxial compressive strength in the high water backfill materials.And the quantitative relation equation were fitting out between with the uniaxial compressive strength and electrical resistance of the high water backfill materials.It is found that there is a positive linear correlation between the electrical resistance and uniaxial compressive strength in the high water backfill materials.(7)Compound modification of the high water backfill material by doping and electrochemical treatment.It is found that the addition of mineral powder and fly ash and electrochemical treatment can effectively change the mechanical properties and deformation characteristics of the high water backfill materials,which has the largest influence on the properties of the high water backfill materials and less electrochemical treatment.However,the influence of electrochemical treatment on high water filling materials is mainly reflected in the change of the deformability of the high water backfill materials,which has great engineering significance to improve the toughness of the filling body in engineering practice and ensure the supporting strength of the filling body under the pressure of roof.By analyzing the changes of the hydration products and microstructural in the different regions of the high water backfill materials before and after the composite modification,the coupling mechanism of doping modification and electrochemical modification on the changes of the mechanical properties in the high water backfill materials was revealed. |
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