Research On The Fly Ash Cemented Filling Materials And Its Modification And Further Application

In recent years,the paste-backfill mining technology with fly ash and coal gangue as main raw materials has attracted more and more attention.It is beneficial for solving coal seam mining under river,building and railways as well as large-scale utilization of large solid wastes such as coal gangue and fly ash.However,the research and practice have proved that the paste-filling materials had the defects of low early strength and large deformation,which affects the control effect of surface subsidence.To solve this problem,this paper used fly ash cemented filling materials（FCFMs）,composed of fly ash,coal gangue,cement and water,in which fly ash from Xingneng Power Plant of the XiShan Coal Electricity Group Co.,Ltd.,coal gangue from Zhenchengdi mine and ordinary Portland cement produced by Xishan Cement Plant as the research object.The paper focused on accelerating the hydration reaction rate,increasing the hydration products production,improving the microstructure,and strengthening the interfacial cementing property and stability of the FCFMs by means of laboratory tests,theoretical analysis and numerical simulation and based on the theories and testing methods of Mining Science,Rock Mechanics,Physical Chemistry,Electrochemistry and Materials Science,and carried out research on the physical and chemical properties and modification of the FCFMs.Some research results with theoretical guidance and practical value have been obtained,as follows.（1）Starting from the basic properties of the FCFMs,the research works clarified the physical and chemical characteristics of raw materials and their respective roles were clarified.Through a large number of proportioning tests,the optimal proportioning was recommended:cement:fly ash:gangue=1:3:5（mass ratio）,and mass concentration between 78%and 80%.The physical and mechanical properties and rheological characteristics of the FCFMs were further analyzed.（2）The hydration kinetics and hydration mechanism of the FCFMs were discussed based on the characteristics of hydration heat and volume resistivity.The curve of the exothermic rate of hydration and the variation rate curve of volume resistivity well reflected the variation characteristics of hydration process in the process of setting and hardening.The hydration kinetic parameters were calculated by using the Krstulovic-Dabic hydration kinetic model.It was found that the variation characteristics of the hydration dynamic process of filling materials were the nucleation and crystal growth controlled by thephase boundaries effect first,and then the transformation mode from the phase boundaries reaction control to the diffusion control process.A descriptive model of hydration mechanism was constructed,and the hydration process of the FCFMs was divided into the dissolution stage,cement-fly ash hydration stage and flocculated-structure formation and growth stage.Low hydration reaction rate and low interfacial cementing strength were the main reasons for low early strength and large deformation of the FCFMs.（3）The leaching rules of active ions such as Si⁴⁺,Al³⁺and Ca²⁺from low-calcium fly ash under the activation of NaOH solution with different concentrations was studied.The change law of chemical groups on the surface of low-calcium fly ash,the formation characteristics of hydration products and the evolution characteristics of micro-morphology on the surface of particles during alkali activation were analyzed.A model for analyzing the reaction process of alkali-activated low-calcium fly ash and the formation mechanism of hydration products was constructed.It was found that the effect of alkali activation on low-calcium fly ash mainly concentrated on the surface of the particles,and the particles experienced four stages of changes through surface modification,namely,dissolution,depolymerization,polycondensation and polymer gel and diffusion.The effect of alkali activation on the physical and mechanical properties of fly ash filling materials was further discussed.The results showed that the early strength of filling materials increased first and then decreased with the increase of NaOH concentration.When NaOH concentration was 0.25mol·L^-1,the uniaxial compressive strength of the FCFMs after 3 d and 7d under alkali activation increased by 39.92%and 21.46%,respectively.（4）The modification test and the effect and mechanism were conducted on FCFMs based on the basic principle of the electrochemical treatment method.The optimum modification parameters under the combined action of three factors,namely,power-on time（t）,potential gradient（P）and NaOH dosage（D）,were determined by using the response surface optimization method.The order of the effect of each factor on the 7 day uniaxial compressive strength was clarified.The effect of electrochemical action on changing the strength and deformation of the FCFMs was analyzed.It was found that after electrochemical modification,the uniaxial compressive strength of the FCFMs after 7d increased by 63.20%,51.21%and 74.39%in the anode,middle and cathode regions,respectively,and the elastic modulus increased by 90.12%,75.93%and 112.98%respectively.The overall modification effect of the cathode region was the best,followed by the anode region and the middle region.It was revealed that the mechanism of electrochemical modification changing the strength and deformation of the FCFMs was the result of synergism between the alkali-activated effect and the electrokinetic effect.Under the action of DC electric field,electrolysis,electroosmosis and electrophoresis（electrokinetic effect）would strengthen the alkali-activated effect,which further improved the hydration reaction rate,increased the amount of hydration products,reduced the porosity of matrix and raised the compactness of the FCFMs in the setting and hardening process.（5）With partial backfill mining in 22618 working face of Zhenchengdi mine of the XiShan Coal Electricity Group Co.,Ltd.for protecting the operation safety of fly ash conveyor belt as the engineering background.The control effect of different backfill mining lengths on surface subsidence was simulated by FLAC^3D numerical simulation software.It was found that the reducing subsidence coefficient and its growth rate were relatively high when the backfill mining length was 700 m.The control effects of surface subsidence unmodified and modified of the FCFMs were compared.Results showed that the maximum surface subsidence value after backfill mining with unmodified FCFMs was375mm;modified FCFMs,the maximum surface subsidence value was 211mm,a decrease of 164mm.The growth rate of the reducing subsidence coefficient was further increased by 43.73%.The maximum horizontal deformation of the ground surface was 0.67mm/m,smaller than the limit deformation values of the"rules for coal seam mining under building,river and railways and coal pillars of main shaft and roadway".