Study on preparation, working performance and microstructure of coal mine filling material with large amount of fly ash

In the process of coal resource exploitation, processing and utilization, there are a large number of coal-based solid wastes, such as coal gangue, fly ash, coal gasification slag, which pollute the environment, occupy land resources and are difficult to be disposed. Applying coal-based solid waste in the preparation of mine filling materials to solve the problems of stratum collapse and ecological damage caused by coal mining can scientifically deal with solid waste, protect the environment, and ensure the sustainable exploitation of coal resources. By studying the flowability, rheological properties, and compressive strength and shrinkage changes of the newly mixed slurry of coal-based solid waste filling material under different fly ash dosages, the influencing factors and performance trends of coal gangue aggregate and traditional windblown sand aggregate on the working performance of the filling material were compared. By means of X-ray diffraction (XRD), scanning electron microscope energy dispersive spectrometer (SEM-EDS), inductively coupled plasma emission spectrometer (ICP-OES) and other analytical means, the hydration products, microstructure evolution process and heavy metal solidification ability of hardened body of high content fly ash based mine filling materials at different water binder ratios and water reducer additions were analyzed. The results show that fly ash can effectively improve the flowability of the newly mixed slurry of coal-based solid waste filling materials, but the addition of fly ash exceeding 80% is not conducive to further enhancing the compressive strength of the filling material. When the fly ash content is 60% and the water cement ratio is 1.0, adding 0.2% polycarboxylic acid water reducing agent, under the same fluidity conditions, the filling material shows good rheological properties in the pure slurry fluid. The compressive strength of the newly mixed hardened slurry after 7 days and 28 days is 4.7 MPa and 6.2 MPa, respectively. When using crushed coal gangue as aggregate, the compressive strength of the filling material at early age (3, 7 d) is significantly improved, and the filling slurry is relatively stable. When using traditional windblown sand as aggregate, the compressive strength of the filling material at 28 d age deteriorates. Under different fly ash contents, the sedimentation shrinkage rate of the crushed coal gangue aggregate filling material is always lower than that of the filling material using windblown sand as aggregate. When the fly ash content is 60% and the water cement ratio is 1.0, the microstructure and structure density of the hardened filling body at 28 days of age is higher than that of the filling body at a water cement ratio of 1.2, the EDS scanning shows the highest concentration of Ca element and the best uniformity. At the age of 28 days, the content of Cd and Cr in the hardened filling with a water cement ratio of 1.0 decreases by 70% and 41.2%, respectively, compared to the age of 3 days. The solidification effect of heavy metals is better than that at a water cement ratio of 0.8 and 1.2. The study on the preparation and working performance of high content fly ash based mine filling materials can provide reference for the scientific utilization of coal-based solid waste and the development of future safe and green mining technologies for coal resources.