Study on Mix Proportion Optimization and Microstructure of Coal-Based Solid Waste (CSW) Backfill Material Based on Multi-Objective Decision-Making Model

Mix proportion and microscopic characterization of coal-based solid waste backfill material based on response surface methodology and multi-objective decision-making

The mix proportion of multi-source coal-based solid waste (CSW) for underground backfilling affects transportation and support performance of backfill materials, and even the backfilling cost. In this study, the optimal mix proportion of desulfurization gypsum (DG), furnace bottom slag (FBS) and gasification fine slag (GFS) is determined by the Response Surface Methodology-Box Behnken Design (RSM-BBD). Then the fluidity, bleeding rate, 3-day strength, 7-day strength and preparation cost are evaluation indicators, the optimal mix proportion of backfill materials is determined by the multi-objective decision-making method (MDM). Finally, the microstructure of the backfill material with optimal mix proportion was studied by TGA, MIP, SEM-EDS and XRD. The results show that the mix proportion of CSW with the optimal comprehensive index is coal gangue (CG): coal fly ash (CFA): DG: FBS: GFS = 1:1.5:0.2:0.1:0.1, the mass concentration is 78%, and ordinary Portland cement (OPC)/CSW = 7.5%. The weight loss phenomenon of the backfill material with the optimal mix proportion occurs continuously during the heating process, mainly due to the evaporation of crystal water, structural water and hydroxyl water. There are dense narrow-necked pores in the backfill material, and the pore connectivity is poor. There is no hydration reaction occurs between CSW particles, and the strength increase of the backfill material mainly depends on the hydration reaction of cement. In ettringite, part of Al2O3 is replaced by SiO2, and part of CaSO4 is replaced by CaCO3. This study provides a reference for the engineering application of underground backfilling with multi-source CSW.

Study on the effect of multi-source solid waste on the performance of its backfill slurry

The preparation of slurry from multi-source solid waste for underground backfill adds a way out for solid waste disposal, which is beneficial to reduce environmental impact. In this paper, the effects of gangue, fly ash, gasification coarse slag and desulfurization gypsum on the fluidity, early strength, thermal stability and other properties of the backfill slurry were studied by fluidity test, strength test, Thermo-Gravimetric Analysis (TGA), Scanning Electron Microscope (SEM) and X-ray Diffraction (XRD). The results show that: (1) When G/SW (mass ratio of gangue to solid waste) < 23%, gangue is beneficial to improve the fluidity and early strength of backfill slurry; with the increase of fly ash content, the fluidity of backfill slurry decreases, but its early strength increases; gasification coarse slag has a negative effect on the fluidity of backfill slurry, but it is beneficial to its early strength when GCS/SW (mass ratio of gasification coarse slag to solid waste) < 33%; desulfurization gypsum can improve the fluidity of the backfill slurry, but it is not conducive to the increase of early strength. (2) The compression failure mode of the backfill is mainly divided into the crack-intensive failure, the single main crack penetration splitting failure, and the double main crack conjugate splitting failure. (3) Endothermic dehydration reactions of adsorbed water and crystallization water generally occur at 55-65 °C and 110-130 °C for backfill with different solid waste contents; As the temperature continues to increase, the backfill material undergoes a slow exothermic decomposition reaction; Increasing the content of gangue, fly ash and gasification coarse slag and reducing the content of desulfurized gypsum can make the backfill less weight loss at high temperature and better thermal stability. (4) The main mineral phases in the backfill material are gypsum and quartz, and there are also a small amount of acicular and hexagonal hydration products thaumasite. After high temperature, the thaumasite is dehydrated and decomposed. The research results are helpful to deeply understand the performance of multi-source solid waste for underground backfilling.