Mix proportion and microscopic characterization of coal-based solid waste backfill material based on response surface...

Area of Science:

• Materials Science and Engineering
• Geotechnical Engineering
• Waste Management and Valorization

Background:

• Underground backfilling utilizes coal-based solid waste (CSW), but its mix proportion significantly impacts transportation, support performance, and cost.
• Optimizing the composition of CSW, including desulfurization gypsum (DG), furnace bottom slag (FBS), and gasification fine slag (GFS), is crucial for effective backfilling.
• Understanding the physical and chemical properties of CSW backfill is essential for its successful engineering application.

Purpose of the Study:

• To determine the optimal mix proportion of multi-source CSW (DG, FBS, GFS) for underground backfilling using Response Surface Methodology-Box Behnken Design (RSM-BBD).
• To evaluate backfill material performance based on fluidity, bleeding rate, strength (3-day and 7-day), and preparation cost using multi-objective decision-making (MDM).
• To investigate the microstructure of the optimized backfill material.

Main Methods:

• Response Surface Methodology-Box Behnken Design (RSM-BBD) was employed to optimize the mix proportion of desulfurization gypsum (DG), furnace bottom slag (FBS), and gasification fine slag (GFS).
• Multi-objective decision-making (MDM) was used to select the optimal mix proportion based on fluidity, bleeding rate, strength, and cost.
• Microstructural analysis was conducted using Thermogravimetric Analysis (TGA), Mercury Intrusion Porosimetry (MIP), Scanning Electron Microscopy with Energy Dispersive X-ray Spectroscopy (SEM-EDS), and X-ray Diffraction (XRD).

Main Results:

• The optimal mix proportion for CSW backfill was determined as coal gangue (CG): coal fly ash (CFA): DG: FBS: GFS = 1:1.5:0.2:0.1:0.1, with a mass concentration of 78% and ordinary Portland cement (OPC)/CSW ratio of 7.5%.
• Thermogravimetric analysis revealed continuous weight loss due to water evaporation. Microstructural analysis showed dense, narrow-necked pores with poor connectivity.
• The strength of the backfill material primarily relies on cement hydration, with no significant hydration reactions observed between CSW particles. Ettringite formation involved substitution of Al2O3 by SiO2 and CaSO4 by CaCO3.

Conclusions:

• The optimized mix proportion of multi-source CSW significantly enhances backfill material properties and provides a cost-effective solution for underground applications.
• The microstructure analysis provides insights into the material's stability and behavior under thermal conditions, highlighting the role of cement hydration in strength development.
• This research offers valuable data and a practical reference for the engineering application of underground backfilling using diverse coal-based solid wastes.