Recycling municipal solid waste incineration fly ash and Si–Al based solid waste to prepare backfill materials for engineering applications: Insights into hydration mechanisms, carbon emissions, and costs

Abstract

Backfilling mining goafs with municipal solid waste incineration fly ash (MSWI FA) and industrial solid waste offers novel possibilities for advancing green mining engineering. However, the recycling of MSWI FA using Si–Al-based solid wastes to prepare MSWI FA-based backfill materials (MBM) remains at the laboratory stage, with limited studies addressing the in-depth mechanisms and sustainability of MBM in actual goafs. This study investigated the leaching safety, chemical bonding, and valence states of Si–Al products and microstructure, carbon emissions, and economic feasibility of a multi-component backfill material (MLBM) composed of MSWI FA, blast furnace slag (BFS), steel slag (SS), coal fly ash (CFA), flue gas desulfurized gypsum (FGDG), and lead–zinc tailings. The binder weight ratio was MSWI FA : BFS : SS : CFA : FGDG = 10:33.6:33.6:20:12.80 under engineering application conditions. The toxicity characteristic leaching procedure test showed that the anion and heavy metal leaching concentrations from MLBM were below the Class II thresholds specified in the Standard for Groundwater Quality (GB/T 14848–2017). The hydration mechanism involves: (i) Cl^– competition that accelerates and promotes the generation of ettringite and Friedel’s salt; (ii) MSWI FA-induced depolymerization and repolymerization of the Si–Al vitreous network, facilitating C–(A)–S–H gel formation with high average connectivity degree/mean chain length (D_c/L_MC) ratio and a low Ca/Si atomic ratio (1.27), resulting in an increased content of Si–O–Si (Q² p and Q2 b) and Si–O–Si (Q3) (bridging oxygen) in the C–(A)–S–H gel; (iii) pore-filling effect by various functional hydration products that increases the proportion of gel pores (3.5–100 nm) in MLBM to 47.31vol% and reduces the porosity to 40.20vol%. Compared with that of traditional cement materials, the production of MLBM led to reduced material costs by 80% and CO₂ emissions by more than 98%. These findings contribute to the sustainable development of green mining, enhance the efficiency of solid waste management, reduce environmental pollution, and support global carbon reduction targets.

Keywords

Municipal solid waste incineration fly ash / Engineering applications / Backfill materials / Chemical bonds/valence state / Microstructure / Carbon emission economics

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