Coal fly ash resource utilization: Effects of inorganic minerals amendments on CFA-originated opal/sand aggregates formation

Abstract

Opal (amorphous silica, SiO₂·nH₂O), a solid waste byproduct of the alkaline extracting alumina from coal fly ash, exhibits strong adsorption properties and is a secondary/clay mineral in the soil. Combining opal with sand to construct opal/sand aggregates for desertification soil remediation holds the potential for large-scale ecological disposal. Unfortunately, the aggregate structure still gaps from natural soil aggregates resulting from inorganic mineral deficiencies. Herein, the effects of five inorganic mineral amendments, limestone (CaCO₃), desulphurization gypsum (CaSO₄·2H₂O), hematite (Fe₂O₃), tricalcium phosphate (Ca₃(PO₄)₂) and gibbsites (Al(OH)₃), on aggregate formation, stabilization, and pore characteristics without the organic matters were investigated in short-term cultivation experiments. Meanwhile, associated adsorption mechanisms were elucidated. Results indicated only gypsum effectively reduced the aggregate’s pH, most enhanced water-holding capacity, albeit increased electrical conductivity. All amendments facilitated aggregate formation and mechanical-stability, with gypsum, CaCO₃, and Fe₂O₃ improving water stability. Various analysis techniques, including XRD, SEM, nano-CT, FT-IR, and XPS, provided insights into the physisorption and chemisorption of minerals onto sand/opal, generating interfaces conducive to aggregation. Compared to CK (control check, without amendment addition), amended macroaggregates demonstrated increased porosity, reduced pore quantity and mean pore diameter (MPD), denser pore structure, improved interpore connectivity, and more complex pore networks, dominated by <80 µm diameters and boundary pores. Notably, desulphurization gypsum elicited the most significant variations, increasing MPD of microaggregates and 2–5 nm mesopores, and decreasing total pore volume and 0–2 nm micropores, while Ca₃(PO₄)₂ and Al(OH)₃ improved >15 nm mesopores. Overall, inorganic minerals, the “skeleton” of soil, effectively upgraded opal/sand aggregates’ physical structure and accelerated aggregate formation quickly. Therein, desulphurization gypsum optimized macroaggregate formation and stability. Desulphurization gypsumamended aggregates serve as soil-like substrates to accelerate the ecological reconstruction of desertification areas.