Activation of sintered red mud and its application as a cement substitute in foamed lightweight soil for backfilling engineering

Sintered red mud (SRM), a solid by-product from aluminium electrolysis, poses serious environmental challenges due to its high alkalinity, large stockpile, and dust pollution. This study enhances the pozzolanic reaction potential of SRM, aiming to replace cement in foamed lightweight soil (FLS) for large-scale backfilling applications. Twenty mix designs were prepared with densities ranging from 600 to 1200 kg/m 3 and cement replacement ratios of up to 67.81 %. The effects of the water-cement ratio, physicochemical properties of SRM, and activator on the density, workability, and mechanical performance of SRM-FLS were systematically evaluated. The hydration behaviour and microstructural evolution were examined to assess the pozzolanic reactivity and elucidate the underlying enhancement mechanism. Results show that the SRM-FLS achieved a 23 % increase in 28-day unconfined compressive strength (UCS), reaching 1.48 MPa at a 70 % cement replacement level. The material demonstrated good water stability (softening coefficient > 0.8) and deformation resistance (compression modulus: 133–167 MPa). Microstructural analysis revealed a refined pore structure (88.8 % pores < 100 μm) and an enhanced formation of C-S-H and CaCO 3 gels. A mixed design framework integrating mass balance, volumetric compatibility, and flowability control is proposed to guide practical application. The developed SRM-FLS offers both environmental and structural benefits, supporting a sustainable and cost-efficient solution for future infrastructure construction.