Photoreactive red mud cementitious composites for environmental remediation

This study explores the recycling of red mud (RM), an iron-rich and highly alkaline residue from the Bayer alumina refining process, as a partial substitute for cement and its valorisation as a low-cost multifunctional photocatalyst within cementitious composites. RM was incorporated as a partial cement substitute (≤15 wt.%) in mortars in both oven-dried and thermally treated forms (300°C and 600°C), and its effect on hydration, workability, strength, microstructure, leaching behaviour, and embodied CO 2 emissions was assessed. The photocatalytic activity of RM mortars was examined through nitrogen oxide (NO x ) abatement under solar and visible light irradiation. The role of RM in modulating light absorption and its impact on the photocatalytic efficiency of titanium dioxide (TiO 2 )-coated mortars was also assessed. Regardless of RM type, partial substitution up to 10% maintained early-age strength comparable to control mix, whereas higher replacement levels reduced long-term mechanical performance due to limited pozzolanic activity of RM and increased macroporosity of matrix. RM incorporation enhanced photocatalytic efficiency, achieving >8% NO x removal, fulfilling Class 3 air-purification criteria. TiO 2 coated RM samples showed 51% higher visible-light NO x abatement than control, attributed to strong interactions between TiO 2 and RM’s hematite phase. Environmental safety was confirmed by >99% immobilisation of trace elements and controlled alkali release. Moreover, 15% RM substitution reduced CO 2 emissions (∼12%), costs (∼7%), and life-cycle costs (∼3.5%), emphasising its dual environmental and economic advantages. Collectively, these findings highlight RM as a high-value additive that couples waste valorisation with photocatalytic air-pollution mitigation, offering a scalable route toward greener construction materials. • Red mud enhances early-age cement properties at ≤10% but lacks long-term reactivity. • Red mud mortars exhibit >8% NO x removal, offering sustainable photocatalytic air pollution mitigation. • Fe 2 O 3 /TiO 2 heterojunction-driven photocatalysis in TiO 2 -coated mortars improves NO x removal and light sensitivity. • Hazardous trace elements, including nickel, zinc, and chromium, are effectively retained by RM mortars. • 15% RM substitution in mortar cuts embodied CO 2 by 12% and production cost by 7%.