Valorization of red mud as an alternative filler for asphalt mastic: modification treatment, pavement performance and...

Red mud (RM), a highly alkaline industrial residue, poses potential risks to ecosystems. To advance the valorization of RM in road engineering, this study proposed a three-stage RM modification method and subsequently prepared resource-recycling fillers by blending the modified RM with hydrated lime (HL) and natural limestone (NL) based on simplex centroid method. The physicochemical properties of the modified RM were characterized using microstructural tests. The pavement performance and environmental impacts of asphalt mastics containing fillers were systematically evaluated, and the optimal proportion range of the resource-recycling filler was determined. Results show that the modification treatment reduced the sodium content of RM from 7.89% to 1.62% and grafted an aminopropyl organic layer onto the RM surface, thereby mitigating particle agglomeration and enhancing the dispersion of RM within the asphalt binder. The modified RM enhanced the adhesion properties of asphalt mastics, increasing mastic-aggregate bonding strength by 38.79% on limestone and 23.26% on granite. In addition, modified RM exhibited low leaching toxicity and exerted negligible impact on roadside vegetation and aquatic organisms. The combination of modified RM, HL, and NL achieves a synergistic balance across multiple properties of asphalt mastics. The recommended formulation range for the resource-recycling filler is 15∼30% modified RM, 38∼60% HL, and 20∼39% NL.