Characterization of modified red mud and its role in porous asphalt mixtures

Utilizing red mud (RM) as an alternative mineral filler in porous asphalt mixtures (PA) represents a resource-efficient approach to disposing of bauxite residue. However, water-induced interfacial stripping of RM asphalt mortar results in insufficient water stability of PA using RM as a filler (PA-RM). In this study, RM was modified through organic surface modification using a silane coupling agent (SCA) and inorganic modification with hydrated lime (HL) to improve water stability. The organic RM was characterized using LPSA, FTIR and XPS. Furthermore, performance of PA-RM was assessed using dynamic stability, freeze-thaw splitting and Cantabro durability tests. After surface modification, the RM exhibits finer particle size, improved dispersion and reduced agglomeration tendency. The organic functional groups grafted onto the RM transform it from hydrophilic to hydrophobic, creating a relatively stable adhesive interface between the organic modified RM and the asphalt. Replacing limestone powder with RM improves the high-temperature performance and long-term thermal-oxidative aging resistance, but slightly reduce the water stability of PA. However, PA prepared with either organic or inorganic modified RM demonstrates superior water stability and anti-raveling performance compared to PA prepared with limestone powder, with the organic modified RM showing the best performance.