Comparative Effects of Biochar, EVA, and Thermally Treated Bauxite Residue on Bitumen Performance, Rheology, and Short-Term Aging

The increasing demand for sustainable pavement materials has led to growing interest in alternative bitumen modifiers derived from industrial and biomass waste. This study compares the effects of biochar, ethylene-vinyl acetate (EVA), untreated bauxite residue (red mud), and thermally treated bauxite residue on the physical and rheological performance of C600 bitumen. Bauxite residue was tested in its untreated state and after thermal treatment at 350 °C and 600 °C to examine how thermal activation influences its modifying effect. The base bitumen and all modifiers were first characterised, and modified binders were then prepared at selected dosages using high-shear mixing. Performance was evaluated through penetration, softening point, rotational viscosity and short-term aging, and Dynamic Shear Rheometer (DSR) temperature sweeps were used to determine complex shear modulus (G*), phase angle (δ) and the rutting parameter (G*/sin δ) across a range of service temperatures. All modifiers increased binder stiffness and improved aging resistance while remaining within viscosity guideline. EVA produced the largest increase in viscosity and the greatest reduction in phase angle, giving the best rutting resistance at high temperatures. EVA and biochar both reduced the penetration reduction ratio to about 1% or less at their most effective dosages. Biochar at 10% gave the most consistent response across the three aging indices (PRR, SPIR and ΔPI), while bauxite residue treated at 600 °C reached the lowest softening point increment ratio at 30% dosage. The bauxite residue variants acted mainly as mineral fillers and produced a more moderate improvement, with thermal treatment showing only a limited additional benefit. The results establish relationships between modifier characteristics, thermal treatment and binder performance, and clarify the relative advantages and limitations of each modifier. These findings offer a comparative basis for selecting sustainable bitumen modifiers and provide foundational insights for the development of hybrid modifier systems combining these materials.