Rheological performance, damage resistance, and processing effects in a multi-additive modified binder

Multi-additive modification of bitumen is increasingly adopted to tune rheological performance, but the combined influence of rigid mineral/oxide phases and elastomeric modifiers, along with the way processing conditions shape rutting–fatigue trade-offs, is still not fully understood. This study examines the behavior of bitumen co-modified with iron (III) oxide (Fe2O3), wollastonite (CaSiO3), and microwave-activated crumb rubber (MACR), using a Taguchi L25 design to screen six formulation and processing variables. Twenty-five modified binders, and a neat (unmodified) binder as reference, were characterized through conventional binder tests and dynamic shear rheometer (DSR) measurements, frequency sweep, Multiple Stress Creep Recovery (MSCR), and Linear Amplitude Sweep (LAS). Microstructural analysis was conducted to determine whether the performance changes were driven by chemical interactions or primarily by the distribution and dispersion of additives within the binder. The results show that MACR exerts the strongest influence on MSCR outcomes and high-temperature viscoelastic behavior, while fatigue-related indicators appear more sensitive to processing, especially mixing temperature, and shear intensity. Because the measured responses did not improve together, the competing performance indicators were evaluated within a multi-response framework. The neat binder retained the highest LAS life at 15% strain, demonstrating that improved rutting resistance did not translate directly into large-strain damage tolerance. A desirability model, ranking under Dirichlet-distributed weight uncertainty, and Pareto screening identified U6 (2% Fe2O3, 5% wollastonite, 12% MACR, 185 ◦C, 60 min, 6000 rpm) as the leading screening candidate. U6 achieved Jnr,3.2 = 0.49 kPa⁻¹ , R3.2 = 14.78%, a geometric-mean LAS fatigue-life metric Nf,geo ≈ 953, and composite desirability D = 0.6869. These findings provide a starting point for subsequent work on aged binders, mixture-scale validation, and the incorporation of additional functional additives such as fireretardant or anti-aging modifiers.