Sustainable Asphalt Mixtures Incorporating RAP and Magnetite–Chromite Tailings: Mechanical Performance and Microstructural Characterization

Reclaimed asphalt pavement (RAP) and mining wastes offer considerable potential for reducing the consumption of natural resources in asphalt mixtures; however, their combined effects on mechanical performance require further investigation. This study evaluated the individual and combined use of RAP and magnetite–chromite tailings (MCTs), with MCT replacing the basalt mineral filler at different levels. Hot-mix asphalt mixtures were evaluated using Marshall stability, flow, Marshall quotient (MQ), and indirect tensile strength (ITS) tests, while X-ray diffraction (XRD), scanning electron microscopy (SEM), and energy-dispersive X-ray spectroscopy (EDS) were used to interpret their mineralogical and microstructural characteristics. Increasing the RAP content reduced the stability, flow, and ITS, while increasing the MQ. A full MCT filler replacement improved the Marshall performance compared with a partial replacement. The fine crystalline morphology and Mg-Fe-Cr-rich mineral composition of MCT, associated with chromite, magnetite, and Mg-silicate phases, contributed to a filler structure favorable for deformation resistance. Conversely, the aged binder coating of RAP, supported by its high carbon content, was associated with an increased stiffness and reduced tensile deformation capacity. R30–M100 (30% RAP-100% MCT) exhibited the highest MQ and lowest flow, whereas R10–M100 (10% RAP-100% MCT) retained approximately 98% of the control ITS. Overall, R10–M100 provided the most balanced hybrid mechanical performance.