Physicochemical Characterization, Binder Rheology, and Engineering Performance of Diatomite–Microfiber Hybrid Asphalt Systems

Diatomite and microfibers have been investigated as asphalt modifiers, but limited attention has been given to how differences between diatomite sources and microfiber formulations are reflected across binder- and mixture-scale performance. This study investigated two diatomites (D1 and D2) and three microfibers—polyacrylonitrile (PAN), polyethylene terephthalate (PET), and basalt—using a multiscale experimental approach. The diatomites were characterized by particle-size analysis, SEM–EDS, XRF, and XRD, while the fibers were examined using SEM–EDS, XRD, and binder-retention testing. Diatomite-modified binders containing 10% and 15% modifier were evaluated for conventional, rheological, and aging properties. At the mixture scale, selected 15%D1 and 10%D2 systems were combined with the three fibers and compared with unmodified and 5% SBS-modified references. D1 was finer and had a slightly higher SiO2 content than D2, while both showed silica-rich porous structures. Diatomite generally increased binder consistency and viscosity and improved the high-temperature rheological response without changing the BBR critical temperature. The 10%D2-PAN mixture achieved the highest indirect tensile strength (4.29 MPa), while the tensile strength ratios of 10%D2-PAN and 10%D2-PET were 82–83%. The 10%D2 formulations showed Cantabro losses of 5.5–6.7% and permanent deformations of 0.247–0.255 mm. In the adapted 100 mm SCB test, 10%D2-PAN also produced the highest flexibility index (10.6) and fracture energy (7.3 kJ/m2). Overall, the results indicate that diatomite source characteristics and microfiber formulation should be considered together, with 10%D2-PAN exhibiting a favorable combination of strength, moisture-resistance, particle-loss, permanent-deformation, and fracture-related responses under the investigated conditions.