A comparative evaluation of sustainable asphalt binder modifiers for enhanced performance.

Asphalt pavement production accounts for approximately 1.5% of global carbon emissions, underscoring the need for sustainable binder modification strategies that enhance performance without compromising durability. Existing studies on eco-friendly asphalt modifiers remain largely fragmented, limiting cross-comparison and mechanistic understanding. This study develops a unified, multi-scale experimental framework to comparatively evaluate waste-derived polymers and geopolymer-based modifiers under identical processing and testing conditions. Five modifiers, crumb rubber (CR), low-density polyethylene (LDPE), a combination of CR/LDPE, fly ash (FA), and metakaolin–silica fume (MK–SF) geopolymers, were assessed alongside a commercial fiber elastomer modifier (VIATOP), using two virgin binders from Egyptian refineries to capture source dependency, resulting in twelve modified systems. Binder behavior was evaluated through integrated microstructural (SEM), chemical (FTIR and CI + SI indices), thermal and photochemical (TGA and UV–Vis), and rheological (viscosity and performance grading) analyses. Results show that geopolymer modifiers provided the most consistent improvements across scales, reducing chemical aging indices, increasing decomposition onset temperatures by approximately 10–20 °C, suppressing UV-induced aromatic growth, and maintaining workable viscosities (< 500 cP at 135 °C) with moderate Performance Grade (PG) enhancement ( ≈ + 2–4 °C). Crumb rubber offered balanced performance, yielding moderate PG increases ( ≈ + 4–5 °C) with improved aging resistance. In contrast, LDPE and VIATOP produced the largest PG increases (up to ≈ + 10–13 °C) but exhibited poorer dispersion, higher oxidation, and reduced thermal and UV stability, indicating short-term stiffening rather than long-term durability. The hybrid CR/LDPE combination showed intermediate, partially synergistic behavior. Overall, the study demonstrates that rheological gains alone are insufficient indicators of durability and highlights the decisive role of base binder chemistry. The proposed framework provides quantitative, transferable design guidance for selecting sustainable asphalt modifiers that balance stiffness, durability, and aging resistance.