Assessing the influence of SBS and nano-CaO modification in mitigating the adverse effects of variable-pH runoff on the intermediate and low temperature performance of porous asphalt mixtures

• Direct comparison of chemical buffering and viscoelastic modification mechanisms • Nano-CaO neutralizes acidic runoff via interfacial chemical stabilization • SBS improves alkaline resistance through enhanced viscoelastic energy dissipation • pH-dependent binder–aggregate interactions control fracture performance • Multi-scale framework links chemistry, rheology, and fracture behavior. This study fills a critical knowledge gap in pavement engineering by presenting the first direct head-to-head comparison of styrene-butadiene-styrene (SBS) and nano-calcium oxide (nano-CaO) modifications for mitigating variable-pH runoff effects on porous asphalt mixtures, with emphasis on intermediate- and low-temperature performance. Although both modifiers have been studied individually, their contrasting mechanisms, viscoelastic enhancement versus chemical buffering, have not been systematically compared under moderately acidic to mildly alkaline conditions (pH 5-9) in high-void porous asphalt structures. This research introduces a novel multi-scale framework that clarifies modifier–aggregate–pH synergies, paving the way for region-specific, environmentally adaptive pavement designs. Initially, a performance-grade bitumen was modified with 3-5% SBS and 2-4% nano-CaO. Binder evaluations assessed including chemical stability, rheological properties at high, intermediate, and low-temperature behaviour. Optimal dosages (4% SBS, 4% nano-CaO) were incorporated into mixtures with limestone and granite aggregates. Specimens underwent aging process and immersion in pH 5.0-9.0 solutions, followed by fracture assessments at 25°C and -12°C, alongside bonding evaluations under varied pH conditions. Key findings reveal that SBS modification with limestone aggregates under alkaline conditions (pH 7.5-9.0) increased fracture energy by 28-35% and fracture toughness by 22-27%, while nano-CaO modification with granite aggregates under acidic conditions (pH 5.0-6.5) enhanced peak load by 24-31% and bond strength by 18-26%. Overall, SBS superiority in alkaline environments with limestone aggregates, delivering enhanced ductility, fracture energy, and toughness via viscoelastic dissipation. Conversely, nano-CaO excelled in acidic conditions with granite aggregates through proton neutralization and interfacial reinforcement, improving load capacity and toughness. Both modifiers enhanced overall fatigue resistance, relaxation properties, and bonding performance, with aggregate mineralogy dictating optimal pairings; bonding analyses confirmed limestone's inherent stability and nano-CaO's effective pH mitigation, promoting cohesive failure modes. Life-cycle assessment demonstrated 10-11% reduction in embodied greenhouse gas emissions and 14-23% long-term cost savings due to extended service lives of 18-20 years. These insights recommend SBS for alkaline-dominated regions with calcareous aggregates and nano-CaO for acidic zones with siliceous ones, fostering resilient, eco-efficient porous asphalt solutions aligned with local environmental stressors.