Synergistic modification of steel slag coarse aggregate by accelerated carbonation and silane coupling: Performance enhancement mechanism and environmental benefit quantification

To overcome the challenges of poor volume stability and weak interfacial adhesion faced by steel slag coarse aggregates in asphalt pavement application, this study developed a synergistic modification strategy based on accelerated carbonation and silane coupling. Through orthogonal experiments, the carbonation process parameters for steel slag were optimized. By integrating physicochemical performance testing, microstructural characterization, pavement performance evaluation and life cycle assessment, the performance and environmental benefits of synergistically modified steel slag and its asphalt mixture were systematically investigated. The results indicated that, balancing modification efficiency and energy consumption, the optimal carbonation process for steel slag was determined to be: temperature of 85 °C, CO₂ pressure of 11 bar, time of 14 h, and liquid-to-solid ratio of 0.7 mL/g. The carbonated steel slag produced under this process achieved an immersion expansion rate of 0.42% and CO₂ sequestration efficiency of 11.06%, effectively eliminating the risk of volume expansion. The mechanism analysis revealed that the synergistic modification significantly reduced the water absorption rate of steel slag and enhanced the interfacial adhesion strength between steel slag and asphalt through the densification of pores by carbonation products and the hydrophobicity of organic layers formed by silane coupling agent. The synergistically modified steel slag asphalt mixture demonstrated optimal high-temperature rutting resistance and moisture stability, while its low-temperature crack resistance and volume stability approached the levels achieved by basalt asphalt mixture. The LCA indicated that although synergistically modified steel slag asphalt mixture exhibited higher carbon emissions than basalt asphalt mixture during material extraction and pavement construction, the carbon reduction benefits achieved by avoiding steel slag landfill effectively offset part of environmental burden. Furthermore, owing to its superior road performance, the mixture is anticipated to deliver long-term environmental benefits by extending pavement service life. This study would provide a theoretical basis for the high-value application of steel slag in green and low-carbon durable pavement.