Carbonation-induced pore structure modification of steel slag aggregates and its effects on expansion, adhesion, and CO2 emissions of asphalt concrete

Steel slag, especially in its powder form, was regarded as a highly promising material for carbon sequestration. However, the carbonation characteristics and environmental benefits of steel slag aggregates remained unclear, primarily due to the complexity of their surface pore structure. This study investigated how carbonation altered the pore structure of steel slag aggregates and analyzed the resulting effects on the expansion behavior, interfacial adhesion, overall durability, and environmental performance of steel slag asphalt concrete. First, the carbonation process was evaluated by monitoring pH changes and Ca 2+ precipitation under three distinct moisture conditions: water-deficient (10%), water-adequate (100%), and water-excess (1000%). Next, X-ray diffraction (XRD) and computed tomography (CT) were employed to quantitatively characterize the depth-dependent evolution of pore structure during carbonation. Subsequently, aggregate stability and pavement performance were assessed, and a predictive model for pore-induced expansion was developed. Finally, the environmental benefits of carbonated steel slag aggregates were quantified based on net CO 2 emissions. Results demonstrated that a water-adequate environment promoted calcium precipitation via the HCO₃ - /OH - balance, thereby enhancing carbonation efficiency. This yielded two key innovations: (1) a pore sealing rate exceeding 90%, which effectively stabilized the aggregate framework; and (2) the production of carbonized steel slag aggregates that achieved net carbon sequestration. The refined pore structure improved aggregate stability, reducing the expansion rate by 71.9% and enhancing pavement performance. In contrast, water-deficient and water-excess conditions disrupted the HCO₃ - /OH - balance, impeded carbonation, and induced pore coarsening from sub-25 µm to over 85 µm, leading to significantly higher expansion and net CO 2 emissions. Overall, the coupled evolution of pore architecture and functional performance elucidated the fundamental carbonation reaction mechanism of steel slag aggregates, providing a theoretical foundation for the optimized pre-treatment of high-quality steel slag aggregates in low-carbon pavement.