Inhibiting steel slag expansion via poly(AA-co-VPA) copolymer for high-performance asphalt mixtures

The rapid elimination of free calcium oxide (f-CaO) is critical for the large-scale utilization of steel slag (SS). To address this, an acrylic acid-vinyl phosphonic acid copolymer (AA-co-VPA) with high Ca²⁺ chelating capacity was utilized in this study. The chelation mechanism of AA-co-VPA with f-CaO and the modification mechanism regarding the microscopic properties and volume stability of SS were systematically investigated via SEM-EDS, XPS, FTIR, and CT scanning. On this basis, a systematic performance validation was conducted, spanning from aggregates and asphalt mastic to the asphalt mixture. The results indicated that AA-co-VPA exhibited exceptional Ca²⁺ chelation performance attributed to its unique bifunctional molecular structure, achieving a chelation capacity of 375 mg/g. At the optimal modification concentration of 1.0%, the immersion expansion rate of the AA-co-VPA modified steel slag (AAVPA-SS-1.0) significantly decreased from 3.76% for unmodified SS to 0.37%, and its f-CaO content was lowered to 0.44%. Microstructural analysis revealed that the proportion of micropores smaller than 100 μm in AAVPA-SS-1.0 was reduced by approximately 90%, leading to synchronous improvements in water absorption and mechanical properties. Rheological tests demonstrated that substituting mineral fillers with the filler (AAVPA‑SSP) prepared from AAVPA‑SS‑1.0 significantly enhanced the elastic recovery and high-temperature resistance to permanent deformation of the asphalt mastic. Finally, the resulting modified steel slag asphalt mixture (AAVPA-SSAM) exhibited excellent pavement performance, with an immersion expansion ratio of only 0.51%, far lower than the 2.82% of the asphalt mixture with unmodified steel slag (SSAM). While effectively addressing the volume stability issue, its water stability and high-temperature rutting resistance were superior to those of both the basalt asphalt mixture (BAM) and SSAM, and its low-temperature cracking resistance met specification requirements.