The utilization of hot-soaked steel slag as a coarse aggregate in asphalt mixtures introduces distinct physical characteristics, specifically low angularity and high porosity, which necessitate specific design modifications. This study investigates the microstructural and chemical properties of hot-soaked steel slag using scanning electron microscopy and energy dispersive spectroscopy (EDS). Furthermore, the gradation design space was re-evaluated to account for significant variations in water absorption and density. Based on this analysis, an improved method for determining the maximum theoretical density is presented, and the concept of a nominal asphalt-aggregate ratio (NO-AR) is proposed to quantify binder content disparities among aggregates of varying densities. Results indicate that the density and water absorption rates of hot-soaked steel slag are significantly higher than those of natural aggregates. EDS analysis reveals that oxygen, calcium, iron, and silicon collectively constitute 93 % of the atomic composition. X-ray diffraction analysis identifies the primary mineral phases as wüstite, calcium ferrite, and calcium silicate. Consequently, a novel stone matrix asphalt-13 gradation curve and an optimized maximum theoretical density calculation scheme are proposed. Comparative analysis demonstrates that the asphalt-aggregate ratio for hot-soaked steel slag mixtures is 6–9 % higher than that of natural aggregate mixtures. This study validates the feasibility of utilizing hot-soaked steel slag in pavement engineering and provides a reference for gradation design involving aggregates with substantial density variations.