This study investigates the utilization of steel slag as both a partial and complete substitute for natural coarse aggregates in concrete. Various proportions of crushed stone (granite) were replaced with steel slag (SS) aggregate at levels of 0%, 45%, 70%, and 100%. A total of 120 concrete cubes (100 mm × 100 mm × 100 mm) were cast and cured in water for 7, 28, and 56 days using the concrete and cement institute method, with batching done by mass. Fresh concrete was assessed using slump tests, while hardened concrete was evaluated for compressive strength, split tensile strength, and durability indices. The results showed that increasing the percentage of steel slag in the mix increased compressive strength over time. Concrete with steel slag substitution demonstrated higher strength compared to traditional concrete. The durability index results indicated that all samples met acceptable ranges, with oxygen permeability between 9.5–10, water sorptivity between 6–10, and chloride conductivity between 0.75–1.5. Although the split tensile strength of slag concrete was slightly reduced at 56 days compared to 7 and 28 days, the overall performance remained satisfactory. The experimental findings revealed that 100% substitution of coarse aggregate with local steel slag resulted in high compressive strength across all testing ages. This study concludes that steel slag can effectively replace natural coarse aggregates in concrete, thereby contributing to the conservation of natural resources.