This study investigates the feasibility of using steel slag (SS) as a sustainable alternative in concrete by partially replacing cement, coarse, and fine aggregates. Three concrete mixes were designed: M1 (control), M2 (SS as coarse aggregate), and M3 (SS powder and fly ash replacing cement). Workability tests showed SS aggregates reduced slump retention due to their porous structure, while FA in M3 improved it. SS accelerated cement setting, with M2 setting faster than M1, while M3 had a slightly delayed setting due to FA. Strength tests revealed similar compressive strengths among all mixes at 90 days, with M2 enhancing flexural strength by 12.7%. SEM analysis confirmed improved microstructure in M3, with a dense C-S–H gel network reducing voids and cracks. These findings highlight SS’s potential as a viable replacement material in sustainable concrete production.