<p>Incorporating steel slag, a residue derived from the metallurgical industry, into cementitious systems offers a sustainable route for resource recovery and carbon footprint reduction. Milling has been reported to address the low cementitious reactivity of steel slag, which remains a significant barrier to its widespread adoption. However, the influence of storage duration after milling on steel slag performance has not been systematically evaluated. To investigate the effects of post-milling duration as an independent factor, two types of steel slag, derived from a basic oxygen furnace and an electric arc furnace processes, were milled using planetary ball milling and stored under controlled environmental conditions for 1&#xa0;h, 1&#xa0;day, 4&#xa0;days, 7&#xa0;days, 3&#xa0;months, and 1&#xa0;year. The results showed that prolonged storage caused significant particle agglomeration, detected by particle size analysis, and a slight reduction in reactivity after 1&#xa0;year, as determined by the Rapid, Reliable Relevant (<i>R</i>³) reactivity test for supplementary cementitious materials. Replacing 30% of cement with steel slags stored for different times (1&#xa0;h vs. 1&#xa0;year) changed the properties of the blends. Compared with freshly milled slag, slag stored for 1&#xa0;year caused a slight delay in cement hydration, as evidenced by calorimetry results. Consequently, blends containing 1-year-stored slag exhibited prolonged setting times and reduced apparent viscosity compared with the 1-h slag–cement blends. The compressive strength of blended cements was also negatively affected by long-term storage, and the amount of hydration products, such as portlandite, was slightly reduced in 28-day composites containing 1-year-stored slag.</p> Graphical Abstract <p></p>