Performance-Based Evaluation of Supplementary Cementitious Material Synthesized with Basic Oxygen Furnace Slag and Ground Granulated Blast Furnace Slag

Basic oxygen furnace slag (BOFS) is one of the major by-products of the steelmaking industry. Its limited utilization as a construction material is primarily attributed to its chemical properties, which hinder its stability and hydraulic activity due to its high free lime (f-CaO) content. This paper explores the performance of supplementary cementitious material (SCM) synthesized with ground granulated blast furnace slag (GGBFS), freshly produced BOFS (f-BOFS), and stockpiled BOFS (s-BOFS). A total of 10 mixtures with ordinary Portland cement (OPC) replacement percentages were assessed, maintaining a total replacement of 50% OPC, incorporating 15%, 25%, and 35% of each material by weight. The laboratory experimental program encompassed material characterization, fresh and hardened properties, pozzolanic activity, and durability assessment, with comparative studies conducted for each evaluation item. Test results indicate that f- or s-BOFS, when used with GGBFS, can be a viable alternative SCM with the potential for hydraulic activities and pozzolanic reaction. The newly synthesized SCMs demonstrated improved strength development in mortar mixtures. The mixture containing [15% f-BOFS + 35% GGBFS] achieved a 28-day compressive strength of 20.6 MPa, while the [25% BOFS + 25% GGBFS] blend reached a compressive strength of 19.7 MPa. These mixtures meet Grade 80 criteria as per ASTM C989/C989M Standard Specification for Slag Cement for Use in Concrete and Mortars. A performance-based ranking system was developed by integrating results from flowability, air content, strength activity index, drying shrinkage, alkali–silica reaction, and sulfate attack. The novelty of this work lies in assessing BOFS–GGBFS blends as SCMs using this multi-criteria approach to identify the most sustainable and technically viable mixtures. Moreover, the study highlights the influence of storage-induced weathering by directly comparing the reactivity and performance of f- and s-BOFSs in ternary blends, providing new insights into optimizing the utilization of slag. Notably, regardless of f- and s-BOFSs, proportions of [15% BOFS + 35% GGBFS] demonstrated superior strength development and achieved an excellent overall ranking. These findings confirm the potential of such slag blends as suitable SCMs for mortar and concrete applications, thereby advancing the sustainability and efficiency of cementitious materials.