<p>It is essential to reduce the indiscriminate dumping of iron filings (IF), which could generate a leachate that contaminates groundwater, by using them as a sand replacement for mortar production. This study investigated the impact of IF on the properties of blast-furnace slag (BFS) blended paste and mortar. The experiment was designed such that IF varied from 0-15% as a partial replacement, maintaining an equal proportion of BFS and ordinary Portland cement (OPC). IF enhanced the density of paste and mortar and contributed to the reduction of internal voids through the formation of iron carbonate and calcium carbonate. The maximum 28-day compressive strengths of the paste and mortar were 124 and 67&#xa0;MPa, respectively. The presence of IF led to increased formation of brownmillerite (Ca₂(Al, Fe)₂O₅), while BFS reduced the formation of sulfate compounds such as gypsum, as indicated by the reduced S-O vibration in the FTIR spectrum. The synergy between IF and BFS in blended paste reduced the Ca/Si ratio and enhanced Ca/Fe and Mg/Si ratios. The interaction between BFS and OPC led to the formation of secondary hydration products, thereby contributing to further strength development. Utilizing iron filings and BFS offers a promising route to sustainable construction by reducing industrial waste and enhancing microstructural integrity and strength development.</p> Graphical Abstract <p></p>