Effect of phase composition on the reactivity of DRI-EAF slags as SCM

DRI in EAFs will reduce the availability of blast-furnace slag, necessitating alternative SCMs. Five DRI–EAF slags (four engineered, and one industrial) were investigated, spanning CaO/SiO₂ ≈ 1.3 or 2.0, FeOx ≈25 or 40 wt%, and cooling (air-cooled vs water-granulated). At 25 wt% clinker replacement, hydration and performance were quantified by isothermal and R 3 calorimetry, UPV, autogenous shrinkage, XRD/TGA, compressive strength, and leaching. Cooling history, through the control of the amorphous fraction, governed reactivity: water-granulated slags exhibited higher cumulative heat, accelerated set, and superior strength activity indices compared to air-cooled analogues. Basicity and FeOx exerted secondary effects: higher basicity accelerated hydration and reduced autogenous shrinkage, whereas elevated FeOx retarded reaction. All binders formed C–S–H, ettringite, and portlandite; hydrotalcite-like LDHs were detected only for the low-basicity, water-granulated slag. All mortars complied with Dutch leaching limits and met SCM strength benchmarks, evidencing the viability of engineered DRI–EAF slags as low-CO₂ cementitious binders.