Influence of Electric Arc Furnace Slag Basicity on Carbonation–Hydration Kinetics and Microstructural Evolution of the Binder under Simulated Flue Gas Conditions

Abstract

The transition to low - CO 2 steelmaking via hydrogen-based direct reduced iron (DRI) processes is increasing the generation of electric arc furnace slags (EAFS), yet their utilization remains limited due to poor hydraulic and pozzolanic reactivity. This study explores the potential of DRI-EAFS as carbon-negative cementitious materials through carbonation-hydration under simulated flue gas conditions (20% CO 2 , 50°C, ambient pressure, 95% relative humidity). High- and low-basicity slags were tested for reactivity, phase evolution, and strength development using selective dissolution, X-ray diffraction–Rietveld refinement, Fourier-transform infrared, thermogravimetric analysis, and scanning electron microscopy–energy-/wavelength-dispersive spectroscopy (SEM-EDS/WDS) mapping. Both slags exhibited strength gain upon carbonation but followed distinct reaction mechanisms. Low-basicity (LB) slag underwent rapid surface carbonation, forming a calcite-rich shell and achieving ∼ 95 % of its 7-day strength within 24 h. In contrast, high-basicity (HB) slag exhibited a two-step kinetic response: initial fast carbonation, followed by sustained strength development driven by hydration and subsequent carbonation of residual larnite. Aragonite, absent in LB slag, was identified only in HB slag, likely stabilized through interaction with calcium silicate (C─S─H) gels. Microstructural analyses revealed thin surface layers in LB slag versus multizonal reaction rims with multiple C─S─H gel types in HB slag. These results highlight slag basicity as a key factor controlling carbonation pathways and binder performance, supporting the valorization of EAFS under flue gas curing as a practical route to low-energy, CO 2 -sequestering construction materials.