3D nanostructure of CO₂-mineralized steel slag

Steel slag, a major industrial waste in China, possesses significant CO₂ absorption potential. In this study, the CO₂ sequestration of steel slag reached up to 15.6%; however, excessive mineralization resulted in reduced hydration activity. Compared to unmineralized slag, the 1-day compressive strength decreased by 15.9%, and cumulative hydration heat over 72 h dropped by 8%. Using advanced visualization techniques such as scanning electron microscopy-backscattered electron (SEM-BSE), 3D X-ray, and focused ion beam-transmission electron microscopy (FIB-TEM), the study reveals the microstructure of overmineralized steel slag, identifying a composition of a calcite outer layer, an amorphous SiO₂ layer, a transition area, and an unmineralized core. The mineralization reaction affected 84.80% of the steel slag particles, with volume expansion causing dense regions to become porous, increasing porosity from 0% to 1.62%. This expansion also risks lattice distortion. During CO₂ mineralization, a dense calcite layer forms, blocking the hydration of internal silicate gels and calcium silicate minerals, reducing the hydration activity of overmineralized slag. This study offers insights for optimizing CO₂ mineralization techniques and applications for steel slag.