Reutilization of Ca-Leached Steel-Making Slag Residues in Cement Mortar: Hydration Gel Morphology and Performance

Ca leaching from steel-making slag (SS) has been widely used to obtain Ca-rich solutions for CO2 mineralization and calcium carbonate production, but the reutilization of the resulting solid residues remains insufficiently addressed. In this study, the effects of partially replacing cement with acetic acid-dissolved residue (HDR) or NH4Cl-dissolved residue (NDR) on mortar performance were evaluated. XRD analysis showed that Ca leaching altered the mineral assemblages of both residues, with the depletion of several Ca-bearing phases and the relative retention of Ca2(Al,Fe)2O5 and the RO phase. The mechanical performance of the residue-containing mortars depended on the residue source and replacement ratio. Low-to-moderate replacement levels retained useful compressive and flexural strengths, whereas excessive replacement reduced strength development. Appropriate incorporation of HDR or NDR maintained favorable sulfate-resistance coefficients, while 10–20% NDR improved freeze–thaw resistance. Calorimetry and microstructural observations showed that Ca-leached residues affected hydration heat evolution and the morphology of gel-like hydration products, while cement dilution and limited residue hydration contributed to the early-age strength loss at high replacement levels. These findings demonstrate the feasibility of reutilizing Ca-leached SS residues as partial cement replacements, provided that the replacement level is selected according to the required mechanical and durability performance of the mortar.