In this study, the influence of steel slag (SS) content on the strength of the cementitious materials was investigated. The quaternary active cementitious material (CaO-SiO₂-Al₂O₃-SO₃) was prepared using various proportions of steel slag (SS), granulated blast furnace slag (BFS), and desulfurized gypsum (DG). The mechanism of synergistic excitation hydration of the cementitious materials was examined using various techniques such as X-ray diffraction (XRD), scanning electron microscopy (SEM) and Fourier transform infrared spectrometry (FTIR). The strength of the mortar test block was initially increased and decreased later with the increase of the SS content. Mortar test block with 20% steel slag, 65% granulated blast furnace slag, and 15% desulfurized gypsum with 0.35 water-binder ratio showed the highest compressive strength of 57.3 MPa on 28 days. The free calcium oxide (f-CaO) in the SS reacted with water and produced calcium hydroxide (Ca(OH)₂) which created an alkaline environment. Under the alkaline environment, the alkali-activated reaction occurred with BFS. In the early stage of hydration reaction, calcium silicate hydrate (C-S-H) gel and fibrous hydration product ettringite (AFt) crystals were formed, which provided early strength to the cementitious materials. As the hydration reaction progressed, the interlocked growth of C-S-H gel and AFt crystals continued and promoted the increase of the strength of the cementitious system.

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.