To investigate the mechanism by which ground granulated blast-furnace slag (GBFS) affects the performance of steam-cured cementitious materials, this study systematically analyzes the effect of GBFS on the mechanical strength and hydration products of mortar by adjusting the GBFS content (0%, 20%, 30%, 50%), curing temperature (50 °C for 7 h, 80 °C for 7 h), and curing time (3 d, 28 d). The results show that although increasing the steam-curing temperature can improve the strength activity index of GBFS-containing mortar, higher temperatures tend to induce later-age strength retrogression in such mixtures. Steam-curing not only promotes the massive formation of calcium hydroxide with coarse crystals but also increases the initial Ca/Si ratio of calcium silicate hydrate (C–S–H) gels, which is a crucial factor contributing to the high susceptibility of steam-cured concrete to brittle fracture; however, the incorporation of GBFS can effectively mitigate this issue. Furthermore, under the steam-curing condition of 80 °C, the addition of GBFS facilitates the formation of hydrogarnet and delayed ettringite, which is unfavorable for the long-term strength development and durability improvement in concrete.

This study investigates the activation potential of various activators for ferronickel slag (FNS) and the associated phase evolution. First, the existing forms of MgO in FNS were identified by analyzing its phase composition across different particle sizes. Subsequently, FNS was activated using six types of activators—Ca(OH)₂, CaO, NaOH, KOH, Na₂CO₃, and a Ca(OH)₂–gypsum composite—under steam curing at 80 °C for 7 days. The setting time, fluidity, hydration products, and mechanical properties of the activated systems were systematically examined. The results show that finer water-cooled FNS particles contain abundant amorphous phases, including amorphous MgO, which can react with Ca-based activators to form hydrotalcite—a reaction not observed with Na- or K-based activators. Compared with Na- or K-based activators, Ca-containing activators, particularly the Ca(OH)₂–gypsum combination, exhibited superior activation performance. In addition, distinct microstructures were observed: NaOH activation promoted the formation of a yarn ball-like N–A–S–H gel, while KOH activation led to a knotted-fiber-bundle-like K–A–S–H phase, the latter showing potential for enhancing the crack resistance of cement-based materials. These findings provide new insights into the activator-dependent hydration mechanisms of FNS and support its value-added utilization in sustainable construction materials.