<p>Blast furnace ferronickel slag (FNS), a potential supplementary cementitious material derived from the ferronickel smelting plants, is rich in reactive CaO, SiO₂, and Al₂O₃, which can contribute to long-term concrete performance through pozzolanic and latent hydraulic reactions. However, its incorporation significantly decreases the early-age strength of blended cementitious systems. To address this drawback, this study introduced calcium–silicate–hydrate (C–S–H) seed, a novel admixture synthesized via a solution precipitation method, to accelerate the hydration of FNS-Portland cement (PC) binary system. The influence of C–S–H seeds on setting time, chemical shrinkage, hydration kinetics, mechanical properties, microstructure, and phase assemblages of the composite binder was systematically investigated. Results showed that while 15% FNS addition slightly delays the setting behavior, the introduction of C–S–H seeds effectively shortened both initial and final setting times. Furthermore, the seeds significantly accelerated early reaction of PC and FNS. Consequently, the early-age compressive strength of the binder containing 15% FNS became comparable to that of the reference cement paste. Meanwhile, the seeds do not compromise the late-age compressive strength, and the 28-day compressive strength was higher than that of the reference cement paste due to the latent hydraulic reaction of FNS. This study highlights the potential of seeding technology to enhance the early performance of FNS-blended cement binders, thereby promoting the sustainable utilization of FNS in cement-based materials.</p> Graphical Abstract <p></p>