The use of electric furnace ferronickel slag (FNS) as a supplementary cementitious material is the current focus of research. This study investigates the effect of mechanical grinding and chemical additives on the activity excition of FNS, as well as the associated synergistic mechanisms. This study shows that the addition of triethanolamine (TEA) increases the fine-grained content in FNS powder, which facilitates the depolymerization of FNS and the early hydration of aluminum tricalcium. Furthermore, the addition of Ca(OH)₂ raises the alkalinity of the cementitious system, which promotes the availability of Ca²⁺ ions and accelerates the hydration process, resulting in the generation of additional hydration products. The enhancement of late hydration of C₃S by TEA and its combination with the secondary hydration of Ca²⁺ at high alkalinity are the pivotal factors to improve the strength of cementitious composite. A mixture of FNS and 0.03% TEA is subjected to grinding for 90 min, using the obtained micropowder which replaces 20% of the cement, and subsequently, after being excited with 3% Ca(OH)₂, the FNS micropowder reaches the quality standards of S95 slag powder. It is worth remarking that the micropowder prepared by mixing FNS with 3% Ca(OH)₂ and 0.03% TEA and grinding it for 81 min also meets the S95 standard for slag powder. The larger dosage of FNS in cement is supported by the observed synergy between TEA and Ca(OH)₂. This research will provide valuable insights for the expanded application of FNS in construction materials.