Understanding Cr speciation and its leaching behavior in electric-furnace ferronickel slag (EFS) is crucial for the safe utilization of this waste. This study first evaluated the properties of EFS as a SCM and then combined synchrotron-based X-ray absorption spectroscopy (XAS) and thermodynamic modeling to analyze Cr speciation and leaching behavior. The results reveal that EFS itself exhibits low reactivity in water or alkaline environments but can be used as a supplementary cementitious material (SCM). Cr exists primarily as Cr(OH)₃ (60 wt%) and Cr₂O₃ (40 wt%). Cr(OH)₃ dissolves below pH 6, while Cr₂O₃ dissolves below pH 4. In cement-EFS composite pastes, the hydrated cement matrix provides an acid-buffering effect, further restricting Cr leaching. The results indicate that the actual leaching risk is significantly lower than the estimation that assumes Cr as soluble Cr^{3 +} or CrO₄^2-. This work provides a scientific basis for assessing Cr leaching risks in EFS-cement composites and offers insights for future standardization efforts.