Particle-Size-Dependent Reactivity and Graded Utilization of Water-Cooled Ferronickel Slag in Cement-Based Materials

The utilization of water-cooled electric furnace ferronickel slag (EFFS) in concrete remains constrained by its intrinsically low pozzolanic reactivity as a supplementary cementitious material (SCM) and its inadequate volumetric stability when employed as aggregate. This study systematically investigates the compositional characteristics of this slag across different particle-size fractions and proposes a wet-grinding activation strategy to enhance its pozzolanic performance. In particular, cement pastes incorporating 10%, 30%, and 50% ultrafine EFFS derived from three original size fractions are comprehensively evaluated in terms of rheological behavior, compressive strength, hydration characteristics, and microstructural evolution. The results demonstrate pronounced size-dependent differences in phase composition. The amorphous phase content of EFFS particles smaller than 1.0 mm (EFFS0) reaches 54.04%, which is 19.72% higher than that of particles larger than 1.8 mm (EFFS2). The higher amorphous content leads to enhanced pozzolanic activity and improved mechanical performance. Mortars containing 30% ultrafine EFFS0 exhibit favorable strength development, with the 28-day compressive strength reaching 96% of that of the plain cement control and showing a 29% increase relative to the corresponding EFFS2 mixture. Based on these findings, a graded utilization strategy for water-cooled EFFS is proposed: The finer and more reactive fractions are suitable for SCM applications after activation, whereas the coarser fractions are more appropriate for non-cementitious applications, such as aggregate utilization.