Ferrochrome slag (FCS), a by-product of ferrochrome production, is a largely underutilized industrial residue with strong potential as a supplementary cementitious material (SCM) to reduce clinker demand and support sustainable construction. While FCS has been widely studied as an aggregate and in geopolymer systems, research on its application as a SCM is scarce, and no study has systematically examined how cooling practices, crystalline phases, and glass structure together govern its pozzolanic reactivity. This study investigates the relationships between cooling rate, glass structure, crystalline phase content, and pozzolanic reactivity in FCS. Industrial slag samples cooled by either water granulation or ambient air, were characterized using X-ray diffraction (XRD) with Rietveld refinement, scanning electron microscopy with energy-dispersive spectroscopy (SEM-EDS), thermodynamic modeling (FactSage), and isothermal calorimetry-based R3 testing. To isolate phase-specific effects, spinel (A(II)B(III)2O4; MgAl2O4-type) and forsterite (A(II)2C(IV)O4; Mg2SiO4-type olivine) samples were synthesized and tested under the same R3 framework. For the studied ferrochrome slag system, results show that water-granulated slags contain higher amorphous content and more depolymerized glass networks, leading to enhanced pozzolanic activity, whereas spinel and forsterite behave as essentially inert phases. This study systematically quantifies the effects of crystalline phases in FCS and demonstrates that glass chemistry and network depolymerization are as influential as the amorphous fraction in controlling pozzolanic reactivity, highlighting that both the quantity and structure of the glass phase govern slag performance. These findings provide a clear understanding of the factors controlling FCS pozzolanic reactivity and offer practical guidance on slag processing to enhance its performance, supporting its potential use as a sustainable SCM in low-carbon concrete.