Pyrometallurgical copper extraction yields large slag volumes—typically 2.2 to 3.0 tons per ton of copper produced—underscoring the need for valorization strategies to improve resource efficiency. Utilizing these slags as supplementary cementitious materials (SCMs) offers a promising pathway. However, data on how composition influences the inherent reactivity of industrial slags remain scarce. Previous laboratory studies have shown that increasing CaO enhances slag performance as an SCM, but data on industrially CaO-modified slags remain limited. This study evaluates the inherent reactivity of industrial modified copper slags with 3.2–15.9 wt.% CaO. Electric smelting furnace slags were modified in a fuming furnace, tapped into a settling furnace, and water-granulated. Reactivity was assessed using the R³ isothermal calorimetry test after laboratory milling. X-ray diffraction (XRD) and scanning electron microscopy (SEM)–energy-dispersive x-ray spectroscopy (EDS) confirmed an amorphous structure with dispersed matte, speiss, and spinel phases. Increasing CaO depolymerized the silicate network, promoting dissolution and enhancing early-stage heat flow. When comparing fineness levels, the cumulative heat after 7 days correlated most strongly with specific surface area (R² = 0.92), confirming that fineness governs overall reactivity. Normalization of the cumulative heat for reactive oxides (SiO₂ and Al₂O₃) and specific surface area revealed that intrinsic reactivity increases with NBO/T ratio, demonstrating that a more depolymerized glass structure promotes chemical dissolution and higher reactivity. These findings demonstrate that industrial CaO modification enhances early hydration kinetics without reducing pozzolanic activity, confirming the suitability of CaO-modified copper slags as SCMs for cementitious systems.