Pyrometallurgical extraction of copper generates an average of 2.2–3.0 tons of slag per ton of copper. With rising copper demand, future increases in slag volumes are expected, making the exploration of sustainable slag utilization essential. There are several applications for iron silicate slag. However, from a global perspective, a large amount of slag still ends up in landfills due to environmental concerns related to the potential weathering of metals present in the slag. This occurs despite results indicating that an improved slag-cleaning process makes iron silicate slag suitable for external applications. One potential application is utilizing iron silicate slag in the cement industry as a supplementary cementitious material (SCM). Given the wide range of ZnO content found in iron silicate slags worldwide, along with previous studies on Portland cement indicating that ZnO negatively affects the cement’s hydration process, there is a clear need for a structural investigation into how ZnO impacts the reactivity of iron silicate slag. This understanding is important for optimizing the use of iron silicate slag as an SCM. Therefore, this study covers a systematic investigation of the effect of ZnO on the inherent reactivity of iron silicate slag. Synthetic slags in the FeO–SiO2–CaO–Al2O3–MgO–ZnO system, with systematic variations in ZnO concentrations between 0 and 14.6 wt.%, were produced on a laboratory scale using water granulation. Results from isothermal calorimeter testing indicate that increasing ZnO concentrations decrease the slag’s inherent reactivity. These findings highlight the necessity of implementing a slag-cleaning process that minimizes zinc concentrations to facilitate the successful use of iron silicate slag as an SCM.