The metallurgical and cement industries account for a substantial share of anthropogenic carbon dioxide emissions. Utilizing oxidic by-product materials from the metallurgical industry as supplementary cementitious materials (SCMs) is a means to improve resource efficiency and lower the emissions from cement production. While the former effect is self-explanatory, the latter is a consequence of the partial replacement of Portland cement with the SCM, which requires neither calcination nor clinkering. The previously published literature for incorporating iron silicate slags in SCM applications includes various testing procedures with multiple parameters varying between studies. Therefore, the present paper offers a first insight into the effect of processing parameters on the inherent reactivity of an industrially produced iron silicate slag. More specifically, the effect of onset granulation temperature and time of grinding on the evolved heat in isothermal calorimetry experiments were studied. The results showed that granulation temperature had an insignificant effect on the evolved heat. Increased grinding time showed contrasting trends in the juxtaposition of evolved heat normalized against mass and surface area. The former increased with prolonged grinding, while the latter decreased. Based on the results, previously reported data were considered, highlighting the need for future studies on controlled variations in chemical composition.