Pyrometallurgical copper production is associated with high slag rates, typically ranging from 2.2 to 3.0 tons of slag per ton produced copper. Therefore, slag valorization is necessary to maintain a resource-efficient operation. An attractive application for these iron silicate slags is as supplementary cementitious materials (SCMs), which effectively lowers the CO2 emissions per ton of concrete. Although utilizing iron silicate slags as SCMs has been studied in previous work, the scientific literature has limited data on the isolated effect of composition on the inherent reactivity in cementitious systems. In particular, no reports on the impact of the FeO/SiO2 ratio have been presented in previous publications. Therefore, the present study aimed to isolate this parameter in a synthetic FeO–SiO2–Al2O3–CaO–MgO–Cr2O3 system. Since the amorphous content is a pertinent parameter for SCMs, a hot-stage confocal laser scanning microscope was utilized to assess the crystallization behavior at continuous cooling conditions. Furthermore, high-temperature rheological experiments were conducted to measure the viscosities of the slags in relation to the crystallization behavior. The experiments highlighted that depolymerizing the slag by increasing the FeO/SiO2 ratio poses increased demands on the cooling rate to avoid crystallization, which was consistent with the rheological data on the impact of temperature on structural changes in the slags. Furthermore, Rapid Reliable Relevant (R3) isothermal calorimeter experiments, assessing the inherent reactivity as an SCM, showed that increasing FeO/SiO2 ratios improves early reactivity at the expense of seven-day reactivity, i.e., a balance between the degree of polymerization and contribution of silicon to the reactions.