Non-ferrous metallurgy slags are currently do not reach their full valorization potential. On a global scale, these slags are landfilled or used in low-value applications. Upcycling these slags to produce a sustainable cementitious binder, e.g. inorganic polymers, does not only substitute the vast CO₂ emitting Portland cement, but also gives economic motivation to metallurgical companies to invest in the valorization of their slags. The basic chemistry of these slags can be reduced to the ternary system CaO-FeO-SiO₂. The aim of this work is to optimize these slags as precursors for inorganic polymers. Slags with molar ratios of (FeO + CaO)/SiO₂ = 1.1–2.3 and FeO/CaO = 1.9–5.4 were synthesized, as they represent slags found within the non-ferrous metallurgical industry. Higher SiO₂ content resulted in increased glass formation. Slags with higher glass fraction reacted more quickly with the sodium silicate solutions used (molar ratios SiO₂/Na₂O = 1.6–2.0 and H₂O/Na₂O = 25). For the 2 d compressive strength, higher silica content of the slag and activating solution resulted in higher early strengths, reaching 19 MPa for the slag with higher FeO/CaO molar ratio. At 28 d, slags with higher CaO content showed a higher relative strength increase with respect to the 2 d strength. The optimum slag was found in the middle of the studied compositional range, resulting in an inorganic polymer reaching 53 MPa. In conclusion, the chemistry of the slags influences the mechanical properties at all ages, while the activating solution can be used to fine-tune the kinetics and early age properties.