Fe-rich silicate slags are promising resources in the production of alternative cementitious materials. In this investigation, molecular dynamics simulations have been employed to provide a comprehensive structural description of a range of binary FeO-SiO2 and ternary CaO-FeO-SiO2 glasses, which are the basis of the slag systems that are observed in the non-ferrous metallurgy. The impact of the chemical variations on the structure of the glasses is assessed. Fe atoms have a slightly more profound role as network modifiers, and Ca as charge compensators for lower SiO2 content. There is a preference of Fe atoms to be associated with free oxygens, as compared to Ca atoms, forming small scale clusters. The obtained results enrich our understanding of the relationship between the composition, the structure and the reactivity of such complex glasses.