First principles calculations are employed to provide a fundamental understanding of the relationship between the reactivity of synthetic calcium silicate phases and their electronic structure. Our aim is to shed light on the wide range of hydration kinetics observed in different phases of calcium silicate. For example, while the dicalcium silicate (Ca₂SiO₄) phase slowly reacts with water, the tricalcium silicate (Ca₃SiO₅) shows much faster hydration kinetics. We show that the high reactivity of Ca₃SiO₅ is mainly related to the reactive sites around its more ionic oxygen atoms. Ca₂SiO₄ does not contain these types of oxygen atoms, although experiments suggest that impurities may play a role in changing the reactivity of these materials. We analyze the electronic structure of a wide range of possible substitutions in both Ca₃SiO₅ and Ca₂SiO₄ and show that while the influence of different types of impurities on structural properties is similar, their effect on reactivity is very different. Our calculations suggest that the variation of electronic structure is mainly related to the formation of new hybridized orbitals and the charge exchange between the impurity atoms and the bulk material. The charge localization upon introducing impurities is quantified to predict candidate substitutions that could increase the reactivity of Ca₂SiO₄, which would broaden the applicability of this lower temperature and thus less costly and energetically less demanding phase.