The reactivity of synthetic glasses depends on their chemical compositions. In far from equilibrium dissolution experiments, the reactivity of Ca-rich glasses with compositions similar to blast-furnace slag is found to be much higher (up to similar to 60wt.% after 7days) compared to Si-rich glasses with compositions similar to type F fly ash (up to similar to 20wt.% after 7days). Isothermal calorimetry and TGA experiments conducted on model systems containing portlandite and calcite and on glass-blended Portland cement confirmed the higher reactivity of the Ca-rich glasses. The degree of glass reaction after 91days ranged from 7 to 20wt.%. The results showed also a higher reactivity of the glasses containing more aluminum (both for Ca-rich and Si-rich glasses) indicating that not only calcium but also aluminum acted rather as network modifier than as network former. The results confirm a strong dependence of the glass reactivity on the degree of polymerization of the glass network.