Reactivity of glasses is predominantly linked to the degree of depolymerization of the glass structure. In this work, eight calcium aluminosilicate glasses were synthesized with compositions reflecting the glassy phases of industrial fly ashes and slags. The chosen composition ranges allow for isolation and identification of the roles of Ca and Al. The reactivity of the glasses is found to be mainly dependent on the CaO content as calcium depolymerizes the glass structure and increases the rate of reaction of the glasses. The effect of Al ₂ O ₃ on glass reactivity is less pronounced and relies on chemical weakening of the glass structure. However, an increasing alumina content has a pronounced effect on the hydrate phase assemblage, bound water content, portlandite consumption and the heat of reaction. The resulting compressive strength of composite cement mortars incorporating the synthesized CaO-Al ₂ O ₃ -SiO ₂ glasses is found to be a function of both the degree of glass reaction and the actual phase assemblage.