Blast-furnace slags are widely used as supplementary cementitious materials in blended cements because of their relatively high reactivity. This study describes the effect of substituting Ca by Mg on the reactivity of synthetic glass with similar composition to slags, keeping the basicity ratio constant. The glass reactivity was assessed using a multi-technique approach including dissolution rate experiments at high dilution, calorimetry, X-ray diffraction combined with Rietveld quantitative phase analysis and electron microscopy image analysis of blended cement pastes. The effect of glass hydration on the hydration product assemblage was assessed by comparing experimental data to thermodynamic modelling. The results show that reactivity was not affected significantly by Ca by Mg substitution at constant basicity. The relative reactivities from the dissolution experiments, calorimetry and image analysis are in agreement and show a dependence of reactivity on basicity but not on Mg content. The release of Mg from the glasses led to the through-solution precipitation of hydrotalcite-like hydration products. Under the high pH conditions studied, there was no indication of incongruent glass dissolution or the formation of a leached passivating layer. The experimental results on changes in the cement hydration product assemblage by glass addition were supported by thermodynamic modelling and generally show an increase in calcium-aluminosilicate-hydrate, AFm, and hydrotalcite-type reaction products.