Supplementary cementitious materials (SCM) have been used by the cement industry for decades to partly replace the portland cement fraction of concrete binders. This is particularly important today in addressing CO2 emissions from the cement manufacturing process. However, defining the reactivity of these mainly aluminosilicate-based materials and their influence on portland cement hydration chemistry has challenged the research community and has limited SCM replacement levels in cementitious binders. In this study, aluminosilicate glasses as models for blast furnace slag and fly-ash systems were synthesized and exposed to different activator solutions in a continuously stirred closed system reactor for a period up to 3 hours. Solution compositions were measured from the very first minutes of dissolution and correlated with results from complementary solid surface analysis. Initial Ca concentration maxima in the first 30 minutes of exposure to the activating solution was a common feature in most dissolution profiles with a subsequent rapid decline attributable to Ca-reincorporation on the reacting surface. Surface-specific analysis confirmed Ca and Al enrichment at the surface, suggesting the formation of a Ca-modified aluminosilicate layer, supporting a dissolution-reprecipitation mechanism for SCM reactivity. Differing chemistries are thought to be responsible for the Ca and Al reintegration on the reacting surface depending on the pH of the solution; near-neutral conditions favor Ca-readsorption and surface condensation reactions, whereas alkaline solutions favor Ca-reintegration via covalently bound phases.