Abstract To assess the chemical durability of the Belgian reference glasses SON68, SM513, and SM539 under hyper-alkaline conditions, maximum glass dissolution rates were measured at 30 °C and pH 13.5 by performing Single-Pass Flow-Through tests with a KOH solution and a synthetic young cementitious water containing Ca (YCWCa). In both leachants, the highest dissolution rate was determined for the Al-rich glass SM539, while similar rates were found for SM513 and SON68 glasses, whose compositions are comparable. For all glasses, the dissolution rates in YCWCa were lower than in KOH due to the presence of Ca, which leads to the formation of a slightly protective calcium silicate layer. The dissolution rates in YCWCa were similar to those determined from glass monolith mass losses in static tests carried out at 30 °C in YCWCa in the presence of hardened Ordinary Portland Cement paste powder with a cement-to-glass ratio of 1. Graphical abstract