The interaction between simulated reference waste glasses SON68 and SM539, and cement has been studied in suspensions of ordinary Portland cement and synthetic cement water with pH 13.5 at 30°C. The cement appears to trigger glass dissolution by consumption of glass matrix components. This leads to fast glass dissolution at a constant rate with formation of a porous gel layer on the glass. A glass dissolution model has been proposed considering that the silicon coming from the glass reacts with portlandite to form C-S-H phases. The transformation of C-S-H into C-A-S-H phases is a second parallel driving force especially for the Al-rich SM539 glass. After consumption of the portlandite, the glass alteration rate is expected to decrease.