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In a model system for alkali–silica reaction consisting of microsilica, portlandite (0–40 mass%), and 1M alkaline solutions (NaOH, KOH), the influence of calcium on silica dissolution and on the formation of reaction products is investigated. The reaction and its products are characterized using calorimetry, X-ray diffraction, thermogravimetric analysis, nuclear magnetic resonance, desorption experiments, and pore solution analysis in combination with thermodynamic modeling. Silica dissolution proceeds until portlandite is consumed due to the formation of C–S–H, and subsequently, saturation of dissolved silica in the alkaline solution is reached. As a result, the amount of dissolved silica increases with the increasing portlandite content. Depending on the amount of portlandite added, the reaction products show differences in the relative amounts of Q1, Q2, and Q3 sites formed and in their average Ca/Si ratio. The ability of the reactions products to chemically bind water decreases with the decreasing relative amount of Q3 sites and with the increasing Ca/Si ratio. However, the amount of physically bound water in the reaction products reaches a maximum value at a Ca/Si ratio between 0.20 and 0.30.