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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.

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In this study, the effect of CO 2 concentration and ambient relative humidity (RH) on accelerated and natural carbonation of 18 concrete mixtures produced with nine different cement types is investigated. Increasing the CO 2 concentration from 0.045 to 1 and 4 % at 57 % RH does not alter the relative carbonation resistance between the concrete mixtures. The increase of RH from 57 to 70 and 80 % RH at 4 % CO 2 shows a water-to-cement ratio and cementspecific effect that affects the relative carbonation resistance between the concrete mixtures. The carbonation resistance at 4 % CO 2 and 57 % RH allows assessing the carbonation resistance of concrete in sheltered and with restrictions in unsheltered outdoor exposure. The carbonation resistance below 70 % RH is mainly governed by the CO 2 buffer capacity. However, in the accelerated tests at 80 % RH and in the unsheltered outdoor exposure capillary condensation is of increased importance.