Alkali–Silica Reaction: The Influence of Calcium on Silica Dissolution and the Formation of Reaction Products

Swiss Federal Laboratories for Materials Science and Technology, Empa, 8600 Dübendorf, Switzerland
Journal of the American Ceramic Society (Impact Factor: 2.61). 11/2010; 94(4):1243 - 1249. DOI: 10.1111/j.1551-2916.2010.04202.x


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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Available from: Andreas Leemann, Jan 14, 2015
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    • "Ca) instead of monovalent alkalis (e.g. Na, K) was shown to generate lower repulsive forces and reduced water adsorption, thus causing less expansive pressure [5] [6] [7]. Cole and Lancucki [8] used XRD to identify crystalline ASR products from a dam built in Australia as okenite, CaSi 2 O 5 ·2H 2 O or its precursor, CaSi 2 O 5 ·4H 2 O, in which some Ca atoms are replaced by K and Na. "

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    • "The suggested sequence includes the formation of C–S–H as the first reaction product in the ASR process. The formation of C–S–H continues up to a point of local depletion of Ca(OH) 2 [8] [9] [10] [11] [12]. "
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    • "In the microsilica samples with portlandite addition, the amount of reacted microsilica increases with increasing portlandite content as shown by calorimetry, XRD and NMR results. In these samples the formation of C–S–H removes silicon from the pore solution, enabling further microsilica dissolution [31]. The reaction proceeds until calcium is depleted. "
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