Article

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

ABSTRACT

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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    • "The present paper aims to improve the understanding of the role of lithium ions in controlling the ASR using the results of various studies which were conducted on both reactive and non-reactive systems and with and without the addition of LiNO 3 . These studies included: (a) the model reactant (MR) experiments simulating ASR during exposure of reactive aggregate to the potassium hydroxide (KOH) solution [23] [24] [25], (b) the analysis of pore solution from mortars containing the reactive aggregate (Jobe sand) or non-reactive aggregate (Ottawa sand) with three different dosages of LiNO 3 (molar ratio of lithium to alkali ions of, respectively, 0, 0.26, and 0.74), and (c) the mortar bar expansion tests performed on specimens containing the same three different dosages of LiNO 3 as those mentioned in point (b). "
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    ABSTRACT: This paper presents the results of the investigation on the effects of Li+ ions on the chemical and physical changes in the cementitious system undergoing alkali-silica reaction (ASR). Specifically, this paper focuses on determining which chemical steps of ASR processes are affected by the presence of Li+ ions in the pore solution in order to provide better understanding of the role of Li+ ions in the mitigation process of ASR. The results presented in this paper strongly support the hypothesis that the Li+ ions facilitate the formation of a physical barrier on the surface of reactive silica, and thus prevent further attack on the reactive sites by hydroxyl ions.
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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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