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Influence of alkali supply from outside on the dissolution behavior of aggregates

  • Reactive aggregates, humidity and alkalis are necessary for ASR in concrete structures. If alkalis come additionally from outside, the damaging reaction can be strongly accelerated. However, the reasons therefore are not completely understood. Some researchers discuss a direct attack of alkali ions on the Si-O-Si bonds of quartz structures. This paper provides another point of view to understand this phenomenon. Different aggregates (grain size 2-8mm) were stored in a highly alkaline KOH solution. Additionally to this solution, sodium chloride with different concentrations was added. The aggregates were selected on the one hand according to their reaction behavior in concrete structures, on the other hand to their dissolution behavior. Long-term dissolution experiments were performed over several months. The silica, the aluminum and of course the sodium concentrations were determined. It can be expected, that under the influence of sodium the silica concentrations in the alkaline solution raise up. The question here is: What happensReactive aggregates, humidity and alkalis are necessary for ASR in concrete structures. If alkalis come additionally from outside, the damaging reaction can be strongly accelerated. However, the reasons therefore are not completely understood. Some researchers discuss a direct attack of alkali ions on the Si-O-Si bonds of quartz structures. This paper provides another point of view to understand this phenomenon. Different aggregates (grain size 2-8mm) were stored in a highly alkaline KOH solution. Additionally to this solution, sodium chloride with different concentrations was added. The aggregates were selected on the one hand according to their reaction behavior in concrete structures, on the other hand to their dissolution behavior. Long-term dissolution experiments were performed over several months. The silica, the aluminum and of course the sodium concentrations were determined. It can be expected, that under the influence of sodium the silica concentrations in the alkaline solution raise up. The question here is: What happens with the aluminum concentration at the same time? Very surprisingly was, that Al reaches a maximum after some days and then it decreases. Exactly from this time when the Al concentration decreases the silica concentration raises up extremely. The reason therefore can only be a precipitation process. Obviously, there is a correlation between the aluminum content in the solution and the extreme silica release of aggregates, both affected by alkali ions from outside. Aggregates, which do not release aluminum because of their composition (e.g. quartzite), do not show this behavior. The conclusion is that such aggregates are not so sensitive against alkali attack from outside.show moreshow less

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Metadaten
Author: Klaus-Jürgen HüngerGND, Mario Kositz, Matti Danneberg
ISBN:978-972-49-2315-4
Title of the source (English):Proceedings of the 16th International Conference on Alkali-Aggregate Reaction in Concrete
Publisher:LNEC
Document Type:Conference publication not peer-reviewed
Language:English
Year of publication:2021
Tag:aggregate grains; aluminium concentration; deicing salt; dissolution experiments; silica release
First Page:63
Last Page:72
Faculty/Chair:Fakultät 6 Architektur, Bauingenieurwesen und Stadtplanung / FG Baustoffe und Bauchemie
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