Alkali Silica Reaction

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Alkali-Silica Reaction (ASR) is a reaction between reactive silica (in the concrete aggregates) and an alkali (usually present in the cement), which results in the formation of a gel. This gel increases in volume with water and exerts expansive pressure on the concrete, causing failure of the concrete.

ASR can cause serious expansion and cracking in concrete, resulting in critical structural problems that can even force the demolition of a particular structure.^[1]

ASR is eliminated in concrete by three approaches:

Limit the alkali metal content of the cement. Many standards impose limits on the "Equivalent Na₂O" content of cement.
Limit the reactive silica content of the aggregate. Certain volcanic rocks are particularly susceptible to ASR because they contains volcanic glass (obsidian). In principle, pure limestone (CaCO₃) are not expected to contain high level of silica. However, some limestones (e.g., Kieselkalk) may be cemented by amorphous silica and can be very sensitive to the ASR reaction such as observed with the limestone of the area of Tournai in Belgium.
Add materials such as pozzolans to the mix. These react preferentially with the cement alkalis, without formation of an expansive gel.