Alkali Silica Reaction

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. [cite web | publisher = Understanding Cement | url = http://www.understanding-cement.com/alkali-silica.html | title = Alkali-silica reaction in concrete | accessdate = 2007-08-11]

The following is the mechanism of ASR causing the deterioration of concrete. 1) The alkaline solution attacks the surface of the siliceous aggregate to convert it to viscous alkali silicate gel. The aggregate is covered with the gel. 2) Consumption of alkali by the reaction induces the dissolution of Ca ions into the solution, which then react with the gel to convert it to hard calcium silicate (reaction rim). 3) The reaction rim allows the penetration of not the alkali silicate but the alkaline solution. The penetrated alkaline solution converts the remaining siliceous minerals into bulky alkali silicate gel. The resultant expansive pressure is stored in the aggregate. 4) The accumulated pressure cracks the aggregate and the surrounding cement paste when the pressure exceeds the tolerance of the aggregate [Modified model of Alkali-silica reaction T. Ichikawa, M. Miura, Cement and Concrete Research, 37 (2007) 1291–1297] .

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 fine siliceous materials such as fly ashs to the mix. These react preferentially with the cement alkalis without formation of an expansive pressure, because siliceous minerals in fine particles totally convert to alkali silicate without formation of semipermeable reaction rims.

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