State-of-the-Art Report on the Mechanism of Alkali-Aggregate Reaction in Concrete Containing Fly Ash

Although aggregates were once thought to be inert, it is now known that all aggregates are chemically reactive. The chemical reactions between the aggregates and the cement paste are responsible for beneficial effects such as enhanced bond, but also of other effects that can be deleterious to the durability of the concrete. Alkali-aggregate reaction is one of such chemical reactions. Its chemistry and mechanism are not yet very well known but several hypotheses have been presented over the years and are reported herein.

The prevention of expansion in concrete due to alkali-aggregate reaction has been widely investigated by many researchers around the world. It is now known that the proper use of mineral admixtures in concrete can reduce the cost of concrete, improve many material properties, and inhibit alkali-silica reaction.

The effect of fly ash and silica fume and a summary of the probable mechanisms in which pozzolans affect the expansion caused by alkali-aggregate reaction in concrete is also reviewed, as well as the different methods of predicting the reactivity of aggregates and a discussion of their relative accuracy and validity.