The alkali-aggregate reaction in concrete involves natural siliceous minerals in aggregates reacting with alkaline hydroxides derived from cement alkalis. This reaction forms an alkali-silica gel that absorbs water, swells, and increases in volume, which is confined by the surrounding cement paste, creating internal pressures that crack and disrupt the concrete. The extent of expansion and damage can be partly attributed to the alkali-silica reaction's osmotic hydraulic pressure and the swelling pressure of the solid products formed.

The rate of reaction is influenced by the size of the siliceous particles; finer particles may cause expansion in a matter of weeks, whereas larger particles could take years to manifest similar damage. Factors such as the porosity of the aggregates, the alkali content of the cement, the amount of water in the paste, and the permeability of the cement paste all affect the progression of this reaction. This reaction typically occurs on the surface of concrete under continuous moisture or during wetting and drying cycles, especially at temperatures between 50 and 100 degrees Fahrenheit. To mitigate these reactions, it is advisable to use low-alkali cement, incorporate a pozzolan, and use at least 30 percent limestone coarse aggregate by mass in the concrete mix.