Alkali Aggregate Reaction in Concrete

Aggregates naturally contain siliceous minerals that react with the alkaline hydroxides from cement alkalis.

The alkali-silica gel formed attracts water and swells, which is constrained by the cement paste around it, leading to internal pressure, causing the cement paste to crack and disrupt, evidenced by expansion.

Expansion is partly due to the alkali-silica reaction's osmotic hydraulic pressure and swelling pressure of solid products.

The size of the siliceous particles influences the reaction rate, with fine particles causing expansion in weeks, while larger ones may take years to cause damage.

The progression of the alkali-aggregate reaction is affected by factors like aggregate porosity, cement alkali content, water in the paste, and cement paste permeability.

This reaction occurs mainly on concrete's surface under continuous moisture or during wetting and drying cycles, particularly at temperatures between 50 and 100 degrees Fahrenheit.

If avoiding harmful cement-aggregate combinations is difficult, incorporating a pozzolan and at least 30 percent limestone coarse aggregate by mass is advisable. Using low-alkali cement also helps reduce the alkali aggregate reactions.