Alkali Aggregate Reaction in Concrete

Alkali-aggregate reaction occurs when siliceous minerals naturally present in aggregates react with alkaline hydroxides from cement alkalis. This reaction forms an alkali-silica gel that absorbs water and swells. The swelling is confined by surrounding cement paste, creating internal pressure that cracks and disrupts the concrete structure.

Siliceous particle size significantly influences reaction progression. Fine particles cause expansion within weeks, while larger particles may take years to produce similar damage. This variation in reaction rate depends on the surface area available for chemical interaction between the aggregate and cement alkalis.

The alkali-aggregate reaction's progression is affected by aggregate porosity, cement alkali content, water present in the paste, and permeability of concrete. These factors work together to determine how quickly and severely the reaction develops, influencing the timeline and extent of concrete damage.

Alkali-aggregate reaction primarily occurs on concrete surfaces under continuous moisture or during wetting and drying cycles. The reaction is most active at temperatures between 50 and 100 degrees Fahrenheit. These conditions provide the moisture and temperature environment necessary for the silica-alkali interaction to proceed.

Expansion results from two mechanisms: osmotic hydraulic pressure and swelling pressure of solid products formed during the reaction. The alkali-silica gel absorbs water and expands, but confinement by cement paste creates internal stresses. These pressures accumulate until they exceed the concrete's tensile strength, causing cracking.

Prevention strategies include using low-alkali cement to reduce available alkalis for reaction. Incorporating a pozzolan and at least 30 percent limestone coarse aggregate by mass also mitigates the reaction. These measures either reduce reactivity or dilute the harmful cement-aggregate combinations that trigger expansion.

Alkali-silica gel attracts water and swells, increasing in volume. Since the gel is confined within the cement paste matrix, this expansion generates internal pressure. When internal pressure exceeds the concrete's tensile strength, the cement paste cracks and disrupts, leading to visible expansion and structural damage.