Alkali–aggregate reaction (AAR) is a chemical reaction between the alkaline components of Portland cement and certain types of aggregates that contain reactive silica. This reaction produces a gel that absorbs water and swells, leading to internal stress within the concrete. There are two main types of AAR: alkali–silica reaction (ASR) and alkali–carbonate reaction (ACR). Both reactions can cause significant expansion and cracking of the concrete, ultimately affecting the structural integrity and durability of concrete structures.