Alkali-aggregate reactions (AAR) are chemical reactions that occur between certain types of mineral of aggregates and the alkaline (Na+ and K+) and hydroxyl (OH-) ions present in the interstitial solution of cement paste in concrete. These dissolution reactions occur due to the high solubility of certain amorphous, disordered or poorly crystallized forms of silica present in very alkaline solutions. This reaction leads to formation of a hygroscopic alkaline gel. In general, this gel is expansive in nature, resulting in internal stresses in concrete and consequently cracking. It is often accompanied by the appearance of efflorescence and exudations on the surface of the concrete.

These reactions are not normally the primary cause of collapse of a structure, however they significantly decrease the durability of concrete as a result of cracking favouring other processes of deterioration, particularly in the cases of water saturation leading to frost attack, and carbonation or chloride penetration resulting in reinforcement corrosion.

Of the types of AAR, alkali-silica reaction (ASR) is the most common and is the subject of intense international research. There is still no general consensus on the mechanisms and the development of the expansive forces. ASR essentially corresponds to an attack on certain forms of reactive silica with a more or less disordered structure (opal, chalcedony, chert, certain types of quartz - proving to be unstable in an alkaline environment of high pH), by alkaline and hydroxyl ions of the interstitial solution of the concrete, producing an alkaline silicate gel. The speed of this attack depends on the concentration of alkali hydroxides in the interstitial solution and the structure of silica. From the hydration reactions of cement, calcium hydroxide (Ca(OH)2) is the main source of calcium ions (Ca +) which penetrate the gel formed. A gel of calcium silicate, sodium and potassium of varying composition is formed in the concrete around the aggregates. This gel, due to its ability to absorb water, generates expansive forces.

The equivalent Finnish termns for Alkali Aggregate Reactions and Alkali Silica Reactions are "Alkalikiviainesreaktio" and "Alkalipiihapporeaktio", respectively. These should not be confused with alkali-silicate reaction which in Finnish is "Alkalisilikaattireaktio".