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".