Chemical reaction in either mortar or concrete between alkalis ( hydroxyl ions associated with sodium and potassium ) from portland cement and other constituents, such as admixtures and pozzolans, and certain siliceous rocks or minerals, such as opaline chert, strained quartz, and certain volcanic glasses, present as constituents of some aggregates. The product of the reaction, a silica gel, can cause expansion and cracking of the concrete or mortar.

Alkali in portland cement can react with silica present in some types of aggregate. ASR can produce efflorescence and expansive forces that can crack concrete. MetaMax® reduces the potential for damage to concrete by converting the products of the ASR into compounds which contribute to the strength and durability of concrete.