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 constituents of some aggregates. Under certain conditions, this reaction can result in deleterious expansion of the mortar or concrete resulting in cracking.

Alkali-aggregate reaction is a reaction which occurs over time in concrete between the highly alkaline cement paste and non-crystalline silicon dioxide, which is found in many common aggregates. This reaction can cause expansion of the aggregate, leading to spalling and loss of strength of the concrete.

Older terminology for Alkali-Silica Reactivity (ASR).