Alkali–aggregate reaction is a term mainly referring to 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 the expansion of the altered aggregate, leading to spalling and loss of strength of concrete.
The alkali–aggregate reaction is a general, but relatively vague, expression which can lead to confusion. More exact definitions include the following:
The alkali–silica reaction is the most common form of alkali–aggregate reaction.
Two other types are:
The pozzolanic reaction which occurs in the setting of the mixture of slaked lime and pozzolanic materials has also features similar to the alkali–silica reaction, mainly the formation of calcium silicate hydrate (C-S-H).