The alkali–silica reaction (ASR) is a deleterious chemical reaction between the alkali ( and ), dissolved in concrete pore water as NaOH and KOH, with reactive amorphous (non-crystalline) siliceous aggregates in the presence of moisture. The simplest way to write the reaction in a stylized manner is the following (other representations also exist): 
:  (young N-S-H gel)
This reaction produces a gel-like substance of sodium silicate ( • n ), also noted  • n , or N-S-H (sodium silicate hydrate). This hygroscopic gel swells inside the affected reactive aggregates which expand and crack. In its turn, it causes concrete expansion. If concrete is heavily reinforced, it can first cause some prestressing effect before cracking and damaging the structure. ASR affects the aggregates and is recognizable by cracked aggregates. It does not directly affect the hardened cement paste (HCP).