Concrete cancer

The alkali–silica reaction (ASR), more commonly known as "concrete cancer", is a swelling reaction which occurs over time in concrete between the highly alkaline cement paste and the reactive non-crystalline (amorphous) silica found in many common aggregates, given sufficient moisture. The expansion of concrete through reaction between cement and aggregates was first studied by Thomas E. Stanton in California during the years 1930 with his founding publication in 1940.

This reaction causes the expansion of the altered aggregate by the formation of a soluble and viscous gel of sodium silicate (Na₂SiO₃ · n H₂O, also noted Na₂H₂SiO₄ · n H₂O, or N-S-H (sodium silicate hydrate), depending the adopted convention). This hygroscopic gel swells and increases in volume when absorbing water: it exerts an expansive pressure inside the siliceous aggregate, causing spalling and loss of strength of the concrete, finally leading to its failure.

ASR can cause serious cracking in concrete, resulting in critical structural problems that can even force the demolition of a particular structure.

For the sake of simplicity, to more easily understand ASR from a chemical point of view, this reaction can be compared to the pozzolanic reaction which would be catalysed by the undesirable presence of too high concentrations of alkali hydroxides (NaOH and KOH) in the concrete. It is a mineral acid-base reaction between NaOH or KOH, calcium hydroxide, also known as Portlandite, or (Ca(OH)₂), and silicic acid (H₄SiO₄, or Si(OH)₄). When complete and to simplify, this reaction can be schematically represented as following:

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