- For less common types of alkali-driven concrete degradation see alkali-aggregate reaction (disambiguation page).
The alkali–silica reaction (ASR) is a reaction which occurs over time in concrete between the highly alkaline cement paste and reactive non-crystalline (amorphous) silica, which is found in many common aggregates.
The ASR reaction is the same as the pozzolanic reaction, which is a simple acid-base reaction between calcium hydroxide, also known as Portlandite, or (Ca(OH)2), and silicic acid (H4SiO4, or Si(OH)4). For the sake of simplicity, this reaction can be schematically represented as following:
- Ca(OH)2 + H4SiO4 → Ca2+ + H2SiO42− + 2 H2O → CaH2SiO4 · 2 H2O
This reaction causes the expansion of the altered aggregate by the formation of a swelling gel of calcium silicate hydrate (C-S-H). This gel increases in volume with water and exerts an expansive pressure inside the material, causing spalling and loss of strength of the concrete, finally leading to its failure.
ASR can cause serious expansion and cracking in concrete, resulting in critical structural problems that can even force the demolition of a particular structure.
The mechanism of ASR causing the deterioration of concrete can be described in four steps as follows:
- The alkaline solution attacks the siliceous aggregate, converting it to viscous alkali silicate gel.
- Consumption of alkali by the reaction induces the dissolution of Ca2+ ions into the cement pore water. Calcium ions then react with the gel to convert it to hard C-S-H.
- The penetrated alkaline solution converts the remaining siliceous minerals into bulky alkali silicate gel. The resultant expansive pressure is stored in the aggregate.
- The accumulated pressure cracks the aggregate and the surrounding cement paste when the pressure exceeds the tolerance of the aggregate.
ASR can be mitigated in concrete by three complementary approaches:
- Limit the alkali metal content of the cement. Many standards impose limits on the "Equivalent Na2O" content of cement.
- Limit the reactive silica content of the aggregate. Certain volcanic rocks are particularly susceptible to ASR because they contain volcanic glass (obsidian) and should not be used as aggregate. The use of calcium carbonate aggregates is sometimes envisaged as an ultimate solution to avoid any problem. However, while it may be considered as a necessary condition, it is not a sufficient one. In principle, limestone (CaCO3) is not expected to contain a high level of silica, but it actually depends on its purity. Indeed, some siliceous limestones (a.o., Kieselkalk found in Switzerland) may be cemented by amorphous or poorly crystalline silica and can be very sensitive to the ASR reaction, as observed with some siliceous limestones exploited in quarries in the area of Tournai in Belgium. So, the use of limestone as aggregate is not a guarantee against ASR in itself. The silica content of the limestone and its reactivity must remain below a threshold value that has to be carefully experimentally assessed by the aggregate producer.
- Add very fine siliceous materials to neutralize the excessive alkalinity of cement with silicic acid by voluntary provoking a controlled pozzolanic reaction at the early stage of the cement setting. Convenient pozzolanic materials to add to the mix may be, e.g., pozzolan, silica fume, fly ash, or metakaolin. These react preferentially with the cement alkalis without formation of an expansive pressure, because siliceous minerals in fine particles convert to alkali silicate and then to calcium silicate without formation of semipermeable reaction rims.
In other words, as it is sometimes possible to fight fire with fire, it is also feasible to combat the ASR reaction by itself. A prompt reaction initiated at the early stage of concrete hardening on very fine silica particles will help to suppress a slow and delayed reaction with large siliceous aggregates on the long term. Following the same principle, the fabrication of low-pH cement also implies the addition of finely divided pozzolanic materials rich in silicic acid to the concrete mix to decrease its alkalinity.
The concrete microbar test was proposed by Grattan-Bellew et al. (2003) as a universal accelerated test for alkali-aggregate reaction. 
- FHWA (2010-06-22). "Alkali-Silica Reactivity (ASR) – Concrete – Pavements – FHWA". Alkali-Silica Reactivity (ASR) Development and Deployment Program. Archived from the original on 8 August 2010. Retrieved 2010-07-28.
- Faridazar, Fred (2009-02-10). "TECHBRIEF: Selecting candidate structures for lithium treatment: What to provide the petrographer along with concrete specimens, FHWA-HRT-06-069 – Pavements – FHWA". FHWA-HRT-06-069. Retrieved 2010-07-28.
- "Alkali–silica reaction in concrete". Understanding Cement. Archived from the original on 10 August 2007. Retrieved 2007-08-11.
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- Funk, Hanspeter (1975). "The origin of authigenic quartz in the Helvetic Siliceous Limestone (Helvetischer Kieselkalk), Switzerland". Sedimentology 22 (2): 299–306. Bibcode:1975Sedim..22..299F. doi:10.1111/j.1365-3091.1975.tb00296.x.
- Monnin, Y.; Dégrugilliers P., Bulteel D., Garcia-Diaz E. (2006). "Petrography study of two siliceous limestones submitted to alkali-silica reaction". Cement and Concrete Research 36 (8): 1460–1466. doi:10.1016/j.cemconres.2006.03.025. ISSN 0008-8846. Retrieved 2009-03-17.
- Ramlochan, Terrence; Michael Thomas, Karen A. Gruber (2000). "The effect of metakaolin on alkali-silica reaction in concrete". Cement and Concrete Research 30 (3): 339–344. doi:10.1016/S0008-8846(99)00261-6. ISSN 0008-8846. Retrieved 2009-03-18.
- Grattan-Bellew, P.E.; G. Cybanski, B. Fournier, L. Mitchell (2003). "Proposed universal accelerated test for alkali-aggregate reaction: the concrete microbar test". Cement Concrete and Aggregates 25 (2): 29–34.