The alkali–carbonate reaction is an alteration process first suspected in the 1950s in Canada for the degradation of concrete containing

dolomite Dolomite may refer to: *Dolomite (mineral), a carbonate mineral *Dolomite (rock), also known as dolostone, a sedimentary carbonate rock *Dolomite, Alabama, United States, an unincorporated community *Dolomite, California, United States, an unincor ...

aggregates.Swenson, E.G. (1957a) A reactive aggregate undetected by ASTM test. ASTM Bulletin, 226: 48–50.Swenson, E.G. (1957b) Cement-aggregate reaction in concrete of a Canadian bridge. ASTM Proceedings, 57: 1043–1056.

Alkali In chemistry, an alkali (; from ar, القلوي, al-qaly, lit=ashes of the saltwort) is a basic, ionic salt of an alkali metal or an alkaline earth metal. An alkali can also be defined as a base that dissolves in water. A solution of a ...

from the cement might react with the dolomite crystals present in the aggregate inducing the production of brucite, (MgOH)₂, and

calcite Calcite is a Carbonate minerals, carbonate mineral and the most stable Polymorphism (materials science), polymorph of calcium carbonate (CaCO3). It is a very common mineral, particularly as a component of limestone. Calcite defines hardness 3 on ...

(CaCO₃). This mechanism was tentatively proposed by Swenson and Gillott (1964) and may be written as follows: :CaMg(CO3)2 + 2 NaOH -> Mg(OH)2 + CaCO3 + Na2CO3 Brucite (Mg(OH)₂), could be responsible for the volumetric expansion after de-dolomitisation of the aggregates, due to absorption of water. The alkali–carbonate reaction is also catalyzed by the soluble NaOH produced by the reaction of with (

portlandite Portlandite is a hydroxide-bearing mineral typically included in the oxide mineral class. It is the naturally occurring form of calcium hydroxide (Ca(OH)2) and the calcium analogue of brucite (Mg(OH)2). Occurrence Portlandite occurs in a variety ...

) present in the hardened cement paste (HCP), therefore perpetuating the reaction indefinitely as observed by Fournier and Bérubé (2000) and Bérubé ''et al.'' (2005).Bérubé, M. A., Smaoui, N., Bissonnette, B., & Fournier, B. (2005)
Outil d’évaluation et de gestion des ouvrages d’art affectés de réactions alcalis-silice (RAS)
Études et Recherches en Transport, Ministère des Transports du Québec. See the chemical equations on pp. 3-4. :Na2CO3 + Ca(OH)2 -> CaCO3 + 2 NaOH The sum of the two above mentioned reactions leading to the ultimate production of brucite and calcium carbonate can be written as follows: :CaMg(CO3)2 + Ca(OH)2 -> Mg(OH)2 + 2 CaCO3 The alkali-carbonate reaction is much less understood than the alkali-silica reaction. Both reactions share in common the continuous regeneration of the

sodium hydroxide Sodium hydroxide, also known as lye and caustic soda, is an inorganic compound with the formula NaOH. It is a white solid ionic compound consisting of sodium cations and hydroxide anions . Sodium hydroxide is a highly caustic base and alkali ...

(NaOH) after the reaction of soluble

sodium carbonate Sodium carbonate, , (also known as washing soda, soda ash and soda crystals) is the inorganic compound with the formula Na2CO3 and its various hydrates. All forms are white, odourless, water-soluble salts that yield moderately alkaline solutions ...

sodium silicate Sodium silicate is a generic name for chemical compounds with the formula or ·, such as sodium metasilicate , sodium orthosilicate , and sodium pyrosilicate . The anions are often polymeric. These compounds are generally colorless transparent ...

with

calcium hydroxide Calcium hydroxide (traditionally called slaked lime) is an inorganic compound with the chemical formula Ca( OH)2. It is a colorless crystal or white powder and is produced when quicklime (calcium oxide) is mixed or slaked with water. It has m ...

, . However, impure dolomitic aggregates also often contain clay impurities, and small amounts of pyrite () and

organic matter Organic matter, organic material, or natural organic matter refers to the large source of carbon-based compounds found within natural and engineered, terrestrial, and aquatic environments. It is matter composed of organic compounds that have c ...

. The alkali-carbonate reaction could therefore also simply hide an alkali-silica or an alkali-silicate reaction. Anyway a chemical coupling between ACR and ASR cannot be ruled out.

External links

Cement.org , Alkali-aggregate reaction

References

Building materials Carbonates Catalysis Cement Chemical reactions Concrete Fracture mechanics Magnesium compounds Materials degradation Mechanical failure modes Patterns Pavements Silicates {{reaction-stub

See also

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References