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

concrete Concrete is a composite material composed of aggregate bound together with a fluid cement that cures to a solid over time. It is the second-most-used substance (after water), the most–widely used building material, and the most-manufactur ...

containing dolomite 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 the Arabic word , ) is a basic 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 soluble base has a pH greater than 7.0. The a ...

from the

cement A cement is a binder, a chemical substance used for construction that sets, hardens, and adheres to other materials to bind them together. Cement is seldom used on its own, but rather to bind sand and gravel ( aggregate) together. Cement mi ...

might react with the dolomite crystals present in the aggregate inducing the production of poorly

soluble In chemistry, solubility is the ability of a substance, the solute, to form a solution with another substance, the solvent. Insolubility is the opposite property, the inability of the solute to form such a solution. The extent of the solubi ...

brucite Brucite is the mineral form of magnesium hydroxide, with the chemical formula Magnesium, Mg(hydroxyl, OH)2. It is a common alteration product of periclase in marble; a low-temperature hydrothermal Vein (geology), vein mineral in metamorphosed li ...

, (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) 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 . It is a white solid ionic compound consisting of sodium cations and hydroxide anions . Sodium hydroxide is a highly corrosive base (chemistry), ...

(NaOH) after the reaction of soluble

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

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 tra ...

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 with water. Annually, approxim ...

, . However, impure dolomitic aggregates also often contain

clay Clay is a type of fine-grained natural soil material containing clay minerals (hydrous aluminium phyllosilicates, e.g. kaolinite, ). Most pure clay minerals are white or light-coloured, but natural clays show a variety of colours from impuriti ...

impurities, and small amounts of

pyrite The mineral pyrite ( ), or iron pyrite, also known as fool's gold, is an iron sulfide with the chemical formula Fe S2 (iron (II) disulfide). Pyrite is the most abundant sulfide mineral. Pyrite's metallic luster and pale brass-yellow hue ...

() and

organic matter Organic matter, organic material or natural organic matter is 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 come fro ...

. 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 Inorganic reactions Concrete Fracture mechanics Magnesium compounds Materials degradation Mechanical failure modes Pavements Silicates {{reaction-stub

See also

External links

References