Alkali dolomite reaction

Limestones generally do not contain sufficient reactive silica or silicates to cause expansion, and damaging alkali-carbonate reaction has rarely been reported. The reactions involving carbonate rocks can be either expansive or non-expansive and are more likely to occur when the limestone contains appreciable quantities of dolomite and clay minerals [8.1]. ASTM C586 [8.8] gives a test method for determining the potential alkali reactivity of carbonate rock aggregates. 

Some types of aggregate can react with Na, K and OH ions in the pore solution, giving rise to detrimental expansion. The principal reactions can take place with aggregate containing certain forms of amorphous or poorly crystalline sihca (alkah sihca reaction, ASR) and with dolomitic hmestone aggregate (alkali carbonate reaction). 

Certain carbonated rocks participate in reactions with alkalis that, in some instances, produce detrimental expansion and cracking. These detrimental alkali-carbonate reactions are usually associated with argillaceous dolomitic limestones that have a very fine-grained structure (ACI 2007). 

An alkali-carbonate reaction may takes place if an argillaceous (illitic) dolomitic hmestone is used as concrete aggregate. Here expansion takes place as the consequence of swelling of the illite eonstituent, following a de-dolomitization of the dolomitic... 

Beside the alkali-silica reaction described above, similar phenomena may occur in the case of reactive dolomites and limestones. The so-called alkali-carbonate reaction (ACR) is less frequent and not completely understood. When the effects of ACR are observed, two similar remedies are also necessary either to keep the content of alkalis in concrete as low as possible, or to decrease the percentage of deleterious aggregate in the concrete mix. 

Alkali-carbonate reaction. The alkali-carbonate reaction is different from the alkali-silica reaction in forming different products. Expansive dolomite contains more calcium carbonate than the ideal 50 % (mol) proportion and frequently also contains illite and chlorite clay minerals. 

In the presence of alkali metals, thermal sulfate reduction will result in precipitation of carbonate cements (mainly calcite and dolomite) or carbonate replacement of dissolving sulfates (gypsum/anhydrite) (2. The reaction of polysulfides with bicarbonate has been suggested as a cause of calcite precipitation. Further, transition and base metals present in formation waters during thermal sulfate reduction could lead to the deposition of disseminated or stratiform base... 

The decomposition of dolomite aggregate in the reaction with alkalis from cement, is also ranked among the cause of concrete destruction. On the other side, the... 

There are various hypotheses explaining this aggregate expansion mechanism. The most wide spread refer to the swelling of clay minerals and of osmotic pressure formation [141,142]. All these hypotheses agree that this phenomenon relates to the reaction of soluble alkalis from cement with the aggregate leading to the decomposition of dolomite ... 

Unlike the alkali-siliea/silicate reaction a pessimnm concentration of the expansive constituent does not exist, and the expansion increases continnonsly with increasing amounts of dolomitic limestone (Tong and Tang, 1995). The use of low-alkali Portland cement (see section 2.10) as binder may also effectively prevent this form of alkali-induced expansion. 

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