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Reaction alkali aggregate - The silent destroying factor


In many cases, the alkali-Aggregate Reaction (Alkali-Aggregate Reaction) is very slow, indistinct making it difficult to control and detect. But buildings in cold and dry climates like North America could pay the price for this phenomenon.

In most concrete, the aggregate is chemically inert, however some aggregates react with the alkali hydroxides in the concrete expanding and cracking for a long time or more possibly years. This alkali-aggregate reaction has two forms: alkali-silicate reaction (Alkali-Silica Reaction, ASR) and alkali-carbonate reaction (Alkali - Carbonate Reaction, ACR).
Alkaline reaction can cause structural cracking after many years.

The alkali-silicate reaction (ASR) is of greater concern because aggregates containing reactive silicon oxide are more common. In the ASR reaction, the aggregate containing some form of silicon oxide reacts with the alkali hydroxide in the concrete to form a baking gel because it absorbs water from the surrounding cement or the environment. These gels can exert enough pressure to damage the concrete.

A conspicuous point of the ASR reaction is a map of random cracks and in severe cases, near the joint and accompanied by concrete cracking. Cracks usually occur in areas where there is a frequent source of moisture such as near water levels at the jetty near the ground behind retaining walls, near seams and edges on pavements or in pillars or depending on the action of wick absorbing moisture. Petrographs can determine the ASR reaction.

Simulation images of Alkali-silicate Reaction (ASR).

The alkali-silicate reaction can be controlled using a number of additives with properties of puzolan (Supplementary Cementious Materials (SCM)). Using the correct proportions, silica fume, fly ash and blast furnace slag have been significantly reduced or eliminated due to the alkaline-silica reaction. In addition, lithium compounds have been used to reduce ASR reactions. 

Although reactive aggregates exist across large areas of North America, the alkaline-silica reaction poses a risk of expansion in concrete which is not as common as measures are taken to control it. It is also important to note that not all ASR gel reactions cause destruction.

The alkali-carbonate (ACR) reaction has been observed with certain dolomite rocks. Dolomitization, the breakdown of dolomites is often associated with expansion. This reaction and late crystallization of brucite (Mg (OH) 2) can cause significant expansion. The destruction caused by alkali-carbonate reactions is similar to that of the ASR reaction;2) can cause significant expansion. The destruction caused by the alkali-carbonate reaction is similar to that of the ASR reaction; 

However, ACR reactions are relatively rare as aggregates susceptible to this phenomenon are less common and generally unsuitable for use in concrete for other reasons. Aggregates susceptible to ACR reactions tend to have a characteristic cleavage (texture) orientation that can be determined by petrologists. Unlike the alkali-carbonate reaction, the use of pozzolanic additives (SCMs) does not prevent harmful expansion due to the ACR reaction. It is recommended that aggregates susceptible to ACR reactions are not used in concrete.

Whether it is alkali-carbonate reaction (ACR) or alkali-silicate reaction (ASR), they all need to be controlled very closely to ensure the quality of the building in accordance with its life expectancy. The subjective negligence in this matter especially for large, key projects can cost investors and society high prices.

Building materials.org


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