Alkali Aggregate Reaction

ASR is an example of a more general problem known as alkaline agglutination reaction [AAR]. ASR is a chemical process in which a base (usually mainly from cement) binds to certain silica in aggregate in the presence of moisture. This reaction absorbs water and swells to produce basic silica gel which causes cracking and fracture of the concrete. In order to cause a destructive reaction, it is necessary to make the following substances sufficient. High alkali cement reactive aggregate [for example

Certain sand and aggregates such as alkali agglutination reaction opal, vermiculite and vermiculite, or volcanic rocks with high silica content react with calcium hydroxide, sodium hydroxide and potassium hydroxide in Portland cement concrete . Although it has been observed and studied for more than 50 years, these reactions are not clearly defined yet and are rarely understood. Some concrete containing alkaline reactive aggregate shows direct evidence of destructive expansion and deterioration. Other concrete may not be disturbed for many years. Petrological examination of reactive concrete showed formation of gel around reactive aggregate. The gel expands extensively in the presence of water or water vapor (80% to 85% relative humidity is all the required water), causing tensile cracks around the aggregate and causing the concrete to expand. If not restricted, the expansion of the concrete first appears as a pattern crack on the surface.

It is believed that the ASR in concrete is the formation of a foamed gel caused by the reaction between aggregate containing silica in a specific form and alkali from cement slurry. When forming a gel in concrete, residual stress is generated. These residual stresses are the main cause of breakage of the concrete hydrated cementitious substrate. Traditionally, fly ash, silica fume and lithium have been used to alleviate the swelling properties of ASR gels. Current mitigation techniques fail in the field and in the laboratory, and as a result it has been found that the expected lifetime of concrete structures will be shortened. Another ASR gel relaxation method is needed. The use of nanosilica in concrete strengthens the hydrated cement matrix of the concrete composite to isolate the formation of ASR gels and reduces the catastrophic stress caused by the expansion of ASR gels.