Alkali-Aggregate Reaction is called as concrete cancer. The alkali-aggregate reaction is an important phenomenon that influences the strength of the concrete structure, how big or small the structure is.
Fig.1.Bridge girders and piers after alkali - aggregate reaction

Some of the aggregates are not completely inert. Some contain reactive silica which will react with the alkalies present in the cement ( i.e. Sodium Oxide and Potassium Oxide). The failures of many structures like pavements, piers and sea wall may be due to the alkali-aggregate reaction.

Alkali -Aggregate reaction 

The alkali-aggregate reaction is the reaction between the alkalies present in the cement and the reactive siliceous minerals in the aggregate. During the alkali-aggregate reaction, alkali silicate gels of unlimited swelling type are formed. Swelling of alkali silicate gels results in the disruption of concrete with the spreading of pattern cracks and eventual failure of the concrete structures.

Factors That Promote Alkali-Aggregate Reaction

Reactive Type of Aggregate: The potential reactivity of aggregates causes the alkali-aggregate reaction. The aggregate will be harmful if it expands more than 0.05% after 3 months or more than 0.1% after 6 months.

High Alkali Content in Cement: The content of alkali in cement should be less than 0.6% by its weight ( IS Specification). If the content exceeds this IS recommendation, there is a possibility of alkali - aggregate reaction. Generally, Indian cement does not contain high alkali as in US or UK.

Availability of Moisture: The alkali-aggregate will progress in the presence of water. The deterioration due to the alkali-aggregate reaction will no occur in the interior of the mass concrete. The deterioration will be more seen on the surface. Therefore by the application of waterproofing, agents to the surface of the concrete, effects of the alkali-aggregate reaction can be reduced.

Temperature Condition: The ideal temperature for the promotion of this reaction is 10 to 30-degree celsius. If the temperature condition is more or less than the above value, it may not provide an ideal situation for the alkali-aggregate reaction.