WHAT IS ALKALI AGGREGATE REACTION AND MEASURES TO CONTROL IT?

ALKALI AGGREGATE REACTION

Aggregates gives the strength to concrete but some of the aggregates contains reactive type of silica, which reacts with alkalies present in cement. For example, sodium oxide and potassium oxide. As a result, the alkali silicate gels of unlimited swelling type are formed. This reaction is known as alkali aggregate reaction.

The types of rocks which contain reactive constituents include traps, andesite, rhyolites, siliceous limestones and certain types of sandstones. The reactive constituents may be in the form of opals, cherts, volcanic glass, zeolite, chalcedony etc.

The alkali silica gel formed by alkali aggregate reaction is confined by the surrounding cement paste and internal pressure is developed leading to expansion, cracking and disruption of cement paste. This expansion appears to be due to hydraulic pressure generated through osmosis, but can also be due to the swelling pressure of the still solid products of alkali silica reaction. This indicates that the swelling of the hard aggregates is most harmful to concrete. 

The reactivity of aggregate depends upon its particle size and porosity as these influence the area over which the reaction take place.

Factors promoting the alkali aggregate reaction :
  1. Reactive type of aggregate.
  2. High alkali content in cement.
  3. Optimum temperature.
  4. Availability of moisture.
  5. Fineness of cement particles.
  • Certain types of rocks like traps, andesite, rhyolites, siliceous lime stones and certain types of sand stones contains reactive constituent.
  • The high alkali content in cement is also an important factor contributing to the alkali aggregate reaction. To prevent the deterioration of concrete due to alkali aggregate reaction alkali content in cement should not exceed 0.6 percent.
  • The ideal temperature for the promotion of alkali aggregate reaction is in the range of 10 degree to 38 degree C. If the temperature is below 10 degree C or more than 38 degree C, It may not provide an ideal situation for the alkali aggregate reaction.
  • The alkali aggregate reaction in concrete gets promoted in the presence of moisture and the lack of water greatly reduces this kind of deterioration. The deterioration will be be  more on the surface of concrete and interior of mass concrete will not deteriorate due to alkali aggregate reaction. 

Measures to control alkali aggregate reaction :
  1. Selection of non reactive aggregates.
  2. By restricting alkali content in cement below 0.6 %.
  3. By controlling temperature.
  4. By controlling moisture condition.
  5. By the use of corrective admixtures such as pozzolanas.
  6. By controlling the void space in concrete.
  7. By not using too fine ground cement.

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