Question

what is Alkali Silicate Reaction and what is the holistic approach of Alkali Silicate Reaction?

Answer 1

Alkali silicate reaction (ASR) is a very important reaction in concrete technology.It is also called concrete cancer.Because it is like the disease cancer,it occurs at one point and then it get distribute to the whole structure and just decrease the life of the whole concrete structure.This reaction is basically coming from the two basic components in the concrete one is the cement and the other one is aggregates.Alkali content in the cement and in aggregates there is some amorphous silica content.These alkali and silica reacts in the presence of water and form the reaction called alkali silicate reaction.As a result of this reaction a gelatinous product is formed which imbbes the pore fluid and expands.This expansion increases the internal stresses in concrete and there by lead to the formation of cracks eventually leads to the deterioration of concrete structures.

  ${\color{Red} Silica + Alkali \overset{Water{\color{Red} }}{\rightarrow}Alkali-Silica Gel}$

  ${\color{Red} SiO_{2} + 2NaOH \overset{H^{_{2}}O}{\rightarrow} NaSiO_{3}.2H_{2}O}$

Here in the reaction various types of silica present in the aggregate reacts with sodium hydroxide present in the pore solution of concrete to form a volumetrically unstable alkali silicate gel.

• Effects of alkali silicate reaction

* Decrease the compressive and tensile strength of the concrete structures

* Creates expansion of the concrete which leads to the formation of cracks.

* Reduces the elastic modulus of the concrete

* Reduces the fatigue strength of the concrete

* Decreases durability of the concrete structure.

• Methods to avoid alkali silicate reaction

* Use non reactive aggregate for preparing concrete.

Aggregates should be tested in the lab before using it in concrete to determine whether it is reactive or not.River gravel is often the most adoptable aggregates in this case since it contain only small amounts of reactive rocks.

* Maintain low alkali content in the cement

This is not a much viable solution toavoid ASR because if a highly reactive aggregate is used in the concrete eventhough a very small amount of alkali is there it will initiate alkali aggregate reaction.

* Maintaining low water cement ratio

By reducing water cemnt ratio the hardened concrete become less permeable thus a less amount of water is available for the ASR gel to imbibe and expands.It also limits the mobility of alkali and water around the concrete .

* Using special admixtures to avoid expansion of the gel formed as result of alkali silicate reaction.This admixtures are very costly.So it is not mostly preferred.