Question

Write a note on decay of concrete. 

Answer 1

The cement concrete although mechanically strong I but due to the presence of free lime it become susceptible to the attack of acidic water. Alkaline water do not have marked effect on concrete strength. Also lime is more soluble in soft water than hard water and hence deterioration of concrete in contact with soft water is more. Presence of sulphates cause maximum damage because it reacts with tricalcium aluminate to form sulpha aluminates which occupies more volume and hence undergo expansion making the cement structure weak.
PROTECTION OF CONCRETE 
• By giving a coating of bituminous material. This prevents direct contact between concrete and water. This is a very inexpensive method.
• By coating the surface with silicon fluoride in a soluble form together with oxides of Zn , Mg or AI. the precipitate of calcium fluoride so-formed in the capillaries prevents dissolution of lime.