Question

Pozzolana used as an admixture in concrete has the following advantages:
1. It improves workability with a lesser amount of water.
2. It increases the heat of hydration and so lets the concrete set quickly
3. It increases the resistance of concrete to attack by salts and sulphates.
4. It leaches out calcium hydroxide.
Select the correct answer using the codes given below:

• A. 1, 2 and 3 only
• B. 1, 2 and 4 only
• C. 1, 3 and 4 only
• D. 2, 3 and 4 only

Answer 1

The correct option is C 1, 3 and 4 only

Pozzolana due to its ball hearing action increases the workability without increasing the amount of water but the amount of pozzolana should be in the appropriate amount otherwise it will make the paste sticky.
Pozzolana reacts slowly so initially heat of hydration is low and concrete: setting becomes slow.
Pozzolana $+Ca(OH{)}_2\to C-S-H.$ (low density). In the later stage more $C-S-H$ gel forms and fill the void so making the concrete durable and increasing the resistance against sulphate attack.
Pozzolana reacts with excess $Ca(OH{)}_2$ so $Ca(OH{)}_2$ amount reduces.