Assertion (A): Pozzolana is added to cement to increase early strength. Reason (R): It offers greater resistance to the permeability of water.
- ABoth A & R are true and R is the correct explanation of A
- BBoth A & R are true but R is not the correct explanation of A
- CA is true but R is false
- DA is false but R is trueCorrect
Solution
Concept
Effects of Pozzolana in Cement
Formula
No
Step-by-step solution
1. Evaluate Assertion (A): Adding pozzolana reduces early strength because the pozzolanic reaction gains strength slowly over time. Thus, Assertion (A) is false. 2. Evaluate Reason (R): Pozzolanic reactions fill internal pores, making the structure denser and reducing water permeability. Thus, Reason (R) is true. 3. Conclusion: Since A is false and R is true, the correct option is D.
Explanation
Option D is correct because Assertion (A) is false and Reason (R) is true. Assertion (A) is false because adding pozzolana to cement actually lowers its early strength development rather than increasing it, as the pozzolanic reaction takes time to gain strength. Reason (R) is true because the pozzolanic reaction fills internal capillary pores, making the concrete denser and significantly increasing its resistance to water permeability.
Shortcut
Remember "Pozzolana = Slow & Tight": - Slow: Pozzolana reduces early strength gain (takes time to react), making Assertion (A) False. - Tight: Pozzolana fills pores to make concrete dense and watertight (low permeability), making Reason (R) True.
Common mistake
Students often incorrectly assume that adding pozzolana increases early strength along with long-term strength, leading them to believe Assertion (A) is true and choose Option A or B instead of recognizing that pozzolana actually lowers early strength.
Correct answer: D) A is false but R is true