Consider the following statements: 1.Slaking takes place on adding water in lime. 2.Hardening takes place on adding water in cement. Which of the above statements is/are correct?
- A1 only
- B2 only
- CBoth 1 and 2Correct
- DNeither 1 nor 2
Solution
Concept
Quicklime slakes with water. Cement hydrates with water, then sets and hardens.
Formula
CaO + H2O → Ca(OH)2 + heat
Step-by-step solution
1. Statement 1: Quicklime (CaO) reacts with water to form slaked lime Ca(OH)2. Heat is given out and the lumps break into powder. This is slaking, so statement 1 is correct. 2. Statement 2: Cement compounds (C3S, C2S etc.) react with water (hydration). This forms C-S-H gel, and the paste sets and then hardens. So statement 2 is correct. 3. Both statements are correct. Answer is (C).
Explanation
When water is added to quicklime, it swells, gives heat and breaks into powder. This is called slaking. When water is added to cement, hydration starts and the paste sets and slowly gains strength, which is hardening.
Why the other options are wrong
A: Statement 2 is also correct. B: Statement 1 is also correct. D: Both statements are correct.
Shortcut
Lime + water = slaking; cement + water = hydration, setting and hardening.
Common mistake
Saying hardening is only due to drying. Cement hardens by hydration, not by drying.
Correct answer: C) Both 1 and 2