Which of the following statement(s) is/are true? (i) Hydraulic lime contains a significant proportion of clay impurities (silicates and aluminates). (ii) During calcination, these form calcium silicates which give the lime the property of hydraulicity – the ability to set and harden underwater via chemical hydration, unlike fat lime which requires CO2 from the air to set.
- AOnly (i)
- BOnly (ii)
- CBoth (i) and (ii)Correct
- DNeither (i) nor (ii)
Solution
Concept
Hydraulicity and composition of lime
Formula
NONE
Step-by-step solution
1. Hydraulic lime is produced by calcining argillaceous limestone containing 10% to 30% clay (silicates and aluminates); statement (i) is true. 2. During burning, these minerals form calcium silicates and aluminates, enabling the lime to set and harden under water without carbon dioxide; statement (ii) is true. 3. In contrast, pure fat lime hardens solely by absorbing CO2 from the atmosphere. 4. Therefore, both statements (i) and (ii) are true. Answer: C
Explanation
Hydraulic lime is produced by calcining limestone that contains 10% to 30% clay minerals (silicates and aluminates). During burning, calcium silicates and aluminates are formed, endowing the lime with hydraulicity—the capacity to set and harden under water without requiring atmospheric carbon dioxide. Conversely, pure fat lime hardens exclusively through slow atmospheric carbonation.
Why the other options are wrong
A) Statement (ii) is also correct because calcium silicates enable underwater setting. B) Statement (i) is also correct because clay impurities provide the necessary silicates and aluminates. D) Both statements correctly describe hydraulic lime composition and setting mechanism.
Shortcut
Hydraulic lime = Clay impurities form silicates -> Sets underwater by hydration.
Common mistake
Assuming hydraulic lime sets by absorbing atmospheric CO2 like pure fat lime.
Correct answer: C) Both (i) and (ii)