Which of the following statement(s) is/are true regarding Portland Pozzolana Cement (PPC)? (i) It has better resistance to sulphate attack than Ordinary Portland Cement (OPC). (ii) It generates more heat of hydration than OPC.
- AOnly (i)Correct
- BOnly (ii)
- CBoth (i) and (ii)
- DNeither (i) nor (ii)
Solution
Concept
Portland Pozzolana Cement hydration and durability
Formula
Pozzolan + Ca(OH)2 + H2O -> C-S-H gel
Step-by-step solution
1. In PPC, pozzolanic materials react with calcium hydroxide [Ca(OH)2] to produce additional stable C-S-H gel. 2. This reaction consumes vulnerable free lime, lowers matrix permeability, and dilutes C3A content, imparting superior sulphate resistance; statement (i) is true. 3. The pozzolanic reaction is slower than standard clinker hydration, generating lower rate and total heat of hydration than OPC; statement (ii) is false. 4. Therefore, only statement (i) is true. Answer: A
Explanation
Portland Pozzolana Cement (PPC) reacts with liberated calcium hydroxide from cement hydration to form secondary calcium silicate hydrate (C-S-H) gel, significantly reducing free lime, matrix permeability, and C3A content to enhance sulphate resistance. Furthermore, the secondary pozzolanic reaction proceeds more slowly than clinker hydration, producing a lower rate and total heat of hydration compared to Ordinary Portland Cement (OPC).
Why the other options are wrong
B) Statement (ii) is incorrect because PPC generates less heat of hydration than OPC, making it suitable for mass concreting. C) Statement (ii) is false. D) Statement (i) is undeniably true as pozzolanic action mitigates sulphate attack.
Shortcut
PPC = High sulphate resistance + Low heat of hydration (ideal for mass concreting and marine works).
Common mistake
Assuming PPC generates more heat because it forms additional C-S-H gel.
Correct answer: A) Only (i)