Which of the following type of cement is preferred for concrete making which is used in construction of sewage treatment plants located where soil is infested with sulphates?
- ARapid-hardening cement
- BSulphate-resisting cementCorrect
- CExtra rapid-hardening cement
- DPortland slag cement
Solution
Concept
Sulphate-resisting cement has low C₃A content, so it resists sulphate attack from soil and sewage.
Step-by-step solution
1. Sulphates in soil and sewage react with C₃A in hardened cement. 2. This reaction forms ettringite, which expands and cracks the concrete. 3. Sulphate-resisting cement has very low C₃A (about 5% or less). 4. So it is the preferred cement for sewage treatment plants in sulphate-bearing soil.
Explanation
Sulphate attack happens mainly because of C₃A in cement. Sulphate-resisting cement keeps C₃A low, so it gives better durability in sulphate-rich soil and sewage works.
Why the other options are wrong
(A) Rapid-hardening cement gives early strength only, with no special sulphate resistance. (C) Extra rapid-hardening cement contains CaCl₂ for fast setting and has no sulphate resistance. (D) Portland slag cement has some sulphate resistance, but sulphate-resisting cement is the specific and preferred choice.
Shortcut
Sulphate in soil or sewage → low C₃A → Sulphate-resisting cement.
Common mistake
Choosing Portland slag cement. It has only moderate resistance and is not the best answer here.
Correct answer: B) Sulphate-resisting cement