Which of the following is NOT a field of application of sulphate-resisting cement?
🧪 Understanding Sulphate-Resisting Cement (SRC)
Sulphate-Resisting Cement is a special type of Portland cement manufactured to be durable in environments with high sulphate concentrations. Sulphate attack occurs when sulphates from soil or water react with Tricalcium Aluminate (C₃A) in the cement paste, forming expansive compounds that cause the concrete to crack and disintegrate. To counter this, SRC is made with a very low C₃A content (less than 5%), making it highly resistant to this form of chemical attack.
🔬 Detailed Analysis of the Options
The question asks where SRC is NOT the preferred choice. We need to identify the application where sulphate resistance is not the primary concern.
D. Dams
This is the correct answer. The primary concern in mass concrete structures like dams is the heat of hydration. The large volume of concrete makes it difficult for the heat generated during hydration to dissipate, leading to a high internal temperature. This causes thermal expansion, followed by contraction upon cooling, which can lead to severe cracking. Therefore, the ideal cement for dams is Low Heat Cement (LHC), which is specifically designed to hydrate slowly and generate minimal heat. While SRC offers chemical resistance, it is not primarily a low-heat cement, making it unsuitable for this application.
A. Sewage treatment works
This is a typical application for SRC. Sewage and industrial wastewater are often rich in sulphates, which are highly aggressive to normal concrete. Using SRC is essential to ensure the long-term durability of the tanks, pipes, and structures in a treatment plant.
B. Foundation and basement
This is a correct application. Foundations and basements are in direct contact with soil and groundwater. If the soil or water has a high sulphate content, using SRC is crucial to prevent the deterioration of these critical structural elements from below.
C. Marine structures
This is a correct application. Seawater contains a moderate concentration of sulphates. For structures like piers, jetties, and seawalls, SRC is used to combat the sulphate attack and improve the overall durability of the concrete in the harsh marine environment.
📊 Summary: Cement for Special Conditions
| Condition | Primary Problem | Preferred Cement |
|---|---|---|
| Sulphate-rich soil/water | Chemical Attack (Sulphates) | Sulphate-Resisting Cement (SRC) |
| Mass Concrete (e.g., Dams) | High Heat of Hydration | Low Heat Cement (LHC) |
| Emergency Repairs / Cold Weather | Slow Strength Gain | Rapid Hardening Cement (RHC) |
💡 Study Tips
- Match the Problem to the Solution: Sulphate problem → Sulphate-Resisting Cement. Heat problem → Low Heat Cement. Speed problem → Rapid Hardening Cement.
- Dams = Heat Problem: The defining issue for dams is their massive size, which traps heat. Always associate dams with the need for Low Heat Cement.
- SRC's Job: Remember the main purpose of SRC is to have very low C₃A content to fight chemical attack, not necessarily to control heat.