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?
🧪 Understanding Sulphate Attack
Sulphate attack is a chemical reaction that occurs when concrete is exposed to water containing dissolved sulphates (commonly found in some soils, groundwater, and industrial effluents like sewage). The sulphates react with compounds in the hydrated cement paste, particularly Calcium Hydroxide and Tricalcium Aluminate (C₃A). This reaction forms expansive products like ettringite, which grow within the concrete, causing internal pressure, cracking, spalling, and eventually, a complete loss of strength. Therefore, in such environments, a special type of cement is required.
🔬 Detailed Analysis of the Options
B. Sulphate-resisting cement (SRC)
This is the correct answer. As its name implies, this cement is specifically manufactured to resist sulphate attack. Its key feature is a very low percentage of Tricalcium Aluminate (C₃A), typically kept below 5%. Since C₃A is the primary compound that reacts with sulphates to cause expansion, limiting its quantity makes the cement inherently resistant. Some forms of SRC are also made by grinding ordinary Portland cement with a high percentage (around 70%) of blast furnace slag. SRC is the preferred choice for structures like sewage treatment plants, canal linings, foundations in sulphate-rich soils, and marine structures.
A. Rapid-hardening cement (RHC)
This is incorrect. RHC is designed for faster strength gain, which is achieved by grinding the cement finer than OPC and slightly increasing the C₃A content. A higher C₃A content makes it *more* susceptible to sulphate attack, making it completely unsuitable for the application described.
C. Extra rapid-hardening cement
This is incorrect. This cement is essentially RHC with an accelerator (calcium chloride, CaClâ‚‚) added during manufacturing. Not only does it have the same vulnerability to sulphates as RHC, but the presence of chlorides can also accelerate the corrosion of any steel reinforcement in the concrete, making it a very poor choice for this environment.
D. Portland slag cement (PSC)
This is an interesting but less precise answer. PSC is made by blending ground granulated blast-furnace slag (GGBS) with OPC clinker. It exhibits good resistance to sulphate attack, but for a different reason than SRC. The slag reacts with the calcium hydroxide (a byproduct of cement hydration), reducing the amount of "fuel" available for the sulphate reaction. While PSC is a good option for sulphate environments, "Sulphate-resisting cement" is the cement *specifically manufactured* for this primary purpose by controlling the C₃A content, making it the most direct and correct answer.
📊 Summary: Cement Suitability in Sulphate Environments
| Cement Type | Key Feature | Suitability for Sulphate Attack |
|---|---|---|
| Sulphate-resisting (SRC) | Low C₃A content (<5%) | Excellent (Specifically designed for it) |
| Rapid-hardening (RHC) | High fineness, higher C₃A | Poor (More vulnerable) |
| Portland Slag (PSC) | Contains GGBS, consumes Ca(OH)â‚‚ | Good to Very Good |
| Ordinary Portland (OPC) | Standard, moderate C₃A | Poor to Moderate (Vulnerable) |
💡 Study Tips
- Match the Name: The most straightforward tip is to match the problem (sulphate attack) with the solution named for it (Sulphate-resisting cement).
- C₃A is the Culprit: Remember that Tricalcium Aluminate (C₃A) is the bad guy in sulphate attack. Any cement with low C₃A will be resistant.
- Speed vs. Durability: Understand the trade-off. Cements designed for speed (Rapid Hardening) often sacrifice chemical resistance because the same compound (C₃A) that contributes to early strength is also vulnerable to chemical attack.