Which of the following is NOT a field of application of rapid-hardening cement?
š Understanding Rapid-Hardening Cement (RHC)
Rapid-hardening cement is a type of Portland cement that, as the name suggests, gains strength more quickly than Ordinary Portland Cement (OPC). This is achieved primarily by grinding the cement clinker to a higher fineness and slightly increasing the proportion of Tricalcium Silicate (CāS). Its key characteristic is that its strength at 3 days is roughly equivalent to the 7-day strength of OPC. This makes it ideal for applications where speed is essential.
š¬ Detailed Analysis of the Options
The question asks where RHC is NOT used. This means we need to find the application where its primary properties (rapid strength gain and consequently, high heat of hydration) would be detrimental.
C. Massive dams
This is the correct answer. RHC is completely unsuitable for mass concrete works like dams. The rapid hydration process generates a large amount of heat. In a massive structure like a dam, this heat cannot dissipate easily, leading to a significant rise in the internal temperature of the concrete. This causes thermal expansion. As the concrete later cools and contracts, severe thermal cracks can form, compromising the structural integrity and water-tightness of the dam. For such structures, Low Heat Cement is used, which has a lower CāS content and higher CāS content to ensure a slow, controlled hydration with minimal heat generation.
A. Road repair works
This is a primary application of RHC. For road repairs, it is crucial to reopen the road to traffic as quickly as possible. The rapid strength gain of RHC allows the repaired patch to achieve sufficient strength in a short period, minimizing disruption.
B. Cold weather concreting
This is a suitable application. Low temperatures slow down the chemical reaction of cement hydration, delaying setting and strength gain. The high rate of heat generation from RHC helps to offset the cooling effect of the ambient temperature, protecting the concrete from frost damage and allowing it to gain strength at a more normal rate.
D. Pre-fabricated concrete production
This is a very common application. In a pre-fabrication yard, the goal is to produce concrete elements (like beams, panels, or pipes) quickly to reuse the moulds and increase production turnover. RHC allows the concrete to gain enough strength for early demoulding and handling, significantly speeding up the manufacturing cycle.
š Summary: RHC vs. Low Heat Cement Applications
| Property | Rapid-Hardening Cement (RHC) | Low Heat Cement (LHC) |
|---|---|---|
| Key Feature | Fast strength gain, high heat | Slow strength gain, low heat |
| Suitable For | Road repairs, cold weather, pre-fabrication | Mass concrete (dams, large foundations) |
| Unsuitable For | Mass concrete (dams) | Emergency repairs, cold weather work |
š” Study Tips
- Rapid = Fast & Hot: Associate Rapid-Hardening Cement with speed and heat. This makes it good for quick jobs but bad for big jobs.
- Massive = Slow & Cool: For massive structures like dams, you need the opposite: a slow, cool reaction to prevent thermal cracking. This is the job of Low Heat Cement.
- Think about the Problem: For each application, ask "Is speed the most important factor?" If yes, RHC is a good choice. If "controlling heat" is the most important factor, RHC is a bad choice.