The rapid hardening Portland cement is obtained by.............
Try it first: pick an option, then read the explanation.
Achieving Rapid Strength Gain
Rapid Hardening Cement (RHC) is designed to achieve high strength much earlier than Ordinary Portland Cement (OPC). Its 3-day strength is roughly equivalent to the 7-day strength of OPC. This is achieved through two main modifications during manufacturing:
- Chemical Composition: RHC has a higher proportion of Tricalcium Silicate (C₃S), the compound responsible for early strength gain.
- Physical Properties: The cement clinker is ground much finer than for OPC.
Detailed Analysis of the Options
(a) Grinding the clinker to a high degree of fineness
This is the correct answer. Grinding the cement more finely increases its specific surface area. A larger surface area allows more cement particles to be exposed to water simultaneously, which significantly speeds up the rate of hydration. This accelerated hydration leads directly to faster development of strength, which is the defining characteristic of RHC.
(b) Adding calcium sulphate to the mixture
Calcium sulphate (gypsum) is added to all Portland cements, not just RHC. Its purpose is to act as a retarder to control the initial setting time, which is the opposite of accelerating hardening.
(c) Adding gypsum after grinding
This is procedurally incorrect. Gypsum must be inter-ground with the clinker to be effective. Adding it after grinding would not distribute it properly.
(d) Burning the mixture at a lower temperature
This is incorrect. To produce more C₃S (which gives early strength), the clinker needs to be burned at a very high temperature (around 1450°C). A lower temperature would be counterproductive to creating a rapid hardening cement.
OPC vs. RHC: Key Manufacturing Differences
| Parameter | Ordinary Portland Cement (OPC) | Rapid Hardening Cement (RHC) |
|---|---|---|
| Fineness (Specific Surface) | Min. 2250 cm²/g | Min. 3250 cm²/g (Significantly higher) |
| C₃S Content | ~ 45% | ~ 55-60% (Higher) |
| Resulting Property | Standard rate of strength gain | High early strength gain |
Study Tips
- Finer = Faster: This is a key relationship in cement chemistry. Finer grinding leads to faster hydration and faster strength gain.
- RHC = Finer + More C₃S: Remember the two main reasons for RHC's properties: it's ground finer and has more Tricalcium Silicate.
- Don't Confuse with Setting: Reiterate that RHC has the same setting time as OPC. The "rapidity" refers only to the hardening (strength gain) phase.