Identify the component content which has to be reduced at the time of clinker grinding in the preparation of Quick Setting Cement :
š¬ Understanding Quick Setting Cement
Quick Setting Cement is a special type of cement designed to set very fast. Its key characteristic is a very short initial setting time (around 5 minutes) and final setting time (around 30 minutes). This property is essential for specific applications, such as underwater concreting or construction in running water, where the mortar or concrete needs to stiffen quickly before it can be washed away. It's important not to confuse this with Rapid Hardening Cement, which gains strength quickly but has a normal setting time.
š Detailed Analysis of the Components
(b) Gypsum
This is the correct answer. In the manufacturing of Ordinary Portland Cement, gypsum (calcium sulphate) is added in small amounts (2-3%) during the final grinding of the clinker. Gypsum acts as a retarder; its primary job is to slow down the hydration of Tricalcium Aluminate (CāA) to prevent an instantaneous "flash set". To create a cement that sets quickly, this retarding effect is undesirable. Therefore, the amount of gypsum added is significantly reduced. This allows the CāA to react rapidly, leading to the desired quick setting action.
(a) Alumina
This is incorrect. Alumina is present in cement primarily as Tricalcium Aluminate (CāA), the compound responsible for the initial fast reaction. To promote a quick set, the amount of alumina might be slightly increased, not reduced. Reducing it would slow down the setting.
(c) Magnesium
This is incorrect. Magnesium oxide is a minor component in cement whose content is limited to prevent unsoundness (long-term expansion). It does not play a primary role in controlling the setting time.
(d) Sulphate
This is incorrect, but related. Gypsum is calcium sulphate. So, by reducing gypsum, the sulphate content is also reduced. However, "Gypsum" is the specific material that is added or reduced during manufacturing, making it the more precise and correct answer.
š Summary: Quick Setting vs. Ordinary Cement
| Property | Ordinary Portland Cement (OPC) | Quick Setting Cement (QSC) |
|---|---|---|
| Gypsum Content | Normal (~2-3%) | Reduced Amount |
| Initial Setting Time | ā„ 30 minutes | ā„ 5 minutes |
| Final Setting Time | ā¤ 600 minutes | ā¤ 30 minutes |
| Primary Use | General construction | Underwater or running water concreting |
š” Study Tips
- Gypsum is the Brake Pedal: Think of gypsum as the brakes for the setting process. To make the cement "stop" (set) faster, you need to use less brake. Therefore, reduce gypsum for Quick Setting Cement.
- Quick Setting ā Rapid Hardening: This is a classic point of confusion. Quick Setting = fast stiffening. Rapid Hardening = fast strength gain. They are two different properties achieved by different means.
- Link to Application: Remember why QSC is needed: for concreting in running water. The cement must set before the water can wash it away. This helps solidify the purpose of its fast setting time.