Gypsum is added to cement in small quantity to:
🧪 The Chemistry of "Flash Set"
During cement manufacturing, a compound called Tricalcium Aluminate (C₃A) is formed. If gypsum were not added, this C₃A would react almost instantly with water upon mixing. This phenomenon is called a "flash set" or "quick set," where the cement stiffens within minutes, leaving no time for transportation, placing, or compaction.
Gypsum (Calcium Sulfate Dihydrate - CaSO₄·2H₂O) is introduced during the final grinding of cement clinker specifically to prevent this.
📝 Detailed Analysis of the Options
(a) control final setting time
This is incorrect. While gypsum does influence the overall hydration process, its primary and most critical role is to delay the *initial* set. The final setting time is less directly controlled by the addition of gypsum.
(b) make cement hydrophobic
Hydrophobic (water-repellent) cement is made by adding substances like oleic acid or stearic acid during grinding. Gypsum does not impart this property.
(c) control initial setting time
This is the correct answer. Gypsum acts as a retarder. It reacts with the C₃A and water to form a protective layer of ettringite (a calcium sulfoaluminate) around the C₃A particles. This layer prevents the C₃A from hydrating too quickly, thus slowing down the initial setting process and increasing the initial setting time from a few minutes to a workable period (e.g., >30 minutes).
(d) give colour to the cement
The characteristic grey colour of Portland cement comes from iron oxides and other compounds in the clinker. Gypsum is a white/translucent mineral and does not contribute significantly to the color.
📊 Effect of Gypsum on Cement
| Property | Cement WITHOUT Gypsum | Cement WITH Gypsum (2-3%) |
|---|---|---|
| Initial Setting Time | Very short (few minutes) - "Flash Set" | Controlled and delayed (> 30 minutes) |
| Workability | Practically zero | Sufficient time for mixing and placing |
💡 Study Tips
- Gypsum = Retarder: The single most important function of gypsum in cement is to act as a retarder for the initial set.
- The Enemy is C₃A: Remember that gypsum's job is to fight the rapid reaction of Tricalcium Aluminate (C₃A).
- The Result is Time: The purpose of adding gypsum is to give engineers and workers enough time to handle the concrete before it hardens.