The most commonly used retarder in cement is natural gypsum.
Understanding Cement Set Retarders
Cement hydration, the chemical reaction that makes cement harden, can sometimes proceed too quickly. This rapid setting can make the cement mix difficult to handle, transport, and place properly before it becomes stiff. To control this process and extend the working time, chemical admixtures known as retarders are added.
The Primary Retarder: Natural Gypsum
According to industry sources, the most common material used as a cement setting retarder is natural gypsum.
What is Gypsum?
Natural gypsum is primarily composed of calcium sulfate dihydrate (CaSO₄ · 2H₂O). It is interground with the cement clinker during the manufacturing process.
Why is Gypsum Used?
Gypsum plays a crucial role in controlling the initial setting of cement. Without it, the tricalcium aluminate (C₃A) phase in cement would react very rapidly with water, leading to flash set, which is undesirable. Gypsum reacts with C₃A to form insoluble calcium sulfoaluminates, which deposit as a protective layer around the C₃A particles, slowing down their reaction and thus retarding the initial set of the cement.
The amount of gypsum needed in cement is not fixed; it varies based on several factors:
- C₃A Content: Higher amounts of C₃A in the clinker require more gypsum to prevent flash set.
- Alkalies: The presence of alkalies in the clinker can influence the reaction rates, potentially requiring adjustments in gypsum content.
- Fineness of Cement: Finer cement has a larger surface area, leading to faster initial reactions, which may necessitate more gypsum to maintain the desired setting time.
Factors Influencing Gypsum Requirement
| Factor | Effect on Gypsum Need | Explanation |
|---|---|---|
| High C₃A Content | Increases | More reactive phase needs more control. |
| High Alkali Content | Increases | Can accelerate early hydration reactions. |
| Increased Fineness | Increases | Larger surface area leads to faster reaction. |
Using the correct amount of gypsum is vital for producing cement with optimal handling and performance characteristics. Too little can lead to flash set, while too much can cause excessive retardation or expansion issues.
