Gypsum functions primarily as a retarder, specifically a set regulator, in cement hydration, although it also plays a role in accelerating early strength development.
Gypsum's Primary Role: Set Regulator (Retarder)
As stated in the reference, gypsum is popularly identified as a set regulator or retarder. Its main function in cement is to control the setting time. Cement without gypsum can undergo a rapid, uncontrolled reaction known as "flash set," where the cement hardens almost instantly upon contact with water, making it impossible to work with.
Gypsum prevents this by:
- Changing Calcium Aluminate Hydration: Gypsum interacts with the calcium aluminate phases in cement. This interaction modifies their hydration process, which directly prevents the rapid setting reaction.
- Controlling Setting Time: By regulating the initial hydration of calcium aluminates, gypsum ensures that the cement sets gradually over a usable timeframe, allowing for proper mixing, placing, and finishing of concrete or mortar.
This role of slowing down the initial setting process is why gypsum is widely known and used as a retarder.
Gypsum's Contribution to Early Strength
While primarily controlling the initial set, the reference also notes that gypsum "contributes for strength acceleration in the early stages of hydration." This highlights a dual nature where, beyond regulating the initial setting reactions, gypsum's presence also aids the development of early strength in the hardening cement paste.
- Facilitating C-S-H Formation: While its main interaction is with aluminates, gypsum's presence influences the overall hydration process, indirectly promoting the formation of calcium silicate hydrate (C-S-H), the main binder responsible for strength in cement, leading to quicker strength gain in the initial hours or days.
Summary of Gypsum's Effects
| Effect | Description | Primary Role |
|---|---|---|
| Set Regulation | Controls the initial hardening time of cement, preventing flash set. | Retarder / Set Regulator |
| Early Strength Gain | Contributes to faster development of mechanical strength in early stages. | Accelerator (for strength development) |
In conclusion, while popularly identified as a retarder due to its critical function in controlling cement setting time by changing calcium aluminate hydration, gypsum also uniquely contributes to strength acceleration during the early phases of hydration, making it an essential set regulator in cement.
