In concrete, Calcium sulphate may act as:
⏱️ Controlling the Setting Time of Cement
One of the most critical aspects of working with concrete is controlling how quickly it hardens. If it sets too fast, there isn't enough time to transport, place, and finish it properly. This uncontrolled, rapid hardening is called a "flash set".
Flash set is primarily caused by the rapid hydration of Tricalcium Aluminate (C₃A), the most reactive compound in cement. To make concrete workable, this reaction must be slowed down or "retarded". This is where calcium sulphate plays its vital role.
📝 Detailed Analysis of the Options
(a) An accelerator
An accelerator does the opposite; it speeds up the rate of hydration and hardening. This is desirable in cold weather or when rapid strength gain is needed. Calcium chloride is a common accelerator, not calcium sulphate.
(c) An air entraining agent
These admixtures introduce microscopic air bubbles into the concrete. This greatly improves its resistance to freeze-thaw cycles. Resins and fatty acids are common air-entraining agents.
(d) An air detraining agent
Also known as de-foamers, these are used to remove excess air from the concrete mix, which can sometimes occur with certain admixtures or aggregates, to avoid a loss in strength.
(b) A retarder
This is the correct answer. Calcium sulphate, in the form of gypsum (CaSO₄·2H₂O), is added to the cement clinker during the final grinding stage (typically 2-3% by weight). When water is added to the cement, the gypsum quickly dissolves and reacts with the Tricalcium Aluminate (C₃A) to form a layer of insoluble calcium sulfoaluminate around the C₃A particles. This layer acts as a barrier, preventing the C₃A from hydrating too quickly and thus preventing a flash set. This retardation gives workers enough time to handle the concrete.
📊 Common Concrete Admixtures and Their Functions
| Admixture Type | Primary Function | Common Example |
|---|---|---|
| Retarder | Slows down setting time | Gypsum (Calcium Sulphate), Sugar |
| Accelerator | Speeds up setting time | Calcium Chloride |
| Air Entraining Agent | Improves freeze-thaw resistance | Vinsol Resin |
| Superplasticizer | Greatly increases workability/flow | Sulphonated Melamine Formaldehyde |
💡 Study Tips
- Gypsum is the Guardian: Think of gypsum (calcium sulphate) as the "guardian" that protects the concrete from setting too early by controlling the hyper-reactive C₃A.
- Retarder = Relax: A retarder gives the concrete more time to "relax" before it hardens, making it easier to work with.
- Not Just an Admixture: Remember that gypsum is not an optional admixture added on-site; it is an essential component inter-ground with the cement at the factory. Without it, Portland cement would be practically unusable.