The chemical that increases the setting time of cement is Calcium Sulphate.
Calcium Sulphate (CaSO₄), commonly found in the form of Gypsum, is a crucial additive in the production of cement. Its primary function, as highlighted in the reference, is to control the hydration process of cement when water is added.
The Role of Calcium Sulphate (Gypsum)
When cement is mixed with water, several chemical reactions occur, leading to hardening. One of the main components in cement clinker is Tricalcium Aluminate (C₃A). C₃A reacts very rapidly with water, which could cause the cement paste to stiffen almost instantly in a phenomenon known as "flash set." This rapid setting would make it impossible to mix, transport, and place the concrete or mortar effectively.
To prevent this unwanted flash set and allow sufficient time for construction activities, a small percentage of Calcium Sulphate, typically in the form of Gypsum (CaSO₄·2H₂O), is interground with the cement clinker during manufacturing.
Key Function:
- Increases Initial Setting Time: By reacting with the C₃A component, Gypsum forms calcium sulfoaluminate hydrates. These hydrates precipitate on the surface of the C₃A particles, creating a barrier that slows down the rate at which C₃A reacts with water. This delay effectively increases the initial setting time of the cement, providing a workable period before the paste begins to harden significantly.
In summary, the addition of Calcium Sulphate as Gypsum is essential for controlling the setting properties of cement, ensuring it remains plastic long enough to be properly used in construction applications.
