The primary purpose of adding gypsum to cement is to control and slow down the setting process, specifically preventing premature hardening.
When cement is mixed with water, it undergoes a chemical reaction called hydration, which leads to hardening. Without gypsum, this reaction can occur too quickly, causing the cement paste to stiffen rapidly – a phenomenon known as "flash setting". This rapid hardening leaves little or no time for the mixture to be transported, placed, and finished properly on a construction site.
Why is Slowing Down Setting Important?
Controlling the setting time is crucial for practical construction. The addition of gypsum interacts with the clinker components in cement, particularly tricalcium aluminate (C₃A), which reacts very rapidly with water. Gypsum forms a protective layer around these particles, slowing down their immediate reaction.
Based on the provided reference:
When cement is mixed with water, it becomes hard over a period of time. This is called setting of cement. Gypsum is often added to cement to prevent early hardening or “flash setting”, allowing a longer working time. Gypsum slows down the setting of cement so that cement is adequately hardened.
Key Benefits of Adding Gypsum
Adding the correct amount of gypsum provides several essential benefits:
- Prevents Flash Setting: This is the most critical function, stopping the cement from hardening almost instantly upon contact with water.
- Extends Working Time: By slowing down the initial set, gypsum allows a longer working time. This gives workers ample time to mix, transport, place, consolidate, and finish the concrete or mortar before it becomes unworkable.
- Ensures Adequate Hardening: While slowing the initial set, gypsum also helps manage the overall hydration process, contributing to the development of strength over time so that cement is adequately hardened.
- Improves Workability: A controlled setting time contributes to better workability and easier handling of the cement mixture.
In essence, gypsum acts as a crucial retarder in the cement hydration process, making cement a usable and versatile construction material. Without it, modern construction techniques would be significantly more challenging or even impossible due to the extremely rapid setting of plain cement clinker when mixed with water.
