Addition of gypsum to cement:

Gypsum (calcium sulfate dihydrate) is added to Portland cement during the final grinding process to act as a setting time regulator [3]. Without gypsum, cement clinker would react immediately with water, leading to a 'flash set' that makes the mixture unworkable [2]. The chemical mechanism involves gypsum reacting with tricalcium aluminate (C3A) to form ettringite, which creates a protective film around cement particles. This film hinders the rapid hydration of C3A, thereby delaying the initial setting and increasing the setting time of the cement [2]. This retardation allows for sufficient time for mixing, transporting, and placing the concrete in construction projects [2]. While gypsum is naturally white or opaque, its primary functional role in the cement industry is specifically as a retarder to prolong plasticity [1].

Sources

1. [3] https://www.science.gov/topicpages/g/gypsum+cement
2. [2] https://nora.nerc.ac.uk/id/eprint/539924/1/B02695.pdf
3. [1] https://www.sciencedirect.com/topics/engineering/natural-gypsum