Natural gypsum as a retarding agent

The natural gypsum we extract is an excellent set retarder and is especially used in cement manufacturing. Its unique qualities are the naturally occurring blend of gypsum and anhydrite as well as its purity. anhydrite rock and the absence of pollutants: 

To make a cement mixture workable, cement clinker is ground together with natural gypsum and anhydrite. When water is added, the gypsum-anhydrite blend prevents the cement from rapidly forming tricalcium aluminate hydrate and flash setting. It does this by creating a thin coat of ettringite on the cement clinker grains, which prevents the cement mixture from setting too quickly – instead of setting within ten minutes, it stays workable for over two hours.  

The key factors here are the proporGon of gypsum and anhydrite and their binding properties. The gypsum is partly dehydrated during clinker grinding, turning into a hemihydrate that is especially important early in the setting process. The anhydrite starts dissolving later and is therefore more important in the final stages. 

However, grinding the clinker and set retarder too finely is not recommended, because that would lead to a larger overall particle surface area and a stronger reaction, as well as higher energy consumption and costs. 

The gypsum and anhydrite blend inherent to natural gypsum is an ideal solution as it regulates setting perfectly every time, in all situations. The same cannot be said of synthethic gypsum. During setting, a thin coat of ettringite usually forms around cement clinker particles. Contaminants can prevent this from happening, leading to issues with the texture of the cement mixture and its workability. 

Contaminants can impair this and, in addition to material problems, can also make the cement more difficult to process.