Gypsum is used in cement to control the hydration kinetics as well as in the set time, compressive strength development and durability properties of concrete. The optimum SO3 content in cement (therefore gypsum addition) vary depending on the quality of the clinker and the quality / type of calcium sulfate. Several types and origin are typically used:
- Natural Gypsum
Main product used by cement producers. Natural gypsums are of variable purity, but easy to handle in transport.
Typically, of lower cost than natural (lesser extraction, crushing, grinding costs)
- FGD Gypsum : a by-product of desulfurization of flue gas from the stacks of fossil-fueled power plants; available in large amount, high purity can be found (> 95%), typically higher than mined gypsum. FGD is a lower cost product, but with a few constraints (fine powder, humidity up to 25%, difficult to transport, some presence of impurities: soluble ions, heavy metals, organics, quartz, clays)
- Phospho Gypsum: a by-product from phosphoric acid production, available in large amount. It is a lower cost product, with high purity, > 95%, with few constraints as well: humidity up to 25%, transport constraints, sometimes residual acidity, presence of impurities, radioactivity