The construction industry uses both the natural and the synthetic forms of gypsum in Portland cement manufacturing, according to the University of Wisconsin. Although the amount of gypsum used in the mix represents a small percentage of the volume of all ingredients, gypsum has important functions.
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The University of Washington calls Portland cement "the most widely used building material in the world." Production averages about 1.72 billion tons annually. Portland cement manufacturing involves adding either natural or FGD gypsum. Gypsum occurs naturally as a mineral or as a soft sedimentary rock. Synthetic gypsum comes from the byproducts of flue gas desulfurization (FGD) in coal powered plants.
Gypsum comprises about 3.5 per cent of the weight of the dry materials in Portland cement. The other ingredients in the dry mix are calcium, silica, alumina and iron. Those dry materials provide the chemicals---tricalcium silicate, dicalcium silicate, tricalcium aluminate, and tetracalcium aluminoferrite---that form reactions with water.
Mixing dry Portland cement with water sets off a chain of chemical reactions. Tricalcium aluminate hydrates and hardens quickly. Adding gypsum to the mix prevents the concrete from hardening almost immediately after adding water.
Control Setting Time
Adding gypsum helps to control the amount of time it takes portland cement to cure, also known as the setting time for cement. According to Washington University, gypsum helps to affect the setting, or curing time of cement, to ensure it does not set too soon or take too long.
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- University of Washington: 4 Portland Cement
- Georgia Institute of Technology, School of Civil Engineering: Portland Cement Hydration, by Dr. Kimberly Kurtis
- University of Wisconsin Madison: FGD Gypsum Utilization: Wallboard, Cement Manufacturing and Agricultural Applications, by Bruce Ramme
- Mineral Information Institute: Gypsum
- University of Kentucky, Center for Applied Energy Research: What are Coal Combustion By-Products (CCBs)? Synthetic Gypsum