Role of Gypsum in Cement

 

Role of Gypsum in Cement

Gypsum is a soft sulfate mineral composed of calcium sulfatedihydrate, with the chemical formula CaSO4·2H2O. It is used for a variety of manufacturing, industrial and agriculture uses. The colour of gypsum is white, grey, brown and red.

When the production process of cement, clinkers are formed and gypsum is added when it is cooled down then the mixture sent to final grinding process. For ordinary Portland cement it uses upto 3-4% and for quick setting cement it can be reduced to 2.5%.

Role of gypsum in Cement

When water is added into cement hydration process starts, water reacts with C3A and hardens. This process occurs very quickly and doesn’t allow time for transporting, mixing and placing. So Gypsum is used to slow down the hydration process.

When gypsum is added into the cement and water is added to it, reaction with C3A particles tales place to form ettringite. This ettringite is initially formed as very fine-grained crystals, which form a coating on the surface of the C3A particles .These crystals are too small to bridge the gaps between the particles of cement. The cement mix therefore remains plastic and workable.

The time allowed for mixing, transporting and placing plays a important role in strength, composition and workabilty of concrete. As gypsum delays the process of hydration, it is termed as retarding agent of cement.          

Effects of Gypsum on Cement

  • It delays setting time of cement
  • It provides greater strength and hardness compared to Non-gypsum cement.
  • It gives longer working time for transporting, mixing and placing.
  • Water requirement is less in this type of cement.
  • Gypsum prevents Flash Setting of cement during manufacturing.
  • When water is mixed to cement Aluminates and sulfates get react and evolve some heat but gypsum acts as coolant and brings down the heat of hydration.


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