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
Ø It delays setting time of
cement
Ø It provides greater
strength and hardness compared to Non-gypsum cement.
Ø It gives longer working
time for transporting, mixing and placing.
Ø Gypsum prevents Flash
Setting of cement during manufacturing.
Ø 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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