What is Gypsum?
Gypsum is a mineral and is hydrated calcium sulfate and chemical form. Gypsum is also known as retarding agent in cement because its main function in cement is to control the initial and final setting time of the cement.
The chemical formula for Gypsum is :- CaSO4.2H2O
Gypsum is a naturally occurring mineral mined from deposits formed by ancient sea beds as a raw material and sedimentary rocks composed of calcium sulfate and water.
Properties of Gypsum
- It is colorless, transparent, translucent crystalline minerals.
- It can be white, grey, yellow, or blue in color.
- It is soft in nature and can be a breakdown with the help of a figure.
- It has low thermal conductivity.
Other uses of gypsum
- It is used for a variety of manufacturing industrial and agricultural uses.
- Gypsum is also used as a fire-resisting material.
- It is used to prepare plaster of pairs.
- It can be used as a fluxing agent.
Why gypsum is added to cement? Role of gypsum in cement
Cement is a binding material used in the construction of concrete structures. Cement plays a vital role in terms of economy and the durability of the structure. So, cement should maintain quality enough while mixing the ingredient of concrete i.e sand, aggregate, and admixture.
Cement is prepared by mixing four main components. They are Tri-calcium silicate, tri-calcium aluminate, di-calcium silicate, and tetra calcium aluminum ferrite. These components are provided in some percentage to prepare the cement.
Components of cement
- Tri-calcium aluminate
- Tri-calcium silicate
- Di-calcium silicate
- Tetra calcium aluminum ferrite
When water is added to cement, tri-calcium silicate quickly reacts with water among all the above ingredients to get hardened. In this case, other ingredients remain unreacted till that time.
Mixing, transporting, and placing concrete correctly and at the appropriate time is very important for concrete to get workability, proper strength, and durability.
When the water is added to cement it starts reacting with the tricalcium aluminate and hardens. The time taken in this process is very much which doesn’t allow time for transporting mixing and placing.
So, to decrease the rate of reaction of tri-calcium aluminate with water we add gypsum in cement.
When is gypsum added to the cement?
We know, that clinkers are formed in the process of manufacturing cement. These cement clinkers are then cooled down and added with a small amount of gypsum.
The mixture is then sent to the final grinding process for ordinary Portland cement. It remains between three to four percent and in the case of quick-setting cement, it can be reduced up to 2.5%.
Effects of gypsum on the cement
- It slows down the setting time of cement by increasing the rate of hydration.
- When water is mixed with cement illuminates and sulfates get to react and evolve some heat but gypsum acts as a coolant and brings down the heat of hydration.
- Gypsum helps to increase the time for concreting activities like mixing, placing, and transporting concrete.
- Gypsum cement possesses considerably greater strength and hardness when compared to non-gypsum cement.
- The cement which contains gypsum needs less water that the cement without gypsum.