Why Gypsum is Added to Cement?

Portland cement is hydraulic cement. This means that it sets and hardens faster due to a chemical reaction with water. As a result, it will also harden under water. Whenever water is mixed with cement, a smooth paste is formed that remains plastic for a short time. During this period, the paste can be disturbed and remixed without injury. As the reaction between water and cement continues, the plasticity of the cement paste is lost. This early period in the hardening of cement is known as `Setting of Cement’.

Why Gypsum is added to Cement?

When cement is mixed with water, it becomes hard over a period of time. This is called setting of cement. Gypsum is often added to Portland cement to prevent early hardening or “flash setting”, allowing a longer working time. Gypsum slows down the setting of cement so that cement is adequately hardened.

Setting of cement

When cement reacts with water, the silicates and aluminates of calcium gets converted to the hydrated colloidal gels and following reactions take place:

1. 2CaO.SiO₂  + x H₂O→2CaO.SiO_{2 .x}H₂O   (This is hydrated colloidal gel of Tricalcium silicate)
2. 3CaO. Al₂O₃ + 6 H₂O→3CaO. Al₂O₃ .6 H₂O   (This is hydrated colloidal gel of Tricalcium aluminate
3. 3CaO.SiO₂→2CaO.SiO₂ + Ca (OH)₂
4. 3CaO. Al₂O₃ .6 H₂O→3 Ca(OH)₂ + 2 Al(OH)₃

In the above reactions we see that finally we have three products:

1. Calcium Hydroxide
2. Aluminium Hydroxide
3. Dicalcium silicate

The job of the Calcium Hydroxide is to bind the silicate participles together, while the job of Aluminum Hydroxide is to fill the space in the lattice.  When Gypsum is added to the Cement it binds with Tricalcium aluminates to make a colloidal gel and the above reaction don’t take place. So it slows the process of fastening.

3CaO. Al₂O₃ + 3CaSO₄+ 2H₂O→3CaO. Al₂O₃ .3CaSO₄.2H₂O

Tricalcium silicate has the property of setting quickly and is responsible for considerable strength in 2-7 days. Normally half of the cement has Tricalcium silicate

Dicalcium silicate has the property of slow setting but becomes strong after a month of Hydration.

Tricalcium aluminate has the property of instantly setting in presence of water and the internal strength of cement is provided by it as it fills the space in the lattice.