Slurry and Clinker in Cement Making

• Limestone (CaCO3) in 3 parts, and Clay in 1part are the raw material to make cement. These are mixed, grinded and water is added to make “slurry”.
• This slurry is heated to 1400-1500°C in rotary kiln. At 1000°C, the CaCO₃ breaks into Quick Lime (CaO) and CO₂. CaO reacts with Silica (SiO₂), Alumina (Al₂O₃) and Ferric oxide (Fe₂O₃) in clay and gives a hard substance called “Clinker”, which is actually a mixture of dicalcium silicate, tricalcium silicate and tricalcium aluminate.
• Clinker is mixed with some 2-5% Gypsum and then grinded to fine powder which is called Cement. Gypsum is often added to Portland cement to prevent early hardening or “flash setting”. The more time it takes, more adequately is it hardened.
• Virtually all the cement produced globally is mixed with sand, aggregates and water, and used to make concrete and mortars. Concrete is second only to water as the most consumed substance on earth, with nearly three tons used annually for each person on the planet.

Grades of Cement

• Various grades of cement such as 43, 53 etc. correspond to the average compressive strength attained after 28 days. Better is the grade, better is compressive strength.

Hydraulic Cement

• Portland Cement is called hydraulic cement because it hardens and sets faster due to chemical reaction with water.

Dry Process v/s Wet Process

• Cement is produced using the wet, the semi-dry, and the dry processes. In 1960, the wet process accounted for 90% of the production. It has now decreased to around 10%. The dry process accounts for around 90% of India’s cement production. The semi-dry process never played an important role in Indian cement production. Its share in total installed cement capacity has been small over time. It currently accounts for 2% of total production