Cement | NEB Class 12 Chemistry | NEPALI EDUCATE

Unit 19 - Cement

Introduction:

• Cement is the constructive binding material which gets set to the hard mass after hydration. The most important cement is Portland cement for construction.

Raw materials for cement:

Portland cement consists of the following essential raw materials:

1. Lime (CaO): 60% - 65%
2. Silica (SiO2): 20% - 25%
3. Alumina (Al2O3): 4% - 8%
4. Iron oxide (Fe2O3): 1% - 4%
5. Magnesia (MgO): 1% - 3%
6. Gypsum (CaSO4.2H2O): About 5%

Other additional raw materials such as bauxite, silica sand, etc. are added in small amounts to get the desired amount of cement.

Manufacture Of Cement:

Manufacture of Portland cement involves the following four stages:

1. Crushing and Grinding:
• The approximate required raw materials for cement are first crushed and then ground in the rotary cylindrical ball mills or in the tube mills fitted with steel grinding balls. Grinding is carried out in two ways:
• Wet or Dry Process:

For the dry process, raw materials should be dried in the cylindrical drier while wet raw materials are directly mixed with a little water. Soft material is broken down by vigorous stirring with water in a wash mill to make slurry. The slurry is then passed through the screen to remove oversize particles.

2. Blending:
• The ground raw materials are then carried for the blending process. The chemical composition of the raw materials for particular cements is obtained by selective quiring.
• In the dry process, the ground raw material is stored in the silos and mixed by compressed air.
• In the wet process, the slurry is taken and stirred by mechanical means or compressed air. Sometimes, slurry is filtered to reduce water content up to 20 – 30%. The filter cake is then fed to the rotary kiln for burning.

3. Burning:
• The blended raw materials are fed to the rotary kiln, which is 200m long and 6m in diameter, containing a steel cell lined with refractory materials. The raw materials feed slowly down to the lower fire end at temperatures ranging about 1350°C - 1500°C depending upon the raw materials taken. The burned products emerge from the rotary kiln and pass to the coolers where heat is transferred to the incoming air.

4. Grinding:
• The clinker and the required amount of gypsum are finely ground in the horizontal grinding mill. The materials may pass straight through the mill, or coarser material may be separated from the ground products and returned to the mill for further grinding. The finished cement is pumped pneumatically (with air) to the silos for storage, from which it is drawn for packing in paper bags or for dispatch in paper containers.

Flowsheet Of Manufacture of Cement:

Types Of Cement:

1. Ordinary Portland cement (OPC):

Ordinary Portland cement (OPC) is made up of four major components:

• Tricalcium Silicate (3CaO.SiO2)
• Dicalcium Silicate (2CaO.SiO2)
• Tricalcium Aluminate (3CaO.Al2O3)
• Tetra Calcium Aluminoferrite (4CaO.Al2O3)

OPC has three grades in the market: OPC-23, OPC-33, OPC-53. It has higher initial strength and gains maximum strength in 28 days.

2. Portland pozzolana cement:

Portland pozzolana cement is obtained from pozzolanic materials such as volcanic ashes, silica fume, fly ashes, and clinker. It is cheaper than OPC and has lower initial strength but higher strength for a long duration of time.

Difference Between OPC and PPC:

S.No.	OPC	PPC
1.	Manufactured from calcareous and argillaceous materials.	Manufactured from pozzolanic materials like volcanic ashes, silica fumes, etc.
2.	Expensive than PPC.	Cheaper than OPC.
3.	Not favorable for aggressive weather.	Favorable in aggressive weather.
4.	Initial strength is higher than PPC.	Initial strength is lower than OPC.
5.	Available in three grades as grade-33, grade-43, grade-53.	No specific grades.
6.	Suitable for fast construction.	Suitable for all types of construction.
7.	Setting time is lower than PPC.	Setting time is higher than OPC.

Cement Industry in Nepal:

In Nepal, cement was used in construction from 1950. In the early days, cement was imported from China, India, Korea, etc. In 1972, Himal Cement Industry was first established by the government in Chovar, Kirtipur. Later, Hetauda Cement (1976 A.D) and Udayapur cement were established and owned by the government, but Himal cement was dissolved in 2002 due to environmental issues.

Recently, around 42 cement industries are running in Nepal. Among them, two are owned by the Government of Nepal. Fifteen industries are self-clinker producing while others are dependent upon clinker importers.

Domestic manufacture can fulfill about 80% of cement demand while 20% is fulfilled by importing the cement. The market share of self-clinker producers is 40%, and the remaining 60% is fulfilled by clinker importers or imported cement.

Cement Industry of Nepal:

1. Jagdamba Cement Industry Pvt. Ltd.
2. Samrat Cement Industry Pvt. Ltd.
3. Udayapur Cement Industry Ltd.
4. Bishal Cement Pvt. Ltd.
5. Hetauda Cement Factory