Cement

Cement is an extremely fine material having adhesive and cohesive properties which provides a binding medium for the discrete ingredients. It is a binder, a substance used for construction that sets, hardens, and adheres to other materials to bind them together. They are not very water-soluble and so are quite durable in water and safe from chemical attacks. Cement mixed with fine aggregate produces mortar for masonry, or with sand and gravel, produces concrete.

Concrete is the most widely used material in existence and is behind only water as the planet’s most-consumed resource. It is capable of bonding constructional materials like sand, stones, bricks, buildings, blocks and many more. The principal constituents of cement are the compounds of calcium, aluminium and silicon. Cement containing calcium is called calcareous whereas containing aluminium and silicon is called argillaceous. Cement has a unique property of setting and hardening of calcareous and argillaceous material underwater by virtue of certain chemical reactions.

Raw materials for cement production

1. Limestone (CaCO₃)
2. Clay (SiO₂, Al₂O₃ and Fe₂O₃)
3. Gypsum (CaSO₄.2H₂O)
4. Magnesia (MgO)
5. Sulphur
6. Alkalies

Main steps in cement production

1. Crushing: The raw materials are extracted from quarries by blasting, drilling or ripening using heavy machinery. The raw materials are then crushed in a rotating cylindrical ball or tube mill. The ores are crushed first to 6 inches and reduced them by a secondary crusher or hammer mill to 3 inches.

2. Grinding: The crushed ore is mixed either in dry or wet ways is then grounded and then made homogeneous by means of compressed gas in a grinding mill. The resulting material is known as slurry having 35-40 % water.

3. Heating: Slurry is then introduced in a rotary kiln with the help of a conveyor. The rotary kiln consists of a large cylinder, 8 to 10 feet in diameter and a height of 300-500 feet. It is made of steel and lined with firebricks. In a rotary kiln, the slurry is passed through different zones of temperature. The whole process in the kiln is completed in 2 to 3 hours.

Different temperature zones

i. Preheating zone: In this zone, the temperature is kept to 500°C and usually the moisture is removed and clay is broken into silica, aluminium oxide and iron oxide.

ii. Decomposition zone: Temperature is raised to 800°C. In this zone, limestone decomposes into quick lime and CO₂.

\text{CaCO}_{3}\ \rightarrow\ \text{CaO}+\text{CO}_{2}

iii. Burning zone: In this zone, the temperature is maintained up to 1500°C and the oxide formed in the above zone combines and forms respective silicate, aluminate and ferrite.

\text{2CaO}+\text{SiO}_{2}\ \rightarrow\ \text{2CaO.SiO}_{2}

\text{3CaO}+\text{SiO}_{2}\ \rightarrow\ \text{3CaO.SiO}_{2}

\begin{aligned} \text{3CaO}+\text{Al}_{2}\text{O}_{3}\ &\rightarrow\ \text{3CaO.Al}_{2}\text{O}_{3}\\
\text{4CaO}+\text{Al}_{2}\text{O}_{3}+\text{Fe}_{2}\text{O}_{3}\ &\rightarrow\ \text{4CaO.Al}_{2}\text{O}_{3}.\text{Fe}_{2}\text{O}_{3}
\end{aligned}

iv. Cooling zone: This is the last stage where the whole assembly is cooled up to 150-200°C.

4. Final grinding: The product which is obtained from the rotary kiln is called clinker. It is usually in the form of a greenish-black or grey-coloured solid. It is then air-cooled and the required amount of gypsum is mixed with the clinker to slow down the setting of cement so that cement is adequately hardened.

5. Packing and Shipping: Cement is conveyed from grinding mills to silos (large storage tanks) where it is packed in 20-40 kg bags. Most of the product is shipped in bulk quantities by trucks, trains or ships, and only a small amount is packed for customers who need small quantities.

Types of cement

Depending upon the hardening and setting behaviour, cement is mainly classified into two types:

i. Hydraulic cement

The cement that sets very quickly and hardens in addition to water is called hydraulic cement. It is mainly composed of limestone, gypsum and clay.

ii. Non-hydraulic cement

The cement which sets very slowly by the absorption of CO₂ from the atmosphere and cannot harden while in contact with water is called non-hydraulic cement. It is composed of lime, gypsum, clay and oxychloride.

Depending upon the composition and characteristics, two commercially available important types of
cement are:

1. Ordinary Portland Cement (OPC)
2. Portlant pozzolana cement (PPC)

Some Important Key Terms

Marl: Limestone with admixture of silica, clay and iron oxide.
Corrective ingredients: The essential chemical component added in the cement raw mixture if not present in the required amount.
Clinker: The fused products of aluminates and silicates of calcium in the form of hard and small stones obtained by fusing limestone and clay.
Slurry: A mixture of raw materials and water.

Manufacture of portland cement process with flow sheet diagram

1. Quarrying: Raw materials are extracted from quarries by blasting or by ripping using heavy machinery. Wheel loaders and dumper trucks transport the raw materials to the crushing installations.

2. Crushing: Limestones produced are then crushed with the help of crushers installed at the mine sites which is transported to plant stack pile with the help of Belt conveyer or Ropeway.

3. Grinding, blending and homogenization: Crushed quarried limestone is further crushed into small pieces to provide fine materials for blending which is blended with corrective ingredients like clay, silica and iron ore in required proportion and mixed in a homogeneous way to produce clinker of the desired composition.

4. Pyro processing: In this process, materials are subjected to high temperatures so as to cause a chemical or physical change. Its control improves efficiency in energy utilization and hence enhances production for good quality assurance.
Here, the blended material is heated at a high temperature at about 1300-1500°C in a rotatory kiln to produce clinker. The kiln is heated with the help of powdered coal so that hot flames are produced.

5. Storing clinker and grinding: The clinker coming from the burning kiln is very hot. It is cooled by admitting the air in a counter-current direction at the base of the kiln and collected in small trolleys. The clinker is grinded in powder in a ball mill along with gypsum to produce cement.

Cement industry in Nepal

Cement was used first in Nepal during the 1950s and its supply was diversed in 1965. Himal cement company was the first cement plant established in 1975 had a production rate of 160 tons a day but it was later shut down in 2002. After this, the Hetauda cement industry and Udaypur cement industry limited were established. Nepal’s domestic cement production has been growing steadily over the past few years as the demand for cement continues to rise. Nepali cement producers are producing OPC, PPC of which OPC is widely used by consumers. Hongshi-Shivam Cement Private Limited, a Nepal-China joint venture company has started cement production in 2018. It is the largest cement factory in Nepal with a daily production capacity of 6000 tons. According to the department of industries, 27 mini, medium and large scale cement industries and more than 17 mini and mini clinker based cement industries have been registered in Nepal. Some cement industries established in the public and private sectors are mentioned below:

Public sectors
i. Himal cement company
ii. Hetauda cement industry
iii. Udaypur cement industry

Private sectors (some list)
i. Jagadamba cement
ii. Butwal cement
iii. Shivam cement
iv. Agni cement
v. Arghakachi cement

Clinker based cement industries (some list)
i. Mittal cement
ii. Pashupati cement
iii. Ambe cement