19 Cement

Learning outcomes

The students should be able to:

19.1 Explain the fundamental importance of cement and identify its raw materials.

19.2 Describe the main steps in cement production (crushing and grinding, strong heating, and final grinding).

19.3 Differentiate between OPC and PPC cement.

19.4 Detail the Portland cement manufacturing process with a flow-sheet diagram

19.5 Detail the Portland cement manufacturing process with a flow-sheet diagram

19.6 Analyze the current state, challenges, and strategic importance of the cement industry in Nepal.

                                              

• Introduction
• Raw materials
• Manufacturing Process
• OPC vs PPC
• Flow sheet diagram
• Cement industries in Nepal

Introduction 

• One of the most important building materials
• The second most used resource in the world, after petroleum (ignoring water)
• Cement is a binder, meaning it binds other materials together. It is not used alone but:
• With sand and gravel → Concrete
• With sand, gravel, and steel → Reinforced Cement Concrete (RCC)
• Cement is a fine powder that, when mixed with water, forms a paste that sets and hardens due to a chemical process called hydration.
• Cements that can set under water are called hydraulic cements.
• The most dominant and well-known type is Portland cement, named for its resemblance to a high-quality building stone quarried on the Isle of Portland in England.
• Not a single composition or definite composition, but a mixture of different compounds.
• Essentially contains- CaO, SiO₂, Al₂O₃, Fe₂O₃, SO₃ etc. does not have specific formula
• The approximate composition of cement is

CaO = 50-60%

SiO₂ = 20-25%

Al₂O₃ = 5-10%

MgO = 1-2%

Fe₂O₃ = 1-2%

SO₃ = 1-2 %

19.2 Raw materials for cement production

• Cement is manufactured from natural raw materials:

Broadly two as major ingredients:

1. Calcareous materials: These provide the essential Calcium Oxide (CaO, or lime). The most common source is limestone (CaCO₃). Other sources include chalk, shells, and marble.
2. Argillaceous materials: mainly provide silica(SiO₂), alumina (Al₂O₃), and iron oxide(Fe₂O₃). The most common source is clay or shale.
3. The raw materials are obtained by blasting rock, boring the rock, and setting off explosives, or simply by digging with an excavator.
4. Beyond these primary ingredients, cement production requires:

• Coal/Petcoke: Used as fuel to heat the kiln to extreme temperatures (up to ~1450°C).
• Gypsum (CaSO₄·2H₂O): Added in the final grinding stage (typically 2-5%) to regulate the setting time of the cement. Without it, cement would set almost instantly after mixing with water.
• Supplementary Materials: Fly ash, slag, or sand may be added to achieve specific properties in the final product, such as improved durability or lower cost.

 Important Distinction: While its raw materials are found in nature, cement is a manufactured product. It does not exist as a natural material. It is a product of complex chemical reactions at high temperatures.

19.3 Main steps in cement production

The journey from rock to powder involves four primary stages: preparation, proportioning, burning, and final grinding.

Stage 1: Crushing and Grinding of Raw Materials

The raw limestone and clay are blasted or dug from quarries and then crushed. Depending on the nature of the materials (e.g., moisture content), one of two processes is chosen:

In the wet process,

• The clay is washed with water in wash mills to remove impurities like flint.
• The crushed limestone is then mixed with the clay paste in a proportion of about 75% limestone to 25% clay
• The mixture is finely ground and made homogeneous by means of a compressed air mixing arrangement.
• The resulting thick fluid paste is known as slurry which contains about 35-40% water.

In the dry process,

• The raw materials are dried and mixed in the proper proportions and then powdered.
• The mixture is then homogenized into raw meal.
• This process is more energy-efficient and is now the dominant method globally.

Stage 2: Strong heating – Calcination

• most crucial and energy-intensive step
• Slurry or raw meal is fedwith the help of a screw conveyor in a rotary kiln, a massive, slightly-inclined steel cylinder (up to 250m long and 6-10m in diameter) lined with refractory firebricks. The kiln rotates slowly (~1-3 rpm).
• The rotary kiln may be a horizontal cylinder or vertical.
• The material is fed from the upper (cool) end and travels slowly towards the lower (hot) end.
• Here, the contents are gradually heated with a flame from pulverized coal to a temperature of 1400 °C – 1600 °C.
• The lime (CaO) chemically combines with silica, alumina, and iron oxide from the clay to form new complex compounds. The material partially melts and fuses into hard, pea-sized, grey balls called clinker.

Stage 3: Final grinding

• The red-hot clinker is rapidly air-cooled and then fed into a grinding mill with a small amount of gypsum (2-5%).
• The mixture is finally ground into a fine, uniform powder called cement.
• Gypsum slows down the hydration process, allowing the cement to set at a practical pace (e.g., 30-60 minutes) rather than instantly and gives compressive strength to the cement.

19.4 Types of cement- OPC and PPC

Ordinary Portland Cement(OPC) and Portland Pozzolana cement(PPC)

Portland cements are most common due to the abundance and low cost of production.

PPC is a variation of OPC,

PPC has a low initial setting strength compared to OPC, but hardens over a period of time with proper curing.

Feature	OPC(OrdinaryPortlandCement)	PPC (Portland Pozzolana Cement)
Composition	Primarily clinker + gypsum. (90-95% clinker)	Clinker + gypsum + pozzolana (fly ash, volcanic ash). (60-80% clinker)
Initial Strength	High. Sets and gains strength quickly.	Lower initially due to the slower reaction of pozzolana with lime.
Long-term Strength	Good, but not as high as PPC.	Higher and continues to increase for a longer period.
Heat of Hydration	High, which can lead to thermal cracking in large pours.	Lower, making it ideal for massive concrete structures like dams.
Durability	Good.	Excellent resistance to water, chemicals (sulfates), and corrosion.
Cost & Environment	More expensive to produce and less eco-friendly due to high clinker content.	Cheaper and more sustainable as it uses industrial waste (fly ash) and reduces CO₂ emissions.
Best Uses	Projects requiring rapid strength gain and quick setting, such as urgent repairs, beams, columns, slabs, and high-rise building frames.	General construction, plastering, foundations, underwater structures, and mass concreting where long-term durability is key.

19.5 Portland cement manufacturing process

Step 1: Quarrying & Crushing → Limestone and clay are blasted or dug from the earth and transported to crushers.

Step 2: Raw Grinding & Blending → The crushed materials are ground (using the wet or dry process) to a fine powder. The chemical composition is precisely controlled to the desired proportions.

Step 3: Preheating → The raw meal is passed through a series of cyclones, where hot exhaust gases from the kiln preheat the material, recovering energy and beginning the calcination process.

Step 4: The Rotary Kiln (Sintering) → The preheated material enters the top of the long, rotating kiln. At 1450-1500°C in the burning zone, complex chemical reactions occur, the most important of which are:

• 2 CaO + SiO₂ → 2CaO · SiO₂ (Di-calcium silicate, C₂S)
• 3 CaO + SiO₂ → 3CaO · SiO₂ (Tri-calcium silicate, C₃S – responsible for early strength)
• 3 CaO + Al₂O₃ → 3CaO · Al₂O₃ (Tri-calcium aluminate, C₃A – responsible for initial set)
• Similarly, magnesium oxide is changed into magnesium silicate.
• These reactions are exothermic. Therefore, the temperature has to be controlled

Step 5: Cooling → The molten clinker exits the kiln into a cooler, where a blast of cold air rapidly drops its temperature, locking in its reactive mineral phases.

Step 6: Final Grinding → The cooled clinker is mixed with 2-5% gypsum and fed into a cement mill. The gypsum prevents “flash setting” by retarding the reaction of C₃A.

Step 7: Packing & Dispatch → The fine cement powder is conveyed to silos for storage or directly to packing machines, where it is filled into air-tight bags (to prevent moisture absorption, which would spoil it) and shipped.

19.6 Cement Industry in Nepal

• The use of cement in Nepal began in the 1950s .
• Only in 1967 was the first cement industry, Himal Cement Company Limited, established
• Its operation started in 1975 and accounted for 18% of the total demand at that time.
• Sadly, it was shut down in 2002 after a large opposition by the local people concerning dust and air pollution.
• For decades, the industry was dominated by state-owned plants like Hetauda Cement Industry Ltd. (est. 1976) and the once-largest, Udayapur Cement Factory Ltd. (est. 1987), which sells its product under the brand name “Gaida.”
• However, a major transformation began in the 1990s with economic liberalization, leading to an explosion of private sector investment.

Current Status

• The cement industry is now one of the leading industrial sectors in Nepal, playing a pivotal role in the nation’s infrastructure development
• Main raw materials- limestone and clay are locally mined, and gypsum, coal, and iron ores are imported
• Domestic cement production has been growing steadily over the past decade, and demand has also been rising.
• Earlier, most of the clinker used to be imported from India, but recently, there has been a rapid expansion of domestic clinker production.
• Installed Capacity: approximately 25 million metric tons per year.
• Domestic Demand: Annual domestic consumption hovers around 8 to 8.5 million metric tons. This reveals a massive underutilization, with the industry operating at only ~35-40% capacity.
• Number of Factories: While 55-62 plants are registered, only about 42-43 are currently operational. Over 18 plants have shut down due to market constraints.
• Self-Sufficiency: Despite low capacity utilization, Nepal has achieved near self-sufficiency. Imports have declined sharply as the country now fulfills most of its own demand. The government even imposes duties on clinker and cement imports to protect local manufacturers.
• Dominant Player: The largest cement plant in Nepal is Hongshi-Shivam Cement Pvt. Ltd. in Nawalparasi. It is a joint venture: 70% owned by China’s Hongshi Holding Group and 30% by Nepal’s Shivam Cement. The plant has a capacity of over 2 million tons per year.

New Investments: Over NPR 300 billion is estimated to have been invested in this capital-intensive industry.