Cement: Materials and manufacturing process

Cement, a fundamental component in construction, is produced through a complex manufacturing process involving various stages and raw materials. The process begins with extracting raw materials through quarrying or mining methods, depending on the type of rock. Hard rocks like limestone, slates, and certain shales are typically quarried and may require blasting, while softer rocks like chalk and clay can be excavated directly.

Once extracted, the raw materials are transported to the crushing plant using trucks, railway freight cars, conveyor belts, or ropeways. They can be transported in either dry or wet states, with some regions employing pipelines for slurry transportation. In areas lacking limestone with sufficient lime content, beneficiation processes such as froth flotation may be used to upgrade the raw materials, albeit at a higher cost.

The manufacturing of Portland cement, the most commonly used type of cement, involves four main stages: crushing and grinding of raw materials, blending of materials in correct proportions, burning the prepared mix in a kiln, and grinding the resulting product known as ‘clinker’ along with gypsum to control setting time. These stages can be carried out using wet, dry, or semi-dry processes, depending on the moisture content of the raw materials.
Measures such as enhancing energy efficiency, utilising renewable energy sources, and implementing carbon capture and storage technologies are being explored to mitigate these environmental impacts. Additionally, research into novel cement and alternative formulations that reduce the reliance on clinker production is underway to address these challenges.

The crushing and grinding stage involves reducing the raw materials into fine particles using crushers and mills. Blending ensures the desired chemical composition of the cement by combining materials from different batches. Burning occurs in kilns, with modern plants predominantly utilising rotary kilns, where the raw materials are heated to high temperatures to form clinker. Dust emissions from kilns are managed through various systems, such as cyclone arrestors and bag-filter systems.

Grinding the clinker and gypsum mixture produces the final cement product, which is then stored in silos before packaging or bulk dispatch. Portland cement primarily consists of four main compounds: tricalcium silicate (C3S), dicalcium silicate (C2S), tricalcium aluminate (C3A), and a tetra-calcium aluminoferrite (C4AF), along with other minor components. Upon hydration, these compounds form calcium silicate hydrate (C–S–H) and calcium hydroxide (CH), which bind the cement and concrete together.

The strength and durability of cement are influenced by its composition and fineness. Various factors, including the presence of alumina and iron compounds, influence the cement's resistance to chemical attack and its heat of hydration. Despite the challenges posed by its production, ongoing research and technological advancements aim to improve the sustainability of cement manufacturing while meeting the demands of construction projects worldwide.

In India, Portland cement is manufactured according to the Indian Standard IS 269:2015, which categorises Portland cement into various types based on their composition and intended use. The most commonly used types of Portland cement in India are:

Ordinary Portland Cement (OPC):

• OPC is the most commonly used type of cement in India and is suitable for general construction purposes.
• It is available in three grades of cement: OPC 33 Grade, OPC 43 Grade, and OPC 53 Grade, with OPC 43 Grade being the most commonly used.

Portland Pozzolana Cement (PPC):

• PPC contains pozzolanic materials like fly ash, volcanic ash, calcined clay, and clinker and gypsum.
• Compared to OPC, it offers better workability, durability, and resistance to chemical attacks.
• It is commonly used for mass concrete works, such as dams, foundations, and sewage pipes.
• The code for Portland Pozzolana Cement (PPC) in India is IS 1489 (Part 1):1991. This code specifies the requirements and the method of sampling and tests for PPC.

Portland Slag Cement (PSC):

• PSC contains granulated blast furnace slag in addition to clinker and gypsum.
  It provides better durability, resistance to chemical attacks, and reduced heat of hydration compared to OPC.
• It is suitable for marine constructions, mass concrete works, and structures exposed to sulphates.
• The code for Portland Slag Cement - Specification in India is IS 455-2015.

Sulphate Resisting Portland Cement (SRPC):

• SRPC is specially formulated to resist the action of sulphates, making it suitable for use in marine environments or areas with high sulphate content in soil or water.
• It is often used in foundations, sewage treatment plants, and other structures exposed to sulphates.
• IS 12330:2009 covers sulphate-resisting Portland cement. This standard specifies the manufacturing, chemical & physical requirements and sulphate resistance portland cement testing.

These are the main types of Portland cement available in India, each designed to meet specific construction requirements and environmental conditions. It's essential to select the appropriate type of cement based on the project specifications and environmental factors to ensure the desired performance and durability of the structure.