Production process of Portland cement (OPC)

The main raw materials are:

<aside> 🪨 Limestone: CaCO3

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<aside> 🧱 Clay: SiO2, Al2O3, Fe2O3

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<aside> ◽ Gypsum: CaSO4 ⋅ 2 H2O

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The mixing procedure can be done in two main methods (dry and wet). There can be slight variations (semi-dry and semi-wet).

Dry process

Figure 5: Manufacturing of cement via dry process. Source: The Constructor (2021) [23].

Wet process

Figure 6: Manufacture of cement via wet process. Source: The Constructor (2021).

Even though the processes seem the same at a first glance, with the exception of added water in the first process, the characteristics are somewhat different, and they are collected in Table 1 below.

Table 1: Comparison of dry and wet processes - Source: The Constructor (2021).

Criteria	Dry process	Wet process
Hardness of raw material	Quite hard	Any type of raw material
Fuel consumption	Low	High
Time of process	Lesser	Higher
Quality	Inferior quality	Superior quality
Cost of production	High	Low
Overall cost	Costly	Cheaper
Physical state	Raw mix (solid)	Slurry (liquid)

Manufacturing of cement, step by step

These are the main chemical and physical processes that take place.

Calcination of limestone into lime (stage where most of the emissions happen):

<aside> 🔥 CaCO3 —> CaO + CO2

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Calcination of mixture of limestone (CaCO3) and clay (SiO2, Al2O3, Fe2O3).
- Takes place between 900 and 1200 °C. At this stage, fuel combustion emissions happen, since high temperatures are needed for the calcination process to occur. Fuel combustion related-emissions amount to around 35% of total emissions produced during this process [24].
  - Fuel to heat the kiln:
    - Powdered coal
    - Oil
    - Hot gases
    - Derived fuel (high calorific municipal solid waste, mostly plastics and tyres)
- Limestone decomposes into lime and CO2 due to the high temperatures. This is where process emissions happen, which amounts to around 65% of total produced emissions in the clinker manufacturing process.

Reaction of the mixture of lime and clay:

<aside> 🔄 CaO & (SiO2, Al2O3, Fe2O3)

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<aside> <img src="/icons/science_blue.svg" alt="/icons/science_blue.svg" width="40px" /> 2 CaO + SiO2 —> Ca2SiO4

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<aside> <img src="/icons/science_blue.svg" alt="/icons/science_blue.svg" width="40px" /> 3 CaO + SiO2 —> Ca3SiO5

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<aside> <img src="/icons/science_blue.svg" alt="/icons/science_blue.svg" width="40px" /> 3 CaO + Al2O3 —> Ca3Al2O6

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4 CaO + Al2O3 + Fe2O3 —> Ca4Al2Fe2O10

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Lime reacts with the silicates and aluminates to form aluminates and silicates of calcium.
Aluminates and silicates of calcium fuse together, creating clinkers.

Powered clinker and gypsum are mixed to form cement: