Production Process of Cement: An Overview

🏗 Input (Raw / Wrought Materials)

Material	Function	Typical per 1t Cement
Limestone (CaCO₃)	Primary source of CaO	$\approx$ 1.25t (0.65-0.70t CaCO₃)
Clay/Shale	Source of SiO₂, Al₂O₃, Fe₂O₃	$\approx$ 0.2t
Sand/Silica additive	SiO₂ for low silica clay	optional (0.03-0.05t)
Iron ore/Mill scale/Bauxite	Fe₂O₃, Al₂O₃ balancing	optional (0.01-0.03t)
Gypsum (CaSO₄·2H₂O)	Sets retarder during grinding	(0.03-0.05t)

🏗 Input (Optional additives)

Material	Function	Typical per 1t Cement
Fly ash	Pozzolanic filler	15-35%
Ground Granulated Blast Furnace Slag (GGBFS)	Latent hydraulic material	30-70%
Limestone	Filler + reaction surface	5-35%
Natural pozzolan	Reactive silicates	up to 15%

The manufacture of cement - the invisible backbone of modern construction - is a sophisticated, multistage industrial process that transforms raw geological materials into a highly engineered construction material. The procedure integrates mechanical processing, high-temperature reactions, and carefully monitored chemical transformations. It encompasses distinct stages that begin with raw material extraction and culminate in the packaging of the final product.

The process starts with the extraction and preparation of raw materials, typically involving limestone as the primary source of calcium carbonate (CaCO₃), along with clay or shale that contribute essential quantities of silica (SiO₂), alumina (Al₂O₃), and iron oxide (Fe₂O₃). These materials are excavated from open-pit or underground mines. Blasting techniques are employed to dislodge large sections of rock, which are subsequently reduced in size through crushers to produce manageable aggregates suitable for further processing.

These crushed raw materials are then conveyed to the pre-homogenization yard, where they are layered in longitudinal piles using a stacker-reclaimer system. This system ensures a consistent chemical profile through the mechanical mixing of materials over time. Uniformity in composition is crucial to maintain consistent burning behavior in the kiln and to achieve predictable clinker phase formation.

The next stage involves the drying and grinding of raw materials to form the raw meal. In modern dry process plants, raw materials are introduced into a rotary mill—a large rotating drum where coarse materials are ground into fine particles. This mill often utilizes hot exhaust gases from the rotary kiln to remove residual moisture from the material, thus avoiding the need for separate drying equipment. The dried and powdered mixture is referred to as the raw meal and is characterized by a specific target composition: approximately 75% CaCO₃, 13% SiO₂, 4% Al₂O₃, and 3% Fe₂O₃. This composition is critical for ensuring that the correct mineral phases develop during subsequent thermal treatment.

Once ground, the raw meal is fed into a multi-stage cyclone preheater tower. These cyclones employ the hot gases from the rotary kiln to raise the temperature of the raw meal progressively, often reaching ~850–900°C. During this stage, a critical chemical reaction occurs: the thermal decomposition of calcium carbonate. This calcination reaction liberates carbon dioxide and forms reactive calcium oxide:

$\text{CaCO}_3 \xrightarrow{\Delta} \text{CaO} + \text{CO}_2 \uparrow$

This endothermic reaction demands substantial heat and represents the largest contributor to the CO₂ emissions associated with cement production.

In many modern facilities, a separate calciner is placed between the preheater and kiln. The calciner allows a portion of the fuel to be burned earlier in the process, increasing thermal efficiency and enabling the majority of calcination to be completed before the material reaches the rotary kiln. The high gas–solid contact efficiency in the calciner ensures that most of the CaCO₃ is decomposed under relatively low residence times.

The calcination of calcium carbonate is responsible for over 60% of the total CO₂ emissions in cement production. Each ton of clinker releases approximately 0.52-0.54 tons of CO₂ purely from this chemical reaction. Combined with fuel combustion the overall carbon footprint reaches about 0.8-0.9 tons of CO₂ per ton of cement produced. This makes cement one of the most carbon intensive industrial materials in the world - it is assumed that cement production accounts for about 8% of all human caused emissions.

The preheated and largely decarbonated material then enters the rotary kiln, the central unit of the cement production process. This long, refractory-lined rotating cylinder operates at its hottest zone around 1450°C. Here, complex solid-state and partially molten phase reactions take place. The fundamental chemical transformations in the rotary kiln include the following sequence:

1. Formation of belite (C₂S): $2\text{CaO} + \text{SiO}_2 \rightarrow \text{Ca}_2\text{SiO}_4 \ (\beta\text{-C}_2\text{S})$

2. Formation of alite (C₃S), the most reactive phase responsible for early strength development: $\text{Ca}_2\text{SiO}_4 + \text{CaO} \rightarrow \text{Ca}_3\text{SiO}_5 \ (\text{C}_3\text{S})$

3. Formation of tricalcium aluminate (C₃A) and tetracalcium aluminoferrite (C₄AF): $3\text{CaO} + \text{Al}_2\text{O}_3 \rightarrow \text{Ca}_3\text{Al}_2\text{O}_6 \ (\text{C}_3\text{A})$ $4\text{CaO} + \text{Al}_2\text{O}_3 + \text{Fe}_2\text{O}_3 \rightarrow \text{Ca}_4\text{Al}_2\text{Fe}_2\text{O}_{10} \ (\text{C}_4\text{AF})$

Rotary kilns usually utilize multi-channel burners. The central channel delivers the main fuel - like pulverized coal, natural gas, fuel oil and shredded tire or plastic chips, etc. - and annular channels that provide primary air and gaseous or liquid fuels. The swirl and momentum of air ensures flame shape, flame length and burnout efficiency. Some kilns even have dual feed systems for both fossil and alternative fuels.

On some rotary kilns whole tires or coarse solids like waste wood or plastics are introduced through a separate chute into the calciner or kiln inlet riser duct. These zones have longer residence time and adequate temperature to ensure complete burnout. Those processes can be designed in a way that the ash becomes part of the clinker if it’s non-volatile and compatible. Some plants - though rare - use mid-kiln ports to feed whole tires or large fuel chunks directly into the rotating kiln shell. Combustion then occurs inside the kiln body while the clinker material descends. This is technically challenging and less favored due to mechanical stress and control issues.

When used, old tires offer additional benefits beyond energy: their high calorific value (around 30-35 MJ/kg which is compareable to coal) contributes significantly to heat input while steel content - for example from steel belts inside the tires - supplements iron oxide (Fe₂O₃) in the clinker. Similarly ash from refuse-derived fuel contributes SiO₂, Al₂O₃ or minor oxides.

The above mentioned reactions yield the grey, sintered nodules known as clinker, composed of intergrown crystals and a residual glassy phase. The clinker emerges at the kiln exit and must be rapidly cooled to preserve its reactivity. This is achieved in a clinker cooler, typically a reciprocating grate or planetary system, where ambient or forced air reduces the temperature to around 60–100°C. The heat extracted from the clinker is recovered and recycled into the preheater, enhancing the plant’s thermal efficiency.

The cooled clinker is now ready for final grinding, where it is conveyed into a ball mill - a large rotating cylinder filled with steel balls. Here, the clinker is ground together with 3–5% gypsum (calcium sulfate dihydrate, CaSO₄·2H₂O). The gypsum acts as a set regulator, preventing the flash setting of cement during mixing with water. Again, hot gases from the kiln system may be used to ensure any remaining moisture is removed during grinding.

To produce different types of cement, various additional materials can be blended into the grinding process. These include fly ash, ground granulated blast furnace slag (GGBFS), natural pozzolans, or limestone. These additions can improve workability, reduce the heat of hydration, and lower the carbon footprint of the final product. The resulting material is a fine grey powder: cement.

Once ground, the cement is stored in large silos, which are equipped with aeration systems to maintain powder flow and preserve product quality by minimizing moisture ingress. From the silos, the cement is either packed into bags (commonly 25 or 50 kilograms) or dispatched in bulk form by trucks, railcars, or ships.

Throughout this entire process, controlling dust and emissions is a major concern. The kiln and grinding exhaust gases are passed through dust collection systems, such as baghouse filters and electrostatic precipitators, which capture particulate matter before it can enter the atmosphere. Modern systems also include desulfurization and NOₓ reduction units to limit air pollution.

Energy efficiency and emissions reduction are key priorities in cement manufacturing. The use of alternative fuels - such as refuse-derived fuel (RDF), biomass, waste oil and used tires - has become increasingly common. Moreover, efforts are being made to reduce CO₂ emissions through clinker substitution (for example using fly ash, slag or pozzolans), carbon capture technologies, and improved process integration (optimizing kiln control, online process monitoring, etc.).

💡 Energy and emissions

Metric	Value
CO₂ per ton of clinker	0.52-0.54t
CO₂ per ton of cement	0.8-0.9t
Share of global CO₂ emissions	$\approx$ 8%
Thermal energy per ton clinker	3.0-3.4 GJ
Electrical energy per ton clinker	90-130 kWh

To put the energy consumption into a perspective: Producing 1 ton of clinker requires about 3.0-3.4 GJ of thermal energy and 90-130 kWh of electricity. Modern dry-process plants operate near these lower limits, older or wet-process plants may consume significantly more. The rotary kiln, the preheater tower and the calcinator are the largest energy consumers accounting for up to 90% of the thermal demand. This shows that the design of efficient preheaters, calcinator and heat recovery is crucial when designing or operating a cement plant.

In summary, the production of cement encompasses a highly integrated series of mechanical, thermal, and chemical operations: from raw material blasting and grinding in rotary mills, through high-temperature reactions in preheaters, calciners, and rotary kilns, to final grinding in ball mills and packaging from silos. The process represents a remarkable orchestration of chemistry and engineering, yielding a material that is foundational to modern infrastructure and construction.

This article is tagged: Basics, How stuff works, Industry