Cement production : explained step by step

Cement is an essential material for the construction of buildings and large structures in general such as dams, bridges, power plants... as it allows to produce concrete once mixed with other materials such as sand and gravel. Mastering cement production and use has been a cornerstone of the development of modern civilization. Having a quality cement is mandatory to be able to build more complex, efficient, resistant buildings. Optimizing the production process of cement is also key for our future as the process is very energy intensive and generates a large part of the CO2 emissions worldwide. This article is focusing on the manufacturing process of cement, its carbon footprint, and gives an overview of the production process step by step, other articles on the website are focusing on unit operations used within this process (mixing, milling...) to which the reader can refer for more information.

1. Cement production process overview

What are the different steps to manufacture cement ?

The production process of cement is not very complex but has many steps to be able to go from raw limestock rocks to a quality cement. The steps are the following [Britannica] :

• Limestone rocks production in a quarry
• Crushing the limestone rocks to get fine grounded limestone
• Addition of clay and sands
• Milling to have a fine mix of limestone, clay and sands
• Pyroprocessing in a kiln that leads to the production of clinker
• Cooling and addition of gypsum
• Milling of gypsum and clinker that will together make the cement

The block diagram of the production process of cement is given below :

Figure 1 : Cement production process block diagram

2. Cement production step by step

2.1 Limestone rock crushing

Limestone rock crushing typically involves the use of a crusher machine. The type of machine used for this purpose can vary depending on the specific application and the desired final product size.

One common machine used for limestone rock crushing is a jaw crusher. This type of crusher uses a fixed jaw and a moving jaw to compress and break down the rock into smaller pieces. Another type of machine that can be used is an impact crusher, which uses high-speed rotating hammers to shatter the rock into smaller fragments.

Both of these machines are effective for crushing limestone rock, but the choice of which one to use will depend on factors such as the desired final product size, the amount of material to be processed, and the specific properties of the limestone being crushed.

2.2 Fine milling of limestone, clay and sands

The production of cement typically involves milling of grounded limestone, clay, and sands. Raw materials are milled into a fine powder, which is then blended and heated to form cement.

One type of machine used in this process is a ball mill. This machine uses rotating cylinders filled with steel balls to grind the raw materials into a fine powder. The cylinders rotate at a relatively low speed, which allows the balls to cascade and grind the raw materials. The resulting powder is then passed through a classifier to separate any oversized particles before being blended and heated to form cement. The use of a ball mill is common in the cement production process, as it is efficient, cost-effective, and able to produce a consistent product size.

2.3 Pyroprocessing in a kiln for clinker production

The production of clinker in a kiln is a key process in the production of cement. The raw materials used in this process typically include limestone, clay, and sands while other minerals such as iron ore and shale can also be used. As explained above these materials must be crushed and ground into a fine powder before being mixed and fed into a kiln.

The kiln is a large, cylindrical furnace that is heated to high temperatures, typically around 1400-1500°C, using coal, natural gas, or other fuels. As the raw materials are fed into the kiln, they are gradually heated and transformed into a molten mixture known as clinker. This process involves a series of chemical reactions, including the release of carbon dioxide and the formation of new minerals. This process step is very energy intensive and is one of the main reasons why cement production is representing a large share of global greenhouse gas emissions.

The objective of using a kiln in the production of clinker is to create the necessary conditions for chemical reactions to occur, which ultimately results in the formation of a hard, solid material that can be ground into a fine powder and used to make cement. Clinker is a key component in the production of cement, as it provides the binding properties that allow the final product to harden and set.

2.4 Clinker milling to produce cement

Once the clinker is produced in the kiln, it is cooled and then ground into a fine powder using a combination of ball mills and vertical roller mills. During the milling process, gypsum is typically added to the clinker powder to control the setting time of the resulting cement. Other additives may also be included to modify the properties of the final product, such as slag, fly ash, or pozzolanic materials.

The milling process involves the use of machines such as ball mills and vertical roller mills. Ball mills are cylindrical devices that rotate around a horizontal axis and use steel balls to grind the clinker and additives into a fine powder. Vertical roller mills, on the other hand, use rollers to grind the clinker and additives, which are fed into the top of the mill and then ground between the rollers and the table below. Both types of mills are commonly used in the production of cement.

Once the clinker and additives have been milled into a fine cement powder, the cement is transported to storage silos and then to packaging and distribution facilities for shipment.

The resulting cement can be used for a wide range of construction applications, including the production of concrete, mortar, and other building materials.

3. Energy requirements and CO2 emissions of cement production

3.1 How much CO2 is emitted per ton of cement produced ?

Cement production is an energy-intensive process due to the high temperatures required to produce clinker in a kiln but also due to the chemical reactions happening during pyroprocessing.

The process of creating clinker involves heating a mixture of raw materials to extremely high temperatures, typically around 1400-1500°C, which requires significant amounts of energy. The use of fossil fuels such as coal, natural gas, and oil is common in cement production, both as a fuel for the kiln and as a source of heat for other parts of the process which leads to CO2 emitted by combustion.However chemical reactions happening in the kiln (calcination) are also responsible for up to 60% of the total of emissions of the cement production process [PCA].

In addition, grinding and milling operations, used upstream (limestone crushing and grounding) and downstream (milling of clinker into cement powder) also requires significant amounts of energy.

The amount of CO2 produced for 1 ton of cement can vary depending on a number of factors, including the specific production process, the type of kiln used, and the source of the fuel used. However, on average, it is estimated that the production of 1 ton of cement results in the emission of around 900 kg of CO2 [PCA]

The share of cement production in global greenhouse gas emissions varies according to the sources consulted. The Portland Cement Association is rating the emissions of CO2 due to cement production at about 1.5% of US total, while other sources [BBC] are considering that up to 8% of global CO2 emission is due to cement. In any case this represents large amount so of CO2, there is therefore a lot at stake to optimize the cement production process and make it more energy efficient.

3.2 How to reduce the environmental impact of cement production ?

Cement industry is under pressure to find solution to decarbonate the cement manufacturing process. Among the solutions being put in place or explored are the following :

• Process Efficiency / Energy savings : as for any process, some improvement can be done by better mastering the process, for example having better automation and control of kiln, or making sure that the heat can be recovered from the kiln and be used to pre-heat materials.

• Alternative fuels: One approach to reducing CO2 emissions in cement production is to replace fossil fuels with alternative fuels such as biomass, waste materials, or renewable energy sources. This can help to reduce the overall carbon footprint of the production process.

• Carbon capture and utilization: Another approach is to capture the CO2 emissions generated during the production process and either to store it or use it for other purposes, such as the production of other materials.

• Alternative cementitious materials: Researchers are also exploring the use of alternative cementitious materials, such as fly ash, slag, or pozzolanic materials, which can be used as a partial replacement for clinker in cement production (reduced the clinker to cement ratio). This can help to reduce the amount of energy and emissions associated with the production of clinker.

• Innovative production processes: There are several innovative production processes that are being developed to make cement production more efficient and sustainable, such as the use of calcined clay, the use of novel kiln designs, and the use of carbon-neutral fuels such as hydrogen.

Source

[Britannica] Cement, Encyclopedia Britannica online
[PCA] Carbon Footprint, Portland Cement Association
[BBC] Climate change: The massive CO2 emitter you may not know about, BBC News online