The production of cement contributes to the emission of carbon dioxide (CO2) and if the cement industry were a country, it would be the third largest emitter in the world. This is consistent with the findings of an extensive analysis conducted by ZENODO that after fossil fuels and land-use change, cement is the third largest source of anthropogenic emissions of CO2.
Just to set the record straight, while people feel free to use the terms cement and concrete interchangeably, the reality is that cement is a binder, a substance used for construction that sets, hardens, and adheres to other materials to bind them together. Cement is:
- An ingredient of concrete;
- Cement comprises from 10 to 15 percent of the concrete mix, by volume;
- Concrete is basically a mixture of aggregates and paste;
- The aggregates are sand and gravel or crushed stone;
- The paste is water and Portland cement; and
- Concrete gets stronger as it gets older.
Portland cement is not a brand name, but the generic term for the type of cement used in virtually all concrete, just as stainless is a type of steel and sterling a type of silver.
Cement is the basic component of construction and it is also the most broadly used construction material.
Cement production has undergone tremendous development since its beginnings some 2,000 years ago. The industrial production of cements started in the middle of the 19th century, first with shaft kilns, which were later replaced by rotary kilns as standard equipment worldwide. The current annual global cement production has reached 2.8 billion tonnes, and is expected to increase to some 4 billion tonnes per year in 2050.
The cement production process, for example, starts with mining of limestone, which is then crushed and ground to powder. It is then preheated to save energy before being transferred to the kiln, the heart of the process. The kiln is then heated to a high temperature of up to 1480 degrees to convert the material to a molten form called clinker. The clinker is then cooled and ground to a fine powder with other additives and transferred to storage silos for bagging or bulk transportation.
The production of cement is either through the wet or dry process with the dry process as the preferred option because of the lower energy intensity:
- Wet Process: It refers to grinding raw material into slurry after mixing with water and then feeding them into the wet process kiln for drying and calcination and finally forming clinker. The slurry’s water content is usually between 32 to36 percent; and
- Dry Process: It means that after raw materials with different particle sizes are dried, broken and ground into powders of certain fineness, they will be sent into the dry process kiln for calcining, finally forming clinker. Besides, the raw material powder can also be made into raw material balls by adding a proper amount of water and then be directly sent to the Lepol kiln for calcining. This method is called a semi-dry process.
It is a known fact that the construction industry drives the production of cement to meet the demand for constructing new buildings, airports, highways etc. and is thus an important determinant of cement subsector energy consumption and CO2 emissions. Just as a point of reference:
- The buildings and construction sector accounted for 36 percent of final energy use and 39 percent of energy and process-related carbon dioxide (CO2) emissions in 2018 and 11 percent of which resulted from manufacturing building materials and products such as steel, cement and glass.
Initial estimates suggest that 4.1 Gt of cement were produced globally in 2019. Production reached a high of 4.2 Gt in 2014 and has since remained at around 4.1 Gt. China is the largest cement producer, accounting for about 55 percent of global production, followed by India at 8 percent.
According to a report, Global Construction 2030, published by Oxford Economics, the volume of construction output will grow by 85 percent to $15.5 trillion worldwide by 2030, with three countries – China, US, and India – leading the way and accounting for 57 percent of all global growth. At the same time the benchmark global study – the fourth in a series from Global Construction Perspectives and Oxford Economics – shows average global construction growth of 3.9 per annual to 2030, outpacing that of global GDP by over one percentage point, driven by developed countries recovering from economic instability and emerging countries continuing to industrialize.
Cement production is an enormous source of CO2 globally. The main reason cement emits so much CO2 because more than half of emissions from cement are Process Emissions and the remaining emissions come from Energy Emissions.
- Process Emissions: The direct emissions of cement occur through a chemical process called calcination. Calcination occurs when limestone, which is made of calcium carbonate, is heated, breaking down into calcium oxide and CO2. This process accounts for more than 50 percent of all emissions from cement production. The CO2 generated from this process is estimated to be 1.5 Gt per year.
- Energy Emissions: Indirect emissions are produced by burning fossil fuels to heat the kiln. Kilns are usually heated by the energies and the combustion of these fuels produces additional CO2 emissions, just as they would in producing electricity. This represents around 40 percent of cement emissions. Additionally, the electricity used to power additional plant machinery, and the final transportation of cement, represents another source of indirect emissions. The CO2 generated from this process is estimated to be 0.8 Gt per year.
This process consumes a total of 357 Mtoe Per Year and the configuration of the energies involved in the process is illustrated in the following graph: