This blog explores how concrete production generates CO₂, why cement is the main contributor, and the correlation between these emissions and local air quality. Key points include the impact of limestone calcination, kiln operations, and fuel use in driving emissions, and the potential for mitigation through material optimization and process efficiency. While concrete is integral to modern infrastructure, even minor, targeted actions in production and design can deliver significant environmental benefits, making sustainable concrete practices both necessary and achievable.

Concrete is the basic building block of today’s cities and other infrastructure. Concrete contributes significantly to greenhouse gas emissions during cement production. Cement, which is the primary binding agent in concrete, produces a great deal of CO₂ emissions due to the chemical reaction that occurs when limestone is heated in the kiln (to produce clinker). In addition to cement, concrete is made from other constituent materials, such as sand and gravel. To make responsible decisions about the construction and use of cement, concrete, and similar materials, it is important to understand how these emissions affect climate change and air quality at local and global levels.

The carbon footprint of concrete is heavily influenced by the cement it contains. However, its production process is highly carbon-intensive, making it the primary contributor to CO₂ emissions in the concrete industry. The first significant source of emissions from the cement industry is calcination, the chemical reaction that produces cement. The calcination process occurs when limestone (which is mainly CaCO₃) is heated in a kiln to a temperature high enough that CaCO₃ will decompose into lime (which is primarily CaO) and CO₂ gas.

CaCO₃ → CaO + CO₂

The CO₂ released during this process is considered to be a ‘process emission’ because it is released as a result of calcination and the use of thermal energy in the production of clinker (which is the primary type of cement used in construction).

To produce clinker from CaO, the clinker must be heated to 1,400-1,450°C. This requires each cement manufacturing plant to consume substantial amounts of fuel, thereby increasing process-related CO₂ emissions. Today, many cement plants also use alternative fuels in place of/ in addition to coal and petcoke. Regardless of the fuel type chosen, the energy required to maintain the high-temperature conditions for thermal clinker production remains a significant source of CO₂ emissions from the cement industry.

Together, chemical decomposition of limestone and energy-intensive kiln operations make cement production one of the most carbon-heavy industrial processes globally. As a result, most of concrete’s CO₂ emissions are effectively “locked in” at the cement stage, shaping why emission reduction efforts must focus heavily on how cement is produced rather than on concrete use alone.

Cement and concrete are often confused, leading to confusion about their respective carbon footprints. Cement is an industrial product with very high carbon emissions during its manufacturing process; however, concrete is a mixed material consisting primarily of cement combined with various aggregate materials, water, and other additives. Recognizing the difference between the two is key to understanding the primary sources of CO₂ emissions.

The majority of CO₂ emissions associated with concrete are attributable to cement production. Although the total amount of cement is only 10-15% by weight of the finished concrete mix, it represents the greatest share of the embodied carbon in concrete; therefore, it is consistently identified as the carbon footprint of cement across the entire concrete supply chain. The primary impact of aggregate material is sorting, crushing, and transporting it from the quarry to the end point. These procedures account for only a fraction of the energy and emissions generated by the processing of cement and the chemical reactions associated with cement production.

While it is true that concrete-related decisions may seem less significant than other industry decisions, they are multiplied by the amount of concrete manufactured and consumed. Even a cubic meter of concrete production results in significant carbon savings at scale by optimizing concrete mix designs and specifying concrete’s unique performance characteristics based on its use rather than its physical properties. By adopting more efficient concrete construction methods, it is possible to use less cement while still achieving the desired strength and durability. This will allow for significant reductions in carbon emissions from cement production, even as the production process continues to generate substantial greenhouse gas emissions.

Cement kilns have been used since antiquity; they are primarily equipped with a rotary kiln preheater and a rotary kiln precalciner. The rotary kiln is the most energy-consuming part of the cement production process and is also a major source of air pollution.

The type of fuel burned in a kiln (coal, petroleum coke, or alternative fuel) affects the amount of CO₂, NOx, SO₂, and particulate matter emissions produced during combustion in addition to the CO₂ emissions from the calcination of limestone and fugitive dust generated during the handling and grinding of materials, which contributes to the increased local PM₁₀ and PM₂.₅ concentrations. Fuel selection and the raw material production are the primary drivers of the emissions.

The CO₂ emissions from concrete production and the combustion of fossil fuels affect air quality, as both processes release CO₂ and other harmful substances into the atmosphere. Cement manufacturing and fuel combustion generate air pollutants, such as particulate matter and reactive gaseous compounds (i.e., GHGs), which also affect local air quality. Given the strong correlation between emissions from cement production and fuel combustion, and those associated with other air quality issues, concrete production and CO₂ emissions must be viewed as climate and air quality issues.

The cement/concrete industry contributes substantially to global industrial CO₂ emissions, many of which stem from rapid urbanization and infrastructure expansion, as well as the longevity of many of these products (40+ years). With these factors contributing to a future increase in demand for cement/concrete products, the responsibility of this industry to help create a climate change and healthy air is also increasing.

From an industry standpoint, cement production has a unique place because much of the CO₂ produced is a byproduct of the production process; thus, it poses a challenge for manufacturers, regulators, and stakeholders in the construction process. The increased attention around air quality regulations and sustainable reporting has resulted in a new normative level of operations, where emissions reduction will no longer be considered a bonus but rather a baseline requirement for operating at an industry level.

To reduce CO₂ emissions from concrete production, actions must be taken on both the materials side (e.g., optimizing mix designs) and the process side (e.g., improving kiln efficiency, using alternative fuels, and optimizing firing processes). The environmental benefits of the two approaches will be substantial in improving air quality and reducing greenhouse gas emissions. However, there must also be a commitment to using data to drive decision-making around lowering carbon intensity. Therefore, tracking data on emissions, fuel performance, and other operational parameters is essential to achieving continuous improvement in carbon emissions reduction and to measuring tangible, sustained results.

The monitoring of air quality has enabled the achievement of emissions-reduction targets through a decision-support mechanism. Monitoring in Cement & Concrete manufacturing allows operations managers to evaluate how changes in processes can impact actual emissions, rather than relying on the expected output of the changing process. At the plant level, monitoring around the kiln(s) and across the site boundaries enables kiln and operational managers to assess the efficacy of control methods and refine operational processes to reduce CO₂ intensity and co-pollutant emissions.

Beyond the manufacturing plant, it is necessary to enable regulators and nearby communities to better understand how manufacturing practices impact the facility’s and the local community’s health. Data from both plant emissions monitoring and the ambient environment can be used to establish and sustain emission reductions, ensure accurate measurement of emissions in the local ambient air, and support continual decarbonization of the facility and long-term carbon emissions objectives.

The carbon footprint of concrete illustrates the difficulty in reconciling the need for infrastructure with the need for environmental stewardship. All stakeholders involved in cement and concrete production should work together to develop strategies that minimize material use and increase product efficiency. It should be noted that while concrete emissions are nearly unavoidable during the cement stage, proactive measures such as real-time monitoring can promptly control them. Awareness of these issues and informed decision-making will help align construction industry practices with climate and air-quality-related goals.