Heating Limestone: A Major CO₂ Culprit in Construction
Why USGS scientists are talking about cement’s carbon footprint
While most people realize that cars, planes, and power plants contribute to climate change, there's another major source of greenhouse gases that's often overlooked. It's all around us –perhaps even right where you are as you read this. It's concrete, and the cement used to make it.
Most Consumed
Concrete is the most widely consumed human-made material on earth and a significant contributor to climate change.
Cement is the main ingredient in concrete and its claim to fame is its role as the essential foundation for buildings, roads, sidewalks, and many other structures. Concrete itself is composed of three key components: cement, water, and aggregate –a blend of gravel and sand. In some states, it’s possible to source all these ingredients locally, but it’s not as easy as it used to be because many cement plants have closed. As a result, cement is often imported from one state, or even country, while aggregate may come from another, reaching their final destination via barge and other means of transport.
Once the ingredients reach their destination, everything is mixed up and channeled into trucks with round drums which are sent out to job sites. This is happening every day all over the world.
Carbon dioxide, or CO2, is the main greenhouse gas released through cement production, significantly impacting global warming by contributing to 7-8% of global carbon emissions. To put this in perspective, it’s roughly equivalent to the CO2 emissions from all the cars in the world.
Cement’s Frequently Overlooked Carbon Footprint
You might assume that the emissions from concrete are primarily due to transporting ingredients from distant locations. While transport is a significant contributor, making it important to find nearby sources, the main contributor is actually a single ingredient: cement. The primary source of CO2 emissions in cement production comes from cooking limestone.
Here’s how it works: Cement is made by grinding up limestone and then heating it, along with clay or shale, in a kiln. The temperature of the kiln ranges from 2,700 – 3,000 degrees Fahrenheit. During this process, CO2 is released in two ways: 1) from the fossil fuels used to heat the kiln and 2) from gases released from the limestone itself. When limestone (CaCO₃) is heated in a kiln, it undergoes a chemical reaction called calcination. This process produces calcium oxide (CaO) and releases CO2 as a byproduct. Calcination accounts for almost two-thirds of the total CO2 emissions from cement production. The other one-third comes from the energy used to heat the kiln.
When mixed with cement, a pile of wet sandy gravel transforms into the cohesive mixture we call concrete.
Future Advisory Role
In April of this year, Darcy McPhee, PhD, a USGS expert who manages the Earth Mapping Resources Initiative was invited by the Department of Transportation's Federal Highway Administration to help select candidates, along with representatives from state agencies, for a Working Group that will conduct a study on access to covered resources for infrastructure projects. In this context, ‘covered resource’ means a common variety material used in transportation infrastructure construction and maintenance, including stone, sand, and gravel.
USGS scientists have had a history of playing an advisory role in the federal inter-agency community on diverse science issues. The inclusion of the USGS in an early Federal Highway Administration meeting highlights its potential to play a future advisory role in tackling the environmental challenges of cement production.
Recognizing the environmental impact caused by heating limestone, government and industry are coming together to explore new solutions. In July, the White House convened the Concrete Innovation Summit, bringing together leaders from government agencies and innovative U.S. cement manufacturers. The summit highlighted progress in accelerating the commercialization of clean cement creation. Part of the focus of the discussion included new minerals and materials that could replace or offset the impact of heating limestone.
Supporting State Efforts
While altering cement production methods would have the biggest impact on the 7-8% of global carbon emissions attributed to this sector, the most immediate and accessible approach is through supply chain changes, such as locally sourcing industrial minerals like onshore and offshore sand and gravel, crushed stone, and ingredients of cement closer to where they will be used. Another important strategy involves replacing a portion of the cement in the concrete mix with fly ash, a waste byproduct of coal combustion. Currently, fly ash is used to replace up to approximately 30% of the cement in concrete.
While state agencies are typically responsible for identifying and managing local resources for construction projects, including locating sources of high-quality fly ash within the state, the Association of American State Geologists reports that most states lack the detailed mapping and data on construction materials needed to fully meet their infrastructure goals. This is where the USGS Earth Mapping Resources Initiative, or Earth MRI will play a crucial role.
Boosting U.S. Sustainable Cement Manufacturing
Most solutions from the cement industry to reduce emissions focus on integrating alternative materials, with fly ash being widely used in concrete production. In contrast, calcium-bearing silicates, used in the cement production process as a substitute for traditional raw materials, are still in the early stages of adoption. The hope among industry and government leaders is to boost the competitiveness and environmental sustainability of U.S. cement production. One way to achieve this is through the commercial application of technologies and public-private partnerships, such as Manufacturing USA, in supporting the development and testing of alternative materials.
The USGS's advanced geological mapping capabilities are crucial not only for identifying valuable new materials, but also for pinpointing materials that should be avoided. According to the American Society for Testing Materials, there are 14 groups of minerals that, when present in aggregates, can reduce the strength and longevity of concrete. By providing detailed geological maps, the USGS helps engineers and builders identify and steer clear of these detrimental minerals, ensuring the production of more durable and long-lasting concrete structures. This dual capability of locating both beneficial and harmful materials underscores the comprehensive value of USGS assessments in quantifying resources, including how much of the resource is of appropriate quality.
The ongoing discussions about cement closely align with the USGS’s commitment to examining the full lifecycle of resources, from their occurrence in the ground to the impacts of their development, like CO₂ emissions.
"There are no modern maps or assessments for many of the proposed innovative construction materials, which are essential for evaluating their producibility,” said Colin Williams, USGS Minerals Resources Program. “The USGS's expertise in geological mapping and assessments uniquely positions it to address these gaps."
Looking ahead, the USGS will be instrumental in evaluating and interpreting data on new materials considered as replacements for traditional ones. By leveraging its capabilities, the USGS can provide comprehensive analysis and insights that support sustainable and innovative construction practices, paving the way for a greener future
This story is part of the “Unleashing the Science” series, showcasing how bureaus within the Department of the Interior produce and apply science to ensure responsible management decisions for our planet now and for the future.
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