The US Needs to Lower Cement Emissions. ‘Blended Cements’ Can Help.

Cement manufacturing is a major contributor to global warming - if the industry was a country, it would be the world's third-largest greenhouse gas emitter. To decarbonize the industry and reach net zero, a range of existing and emerging technology solutions will be needed - there's no silver bullet. But in the U.S., there is a relatively untapped solution that can help reduce emissions and make the cement sector more competitive with the rest of the world.

U.S. cement is more emissions intensive than cement produced in other countries. This is in part because the U.S. is lagging in the adoption of low-carbon "blended cements," a readily available decarbonization solution used in many other countries. While shifting U.S. manufacturing processes to embrace blended cements would make sense both economically and environmentally, it's not just manufacturers who need to act - the construction industry and policymakers at all levels need to get on board.

Why US Cement Is Emissions Intensive

To understand why U.S. cement is especially emissions intensive, it's important to discuss its relationship to concrete. While everyday language often confuses concrete and cement, the two materials are distinct. Concrete is the final product we see in buildings, roads and bridges. Cement is the key ingredient that holds it all together and is the primary source of greenhouse gas emissions from making concrete.

The main source of emissions in cement comes from the clinker, an intermediary binding material in cement. Clinker is created using an emissions-intensive process that heats up limestone and other materials. In total, around 70% of carbon dioxide emissions from making concrete results from the production of clinker.

U.S. cement produces more emissions than cement made in many other countries because it contains more clinker. The average clinker-to-cement ratio in the U.S. is 0.88 (i.e., 880 kilograms clinker per ton of cement). The world average is 0.76, and in the EU, India and China, the ratio is between 0.64 and 0.76. In 2019, the emissions intensity of cement production in the U.S. was around 20% higher than that of other major cement-producing parts of the world.

The reason for the difference is unlike the U.S., other countries largely produce cement by swapping some of the clinker with low-carbon materials known as supplementary cementitious materials (SCMs). SCMs include waste byproducts such as slag and fly ash or other natural materials such as clay. This clinker substitution is typically done while making cement at the plant to produce "blended cements." U.S. manufacturers do not often include SCMs in significant quantities while making cement, though SCMs are generally added to the cement during the mixing of the concrete itself.

How Clinker Substitution Plays a Role in Reducing Cement Emissions

Since SCMs are included in concrete in the U.S., it is important to consider whether the difference in cement emissions intensity across countries is really a fair "apples-to-apples" comparison. Comparison of total binder (i.e., binding materials including cement and SCMs) to concrete ratio would be a better measure to compare clinker substitution in the U.S. to other countries. However, data on the extent to which SCMs are replacing cement, and in effect clinker, in U.S. concrete is hard to come by. Consistent public data on the concrete mixes and related greenhouse gas emissions is not available for the thousands of concrete suppliers across the United States.

Cement in its powder form before water is mixed in. Cement production in the U.S. is more emissions intensive than many other parts of the world, in part because it doesn't swap clinker with lower-carbon supplementary cementitious materials. Photo by Aumada/iStock.

Recent estimates of the average amount of cement used to make concrete in the U.S - 10% to 15% by volume - are consistent with the rest of the world. And a recent report indicated that U.S. concrete emission intensities are higher than some other countries. This suggests that the higher emissions intensity of cement production in the U.S. is a real concern that isn't addressed in the process of making concrete. A contributing factor could be the prescriptive content-based standards for concrete, set by various state agencies in 37 U.S. states, that define the maximum amount of cement substitution allowed.

Since there are far fewer cement producers compared to concrete producers, the fastest and most transparent way to address this would be to encourage clinker substitution by adding SCMs when producing cement.

The U.S. Department of Energy (DOE) considers clinker substitution to be the most powerful lever to decarbonize cement through 2030 and estimates that deploying blended cements would reduce U.S. cement emissions by 20% to 25% in its latest Low-Carbon Cement Commercial Liftoff report. In addition to reducing the emissions intensity of concrete, adding SCMs can increase the strength of cement and reduce the overall cost of making cement, the latter of which could generate $1 billion in additional value annually, according to the DOE.

So Why Is the US Lagging on Blended Cements?

The U.S. cement sector is one of the oldest in the modern world and makes cement in a way that has worked well for two centuries. It is also a relatively cheap material with no incentives to use it and its ingredients more judiciously. There are many headwinds to shifting the industry, including long-standing historical practices, institutional inertia, a lack of information about the use of blended cements and the risk-averse nature of the construction industry. This combination leads to low acceptance levels of lower-carbon blended cements by purchasers.

For example, while standards that allow for clinker substitution in cement have been available in the U.S. for more than a decade - such as the ASTM's C1157 performance-based standard for cement and C595 content-based standard for the addition of SCMs - they have not been readily adopted by government procurement agencies and private sector contractors that decide which materials will be used in construction projects. The most commonly used cement in the U.S., known as Portland cement, is produced using the ASTM C150 standard, which allows for only a 5% replacement of clinker. Meanwhile, in the EU, the most common type of cement (CEM II) has a clinker substitution rate between 6% and 35%.

Efforts to Decarbonize the US Cement Industry

U.S. government and industry are both ramping up action and investment to decarbonize the cement sector. The DOE recently awarded $1.6 billion to six projects focused on decarbonizing cement. Three of the six projects funded aim to demonstrate commercial scale production of limestone calcined clay (LC3), a type of blended cement.

There are also currently two bipartisan bills in Congress - the Concrete and Asphalt Innovation Act and the IMPACT Act - which seek to promote research, development and demonstration projects in low-carbon cement and concrete, and create markets for these products. At the 2023 UN climate change conference (COP28), the U.S. reaffirmed its pledge to the UN Industrial Deep Decarbonization Initiative, promising to source low-carbon cement, steel and concrete for publicly-funded projects.

A 2023 DOE report showcased the standard for the blended cement known as Portland Limestone Cement (PLC) which has recently been adopted by all state departments of transportation. This has led to blended cements accounting for 35% of cement consumed in the U.S. in 2023, up from just 3% in 2020. While this is good progress, it has likely reduced cement emissions intensity by a nominal amount (about 5%) because the standard for PLC only allows up to 15% replacement of clinker with raw limestone. Therefore, government agencies need to encourage the use of blended cements with much higher clinker substitution rates such as LC3, which can replace up to 40% of clinker using combinations of SCMs based on existing standards. This case study demonstrates that promoting material standards for similar blended cements like LC3 can be an effective tool for scaling clinker substitution and the use of lower-carbon blended cements.

Cement trucks are parked inside a cement factory in Brooklyn, N.Y. State departments of transportation across the U.S. have adopted the use of Portland Limestone Cement, which has led to blended cements accounting for 35% of cement consumed in the U.S. in 2023. Photo by David Grossman/Alamy Stock Photo.

The Portland Cement Association (PCA), which represents many U.S. cement producers, has set a target of reducing its clinker-to-cement ratio to 0.75 by 2050. And the Global Cement and Concrete Association, which represents 40% of the world's cement producers and 85% of U.S. cement producers, aims to decrease the global average clinker ratio to 0.58 by 2050.

To keep abreast of international targets, the U.S. cement and concrete sector clearly needs to take further steps and needs better policies to support a 0.58 or lower clinker-to-cement ratio target by 2050.

How the US Can Reduce Emissions from Cement and Concrete

Reducing the amount of clinker in cement, as well as the amount of cement in concrete, are decarbonization solutions that must be deployed in tandem and across the value chain.

Government policies can help propel this change. Some ways U.S. policymakers can help increase uptake could include:

• Promoting materials standards. At the cement stage of the value chain, performance and content standards that allow greater clinker substitution, such as ASTM's C1157 and C595 for blended cements, need to be promoted among state agencies and government and private contractors in lieu of the more rigid C150 Portland cement content standard. Similarly, performance standards need to be adopted for concrete to allow greater cement substitution. Standards should be frequently updated to accommodate innovation and newer cement blends. Policymakers should also encourage further production of SCMs through grants and permits, particularly for natural products like clay, to ensure robust supply chains and future availability of these substitutes.
• Developing emissions reporting and benchmarking standards. To improve data availability, cement and concrete manufacturers should be encouraged to adopt standards such as Type III environmental product declarations (EPDs) to ensure consistent reporting of emissions intensity. The U.S Environmental Protection Agency (EPA), which is developing the EPD assistance and carbon labeling programs for harmonized standards, must ensure manufacturers are able to demonstrate lower emissions with the use of blended cements and SCMs in the concrete mix in a consistent and accessible way.
• Leveraging demand-side policies. Concrete producers, who are the primary purchasers of cement, need incentives and market signals to purchase low-emission, blended cements. Procurement policies like Buy Clean and advance market commitments, which aim to use the government's purchasing power to create demand for green products, can be used to build the market for blended cements.
• Increasing awareness across the value chain. To increase awareness and acceptance throughout the value chain, pilot programs and demonstration projects should be launched by agencies such as DOE, EPA and state departments of transportation showcasing the environmental and economic benefits as well as strength and durability of blended cements to contractors and architects.

Why the US Should Ramp Up Efforts on Blended Cement

Clinker substitution to create blended cements is one solution that can be implemented today to reduce emissions from the cement sector. Its potential in the U.S., in particular, needs to be harnessed by increasing the types of blended cements, lowering the amount of clinker in cement blends and improving data reporting to verify reduced cement quantities in and emissions from concrete.

At the same time, multiple emerging technologies such as novel cements, alternative fuels and carbon capture will be needed to completely decarbonize the cement sector. The recent federal investments in these technologies offer unparalleled opportunities toward decarbonizing the U.S. cement sector in the medium- to long-term.

Scaled adoption of blended cements in the U.S. in the near-term would therefore ensure that the U.S. cement sector continues to innovate, positioning the country to become a front-runner in low-carbon cement production.