Laying the Foundation of Cement and Concrete Decarbonization

A version of this was originally published by the Industrial Innovation Initiative (i3). I3 is convened by the World Resources Institute and the Great Plains Institute. Click here to view the full article.

From skyscrapers to scraped knees, concrete is foundational in our everyday lives. Although it is the most used human-made resource in the world, the emissions packed into it are less noticeable. Cement, the key ingredient in concrete, makes up around 8% of global CO₂ emissions. In 2022, the world produced 4.4 billion tons of cement releasing 2.7 billion metric tons of CO₂. In the same year, the United States produced 90 million tons of cement releasing 70 million metric tons of CO₂. That is the equivalent to over 15 million fossil fuel-powered cars driven for one year.

Concrete is a mix of cement, different sizes of crushed rock and sand (known as aggregate), air, and water. While cement only constitutes around 10% to 15% of concrete by mass, it accounts for nearly 80% of its CO₂ emissions. These numbers help explain why, if the cement industry were a country, it would be the third-largest emitter of CO₂ behind China and the United States.

Meanwhile, demand for cement and concrete is increasing. This is largely due to a growing global population and urbanization. This sector is also critical to the global economy and workforce:  it contributes over 5% of global gross domestic product, or GDP, and nearly 8% of employment worldwide.

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How Concrete is Made: Cement Production, Emissions, and Emissions Solutions

How does a single industry end up producing such a large share of global CO₂ emissions?

Before diving into the where and how of CO₂ emissions from cement, it’s important to take a step back to understand why industrial emissions present a particular challenge when it comes to meeting midcentury net-zero targets in the U.S. and abroad.

In industrial production, CO₂ is emitted into the atmosphere in two ways: at the facility or at the power plant supplying the facility with electricity. Emissions released at the facility are called “direct emissions” and can come from combustion or non-combustion sources. Most industrial sector emissions originate from fossil fuels that are combusted for heat. The second type of direct emissions come from chemical reactions inherent to the production process that release CO₂ as a byproduct and are called “process emissions.” The main example of an industrial product that emits more process emissions than combustion emissions is cement. Roughly 60% of cement’s emissions are due to chemical reactions inherent in turning limestone into cement and 40% come from the heat that drives that process. Industrial processes also require energy from off-site electricity generation. These emissions from the power plant are called “indirect emissions.”

The difference between direct and indirect emissions is important when considering the full scope of solutions required for industrial decarbonization. However, direct emissions contribute most to the carbon intensity of cement.

Figure 1. Breakdown of Emission Sources from Concrete Production

Cement production accounts for 80% of emissions in concrete. This is predominantly a mix of process and combustion emissions from the kiln as explained in Step 2 of the production process outlined in the paragraphs below.

Graph based on data from the 2018 Energy Transitions Commission’s Mission Possible Cement Report.

Production Process 

The concrete production process can be simplified to four major steps: quarrying, grinding, and pretreating; the production of clinker; the production of cement; and finally, the creation of the end product, concrete. This process outlined below includes possible decarbonization solutions applicable at each step.

STEP 1: The first step in the cement production process is the quarrying and grinding into powder of raw materials, primarily limestone. Depending on the facility, these materials undergo various pretreatments before entering the kiln for calcination.

ENERGY EFFICIENCY: Investment in upgraded equipment for quarrying and grinding, or equipment for pretreatments, such as preheaters and precalciners, can improve the energy efficiency at the plant.

STEP 2: In a rotary kiln, the limestone mixture is heated to around 1250 C to produce clinker, the main, and most critical, ingredient in cement. This process is called calcination.

As the mixture is heated, it breaks down into calcium oxide (clinker) and CO₂. The equation below shows how this transformation occurs. Clinker production accounts for roughly 90% of the emissions associated with cement production, and is a mix of process and combustion emissions.

Calcination: CaCO₃ (calcium carbonate from limestone) + heat  = CaO (calcium oxide – clinker) + CO₂ (emissions)

• CARBON CAPTURE TECHNOLOGIES: To address process emissions, carbon capture technologies can play a critical role. Because these carbon dioxide emissions are released during the chemical transformation of the material, they are unavoidable in conventional cement production. However, cement production facilities can be retrofitted with carbon capture equipment that captures the emissions from the precalciner or the kiln before they are released into the atmosphere. This captured CO₂ is then transported to where it can be used beneficially in another product or process, such as concrete curing, or sequestered underground.
• FUEL SWITCHING: The high temperature heat needed in the kiln to produce clinker presents a challenge to reducing combustion emissions. Currently, many facilities use coal and natural gas to achieve high heat. Substituting these fossil fuels with lower-carbon feedstocks – such as biofuels, hydrogen, or waste feedstocks – can help reduce these emissions while achieving the same temperatures.
• ENERGY EFFICIENCY: Improving the efficiency of kilns and clinker coolers can also help maximize energy productivity throughout the clinker production process.

STEP 3: The clinker is then cooled before being blended with a small amount of gypsum, limestone, or other additives. The mixture is then ground to a powder to form cement. Cement produced in this manner is called Portland cement and is the most commonly used cement worldwide.

• BLENDED & NOVEL CEMENTS: Emissions from cement production can be reduced by lowering the clinker content and substituting the cement with supplementary cementitious materials (SCMs) to create ‘blended cement’. Clinker acts as the binding agent, or glue, in cement. When the amount of cement is reduced, it needs to be replaced with another glue. SCMs include materials such as natural pozzolans or some industrial waste materials, including fly ash or steel slag. Novel, low-carbon alternatives to traditional Portland cement are also being explored, such as alkali-activated cements. These cement types have different ingredients and can potentially lower process and combustion emissions during cement production. However, many of these mixes face barriers to deployment, such as high cost and lack of demand, incompatibility with prescriptive standards, and low public awareness. A supportive policy and regulatory landscape is needed to help these novel cements accelerate development and commercialization.

STEP 4: Lastly, the cement is mixed with water, aggregates, and sand to form the end product, concrete. Roughly 80% of the CO₂ emissions from concrete come from the cement production process. The other 20% comes from the energy needed for activities across the value chain, including quarrying, transport, and grinding.

Concrete = Cement + water + aggregates + sand

• MATERIAL SUBSTITUTION & EFFICIENCY: The carbon intensity of concrete can be further reduced by lowering the amount of cement content and substituting again with SCMs. Alternative aggregates, such as recycled concrete, waste products, or synthetic aggregates, can also be substituted to reduce the carbon intensity of the material. Total emissions from the concrete supply chain can also be lowered by optimizing the use of concrete in construction to ultimately use less of it.

Policy Landscape & Market Solutions

The applicability of each of these decarbonization solutions is often dependent on location and facility-specific factors, such as the facility’s age, the availability of alternative fuels and materials, and the proximity of infrastructure for transport, use, or storage of captured CO₂. Additionally, many of the solutions described have various levels of technological readiness and cost, making a supportive policy landscape crucial for spurring deployment.

Procurement is one type of policy instrument that can create favorable market conditions for faster development and deployment of a new product or technology. Procurement of low-embodied carbon materials requires disclosure, incentives, and standards to leverage the purchasing power of public agencies at the federal, state, and local level and of private companies.

Several best practices can be considered when designing effective procurement policy:

• Implementing effective procurement practices requires comparable, easily reported data on emissions intensity across products (i.e., disclosure). This is commonly achieved through environmental product declarations (EPDs) and carbon labels, which are sometimes compared to a nutrition label for construction materials.
• Complementary incentives can encourage early participation and broader implementation from industry through financial support for technological upgrades, technical assistance or bid incentives.
• Setting emissions limits and benchmarks for eligible products encourages domestic manufacturing and innovation while discouraging emissions outsourcing. To discourage emissions leakage, some countries are also considering a carbon border adjustment mechanism to tax imported cement and concrete based on their carbon intensity.

Effective procurement policies can leverage government purchasing power to play a significant role in supporting domestic manufacturers by creating a demand for green products. With the Bipartisan Infrastructure Law and Inflation Reduction Act increasing demand and support for low-carbon cement in domestic infrastructure projects, both state and federal agencies are poised to play an essential part in defining sustainable procurement policy informed by industry.

In December 2021, the Biden administration announced its Federal Sustainability Plan and Executive Order 14057, launching a Buy Clean Task Force to promote the use of low-carbon, made-in-America construction materials. In response, the General Services Administration issued its first “Buy Clean” standard for concrete, and the Department of Transportation released its first agency-wide “Buy Clean” policy to bolster more sustainable procurement practices across its programs. Additionally, in September 2022, the federal government announced that it will prioritize the purchase of concrete with lower levels of embodied greenhouse gas emissions.

Meanwhile, many states have passed or introduced legislation to promote the use of low-carbon concrete in public projects, including California, Colorado, New Jersey, and Oregon.

Procurement policies are only one side of the equation. To use novel and blended lower carbon cement and concrete mixes in construction projects, less prescriptive, performance-based material standards must also be adopted and promoted for use of these mixes.  

As government officials design domestic decarbonization policies, it will also be key to consider trade policy and the global marketplace such as border carbon adjustments. International cooperation can play an important role as well. Coalitions like the Industrial Deep Decarbonization Initiative, with the U.S. as a member, can encourage the use of green public procurement and the adoption of harmonized and ambitious standards for low-emission cement.

Several emerging technologies will be required to achieve cement sector decarbonization on a 2050 timeline—but these vary widely in terms of cost, timeline for deployment, and maturity. For instance, while carbon capture technologies will be essential for the industry to reduce emissions in the long term, carbon capture has not yet reached a commercial scale of deployment. Likewise, novel cements and alternative materials will require further research and development to enable near-term use. For the cement sector to decarbonize, research, development, and demonstration for both near- and long-term solutions will be critical to achieving cost-effective and timely deployment.

By creating a favorable market and policy environment, these policies can drive decarbonization along each step of the value chain while retaining and generating workforce opportunities and bolstering the U.S. manufacturing sector.

For more information, contact Ankita Gangotra and Katie Lebling.