Greening Concrete: A Major Emitter Inches Toward Carbon Neutrality - Yale E360

The Holcim cement plant in Lägerdorf, Germany. Christian Schwägerl

Concrete is the most ubiquitous manmade building material on the planet, but making it generates massive amounts of CO2 pollution. Companies are experimenting with ways to green the process, from slashing the use of limestone to capturing the carbon generated when it’s burned.

By Christian Schwägerl • October 29, 2024

Bernd Soboll’s favorite spot at his workplace — a cement plant 30 miles north of Hamburg — is an open-air platform almost 300 feet high. From here, the construction manager can see all the steps that go into making the material that has made the modern world possible — its roads, bridges, airports, houses, and skyscrapers.

Near the horizon, a bucket wheel excavator churns through a limestone quarry. From there, chalk is transported to a drying plant, then mixed and ground into a fine powder. This so-called “raw meal” is then pumped up the large tower that holds the viewing platform. While falling back down in large pipes, the chalk is heated until it enters a rotating kiln that reaches 1,500 degrees Celsius.

Cement — a gray powder that acts as a glue when mixed with sand, gravel, and water — is the key ingredient for concrete, the world’s most widely used man-made material. It’s also one of its most problematic, climate-wise. Since the early days of the industrial revolution, coal and other fossil fuels have been used to heat cement kilns to 1,500 degrees. And when limestone is incinerated to form clinker, the precursor to cement, it releases carbon dioxide into the atmosphere. Thousands of plants around the world produce some 4 billion tons of cement a year, generating between 5 and 8 percent of global greenhouse gas emissions, a share larger than that of the entire aviation industry.

“We’re emitting close to 1 million tons of CO2 per year from our plant” in Lägerdorf, Soboll says. But perhaps not for too much longer. Last April, Holcim, the owner of the plant and one of the largest building materials companies in the world, broke ground on a project that costs several hundred million U.S. dollars and aims to convert the Lägerdorf campus, by 2028, into one of the world’s first carbon-neutral cement plants by capturing its CO2 emissions.

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Geologists have calculated that since the 19th century, enough concrete has been produced to pour two pounds of it on every square yard of the Earth’s surface. In recent years, China has become the main producer and consumer of cement and concrete. Between 2011 and 2013, the Asian superpower used as much concrete as the United States did during the entire 20th century. In a study published in Nature Communication in 2023, a team of scientists projected that developing nations alone (excluding China) could, by 2050, double or even quadruple their CO2 emissions from cement production to up to 3.8 billion tons annually

The International Panel on Climate Change, the United Nations scientific body advising governments, says humankind must become carbon-neutral by mid-century by both reducing greenhouse gas emissions and storing carbon removed from the atmosphere using nature-based or technical means. For makers of cement, finding new ways to reduce their carbon footprints, even as demand for their product soars, can’t happen soon enough.

Globally, the industry is working on several fronts at the same time. Some companies are replacing their fossil heat and electricity sources with renewables. Others are reducing the proportion of limestone in clinker and the proportion of clinker in cement, switching to fossil-free materials for additives, and capturing remaining carbon dioxide for either disposal or recycling.

According to Sven Weidner, director of the Lägerdorf “Carbon2Business” project, the plant has already reduced emissions by sourcing electricity from windmills the company erected near its property, by replacing some of the fossil fuel used for heating the kiln with energy derived from burning biomass and nonrecyclable waste, and by bringing down the share of clinker in cement and replacing it with alternative materials. A German federal registry of emissions shows that the Lägerdorf plant has reduced about 20 percent of its CO2 emissions since 2010.

Still, the bulk of the plant’s emissions comes from the very process of turning limestone into clinker, which is all about extracting CO2 from the raw material itself. “The truth is that as long as we use chalk or limestone, there will be CO2,” Weidner says. To reduce those “unavoidable emissions,” he says, the plant’s carbon needs to be captured.

To reach this goal, the new kiln at the plant will use pure oxygen instead of ambient air to burn the raw meal, a change that increases combustion efficiency and excludes nitrogen from exhaust gases, leading to almost pure CO2. Next, that gas is cooled to a liquid, to make it ready for transport. While this will eliminate the plant’s CO2 emissions, capturing carbon comes at a price: it will quadruple the plant’s energy requirement.

As carbon air capture ramps up, major hurdles remain. Read more.

“Luckily, there’s plenty of renewable electricity from wind here in Northern Germany which we can buy from the grid,” Weidner says, referring to the recent boom of renewable electricity sources, which have, to date, met 60 percent of Germany’s needs in 2024. The European Union will support the project with 110 million euros ($120 million) from its innovation funds.

Starting in 2028, the Lägerdorf plant plans to collect up to 1.2 million tons of carbon dioxide a year, then compress and transport it by pipeline to a newly built “CO2 hub” in a chemical park on the banks of the Elbe River, in Brunsbüttel, about 20 miles away. From the hub, the gas could travel in two directions: One goes out to the North Sea by ship or pipeline, where the gas would be injected and stored, permanently, hundreds of feet below the seabed; the other direction involves reusing the gas.

Carbon capture equipment being installed at the Heidelberg Materials cement plant in Brevik, Norway, August 2023. Heidelberg Materials

The sequestration strategy is currently pursued by cement companies all over Europe and in the U.S. And while environmentalists warn about possible leaks, Susanne Buiter, chief scientist of the German Research Centre for Geosciences in Potsdam, says that “carbon capture and storage” (CCS) can be done safely in the saltwater and limestone pores at 600 to 1,200 meters below the seabed. “It will either dissolve as carbonic acid or bind with the limestone,” she says. Injection sites in the North Sea are the German government’s main solution for so-called “unavoidable emissions,” like those from the cement and other industries.

CCS is already being used in countries like Norway, where cement producer Heidelberg Materials plans to capture half of the CO2 emissions of its Brevik plant, starting in 2025, and store them in former natural gas deposits under the North Sea. In the U.S., 15 plants — none of them making cement — captured about 24 million tons of CO2 last year, according to the Congressional Budget Office. CCS is used in the U.S. mainly by the oil industry to force more oil out of partially depleted wells. No such plans exist for Germany, where domestic fossil fuel production is being aggressively phased out. The country’s economy minister, Robert Habeck, from the Green Party, states, however, that without disposing of “unavoidable emissions” from a range of industries below the North Sea, Germany will not reach its national goal of climate neutrality by mid-century.

Holcim and other companies are also pursuing another approach, called “carbon capture, utilization, and storage” (CCUS), in which the liquified gas could be sold as a raw material to other industries. That’s the option Weidner prefers for the Lägerdorf plant: “We should build a circular carbon economy and use CO2 as much as possible as a resource,” he says.

Possible customers for his future purified CO2, he says, include companies growing food in greenhouses and industries looking for ways to replace fossil carbon with new sources. “The chemical industry could use our gas to make synthetic fuels or plastics without fossil oil,” he says.

The climate benefit of CCUS is controversial, though. Economically, selling CO2 from a cement plant as a product trumps paying a hefty fee for its underground disposal. But in terms of keeping carbon out of the atmosphere, CCUS is not a perfect solution. “Where carbon is used several times, emissions are shifted all the way downstream of the last use,” the Umweltbundesamt, Germany’s Environmental Protection Agency, warns, adding: “This recirculation only leads to a temporal and local shift, but not to a reduction of the original emissions.”

The new Lägerdorf plant and the nearby CO2 hub will be designed to facilitate both options. “The hub can be used flexibly and facilitate both carbon use and storage,” Weidner says. “That’s the sensible thing to do as we don’t yet know which options will be available to us.”

Workers in Sugar Land, Texas, pour concrete bound by Terra CO2 cement, which is made with mining waste. G. Lyon Photography / Terra CO2

Others in the cement industry are pursuing more radical changes, like replacing limestone altogether.

Terra CO2 Technologies in Utah, for example, aims to replace a portion of limestone with powders made from mining waste — reducing emissions by 70 percent compared to pure Portland cement, the most common type used globally, according to the company’s CEO. Oliver Blask, a concrete researcher at Germany’s Ingolstadt University of Applied Sciences, thinks the potential of this approach is limited. “The time when we could achieve great effects through dilution is over,” he says. Blask believes that the most promising approach is to replace limestone at the very beginning of the process with alternative materials that don’t release CO2 when burned. Experiments have been underway for some time with special clays or ground slag from iron and steel production. “These alternatives take carbon out of the equation and could reduce CO2 by up to 100 percent,” he says.

Earlier this year, the U.S. Department of Energy (DOE) offered $1.6 billion in Inflation Reduction Act funding to companies pioneering the use of alternative materials. Brimstone Energy, based in Oakland, California, is replacing limestone with carbon-free silicate rock; Virginia’s Roanoke Cement Company and Denver-based Summit Materials are developing methods that use so-called calcined clays.

According to the DOE, Summit’s four planned plants have the potential to annually reduce CO2 emissions by 1.1 million tons while “also addressing 2 percent of the U.S. 2030 projected demand for cement.” Another cement start-up called Sublime Systems, founded by two MIT scientists and based in Somerville, Massachusetts, relies on an electrolytic reactor instead of a kiln to process non-carbonate materials, claiming to generate no carbon emissions.

Many different solutions, from carbon capture to new ingredients, will have to be deployed to reach climate neutrality for the world’s most important building material. But even at the very end of concrete’s life cycle, there is potential to draw down emissions.

The Swiss start-up Neustark, founded in 2019 as a spin-off from ETH Zurich University, describes concrete from demolished buildings as “the largest waste stream in the world,” with around 900 million tons of waste produced annually. The company has developed a technology to accelerate crushed concrete’s ability to absorb and bind carbon dioxide by injecting CO2 produced in biogas plants into concrete granulate in recycling plants.

Scientists are trying to coax the ocean to absorb more CO2. Read more.

It takes decades for concrete to absorb CO2 because the gas is highly diluted. But according to the company, a ton of crushed concrete can absorb 20 pounds of concentrated carbon dioxide within hours. Northeast of Berlin, a plant that stores 1,000 tons of CO2 per year in crushed concrete has been in operation since 2023. Neustark has 19 plants in operation, with 40 more planned or under construction. The company aims, by 2030, to store a million tons of greenhouse gases in concrete granulate per year.

Exactly 200 years after Portland cement was patented in the United Kingdom, one thing is clear: Both Earth’s climate and the future of the construction industry depend on the rapid decarbonization of mankind’s most important building material.

Christian Schwägerl is a Berlin-based journalist who writes for the German newspaper Frankfurter Allgemeine, Spectrum of Science, and other media outlets. He is cofounder of RiffReporter, a freelance cooperative, and author of The Anthropocene: The Human Era and How it Shapes Our Planet. Follow him on Mastodon. More about Christian Schwägerl →

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