For Cement’s Massive Carbon Footprint, Some Concrete Steps

On a scorching July morning at a testing facility outside of Paris, a cadre of scientists, engineers, and architects wearing hard hats and safety goggles watched through protective glass as a machine molded a soupy, grey mixture into batches of brick-sized blocks. Further along the line, a forklift operator carefully loaded the blocks into a curing chamber like loaves of bread in a bakery.

What they were witnessing was a trial run of a new concrete-making process developed by Solidia Technologies, one that the New Jersey-based company hopes will dramatically reshape the way this building material is made. By tweaking the chemistry of one of concrete’s essential ingredients — cement — and altering its curing process, the company says it can make concrete cheaper than the traditional process, while at the same time drastically cutting the carbon emissions associated with cement production.

Cement is one of the global economy’s most carbon-polluting industries. Responsible for about 8 percent of global carbon dioxide (CO2) emissions in 2015, if it were ranked with individual countries, the cement industry would be the third-largest greenhouse-gas emitter in the world behind only China and the United States. And this already outsized footprint is only projected to grow in the coming decades as economic development and rapid urbanization continue across Southeast Asia and sub-Saharan Africa. According to the International Energy Agency and the Cement Sustainability Initiative, by 2050 cement production could increase by as much as 23 percent.

This poses a significant challenge for combating climate change. One 2018 study estimated that cement-related emissions will have to fall by at least 16 percent by 2030, and by far more after that, if nations are to meet the 2015 Paris Climate Accord target of staying below 2 degrees Celsius of warming this century.

According to industry experts, reductions on that scale will require the widespread adoption of less carbon-intensive cement alternatives now under development in labs around the world. But in a market ruled by a handful of major producers wary of making changes to their existing business models, an absence of strong policies incentivizing greener technologies, and a construction industry reasonably cautious about novel building materials, the prospects for such a radical shift are far from certain.

The concrete industry’s staggering carbon footprint is mainly due to the sheer scale of the material’s use. A mundane combination of sand and gravel glued together by cement, this man-made stone is so ubiquitous that it is part of nearly every structure of our modern built environment.

“Today’s society would not have been possible without concrete,” said Robert Courland, author of the book “Concrete Planet.” It’s the most abundant synthetic material in existence, and according to the Cement Industry Federation, an Australian trade group, if you divvied up all of the concrete used around the world each year, three tons of concrete would go to every person on the planet, making it the world’s second most-consumed resource after water.

With its abundance, concrete takes a mammoth toll on the environment. The process for making Portland cement, the most common form used to produce concrete, for example, is one of the most carbon-intensive manufacturing processes in existence; manufacturing just one ton yields upwards of 1,000 pounds of carbon dioxide.

The process begins with crushed limestone, which is mixed with other raw materials and then fed into a large, rotating, cylindrical kiln heated to more than 2,600 degrees Fahrenheit. The kiln is inclined at a slight angle, and materials are poured into the raised end. As they move toward the roaring blast of flame at the kiln’s lower end, some components are burned off as gases, while the remaining elements unite to produce gray balls known as clinker. The lumps of material — about the size of marbles — are cooled and then ground into a fine powder to form the key binding element that allows concrete to harden when cured with water.

This process, which has barely changed since it was invented nearly two centuries ago, produces carbon emissions in two ways. First, fossil fuels are typically burned to heat the kiln to the high temperatures required to break the materials down, emitting carbon in the process. In addition, the thermal decomposition process itself results in emissions, as carbon trapped in the limestone combines with oxygen in the air to create carbon dioxide as a byproduct.

As much as two-thirds of cement-related carbon emissions arise from this reaction, which is why cement making is considered such a particularly difficult process to decarbonize, said Gaurav Sant, a professor of civil and environmental engineering at the University of California, Los Angeles (UCLA). Since carbon dioxide emissions are a part of the chemical process itself, he said, even a complete switch to low- or zero-carbon energy sources for heating the kilns would solve only part of the problem.

Cement producers have already taken steps to reduce emissions. Thanks to improvements in energy-efficiency and tweaks to concrete mixtures, the average carbon dioxide intensity of cement production has decreased by 18 percent globally over the past 20 years. Some companies have also installed technology to prevent carbon dioxide emissions from ever entering the atmosphere, though such systems can only capture so much and may not prove to be feasible on the scale required to make a significant impact.

While industry leaders have recently pledged even further decreases, Sant warns that existing technologies can only deliver a part of the carbon dioxide savings needed to achieve the Paris goals. “What the industry really needs to do is plow money and efforts into producing new or alternative types of cement that require less to no clinker,” he said. “It’s the only way they can address the issue of upstream CO2 emissions from cement production.”

Companies are trying different methods to reduce or eliminate the amount of clinker required to make concrete. North Carolina-based bioMASON, for example, employs naturally occurring bacteria as a binder to make concrete bricks, while CO2Concrete, a spin-off from UCLA, has developed a technology that takes carbon dioxide directly from the flues of power plants to produce solid mineral carbonates that can then be used to replace traditional Portland cement. Others, like banah in the United Kingdom and Zeobond in Australia, are focused on using byproducts from other industrial processes to create so-called “geopolymers” to replace clinker in making cement.

Experts say Solidia, the New Jersey company that was put through its paces in France over the summer, is one of the most promising. Its process, which was first developed in 2008 at Rutgers University, involves manipulating the cement chemistry to significantly lower the kiln temperature required to produce the clinker, and then curing the concrete made with their cement with waste carbon dioxide instead of water.

“Those technologies combined garner a carbon footprint reduction up to 70 percent compared to ordinary Portland cement-based concrete — and for a lower cost,” said Tom Schuler, president and CEO of Solidia, which has gathered financial support from well-known venture capital firms Kleiner Perkins and Bright Capital, oil giant BP, and Switzerland-based LafargeHolcim, the largest cement producer in the world.

Another company working on alternative cement solutions is CarbonCure, headquartered in Halifax, Nova Scotia. The brainchild of civil engineer Rob Niven, CarbonCure has developed a system where liquefied carbon dioxide is pumped into wet concrete as it’s being mixed. As the concrete hardens, the carbon from the carbon dioxide reacts with the concrete to become a mineral, effectively reducing the need for cement without compromising the concrete’s strength or price.

“On any given building or infrastructure project, this CO2 mineralization process reduces as much carbon as hundreds, if not thousands of acres of trees would absorb in the course of a year,” said Christie Gamble, CarbonCure’s director of sustainability. Worldwide deployment, she said, could reduce about 550 million tons of carbon dioxide each year, the equivalent of taking 150 million cars off the road.

For now, CarbonCure’s technology, which requires a small retrofit consisting of a computer system, a tank to store the carbon dioxide, and a tube to pump it into the concrete mix, is now installed in nearly 150 concrete plants across North America. The company said it’s also expanding into Southeast Asia and Europe.

A real-world demonstration of their product is taking shape in Georgia at a multi-story commercial office building under construction in one of Atlanta’s hippest neighborhoods. Set to be completed by the end of the year, the building will be the first large-scale development to use concrete made with CarbonCure throughout the entire structure. According to Gamble, this project alone will prevent more than 750 tons of carbon dioxide from being released into the atmosphere, an amount equivalent to 800 acres of forestland sequestering carbon dioxide for a year.

Although companies like Solidia and CarbonCure are starting to make headway, they still have a long way to go before capturing even a small share of the market. Schuler says a major hurdle is the building sector’s widespread conservatism. “The industry’s general attitude is to see it to believe it,” Schuler said. The company has spent around $100 million on research and development and trials like the one in France to convince commercial clients.

Reluctance to adopt newer technologies is understandable. “When it comes to ensuring life safety in structures, you have to be sure that what you’re doing will work,” Sant said. But he also argued that today’s safety regulations are not capable of evaluating the novel processes for producing concrete that will be required to significantly cut the industry’s carbon emissions.

“The problem is that we have relied too long on prescriptive codes and standards that tell us to make concrete a certain way, rather than using performance-based criteria that would spur sectoral innovation,” he said.

Another major issue is cost. “Though novel solutions do not always cost more than conventional solutions, in cases where they are, there is limited willingness to pay for the additional cost,” said Jeremy Gregory, executive director at MIT’s Concrete Sustainability Hub, a research group focused on sustainable concrete production and use. A 2015 study found that geopolymer-based cements, for example, can cost triple what traditional ones do.

Policies to offset those higher costs and encourage investment in climate-friendly cements are also lacking, Gamble said, suggesting, “technological advancement cannot on its own drive cement emissions down.” What’s needed, she says, “are measures such as emission caps and penalties to send market signals and encourage the widespread adoption of greener technologies.”

In the end, she conceded that low-carbon cement is still quite far from reaching wide-scale adoption. Yet she remains positive: “Perhaps it will take 20, 30 years, maybe more. But we are starting to see the first glimmers of that path.”

Given the monumental scale of its carbon footprint, cement alone could make or break efforts to slow global warming. For Gregory, the only way forward is to keep pushing the whole industry to accelerate its efforts.

“Delaying or avoiding this challenge,” he said, “isn’t really an option.”

Marcello Rossi is a freelance science and environmental journalist based in Milan, Italy. His work has been published by Al Jazeera, Smithsonian, Reuters, Wired, and Outside among other outlets.