The Race to Reinvent Cement

What if we could transform the material that built the modern world from a climate wrecker into a carbon sponge?

Fast-forward Earth about a million years. Humanity has come and gone—perhaps wiped out by a pandemic or some cataclysmic event, like the dinosaurs before them. 

Aliens or maybe even intelligent descendants of today’s apes are digging for clues to what happened long ago. They may or may not find human bones, and they may or may not find metallic remnants of our modern lives. But what they almost certainly would find are crumbling, angular gray blocks covered with dust and tinged with a reddish chemical formed by the oxidation of steel rebar.

If there is a signature material of the Anthropocene, it is concrete. It is the most common human-made substance. We build houses, skyscrapers, bridges, and memorials from it; we drive on it; and we even bury our dead in it. But most of us hardly give the stuff a glancing thought.

Except Robert Courland, that is. He wrote the book on it, quite literally. In Concrete Planet, Courland—a retired tech executive turned writer and historian—traces the nuanced and intertwined history of concrete and modern human civilization. The Romans were the first to capitalize on the many useful properties of concrete. The nearly 2,000-year-old dome of the Pantheon was built using it. Then, after a brief pause when concrete’s recipe was lost, the material’s rediscovery helped structure the boom of the industrial age. But Courland finds that the modern use of concrete—supported by steel reinforcement bars—creates buildings that are capable of standing for only a fraction of the time that the Pantheon has survived. Most of today’s structures will start to wither away within a hundred years.

No matter the short life. Courland estimates there is about 40,000 kilograms of concrete for each person alive, and we’re adding about 500 kilograms per person each year. Vaclav Smil, author of Making the Modern World, estimates that China poured more concrete between 2011 and 2013 than the US had done in all of the twentieth century. 

Everything about concrete is huge—including its carbon footprint. It’s fair to say that without concrete, the modern world wouldn’t exist. Ironically, it’s also fair to say that, in an unexpected way, concrete is contributing to the destruction of the modern world. That’s because the chemistry of cement, which is the glue that holds concrete together, produces carbon dioxide as a major byproduct. According to a 2018 study, the cement industry is responsible for about 4 percent of global annual greenhouse-gas emissions, nearly twice that of the aviation industry.

China poured more concrete between 2011 and 2013 than the US had done in all of the twentieth century.

That’s why the world’s most common material is urgently in need of an upgrade. Even if we manage to burn fewer fossil fuels, deploy massive solar- and wind-power installations, and transition to electric cars, it won’t be enough to make our emissions goals. For the world to hit net-zero emissions within the next few decades, we will also need to find ways to pull CO 2 out of the atmosphere. And one way to do that would be to store it in our ever-expanding built environment. In fact, concrete could be a remarkably efficient carbon sponge.

Fortunately, work on that upgrade has already begun. 

Over the past year, while working on a series about solutions to climate change, I stumbled across fascinating developments under way in the cement industry, where both large multinationals and small startups are racing to create a greener product. Some of these companies have already created zero-emissions cement, and others are looking to create “negative emissions” products that would suck carbon dioxide from the air.

The environment will be the prime beneficiary of this race, but so will the companies that take the lead. A McKinsey report estimates that a $1 trillion market beckons for those who can lock up carbon dioxide in useful products, rather than spewing it into the atmosphere.

The biggest prize lies in tweaking the recipe for cement, such that the process locks away carbon dioxide from the air.

To understand why such a market might exist for concrete, you need to know a bit about the recipe for making it.

No two batches of cement are 100 percent chemically identical. In fact, this is how the European Standard defines the most widely used type, called Portland cement: “[It] shall consist of at least two-thirds by mass of calcium silicates, the remainder consisting of aluminum- and iron-containing [compounds] … and other compounds. The ratio of calcium oxide to silica shall not be less than two.”

You don’t need to be a chemist to realize that even a recipe for the simplest cake has less room to maneuver. To get cement, you can throw any decent-quality limestone and some clay into a coal-fired kiln, then heat it to 1400 degrees Celsius. In goes a mixture of yellow, brown, gray, and black particles, and out come uniformly colored gray particles. Walking the length of a kiln floating only meters above my head, which I got a chance to do at a cement plant in Sweden, feels like going from the Arctic to the Sahara. 

Cement’s chemical flexibility, along with its high strength and moldability, plus the fact that it’s made of easily accessed raw materials, makes it affordable and universal.

The building material’s carbon emissions come mainly from the use of limestone—that is, calcium carbonate (CaCO 3). Inside the kiln, heat converts limestone to lime—that is, calcium oxide (CaO)—while releasing carbon dioxide. The calcium oxide then reacts with clay (SiO 2) to form different types of calcium silicates, which we call cement.

Cement’s role in construction is like that of glue in craft projects. When you add water to cement, it starts to react with carbon dioxide in the air to form calcium carbonate again. If during this process you add aggregates such as gravel, it will hold them together to form concrete. Typically, cement forms less than 20 percent of the end product we see in gray walls of unfinished buildings.

The reason why cement’s emissions remain high, however, is that the reaction is never completely reversed—not all calcium oxide is converted back to calcium carbonate. Thus, not all of the carbon dioxide released in the process of creating cement is sequestered back when concrete is formed. So one fix would be to force that reaction to go to completion.

And therein lie both challenge and opportunity. Manufacturers can change the carbon output of cement on two basic fronts. They can reduce emissions by replacing with renewables the coal used to fire up the kilns and by using more efficient vehicles for transport. Big companies are making progress on this front. But the bigger prize lies in tweaking the recipe for cement, such that the process doesn’t produce as much carbon dioxide or—even better—locks away carbon dioxide from the air. Work on that front is coming from smaller startups.

Last summer, I visited one of the most promising such startups, in Piscataway, New Jersey. Solidia Technologies was created based on work done at Rutgers University. Researchers there had developed a new cement recipe that replaced limestone with wollastonite, a whitish-gray mineral used as a component in making ceramic tiles, as a paint additive, and even as a substitute for asbestos.

As a type of calcium silicate, wollastonite doesn’t need to drive out carbon dioxide. Moreover, because the material needs no heating, Solidia’s innovative recipe wouldn’t consume the coal needed for conventional cement. Better still, this type of cement would go one step further when used to produce concrete. It would trap carbon dioxide from the air during the curing process, in effect creating a product with negative emissions.

The recipe worked well in the lab but failed the test of commercialization. The problem was supply: even if Solidia were able to make a dependable product, there’s just not enough wollastonite in the world. About 1.5 million kilograms of wollastonite is mined each year in the US, enough to make some 1.5 million kilograms of low-emissions cement. That sounds like a lot—until you realize that US factories make nearly 100 billion kilograms of cement each year. The problem is similar in other countries, where the demand for cement far outstrips wollastonite deposits available. 

Solidia’s early failure wasn’t a surprise. Other companies have tried and failed to replace limestone in cement’s recipe. UK-based Novacem, for instance, wanted to use magnesium oxide instead of calcium oxide. But it, too, failed to commercialize the technology, eventually selling its intellectual property rights to a competitor before folding.

Where Solidia’s story differs is that it stuck with the goal of creating a greener product. Though they could not eliminate limestone entirely, during the many years of testing different cement chemistries and learning from their work on wollastonite, they created a recipe that drastically reduced carbon emissions by using less limestone.

The startup now needs to show that this lower-emission cement can be made into concrete that’s at least as good as others—and can be scaled up in a way that’s affordable. That’s what Solidia is working on right now. In their New Jersey factory, after putting on protective gear—hard hat, shoe gloves, and lab glasses—I got to see the process of making concrete using Solidia’s potentially game-changing cement.

concrete blocks made from Solidia’s cement capture about 240 kilograms of carbon dioxide for every 1,000 kilograms of cement used in the mixture.

The off-white-colored cement (as opposed to the gray Portland cement) is drawn from a large hopper and added to a mixer machine. A proprietary aggregate—some combination of particulate material such as sand, gravel, and crushed stone—and water are poured into the machine, which is rotated until a thick, soupy mixture forms. The mixture is then transferred to a “vibratory press” where it’s poured into molds, which are then moved to an enclosure full of carbon dioxide.

Unlike Portland cement, Solidia’s mixture doesn’t simply harden after adding water; it requires the absorption of climate-killing CO 2. That’s because using lower amounts of limestone in producing the proprietary cement creates a material that doesn’t react with carbon dioxide in the air—where its concentration is a mere 400 parts per million. Instead, the reaction happens in a chamber with 100 percent carbon dioxide in it. 

The upshot is that the concrete blocks made from Solidia’s cement capture about 240 kilograms of carbon dioxide for every 1,000 kilograms of cement used in the mixture. That’s on top of fewer emissions produced during the manufacture of Solidia’s cement. 

Over concrete’s lifecycle—from limestone to cement to concrete—Solidia produces up to 70 percent fewer emissions, compared to Portland cement. So if 1,000 kilograms of Portland cement releases 1,000 kilograms over its lifecycle, then Solidia cement releases only 300 kilograms. That’s not zero, but it’s a step-change for an industry that has been making only marginal progress.

What’s more, concrete produced using Solidia’s cement exceeds building standards and takes fewer than 24 hours to cure, compared to weeks for curing Portland cement. These claims have been verified by the US Department of Energy, which has provided some funding to the startup. Solidia has also used already existing cement factories to produce 10,000 metric tons of its product—demonstrating scalability.

On the tour, the company’s chief technology officer, Nicholas DeCristofaro, gave an example of just how much carbon dioxide is trapped by Solidia’s cement. He placed a concrete brick (about 12 inches x 5 inches x 5 inches) on a table. “This block,” he said, “has captured as much carbon dioxide as you can find in the air in this whole room.” (The room was a mid-sized office, 15 feet x 15 feet x 10 feet.)

Solidia’s success so far has attracted a lot of interest, and that’s helped them raise more than $60 million. The company’s board features members from LafargeHolcim and HeidelbergCement, the world’s largest and fourth-largest cement companies, respectively; Kleiner Perkins Caufield Byers, one of the world’s largest investors; and BASF and Air Liquide, two of the world’s largest suppliers of bulk chemicals. In October, Solidia announced a big round of funding from the Oil and Gas Climate Initiative, a group of some of the world’s biggest oil companies that is looking at technology development as a means of surviving in a carbon-constrained world. 

Meanwhile, other cement startups are finding their own niches. The Canadian startup CarbonCure has 50 concrete-making plants across North America using its technology. Unlike Solidia, CarbonCure hasn’t reinvented cement. Instead, it found the inefficiencies in the process and filled the gap with, well, carbon dioxide.

When cement is mixed with water and gravel to form concrete, it starts capturing carbon dioxide from the air to form calcium carbonate. But the reaction never goes to completion because the concrete hardens to the specifications builders seek without all of the cement in the mixture reacting to trap carbon dioxide. CarbonCure works with concrete makers who use conventional Portland cement. Instead of heat or steam curing, CarbonCure sets up chambers where the concrete blocks are cured with carbon dioxide. In an environment of 100 percent carbon dioxide, much more of the cement reacts than would be the case in air where the concentration of carbon dioxide is merely 0.4 percent.

The resulting concrete product has greater tensile strength and hardness than that produced without the carbon-dioxide curing process. The pure carbon dioxide is sourced from suppliers, who are increasingly using off-take from chemical processes that would otherwise have dumped it into the atmosphere. Concrete makers are able to recover the costs of CarbonCure’s technology by charging its customers a higher price for what they claim is a better product. In the 10 years since being founded, the company has raised more than $10 million. Last year, the California high-speed rail project announced that it would use CarbonCure’s concrete for a 30-mile stretch between Madera and Fresno.

Another startup, Calix, based in Australia, has developed a process that reduces the amount of time and energy needed to heat up limestone. Moreover, instead of throwing coal into the kiln for heat, it uses natural gas, which produces fewer than half the carbon emissions per unit of energy produced. 

Here’s how it works. In conventional kilns, limestone and coal are burned in the same chamber. Instead, Calix uses a double-barreled structure where natural gas is burned in the outer barrel and limestone is poured from top to bottom into the inner barrel. Because the heat isn’t in direct contact with limestone, the reaction should be less effective than in the conventional kiln. But Calix overcomes the problem by pre-grinding the limestone into a fine powder, which increases its surface area and allows indirect heat to complete the same reaction. 

The carbon dioxide released in the inner chamber as limestone is converted to lime and can then be trapped as a pure gas and either stored in cans to be used by startups such as CarbonCure or buried underground in depleted oil and gas fields, where it can remain safely away from the Earth’s atmosphere.

Overall, the process drastically reduces the emissions of cement production. Working with HeidelbergCement, Calix won a $15 million grant from the European Union in February to start construction on a plant in Italy that can prove the technology works commercially.

On a smaller scale, Carbicrete is a company spun off research conducted at McGill University in Canada. The company does away with the use of cement altogether. Instead, it makes use of industrial slag, a waste product of metal manufacturing, as the binding agent in concrete. Chemically speaking, slag is a complex mixture that contains lots of silicates and oxides similar to those found in cement. Like the other startups, it cures the concrete in the presence of carbon dioxide, producing what the company claims is negative-emissions concrete with specifications better than those of conventional concrete. Though there isn’t enough slag in the world to produce all the concrete we need, the company may just find a niche application. After all, unlike the limited supply of wollastonite, which needs to be mined, slag is available as a waste product that the metal industry often pays to dispose of.

Right now, wherever I see concrete, I also see carbon pollution. But if I look again, this time from a different angle, I can imagine all those new buildings, roads, and bridges handily locking up a lot of CO 2

A niche application, in the case of concrete, may be necessary for experimentation, but ultimately it’s insufficient. Our concrete world is by definition economy-sized.

And in most economies, there is currently no price on carbon. That means there is no financial incentive to cut CO 2 emissions. Nevertheless, cement makers comprise some of the world’s largest companies, where some of the smartest investors put their money. These companies are also among the world’s biggest greenhouse-gas emitters. As a result, they’re now facing investor pressure to cut emissions and ensure their factories won’t become stranded assets in the future.

New cement factories and many existing ones will last decades, and many of these companies estimate that most of their markets will institute a carbon price soon. China, for instance, is launching the world’s largest carbon market this year. Though initially it will cover only the electricity sector, there are plans to expand to all greenhouse-gas producers.

 “The whole cement industry has the objective to deeply decarbonize in the future,” says Jan Theulen, director of alternative resources at Heidelberg, the world’s fourth-largest cement maker. Heidelberg has made a public commitment to reach carbon neutrality by 2030.

That’s why they’re not only keeping a close eye on startups but also investing in other approaches to reduce cement’s emissions. Heidelberg has partnered with researchers at Linnaeus University in Sweden to capture algae emissions; these plants use the same process as trees to convert carbon dioxide and water in the presence of sunlight into sugars and other nutrients needed to grow. The carbon-trapping system supercharges the photosynthesis the little green critters have mastered over billions of years. In a single pass through the algae mixture, as much as 40 percent of carbon dioxide is absorbed. Run it through the system a few times, and almost all of the greenhouse gas is removed. As the algae multiply, the excess is removed, dried, and sold as animal feed. The process exists only on a pilot scale so far, but it seems promising.

Likewise, Solidia is forging ahead. They insist that their cement can be used for all sorts of concrete applications. I was less convinced, because most concrete construction requires pouring and curing on site. Ensuring that such uses are covered in chambers full of carbon dioxide seems difficult. Still, even if we assume Solidia’s cement can be used only for precast concrete made into bricks and slabs before being transported to where they need to be used, it’s a significant chunk of the market. The most recent estimate, from 2016, says precast concrete is at least 15 percent of the market globally. That proportion rises to as much as 50 percent in wealthier regions, where labor required to pour concrete is expensive.

All concrete roads, it seems, lead back to the issue of scale. Cement and concrete may be low-value products, but their sales volumes are huge and the demand for them is expected to remain stable for decades. If a startup can find inefficiencies in these industries, there is plenty of money to be made. If a multinational were to do the same, there is plenty of money to be saved. 

In 2016, I moved into a newly built property in London in an area that will see new construction for another 10 years before the planned development is complete. Over the past two years, I’ve watched workers pour many tons of concrete around steel structures, build multi-story homes for new residents, and convert an empty piece of land into a thriving community. Right now, wherever I see concrete, I also see carbon pollution. But if I look again, this time from a different angle and taking a longer view, I can imagine all those new buildings, roads, and bridges handily locking up a lot of climate-killing CO 2.


Akshat Rathi is a reporter for Quartz, where he covers science, energy, and environment. In 2017, he published The Race to Zero Emissions series investigating the role of carbon-capture technology in mitigating climate change. This article was adapted from a piece on