RECASTING CEMENT
The Race to Decarbonize Concrete
THE MOONSHOT If you make one ton of cement, you generate up to a ton of waste carbon dioxide. With construction in developing countries at historical highs, the manufacturing of cement — the binding ingredient in concrete — now accounts for some 8% of all the carbon dioxide humanity spews into the atmosphere. By developing and deploying new low-carbon cement and carbon-capture technologies, and by changing the way we design and build, we could remove concrete and cement-making from the list of human-made materials that force climate change.
THE PHILANTHROPY OPPORTUNITY The cement industry and materials engineers are already developing low-carbon cement and concrete technologies, but no one fix can apply on a global scale. In addition to accelerating R&D of new cementitious materials and decarbonization technologies, philanthropic support could help reduce psychological, economic, and regulatory risks associated with adopting new construction materials. Specific initiatives could include demonstration projects that validate the safety and performance of new materials; cost-assistance programs that make low-carbon cement cost-competitive with traditional materials; and community-building organizations that can unify the many stakeholders in the production and use of cement and concrete in the cause of dramatically reducing cement’s carbon footprint.
Of all the legacies of the ancient Romans, perhaps none is as obvious as the way we build. The elegant coffered ceiling of the Pantheon temple, the first of its kind and still the world’s largest unreinforced concrete dome, has stood in Rome for nearly 2,000 years, now joined on Earth by concrete marvels as exquisite as the Sydney Opera House and as massive as the Three Gorges Dam. The only material we use more of is water. Every year more than 20 billion metric tons of concrete — enough to build 175,000 Empire State Buildings — are produced worldwide.
The basic concrete formula used since 1824, when English bricklayer Joseph Aspdin patented what’s known as Portland cement, consists of about 10% — 15% calcium silicate cement (the binder), 60% — 75% aggregate (sand and gravel) and 15% — 20% water. It’s a recipe that has enabled great engineering feats with the humblest and most abundant of materials: rocks and water. But it now presents humanity with a serious problem.
Portland cement is made by heating limestone, clay and sand in a kiln to the point where their minerals fuse into “clinker,” nuggets that can be ground into a fine powder and mixed with water to form a paste of calcium silicate hydrate (C-S-H) — the binder that cures and hardens around concrete’s inert aggregates to form the most intensely used material on Earth. This is an unusually carbon-intensive process, for two reasons: First, burning rocks to the temperature required to make clinker — about 1,450° Celsius — requires a lot of energy, typically supplied by burning fossil fuels. Fuel combustion accounts for about 40% of cement’s carbon dioxide (CO2) emissions. The other 60% is inherent in the chemical reaction caused by this heat: When roasted, or calcined, a molecule of calcium carbonate (CaCO3) in limestone releases a CO2 molecule into the air, leaving calcium oxide, or lime (CaO), a principal component of Portland cement.
In the summer of 2018 the Royal Institute of International Affairs, the English think tank commonly known as Chatham House, released a report titled Making Concrete Change, one of several recent warnings about concrete’s carbon footprint. The 4 billion tons of cement produced annually, wrote the authors, account for about 8% of the CO2 emissions that contribute to increasing global temperatures. If the cement industry were a country, it would be the world’s third-largest CO2 emitter, right behind China and the United States.
According to the World Business Council for Sustainable Development, cement producers have acknowledged the problem, taken steps to reduce their product’s carbon footprint and achieved significant reductions in direct CO2 emissions per ton of material since 1990. The key phrase there is “per ton.” These changes might have made a dent if the world were making the same amount of cement every year — but global cement consumption has quadrupled over the past three decades.
We’re building at a pace that’s unprecedented and, in its particulars, approaching the unbelievable: A March 2015 article in the Washington Post pointed out that China used more cement in 3 years, from 2011 to 2013, than the United States did during the entire 20th century — a century in which both the Panama Canal and the Hoover Dam were built. Though China’s cement production has begun to decline since its 2014 peak, it’s still five times higher, per capita, than the United States’. And other rapidly urbanizing nations, such as India, Indonesia and several African countries, including Nigeria and South Africa, are poised for dramatic increases in concrete consumption.
The Paris Agreement’s goal of reducing the risks of catastrophic climate change involves keeping global average temperatures from climbing higher than 2°C above pre-industrial levels by 2100 (the “2°C Scenario,” or 2DS). The agreement also calls for reducing net anthropogenic (human-caused) emissions of greenhouses gases, such as CO2, to zero in the second half of the century.
There are levers that might help the cement and concrete industries, with help from investors and policymakers, get to zero.
The changes made by the cement industry thus far, while impressive, won’t meet these goals, particularly when the International Energy Agency (IEA), an intergovernmental organization based in Paris, expects the current demand for cement to grow by as much as 23% by 2050. In April of 2018 the IEA, in conjunction with the Cement Sustainability Initiative (CSI), a cooperative effort by major cement producers to advance sustainable development, published their own report, Technology Roadmap: Low-Carbon Transition in the Cement Industry. Given today’s commercially viable technologies, the report concludes, the industry could reduce its direct CO2 emissions — the combustion and process emissions produced during its manufacture — by an additional 24%.
Ian Riley knows cement. In China — which produces more cement than the rest of the world combined — Riley led the operations of the world’s largest cement company, LafargeHolcim, for about a decade, some of which he spent working with the CSI. He left the industry last year, he said, because “I wanted to find some way to make a contribution on climate change.” He was recently named the first CEO of the World Cement Association (WCA), an organization founded in 2016 that includes more than 40 cement companies around the world. WCA released its own Climate Action Plan last year. Getting 24 percent of the way to zero, Riley pointed out, isn’t ambitious enough. “If you’ve got society demanding zero net emissions,” he said, “that’s still 76 percent you’ve got left. The problem is that we don’t have a financially viable technology roadmap to address that 76 percent.”
It’s a grim reality, aggravated by the fact that CO2 emissions are literally baked into the cement-making process. But there are levers that might help the cement and concrete industries, with help from investors and policymakers, get to zero.
The Supply Side: Material Solutions
The technologies that will achieve currently attainable cement-related emission reductions are the tried and true:
- Boosting the fuel efficiency of cement kilns through redesign or retrofit, or by using waste heat recovery units that recapture some of the energy produced during combustion.
- Using alternative kiln fuels such as biomass, solid waste, solar energy, or geothermal energy.
- Reducing clinker content by blending Portland cement with supplementary cementing materials (SCMs) that resemble, in their composition, the sandy volcanic ash — pozzolana– the Romans used to make their durable concretes. SCMs react with slaked lime (water-saturated calcium oxide) to form powerfully cementitious aluminosilicate compounds. The most commonly used SCMs today are metakaolin (calcined clay) and industrial by-products such as fly ash (fine particulate matter produced by burning coal), silica fume (ultrafine silica powder collected as a by-product of metal alloy production) and blast furnace slag (a by-product of iron- and steel-making).
Nearly every cement company has introduced blended cements to the market, and they’re among the most mature decarbonization solutions available today, but they appeal to a narrower customer base than ordinary Portland cement. Different clinker ratios mean different physical properties, which are sometimes desirable; sometimes not. Also, the availability of industrial SCMs varies by region, and the low-emission future envisioned by the Paris Agreement actually will decrease the availability of some of the most abundant sources of blending materials. For example, the fewer coal-burning power plants in operation, the less fly ash will be available to cement makers. A new blended cement, LC3 (limestone calcined clay cement), developed by Swiss researchers, cuts cement clinker content in half without using these by-products, instead substituting a mix of limestone and low-grade clay. Pilot plants are producing LC3, which cuts cement’s CO2 emissions by 30%, in India and Cuba.
For the industry to do more than simply slow the rate at which cement-related CO2 emissions increase, these fixes will need to be implemented at a wider scale. Over time, they offer an important additional benefit: They save businesses money. For investors, they’re a safe bet in an industry that shows no signs of slowing.
But they’re only a start, as the authors of Making Concrete Change point out: “The next phase of decarbonization,” they wrote, “will be technically and economically more challenging than efforts to date unless a new wave of innovation redraws the landscape.”
The various action plans drawn up by think tanks and industry groups generally mention at least two promising innovations that could usher in this next phase:
•Alternative binders with lower carbon footprints than Portland cement. Among the commercially available alternatives today are alkali-activated binders, created when silica is dissolved in an alkaline solution to form sodium silicate or “water glass,” which hardens when combined with SCMs. According to John Provis, Professor of Cement Materials Science and Engineering at the University of Sheffield in England, these binders release about 80% less CO2 during production than Portland cement.
Alkali-activated binders are sometimes referred to as “geopolymers,” a term that can start a brawl among concrete scientists (Provis has been threatened with a lawsuit for editing Wikipedia’s “Geopolymer” page). But being familiar with the term will help you realize the materials have been used for decades. Buildings made with geopolymer concretes in Belgium in the 1960s are still in service. 40,000 cubic meters of geopolymer concrete were used to pave everything but the runways at Brisbane West Wellcamp Airport (renamed in 2017 as Toowoomba Wellcamp Airport) in Queensland, Australia, completed in 2014, and the materials are being used in large-scale applications in South Africa, the Netherlands, and the United Kingdom.
“Part of the reason I think alkali-activated cements have an enormous future,” Provis said, “is their ability to make use of whatever’s available locally. You can design different combinations and blends of materials.” Banah, a U.K. company whose tagline is “cement reimagined,” makes geopolymer concrete from metakaolin, an abundant resource there. A leading Australian geopolymer manufacturer, Zeobond, uses fly ash.
Several other alternative binders are in either the pilot or R&D phase, such as belite clinker, pre-hydrated calcium silicates or magnesium-based cements. While these alternatives will never become a universal replacement for Portland cement, they’re attractive niche options that can outperform conventional concrete in certain applications. Banah, for example, offers alkali-activated formulations for different applications including heat resistance, acid resistance and rapid setting. Geopolymers are particularly useful for maintaining the strength and impermeability of concretes that contact seawater — a serious weakness of Portland cement.
•Cement- and concrete-making processes that capture, and perhaps incorporate, carbon. Carbon capture and storage (CCS) is a promising technology whose potential is just beginning to be explored.
Several companies are integrating CO2 as a component of cement and/or concrete. Montreal-based Carbicrete cures slag-based cement with CO2, permanently storing it in precast blocks. Carbon Upcycling, a team of researchers from the University of California-Los Angeles, is scaling up development of what it calls CO2 Concrete, precast blocks formed by combining waste CO2 from power plants with slaked lime.
Another Canadian company, CarbonCure, has developed a retrofit technology that injects waste CO2 (captured from factories of other industrial emitters) into ready-mixed concrete, precast units, or concrete masonry. Once inside these building materials, the greenhouse gas combines with calcium in the cement, mineralizing into calcium carbonate and increasing the material’s compressive strength. CarbonCure technology has been adopted by more than 100 producers in more than a dozen provinces and states — including Hawaii, where the company recently launched a partnership with the largest ready-mix supplier on the island of Oahu — and Singapore.
Solidia Technologies, a New Jersey-based company, reduces concrete’s carbon footprint in two ways: Its patented lower-calcium cement requires less fossil fuel, reducing CO2 emissions by 30% — and costs less to produce than Portland cement. Solidia concrete is also cured with CO2 rather than water, contributing another 30% reduction — for a cumulative reduction of 60%.
Solidia concrete has been formed into precast units, and the company’s chief technology officer, Nick DeCristofaro, said the company is on the verge of its first ready-mixed demonstration. Ironically, one of Solidia’s highest input costs is CO2, which it needs in high enough concentration to cure its concrete. It currently buys industrial food-grade CO2, the kind used to carbonate drinks.
CCS technology, said Ian Riley, could make Solidia concrete a truly revolutionary product. “If you combined the technology of Solidia with the technology to capture CO2 from the cement kiln,” he said, “you’d not only have CO2 capture reducing emissions from the cement kiln, but you’d also potentially have the source of the CO2 for curing the concrete.”
The technology exists to do this, Riley said, but it’s not economical yet. There are a few pilot carbon-capture projects around the world, including an Anhui Conch Cement plant in China and HeidelbergCement plants in Belgium and Norway. At its current stage of development, CCS, which has the potential to zero out cement-related emissions or even to make concrete carbon-negative, still at least doubles the cost of making cement. It’s an area Riley thinks is ripe for further investment. In a world where promising innovations are backed by billions of dollars, he said, his guess is that investments in carbon capture are probably in the hundreds of millions: “Globally, there’s not enough being spent on these decarbonization technologies to really generate quick progress.”
In the United States, at least one concrete innovator, Brent Constantz, is all-in on carbon capture, though he’s given up on the idea of creating a new cement. Few people know more about cement than Constantz, a Silicon Valley entrepreneur who made his fortune inventing and developing bone cements for orthopedic surgeons. About a decade ago Constantz founded a company, Calera, that captured waste CO2 from a natural gas power plant on the California coast and bubbled it through seawater, creating calcium carbonates similar to those found in coral reefs. Calera cement performed well when used to pave a section of the coastal highway, but the circumstance Constantz anticipated would make his product attractive to California builders — a price on carbon — never materialized. Today Calera sells value-added products such as wallboard, and California — one of a handful of states to have any kind of carbon-pricing scheme at all — is still working the bugs out of a cap-and-trade program enacted in 2013.
Constantz’s new company, Blue Planet Ltd., is focused on the 70% of concrete that’s not cement: the sand and gravel in aggregate which, for reasons ranging from geochemistry to NIMBYism, is in short supply in urban California. Blue Planet makes fake sand and rocks, mineralizing waste CO2 (from power plants and cement kilns) and calcium oxide (from industrial wastes and old concrete) into custom-sized limestone nodules. Blue Planet aggregate, about 44% CO2 by mass, has been used to pour a section of the Terminal One boarding area at San Francisco International Airport.
“We’re saying we’re going to go out to the cement kiln and capture the CO2 from the regular Portland cement-making process, so the Portland cement itself doesn’t have a carbon footprint,” Constantz said. “And then we’ll take the CO2 that’s been spewing out of that kiln and convert it into rock. So you can use as much Portland cement as you want — which is what the structural engineers want anyway.”
The Demand Side: Tools for Change
With Blue Planet, Constantz has tweaked conventional wisdom enough to make an important point: Concrete’s carbon footprint is a problem bigger than cement.
It’s a problem, actually, that encompasses more than the material itself: Alkali-activated binders, for example, have performed well for decades, but many in the architecture, engineering, and construction community remain only vaguely aware of them. This may be due in part to inertia. Provis calls Portland cement the Big Mac of construction materials: No matter where you buy it, you know what you’re getting. It’s usually what ends up on the construction site after all the exotic choices have been rejected. Companies that profit from Portland cement have no reason to change, nor to encourage competitors.
This is a cultural problem, not a material one, and the industry knows it. Within months of co-producing the Technology Roadmap, the CSI was absorbed by a new organization, founded by several of the world’s largest cement and concrete makers: the Global Cement and Concrete Association, or GCCA, whose aim is “to strengthen the sector’s contribution to sustainable construction and to foster innovation along the entire value chain.” Claude Lorea, GCCA’s cement director, says the organization aims to build on the work of the CSI by improving the sector’s social and environmental impacts, and fostering innovation and collaboration. “We’ve been formed to be the global voice not only of cement but also of concrete,” she said. “It’s a broader agenda, and we want to continue to demonstrate leadership, and we’ve already engaged with other organizations such as the World Green Building Council.” The Green Building Council is the non-profit that established the Leadership in Energy and Environmental Design (LEED) rating system for the design, construction, operation and maintenance of buildings.
A look at concrete over its entire life cycle, from drafting table to wrecking ball, reveals a couple of leverage points that could further reduce its carbon footprint:
•A shift to performance-based material specifications can increase the demand for lower-carbon alternatives. According to Jeremy Gregory, executive director of the Concrete Sustainability Hub, a research initiative at the Massachusetts Institute of Technology (MIT), the architecture, engineering and construction culture must change the way it views the codes and standards that determine how concrete is used — standards that specify ingredients, rather than performance outcomes. Designers and builders rely on a time-tested cookbook. “The alternative would be to institute performance-based specifications,” Gregory says. “Instead you’d say: ‘I need this much strength, this much stiffness, this kind of durability.’ And you’d say: ‘Do whatever mixture you want that meets those specifications — and oh, by the way, I have these environmental impact goals as well.”
The impetus for this cultural change, Gregory says, “is going to have to come from the top. Policymakers need to say it’s necessary.” So far, baby steps: The LEED standard requires environmental product declarations (EPDs) for the environmental impacts of materials over their life cycles, but statutory regulation has been more challenging in Western market economies. The Buy Clean California Act of 2017, the first of its kind in the United States, placed limits on the acceptable global warming potentials, including carbon footprints, of materials used in state-funded construction projects — but cement and concrete, after input from powerful industry lobbyists, remain conspicuously absent from the list of covered materials.
•Tools for life-cycle analysis and design can help architects, engineers and builders use less concrete — and build smarter with the concrete they use. At MIT, Gregory and other researchers don’t focus merely on the “embodied” environmental impacts of concrete — the CO2 released during its manufacture — but also on the “operational” impacts of concrete structures over their lifetimes.
Those lifetimes are often shorter than they need to be, and impacts are multiplied every time a structure is torn down and replaced. In the United States, concrete structures are typically built for a design life of about 50–60 years. The Pantheon, built of slow-curing lime-and-pozzolan concrete, has been repurposed as a Christian church, but modern buildings aren’t designed or built to last 2,000 years. Actually, Gregory said, we don’t really know how long our buildings might last. We don’t destroy them because they’re failing: “We usually do that because they’ve gone out of style.”
Urban builders could disrupt this build-and-wreck cycle by designing solid core structures that could be reconfigured and adapted. Expensive lofts in New York City and San Francisco, for example, were once industrial spaces, built for modularity.
Johanna Lehne, one of the co-authors of Making Concrete Change, believes this is an activity poised for digital disruption. Calculating the life-cycle impact of a building involves more variables than any human can identify and sort — but a digital modeling platform could help design structures that use less concrete, or use concrete more efficiently. A digital building information model, for example, could allow users to integrate embodied carbon calculations into a design model and compare the model’s performance to other buildings. It could allow structural engineers and architects to collaborate and optimize designs and materials at the beginning of a project. Digital tools could even be used to tailor a concrete mix on site, Lehne says, where a supervisor could call up, on a tablet or smart phone, details on the compatibility of a certain clinker substitute with a given mixture. “It just feels like the data availability we have now,” Lehne says, “combined with machine learning that could help find the right solution for any building in any context — that seems to really unlock new opportunities for deep decarbonization.”
Every cement or concrete expert will tell you there is no One Big Fix that will scale up to knock down the sector’s CO2 emissions on a global scale; it will take a toolkit, with tools applied to match a region’s economics and politics. But unlike technical tools, which can be fostered by direct influxes of capital, it’s hard to know how to advance demand-side changes within a culture built around Portland cement, a product that’s served so well — and so inexpensively — for nearly 200 years.
The good news is that the industry associations devoting themselves to sustainability and innovation — the WCA and the GCCA — are on board. “We want to engage with the rest of the value chain and the construction sector,” says Lorea, “in order to maximize the design life of building and infrastructure.” Ian Riley says WCA members are eager to partner with technology companies, universities, and other researchers and nonprofits — anyone with good ideas: “I think putting together networks of people who are working on climate change in the industry is something we need to do to make it work.”
Every cement or concrete expert will tell you there is no One Big Fix that will scale up to knock down the sector’s CO2 emissions on a global scale; it will take a toolkit, with tools applied to match a region’s economics and politics.
Demand management will be indispensable in reducing concrete’s carbon footprint — but it can only go so far. The Energy Transitions Commission, a coalition of energy industry insiders, advisers, non-profits and academics established by Royal Dutch Shell and McKinsey & Company in 2015, released a report, Mission Possible, in November of last year, providing an overview of how “harder-to-abate” sectors, including cement, could achieve zero emissions by 2050. Demand-side measures, the report says, could reduce cement emissions by a maximum of 34%. Increased energy efficiency could cut an additional 10%. The only way to get to zero, the report indicates, is by piloting, developing and implementing decarbonization technologies such as carbon capture.
Mission Possible also points out that available decarbonization options will add up to $100 to the cost of each ton of cement, a non-starter for businesses with the option of buying ordinary Portland cement concrete. Riley believes decarbonization will be Mission Impossible without a cost imposed on emitting carbon — and a cost considerably higher than today’s. Until it costs more to do nothing, technologies such as carbon capture will always be more expensive.
Results so far have validated the concerns of carbon-pricing skeptics: In the United States very few states have attempted to implement carbon-pricing schemes, and Europe is working to repair an emissions trading system that Riley described as “a bloody disaster.” In 2016, the UK think tank Sandbag reported that over the decade from 2005 to 2014, EU cement producers reaped nearly €5 billion in profits from trading emissions allocations, while the carbon intensity of their cement production increased by 3.7%.
Riley’s an optimist, though. Carbon pricing is a new approach that hasn’t really worked yet — but that doesn’t mean it won’t. “As a minimum for success, you have to have the U.S., the Europeans, the Chinese and the Indians on board. If you can get those big four, it won’t be that big an issue in the rest of the world.”
He thinks the big four are close. “The Europeans, for political reasons, have tended to be more eager to get ahead on this,” he said. “And in the case of the other three, their natural environments are more sensitive to warming . . . Storm damage and changes in climate have already started to be very apparent. I think the big players will be motivated for change.”
To be fair, the industry itself has been motivated, and has achieved significant reductions in cement-related CO2 emissions, for three decades now — but they’ve been the easy fixes. The industry will need help — from policymakers and investors, and the designers, engineers and builders who use its products — to make the harder ones. Lehne, for one, senses a convergence of disruptive trends that have the sector poised for the heavy lift: decarbonization technologies, alternative materials and digital tools coming online to shrink concrete’s carbon footprint, as well as investors and consumers who increasingly demand products that reduce environmental risk. It’s a cultural evolution already exerting its influence on the world’s policymakers. “These feel like big shifts,” Lehne says, “that are going to make the construction sector have to change, one way or another.”
Craig Collins is a California-based freelancer who writes about science, technology, agriculture, and government.
