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.

The Three Gorges Dam project in China is one of the largest infrastructure projects in history. The structure required 35.6 million cubic yards of concrete. (Image source: Wikipedia)

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).

Supplemental cementing materials (SCMs), among them fly ash, calcined clay, silica fume, and ground blast furnace slag can reduce the need for clinker, cement’s most carbon-emissive component. (Image Source: Portland Cement Association)

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%.

The look of concrete to come: the same, but with a smaller, neutral, or even negative carbon footprint. Shown here are engineers admiring a batch of precast, low-carbon cement bricks. (Image source: Solidia)

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.”

At this Norcem cement plant in Brevik, Norway, partner company Aker Solutions has been testing a carbon-capture technology with the goal of snagging up to 400,000 tons of the greenhouse gas before it goes up in the air annually. (Image source: Aker Solutions).

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.