Capturing Carbon Emissions in Cement?

Stepped on for more than 2,300 years, cement rarely gets the respect it deserves.

The versatile construction ingredient has been commonplace since the Roman Empire, but production has boomed in recent decades, rising nearly 400 percent since 1970. Global cement output may reach 5 billion metric tons by 2030, driven mainly by growth in the top two producing countries, China and India, according to a December WWF report.

In much of the world, replacing traditional wooden or earthen structures with concrete - the hardened mix of the cement powder, aggregate, and water - is a sign of wealth and prosperity. But cement manufacture is among the most energy-consuming and polluting industrial processes.

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WWF estimates that unless cement plants become more efficient or transition to renewable energy sources, the industry - now responsible for some 6 percent of human-caused greenhouse gas emissions - is likely to become a larger contributor to climate change than the European Union by 2030.

Alternatives have emerged this past year, however, that may redefine the future of cement. One company, California-based Calera Corp., claims to have found a way for cement production to absorb more carbon dioxide than it emits, acting as a vehicle for carbon capture and sequestration, or CCS.

Similar to how certain corals produce reefs by excreting dissolved calcium carbonate, or limestone, Calera sends carbon dioxide emissions through seawater to create a chalky carbonate byproduct. This cement substance is then mixed with aggregate and water to create concrete. Its production avoids the need to heat the cement materials in coal-fired kilns, while sequestering carbon throughout the process, the company says.

For every ton of cement produced, two-fifths of a ton of carbon dioxide is stored, according to founder Brent Constantz.

"Calera appears to be the only viable, economically feasible, no CO₂-footprint way to capture, separate, and sequester billions of tons of carbon dioxide," said Constantz, a Stanford University biomechanical engineer who developed bone-fracture cement before turning his attention to climate change solutions. "We're converting carbon dioxide into carbonate, and we're putting it into the built environment."

Constantz opened his company's first demonstration site in August 2008. Located about 90 meters away from a natural gas-fired power plant, the Calera facility aims to filter more than 90 percent of the plant's carbon dioxide emissions through seven 11-million liter seawater tanks that sit adjacent to the Pacific Ocean.

"Gas-powered plants are not our principal aim," Constantz said. "Our principal aim is coal plants around the world. We're spending lots of time interviewing different coal plants in the U.S. and other parts of the world as well.... The challenge is to decide how many plants to develop in parallel."

Although the test facility so far produces at most five tons of cement per day, the technique is quickly gaining attention around the world. Aided by the connections of its main investor, venture capitalist Vinod Khosla, Calera is discussing projects in Australia, the Netherlands, Saudi Arabia, and the United Arab Emirates (UAE), which is considering using the material in its Masdar City project, Constantz said.

Calera is not alone, however. California-based Carbon Sciences plans to mix cement with power-plant emissions and waste material from coal and steel production. Carbon Sense Solutions in Nova Scotia, Canada, redirects emissions from pre-cast concrete plants into new cement. And London-based Novacem says its magnesium silicate cement will absorb enough emissions to make it carbon negative.

But Calera appears to be gaining the most attention. The company received interest from the U.S. Department of Energy last year - Constantz met with former agency Secretary Sam Bodman. Constantz is also speaking at next month'sWorld of Concrete trade fair in Las Vegas, the commercial concrete and masonry construction industry's only annual international event.

In addition, Khosla promoted his investment at the World Future Energy Summit in Abu Dhabi, UAE, on Thursday as a better CCS strategy than geological storage. The world's eight largest industrialized nations, the G8, agreed last year to establish 20 CCS demonstration projects by 2010 [PDF], most of which would inject filtered carbon dioxide emissions into underground reservoirs.  

Calera and other emerging cement companies still need to prove that the carbon-sequestering cements can be cost efficient and durable compared to traditional cement.

Meanwhile, the cement industry's interest in reducing energy costs has led many countries to replace small-scale cement plants with larger, more efficient models. Cement kilns are also turning to greater use of renewable energy sources, such as biomass, which typically provides only 5 percent or less of kiln fuel in developing countries.

But green building designers are searching for more dramatic changes in cement options. "Let's ...begin to lessen the CO₂ consequences of our current dependence on cement," said Gail Vittori, co-director of the Austin, Texas-basedCenter for Maximum Potential Building Systems, whose headquarters is built with concrete made from recycled sources. "Part of that is how we identify opportunities for some more-benign cement materials such as naturally occurring elements or recycled materials."

Alternative blends have included ash from coal-fired power plants, known as fly ash, and pozzolana, a volcanic ash. Although limited in their supply, these natural materials replace some of the fused minerals used in traditional cement, and therefore avoid the need to heat limestone to 1,450 degrees Celsius. The substitutes are becoming more common, mostly in continental Europe, according to the International Energy Agency.

Vittori said she is also keeping an eye on companies such as Calera. Even if they cannot replace cement entirely, their impact could be meaningful, she said. "Adding 1 percent here or there could make a big difference in CO₂ emissions," she said. "It's certainly a worthy pursuit."

Ben Block is a staff writer with the Worldwatch Institute. He can be reached at This email address is being protected from spambots. You need JavaScript enabled to view it..

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