Burying Carbon in Concrete

Here at Bentley, we make products which help engineers, architects, and other design professionals sustain the world's infrastructure. Another big part of building a sustainable environment for humans is in the process of material selections. Many of the materials which go into structures or roads take a large toll on the environment in their extraction, refinement, and fabrication. However, that may soon no longer be true.

    New cement technology may help to trap greenhouse gas emissions. (Photo: Concrete Bench by Stewart Leiwakabessy).

Several companies are now developing methods of generating carbon negative cement. That's right, not only can this cement be carbon-free to produce, but actually sequester carbon emissions from other sources. This would take a product that the EPA estimates is the third largest source of greenhouse gases in the U.S. and make it a place to store carbon from other sources.

And this solution is feasible with current technology.

Scientific American reports on one-such company – California-based Calera – that has found a Portland cement substitute  that actually helps reduce global climate change:

While Calera's process of making calcium carbonate cement wouldn't eliminate all CO2 emissions, it would reverse that equation. "For every ton of cement we make, we are sequestering half a ton of CO2," says crystallographer Brent Constantz, founder of Calera. "We probably have the best carbon capture and storage technique there is by a long shot."

Carbon capture and storage has been identified by experts ranging from the U.N.'s Intergovernmental Panel on Climate Change to the leaders of the world's eight richest nations (G8) as crucial to the fight against climate change. The idea is to capture the CO2 and other greenhouse gases produced when burning fossil fuels, such as coal or natural gas, and then permanently store it

The raw materials are essentially limitless for this process, which has the potential of reducing the cost of the cement used:

The Calera process essentially mimics marine cement, which is produced by coral when making their shells and reefs, taking the calcium and magnesium in seawater and using it to form carbonates at normal temperatures and pressures. "We are turning CO2 into carbonic acid and then making carbonate," Constantz says. "All we need is water and pollution."

… 

Nor are there any limitations on the raw materials of the Calera cement: Seawater containing billions of tons of calcium and magnesium covers 70 percent of the planet and the 2,775 power plants in the U.S. alone pumped out 2.5 billion metric tons of CO2 in 2006. The process results in seawater that is stripped of calcium and magnesium-ideal for desalinization technologies-but safe to be dumped back into the ocean. And attaching the Calera process to the nation's more than 600 coal-fired power plants or even steel mills and other industrial sources is even more attractive as burning coal results in flue gas with as much as 150,000 parts per million of CO2.

At this point, the hurdles are mainly regulatory. As anyone in the construction industry knows, the adoption of new technologies and materials is often very slow (and not without good reason). However, global climate change is a very hot issue right now – both on the minds of the public and the politicians they serve (both U.S. presidential candidates have plans for carbon reductions, for example. Further, it's not a problem that is going to be solved overnight and have large-scale, sustainable plans such as this will be crucial to solve it. Just how large scale is carbon emissions from concrete?

The U.S. used more than 122 million metric tons of Portland cement in 2006, according to the Portland Cement Association (PCA), an industry group, and China used at least 800 million metric tons. 

The scale of concrete use creates results in about 5% of the planet's CO2 emissions (source).

Currently, CALTRANS (The California Department of Transportation) and Dynegy (a California energy supplier) have expresses interest in the technology.

The article doesn't mention any of the relative material properties of the Calera carbonate cement to Portland cement. This, of course, will be crucial for using the material as a replacement (or cementatious material addition to a concrete mix).Until some material properties are known, little can be said about what changes – if any – will be needed in specifying concrete which includes Calera's carbon-sequestering cement.