A Bold Attempt to Make Cement Production a Carbon Sink

Unfortunately there turned out to be a catch but inventor Brent Constantz will try again


With last month having turned out to be the hottest July in U.S. history, planet Earth having experienced four highly unusual heat waves in the last decade, and Arctic sea ice at a record low, evidence is mounting that human-induced global warming must be loading the dice. Yet the United States remains without an explicit climate policy, and the world remains deadlocked over how to approach greenhouse gas reduction. So it would be nice if somebody appeared with a magic sword capable of cutting through that Gordian knot.

News writer Robert Service, in the August 10 issue of Science magazine, describes one significant attempt to forge such a sword. Since 2002, an engineering innovator named Brent Constantz, currenlty a consulting associate professor at Stanford University, has been developing a process that would make cement manufacturing a net consumer rather than a net emitter of greenhouse gases. The potential implications are huge. After energy, cement production is the world's second largest source of carbon emissions. Roughly speaking, according to Service, the world makes about 15 billion tons of cement and concrete each year and 32 billion tons of aggregate (mixtures of cement, concrete, sand and gravel). Production of each ton of concrete releases one ton of carbon dioxide.

In 2007, with support from Khosla Ventures, Constantz (in photo above) got the opportunity to test his ideas with the creation of a company, Calera, and the construction of a cement factory near a California seaside power plant. There, in a 33.5 meter tower, a CO2-rich flue gas mixed with seawater droplets to form the calcium and magnesium carbonates that would be the stuff of cement. The net result, said Calera and Constantz, was net consumption of a half ton of carbon dioxide per ton of concrete produced, rather than a ton emitted.

It was a bold and exciting claim. At last, it seemed, there was a possible solution to carbon buildup in the atmosphere that might be commensurate with the scale of the global problem. But there turned out to be a hitch. Ken Caldeira, an eminent climate scientist affiliated with the Carnegie Institution for Science at Stanford University, divined that binding of the carbonates with sea water could scarcely take place unless the seawater was doped with akalines, which would not be cost-free by any means. "Caldeira was right," Service reports. "It turned out Calera engineers were adding sodium hydroxide or other strong bases to their seawater to make it more alkaline, driving its pH as high as 12 or 13."

Confirming that account in an e-mail message, Caldeira says that Calera "had shown figures with seawater and CO2 as inputs and cement and water as outputs, which would be the chemical equivalent of a perpetual motion machine. . . I don't know whether at first they thought they had invented a perpetual motion machine and only later realized that they would need to find bases (i.e., sources of alkalinity) or whether they knew this all along but just weren't forthcoming."

However that might be, Calera soon gave up on the novel cement-making process and has shifted its focus to other endeavors, resulting in the departure of Constantz, who wished to persevere with his ideas for making environmentally friendly cement out of seawater. He is now developing a process, inspired by corals, that greatly speeds the binding of seawater carbonates with CO2. The process still depends, however, on addition of chemical bases.

The moral of the story of this attempt to find a magic bullet carbon sequestration technique, like others before it, would seem to be that it is never as easy as it looks at first.

The Service article is part of a larger Science magazine package devoted to novel waste handling technologies. Another article, just as thought provoking, describes attempts to develop microbial fuel cells and other microbial electrotechnical technologies (METs) that could draw energy from waste waters in treatment facilities. Again, the potentlal is huge. So far, however, experiments are taking place only at a scale of about a liter, and the authors caution that METs may never pan out.


Correction and note (Aug. 16, 2012):

Aggregate, consisting of sand and gravel, is a constituent of concrete.

Caldeira, coincidentally, is the author of an article in the September issue of Scientific American about the long-term and very-long-term effects of global warming. His arresting observations are well worth consulting.



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