Cement From Thin Air

A biologist's plan for radically reducing carbon emissions

As a marine-biology student in the 1980s, Brent Constantz was astonished to discover how simply corals conjure their stony mass from nothing more than seawater. The trick? They combine the calcium and bicarbonate already present in seawater into calcium carbonate, which crystallizes into a durable exoskeleton. Constantz spent the next two decades thinking about how to apply a similar trick to patching human bones, took out more than 60 patents, started two companies, and now his bone cement is in use around the world.

But he also continued thinking about coral, and in 2007 that led him to an ingenious insight about another form of cement-the kind that goes into buildings. Like coral, limestone cement also crystallizes in water. Add an aggregate to the mix, such as sand or gravel, and the result is cheap and durable concrete. But making cement requires heating limestone to about 2,600°F, which causes the limestone to release carbon dioxide. The result, reports the U.S. Department of Energy, is that cement production has become the “largest source of U.S. carbon dioxide emissions other than fossil fuel consumption.” And demand is growing rapidly, especially in the developing world. In China, for instance, some 15 million people move from the country to the city every year, and construction must keep pace.

Constantz realized that cement manufacturers, by emulating coral, could meet that demand even as they actually reduced the total amount of carbon dioxide released into the atmosphere. Moreover, they could sequester the raw materials from the world’s single largest carbon-dioxide emitter, electric power plants. In 2009 his latest company, Calera, started putting that insight into practice at a 1,000-megawatt power plant in Moss Landing, California. Engineers there spray mineral-rich seawater or brine water through flue gas captured from the plant’s smokestacks. The calcium in the water bonds with carbon in the would-be pollution to form cement. Constantz says the demonstration plant is capable of producing up to 1,100 tons of cement a day and, in doing so, sequestering 550 tons of carbon dioxide. Within three years, he says, Calera will be operating plants in Australia and Wyoming.

Constantz notes as well that, unlike other sequestration schemes, his plan for capturing carbon emissions is proven. For at least 600 million years, sea creatures have been “sequestering” carbon dioxide in their skeletons, which have compacted over time to form all the limestone on Earth-the very stuff we now heat to make cement. Instead of turning stone to carbon dioxide, we can turn carbon dioxide into “stone,” locking it away forever in the concrete foundations of our cities. “When we think of climate change,” Constantz says, “the main lever we have is putting carbon back in the geologic record.”
-Benjamin Phelan

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