Is it possible green shoots are starting to appear in the concrete jungle? That’s the prospect being dangled by California’s Calera Corporation, which claims a concrete-making process that is not only carbon neutral but potentially carbon negative.

“What we’re doing is cheaper and less energy intensive than any other option out there,” says CEO Brent Constantz, a consulting professor at Stanford University’s Department of Geological and Environmental Sciences.

Among efforts to stem the tide of global warming, scientists are researching ways to capture and sequester carbon dioxide with geological storage deep underground or under the sea a favored option.

But Constantz reckons his company, recently awarded a $30 million Department of Energy grant, can achieve similar results by embedding CO₂ in concrete, locking it up permanently in roadways, bridges and other structures. (Here’s his testimony before Congress in May.)

Concrete is a mixture of cement, aggregate — usually gravel and sand — and water. Making cement involves a chemical change that releases CO₂ when the main ingredient, limestone, is baked at high temperatures to produce clinker, which is pulverized into cement powder.

Calera does not follow this so-called “heat-and-beat” process; instead, the Los Gatos-based company proposes using CO₂ emissions from sources such as gas or coal-fired power plants to make cement.

The company is testing this at a pilot plant near the Moss Landing gas-fired power station on Monterey Bay, Calif. When raw flue gases belching out of those chimney stacks are bubbled through sea water, the CO₂ is absorbed and produces carbonates, much like the natural mineral deposits that create coral reefs.

These chalky deposits can be used to make cement or as part of the aggregate, which accounts for 80 percent of concrete. Either way, says Constantz, Calera captures as much as half a ton of CO₂ for each ton of cement.

Constantz says all performance testing to date, from Moss Landing and from trials in Australia, suggests a product at least equal to traditional cement and concrete in terms of strength, durability and flexibility.

Hendrik van Oss, cement commodity specialist with the U.S. Geological Survey, says producing one ton of cement releases close to a ton of CO₂, about half from chemical reactions and half from the energy needed to heat the kilns.

That’s a huge and growing factor in global warming. Van Oss says current world cement production is about 2.8 billion tons, and he doesn’t argue with predictions that, driven by China’s rampant growth, production could reach 5 billion tons by 2030.

Like Calera, other young companies, in Canada and England, are also looking to meet this challenge by capturing CO₂ in concrete.

Carbon Sense Solutions, based in Halifax, Nova Scotia, working with pre-cast concrete, is developing a system that reabsorbs CO2 generated during manufacture back into the concrete during curing, the strengthening and hardening process.

Meanwhile, Novacem, a company spun out of London’s Imperial College, also claims to be carbon-negative thanks to a carbonate-free cement that enables concrete to suck CO₂ from the atmosphere.

Van Oss thinks this technique would work best on expansive flat surfaces, such as sidewalks and thin walls, since the process forms a crust of calcium carbonate just beneath the concrete surface, which gradually slows and obstructs further absorption.

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