When you manufacture cement, you heat up limestone and other clay-like materials to an astounding 2,552 degrees Fahrenheit. And creating temperatures that high requires an awful lot of energy and (usually) a large amount of fossil fuels. Not only that, but when you heat up limestone — a carbonate — that limestone then breaks down into calcium oxide and carbon dioxide (CO2). This double whammy of incredibly high energy needs, and the use of a feedstock that directly releases carbon dioxide, means that cement manufacturing is one of the most carbon-intensive industries on the planet.

In fact, according to Columbia University's Earth Institute, this one industry contributes 5 percent of the entire global carbon dioxide emissions. (By comparison, the entire transportation sector contributes about 14 percent.)

An overlooked contributor to the problem
By now, most of us who follow the issue of global climate change know that we should probably drive our cars less, eat less meat, and reduce our energy consumption. But for some reason there's little recognition of the fact that one of the basic building blocks (hah!) of the modern built environment is directly contributing to our planetary crisis on an almost unimaginable scale. That, however, may be about to change.

As Barbara Grady over at Business Green reports, many cement manufacturers are planning for a day when carbon pollution no longer gets a free pass, and they're actively exploring both incremental improvements to their manufacturing methods as well as more radical reconsiderations of how cement is made and what it's made from. Under the banner of the Cement Sustainability Initiative (CSI), 25 of the world's largest cement companies are gunning for a 30 percent reduction in their CO2 emissions. Many of the leading innovators in this space, however, may achieve far greater efficiencies — with some promising to turn the entire process carbon-negative.

Turning cement emissions into liquid fuel
One of the initiatives Grady profiles is HeidelbergCement's partnership with a company called Joule Technologies. Together, the two firms are working on a process that captures CO2 emissions from cement manufacturing smokestacks and, using engineered bacteria as a catalyst, transforms those emissions into a feedstock for liquid fuel. Because that liquid fuel can be used to replace fossil fuel-based transportation fuels, the end result is significantly more "bang" for your CO2 buck. If all goes to plan, Heidelberg and Joule are predicting commercial applications of their technology within five years.

Workers pour and level out cement in a residential neighborhoodCement is the basic building block of the modern built environment, and it directly contributes to our emissions crisis. (Photo: ChameleonsEye/Shutterstock)

Cement as carbon sequestration
Another company profiled by Grady is Solidia, a U.S.-based firm that has developed a method for injecting CO2 captured from industrial operations into cement during the manufacturing process. That CO2 then acts as a binding agent, becoming permanently stored within the cement itself. This creates what the company claims could be the world's first carbon-negative cement, meaning it sequesters more carbon than was produced during manufacturing.

A long way to go
But let's not get too carried away about the potential for carbon negativity. Leading environmental thinker and author Tim Flannery, in his book "Atmosphere of Hope," covered the idea of carbon-negative cement as part of his exploration of "third way" technologies — approaches that could help us draw down some of the carbon that has already accumulated in the atmosphere. For cement to sequester even one gigaton of carbon per year, says Flannery, 80 percent of the world's cement manufacturing would have to switch over to technologies like Solidia's. Meanwhile the combined academies of the United States have estimated we'd need to sequester or otherwise draw down a whopping 18 gigatons of CO2 to start reducing atmospheric concentrations by even one part per million.

Still, you've got to start somewhere. And when a carbon-intensive industry starts exploring new ways that could help solve the problem — as opposed to just cause less harm — we should take note.

The huge carbon footprint of cement (and what we can do about it)
Cement manufacturing contributes 5 percent of global carbon emissions. But what if it could become part of the solution?