Concrete Jungles Store Carbon Dioxide, But Is It Enough?

Called the “lungs of the world,” our planet’s forests store more carbon dioxide in their leaves, soil and wood than is contained in the Earth’s entire atmosphere—often for hundreds of years. Forests keep the Earth cool and give off oxygen, making it possible for us to live here.

But according to a new study, we also have a carbon sequestrator located within our big cities: cement.

The foundation, as some say, of American civilization, cement is the glue that allows concrete to harden. Each year worldwide, four billion tons of cement are manufactured, which translates into a half ton for every person on Earth. Its production creates a full 5 percent of the world’s anthropogenic carbon emissions.

So, does cement’s ability to sequester carbon dioxide outweigh what’s produced in its manufacture? Are our concrete cities not as environmentally detrimental as we previously thought?

Can cement conquer carbon?

To make cement, limestone is heated at high temperatures in order to convert it to lime. That process releases carbon dioxide, which accounts for half of the substance’s total carbon emissions. Burning fossil fuels in the kilns release the other half.

In the recent study, published in the journal Nature Geoscience in November 2016, an international team of researchers calculated the carbon uptake by four cement materials—concrete, mortar, construction cement waste and cement kiln dust—in China, Europe, the United States and the rest of the world between 1930 and 2013. They used data from several studies to estimate how cement is used: its thickness, surface area, quality and building lifespans. They also calculated the sequestration resulting from demolition into different particle sizes and then whether the waste is disposed of or reused. Finally, they computed the carbonation rate in the different materials under different conditions, such as if it was buried or exposed.

As cement-based materials in buildings stand for years exposed to the air, carbon dioxide enters the substances’ pores, reacting with water and other elements. The carbon, then, gets converted into other chemicals that stay buried. What the researchers found is that almost a quarter of cement’s carbon dioxide emissions—or more than 4.5 billion tons—over the past 80 years have been soaked up by the commonplace building material. That’s an offset of 43 percent of the total carbon emitted when limestone is converted to lime, not including the emissions from fossil fuel use.

This carbon uptake is increasing as cement use around the world soars, especially in China. Existing cement stocks worldwide sequester about one billion tons of carbon dioxide every year. That missing piece in the calculations of cement’s carbon footprint is, indeed, good news.

Ferrock may be the future

But sequestering 43 percent of the 5 percent of the world’s industrial carbon emissions over the past 30 years doesn’t indicate that cement will provide enough of a dent in the greenhouse gases we are currently spewing into the atmosphere. Climate change scientists say that even if we stopped all carbon emissions now, we’ll likely surpass the threshold “safe” level of a 2-degree Celsius rise in CO2 in the long term, with major consequences for the planet. The researchers conducting the study published in Nature Geoscience say that policies to reduce emissions from cement production should focus on fossil fuel use rather than the limestone-to-lime process. Only if the emissions from cement factories can be captured and stored can cement become carbon negative, sucking up more carbon dioxide than it produces.

An alternative to cement, however, could provide a quicker, better solution. University of Arizona doctoral student David Stone invented it, and it’s called Ferrock.

Ferrock is a waste-iron-based, glass-aggregate cement. Its developer says Ferrock absorbs CO2 like a sponge. And unlike regular cement that creates one ton of greenhouse-gas-causing carbon dioxide in order to produce one ton of cement, Ferrock produces no carbon dioxide during production. Further, in order to cure Ferrocrete, the cement product made from Ferrock, carbon dioxide is required as a catalyst, thereby making it carbon negative.

Ferrock isn’t available for large projects just yet, such as an airport runway or a six-lane highway. But because it possesses significantly greater flexibility and is immune from many of the reactions that break down normal concrete in saltwater or wastewater, it is ideally suited for structures in marine environments, in seismically active areas and in conveyance systems, such as water pipes and waste pipes.

We may not be able to call our cities of concrete green just yet, but it is good to know that science—both of the study research and product development kind—is making them friendlier to the future.

Here’s to finding your true places and natural habitats,

Candy