Two Companies Cement X-Prize for Decarbonizing Concrete

Manufacturing cement, the key ingredient of concrete, is responsible for anywhere between 7% and 10% of the world's carbon dioxide (CO2) emissions. About half of the emissions are from combustion — cooking calcium carbonate, mostly limestone, at 2,642 degrees with fossil fuels. About another half of it is chemistry, where calcium carbonate (CaCO3) is reduced to calcium oxide (CaO) — also known as lime — and lots of CO2. This is a big problem for the construction industry.

Now, two companies have figured out how to put CO2 back into concrete, thereby reducing its carbon footprint. The companies — CarbonCure Technologies and CarbonBuilt — just received the NRG COSIA Carbon XPRIZE for the solution.

How CarbonCure Does It

It takes a lot of energy to break calcium carbonate into calcium oxide and CO2, and the CarbonCure process reverses it by pumping CO2 into the concrete mix, where any available calcium oxide essentially turns back into limestone. This would happen naturally over a period of years or decades, but CarbonCure speeds it up. It makes the concrete stronger in the process and lets the concrete producer reduce the amount of cement, making it a double win.

Between the sequestered CO2 and the reduction in cement, it can save up to 25 pounds of CO2 per cubic yard of concrete and reducing its embodied carbon. The company explained:

"Embodied carbon reduction is the current hot topic among the sustainable design and construction communities, as it historically has been overlooked and plays a key role in reducing the carbon footprint of the built environment. By 2050, embodied carbon emissions will be responsible for almost half of all construction emissions."

That's actually an understatement: As buildings reduce their operating emissions, the embodied carbon could reach as much as 95% of all construction emissions, making this even more important.

When Treehugger first covered CarbonCure (now archived) the company could only do concrete masonry units. Now its process has been improved to where it can use it in Ready Mix Concrete. CarbonCure's press kit is also very careful to correct a common media mistake by noting that "CarbonCure does not capture carbon dioxide."

However, its XPRIZE-winning project in Alberta, Canada actually appears to do just that. It removed CO2 from the exhaust of a cement kiln, used it to carbonate reclaimed wastewater from washing out Ready Mix trucks, and then used that water for the CarbonCure processing of the concrete. Many would happily call that Carbon Capture, Utilization and Storage (CCUS).

“This breakthrough helped us envision a future with a fully circular economy, where we’ve not only reduced the amount of CO2 emissions that we produce, but any remaining CO2 emissions are used to create valuable products,” said CarbonCure CEO and founder, Rob Niven.

How CarbonBuilt Does It

Treehugger has not covered CarbonBuilt before and is less familiar with its process, but it appears the company is adding calcium hydroxide, Ca(OH)2 known also as slaked lime, to "reduce the usage of traditional cement and increase the use of waste materials like fly ash." Regular concrete is made with calcium oxide and hardens when water is added through a process of hydration, which is why it is known as hydraulic cement.

Non-hydraulic Cement is made with calcium hydroxide and hardens through carbonation in contact with carbon dioxide, and it is usually a much slower process because there isn't that much CO2 in the air. It appears the CarbonBuilt Reversal Process adds some oomph by injecting CO2 into the mix.

This may be why they appear to be making blocks and precast that can fit inside what looks like a shipping container that's probably full of CO2; non-hydraulic cement needs dry conditions and isn't usually used outdoors anymore. Some sources call it obsolete and inconvenient, but CarbonBuilt may be giving it new life.

According to the XPRIZE release,

"UCLA CarbonBuilt, developed technology that reduces the carbon footprint of concrete by more than 50 percent while reducing raw material costs and increasing profitability. The CarbonBuilt concrete formulation significantly decreases the need for ordinary Portland cement while enabling the increased use of low-cost waste materials. During the curing process, CO2 is directly injected from flue gas streams (like power plants or cement factories) into the concrete mixture where it is chemically transformed and permanently stored."

At first glance, decreasing the need for Portland cement, which is made from calcium oxide that comes out of a kiln, doesn't seem like a big deal if it is being replaced with non-hydraulic cement, which is made by adding water to that same calcium oxide to get calcium hydroxide. However, the chemical reaction of calcium hydroxide with CO2 absorbs a lot more of the stuff than the hydraulic cement reaction does, as it turns back into good old limestone (calcium carbonate) and water.

Other companies making non-hydraulic cement have claimed reductions in CO2 footprints of up to 70%. And hey, it won an XPRIZE so it's gotta work.

This is all wonderful news for the construction industry; there really does seem to be serious progress in decarbonizing concrete. I was skeptical when the concrete industry pledged to deliver carbon-neutral concrete by 2050 — I would be so happy to eat those words.

Here is a bit more about the difference between hydraulic and non-hydraulic concrete: