Materials

New recipe makes concrete that absorbs more CO2 than it emits

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A new cement recipe containing up to 30% treated biochar could make for carbon-negative concrete
A new cement recipe containing up to 30% treated biochar could make for carbon-negative concrete
Researchers Zhipeng Li and Xianming Shi with samples of the new carbon-negative concrete
Washington State University

Concrete is one of the most common materials on Earth, thanks to its high strength and low cost – but it’s also one of the largest single sources of carbon dioxide emissions. Engineers at Washington State University (WSU) have developed a new method for making concrete that absorbs more carbon than it emits.

The process of making cement requires very high temperatures, and that usually requires burning fuels which, of course, emits CO2. That can be partly offset by switching to renewable energy sources, but chemical reactions in the mixture also release huge amounts of CO2, and this is harder to avoid. All up, it’s estimated that cement production accounts for as much as 8% of humanity’s total carbon dioxide emissions.

Scientists have been tweaking the formula to try to reduce concrete’s carbon footprint, by substituting limestone for volcanic rock, or adding ingredients like titanium dioxide, construction waste, baking soda or a clay commonly discarded during mining. Other teams have even tried using microalgae to grow the required limestone.

For the new study, the WSU researchers investigated a new method involving biochar, a charcoal made from organic waste. While biochar has been added to cement before, this time the team treated it first using concrete washout wastewater. This boosted its strength and allowed a higher proportion of the additive to be mixed in. But most importantly, the biochar was able to absorb up to 23% of its own weight in carbon dioxide from the air around it.

In experiments, the team created cement that contained 30% treated biochar, and found that the resulting concrete was carbon-negative – it actually absorbed more carbon dioxide than was emitted during production of the material. By the researchers’ calculations, 1 kg (2.2 lb) of the 30%-biochar concrete removes about 13 g (-0.5 oz) of CO2 more than its production releases. That might not sound like much, but considering regular concrete is usually responsible for releasing about 0.9 kg (2 lb) of CO2 per 1 kg of material, it’s a stark difference.

Researchers Zhipeng Li and Xianming Shi with samples of the new carbon-negative concrete
Washington State University

The total gains could be even better, the team says, if downstream differences were accounted for in their analysis. For example, using biochar for environmentally friendly purposes like this concrete diverts the biomass it’s made of away from other fates that could potentially release more CO2. Plus, the new concrete would be expected to continue absorbing CO2 during its working lifetime of several decades.

Importantly, the biochar-concrete also retains its strength. When measured after 28 days, the compressive strength of the concrete was 27.6 MPa (4,003 psi), which is around that of regular concrete.

The researchers plan to continue optimizing and scaling up the method, and testing how well the resulting concrete resists weathering and other types of damage.

The research was published in the journal Materials Letters.

Source: Washington State University

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5 comments
guzmanchinky
quiet lab techs saving the world one experiment at a time, all while the loud politicians get all the attention...
Steve Barry
And yet, CO2 is still not the problem.
GdB
Tensile strength with this formula is important.
rgbatduke
Less impressive than it might be, since it has been known for a rather long time that quicklime based building materials absorb CO2 and recarbonate/cure over a very long time, while "cement" based building materials do not. See e.g. https://biolime.com/lime-vs-cement/. One literally gets stronger over time, the other weaker and more brittle -- see for example the cracks in my driveway only 35 or so years old, the loss of mortar in my house's brickwork over a similar time frame.

However, the choice is a complex one. Cement vs lime mortars actually behave differently and are better suited to different materials, and as you note, BOTH generate 80% or more of their carbon load during the process that converts limestone to lime, which is the real bugaboo here. The chemical fact of the matter is that carbon is a really good way of removing oxygen from ores and rocks to get metals or materials like lime OR cement.

They are also years -- nay, decades -- away from proving that their new concrete is safe to use for anything more stressed than building a garden wall. Failure of building materials can be catastrophic, and things like acid rain, earthquakes, wet vs dry environments, hot summers and freezing winters, and salt can all cause deterioration and eventual failure. Before any actual architect uses this new material, it will have to have become the old material at least somewhere, most likely somewhere safe where e.g. an overpass built with it won't crumble and fall on people if it fails in twenty years.
jerryd
Or just use calcium silicate instead of calcium carbonate, no CO2 to release.