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.

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