This eco-friendly concrete uses biochar to suck out carbon dioxide

The biochar was able to suck up to 23 percent of its weight in carbon dioxide from the air.
Loukia Papadopoulos

Washington State University (WSU) researchers have engineered a carbon-negative, environmentally friendly concrete that is nearly as strong as regular concrete by infusing regular cement with biochar, a type of charcoal made from organic waste.

The biochar was able to suck up to 23 percent of its weight in carbon dioxide from the air while still achieving a strength comparable to ordinary cement.   

“We’re very excited that this will contribute to the mission of zero-carbon built environment,” said Xianming Shi, professor in the WSU Department of Civil and Environmental Engineering and the corresponding author on the paper.

More than 4 billion tons of concrete are produced every year globally and cement production is thought to be responsible for about 8 percent of total carbon emissions by human activities worldwide.

In the past, attempts have been made to add biochar as a substitute in cement to make it more environmentally friendly but adding even 3 percent of the material dramatically reduced the strength of the concrete. 

An improved solution

This is where WSU researchers came up with a better solution: after treating biochar in the concrete washout wastewater, they could add up to 30 percent biochar to their cement mixture. The paste made of biochar-amended cement reached a compressive strength after 28 days compared to that of ordinary cement of about 4,000 pounds per square inch.

“We’re committed to finding novel ways to divert waste streams to beneficial uses in concrete; once we identify those waste streams, the next step is to see how we can wave the magic wand of chemistry and turn them into a resource,” said Shi. 

“The trick is really in the interfacial engineering – how you engineer the interfaces in the concrete.”

“Most other researchers were only able to add up to 3 percent biochar to replace cement, but we’re demonstrating the use of much higher dosages of biochar because we’ve figured out how to engineer the surface of the biochar,” he concluded in the statement.

Furthermore, the concrete made of the new material would be expected to continue sequestering carbon dioxide for its lifetime, typically 30 years on the pavement or 75 years on a bridge. 

Now, the researchers have filed a provisional patent application for their carbon-negative concrete work. Their work was published in the journal Materials Letters.

Study abstract:

This study demonstrates a synergistic carbon-capture strategy, i.e., using concrete washout water and biochar together to achieve carbon-negative concrete. This strategy enabled the biochar to capture 22.85 wt% air-borne CO2, which precipitated calcium carbonate onto the biochar. This carbon-negative cement paste consisted of 30 wt% CO2-weathered biochar and 70 wt% portland limestone cement, which achieved the 7-day and 28-day compressive strengths of 22.1 MPa and 27.6 MPa, respectively. Microscopic investigation revealed the mechanisms underlying the enhanced strengths of such concrete.

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