Biochar-infused carbon-negative concrete developed

A viable formula for a carbon-negative, environmentally friendly concrete that is nearly as strong as regular concrete has been developed at Washington State University (WSU).

A viable formula for a carbon-negative, environmentally friendly concrete that is nearly as strong as regular concrete has been developed at Washington State University (WSU).

In a proof-of-concept work, the researchers infused regular cement with environmentally friendly biochar. Biochar is a type of charcoal made from organic waste that had been strengthened beforehand with concrete wastewater. The biochar was able to suck up to 23 per cent of its weight in carbon dioxide from the air. At the same time, it has reached a strength comparable to ordinary cement.

The research could significantly reduce carbon emissions in the concrete industry. It is one of the most energy- and carbon-intensive of all manufacturing industries. The work, led by doctoral student Zhipeng Li, is reported in Materials Letters. The paper was titled, “Towards sustainable industrial application of carbon-negative concrete: Synergistic carbon-capture by concrete washout water and biochar.”

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

What drove the development to add biochar to concrete?

More than 4 billion tons of concrete are produced every year globally. Making ordinary cement requires high temperatures and the combustion of fuels. The limestone used in its production also goes through decomposition, which produces carbon dioxide. Cement production is thought to be responsible for about 8 per cent of total carbon emissions by human activities worldwide.

Researchers have tried adding biochar as a substitute in cement to make it more environmentally friendly and reduce its carbon footprint. However, adding even 3 per cent of biochar dramatically reduced the strength of the concrete. After treating biochar in the concrete washout wastewater, the WSU researchers could add up to 30 per cent biochar to their cement mixture. The paste made of biochar-amended cement reached a compressive strength after 28 days compared to 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.”

Where did the biochar come from?

The caustic concrete washout water is a sometimes problematic waste material from concrete production. The wastewater is very alkaline but also serves as a valuable source of calcium, said Shi. The researchers used calcium to induce the formation of calcite. It benefits the biochar and, eventually, the concrete incorporating the biochar.

“Most other researchers could only add up to 3 per cent biochar to replace cement. 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 said.

The synergy between the highly alkaline wastewater and the highly porous biochar meant calcium carbonate precipitated onto or into the biochar. The calcium strengthened the biochar-infused concrete and allowed for the capture of carbon dioxide from the air. Concrete made of the material should continue sequestering carbon dioxide for the lifetime of the concrete. That is typically 30 years for pavement or 75 years for a bridge.

The researchers have been working with the Office of Commercialization to commercialise this technology to protect intellectual property. They have filed a provisional patent application for their carbon-negative concrete work. The team has recently received a seed grant from the Washington Research Foundation to produce more data for various use cases. They are also actively seeking industry partners to scale up production for field demonstrations and license this WSU technology.

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