Biochar blend makes concrete more environmentally friendly

In a proof-of-concept work, the researchers infused regular cement with biochar that had been strengthened with concrete wastewater. The biochar was able to absorb up to 23 per cent of its weight in carbon dioxide from the air while still reaching a strength comparable to ordinary cement.   

The work, led by doctoral student Zhipeng Li, is detailed in Materials Letters.

“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.

Researchers have tried adding biochar as a substitute in cement to make it more environmentally friendly, but adding even three per cent of biochar reduced the strength of the concrete. After treating biochar in the concrete washout wastewater, the WSU researchers were able to add up to thirty per cent biochar to their cement mixture.

After 28 days the biochar-amended cement is said to have reached a compressive strength comparable 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 [utilise] chemistry and turn them into a resource,” said Shi. “

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

“Most other researchers were only able to add up to three per cent 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,” said Shi.

The synergy between the highly alkaline wastewater that contains a lot of calcium and the highly porous biochar meant that calcium carbonate precipitated onto or into the biochar, strengthening it and allowing for the capture of carbon dioxide from the air. A concrete made of the material would be expected to continue sequestering carbon dioxide for the lifetime of the concrete, typically 30 years in pavement or 75 years in a bridge. 

A provisional patent application has been filed on the team’s carbon-negative concrete. They are also seeking industry partners to scale up production for field demonstrations and licensing.