Carbon-Negative Concrete Created by Researchers

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

In their proof-of-concept work, the researchers infused regular cement with eco-friendly biochar, a type of charcoal made from organic waste, that had been pre-hardened with sewage concrete. The biochar was able to absorb up to 23% of its weight in carbon dioxide from the air while still achieving strength comparable to regular cement.

The research could significantly reduce carbon emissions in the concrete industry, which is one of the most energy- and carbon-intensive manufacturing industries. The work, led by doctoral student Zhipeng Li, is published in the journal, Material letters.

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

More than 4 billion tons of concrete are produced annually globally. Making ordinary cement requires high temperature and combustion of fuel. The limestone used in its production also goes through decomposition which creates carbon dioxide, so cement production is believed to be responsible for around 8% of all carbon emissions from human activities worldwide.

Researchers tried adding biochar as a substitute in cement to make it more environmentally friendly and reduce the carbon footprint, but adding 3% biochar significantly reduced the strength of the concrete. After treating the biochar in the concrete wastewater, the WSU researchers were able to add up to 30% biochar to their cement mixture. The putty made with biochar modified cement was able to reach compressive strength after 28 days compared to regular cement of about 4,000 psi.

“We are committed to finding new ways to turn waste streams into useful uses in concrete; once these waste streams are identified, the next step is to see how we can wave the magic wand of chemistry and turn it into a resource,” Shi. “The trick is really in the interfacial engineering—how you engineer the interfaces into the concrete.”

Caustic concrete wash water is sometimes a waste material that results from concrete production. Shi said the wastewater is highly alkaline but also a valuable source of calcium. The researchers used calcium to induce the formation of calcite, which benefits biochar and eventually biochar-containing concrete.

“Most other researchers have only been able to add up to 3% biochar to replace cement, but we demonstrate using much higher doses of biochar because we have figured out how to engineer the surface of biochar,” he said.

The synergy between the highly alkaline wastewater that contains a lot of calcium and the highly porous biochar means that calcium carbonate is deposited on or in the biochar, which hardens it and allows carbon dioxide to be absorbed from the air. Concrete made of the material is expected to continue to sequester CO2 for the life of the concrete, typically 30 years in a pavement or 75 years in a bridge.

In order to commercialize the technology, the researchers worked with the Office of Marketing to protect intellectual property and filed a provisional patent application on their carbon-negative concrete work.