Concrete has a substantial carbon footprint, estimated to be responsible for about 8% of total carbon emissions from human activities. This is due to numerous factors, including the fuel combustion and limestone consumption involved in cement manufacturing. Biochar, a material produced through pyrolysis of biomass, has the potential to reduce the carbon footprint of concrete due to its ability to efficiently adsorb and sequester atmospheric carbon dioxide, but previous attempts to incorporate biochar into cement have shown that its addition dramatically reduces the concrete’s strength. Researchers from Washington State University have now demonstrated a new method for producing concrete from biochar, resulting in a carbon-negative cement mixture with strength comparable to ordinary concrete.
Previous studies have shown that cement mixtures containing as little as 3% biochar produce significantly weakened concrete that would not be practical for ordinary use. In order to strengthen the biochar, the researchers treated it with concrete washout wastewater, which is highly alkaline and also has a high calcium content. The alkalinity and calcium content of the wastewater, in synergy with the high porosity of biochar, facilitates the precipitation of calcium carbonate onto the biochar. This approach both strengthens the biochar and enhances CO2 sequestration, in addition to reusing a waste stream from concrete production. The researchers incorporated the pretreated biochar into a cement mixture at 30% by weight and tested the mixture’s compressive strength and CO2 capture over time.
The team found that the compressive strength of the mixture after 28 days was 27.6 MPa, or about 4000 pounds per square inch, comparable to that of ordinary cement. Additionally, the pretreated biochar captured nearly 23 wt% of atmospheric CO2, demonstrating its capability to sequester the greenhouse gas. In real world application, the concrete mixture could sequester the CO2 throughout the lifetime of the material – approximately 30 years for pavement or 75 years in bridge construction, for example. In addition to this factor, the decreased use of limestone and reuse of concrete industry waste streams makes the biochar composite cement developed by the researchers a potential sustainable and eco-friendly alternative to traditional concrete. This research was published in Materials Letters.
“We’re very excited that this will contribute to the mission of a zero-carbon built environment,” said corresponding author Xianming Shi. “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. The trick is really in the interfacial engineering–how you engineer the interfaces in the concrete.”
The researchers have taken next steps toward commercialization of the carbon-negative concrete and have filed a provisional patent application for their technology.