Researchers at Washington State University have demonstrated a new synergistic carbon-capture strategy, i.e., using concrete washout water and biochar together to achieve carbon-negative concrete. Their strategy enabled the biochar to capture 22.85 wt% air-borne carbon dioxide, which precipitated calcium carbonate onto the biochar. This carbon-negative cement paste consisted of 30 wt% carbon dioxide-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.
The carbon-negative, environmentally friendly concrete that is nearly as strong as regular concrete. Image credit: Washington State University.
Global warming poses a threat to the sustainability of human society, and it is essential to find a sustainable and effective strategy to curtail anthropogenic emissions of carbon dioxide.
More than 4 billion tons of concrete are produced every year globally. Making ordinary cement requires high temperature and combustion of fuels.
The limestone used in its production also goes through decomposition which produces carbon dioxide, so that cement production is thought to be responsible for about 8% of total carbon emissions by human activities worldwide.
Scientists have tried adding biochar as a substitute in cement to make it more environmentally friendly and reduce its carbon footprint, but adding even 3% of biochar dramatically reduced the strength of the concrete.
After treating biochar in the concrete washout wastewater, Washington State University researchers Xianming Shi and Zhipeng Li were able to add up to 30% biochar to their cement mixture.
“We’re very excited that this will contribute to the mission of zero-carbon built environment,” Professor Shi said.
“The paste made of the biochar-amended cement was able to reach a compressive strength after 28 days comparable 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.”
“The trick is really in the interfacial engineering — how you engineer the interfaces in the concrete.”
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,” Professor Shi said.
The authors 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 3% 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,” Professor Shi said.
“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.”
The team’s work was published in the journal Materials Letters.
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Zhipeng Li & Xianming Shi. 2023. Towards sustainable industrial application of carbon-negative concrete: Synergistic carbon-capture by concrete washout water and biochar. Materials Letters 342: 134368; doi: 10.1016/j.matlet.2023.134368
