Washington State University researchers reported a proof-of-concept cement paste made with biochar treated using concrete washout water and airborne carbon dioxide. In their tested formulation, the treated biochar captured CO₂, and a paste containing it was described as carbon-negative under the study’s accounting. The findings are laboratory results—not evidence of a market-ready structural concrete product.
How the recipe captures carbon
The method combines three materials in a specific sequence: porous biochar made from pyrolyzed organic waste, alkaline concrete washout water, and airborne CO₂. Concrete washout water, a byproduct of cleaning concrete equipment, contains calcium and has high alkalinity. The researchers used it to pretreat the biochar, then exposed the water-biochar combination to air. This encouraged calcium carbonate to form on or within the biochar, storing captured CO₂ in a mineral form.
The CO₂-weathered biochar was then incorporated into Portland limestone cement to make a cement paste. This is not simply untreated charcoal mixed into conventional concrete: both the washout-water pretreatment and the subsequent exposure to CO₂ are central to the reported approach. The researchers presented the treatment as a way to address a challenge in earlier biochar-cement work, where high biochar additions can reduce strength.
What the researchers measured
In their 2023 study, Zhipeng Li and Xianming Shi reported that biochar treated with concrete washout water took up airborne CO₂ equal to 22.85% of the treated biochar’s weight. Separately, they tested a paste formulated with 30% CO₂-weathered biochar and 70% Portland limestone cement by weight. The 30% figure describes the biochar share of the paste; it is not the biochar’s CO₂ uptake percentage.
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The researchers reported compressive strength of 22.1 MPa at seven days and 27.6 MPa at 28 days for the tested cement paste. These are laboratory measurements of a particular paste formulation at specified ages. They should not be read as strength ratings for a finished structural concrete product or as proof of performance in a building.
What “carbon-negative” means here
Li and Shi describe their tested paste as carbon-negative based on the study’s formulation and accounting. That result is specific to this material and its assessment; it does not establish that biochar concrete in general removes more carbon than its production emits. The available reporting does not establish a commercial-scale lifecycle assessment verifying the claim across manufacturing, transport, construction, and use.
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The peer-reviewed paper, “Towards sustainable industrial application of carbon-negative concrete: Synergistic carbon-capture by concrete washout water and biochar,” appeared in Materials Letters in 2023. Read the study record for the paper’s methods and results. Washington State University’s research announcement describes the project and its development status. Corresponding author Xianming Shi said the team was “very excited that this will contribute to the mission of zero-carbon built environment,” describing the work’s potential rather than a result already demonstrated at industry scale.
Is it ready for construction use?
The result is a proof of concept, not an announced commercial product. WSU reported that the team had filed a provisional patent application and was working with the university’s Office of Commercialization to protect the intellectual property. That indicates commercialization interest, but does not confirm a launched product, field deployment, or availability to builders.
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The study reports laboratory mechanical testing and microscopic investigation, while also identifying technical challenges involving biochar strength, its interface with the cement matrix, and the performance of biochar-containing cementitious composites. The reported evidence does not establish long-term durability, code acceptance, cost, scale-up, or construction-site performance. Those questions would need to be answered before the recipe could be treated as a dependable specification for ordinary building work.
Independent reader supportYour contribution helps us test, update, and keep practical guides available for everyone.What to look for in future comparisons
Any meaningful comparison should distinguish the treated biochar from untreated biochar, and should consider CO₂ uptake alongside mechanical performance—not as a substitute for it. It should also separate controlled laboratory results from commercial-scale production and field performance. The sources describing this study identify no comparable retail product, so the reported paste should be understood as a research formulation rather than an off-the-shelf alternative to conventional concrete.
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