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Coffee Grounds in Concrete: How the Biochar Research Works

See how processed coffee grounds replace sand, why lab and footpath strength results differ, and what carbon assessments measure.

By Edward MillerPublished January 21, 2026Updated October 25, 20263 min read

Quick Answer

Processed coffee grounds can replace part of the sand in concrete. Researchers first turn them into biochar by heating them with oxygen excluded. One laboratory mixture gained 29.3% in compressive strength; a later field formulation improved bending resistance while reducing compressive strength against its own control.

Separate dishes of brown spent coffee grounds and fine black coffee biochar beside a small concrete sample.

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The Transformation Is the Point

Biochar is the carbon-rich material left by pyrolysis, a heating process carried out with oxygen excluded. It changes the grounds' chemistry and creates a porous material. The 2023 study by Roychand and colleagues compared raw spent grounds with biochar produced at 350 °C and 500 °C.

Raw grounds released organic compounds that interfered with cement hydration, the reactions through which cement and water form the hardened binder. That weakened the tested concrete. With the right processing and mixture, coffee biochar could instead become a useful ingredient. Its porous particles can hold moisture that supports continuing hydration, one mechanism discussed by the researchers.

Why Sand Replacement Is Not Cement Replacement

Sand is part of the aggregate, the solid particles held together in concrete. Cement forms the binder when it reacts with water. Replacing some sand therefore changes a different ingredient from replacing cement.

In the highlighted laboratory mixture, coffee biochar replaced 15% of the sand by volume. That means neither 15% of the entire concrete nor 15% less cement. RMIT's original research account describes that partial sand substitution. Its account of the Gisborne footpath trial says the trial retained the cement amount normally used.

From the Laboratory to a Field Trial

Commercial processing was an important part of the transition. The field project's reactor could not make the material at the laboratory's preferred 350 °C, so it produced biochar at 450 °C. The field mix also used a different cement blend. These were different formulations tested against their respective controls.

Laboratory and Field-Mix Strength Results

Laboratory and Field-Mix Strength Results
Material and test	Result against its control
Original laboratory mixture: 350 °C biochar replacing 15% of sand by volume	29.3% higher compressive strength, the result often rounded to 30% in headlines.
Field-trial mixture: 450 °C biochar, cast specimens tested at 28 days	9.3% lower compressive strength and 18.9% higher flexural strength.

Compression measures resistance to being squeezed; flexure measures resistance to bending. The 2025 field paper shows why both matter: one property improved while the other declined. It also reports shrinkage tests and later testing of cores taken from the footpath. Those checks extend the investigation beyond the original laboratory strength result.

A personal note: The detail I find most useful here is the gap between a promising lab mixture and material made with commercial equipment. It makes the project feel like engineering rather than a slogan: the processing changes, the tests ask different questions, and the answer becomes more complicated. I can be interested in a second use for coffee grounds while still wanting the less convenient results included. For me, that makes the progress more persuasive than repeating the largest percentage on its own.

What the Newer Carbon Study Adds

A separate life-cycle study published online in November 2025 assessed environmental impacts. Its abstract reports a maximum 23.2% reduction in CO₂ emissions across the assessed 5–15% coffee-biochar replacement scenarios, with diminishing benefits above 15%.

That result concerns the modeled life cycle. Processing, transport and the material displaced help determine the footprint; a strength increase alone does not establish a carbon saving. Our coffee-footprint article explains how the stages and service counted affect comparisons.

What This Means for the Grounds in Your Kitchen

If a local coffee-waste program collects grounds, check what it accepts, how it wants them stored and where they go. A compost collection and a construction-biochar supply chain serve different purposes. For the garden route, start with used grounds as a compost ingredient.

Do not try to make construction biochar at home or improvise a structural concrete mixture. This application relies on controlled material processing and tests of the finished concrete.

Sources

The life-cycle paper's abstract reports a maximum 23.2% CO₂ reduction; RMIT's accompanying announcement gives 26%. The figure used here follows the paper's abstract. The full model is needed to resolve the difference.