The simple routine of drinking a cup of coffee every day generates one of the largest amounts of organic waste in the world. If you calculate how much used coffee grounds accumulate throughout the day across most of the planet, you end up with 1 billion kilograms of used coffee grounds per year. Most of this waste ends up in landfills, where it causes an environmental problem. A minority of environmentalists use it to make compost for their gardens, but this is a tiny percentage compared to the total amount produced.
However, a group of Australian engineers have discovered how to transform this breakfast waste into a structural solution. A study by RMIT University has succeeded in converting coffee grounds into a special additive that increases the compressive strength of concrete by 30%. Not only will we be able to use coffee grounds twice, but we will also end up with better building materials.
The crisis in construction materials
The environmental impact of coffee is not its transatlantic transport (although this also adds to its carbon footprint), but rather the fact that used coffee grounds end up accumulating in oxygen-depleted landfills. Their decomposition releases large amounts of methane gas, which is a powerful greenhouse gas that contributes to global warming. In fact, its greenhouse effect is 21 times greater than that of carbon dioxide (CO2) itself.
One major challenge is the current shortage of sand, which is an essential ingredient in the manufacture of concrete. Global demand for concrete is depleting natural sand reserves, which are being intensively extracted from riverbeds and banks. This obviously has a major environmental impact. Thanks to Australian engineers, used coffee grounds offer a substitute to replace a portion of this finite material with a sustainable waste material.
Modern Alchemy: coffee grounds vs. pyrolysis
Before you start experimenting with used coffee grounds in your next home renovation, we must warn you that it is not possible to add raw coffee to the concrete mix. This technology has been developed following a fairly precise preparation process. Untreated organic material still contains soluble compounds—such as sugars and fats—which, when filtered, inhibit the chemical reaction that causes cement to harden. If we simply added used coffee grounds to concrete, we would end up with a weak mixture with little resistance.
All of this technology resides in a process called pyrolysis, which consists of heating used coffee grounds to a controlled temperature of approximately 350 °C (662 °F) in an oxygen-free environment. This heat breaks down and removes all volatile organic molecules that are not beneficial to the mixture. The result is incredible: a stable, carbon-rich material called coffee biochar… very similar to charcoal.
This biochar is perfect because it serves as an internal water reservoir: water is absorbed during mixing and slowly released during the concrete curing phase. This allows for more complete and continuous hydration of the cement. The end result is a denser, better-bonded structure, which translates into impressive strength gains.
Economic—and ecologic—impact on construction carbon footprint
Research by engineers has shown that the quality of biochar and its performance in concrete depend on its preparation and dosage. They found that the optimum strength was achieved when 15% of the sand volume in the mixture was replaced with coffee grounds biochar. Specifically, the study reported a 29.3% increase in compressive strength with this dosage.
As mentioned, the peak of the rebirth was achieved by subjecting the coffee grounds to a temperature of 350 °C. If heated to a significantly higher temperature—such as 500 °C—the biochar became too porous and fragile.
From an environmental perspective, replacing 15 percent of sand with biochar from coffee grounds offers a substantial reduction in environmental impact. By reducing this 15 percent of sand, CO2 emissions associated with concrete production could be reduced by up to 23.2%. For now, this coffee ground biochar has been used to make concrete paths. The first one pavement was laid in Gisborne, Victoria, were it earned the nickname Latte Street. Isn’t that cute?