Concrete is made of four basic ingredients: water, gravel, sand and cement.
Roychand and his team partially replaced sand with biochar — a material similar to charcoal — derived from coffee waste; they obtained their best result when they replaced 15% of the sand and baked the grounds at 350 degrees Celsius (662 degrees Fahrenheit). The resulting concrete was 30% stronger than regular concrete by compressive strength — the ability of the material to withstand a load.
In regular concrete, water, its second-largest ingredient by volume, is absorbed by the cement over time, reducing the amount of moisture that’s still inside the concrete, Roychand says. This drying effect, known as desiccation, causes shrinkage and cracking at a microscale, weakening the concrete.
Biochar from coffee waste can reduce this natural process. When the biochar is mixed with concrete, Roychand says, its particles act like tiny water reservoirs, distributed throughout the concrete. As the concrete sets and begins to harden, the biochar slowly releases the water, essentially rehydrating the surrounding material and reducing the impact of shrinkage and cracking.
In the bright future of solarpunk, the concrete structures will be strengthened and vitalized by coffee. Basically this is the polar opposite of the Warhammer 40K vision of the future.