Human Waste Could Help Make Stronger, More Sustainable Concrete

Researchers have discovered a way to convert human waste into biochar, a building material that could be used to create more sustainable and stronger concrete, according to a study published in Scientific Reports. The process begins by drying and heating the waste, known as fecal sludge, to temperatures between 662 and 842 degrees Fahrenheit (350 to 450 degrees Celsius) under limited oxygen in a process called pyrolysis. This transforms the organic material into a carbon-rich solid known as biochar.

The biochar was then ground and sieved before being mixed with cement, the binding ingredient in concrete, at 5, 10, or 15 percent by weight. The researchers tested the resulting concrete by measuring its resistance to crushing and bending, as well as its water absorption and pore space. They found that replacing 5 percent of the cement with biochar resulted in the highest strength during 28, 56, and 91-day curing periods.

Concrete containing 10 percent biochar also showed improved performance, although it took longer to reach its full strength.

The tiny pores in biochar help explain why it enhances concrete's strength. These pores can hold water during mixing and release it later to support the chemical reactions necessary for cement to harden. Additionally, the biochar can fill gaps and contribute to the formation of additional binding compounds. However, adding too much biochar can have negative effects.

At 15 percent replacement, the concrete experienced a reduction in strength due to the formation of voids and cracks, indicating that excessive use of biochar weakens the internal structure.

While using less cement has environmental benefits, as its production consumes significant energy and releases carbon dioxide, further testing is required to ensure the material's performance in real-world conditions. The researchers suggest that freezing and thawing, coastal salinity, and temperature extremes should be investigated. Scaling up production also raises questions about cost and the environmental safety of potentially toxic contaminants leaching from the material.