Flickr/Creative Commons License/Oregon Department of Forestry

When touring Brazil’s Amazon basin in the 1960s, Dutch scientist Wim Sombroek chanced upon a black soil with exceptional nutritional properties. In addition to high levels of phosphorous, nitrogen, potassium, and plant matter, this soil contained high quantities of black carbon. Nicknamed Terra Preta, or black soil in Portuguese, the substance is highly fertile. It is also an effective storehouse for carbon and can sequester the element for a millennium or more.

Biochar, as the black earth is commonly known, is a carbon-rich soil created when organic materials decompose via heat in an oxygen-deprived environment. In the last two decades, it has received increased attention as a means of enhancing soil fertility and sequestering CO2. In What We Need to Do Now: For a Zero Carbon Future (Profile Books), author Chris Goodall suggests the possibility of capturing more than one billion metric tons of CO2, the equivalent of several percentage points of worldwide emissions, by incorporating biochar into the soil. In a similar vein, teams of scientists and student researchers have also been experimenting with incorporating biochar into building materials.

According to Hans-Peter Schmidt, a researcher at the Ithaka Institute for Carbon Intelligence in Arbaz, Switzerland, biochar represents a one-to-one ratio of CO2 emissions reduction. For each ton of biochar employed in buildings, one ton of CO2 is kept out of the atmosphere. The institute has developed biochar-based clay and lime plasters in which black carbon comprises up to 80% of the material. This high proportion is achievable since biochar may be used entirely in place of sand—and given the material’s high porosity, the resulting plaster is five times lighter than the typical version. In addition to carbon storage, the biochar-clay plaster exhibits good insulation, humidity-regulation, and electromagnetic radiation-mitigation properties—all of which enhance a building’s interior climate. If a building is eventually demolished, its biochar-based plaster may be reincorporated in the soil as a compost supplement, extending its carbon-trapping capacity.

A lecture on biochar and the building industry by Kathleen Draper, director of the U.S. branch of the Ithaka Institute

Biochar may also be used in building construction modules such as brick and tile. Early prototypes of biochar bricks at the Ithaka Institute incorporate a binder material such as cement or lime and exhibit a compressive strength of 20 N/mm2 (the minimum compressive strength for common brick is about 3.5 N/mm2). A team of engineering students at the University of Rochester conducted a series of biochar brick experiments based on the Ithaka Institute’s research. Intending to minimize the use of virgin materials like Portland cement and sand, the students created two types of biochar bricks: one with cement and another with repurposed plastic and additives. The first version consisted of 75% biochar and 25% cement. For the latter version the students experimented with different percentages of biochar, high-density polyethylene, and sand. The team found that bricks made with 50% biochar and 50% HDPE exhibited the highest compressive strength, but that the biochar-cement brick performed better when it came to insulating value, hardness, and water absorption. The team estimated that the biochar-cement bricks would enable a 6% decrease in the CO2 emitted from cement production if used globally.

A student team at the Rochester Institute of Technology took a similar approach in creating biochar roof tiles. The idea emerged as part of the 4 Walls Project, a housing initiative in El Sauce, Nicaragua. The students working with the project sought an alternative to the typical sheet-metal roof given its high cost and embodied energy, and the noise it makes during rainfall. They designed a substitute roof tile consisting of 30% biochar added to a mix of cement, sand, water, and reinforcing plastic from shredded waste soda bottles, creating a product that sequesters carbon and consumer waste while reducing overall embodied energy.

Experiments like these are new but compelling. The Ithaka Institute has a bold vision for architectural biochar—or Arquitetura Preta—applied at an urban scale to create Carbon Intelligent Cities. Rather than sequester carbon underground with carbon capture and storage technology, we could make a visible and functional impact by using stored carbon in building construction. Carbon-rich envelope materials can also be used to build green walls and roofs, and could take on a new role once a building reaches the end of its useful life. “Instead of having to use special landfill sites for getting rid of contaminated building materials, we could be seeing tomatoes and potatoes growing on the the remains of demolished houses,” writes Schmidt.