Carbon-Negative Construction

A new generation of building materials aims to actively pull carbon out of the air

By Lucy Wang

Hempcrete
Hempcrete

Brick ties secure the hemp blocks, which can also be used as cladding. ©Coexist Build

Hempcrete

Hemp is making a comeback—again

Industrial hemp has been used in construction since Roman times. But in the mid-twentieth century, it got lumped in with its psychoactive relative, marijuana, and innovation fell on hard times. Now, in the wake of the 2018 Farm Bill’s legalization of production, it has bounced back into the limelight—this time with a distinctive twenty-first-century twist: carbon capture.

Hemp’s rapid growth, dense canopy, and ability to grow in nutrient-poor soils make the plant more efficient than trees at sequestering carbon. In construction, hemp is combined with a lime-based binder to create hempcrete, a carbon-negative biocomposite that can sequester over 100 kilograms of CO2 per square meter.

“Hempcrete’s not only healthy and nontoxic, but is also resistant to fire, pests, mold, and mildew,” says Drew Oberholtzer, cofounder of Coexist Build, an architecture studio with a regenerative organic farm that doubles as a research and development lab for hemp-based building materials. The sustainable building material also boasts benefits for indoor air quality and, as its name implies, can be precast in blocks. This eliminates the time needed for curing on site and makes it easy to incorporate it into existing construction trades.

Although hempcrete can be cast like concrete, it isn’t a substitute. The lightweight material lacks the structural integrity needed for load-bearing. It can, however, be used in place of traditional drywall, insulation, and siding—where its high thermal mass offers energy-saving benefits by reducing the need for heating and cooling.

Because industrial hemp production in the United States was legalized only recently, Oberholtzer still imports raw material from Europe. “The biggest variable in the amount of carbon sequestered in manufacturing hempcrete is transportation of raw material to a manufacturer and project site,” he says. “With the development of local supply chains from seed or farm to gate, the amount of carbon sequestered will be optimal.”

Wil Strubar

When placed into a sand-hydrogel scaffold, photosynthesizing cyanobacteria convert carbon dioxide into calcium carbonate that binds the sand particles together. ©University of Colorado Boulder, College of Engineering and Applied Science

Engineered living materials 

Synthetic biology, materials science, and architecture
are fusing into a new field

Imagine architects using engineered bacteria to grow buildings that self-repair and adapt to the environment. It is a plot line out of science fiction that is steadily making its way into labs around the world. Chief among them is the Living Materials Laboratory at the University of Colorado in Boulder, where an interdisciplinary team has created a type of living biocement that reproduces by capturing carbon dioxide.

“Our living building materials are made by combining sand, a small amount of hydrogel, cyanobacteria, water, and nutrients,” says Wil Srubar, principal investigator at the lab. “The cyanobacteria use sunlight, CO2, and seawater to grow, and they precipitate calcium carbonate as a result of their metabolic process. This helps bind the sand particles together and strengthens and toughens the composite.”

This process, which takes less than a day from start to finish, is similar to how colonies of coral polyps form coral reefs by secreting calcium carbonate layers. Just as living reefs continuously expand, the CO2-absorbing bacteria’s exponential growth drives the biomaterial’s ability to reproduce.

“In our study, we took one fully formed ‘parent’ generation of biobrick, split it into two halves, added other abiotic ingredients (e.g., sand, seawater, nutrients) that enabled the cyanobacteria to grow and mineralize the entire mass into a new, full-size ‘child’ biobrick,” explains Srubar, referring to the study published in January 2020 in the journal Matter. The procedure can be repeated twice, so that a single biobrick can spawn up to eight descendant biobricks.

Changes to temperature and humidity levels switch the living material between a continued growth phase and a dormant phase. Once fully dried, the biobricks achieve compressive strength comparable to that of concrete masonry units and can be used for interior load-bearing applications.

“As we fine-tune our dry-packaged biocement technology, it could be used anywhere in the world to mold and shape load-bearing materials, in much the same way you use portland cement today,” says Srubar, adding that their approach yields more than a 90 percent reduction in CO2 emissions, compared to traditional cement.

Research into the biocement technology began in 2017 with funding from DARPA, the advanced-technology arm of the US Department of Defense, as part of the Engineered Living Materials (ELM) Program. Srubar and his colleagues have recently spun out a startup called Prometheus Materials to further develop the biocement, with the goal of reducing global carbon emissions by a gigaton per year.

“We believe this material is particularly suitable in resource-scarce environments, such as deserts or the Arctic—even human settlements on other planets,” says Srubar. “The sky is the limit, really, for creative applications of the technology.” 

carbon negative concrete
bendable concrete

Left: Carbon dioxide is sequestered into these Extremely Durable Concrete (EDC) specimens via carbonation in a reactor. Right: Engineered Cementitious Composite (ECC) is a flexible material that’s more durable than traditional concrete and can bend like metal, which also gives it self-healing capabilities. ©University of Michigan Center for Low Carbon Built Environment

Bending concrete
to our will

Can the problem become a solution?

There is one building material that dwarfs all others, both in sheer volume and in carbon emissions. Concrete is the world’s most-consumed material, after water—and just the production of cement accounts for eight percent of the world’s anthropogenic CO2 emissions.

According to Dr. Victor Li, director at the Center for Low Carbon Built Environment at the University of Michigan, scientists and entrepreneurs are exploring two main avenues to creating carbon-negative concrete.

The first approach is to simply reduce the carbon footprint of concrete’s ingredients. Most concrete is made by combining cement with aggregates, which typically comprise geological materials such as gravel and sand. Greener concrete often uses recycled materials, such as post-consumer glass and demolition debris, as aggregate and reduces cement content with alternative binders such as fly ash and slag.

The second method is to sequester carbon directly into concrete. Carbon captured from industrial plants can be transformed from a gaseous state into solid mineral carbonates (CaCO3) to be an aggregate, which gives concrete its strength and durability. Recycled carbon can also be injected into the mix to cure concrete and reduce the amount of cement used.

Although Li is optimistic about turning concrete into a carbon sink, he says low-carbon innovation isn’t enough. “When we talk about decarbonizing concrete, we also need to think about long-term durability, since concrete is used for our roads and reservoirs—infrastructure with long lifespans.

If the carbon-negative concrete requires repairs every year, that’s not sustainable.” If the key is durability, one promising solution is Engineered Cementitious Composite (ECC), a high-performance and bendable concrete pioneered by Dr. Li in the 1990s. Unlike typical concrete, which easily cracks due to low tensile strength, ECC is a ductile material that bends under pressure and has self-healing properties. Dr. Li and other researchers are working on combining this highly durable, bendable concrete with CO2 sequestration technologies to create a greener, carbon-negative concrete that saves on carbon emissions in both the short and long term.

mycellium bricks
mycellium panel

Mycelium panels ©BIOHM

Fungus panels insulate and fireproof buildings while capturing carbon

Mycelium is having a growth spurt

Fungus inside your walls may sound like a homeowner’s worst nightmare, but those ancient organisms could be key to making buildings healthier and more sustainable. British biotech company BIOHM has harnessed the power of mycelium—the filamentous root network of a fungus, remarkable for rapid growth and gluelike properties—to grow biodegradable insulation panels that they say perform just as well as standard insulation made from fossil fuels.

The insulation panels are grown from the startup’s specially cultured mycelium strains that feed on organic and synthetic substrates made from industrial byproducts. Some of BIOHM’s mycelium strains have even been engineered to digest plastics and improve the insulation’s fire resistance.

BIOHM estimates that their carbon-negative manufacturing process can sequester at least 17 tons of carbon per month. The startup plans to begin mass-producing its mycelium insulation panels soon. 

Made of Air
made of air panels

©Made of Air

Carbon-negative
thermoplastics

Biochar is the key ingredient

Charcoal could be an unlikely hero for curbing climate change—as long as it is not used for fuel. Berlin startup Made of Air is using biochar, a charcoal-like material first produced in ancient Amazonia, to permanently sequester atmospheric carbon from plant matter into carbon-negative thermoplastics.

Here’s how it works. Forest and farm waste are converted to biochar with pyrolysis, a process of burning biomass at high temperatures in an oxygen-free chamber. Biodegradable binders are then combined with biochar to create malleable thermoplastic granules. The bioplastics, which the startup says are 85 percent carbon, have been successfully tested in a variety of products, from H&M sunglasses to the building-façade panels for an Audi dealership in Germany. 

Lucy Wang is a freelance writer and design editor @Inhabitat. Her work has appeared in Lawn & Landscape Magazine, Landscape Forms’ Insite, and Dwell.com.

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