Led by scientists at the University of Colorado Boulder, the work could help turn construction waste into building materials with lower environmental impact.
“From termite mounds to adobe buildings, humans and animals have been building with earth since the dawn of time,” said Wil Srubar, professor in the university’s Civil, Environmental and Architectural Engineering department.
“There hasn’t been a lot of science to how earthen builders design the materials, so we wanted to use scientific knowledge and tools to understand it.”
Insects use biopolymers – large biological molecules found in saliva, which act like glue – to bind natural materials such as soil and clay together. Inspired by nature’s designs, Srubar and his team – including researchers at Columbia University in New York – set out to investigate which biopolymer could bind earthen materials and make them 3D-printable.
The researchers tested five biopolymers, including legume-derived guar gum, locust bean gum and cassia gum. These compounds are commonly used in food products such as salad dressings to keep oil and water from separating. They also studied sodium alginate, derived from seaweed, and xanthan gum, produced by fermenting sugar.
The team found that locust bean gum could hold earthen materials tightly together, but that made it harder to push through a 3D printer nozzle.
Sodium alginate, often used in ice cream, produced the opposite effect. Instead of functioning like a glue, the polymer changed the electrical charges on clay particles, causing them to repel one another. The resulting material was a stable mixture that could still flow smoothly through a 3D printer.
The researchers added 0.12% of sodium alginate to earth excavated from a granite quarry near Golden, Colorado. They found the material was both strong and printable, withstanding 25% more pressure than earth without the biopolymer and capable of being printed 33% faster.
The team printed an 8mm-thick wall that leaned outward, even remaining stable when tilted to 60º – far steeper than the Leaning Tower of Pisa.
The framework used in the research could test other biopolymers for enhanced properties such as strength and durability, Srubar said.
“There are some good indoor environmental benefits of having earth in a building,” said Samuel Armistead, a research associate in the Colorado Boulder department. “It can regulate indoor moisture and uptake air pollutants. It can also serve as a thermal insulator, keeping things cool in the summer and warm in the winter.”
Construction projects often generate large amounts of excavated soil when workers dig foundations, basements or parking structures. “Our study suggests that there are ways to reuse waste earth material onsite, and that could largely reduce the environmental footprint of construction,” Armistead said.
The work was published in Nature Communications.
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