An ingredient that gives ice cream a creamier texture could make natural earthen materials like clay and sand easier to 3D-print into durable structures, according to new research led by scientists at the University of Colorado Boulder.
The discovery 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 Department of Civil, Environmental and Architectural Engineering. "But 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."
In nature, termites construct towering mounds. Wasps build intricate nests, and honeycomb worms create reef-like structures along coastlines. Rather than relying on cement, these organisms use biopolymers, which are large biological molecules that act like glue, often found in saliva, to bind natural materials like 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 team tested five biopolymers, including legume-derived guar gum, locust bean gum and cassia gum. These compounds are commonly found in food products like 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 researchers found that locust bean gum could hold earthen materials tightly together by binding soil particles into a stronger network. But that same effect made the material harder to push through a 3D-printer nozzle.
Sodium alginate, often found in ice cream and used to make spherical foods like popping boba, 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, similar to how the same poles of two magnets push each other away.
As a result, adding sodium alginate to clay and sand produced materials that allowed the particles to suspend in a stable mixture while still flowing smoothly through a 3D printer.
Then the team searched for the best formulation. To natural earth excavated from a granite quarry near Golden, Colorado, they added just 0.12% of sodium alginate, which produced a material that was both strong and printable. It could withstand 25% more pressure than earth without the biopolymer and could be printed 33% faster.
Using the formula, the team printed an 8-millimeter-thick (0.3-inch) wall that leaned outward at dramatic angles. They found that the structure could remain stable even when tilted to 60 degrees, far steeper than the Leaning Tower of Pisa.
While the current study focuses primarily on improving the printability of earthen materials, Srubar said scientists could use the same framework to test other biopolymers for enhanced properties such as strength and durability.
"There are some good indoor environmental benefits of having earth in a building," said Samuel Armistead , a research associate in the Department of Civil, Environmental and Architectural Engineering. "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. Much of that material ends up in landfills.
"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.
Because clay and sand are widely available, Srubar said the team's findings could help builders around the world to tap into local resources.
"Clay and sand are among the most abundant building materials on Earth," Srubar said. "The science and engineering we're developing can be applied almost anywhere in the world."
