“Impact printing” is a cement-free alternative to 3D-printed structures

Recently, construction company ICON announced that it is close to completing the world’s largest 3D-printed neighborhood in Georgetown, Texas. This isn’t the only 3D-printed housing project. Hundreds of 3D-printed homes are under construction in the US and Europe, and more such housing projects are in the pipeline.

There are many factors fueling the growth of 3D printing in the construction industry. It reduces the construction time; a home that could take months to build can be constructed within days or weeks with a 3D printer. Compared to traditional methods, 3D printing also reduces the amount of material that ends up as waste during construction. These advantages lead to reduced labor and material costs, making 3D printing an attractive choice for construction companies.

A team of researchers from the Swiss Federal Institute of Technology (ETH) Zurich, however, claims to have developed a robotic construction method that is even better than 3D printing. They call it impact printing, and instead of typical construction materials, it uses Earth-based materials such as sand, silt, clay, and gravel to make homes. According to the researchers, impact printing is less carbon-intensive and much more sustainable and affordable than 3D printing.

This is because Earth-based materials are abundant, recyclable, available at low costs, and can even be excavated at the construction site. “We developed a robotic tool and a method that could take common material, which is the excavated material on construction sites, and turn it back into usable building products, at low cost and efficiently, with significantly less CO₂ than existing industrialized building methods, including 3D printing,” said Lauren Vasey, one of the researchers and an SNSF Bridge Fellow at ETH Zurich.

How does impact printing work?

Excavated materials can’t be used directly for construction. So before beginning the impact printing process, researchers prepare a mix of Earth-based materials that has a balance of fine and coarse particles, ensuring both ease of use and structural strength. Fine materials like clay act as a binder, helping the particles stick together, while coarser materials like sand or gravel make the mix more stable and strong. This optimized mix is designed such that it can move easily through the robotic system without getting stuck or causing blockages.

Impact printed structures can be produced in a central facility.

Credit: Michael Lyrenmann / Gramazio Kohler Research / ETH Zurich

The next step is to prepare a digital blueprint. Similar to a 3D printer, the robotic impact printing system also requires a digital model to guide the production of a structure. Once this digital blueprint is ready and uploaded to the system, the robotic tool is mounted on a mobile platform for use at a construction site. Then, the mix of Earth-based materials is put into a large volumetric hopper attached to the robotic tool. 

When the hopper is filled, the system begins moving and performing the necessary processes—extruding, cutting, and spraying the material—to build structures as specified in the digital model. This process continues until the structure is completed.

The construction process works quite differently from the typical building-grade 3D printing. There, the materials on their own are too weak to support a structure. Additives like cement are needed to double the yield stresses (the stress the material can endure before it begins to deform permanently) tolerated by the final structure.

The robotic tool used for impact printing deposits the construction material at high velocity (reaching 32 feet/10 meters per second) in a controlled manner. The high-velocity impact that results facilitates strong bonding between layers of Earth-based materials—even before any binding material is added. “Our material already has a higher strength and stiffness (>28 kPa). Therefore, we have a head start on the strength gain of the material, and we are relying less on additives to enhance the material properties,” Vasey said.

The researchers successfully created 6.5-foot-tall (2 meters) walls using this approach. Each of these structures is strong enough to support another structure of similar weight without relying on a chemical additive like cement. “With our system, if you are printing a 2-meter-high structure, you can already start with the material in a state to withstand 2 meters of load,” Vasey added. But it’s not the right material if you need to build much higher. “Our material has about 2 megapascals compressive strength, [which] is lower than typical concrete, but this is completely fine for building walls, and load bearing behavior up to two stories,” she told Ars.

Impact printing is good for our planet

3D printing reduces labor costs for companies and promises to make housing more affordable. But it’s not necessarily sustainable or environmentally friendly. It relies on cement as an additive, a construction material that is alone responsible for nearly 8 percent of global CO₂ emissions. Moreover, due to the use of additives, 3D-printed structures are generally not recyclable.

“3D printing can allow you to save some material because you can place material directly where it’s needed. However, at the same time, usually, you have a large proportion of mortars, additives, and accelerators in the material mix, which all make the CO₂ per volume very high,” Vasey explained.

This isn’t the case with structures constructed using impact printing, as the method doesn’t require additives like cement and uses naturally occurring, less carbon-intensive materials. However, the researchers currently use 1 to 2 percent of a mineral stabilizer, which is less harmful and more recyclable than cement. “But in the future, we don’t want to use any additives or stabilizers at all. Our method could be completely circular, meaning that the parts could be deconstructed and reused in future buildings without going to landfill,” Vasey told Ars Technica.

Vasey and her colleagues now plan to commercialize this technology. They hope to enter the market once they have a prefabrication facility, a factory where parts will be made, ready to be sold and transported to the construction site. “This is because the prefabricated method is close to being technologically ready. We expect that we could incorporate as a start-up in the next year and that a product would be on the market in about three years," Vasey said.

Circular Economy and Sustainability, 2024. DOI: 10.1007/978-3-031-39675-5_9 (About DOIs)

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