When most people look at piles of mining waste, they see rubble. For Maddie Farrer and Chenming He, two researchers at Harvard’s Graduate School of Design (GSD), those rocks look like building blocks for the future. Inside the Autodesk Technology Center in Boston, the duo is using 3D printing to “weave discarded stone into new kinds of architecture.”

The project is called Geo-Stitch, and it started as part of a class on the circular economy and carbon reuse. Farrer, who grew up in Kentucky, remembers seeing strip mines running across Appalachia, where mountaintops were blown apart for coal, leaving behind entire valleys filled with unwanted waste rock. The sandstone was beautiful, but it was “treated as a nuisance,” covering ecosystems, polluting streams, and constantly pushed aside by restoration crews.

“Growing up, I always saw that waste,” she said. “Beautiful sandstone that just gets pushed aside. I wondered if it could be reused for something structural.”

Harvard GSD researchers Chenming He and Maddie Farrer inside Autodesk’s Boston Technology Center, standing beside the gantry-style 3D printer used to prototype their Geo-Stitch project. Image courtesy of 3DPrint.com.

From Waste to Walls

That question turned into her thesis: what if the rocks stayed in their natural, irregular form, and 3D printing was used to negotiate the gaps between them or “stitch it together”? The idea was to replace industrial bricks with boulders and use a custom mortar, extruded by a printer, to fill the negative space. Some of the sandstone breaks down easily into sand and lime (the perfect ingredients for mortar), while tougher boulders provide the structure, explains Farrer.

Farrer and He are developing the system within the Grinham Research Group at GSD, led by Associate Professor Jonathan Grinham. The group explores sustainable building methods through materials science, and the Geo-Stitch project in particular asks whether irregular stone waste can be upcycled into structural walls when paired with 3D printed mortar.

“Robots do what they’re good at, and that is lifting heavy pieces and following precise toolpaths,” Farrer explained. “Humans contribute the architectural agency, bringing the design intelligence to figure out how the pieces come together.”

At Autodesk, the team finally had the chance to test their concept with a large-format, gantry-style concrete 3D printer by Build Additive, installed at the Technology Center in Boston and shared with resident researchers.

Encouraged by Grinham, who told them simply to “try it,” they began with local materials. The team scanned nearby rocks, generated custom toolpaths, and extruded cement-based mortar around foam stand-ins for stone. The material set quickly — drying in less than 24 hours — but it took about a week to fully cure and reach strength. These first prints were a way to test how mortar could flow around irregular shapes, even “negotiating overhangs and non-parallel layers,” something traditional stone masonry could never offer. Robots handled the heavy lifting and precise line work, while the design decisions — from cavities for wiring and plumbing to insulation pockets — came from the human side. Eventually, the plan is to move from these early foam-and-cement tests to a custom mortar made from Kentucky’s sandstone waste.

“We’re really lucky to be here,” He said. “Through the residency program, we gained access to the 3D concrete printer, which we didn’t have before. Up until now, this was just a design exercise on paper, rendered models, nothing more. Experimenting with their tools is what made it real.”

Designing with Surplus

What makes Geo-Stitch unusual is its attitude toward material use: “In sustainable architecture, the goal is to minimize materials. But here, the starting point is abundance. This is an architecture of surplus,” Farrer said. “We actually have too much stone, and the question is how to reuse it in a creative, structural way.”

Like Farrer mentioned, most of that surplus rock comes from mountaintop removal mining, a common practice in Appalachia. The leftover stone, known as overburden, is dumped into nearby valleys. Between 1985 and 2015 alone, that practice cleared about 720,000 acres of land (roughly 3.5% of Central Appalachia) and buried over 1,200 miles of streams. This huge, unused stone sits idle, a resource waiting for creative reuse.

Valley fills and waste-rock deposits created by mining operations in Appalachia. Image courtesy of Wikipedia.

The prototypes are still early, but the researchers have a long-term vision in mind. One dream is to eventually bring Kentucky’s displaced rocks back into new structures, returning waste to the same landscapes it once damaged.

Working with irregular stone isn’t easy. “The printer can’t rely on standard slicing software, so we had to customize everything,” He said. “Toolpaths, layer heights, extrusion speeds, all of it had to be rethought.”

Cement itself also added complexity. “It’s a messier, more hands-on process than printing plastic,” Farrer noted. “You’re constantly thinking about curing time, consistency, and extrusion speed. So once you start, you can’t stop.”

Still, the early tests were promising. “It came out exactly how we imagined,” He added.

Harvard GSD researchers Chenming He and Maddie Farrer testing their Geo-Stitch prototypes at Autodesk’s Boston Technology Center. Image courtesy of 3DPrint.com.

When I met Farrer at the Technology Center this summer, she had just graduated from Harvard GSD and was preparing to move into architectural practice. She told me she hoped to keep exploring Geo-Stitch beyond school, while her teammate, He, continues the research as part of his doctoral studies. For both, the Autodesk Research Residency Program was a key moment, the place where an academic idea became a printed prototype.

“We’re just at the beginning,” He concluded. “But seeing a wall form from waste rock and mortar, and imagining it as part of a real building someday, that’s an incredible feeling. Here we are finding ways to turn discarded stone into the backbone of new construction.”

This article is part of the “Boston’s Additive Edge at Autodesk” series, highlighting projects and research taking shape inside Autodesk’s Technology Center in Boston.