‘Waste rock’ could become high-value aggregate resource for local roads

Truck hauling waste rock

It’s easy for drivers to appreciate a well-made road. But it might surprise you to learn that we’re running low on the main ingredient in road building. Gravel pits are being depleted of good-quality natural gravel, especially in northern Minnesota.

Researchers at the University of Minnesota Duluth’s Natural Resources Research Institute (NRRI) have identified a waste rock that is almost a perfect replacement for gravel. That has Tom Nemanich, owner of Mesabi Bituminous, pretty excited. The depleting natural gravel resources had him a bit concerned.

“It’s basically getting to where we’ll have to start trucking it in from somewhere and that’s expensive,” he says. “You always want to use what’s close by.”

Back in 2007, NRRI geologists identified a potential high-quality aggregate between the magnetite ore layers targeted for mining. For the taconite industry, this would be waste rock that would need to be moved out of the way to access the desired ore. As an aggregate, it’s a marketable resource. (Their findings were the result of a major research effort led by the NRRI’s Donald Fosnacht and Lawrence Zanko that explored the development and marketing of Iron Range aggregate materials for Midwest and national transportation applications.)

This year in late May, Mesabi Bituminous Inc. began hauling out the rock they’re calling Mesabi Select, from a pit owned by Optimal Mining, Inc. The Mesabi Select will be processed and used in road projects all over the Iron Range.

Chunk of aggregate rock
Photos: Natural Resources Research Institute, UMD

Swapping out taconite waste rock for gravel isn’t straightforward. Nemanich explained that quality testing for bituminous (bitumen is the liquid binder that holds together asphalt) is based on the specific gravity of the aggregate material. Specific gravity defines how many cubic yards of asphalt can be made per ton of aggregate. Typical taconite waste rock’s specific gravity can be all over the map, which makes it difficult to use. Natural gravel is pretty consistent. The waste rock that Nemanich is hauling out—the Mesabi Select—has a consistent, lower specific gravity that makes a quality asphalt.

“Identifying this aggregate is a huge benefit that could open up to something much bigger,” Nemanich says. “It’s helped our business significantly.”

Tom Gardner, an agent for Optimal Mining, credits NRRI for identifying the high-quality aggregate—and his daughter Rachel and her master’s degree focus on sustainability—for educating him in “by-product synergy.” She pointed her father to the work NRRI was doing to identify uses for taconite by-product.

“We’re grateful for the opportunity to allow Mesabi Bituminous to haul out the aggregate and put the rock to good use,” Gardner says. “It’s a great example of by-product synergy and supports mining sustainability efforts. Truly a win-win-win.”

Zanko, now a senior research program manager, and other NRRI researchers have also been exploring the use of taconite mixtures for pothole and pavement repair. Several projects have been funded by MnDOT and the Minnesota Local Road Research Board in recent years, and work is continuing to further refine mixing methods and applications (see CTS Catalyst, May 2020).

(By June Breneman, adapted with permission from NRRI Now e-news, July 2020.)