NRRI's taconite-tailings pothole patch survives winter
Amid one of the worst pothole seasons for Duluth streets, researchers say an experimental patch mix using taconite tailings held up over the winter. Standing at the corner of Truck Center Drive and Chestnut Street Thursday morning, Larry Zanko, a senior minerals researcher with the University of Minnesota Duluth's Natural Resources Research Institute, kicked and poked at several of the test patches set last summer and fall.
"Overall, I'm satisfied," Zanko said. "But I know that we can do better."
The pothole patch uses taconite tailings, the waste rock leftover from the iron ore production process that would otherwise sit in tailings ponds. Although aggregate makes up about 90 percent of the tailings, 10 percent is still concentrate, which contains magnetite.
Just before the mixture is applied to the road, phosphoric acid is added, which reacts with the magnetite. That heats the mixture up before it is poured into the pothole where it cures and hardens.
When it sets up in about 15 to 20 minutes, the patch is ready for traffic.
"It's like an iron cement," Zanko said.
Unlike the common blacktop pothole patch, a temporary fix, researchers and city officials hope the taconite patches won't need to be replaced as often.
Sara Post, a senior research engineer for the NRRI, believes the reaction creates an iron-phosphate cement — but researchers aren't certain.
"We know that there's a reaction, and that it makes this cement between the aggregate grades ... we just don't necessarily understand what we're making," Post said.
To do that, Zanko and Post will bring back samples of the patch to the NRRI for study under a microscope.
"So now we're to the point where we have it fairly well nailed down, but there are things that we still need to understand about why did that one perform better," Zanko said.
Researchers are now hoping to bring this up to scale so road crews can spread it faster. Currently, the patches can only be applied in small test batches, nowhere close to the pace crews can shovel blacktop into potholes from the back of a hopper.
Since it behaves like cement, Zanko said they might try something like a concrete mixing truck.
"You could have more of a constant repair operation going on," Zanko said.
Tailings have been used as aggregate in road surfaces for years and the NRRI sources its tailings from the western Iron Range, ensuring the materials don't contain the asbestos-like fibers at the center of the 1970s Reserve Mining case.
While the NRRI has long-experimented with finding value-added uses for taconite tailings, the project sped up a year ago when Duluth Mayor Emily Larson announced a partnership between the NRRI and the city to combat the city's plague of potholes.
"This is a really challenging town to do roads in," Post said. "Anything that holds up is a plus."
So far, researchers like what they see.
"It's always good to see the things last through the wintertime with the plowing and the ice and the awful weather we've been having," Post said.