Just blocks from Duluth's ore docks, where train cars sat full of iron ore pellets, crews in Lincoln Park on Thursday were using waste left behind in the production of pellets like those to fill a pothole.

The project, a partnership between the city of Duluth and University of Minnesota Duluth's Natural Resources Research Institute, takes taconite tailings, the leftover rock that would otherwise sit in tailings ponds, and use them to fill potholes. If the patch holds up, the mixture could be used more and more as an alternative to typical asphalt patches.

Mixing the taconite tailings with a liquid activator, Larry Zanko, a senior minerals researcher with the NRRI, and Sara Post, a research engineer for the NRRI, then spread the mixture into a large crack on Truck Center Drive using a plastic toy canoe paddle and then smoothed it out with a piece of PVC pipe.

For the NRRI, Thursday morning's demonstration was a chance to see their experiment hit the roads.

"It's one thing to do it in the lab ... but we really need to get it out, have traffic, weather get on it and see how it holds up," Zanko said.

For Duluth Mayor Emily Larson, it was the culmination of months of partnership between the NRRI and the City, a project she announced at her State of the City address in March.

"So far, it's feeling really promising," Larson said, standing above the pothole as the mixture cured.

Down the road, several patches poured earlier in the year seem to be holding up well, but NRRI researches had slightly tweaked the "road repair recipe" before Thursday's demonstration.

So far, Greg Guerrero, Duluth's street maintenance manager, said he likes what he sees in the taconite patching technique.

"It does stay in place a little bit longer ... if we can get 2-3 years out of this, we can be well, well ahead of the game," Guerrero said.

Although cars can travel on asphalt patches right after crews roll it onto the road, the taconite mixture needs 10-20 minutes to cure after the application. It's worth the wait, Guerrero said.

But the city won't abandon asphalt patches anytime soon.

"We'll continue to use asphalt," Greg said. "This is going to be one of the things we use here and there."

If the patches survive the winter, then the city and NRRI will turn their attention to faster, more efficient methods of application.

Once the tailings and activator are mixed together, which triggers an exothermic reaction, or a release of heat, it sits in a container for several minutes before it's poured into the pothole and cures or hardens.

In the research phase, that process is OK, but the city needs something faster so crews can quickly repeat the process on pothole after pothole.

"This isn't really a scale that's going to work right now," Larson said. "And we ask our crews and team to do a lot. They are covering miles, they are working really hard. So what's important is that if we're going to bring this to scale, we do need to talk about adaptive equipment that would make that work."