COAL COUNTRY - Fly ash from a North Dakota power plant is making the new I-35W bridge in Minneapolis, Minn., stronger than its own design standard.

The fly ash - a byproduct of incinerated lignite - was produced at Great River Energy's Coal Creek Station and about 10,000 tons of it were blended with Portland cement in making the bridge concrete.

The I-35W bridge reopened last week, after collapsing about a year ago, killing 13 and injuring another 145.

Al Christianson develops byproduct sales for the Coal Creek Station near Underwood. Christianson said the sale required 400 truckloads to the bridge site and was the single largest one-source sale the plant has ever made.

I-35 reconstruction was on a fast track and design safety was a watchword of the project.

Kevin Macdonald, an engineer for Cemstone, said his company was in charge of the concrete mix used throughout the bridge and he turned to Coal Creek's fly ash for several reasons.

Macdonald said fly ash makes better, stronger concrete because it decreases the amount of lime, which is the weakest component of concrete because it can dissolve with water.

He said parts of the concrete bridge have a 10,000 psi rating, which is 40 percent higher than the design required.

"There's no concrete on that bridge that doesn't have fly ash in it," Macdonald said.

He said fly ash creates more than normal heat in the mix, which is the limiting factor in how much of it can be blended with Portland cement for mass elements.

Another bonus is that fly ash releases zero carbon dioxide, while a cement concrete releases a small amount for every ton, he said.

The cost is right, too.

Christianson said fly ash costs $60 a ton delivered, about half as much as Portland cement.

The amount used in the I-35W bridge is about 10 days' output from Coal Creek. It's fairly widely used, including in the new Memorial Bridge replacement in Bismarck-Mandan, the Fort Mandan visitor center and the Bismarck State College National Energy Center of Excellence.

(Reach reporter Lauren Donovan at 888-303-5511 or lauren@westriv.com.)