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PennDOT's considers using fly ash to make cement

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Tuesday, May 26, 2009
 

To some, fly ash is an environmental nuisance bordering on a hazard, a metal-laden byproduct of burning coal that authorities stash away in empty mines, gravel pits and ponds.

But to PennDOT Assistant Construction Manager Michael McCart, it's a way to make roads, sidewalks and bridges stronger and longer-lasting, thanks to growing attempts to reuse it and other industrial wastes in making concrete.

Concrete used for roads and buildings is a combination of stone, sand and binding agents — usually a mix of lime and silica called portland cement — that create chemical reactions and bind together when mixed with water.

Because fly ash from power plants and ground-up slag left over from the steelmaking process can replace some of that cement and make the final product stronger, PennDOT is considering making their use mandatory.

"I look at quality, and I know it's going to give us a quality product," McCart said. "The department is moving forward with requiring this instead of making it an option, and that's also part of our 'going green.'"

PennDOT is working on specifications for the strength, durability and other properties it expects from the concrete used in its projects, and the newest standards are strict enough that suppliers can't meet them without using fly ash, slag or other additives, he said.

During the past five years, the number of concrete suppliers in the area using fly ash and slag in mixes has increased from two plants to 10.

Up to 15 percent of portland cement in PennDOT's typical mix can be replaced with fly ash, or up to 50 percent can be replaced with slag, McCart said. Most of the products are brought in by rail to Leetsdale from coal plants and steel mills as far away as southern Illinois. PennDOT uses between 3,000 and 5,000 tons of each per year.

When mixed with cement, the kind of fly ash produced by burning anthracite and bituminous coal found across Pennsylvania keeps reacting with water to strengthen the concrete nearly a month after it is poured and set, said Julia Vandenbossche, an assistant professor of civil engineering at the University of Pittsburgh.

Blast furnace slag can have some of the same properties, but only if it is cooled with water after being taken from the furnace in a molten-hot state. Air-cooled slag doesn't create the same chemical reactions, but it can be used as "aggregate" filler, she said.

McCart said slag and fly ash reduces the concrete's susceptibility to being weakened and damaged by water and salt.

Even the portland cement industry is embracing the use of the materials that replace some of their product.

"It's part of the concrete culture now," said Steven Kosmatka, vice president of research and technical services at the Skokie, Ill.-based Portland Cement Association, which represents cement producers across the country. Some cement producers have taken to adding fly ash and slag themselves, instead of leaving it to concrete suppliers.

But as other states such as Minnesota switch to using more fly ash and less cement, Kosmatka said there were trade-offs.

"The typical dosage is about 15 to 25 percent in place of cementing materials, but some places like Minneapolis use 40 percent or more, and there are LEED (green-certified) buildings that use almost all fly ash and slag and very little portland cement," Kosmatka said. "That scares the heck out of some concrete producers and contractors."

About 70 million tons of fly ash are produced each year in the United States, and about 15 million tons of it end up in concrete products, Kosmatka said. Some is used in other applications, such as filling abandoned mine shafts to reduce their likelihood of collapse.

The rest is stored in ponds or dumped in landfills, where the metals contained in the ash can leach out and contaminate groundwater if not properly contained. On Dec. 22, an earthen dam broke at a 40-acre fly ash storage facility 35 miles outside of Knoxville, spilling ash and sludge into a neighborhood. There are 10 such storage facilities in Pennsylvania, including three in Armstrong, Westmoreland and Beaver counties.

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