In 2009, Illinois used 38,881 tons of fly ash in highway construction; IDOT reported spending $1.75 million while reducing landfill use and cement demand

Every time you drive over a stretch of the Illinois highway system, there's a good chance you're driving over waste from a coal plant. Yes, that sounds odd, but it's true; that's documented. According to the Illinois Department of Transportation's 2011 Physical Research Report No. 158, Illinois used 38,881 tons of fly ash in road construction in 2009 alone, at a cost of about $1.75 million to the department. That's a state agency finding a way to use a waste byproduct of a power plant instead of sending it to a landfill or mining new material to make cement.

What fly ash actually is, and why it ends up in your commute

Fly ash is not glamorous. It's the by-product of burning coal in coal-fired power plants. This finely divided powder is captured in smokestacks to prevent it from entering the atmosphere. According to IDOT, fly ash is divided into two classes based on the coal source it comes from: Class F fly ash is produced by burning anthracite or bituminous coal, while Class C fly ash comes from lignite or subbituminous coal. Class C fly ash is self-cementing, while Class F fly ash needs to react with lime to create a pozzolanic reaction. This means that when we add fly ash to concrete, it reduces the amount of Portland cement we need to use in the process of making concrete. Portland cement production is energy intensive, which is why reducing the amount needed is an economically attractive option for concrete producers.


What that 1.75 million dollars actually bought

At first glance, spending 1.75 million dollars on a power plant byproduct seems an odd line item. However, the IDOT report gives context for why it did so. The agency explains that using fly ash as a supplementary cementitious material in 2009 helped reduce the amount of landfill space needed and cut back on greenhouse gas emissions associated with producing new cement. In short, that spending was not an act of charity towards a coal plant. IDOT was using a material that could, in part, do the job of cement and cut back on its use of a resource that is intensive to produce.

The argument fits in with what federal regulators have said for years. The EPA's 2014 evaluation of encapsulated coal ash reuse found that using fly ash in concrete and flue gas desulfurization gypsum in wallboard is appropriate because the materials' environmental releases are comparable to, or below, the health and environmental benchmarks the agency applies to virgin materials. The EPA also found that, between the two uses, they accounted for nearly half of the coal ash beneficially reused around the country, rather than being disposed of.

A supply problem nobody saw coming

Here is where the plot thickens for those interested in current events and clean energy. According to the same IDOT report, Illinois power plants have been modifying their equipment to remove mercury from flue gas, altering the chemical composition of the ash. This rendered part of Illinois' own fly ash production unusable for construction purposes, forcing IDOT to acquire large amounts of fly ash from other states to meet demand, a shift the report notes was already underway by 2009. The Federal Highway Administration's Technical Brief on Supplementary Cementitious Materials has since documented the problem nationwide: the shift to natural gas reduced fly ash supply, while growing environmental protection increased demand.

What this means if you care about infrastructure and climate

The Illinois fly ash numbers are notable both for the cost efficiency and for the waste they kept out of landfills. Here's a state agency that took an industrial byproduct that was headed for a landfill, reduced demand for cement, and put a state dollar figure on the tradeoff in the process. That said, it's worth noting the "diversion" story only tells part of the picture: by the same year IDOT was reporting these numbers, mercury-scrubbing upgrades at Illinois plants were already pushing the state to import a share of its usable fly ash from elsewhere. As the supply squeeze continues, fly ash that was coming from coal plants and going into highway construction will become a more limited, and less local, resource.

As the United States begins to shut down power plants and limit emissions, states will need to find a replacement for the fly ash that has been going into concrete. In the interim, the 2009 numbers from Illinois look like a good case study of what concrete suppliers can do to reduce costs and increase the use of industrial byproducts, while also providing the sort of tax savings that can help a state in lean economic times. The numbers are also instructive, since they point to the material-replacement problems that will arise as the U.S. begins to rely more on cleaner energy sources and less on coal.