This article was published in Scientific American’s former blog network and reflects the views of the author, not necessarily those of Scientific American
In discussions on climate change, much of the conversation focuses on CO2 emissions from the nation’s coal power plants. However, a recent report from the Environmental Integrity Group on how coal ash is contaminating our water supplies provides a needed reminder that our love of coal is killing us softly not just through air pollution but also water pollution. Yet there may be an easy solution to this right under our feet in the concrete we walk on every day.
Coal ash is the ash that remains after coal is burned in power plants, and it is one of the largest forms of industrial waste generated in the United States. In 2018, American coal plants produced approximately 111 million tons of coal ash, equivalent to the mass of about 8.5 million filled garbage trucks. And while more than 64 percent of coal ash is recycled, vast quantities—approximately 40 million tons—are discarded annually into landfills and ash ponds across the country, many of which lack geomembrane liners that can prevent toxic chemicals and wastes from seeping into bodies of water.
But coal ash is not all homogenous, and some of what’s discarded actually contains fly ash, a byproduct of combustion that turns out to be a valuable resource for making concrete. When fly ash is mixed into fresh concrete, the fresh concrete “activates” the ash, which forms a binder that adds strength. Furthermore, the heavy metals and chemicals it contains become immobilized, no longer able to leach out of the concrete and pollute our water sources. And since cement is the most expensive ingredient in concrete, replacing some of it with fly ash, which costs only a fourth as much, saves money.
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Perhaps surprisingly, fly ash also makes concrete greener, since cement manufacturing is responsible for about 5 percent of all human-made CO2 emissions. These facts, coupled with significant improvements in concrete durability are why many engineers design concrete mixtures to replace at least 25 percent of the mass of cement in concrete with fly ash.
Yet in recent years, the quantity and quality of fly ash in many states across the U.S. has declined. This is primarily due to our aging power plants being old and inefficient. Additionally, the rise of natural gas; emission regulations; and seasonal variations in energy use also play a role. As a result, the concrete sector has been working to identify materials that can fill this void in fly ash supply.
One possibility is reclaiming the fly ash from landfills and ponds, something my colleagues and I at the University of Texas at Austin have been studying. Our research shows that these reclaimed waste products can be used in concrete. To be sure, not all the reclaimed coal ash is of high enough quality to be suitable for use in concrete. But the work we have been doing is promising, as are some of the industrial projects that have already been implemented with reclaimed ash.
What is needed now is additional research on how these ashes will interact with some of the other additives used in concrete, understanding the long-term properties of concrete containing these ashes, and improving reclaimed coal ash screening and remediation methods.
A little over five years ago, the coal ash spill that happened in 2014 on the Dan River, in North Carolina, destroyed homes, rivers and lives. But large spills like these should not be our only concern. The daily leakage of chemicals from coal ash landfills and ponds has the potential to contaminate drinking water supplies across the nation. Finding a way to properly remove some of the coal ash from ash ponds and landfills is complex, as it requires changes in how utility companies manages coal waste and also support from industry and governmental agencies to accept these ashes in concrete.
Fortunately, concrete seems to be a good answer to solving part of this difficult problem.