Coal combustion products
Based on Wikipedia: Coal combustion products
Every year, coal-fired power plants around the world produce hundreds of millions of tons of ash. Not the romantic, campfire kind—this is industrial residue on a geological scale, enough to bury entire cities. And here's what makes it fascinating: this waste product, born from burning ancient plant matter compressed for millions of years, has quietly become one of the most recycled industrial materials on Earth.
The concrete in your driveway? Probably contains coal ash. The drywall in your walls? Could be made with it. That highway you drove to work on? Almost certainly built with it. We've essentially found a way to transmute pollution into infrastructure.
The Four Horsemen of Coal Waste
When coal burns in a power plant, it doesn't simply vanish into energy. About ten percent of what goes in comes out as solid residue, and that residue takes four distinct forms, each with its own chemistry, uses, and problems.
Fly ash is the lightweight champion, comprising sixty percent of all coal combustion waste. Picture microscopic glass spheres, most smaller than a human hair, that form when minerals in burning coal vaporize and then rapidly cool as they rise with the exhaust gases. They solidify so quickly that they don't have time to form crystals—instead, they freeze into smooth, spherical beads of amorphous glass. Power plants capture these particles using electrostatic precipitators (devices that give particles an electric charge and then attract them to collection plates) or fabric filter bags before they can escape into the atmosphere.
For most of the twentieth century, nobody bothered to catch this stuff. It simply drifted out of smokestacks, settling on nearby communities like toxic snow. The particles were small enough to penetrate deep into human lungs. Environmental regulations eventually forced power plants to capture fly ash, which created a new problem: what to do with mountains of fine gray powder.
Flue-gas desulfurization materials account for about twenty-four percent of coal waste. These are the byproducts of "scrubbers"—chemical systems that remove sulfur dioxide from power plant exhaust before it can cause acid rain. The most common scrubber type sprays a limestone slurry into the exhaust stream. The calcium in limestone reacts with sulfur to produce calcium sulfate, better known as gypsum. This synthetic gypsum is chemically identical to the natural mineral that's been used in construction for thousands of years. Today, nearly a third of gypsum board panels manufactured in the United States come from this power plant byproduct.
Bottom ash is heavier and coarser than fly ash. Instead of floating up with exhaust gases, it falls to the bottom of the combustion chamber. Imagine sand-sized to gravel-sized particles of fused mineral matter. It accounts for about twelve percent of coal waste.
Boiler slag makes up the remaining four percent. This forms in a specific type of boiler where ash melts into a liquid and drains through an opening at the bottom, where it's quenched in water. The rapid cooling produces hard, angular, glassy particles with a distinctive dark color. It's prized for sandblasting and for making roofing shingles because of its hardness and resistance to weathering.
The Chemistry of Ancient Swamps
Coal is fossilized vegetation—mostly trees, ferns, and other plants from swamps that existed between 300 and 400 million years ago, during the Carboniferous and Permian periods. When these plants died and sank into oxygen-poor water, they didn't decompose fully. Instead, they were buried under sediment, compressed, heated, and chemically transformed over hundreds of millions of years.
The plants themselves were mostly carbon, hydrogen, and oxygen. But they grew in soil, absorbed minerals from groundwater, and accumulated trace elements from their environment. When coal burns, the carbon combines with oxygen to release energy and carbon dioxide. But the mineral components don't burn—they melt, fuse, and transform into ash.
The composition of fly ash reflects this geological history. Silicon dioxide (the same compound as quartz and the main ingredient in glass), aluminum oxide (the same compound as corundum and sapphire), and calcium oxide (quickite) dominate. But buried in this matrix are trace amounts of some genuinely concerning elements: arsenic, lead, mercury, cadmium, chromium (including the carcinogenic hexavalent form made famous by the film Erin Brockovich), and many others.
The exact mix depends on where the coal came from. Coal from Wyoming's Powder River Basin has different trace elements than coal from West Virginia's Appalachian deposits because the ancient swamps where they formed had different geologies. This variability creates both challenges and opportunities for recycling.
The Two Classes
Engineers classify fly ash into two categories that matter enormously for how it can be used.
Class F fly ash comes from burning older, harder coal—anthracite and bituminous types that formed under greater pressure and heat. It contains less than seven percent calcium oxide. This ash is what chemists call a "pozzolan," a term that traces back to the Italian town of Pozzuoli, near Naples, where Romans quarried volcanic ash for their concrete.
Pozzolanic materials don't harden on their own when mixed with water. They need an activator—something alkaline to get the chemical reaction started. In Roman times, that activator was lime. Today, it's usually Portland cement. Once activated, the glassy silica and alumina in Class F fly ash react slowly to form calcium-silicate-hydrate, the same binding compound that gives regular concrete its strength. The result is often stronger and more durable than concrete made with cement alone.
Class C fly ash comes from younger, softer coal—lignite and sub-bituminous types. It contains more than twenty percent calcium oxide, which means it has an important superpower: it's self-cementing. Mix it with water and it will harden on its own, no activator required. This makes it easier to use but also means it must be handled more carefully to prevent premature setting.
The distinction matters because Class C ash can sometimes replace cement entirely, while Class F ash typically supplements cement rather than replacing it. Both offer environmental benefits by reducing the need to manufacture Portland cement, a process responsible for roughly eight percent of global carbon dioxide emissions.
From Pollution to Building Material
The Romans figured out nearly two thousand years ago that volcanic ash made exceptional concrete. The Pantheon in Rome, built in the second century, still stands with its original unreinforced concrete dome—the largest such dome in the world for over a millennium. Roman aqueducts and harbors have survived earthquakes, floods, and centuries of neglect. Modern engineers studying these structures discovered that the volcanic ash in Roman concrete continues reacting with seawater even today, slowly filling cracks and growing stronger over time.
Fly ash works the same way. When mixed into concrete, it produces a material that's more workable when wet, stronger when cured, more resistant to chemical attack, and less permeable to water. The tiny spherical particles act like ball bearings, making fresh concrete flow more easily. As the concrete cures, the pozzolanic reaction continues for months or even years, gradually increasing strength.
This isn't a minor industrial niche. Fly ash concrete is everywhere. The foundations of skyscrapers, the decks of bridges, the barriers of highways, the floors of warehouses—all commonly contain fly ash. A typical mix might replace thirty percent of Portland cement with fly ash, though some specialized applications go higher.
The environmental calculus is compelling. Manufacturing Portland cement requires heating limestone to about 1450 degrees Celsius, driving off carbon dioxide in a chemical reaction and burning massive amounts of fuel. Every ton of cement produces nearly a ton of carbon dioxide. Substituting fly ash avoids this emission while also diverting waste from landfills.
Beyond Concrete
The applications extend far beyond what you might expect. Fly ash appears in bowling balls and countertops, in floor tiles and boat hulls, in PVC pipes and artificial reefs. Companies have developed fly ash bricks that require ninety percent less energy to manufacture than traditional clay bricks. The ash serves as filler in paints, plastics, and wood products.
Synthetic gypsum from scrubbers has found an even more direct path to usefulness. Gypsum board—the drywall that forms the interior walls of most buildings in North America—is simply gypsum sandwiched between paper. It doesn't matter whether that gypsum was mined from the earth or precipitated from a smokestack scrubber; chemically, it's identical. Using synthetic gypsum reduces mining and its associated environmental impacts while solving a waste disposal problem.
The coarser materials—bottom ash and boiler slag—have their own roles. Bottom ash substitutes for gravel in road construction and as fill material. Boiler slag's hardness makes it ideal for blasting grit, used to clean metal surfaces and remove rust. Its dark color and durability have made it a standard material for roofing shingles, where it forms the protective granular coating on the surface.
The Darker Side
All of this beneficial use obscures a troubling reality: more than half of coal combustion waste still ends up in disposal sites, and those sites have a disturbing history of failures.
Coal power plants need enormous quantities of water, which led utilities to build them near rivers and lakes—often the same water sources that supply nearby cities. When the plants produced ash, the easiest disposal method was to mix it with water and pump the slurry into ponds, euphemistically called "impoundments." Many of these ponds were built decades ago with minimal engineering and no liner to prevent leaching into groundwater.
The catastrophic failures make headlines. In 2008, a dike at the Tennessee Valley Authority's Kingston Fossil Plant collapsed, releasing 1.1 billion gallons of coal ash slurry. The wave destroyed homes, contaminated rivers, and created a cleanup challenge that took years and cost over a billion dollars. Workers involved in the cleanup have suffered elevated rates of illness and death, though the exact connection remains legally contested.
In 2014, a storm drain beneath a Duke Energy facility in North Carolina collapsed, releasing up to 39,000 tons of coal ash and 27 million gallons of contaminated water into the Dan River. The spill extended seventy miles downstream and prompted nationwide attention to the hazards of coal ash storage.
Even without dramatic failures, ash ponds pose chronic risks. Heavy metals leach slowly into groundwater over decades. Communities near disposal sites have documented elevated levels of arsenic and other contaminants in their wells. The problem is particularly acute because many ash ponds sit in floodplains, where rising waters can overtop containments or infiltrate from below.
Regulation and Resistance
For decades, coal ash occupied a regulatory gray zone. Was it hazardous waste, subject to stringent handling requirements? Or was it merely an industrial byproduct, handled like any other fill material? The answer had enormous economic implications—hazardous waste designation would have dramatically increased disposal costs.
The coal industry argued, with some justification, that classifying all coal ash as hazardous would stigmatize beneficial uses. Who would want concrete containing hazardous waste? The recycling market would collapse, and more ash would end up in landfills, not less.
Environmentalists countered that the hazardous components don't disappear just because the ash is mixed into concrete or spread on farm fields. The arsenic and lead and mercury are still there, potentially leaching out over time.
The United States Environmental Protection Agency spent years studying the question before finalizing rules in 2015 that treat coal ash as non-hazardous waste but require groundwater monitoring, structural integrity assessments, and eventual closure of unlined ponds. The rules have been challenged and revised multiple times since, with the regulatory future still uncertain.
Other countries have taken different approaches. The European Union classifies some coal ash as hazardous based on its specific composition. Australia has developed extensive guidelines for beneficial use while requiring careful testing. India, with a rapidly growing power sector and limited disposal space, has mandated increasing percentages of ash utilization.
The Scale of the Problem
To understand why this matters, consider the numbers. In 2017, coal-fired power plants in the United States alone produced over 38 million tons of fly ash. About 24 million tons were recycled; the rest went to disposal. Globally, coal ash production approaches a billion tons per year.
This is the debris of the industrial age, the mineral remnant of burning ancient sunlight captured by long-dead forests. It accumulates in ponds and piles, in landfills and abandoned quarries. Even as coal power declines in the United States and Europe, it continues growing in Asia, where coal remains a dominant energy source.
The beneficial uses represent genuine environmental improvements. Every ton of fly ash that replaces Portland cement avoids roughly a ton of carbon dioxide emissions. Every ton used as aggregate avoids mining virgin materials. The closed-loop elegance appeals: waste becomes resource, pollution becomes infrastructure.
But the system works only if the beneficial uses can absorb the waste stream. When recycling markets falter—as they did during construction slowdowns in the 2008 recession—ash accumulates. When power plants close, they leave behind decades of stored ash that someone must manage. The legacy sites, some dating to the early twentieth century, will require monitoring and maintenance essentially forever.
Connections Across Time
There's something poetic about the fact that we're using coal ash to make concrete in essentially the same way that Romans used volcanic ash two millennia ago. Both materials are pozzolans—glassy silicates that react with lime to form durable binders. Both are byproducts of geological violence, whether volcanic eruption or the slower violence of burial and compression followed by combustion.
The connection to rare earth elements, mentioned in the article that prompted this essay, runs deeper than you might expect. Coal ash contains elevated concentrations of rare earth elements—the fifteen lanthanides plus scandium and yttrium—accumulated from the soils where ancient plants grew. Researchers have explored extracting these elements from ash as an alternative to conventional mining, potentially transforming a waste disposal problem into a domestic source of strategic materials.
The economics haven't worked yet. Rare earths in coal ash are dispersed at concentrations of a few hundred parts per million, far lower than in ore deposits. Extracting them requires dissolving massive quantities of ash in acid, then separating and purifying the individual elements through complex chemical processes. But as demand for rare earths grows and ore grades decline, the calculation may shift.
The Future of Coal's Past
Coal power is in decline across much of the developed world, squeezed by cheap natural gas, expanding renewables, and climate policy. The United States retired more coal capacity in the 2010s than it built in any previous decade. Europe is accelerating closures. Even China, despite continuing to build new plants, has begun grappling with overcapacity.
This decline doesn't eliminate the ash problem; in some ways, it makes it worse. Operating plants have revenue to fund ash management. Retired plants become liabilities, their ash ponds orphaned when owners go bankrupt or simply walk away. The cleanup costs for legacy sites may ultimately fall to taxpayers.
The recycling markets face their own challenges. Concrete producers have become accustomed to steady supplies of fly ash from nearby power plants. As those plants close, the ash must be transported from more distant sources, increasing costs and carbon footprint. Some producers are turning to other pozzolans—ground granulated blast furnace slag from steel production, calcined clays, even volcanic ash—to replace diminishing fly ash supplies.
What remains is a material paradox: a waste product that's often more valuable than the coal that produced it, a pollution source that can reduce other pollution, a legacy of the carbon age that might help build whatever comes next. The ash from ancient forests, burned to power the present, now forms the literal foundation of our infrastructure. Whether that's redemption or just very slow irony depends on how well we manage what's left.