Coal combustion products (CCPs), also known as coal combustion wastes (CCWs) or coal combustion residuals (CCRs), are materials created when coal is burned. These materials are divided into four groups. Each group is based on the different physical and chemical forms that result from the ways coal is burned and the controls used to manage emissions during the process.

Fly ash

Fly ash, flue ash, coal ash, or pulverized fuel ash (in the UK)—also called coal combustion residuals (CCRs)—is a product made from tiny particles that come out of coal-fired boilers along with flue gases. Ash that falls to the bottom of the boiler's combustion chamber, often called a firebox, is called bottom ash. In modern coal-fired power plants, fly ash is usually collected by electrostatic precipitators or other particle filters before the flue gases go up the chimneys. Together with bottom ash, it is known as coal ash.

The composition of fly ash depends on the type of coal burned, but it always contains large amounts of silicon dioxide (SiO₂), aluminum oxide (Al₂O₃), and calcium oxide (CaO), which are the main minerals in coal-bearing rock layers.

Using fly ash as a lightweight aggregate (LWA) helps recycle one of the largest waste streams in the US, offering economic and environmental benefits.

The smaller parts of fly ash vary based on the coal's composition but may include trace amounts of elements like gallium, arsenic, beryllium, boron, cadmium, chromium, cobalt, lead, manganese, mercury, molybdenum, selenium, strontium, thallium, and vanadium. It may also contain tiny amounts of dioxins, PAH compounds, and other carbon-based substances.

In the past, fly ash was often released into the air, but modern pollution control rules require it to be captured before release using equipment. In the US, fly ash is usually stored at power plants or landfills. About 43% is recycled, often used as a pozzolan to make hydraulic cement or plaster and as a substitute for Portland cement in concrete. Pozzolans help concrete and plaster harden and protect them from water and chemical damage.

If fly ash or bottom ash is not from coal—such as when waste is burned in a waste-to-energy facility—it may contain more harmful substances and is often classified as hazardous waste.

Fly ash solidifies while floating in exhaust gases and is collected by electrostatic precipitators or filter bags. Because it cools quickly, fly ash particles are usually round and range in size from 0.5 μm to 300 μm. Rapid cooling means few minerals form crystals, and most remain as amorphous glass. Some minerals stay crystalline. This makes fly ash a mix of different materials.

The main chemical parts of fly ash are silicon dioxide (SiO₂), aluminum oxide (Al₂O₃), iron oxide (Fe₂O₃), and sometimes calcium oxide (CaO). Fly ash has a wide variety of minerals, including glass, quartz, mullite, and iron oxides like hematite, magnetite, and maghemite. Other minerals found include cristobalite, anhydrite, free lime, periclase, calcite, sylvite, halite, portlandite, rutile, and anatase. In calcium-rich fly ash, minerals like anorthite, gehlenite, akermanite, and calcium silicates or aluminates similar to those in Portland cement may be present. Mercury levels can reach 1 ppm, but are usually between 0.01–1 ppm for bituminous coal. Trace element levels vary based on the coal burned.

The American Society for Testing and Materials (ASTM) C618 divides fly ash into two classes: Class F and Class C. The main difference is the amount of calcium, silica, alumina, and iron in the ash. The chemical properties of fly ash depend on the coal burned, such as anthracite, bituminous, or lignite.

Not all fly ash meets ASTM C618 standards, but this may not be required for certain uses. Fly ash used as a cement replacement must meet strict construction standards, though no specific environmental rules exist in the US. At least 75% of fly ash must be 45 μm or smaller in size, with less than 4% carbon content (measured by loss on ignition, or LOI). In the US, LOI must be under 6%. Fly ash particle size can change due to coal mill and boiler performance, so it may need processing like mechanical air classification to be used effectively in concrete. If used as a sand replacement in concrete, fly ash with higher LOI can be used. Quality control, such as Bureau of Indian Standards or Dubai DCL marks, is important for ensuring quality.

In the past, fly ash was released into the air, causing environmental and health problems. Laws in industrialized countries like the US now limit fly ash emissions to less than 1% of total ash produced. Globally, over 65% of fly ash from coal plants is stored in landfills or ash ponds.

Ash stored outdoors can release toxic substances into groundwater. This has led to discussions about creating lined landfills to prevent chemicals from seeping into groundwater and ecosystems.

Because coal was a major energy source in the US for many years, power plants were often located near cities. These plants also needed large amounts of water, so they were placed near rivers and lakes used for drinking water. Many fly ash storage basins were unlined and at risk of spilling or flooding. For example, Duke Energy in North Carolina has faced lawsuits over coal ash storage and spills into water basins.

Recycling fly ash has become more important due to rising landfill costs and interest in sustainability. In 2024, US coal plants produced 24.7 million short tons of fly ash, with 78% reused. Recycling reduces the need for new materials and provides an affordable substitute for Portland cement.

Reuse

In 2019, about 52% of coal combustion products (CCPs) in the United States were recycled for "beneficial uses," according to the American Coal Ash Association. In Australia, about 47% of coal ash was recycled in 2020. Recycling helps reduce the harmful effects of CCPs by stabilizing dangerous substances such as arsenic, beryllium, boron, cadmium, chromium, chromium VI, cobalt, lead, manganese, mercury, molybdenum, selenium, strontium, thallium, and vanadium, along with dioxins and polycyclic aromatic hydrocarbons.

In the United States, there is no government system for tracking or labeling how fly ash is used in different industries, such as manufacturing, construction, or agriculture. The American Coal Ash Association publishes incomplete data about fly ash use each year.

Coal ash is used in many ways, including:
– Building materials like concrete, bricks, and insulation.
– Products such as cosmetics, toothpaste, kitchen countertops, floor and ceiling tiles, bowling balls, flotation devices, stucco, utensils, tool handles, picture frames, auto bodies, boat hulls, cellular concrete, geopolymers, roof tiles, roofing granules, decking, fireplace mantles, cinder blocks, PVC pipes, structural insulated panels, house siding, running tracks, blasting grit, recycled plastic lumber, utility poles, railway sleepers, highway noise barriers, marine pilings, doors, window frames, scaffolding, sign posts, crypts, columns, railroad ties, vinyl flooring, paving stones, shower stalls, garage doors, park benches, landscape timbers, planters, pallet blocks, molding, mailboxes, artificial reefs, binding agents, paints, undercoatings, metal castings, and filler in wood and plastic products.

Fly ash is used in concrete because it has special properties that help make concrete stronger and more durable. This use was recognized as early as 1914, but serious studies began in 1937. Ancient Roman structures, like aqueducts and the Pantheon in Rome, used volcanic ash (which has similar properties to fly ash) in their concrete. This helped these structures last for many centuries.

Fly ash can replace up to 30% of the Portland cement in concrete, though higher amounts may be used in some cases. It can improve the strength and chemical resistance of concrete. Fly ash also makes concrete easier to work with, reducing the need for water. Some projects now use up to 50% fly ash in concrete. In India, a dam project used 70% fly ash in its concrete.

Because fly ash particles are round, they help concrete mix more easily and reduce the amount of water needed. Using fly ash instead of cement can lower the amount of carbon dioxide produced, as cement manufacturing releases a lot of CO₂. However, burning coal to make fly ash also produces CO₂. If fly ash replaces a large portion of cement worldwide, it could reduce carbon emissions from construction, assuming fly ash production is already accounted for.

Fly ash has unique properties compared to other materials. Unlike typical soil, fly ash has a uniform particle size and contains clay-like particles. When used in embankments (raised earth structures), factors like particle size, how easily it compacts, strength, and resistance to water or freezing are important. Most fly ash used in embankments is Class F.

Soil stabilization involves changing soil to improve its strength and ability to support structures. It can help reduce soil shrinkage or expansion, making it better for roads and buildings. Stabilization can use materials like lime, fly ash, or cement. Proper planning and testing are essential to choose the right materials and achieve the desired results. Benefits of stabilization include stronger soil, less water absorption, thinner road layers, and reduced construction costs. A related process, soil modification, focuses on reducing soil moisture to speed up construction, but it does not improve strength as much as stabilization. Equipment used for these processes includes spreaders, mixers, storage containers, water trucks, and compactors.

Fly ash is also used in flowable fill, a material used to fill spaces in construction. Flowable fill can range in strength from 50 to 1,200 pounds per square inch, depending on the project. It often includes cement, fly ash, and water. High fly ash mixes use mostly fly ash with a small amount of cement, while low fly ash mixes use more filler materials. Class F fly ash is best for high fly ash mixes, and Class C fly ash is used in low fly ash mixes.

Asphalt concrete, a material used for road surfaces, can include fly ash as a filler. It helps fill gaps between larger materials in the mix. Fly ash must meet specific standards, such as those in ASTM D242, to be used in asphalt pavement. Fly ash is hydrophobic, meaning it repels water, which helps in construction.

Environmental impacts

Most coal combustion products (CCPs) are placed in landfills, stored in mine shafts, or kept in ash ponds at coal-fired power plants. These materials can enter the environment through air or water.

Fly ash contains small amounts of heavy metals and other substances that can be harmful to humans and ecosystems if present in large amounts. Heavy metals found in coal include arsenic, beryllium, cadmium, barium, chromium, copper, lead, mercury, molybdenum, nickel, radium, selenium, thorium, uranium, vanadium, and zinc. The impact of fly ash on the environment depends on the power plant where it is produced and the ratio of fly ash to bottom ash in the waste. This is because the chemical composition of coal varies based on the region where the coal is mined and the burning process in the power plant.

Fly ash particles are very small (less than 2.5 micrometers) and can easily become airborne. These particles can travel long distances, affecting both local and global populations. When inhaled, they can enter the lungs and move deep into the respiratory system, potentially causing heart and lung problems or lung cancer. If these particles enter the lymphatic system, they can move into the bloodstream. Airborne particles can also fall into water, leading to water contamination. Crystalline silica and lime, along with toxic chemicals in fly ash, pose risks to human health and the environment. Crystalline silica, which is found in fly ash, is known to cause lung disease, such as silicosis, when inhaled. The International Agency for Research on Cancer (IARC) and the U.S. National Toxicology Program classify crystalline silica as a known human carcinogen.

Fly ash may contain high levels of heavy metals, increasing the risk of pollution. The most common disposal methods are landfills or settling ponds. Fly ash can pollute surface water through erosion, runoff, airborne particles landing on water, contaminated groundwater moving into surface waters, flooding, or leaks from coal ash ponds. In water where aquatic life exists, these metals can be absorbed by organisms through ingestion or skin and gill contact. Once absorbed, the metals can enter the bloodstream and accumulate in organs like the liver or kidneys. High levels of heavy metals can cause neurological issues, organ damage, or death in aquatic organisms. These metals can also build up in the food chain, eventually affecting humans through the food supply, leading to illness or cancer. Groundwater, which is a source of drinking water, can become contaminated with heavy metals, harming human health.

When coal burns, it creates an alkaline dust with a pH between 8 and 12. This dust can settle on soil, raising the pH and affecting nearby plants and animals. Soil contaminated with fly ash often has higher bulk density and water-holding capacity but lower hydraulic conductivity and cohesiveness. The effects of fly ash on soil and soil microorganisms depend on the pH of the ash and the concentration of trace metals. Contaminated soils may reduce microbial respiration and nitrification. These soils can either harm or benefit plant growth. Fly ash can improve soil by correcting nutrient deficiencies, but high levels of boron can harm plants.

Fly ash can be used as a soil additive. When added in small amounts, it can increase plant growth. However, higher amounts of fly ash may reduce plant biomass, seedling height, and root and leaf length. Studies show that high levels of fly ash can lower the efficiency of photosynthesis in plants.

When stored in large quantities, fly ash is often kept wet to reduce dust. These storage areas, called ash ponds, are usually large and stable for long periods. However, if the dam or wall of an ash pond breaks, the release of materials can be sudden and widespread.

Al, As, Bo, Ca, Mn, Se, N, SO₄²⁻

Human health impacts

The National Academy of Sciences stated in 2007 that "high levels of harmful substances in many CCR (coal combustion residue) leachates may cause health and environmental problems."

Lead is a dangerous heavy metal found in fly ash, a material left after burning coal. Lead can move easily through the environment. If it enters water or soil, it can harm marine life and reduce the variety of living organisms. Exposure to lead can damage blood circulation, the brain, kidneys, and heart. Even small amounts of lead can interfere with the body’s ability to make a substance needed for blood and nerves, which affects brain function. Children are especially at risk from lead exposure.

Mercury can also be found in CCPs (coal combustion products). It is released during coal burning as elemental mercury, mercury attached to particles, or mercury in an oxidized form. Elemental mercury is very reactive. If it enters water, bacteria can change it into a more dangerous form called methylmercury. Methylmercury moves through the food chain and builds up in the bodies of animals. This is dangerous because mercury harms the nervous system. Exposure to methylmercury is especially concerning for pregnant individuals.

Arsenic can exist in many chemical forms, which means it can react in different ways. The effects of arsenic on health depend on its form. If people swallow arsenic directly, it can cause sudden poisoning with symptoms like vomiting or nausea, or more severe effects like mental changes. Long-term exposure may lead to heart failure or blood disorders such as a decrease in white blood cells.

Material Safety Data Sheets suggest several safety steps when handling or working with fly ash. These include wearing protective goggles, face masks, and disposable clothing. Workers should avoid stirring up fly ash to reduce the amount that becomes airborne.

Regulations

In 1980, the U.S. Congress labeled coal ash as a "special waste" that would not be controlled under the strict rules for hazardous waste found in Subtitle C of the Resource Conservation and Recovery Act (RCRA). Congress told the Environmental Protection Agency (EPA) to study this issue and decide if stricter rules were needed. In 2000, EPA said coal fly ash did not need to be treated as hazardous waste. Because of this, most power plants did not have to install special plastic sheets or systems to collect leaked water in ash ponds. After several pollution incidents, EPA created a Coal Combustion Residuals (CCR) rule in 2015. The agency still called coal ash non-hazardous (avoiding strict Subtitle C rules), but added new rules to prevent pond failures and protect groundwater. These included better inspections, record-keeping, and monitoring. Rules for closing ponds were also added, such as covering ponds, using liners, and removing water. Some parts of the 2015 rule were challenged in court, and the court told EPA to revise certain parts of the rule.

In response, EPA released its "CCR Part A" rule on August 28, 2020, requiring all unlined ash ponds to be fixed with liners or closed by April 11, 2021. Some plants could request extra time—up to 2028—to find alternatives for managing ash before closing their ponds. EPA also released its "CCR Part B" rule on November 12, 2020, allowing certain plants to use alternative liners if they proved these would not harm people or the environment. By December 14, 2020, some plants applied for these alternatives. In 2023, EPA denied six of these applications. In January 2025, EPA denied a request from the Coronado Generating Station in Arizona, but in February 2026, it said it would consider approving the request after new information was submitted. As of early 2026, EPA had not finalized decisions on other applications.

In 2024, EPA issued a final rule for "legacy" or inactive ash ponds. On November 28, 2025, EPA proposed extending the deadline for some active ponds from 2028 to 2031.

In 2020, EPA released a rule under the Clean Water Act that changed some parts of its 2015 rule on wastewater from ash ponds and power plants. This rule was also challenged in court. In 2024, EPA updated the rule again, making stricter limits on wastewater for some facilities, including water from flue gas desulfurization, bottom ash transport, and coal ash leachate. In December 2025, EPA extended some deadlines in the 2024 rule.

In China, the largest producer of fly ash, the government manages fly ash through the National Development and Reform Commission and the Ministry of Environmental Protection. Fly ash is treated as solid waste and can be stored in approved facilities if not reused. However, some facilities are too close to homes, breaking storage rules.

The European Union created the European Waste Catalogue (EWC) to classify waste by type and how it is made. By 2014, member countries had to include the EWC in their laws. Under the EWC, fly ash is also classified as non-hazardous.

In India, the Ministry of Environment, Forest and Climate Change first set rules in 1999 requiring all thermal power plants to use 100% of fly ash by a certain date. Later rules in 2003 and 2009 moved the deadline to 2014. By 2015, only 60% of fly ash was being used. A new rule in 2015 set December 31, 2017, as the final deadline for full use. As of 2015, about 55.7% of fly ash was used, mostly in cement (42.3%) and less than 1% in concrete.

Researchers in India are working to increase fly ash use in concrete and new types of cement, such as geopolymer, to meet the 100% use goal. India produced 280 million tonnes of cement in 2016, with the housing sector using 67% of it. There is potential to use more fly ash in cement and concrete. Some people believe Indian rules limit fly ash use to less than 35%, but examples like high-volume fly ash concrete in large projects show this is not always true. To use fly ash more effectively, India needs to develop ultra-high-volume fly ash concrete using local materials.

In the geologic record

During the Permian–Triassic extinction event about 252 million years ago, burning of coal deposits in the Siberian Traps released large amounts of char that resemble modern fly ash into the oceans. This char is found in rock layers from the Canadian High Arctic and is preserved in the geologic record. Some scientists think the fly ash may have caused harmful conditions in the environment.