What is Fly Ash?
Fly ash is also known as flue ash, coal ash or pulverised fuel ash (in the United Kingdom) – plurale tantum: coal combustion residuals (CCRs) – it is a coal combustion product made up of fine particles of burned fuel and flue gases that are expelled from coal-fired boilers. Bottom ash is ash that settles at the bottom of a boiler’s combustion chamber which is also known as a firebox. In modern coal-fired power plants, the fly ash is usually captured by electrostatic precipitators or other particle filtration equipment before the flue gases reach the chimneys.
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When it’s combined with bottom ash scraped from the boiler’s bottom, it’s called coal ash. Fly ash contains significant amounts of silicon dioxide (SiO2) (both in amorphous and crystalline form), aluminium oxide (Al2O3) and calcium oxide (CaO), the main mineral compounds in coal-bearing rock strata. The components of fly ash keep on varying depending on the source and composition of the coal being burned.
Some of the minor components of the fly ash include arsenic, beryllium, boron, cadmium, chromium, hexavalent chromium, cobalt, lead, magnesium, mercury, molybdenum, selenium, strontium, thallium and vanadium. Additionally, a very small amount of dioxins and PAH compounds are available depending on the coal bed composition. And it also contains carbon that hasn’t been burned.
When fly (or bottom) ash is not made from coal, such as when solid waste is burnt to generate power at a waste-to-energy facility, the ash may include higher amounts of toxins than coal ash. The ash created in this situation is frequently designated as hazardous waste.
Fly Ash Concrete
Fly ash was once commonly thrown into the atmosphere, but new air pollution control regulations require it to be gathered first and then released using pollution control technology. Fly ash is often stockpiled at coal power plants or disposed of in landfills in the United States. About 43% of it is recycled, and it is frequently used as a pozzolan in the manufacture of hydraulic cement or hydraulic plaster, as well as a replacement or partial replacement for Portland cement in the construction of concrete. Pozzolans aid in the setting of concrete and plaster as well as provide additional protection for concrete against damp circumstances and chemical attacks.
Fly Ash Bricks
Construction bricks made from fly ash can be made in a variety of ways, resulting in a wide range of products. One sort of fly ash brick is made by mixing fly ash with an equal amount of clay and burning it at around 1000 degrees Celsius in a kiln. The main advantage of this method is that it reduces the amount of clay required. A different type of fly ash brick is formed by combining soil, plaster of Paris, fly ash and water by allowing them to cure. This approach decreases air pollution because it does not require the use of heat. A higher proportion of fly ash is used in modern manufacturing techniques, as well as a high-pressure manufacturing technology, which produces high-strength bricks with environmental benefits.
Fly ash has been used to create concrete building blocks in the United Kingdom for over 50 years. They’re commonly used for the cavity walls’ inner skin. They are naturally more thermally insulating than conventional aggregate-based blocks.
Since 1970, ash bricks have been used in the construction of homes in Windhoek, Namibia. The bricks, on the other hand, have a tendency to break or cause unattractive pop-outs. This occurs when moisture comes into touch with the bricks, triggering a chemical reaction that causes the bricks to expand.
Fly ash bricks are used in construction in India. Leading producers employ a compression technique and an industrial standard known as "Pulverized fuel ash for lime–Pozzolana mixture", which uses over 75% post-industrial recycled waste. This results in a sturdy product with excellent insulation and environmental benefits. Different fly ash bricks sizes are available in the market depending on the requirement and the use.
Fly ash bricks price ranges from Rs. 3 to Rs. 20 depending on the size and quality of the brick.
Fly Ash Cement Relation
Many cement-based products, such as poured concrete, concrete blocks and brick, employ fly ash as a primary ingredient. One of the most common uses of fly ash is in the form of portland cement concrete pavement or PCC pavement. Road-building projects utilising PCC can require a lot of concrete, thus switching fly ash can save you a lot of money. Fly ash has also been utilised as an embankment and mine fill, and the Federal Highway Administration is increasingly accepting of it.
The substitution rate for fly ash for portland cement is commonly set as 1–11/2 pounds of fly ash for 1 pound of cement. To accommodate the higher volume of the fly ash, the amount of fine aggregate in the concrete mix must be lowered.
Disposal and Market Sources
Fly ash from coal burning was previously simply engulfed in flue gases and dispersed into the atmosphere. Fly ash emissions were limited to less than 1% of total ash output in strongly industrialised countries, causing environmental and public health issues. More than 65% of fly ash produced by coal power plants is disposed of in landfills and ash ponds around the world.
Toxic substances can leak into subsurface water aquifers if ash is kept or placed outside. As a result, most of the current discussion over fly ash disposal centres on the construction of specially sealed landfills that prevent toxic compounds from leaching into groundwater and local ecosystems.
Because coal was the most common energy source in the United States for decades, power companies generally built coal facilities near major cities. To add to the environmental concerns, coal plants require a significant amount of water to operate their boilers, causing coal plants (and later their fly ash storage basins) to be located near urban areas, as well as rivers and lakes that are frequently used as drinking water sources by nearby cities. Many of the fly ash basins were not lined, posing a significant risk of spillage and floods from neighbouring rivers and lakes. For example, Duke Energy in North Carolina has been involved in a number of high-profile cases over coal ash storage and spills into the water basin.
Did You Know?
What is the purpose of fly ash in concrete? Fly ash increases the workability of flexible concrete as well as the strength and durability of hardened concrete when used in concrete. The cost of using fly ash is similarly low. The amount of portland cement used in concrete can be lowered by using fly ash.
FAQs on Fly Ash
1. What exactly is fly ash and how is it produced?
Fly ash is a very fine powder that is a byproduct of burning pulverised coal in thermal power plants. As the coal is combusted, the mineral impurities (like clay, quartz, and feldspar) melt in the high-temperature environment. These molten droplets cool and solidify rapidly in the exhaust gases, forming fine, spherical, glassy particles. These lightweight particles are then captured by electrostatic precipitators or baghouses, preventing them from escaping into the atmosphere.
2. What is the typical chemical composition of fly ash?
Fly ash does not have a single chemical formula, as its composition varies depending on the source of the coal being burned. However, it primarily consists of silicon dioxide (SiO₂), aluminium oxide (Al₂O₃), and iron oxide (Fe₂O₃). It also contains smaller amounts of calcium oxide (CaO), magnesium oxide (MgO), and sulphur trioxide (SO₃). These components, particularly the silica and alumina, give fly ash its valuable pozzolanic properties.
3. What are the most important examples of fly ash usage?
Fly ash has a wide range of applications, primarily in the construction industry, turning an industrial waste product into a valuable resource. Some key uses include:
Cement and Concrete: It is widely used as a partial replacement for Portland cement in concrete production, which enhances strength and durability.
Brick Manufacturing: Fly ash bricks are a sustainable alternative to traditional clay bricks, as they are lighter and require less energy to produce.
Road Construction: It is used as a stabilising agent for road embankments and as a filler material in asphalt.
Land Fills: It can be used as a structural fill material, helping to reclaim land in an environmentally safe manner.
4. How do fly ash bricks differ from traditional red clay bricks?
Fly ash bricks and traditional red bricks differ significantly in composition, properties, and environmental impact. Fly ash bricks are made from industrial waste (fly ash, sand, and cement), are lighter in weight, and have a uniform shape. They offer better thermal insulation and lower water absorption. In contrast, red bricks are made from topsoil (clay), require intensive firing in kilns which consumes significant energy, and contribute to soil erosion. Fly ash bricks are generally considered a more sustainable and cost-effective alternative.
5. Why is fly ash considered an environmentally significant material?
Fly ash is environmentally significant for two main reasons. Firstly, its utilisation helps manage a major industrial waste product, preventing it from ending up in landfills or ash ponds where it could contaminate soil and groundwater. Secondly, its use in cement and brick production significantly reduces the carbon footprint of the construction industry. By replacing a portion of cement, it lowers the CO₂ emissions associated with cement manufacturing, which is a highly energy-intensive process. This practice of waste valorisation is a key principle of a circular economy.
6. What are the two main types of fly ash?
Based on their chemical composition, fly ash is primarily classified into two types as per American Society for Testing and Materials (ASTM) standards:
Class F Fly Ash: Produced from burning anthracite or bituminous coal, it has a low calcium content and possesses pozzolanic properties. It requires an external activator, like cement or lime, to form a cementitious compound.
Class C Fly Ash: Produced from burning lignite or sub-bituminous coal, it has a higher calcium content and possesses both pozzolanic and self-cementing properties. This means it can harden on its own when exposed to water.
7. How does fly ash actually improve the strength and durability of concrete?
Fly ash improves concrete through a chemical process known as the pozzolanic reaction. When cement hydrates, it produces calcium hydroxide as a byproduct, which does not contribute to strength and can be a source of weakness. The silica in fly ash reacts with this free calcium hydroxide to form additional calcium silicate hydrate (C-S-H), the same compound that gives concrete its strength. This secondary reaction fills pores, creating a denser, less permeable, and more durable concrete that is more resistant to chemical attacks and cracking over time.
8. What are the major challenges or limitations associated with using fly ash?
Despite its benefits, using fly ash has some limitations. A major challenge is its variable quality, which depends heavily on the coal source and combustion process, requiring stringent testing. Another issue is that adding fly ash can slow down the setting time of concrete, which can be problematic in cold weather conditions, delaying construction schedules. Additionally, the presence of unburnt carbon in some fly ash can interfere with air-entraining admixtures, affecting the concrete's resistance to freeze-thaw cycles.
9. What environmental concerns are linked to the disposal of unused fly ash?
When not properly utilised, the disposal of fly ash poses significant environmental risks. It is typically stored in large open-air pits called ash ponds or dry landfills. A major concern is the leaching of heavy metals like arsenic, mercury, lead, and cadmium into groundwater, contaminating drinking water sources. Another risk is air pollution from fine ash particles becoming airborne, which can cause respiratory problems. Catastrophic failures of ash pond dams can also lead to widespread contamination of rivers and surrounding land.
