Fly ash is a substance that is created during the process of burning coal in power plants. It is a fine, gray powder that is made up of spherical, glassy particles that are carried by the flue gases. Since fly ash contains pozzolanic materials, it can combine with lime to create materials that have cement-like properties.
One of the most common uses of fly ash is in the production of concrete. By mixing fly ash with other materials, such as cement, sand, and water, a strong and durable concrete can be produced. Fly ash can also be used in other construction projects, such as mines, landfills, and dams.
Because fly ash is a by-product of coal combustion, it is considered to be a waste material. However, its use in construction projects can help to reduce the amount of waste that is produced and can also provide a cost-effective alternative to traditional building materials. Furthermore, the use of fly ash in construction can help to reduce the environmental impact of coal-fired power plants.
Chemical Composition of Fly Ash
Fly ash’s chemical makeup is influenced by two key factors: the type of coal utilized and the techniques employed for coal combustion. These two elements play a significant role in determining the chemical composition of fly ash. The type of coal utilized can influence the amount of minerals present in the ash, as different types of coal contain varying levels of minerals. Additionally, the methods used for coal combustion can also impact the chemical composition of the resulting fly ash. Different techniques may produce different chemical reactions that can alter the ash’s composition. Therefore, it’s essential to consider these factors when examining the chemical composition of fly ash.
Table No 1: Chemical composition of fly ash of different coals.
| Component | Bituminous Coal | Sub bituminous Coal | Lignite Coal |
| SiO2 (%) | 20-60 | 40-60 | 15-45 |
| Al2O3 (%) | 5-35 | 20-30 | 20-25 |
| Fe2O3 (%) | 10-40 | 4-10 | 4-15 |
| CaO (%) | 1-12 | 5-30 | 15-40 |
| LOI (%) | 0-15 | 0-3 | 0-5 |
Physical Properties of Fly Ash
The physical properties of fly ash are,
1. Fineness of Fly Ash
According to ASTM standards, the fineness of fly ash must be determined through both dry and wet sieving. The process involves taking a sample of the fly ash and passing it through a 45 micron sieve. The percentage of material retained on the sieve is then calculated.
Additionally, the fineness of the fly ash can also be measured using the LeChatelier method and the Blaine Specific Surface method. These methods are used to further evaluate the particle size distribution of the fly ash sample.
The LeChatelier method involves measuring the increase in length of a cement paste bar that has been exposed to the fly ash. The increase in length is proportional to the amount of fine material in the fly ash sample.
The Blaine Specific Surface method, on the other hand, determines the surface area of the fly ash particles by measuring the amount of air permeability through a compacted bed of the material. The resulting measurement is an indicator of the fly ash’s overall fineness and reactivity.
Overall, these various methods provide a comprehensive evaluation of the fineness of fly ash, which is an important characteristic that can affect its suitability for use in various applications, including construction and cement manufacturing.
2. Specific Gravity of Fly Ash
Fly ash, a byproduct of coal combustion, has a specific gravity that varies depending on the type of coal being burned. The specific gravity can range from as low as 1.90 for sub-bituminous ash to as high as 2.96 for iron-rich bituminous ash. The specific gravity is a measure of the density of the fly ash, and it is expressed as the ratio of the density of the fly ash to the density of water. Therefore, the specific gravity of fly ash is an important characteristic to consider when it comes to its use in various applications.
It should be noted that sub-bituminous ash, which has a lower specific gravity, is typically less dense and lighter than iron-rich bituminous ash, which has a higher specific gravity. This means that sub-bituminous ash may be easier to transport and handle, but it may not have the same properties as iron-rich bituminous ash. On the other hand, iron-rich bituminous ash may have greater potential for use in certain applications due to its higher density and unique properties.
Overall, understanding the specific gravity of fly ash is important when it comes to selecting and using the appropriate type of fly ash for a particular application. Different types of fly ash may have different specific gravities, which can affect their performance and behavior in various settings. Therefore, it is essential to consider the specific gravity of fly ash when making decisions about its use.
3. Size and Shape of Fly Ash
Fly ash is a finely grained material that typically falls within the particle size range of 10 to 100 microns. Its shape is often characterized as spherical and glassy in appearance.
4. Colour
Fly ash is a by-product of coal combustion that is widely used in the construction industry. Its color is determined by the chemical and mineral components present within it. The amount of lime in the fly ash contributes to its light colors, such as tan, while the presence of iron imparts a brownish hue. The presence of un-burned material is usually associated with a dark grey to black color.
Coal combustion is a primary source of fly ash, which is a useful material in construction. The color of fly ash is determined by its chemical and mineral makeup. Light colors such as tan are associated with higher levels of lime, while the presence of iron imparts a brownish hue. Darker shades of grey or black are typically linked to a higher un-burned content.
The color of fly ash is an important consideration for its use in construction. Its various hues are determined by the presence of different chemicals and minerals, with lime contributing to lighter shades and iron resulting in brown tones. Higher levels of un-burned material can lead to a darker grey or black color. By understanding the color of fly ash, its properties can be better utilized in construction applications.
Classification of Fly Ash
Fly ash, a byproduct of coal combustion, is classified differently depending on the codes used. The classification of fly ash is determined by various factors such as its chemical composition, physical properties, and the type of coal burned. The different codes used to classify fly ash include the American Society for Testing and Materials (ASTM), the Indian Standard (IS), and the European Standard (EN). Each code has its own classification system, which may be based on different parameters. Despite the differences in classification, fly ash is generally categorized into two main types: Class F and Class C. Class F fly ash is produced from burning anthracite or bituminous coal and is typically characterized by its low calcium content. On the other hand, Class C fly ash is produced from burning sub-bituminous coal and contains a higher percentage of calcium.
1. Type of Fly Ash as per IS Codes (IS 3812-1981)
A. Grade I
The grade of Fly ash in question is obtained from the combustion of bituminous coal. This type of coal has a composition where the combined weight percentage of its fractions SiO2 (silicon dioxide), Al2O3 (aluminum oxide), and Fe2O3 (iron oxide) is greater than 70%. The Fly ash that results from the combustion process of this particular type of coal is classified under this grade, which distinguishes it from Fly ash derived from other types of coal with different compositions.
B. Grade II
The type of fly ash being referred to is a grade that is derived from lignite coal. This particular grade has a composition where the combined fractions of silicon dioxide (SiO2), aluminum oxide (Al2O3), and iron oxide (Fe2O3) are greater than 50%.
2. Type of Fly Ash as per American Society for Testing and Materials (ASTM C618)
Fly ash is a byproduct of burning coal in power plants, and it can be classified in different ways depending on the type of coal used and its chemical composition. The American Society for Testing and Materials (ASTM) has established a classification system for fly ash based on its chemical and physical properties.
According to ASTM, there are two main classes of fly ash: Class F and Class C. Class F fly ash is produced by burning anthracite or bituminous coal, and it contains a high amount of silica, alumina, and iron oxide. It has a low calcium content and is therefore not suitable for use in concrete mixtures that require high early strength.
On the other hand, Class C fly ash is produced by burning subbituminous or lignite coal, and it contains a higher percentage of calcium, which makes it more reactive than Class F fly ash. It has a lower silica, alumina, and iron oxide content, and it can be used in concrete mixtures that require high early strength.
In addition to these two classes, ASTM also defines two subclasses of fly ash based on the amount of calcium oxide present. Class F fly ash can be further divided into subclasses based on the percentage of calcium oxide, with subclasses designated as Low, Intermediate, and High Calcium Fly Ash. Similarly, Class C fly ash can be divided into subclasses based on its calcium oxide content, with subclasses designated as Class C-H and Class C-L.
Overall, ASTM’s classification system helps to differentiate between the various types of fly ash and allows for their proper use in different applications.
A. Type C
Type C fly ash is a specific kind of fly ash that is generated through the burning of lignite or sub bituminous coals. This type of fly ash is unique in that it contains a high concentration of CaO, with levels exceeding 10 percent. What sets Type C fly ash apart from other types of fly ash is that it possesses both cementitious and pozzolanic properties. The cementitious properties of this ash make it particularly useful in construction applications, as it can be used to create strong and durable concrete. Meanwhile, the pozzolanic properties of Type C fly ash make it an effective additive for enhancing the performance of concrete, particularly in terms of its workability and durability.
B. Type F
3. Type of Fly Ash based on boiler operations
A. Low temperature(LT) fly ash
It is produced when the combustion temperature is below 900o C
B. High temperature(HT) fly ash
It is generated out of combustion temperature below 1000o C
Mechanism of Fly Ash
Portland cement is a mixture containing about 50% of tri-calcium silicate, which is the primary mineral component. Upon hydration, it generates calcium silicate hydrate and calcium hydroxide. However, the calcium hydroxide does not contribute to the strength of the concrete. Therefore, when fly ash is used as a pozzolana, it is treated as silica because non-crystalline silica glass is the primary constituent of fly ash. The reactive silica present in the fly ash reacts with the calcium hydroxide released during the hydration process of Portland cement. As a result, calcium silicate hydrate is generated, which acts as a binder, fills up the empty spaces, and enhances impermeability and strength. This process occurs slowly but progressively, resulting in more robust concrete.
Table No 2: Hydration reaction of Portland cement and fly ash Portland cement
| Hydration Process | Tricalcium Silicate | + | Water | = | Calcium Silicate Hydrate | + | Calcium Hydroxide |
| Portland cement | C3S | + | H | = | C-S-H | + | CH |
| Portland Cement + Fly Ash | S (Silica+flyash) | + | CH | = | C-S-H |
Comparison of Requirements of Fly Ash in ASTM, EN and IS
Table No 3: Comparison of Requirements of Fly Ash in ASTM, EN and IS
| Properties | ASTM C-618 | En-450 | En-197-I | En-3892-I | IS 3812 2003- I |
| Sio2 minimum | 35 | ||||
| Reactive/soluble Sio2, min. | 25 | 25 | 20 | ||
| Sio2+Al2O3+Fi2O3 minimum | 70 | 70 | |||
| MgO, Maximum | 70 | ||||
| LOI(1hour)max | 6 | 5-7 | 5-7 | 7 | 5 |
| Total alkalis, max. | 1.5 | 1.5 | |||
| SO3, maximum | 5 | 3 | 2 | 3 | |
| Free CaO, maximum | 1 | 1 | |||
| Total/reactive CaO, maximum | 10 | 10 | 10 | ||
| Fineness, 45 micron, maximum | 34 | 40 | 12 | 34 | |
| Blaines fineness m2 /kg min. | 320 | ||||
| Cement activity 28 days | 75 | 75 | 80 | 80 | |
| Lime reactivity, N/mm2 | 4.5 | ||||
| Soundness, Le-Chatelier, mm | 10 | 10 | 10 | 10 | |
| Autoclave, Percent | 0.8 | 0.8 |
Uses of Fly Ash
Fly ash, a by-product of coal combustion, finds various applications in different industries. One of the primary uses of fly ash is in the manufacturing of Portland cement. This use of fly ash helps to reduce the environmental impact of cement production while also enhancing its durability.
Another significant application of fly ash is in embankment construction. It is typically used as a low-cost material to fill in voids and stabilize the soil. It can also be used as a soil stabilizer to prevent soil erosion.
Fly ash is also used as a component in the production of flowable fill. This is a mixture of fly ash, water, and cement that is used to fill in large voids in the ground, making it ideal for creating a smooth surface for construction projects.
In addition to these uses, fly ash is used as a filler mineral in asphalt road laying to fill the voids, thus improving the quality and longevity of the road. Fly ash is also utilized as a component in geopolymer technology that involves the use of inorganic, amorphous alumino-silicate materials.
Fly ash is also used in the construction of roller compacted concrete dams. This application requires high-quality fly ash, which can enhance the strength and durability of the concrete.
Furthermore, fly ash is used in the manufacture of fly ash bricks. The bricks made from fly ash are lightweight and have excellent insulating properties, making them suitable for construction purposes.
Lastly, when treated with silicon hydroxide, fly ash can act as a catalyst. This treatment converts the fly ash into a useful material that can help improve the efficiency of various chemical reactions.