Fly Ash, Production, Properties, and Environmental Benefits

Fly Ash

Fly ash, also known as pulverized fuel ash, is produced from burning pulverized coal in electric power generating plants. Mineral impurities included in coal, such as clay, feldspar, quartz, and shale, fuse in suspension during combustion and float out of the combustion chamber with the exhaust gases. Fly ash, which are spherical glassy particles, is formed as the fused material cools and solidifies as it rises. It is a fine-grained, powdery particle that is removed from exhaust gases using bag filters or electrostatic precipitators.

Depending on the collecting method used mechanical, electrical, or bag houses and fabric filters between 85 and 99 percent of the ash from the flue gases is recovered as fly ash. 75–85% of the total coal ash is collected as fly ash, while the remaining portion is bottom ash or boiler slag. Fly ash produced from thermal power plants is a variable material because of several factors. Due to a number of variables, fly ash generated by thermal power plants is a changeable material.

These factors are
(1) type and mineralogical composition of the coal;
(2) degree of coal pulverization;
(3) type of furnace and oxidation conditions;
(4) the manner in which fly ash is collected, handled, and stored before use.

Fly ash from different power plants is likely to differ since no two utilities or facilities may have all these elements in common. Due to load conditions during a 24-hour period, fly ash characteristics can also change within the same plant.

The size of the fly ash particles is mostly influenced by the dust collection system used. Generally speaking, it is finer than Portland cement. Fly ash particles range in diameter from less than $1 \:to \:150\: \mu m$ . The kinds and proportions of incombustible material present in the coal utilized to determine the chemical makeup of fly ash. The components of fly ash differ significantly depending on the origin and make-up of the coal being burned, but they all contain sizeable amounts of silicon dioxide (SiO2), both amorphous and crystalline, aluminum oxide (Al2O3), and calcium oxide (CaO), both of which are endemic components in many coal-bearing rock strata.

Environmental Benefits of Using Fly Ash

Fly ash use in cement and concrete has significant environmental advantages, including:

Increasing the lifespan of concrete roads and structures by improving concrete durability;
Reducing energy use and harmful air emissions when fly ash is used
Conservation of other natural resources and materials;
Decrease in the amount of coal combustion products that must be disposed of in landfills places or displace manufactured cement.

Properties of Fly Ash

1. Size, Shape, and Colour

The particle size of fly ash is finer than that of regular Portland cement. Fly ash is made up of silt-sized particles, most of which are spherical in shape and usually small. The size of fly ash varies from $10 \:to \:100\: \mu m$ . Fresh concrete is more fluid and workable thanks to these tiny glass spheres. One of the key characteristics influencing fly ash’s pozzolanic reactivity is its fineness.

The chemical and mineral components that makeup fly ash determine its color. Tan to dark gray is possible. Tan and light hues are typically linked to higher lime concentration, while brownish hues are linked to higher iron levels. High unburned carbon (LOI) content is what gives the color, which ranges from dark gray to black. Each power plant’s and coal source’s fly ash color is often quite constant.

2. Fineness

The coal crushers’ state of operation and the coal’s own grindability are directly related to how fine the fly ash is. The pozzolanic activity of fly ash is correlated with its fineness. For the purposes of using fly ash in concrete applications, fineness is referred to as the percentage of the material by weight that is retained on the 5 lm (#325) sieve.

The maximum amount of fly ash retained on the $45 \mu m$ (#325) mesh sieve while wet screening is 34%, according to ASTM C618. The majority of ash particles are typically smaller than $1 \mu m$ in size. Particle sizes in bituminous ashes range from less than 1 to more than $100 \mu m$ . According to Joshi [53], the typical size is between 7 and 12 lm. More coarsely graded

3. Specific Gravity

The shape, color, and chemical makeup of fly ash particles all affect the specific gravity of the material. Fly ash’s specific gravity can range from 1.3 to 4.8 in general. Fly ashes from Canada range in specific gravity from 1.94 to 2.94, while those from the United States range from 2.14 to 2.69.

4. Pozzolanic Activity

Pozzolanic reactivity is the ability of fly ashes, which have little to no cementing value, to react with calcium oxide in the presence of water and form highly cementitious water-insoluble compounds. When moisture is present, the meta-stable silicates in self-cementitious fly ash interact with calcium ions to generate water-insoluble calcium-alumino-silicate hydrates.

A fly ash’s pozzolanic activity is influenced by its fineness, calcium content, structure, specific surface, particle size distribution, and LOI concentration, among other factors. According to numerous researchers, fly ash’s pozzolanic activity considerably rises as it is ground to a finer consistency. Beyond 6,000 cm2, the effect of increased specific surface area is said to be negligible.

7. Chemical Composition

Fly ashes contain the following elements in their chemical makeup: silica (SiO2), alumina (Al2O3), and oxides of calcium (CaO), iron (Fe2O3), magnesium (MgO), titanium (TiO2), sulfur (SO3), sodium (Na2O), potassium (K2O), and unburned carbon (LOI). SiO2 and Al2O3 combined account for between 45 and 80 percent of the total ash among them.

In comparison to bituminous coal ashes, sub-bituminous and lignite coal ashes include proportions of CaO and MgO that are considerably higher, and lower, for SiO2, Al2O3, and Fe2O3. The chemical analysis of different fly ashes.