Fly ash is the most common waste material used in construction. It is a by-product of coal combustion in power plants and is collected from the chimneys of these plants. Fly ash is a powdery substance that is primarily composed of small, spherical particles of glassy material. It is a versatile material that has several applications in the construction industry.

Fly ash is used in construction due to its numerous advantages such as:

1. Pozzolanic properties: Fly ash is a pozzolanic material, which means it reacts with calcium hydroxide to form cementitious compounds. When mixed with lime and water, fly ash undergoes a chemical reaction that results in the formation of additional cementitious compounds. This reaction enhances the strength, durability, and workability of the concrete.

2. Improved workability: Fly ash improves the workability of concrete by providing a lubricating effect. It reduces the water demand and increases the cohesiveness of the mix, making it easier to handle and place.

3. Reduced heat of hydration: The addition of fly ash in concrete reduces the heat generated during the hydration process. This is beneficial in large concrete placements where excessive heat can cause cracking and thermal stresses.

4. Increased durability: Fly ash reduces the permeability of concrete, making it more resistant to water penetration and chemical attack. It also enhances the resistance to sulfate attack, alkali-silica reaction, and carbonation.

5. Environmental benefits: By utilizing fly ash in construction, the volume of waste sent to landfills is reduced. This helps in conserving landfill space and reducing the environmental impact associated with waste disposal.

6. Cost-effective: Fly ash is a cost-effective material as it is readily available in large quantities as a by-product of coal combustion. Its use in construction can lead to cost savings without compromising on the quality and performance of the structures.

In conclusion, fly ash is the most common waste material used in construction due to its pozzolanic properties, improved workability, increased durability, environmental benefits, and cost-effectiveness. Its utilization in construction helps in reducing waste generation, conserving natural resources, and improving the overall sustainability of the construction industry.