Fly ash is an important supplementary material for concrete. Its inclusion has numerous advantages and improves concrete performance in both fresh and hardened states. Fly ash-based concrete is also cost-effective. This blog provides information on fly ash, its properties, benefits, the effects of fly ash in concrete, and its uses in construction.
What is Fly Ash or Pulverised Fuel Ash?
Fly ash is a by-product created during the burning of coal used in thermal power plants. It is a fine grey powder containing spherical glassy particles that ascend with the exhaust gases. Fly ash contains pozzolanic material components that combine with lime to generate cementitious materials. Silica (SiO2), calcium oxide (CaO), alumina oxide (Al2O3), and ferric oxide (Fe2O3) are the primary constituents of fly ash. It is commonly used as a supplementary cementitious material in concrete.
The majority of fly ash is pozzolanic, which means it contains siliceous or siliceous-and-aluminous material that reacts with calcium hydroxide to make cement. When Portland cement reacts with water, it forms hydrated calcium silicate (CSH) and lime. The hydrated silicate gains strength, while the lime fills the gaps. When properly selected fly ash combines with lime, it produces CSH, the same cementing component found in Portland cement. To ensure enhanced characteristics in concrete, fly ash must be carefully selected.
What is the Effect of Fly Ash in Concrete
The following are the effects of fly ash in concrete in its fresh and hardened state.
1. Effect of Fly Ash on Fresh Concrete:
- The addition of fly ash can raise the water need for a given workability. Because fly ash is so fine, adding it to cement means using a bit more water to achieve the same slump.
- Fly ash in concrete helps in minimizing segregation and bleeding.
- The heat of hydration is decreased by adding fly ash in concrete.
- A 30% substitution of fly ash may increase the initial setting time by up to two hours.
2. Effect of Fly Ash on Hardened Concrete:
- Fly ash concrete construction often has a lower initial strength than regular concrete but higher strength at a later stage.
- The use of fly ash in concrete reduces its permeability. Pulverised fuel ash concrete maybe three times more permeable in the first 28 days than regular concrete, although it may become less permeable in less than six months.
- The addition of fly ash in concrete helps in improving the resistance to chemical attack.
Fly Ash Content in Concrete
Usually, the fly ash content in concrete is at a rate of 15-35% by weight of cement, but up to 70% is added for mass concrete used in the construction of dams, roller-compacted concrete pavements, and parking lots.
Different Types of Fly Ash for Construction
According to “ASTM C618-08a” (American Society for Testing and Material), there are two types of fly ash depending on the type of coal the ash is made of.
Class C Fly Ash
This type of fly ash is made from lignite and sub-bituminous coal. It has both pozzolanic and self-cementing properties. The most popular uses for class C fly ash are in structural concrete, PCC pavements, and as a main ingredient in bricks, blocks, tiles, and other construction materials.
Class F Fly Ash
Fly ash classified as Class F is more prevalent than Class C fly ash and is derived from bituminous coal. It has pozzolanic properties and is effective in reducing sulfate attack.
Benefits of Fly Ash in Concrete
The major benefits of fly ash in concrete are as follows:
- Improved Workability:
Fly ash particles are spherical and fine, which can enhance the workability of fresh concrete, making it easier to place and finish. The spherical particles work as small ball bearings in the concrete mix, providing a lubricating effect. This also helps in enhancing the concrete pumpability by lowering frictional losses during the pumping operation.
- The Fly Ash Concrete Mix Requires Less Water:
The use of fly ash instead of cement reduces the amount of water required for a particular slump. When fly ash is employed at around 20% of the total cementitious material, water demand is reduced by around 10%.
- Enhanced Strength:
Fly ash cement concrete has higher ultimate strength when compared with Portland cement concrete. The additional binder created by the fly ash interaction with accessible lime enables fly ash concrete mix to increase strength over time.
- Reduced Heat of Hydration:
Fly ash-based concrete generates less heat of hydration during the hydration process. This reduction in hydration heat does not mean sacrificing long-term strength improvement or durability. Hence, it is beneficial in mass concreting construction to reduce the risk of thermal cracking.
- Reduced Permeability:
The reduction in water content, along with the creation of extra cementitious compounds, reduces concrete’s pore interconnectivity, hence decreasing permeability.
- Increased Durability
Using fly ash in construction can enhance the durability of concrete by making it more resistant to chemical attacks, such as sulfates and chlorides. It also reduces permeability, which can improve the longevity of the concrete structure.
- Environmental Benefits
Utilizing fly ash in concrete reduces the need for Portland cement, which in turn lowers the carbon footprint of concrete production. It also helps in managing waste from coal-fired power plants.
- Cost Savings
As fly ash is a by product and available in abundance, it is cheaper to obtain. So, replacing a portion of cement with fly ash can reduce the overall cost of the concrete mix. As a result of this, fly ash concrete price is less than that of Portland cement concrete.
Applications of Fly Ash in Construction
The following are the common fly ash uses in construction:
- Fly ash is mixed with Portland cement to make Portland Pozzolana Cement.
- The durability and reduced heat of hydration make fly ash-based concrete suitable for large-scale infrastructure projects such as pavements and roads.
- Fly ash is typically used for embankment construction.
- Fly ash concrete mix can be used in structural elements, providing benefits in terms of strength and durability.
- Due to reduced heat generation, fly ash cement concrete is ideal for mass concrete applications such as dams and large foundations.
- Fly ash is used in the manufacturing of fly ash cement bricks.
- It is used as a soil stabilizing agent.
- Geopolymers include fly ash as a component.
Conclusion
Fly ash has been used successfully in a variety of construction projects across the country. The use of fly ash in concrete was also found to minimize carbon emissions on the construction site. While there is now awareness of the benefits that fly ash may provide in the quest for sustainable construction material, considering the volume of fly ash produced and technological improvements in the construction industry, there is still a lot of room to utilize its advantages.
FAQ
Fly ash is not harmful to cement; in fact, it often improves the quality of the concrete. It enhances the concrete’s strength, durability, and workability. However, if not used correctly or if low-quality fly ash is used, it can lead to issues like slower setting times or reduced strength. When used properly, fly ash is beneficial and not harmful to cement.
Ordinary Portland Cement (OPC) is typically made without fly ash. OPC is a pure form of cement that doesn’t contain any additives like fly ash. If you’re looking for cement without fly ash, OPC is the standard choice. Other types like Portland Pozzolana Cement (PPC) do contain fly ash.
Yes, fly ash can partially replace cement in concrete, but not completely. Typically, fly ash can replace 15-30% of the cement in a mix. This reduces costs and improves certain properties like workability and long-term strength. However, you can’t replace all the cement with fly ash, as cement is still needed to bind the concrete properly.