Fly ash bricks are made from various raw materials. They are known for their high compressive strength and uniform size. The fly ash brick industry is witnessing significant growth, fuelled by the booming construction industry and advancements in processing technologies.
Fly ash bricks account for about 1/6th of India’s annual brick production and are an eco-friendly alternative to other types of building blocks. Therefore, their use is a popular solution for residential and commercial structures. Let’s understand the composition and production process of fly ash bricks.
Raw Materials Used in Fly Ash Brick Manufacturing
Fly ash bricks are made up of various raw materials, each with different proportions. The major raw materials used in fly ash brick manufacturing are, namely:
- Fly Ash
- Sand
- Sludge LIme
- Gypsum
- Grit 4 mm to 8 mm
- Crusher Dust
- Cement
Let’s learn the significance of each of the raw materials, their mix proportion and their attributes below.
Fly Ash: 55-60%
Fly ash is a by-product of coal combustion from thermal power plants. It includes silica, alumina, iron oxide, and calcium oxide. According to Indian Standards, it is of two types, namely:
- Grade 1 with more than 70% of SiO2+Al2O3+Fe2O3 content.
- Grade 2 with 50-70% content of SiO2+Al2O3+Fe2O3.
Fly ash in general provides good thermal insulation and is highly durable.
Sand and Grit: 20-25%
- The sand acts as a binding material and helps hold the fly ash, lime, and gypsum together.
- It fills the voids between fly ash particles, providing a compact structure.
- The sand contributes to the compressive strength of the brick and improves its workability.
- 4 mm to 8 mm grit serves as an aggregate in fly ash bricks. It provides structural integrity and strength. The specific size range from 4 mm to 8 mm ensures the optimal compaction and bonding of materials with each other.
- It also enhances the comprehensive strength of the fly ash brick, making it suitable for heavy loads and helps reduce the water absorption rate of the brick.
Sludge Lime: 15-22%
- Sludge lime is a by-product of the paper and pulp industry. It consists mainly of calcium carbonate and is less expensive than hydrated lime.
- Using sludge lime helps in waste management by utilising the industrial by-products.
- Sludge lime is usually wet and contains lumps, which need to be broken down before its use in fly ash bricks.
Gypsum: 5%
- Gypsum acts as a binding agent in fly ash bricks, bringing all the materials together.
- Gypsum is chemically known as calcium sulphate dihydrate. During the hydration process, gypsum reacts with water to form a crystalline structure that contributes to the strength of the bricks.
- The setting time of the fly ash brick is controlled by gypsum, which ensures the hardening process.
- It enhances the long-term strength of the brick performance and leads to improving the brick’s workability.
- The presence of gypsum also enhances the durability of the fly ash brick, making it more resistant to weathering.
Crusher Dust
- Crusher dust is defined as the fine particles produced during the crushing of stone used as filler material.
- It is cheaper than river sand, which reduces the overall cost of fly ash bricks.
- It reuses waste from stone crushing to reduce environmental impact.
- The crusher dust minimises the white salt deposits on the brick surface. It also helps improve the durability of the fly ash brick as it reduces the water absorption rate of the bricks.
Cement: 8-12%
- Cement can be used as a substitute for gypsum and lime. When cement is used, the sand and grit content increases to 30-40%.
- Cement acts as a binding material in fly ash bricks, holding other materials together to form a solid, durable brick. When the cement is mixed with water, it undergoes a hydration reaction. It forms a hard substance that binds the fly ash and sand particles.
- It contributes to the comprehensive strength of the fly ash and makes it suitable for load-bearing applications.
- The use of cement enhances the durability of fly ash bricks. It makes them resistant to weathering and harsh chemicals.
Fly Ash Brick Manufacturing Process
The manufacturing process of fly ash bricks involves a series of controlled steps. It transforms industrial waste into high-quality construction material. Fly ash is collected and processed. This results in the formation of fly ash bricks that offer superior strength and durability.
The following steps are adopted in the manufacturing process:
Mixing the Raw Materials
The raw materials are measured and used according to the required proportions. First, the dry materials like fly ash, sand lime, and gypsum are mixed well to ensure correct proportions and uniform distribution.
Then the water is gradually added to the dry ingredients to achieve the desired consistency.
The mixing process takes approximately about 5 to 10 minutes, depending on the equipment used. A mixer or any batching plant can be used for the mixing process. In the end, a uniform mixture with the right consistency must be obtained with no lumps or dry patches. Sometimes additives (chemical admixtures) are added to improve the workability of the content.
Moulding
The mixtures are then transferred to a moulding machine. It is put into a hydraulic or manual moulding machine. These machines are typically made from mild steel or cast iron.
The mixture is compressed in the machine to achieve consistent shape and density. Therefore, the desired brick shape is obtained using the high pressure of the hydraulic system.
To ensure a smooth release of the bricks, the moulds are cleaned and lubricated. This prevents the mixture from sticking to the mould and is essential for easy brick formation.
Curing
After the raw materials are moulded, they are placed in palettes or racks. It helps in easy handling and transportation during the curing process.
The bricks are typically cured using a steam, which is placed in a curing chamber. The chambers maintain a temperature of around 66°C for 24 hours.
The curing process generally lasts for 7 to 14 days. It accelerates the chemical reaction between the materials and provides strength to the brick.
Proper curing ensures the brick achieves maximum compressive strength and durability and also helps reduce the chance of efflorescence.
Drying
The drying step is critical during the manufacturing process of fly ash bricks. This process can involve air-drying (naturally drying under sunlight) or machine-induced drying for faster completion.
Final curing
After the drying, the bricks are in the process of final curing. In this step, the bricks are kept in moisture and can last for several days to weeks. This is continued until the brick attains the appropriate strength.
Sorting and Stacking
Sorting
Once the fly ash bricks are dried and cured, they undergo a quality check. They are sorted according to the qualities required to meet the standards.
Stacking
The sorted bricks are ready for assembly. They are stacked systematically and stored in a designated area for dispatch.
Conclusion
In conclusion, fly ash brick represents the advancement of the construction industry. The modern market highlights the ongoing efforts to promote fly ash bricks. Also, various studies emphasise its potential to enhance sustainability, making it a crucial material in the real estate industry.