6.2.1. Origin and Production

Fly ash is a by-product obtained from the combustion of pulverized coal in thermal power plants. It consists primarily of silicon dioxide (SiO₂), aluminum oxide (Al₂O₃), and iron oxide (Fe₂O₃).

Fly ash is a fine powder created during the burning of coal in power plants. When coal is burned, it produces ash, and when the ash is collected from the exhaust, it is called fly ash. This ash mainly contains minerals like silicon dioxide, aluminum oxide, and iron oxide, which contribute to its properties and performance in concrete.

Think of fly ash like the leftover ash in a fireplace after burning wood or coal. Just as that ash can have different colors and textures based on the type of wood or coal burned, fly ash varies in color and composition depending on the coal source used in power plants.

Types of Fly Ash

• Class F: Low in calcium; pozzolanic in nature.
• Class C: High in calcium; both pozzolanic and cementitious.

Fly ash comes in different classes based on the calcium content. Class F fly ash is low in calcium and mainly acts as a pozzolan, meaning it can react with calcium hydroxide in the presence of water to form additional cementitious compounds. In contrast, Class C fly ash contains a higher level of calcium and can behave both as a pozzolan and as a cement-like material, meaning it can contribute to the structure of concrete on its own.

Imagine baking cookies using different types of flour. Just like how some flour types might work better for certain recipes, the type of fly ash also plays a specific role in enhancing concrete properties depending on its class.

Properties

• Specific surface: 300–500 m²/kg
• Fineness: Varies with grinding
• Pozzolanic activity: Depends on reactive silica content
• Color: Gray to black

The properties of fly ash include its specific surface area, fineness, pozzolanic activity, and color. The specific surface area tells us how much surface is available for reactions and typically ranges from 300 to 500 square meters per kilogram. Fineness indicates how finely the fly ash is ground. The more reactive the silica in the fly ash, the greater its pozzolanic activity, which is crucial for its effectiveness in concrete. The color of fly ash can vary from gray to black, which is influenced by the type of coal burned.

Consider powdered sugar compared to regular sugar. The powdered sugar has a much finer texture and can dissolve easily in liquids, similar to how finer fly ash can improve its reactivity and effectiveness in enhancing concrete.

Effects on Concrete

• Improves workability and pumpability
• Reduces water demand
• Enhances long-term strength
• Reduces permeability
• Slower early strength gain
• Reduces heat of hydration

Adding fly ash to concrete positively affects several properties. It improves workability, making the mixture easier to handle and pump. It also reduces the water needed for mixing, which helps create a denser mix. Over time, fly ash contributes to improving the long-term strength of concrete while also reducing its permeability, making it less susceptible to water and chemicals. However, using fly ash may lead to a slower gain in strength early on, which means concrete might take longer to set. Additionally, fly ash helps lower the heat generated during hydration, which is essential in preventing cracking in large pours.

Think of fly ash as a special ingredient in a recipe. Just like some ingredients make a cake fluffier or help it bake more evenly, fly ash helps create stronger, more durable concrete over time. But, like baking a cake that takes longer, fly ash can slow down how quickly the concrete gains its initial strength.

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