6.3. Silica Fume (Microsilica)

Silica fume is an ultra-fine by-product obtained from the manufacture of silicon and ferrosilicon alloys in electric arc furnaces.

Silica fume is a by-product, meaning it is not a primary product that manufacturers aim to create. Instead, it is formed during the production of silicon and ferrosilicon alloys. These materials are produced in electric arc furnaces where very high temperatures are used to melt raw materials. The silica fume produced during this process is extremely fine, with particles that are much smaller than typical sand or cement particles, which allows it to interact more effectively with cement components in concrete.

Imagine making syrup from fruit. While boiling the fruit, you may produce some residue or foam that rises to the top. This foam is not what you intended to make, but it can still have its uses, just like how silica fume arises during alloy production and can enhance concrete.

• Extremely high surface area: ~20,000 m²/kg
• Particle size: < 1 μm
• SiO₂ content: > 90%
• Highly reactive pozzolan

Silica fume has some unique physical properties that make it beneficial for concrete. First, it has an extremely high surface area of about 20,000 square meters per kilogram, which is much higher than typical cement. This allows it to react more easily with water and other components in concrete. The particle size is also very small, being less than 1 micron, which enhances its ability to fill the gaps between larger particles in the mixture. Furthermore, its chemical composition is primarily silicon dioxide (SiO₂), which contributes to its reactivity. As a highly reactive pozzolan, it means it can participate in chemical reactions that improve the properties of the hardened concrete.

Think of silica fume like fine dust that can fill in the cracks between larger rocks. The more tiny particles you have, the tighter the packing you create. This is similar to how silica fume fills spaces between larger cement grains in concrete, resulting in a stronger material.

• Significantly improves compressive and flexural strength
• Reduces permeability and chloride ion penetration
• Increases cohesiveness and reduces bleeding
• Enhances bond strength with reinforcement
• May increase water demand (requires superplasticizers)

Using silica fume in concrete has profound positive effects on its properties. It significantly boosts compressive strength, meaning the concrete can withstand more force before failing. It also improves flexural strength, which is essential for resisting bending. When it comes to permeability, silica fume helps create a denser mixture, making it harder for water and chloride ions to penetrate the concrete, thus protecting it against environmental damage and corrosion of reinforcement bars. The cohesiveness of the concrete mixture increases, which helps in reducing bleeding—this is when water rises to the surface and can weaken the concrete. Silica fume also enhances the bond strength between the concrete and embedded steel reinforcements, ensuring that they work together effectively. However, it can also increase the water demand of the mix, meaning to achieve the right consistency, superplasticizers may need to be added.

Imagine a dense sponge versus a loose napkin. The sponge (when silica fume is added) has a very tight structure that doesn’t allow water to pass through easily, while the napkin allows water to seep through quickly. Similarly, silica fume helps tighten the structure of concrete, preventing water damage.

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