Microsilica, also known as silica fume, is a byproduct of the silicon metal and ferrosilicon industries. This amorphous silicon dioxide (SiO2) is generated as a gas during the reduction of highly pure quartz in submerged electrical arc furnaces. As the molten metal is produced, a silica-based gas is emitted. This gaseous fume, upon rising, cools rapidly and forms extremely minute, amorphous, spherical particles.

Microsilica: A Closer Look at Its Formation

The microsilica is collected in a bag house, a system designed for filtering the hot air and gases vented from the furnace. This gas vapor condenses in bag house collectors, forming a very fine powder of spherical particles. These particles average 0.1 to 0.3 microns in diameter, boasting a surface area of 17 to 30 m2/g.

Physical and Chemical Characteristics of Microsilica

Microsilica is a gray or gray-white powder with a refractoriness of over 1600℃. Its bulk density ranges from 320 to 700 kg/m3. The chemical composition of microsilica is primarily SiO2, accounting for 75 to 98% of its content. Other components include Al2O3, Fe2O3, MgO, CaO, and Na2O. The pH value of microsilica is neutral.

The fineness of the silica fume is less than 1μm, with an average particle size of 0.1 – 0.3 μm and a specific surface area of 20 – 28 m2/g. This fineness and specific surface area are about 80-100 times that of cement and 50-70 times that of fly ash.

Microsilica in the Refractory Industry

Microsilica has found extensive use in the refractory industry. It plays a crucial role in enhancing the performance of amorphous refractories. Here are some of its applications:

1. It serves as a substitute for pure aluminum oxide in refractories.
2. It is used as an additive in the production of amorphous and shaped refractory products, significantly improving their strength and high-temperature performance.
3. It is used as a coagulant in the overall casting ladle.
4. It is used as a binder, coagulant, and additive for other refractory products.

Microsilica in Concrete

Microsilica powder can fill the spaces between cement particles, making the slurry denser. Its applications in concrete include:

1. Enhancing strength: Adding 5-10% microsilica can increase the compressive strength by 10-30% and the flexural strength by more than 10%.
2. Increasing density: It can enhance the impermeability by 5-18 times and the chemical resistance by more than 4 times.
3. Improving frost resistance: After 300-500 rapid freeze-thaw cycles, the relative elastic modulus of microsilica powder decreases by 10-20%, while ordinary concrete decreases to 30-73% through 25-50 cycles.
4. Providing early strength: Microsilica concrete shortens the induction period and exhibits early strength characteristics.
5. Enhancing anti-abrasion and anti-cavitation resistance: The punching resistance of microsilica concrete improves 0.5-2.5 times compared to ordinary concrete, and its anti-cavitation ability improves 3-16 times.

Other Applications of Microsilica

Microsilica is used as a filler in rubber or plastic in the elastomers/polymers industry. It is used in the defluorination process during the production of dicalcium phosphate (animal feed) in the fertilizer industry. In the oil and gas industry, it is used in down-hole grouting operations to improve flow and decrease permeability. In the fiber cement industry, it is used to enhance the strength and permeability of corrugated roofing panels. In shotcrete, or sprayed concrete, silica fume reduces rebound and increases bonding strength, productivity, and savings in material cost. It is also used in commercial gypsum and concrete wallboard products for increased performance.