Microsilica

Microsilica & Silica Fume
SiO2 85% -97% min
Surface area: 15-30m2/g
Bulk density: 480-720kg/m3

Silica Fume Definition:
Silica fume is very fine non-crystalline silica produced in electric arc furnace as a by-product of the production of elemental silicon or ferrosilicon alloys containing silicon.

It is usually a gray colored powder after being collected from a furnace, somewhat similar to Portland cement or some fly ashes, which is of more importance to the concrete industry.

The silica fume is collected in very large filters in the baghouse and then made available for use in concrete directly.

Usage:
Concrete and mortars

Chemical Properties:
1. Amorphous. This term simply means that silica fume is not a crystalline material. A crystalline material will not dissolve in concrete, which must occur before the material can react. Don't forget that there is a crystalline material in concrete that is chemically similar to silica fume. That material is sand. While sand is essentially silicon dioxide(SiO2), it does not react because of its crystalline nature.

2. Silicon dioxide(SiO2)> 85%. This is the reactive material in silica fume.
Adding silica fume brings millions and millions of very small particles to a concrete mixture. Just like fine aggregate fills in the spaces between coarse aggregate particles, silica fume fills in the spaces between cement grains. This phenomenon is frequently referred to as particle packing or micro-filling. Ever if silica fume did not react chemically, the micro-filler affect would bring about significant improvements in the nature of the concrete.

3. Trace elements: There may be additional materials in the silica fume based upon the metal being produced in the smelter from which the fume was recovered. Usually, these materials have no impact on the performance of silica fume in concrete. Standard specifications may put limits on some of the materials.

Physical Properties:
1. Particle size< 1um.
Silica fume particles are extremely small, with more than 95% of the particles being less than 1um (one micrometer). Particle size is extremely important for both the physical and chemical contributions of silica fume in concrete.

2. Bulk density: (as-produced) 130to430kg/m3; (densified) 480to720kg/m3.
This is just another term for unit weight. The bulk density of the as-produced fume depends upon the metal being made in the furnace and upon how the furnace is operated. Because the bulk density of the as-produced silica fume is usually very low, it is not very economical to transport it for long distances.

3. Specific gravity: 2.2.
Specific gravity is a relative number that tells how silica fume compares to water, which has a specific gravity of 1.00. Silica fume has a specific gravity of about 2.2, which is somewhat lighter than Portland cement, which has a specific gravity of 3.15. Thus, adding silica fume to a concrete mixture will not "densify" the concrete in terms of increasing the density of the concrete.

4. Specific surface: 15, 000 to 30, 000m2/kg.
Specific surface is the total surface area of a given mass of a material. Because the particles of silica fume are very small, the surface area is very large. We know that water demand increases for sand as the particles become smaller; The same happens for silica fume. This fact is why it is necessary to use silica fume in combination with a water-reducing admixture or a super plasticizer. A specialized test called the "BET method" or "nitrogen adsorption method" must be used to measure the specific surface of silica fume. Specific surface determinations based on sieve analysis or air-permeability testing are meaningless for silica fume.