What is Silica Fume (Microsilica)?
Silica Fume Introduction
Silica fume (also known as microsilica) is very fine noncrystalline silica by-product resulting from the reduction of high-purity quartz and carbon in electric arc furnaces during the production of elemental silicon or alloys containing silicon.
Silica fume consists of very fine vitreous particles with a surface area ranging from 15 to 30 m2/g, with particle approximately 100 times smaller than the average cement particle. Because of its extreme fineness and high silica content, silica fume is a highly effective pozzolanic material.
Silica fume is used in concrete to improve its properties. It has been found that silica fume greatly improves compressive strength, bond strength, and abrasion resistance; reduces permeability; and therefore helps in protecting reinforcing steel from corrosion.
Silica Fume Production
- Wood Chips
Desired ReactionSiO2 + 2C = Si + 2CO
Smelting FurnaceTemperature 2000℃
As-Produced Silica Fume
Overview of Facility
Capture as-produced silica fume by bag house
After being collected over the furnace, the silica fume must be transferred, cooled, and physically trapped. The large pipe on the left is bringing the silica fume from the furnaces. The vertical elements are cyclones that are used to remove oversize and other unwanted materials. The large building is the bag house where the fume is captured.
Densification silo for production of densified form of silica fume — The as-produced fume is brought into this silo. Compressed air is used to aerate and tumble the fume particles. Electrostatic charges develop and cause the individual particles to agglomerate. Once the densified silica fume reaches the desired bulk density, it is taken from this silo for packaging or shipping.
Silica fume can fill the pores between cement particles. It reacts with calcium hydroxide in water to form calcium silicate hydrate (C-S-H) gels, and also with magnesium oxide and water to form magnesium silicate hydrate (M–S–H) gels. For cement-based concrete, mortar, mixing the right amount of silica fume can significantly improve its compressive strength, flexural strength, impact resistance, corrosion resistance, wear resistance; as well as reduce its permeability.
Silica fume products are also used in the refractory industry, of which high-grade undensified silica fume is currently the most used. As an additive of refractory products, silica fume improves their flow ability, strength, durability, and performance under high-temperature conditions.
For Other Applications
· To produce silicon carbide.
· Used for fertilizers, pesticides, etc., as a chemical dispersant to prevent some synthetic powders from caking or enhance effects.
· As a metallurgical pellet binder.
SiO2 Content Comparison
- Collected from ferrosilicon: SiO2 > 85%.
- Collected from high purity silicon: SiO2 > 92%.
- Collected from silicon metal: SiO2 > 94%.
The difference of SiO2 content results in different performance in concrete, like compressive strength, chloride permeability, etc. So does their application: low-grade (low SiO2 content) microsilica can be used for substructures mix (for example: 40Mpa), but for higher strengths or more durable concrete, high-grade microsilica should be selected.
Densification and Bulk Density
According to the different degrees of densification, silica fume can be divided into undensified (as-produced) silica fume, semi-densified silica fume, densified silica fume, with bulk density around 180 kg/m3, 300-360 kg/m3, 630-650 kg/m3, respectively.
Except for bulk density, there is no difference in specifications among the three types. Densified silica fume is commonly used in construction like ready-mixed concrete, while undensified silica fume is often used in refractory, shotcrete, fertilizer. Because the as-produced silica fume has a lower density, its transport cost is higher.
Reference of specs:
The color of silica fume ranges from light gray to dark gray. And there is also white silica fume with both SiO2 and ZrO2 content.
Factors that influence the color of silica fume:
- Raw material, reductant: Since SiO2 is colorless, the color is influenced by other content such as carbon and iron oxide. Under normal conditions, silica fume collected from silicon metal, high purity silicon has darker colors than that collected from ferrosilicon.
- Time: Silica fume produced from smelters just start operating has darker colors than from smelters that have operated for more than 2 or even 6 months.
- Bulk density: Typically, the higher the density, the darker the color.
- Temperature in smelters.
- Conditions of smelters.
The color of silica fume collected from the same smelter in a day is the same.
Normally, a densification silo contains 24 ton of base material with bulk density of 300kg/m3. And around 10 ton per day silica fume will be added in the silo, evenly mix with the base material to densify. An above-40-minute densification process can ensure that the color of silica fume collected in a day is consistent.
The densification silo discharges also about 10 ton quantity once a day, retaining 24 ton as base material.
Silica fume chlorine content from low to high: collected from silicon metal, ferrosilicon and high purity silicon.
- From silicon metal: < 0.02%;
- From ferrosilicon metal and high purity silicon: normally < 0.5%. Calcium chloride is added if chlorine content exceeds 0.5%.
viscosity from high to low: from silicon metal > high purity silicon > ferrosilicon. (due to different reductants)
Factors that influence the color of silica fume:
- Bulk density: The higher the density, the lower the viscosity.
- Content of coal tar: The lower the content, the lower the viscosity.
The pH value of silica fume is mainly influenced by the sulfur content in coal tar or petroleum coke. With coke high in sulfur, the pH of silica fume will be low.