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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.
Desired Reaction
SiO2 + 2C = Si + 2CO
Temperature &#;
off-gas
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.
· 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.
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.
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.
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:
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Reference of colors:
Densified SF
Undensified SF
Zirconia SF
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.
viscosity from high to low: from silicon metal > high purity silicon > ferrosilicon. (due to different reductants)
Factors that influence the color of silica fume:
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.
Silica Fume Introduction
Silica Fume
SupplementaryCementitious Materia
The American Concrete Institute (ACI) defines silica fume as &#;very fine non-crystalline silica produced in electric arc furnaces as a by-product ofthe productions of elemental silicon or alloys containingsilicon&#; (ACI 116R). It is usually a gray colored powder, somewhat similar to portland cement or some fly ashes.
Silica fume is usually categorized as a supplementary cementitious material. This term refers to materials that are used in concrete in addition to portland cement.
Silica fume is frequently referred to by other names:
Silica fume is a by-product of producing silicon metal or ferrosilicon alloys in smelters using electric arc furnaces. These metals are used in many industrial applications to include aluminum and steel production, computer chip fabrication, and productions of silicones, which are widely used in lubricants and sealants. While these are very valuable materials, the by-product silica fume is of more importance to the concrete industry.
Densified and Undensified Silica Fume
Silica fume has historically been available in three basic product forms: undensified, slurried, and densified. There is no data available, after many years of testing, to show that any one of the product forms will perform better in a concrete mixture than any of the others.
Slurried silica fume is no longer available in the U.S. market. Undensified silica fume is available,but it is not frequently used in ready-mixed or precast concrete. Undensified silica fume is primarily used in pre-bagged products such as grouts or repair mortars.
Densified silica fume is produced by treating undensified silica fume to increase the bulk density up to a maximum of about 400 to 720kg/m3. This increase in bulk density is usually accomplished by tumbling the silica-fume particles in a silo, which causes surface charges to build up. These charges draw the particles together to form weak agglomerates. Because of the increased bulk density, this material is more economical for truck transportation.
Densified silica fume works very well in concrete. However, one caution when working with this product form is to ensure that the mixing is adequate to break up the particle agglomerations. Mixing in some types of mixers such as those that are used in dry mix shotcrete, roof tiles, or other applications where coarse aggregate is not present may not be adequate to break up the agglomerations. In those situations, an undensified silica fume may be more appropriate. Contact the Silica Fume Association for assistance in these types of applications.
REFERENCES
Information provided by US DEPARTMENT OF TRANSPORTATION &#;Federal Highway Administration &#;Silca Fume Association -April
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