SILICA FUME

Silica fume is an ultrafine, amorphous silicon dioxide (SiO₂) by-product formed during the production of silicon metal or ferrosilicon alloys in electric arc furnaces, consisting of extremely small spherical particles with very high specific surface area.
Because of its high silica content and extreme fineness, silica fume is a highly reactive pozzolan that reacts with calcium hydroxide in cementitious systems to form additional calcium silicate hydrate, significantly improving strength, durability, and impermeability.
Silica fume is widely used as a mineral admixture in high-performance concrete and also as a functional filler in refractories, polymers, coatings, and specialty industrial applications where enhanced mechanical and durability properties are required.

CAS Number: 69012-64-2
EC Number: 273-761-1
Molecular Formula: SiO₂
Molecular Weight: 60.08 g/mol

Synonyms: Microsilica, Silica Fume, Condensed Silica Fume, Silica Smoke, Amorphous Silica, Amorphous Silicon Dioxide, Silicon Dioxide Fume, Fumed Silica (industrial/by-product grade), Reactive Silica, Ultrafine Silica, High-Purity Silica Fume, Pozzolanic Silica, Siliceous Fume, Silica By-product (Silicon Industry), Silica Dust (industrial term), Micro-Silica Powder, Ultrafine Silicon Oxide, Non-Crystalline Silica, Synthetic Amorphous Silica (by-product), Silica Vapor Condensate, Silicon Furnace Fume, Condensed Silicon Oxide, High-Surface-Area Silica, Reactive Amorphous SiO₂, Industrial Microsilica

Silica fume is an ultrafine amorphous silicon dioxide (SiO₂) material obtained as a by-product during the production of silicon metal or ferrosilicon alloys in electric arc furnaces.
Silica fume consists of extremely small, spherical particles with a very high specific surface area, which gives it exceptional reactivity and pozzolanic properties.
Due to its high silica content and fine particle size, Silica fume reacts readily with calcium hydroxide in cementitious systems to form additional calcium silicate hydrate, significantly enhancing strength, durability, and impermeability.

Silica fume is widely used as a mineral admixture in concrete and cement-based materials to improve mechanical strength, reduce permeability, increase resistance to chemical attack, and enhance long-term durability.
In addition to construction applications, Silica fume is used as a functional filler in refractories, ceramics, coatings, rubber, polymers, and specialty chemicals, where it improves mechanical reinforcement, rheology, and thermal stability.
Silica fume is non-combustible, chemically stable under normal conditions, and valued for its performance-enhancing effects in both structural and industrial formulations.

Silica fume is an amorphous (non-crystalline) polymorph of silicon dioxide, silica.
Silica fume is an ultrafine powder collected as a by-product of the silicon and ferrosilicon alloy production and consists of spherical particles with an average particle diameter of 150 nm.
The main field of application is as pozzolanic material for high performance concrete.

Silica fume is a spherical, sub-micron-sized amorphous silicon dioxide that is used in many applications.
Silica fume is defined as a byproduct of the silicon and ferrosilicon industry, consisting of tiny particles of non-crystalline silica produced when SiO₂ vapors oxidize and condense during the reduction of high-purity quartz to silicon.
Silica fume typically contains 85–95% non-crystalline silica.

Silica fume is a byproduct of producing silicon metal or ferrosilicon alloys.
One of the most beneficial uses for Silica fume is in concrete.

Because of its chemical and physical properties, Silica fume is a very reactive pozzolan.
Concrete containing Silica fume can have very high strength and can be very durable.

Silica fume is available from suppliers of concrete admixtures and, when specified, is simply added during concrete production.
Placing, finishing, and curing silica-fume concrete require special attention on the part of the concrete contractor.

Silicon metal and alloys are produced in electric furnaces as shown in this photo.
The raw materials are quartz, coal, and woodchips.

The smoke that results from furnace operation is collected and sold as Silica fume, rather than being landfilled.
Perhaps the most important use of this material is as a mineral admixture in concrete.

Silica fume consists primarily of amorphous (non-crystalline) silicon dioxide (SiO2).
The individual particles are extremely small, approximately 1/100th the size of an average cement particle.

Because of its fine particles, large surface area, and the high SiO2 content, Silica fume is a very reactive pozzolan when used in concrete.
The quality of Silica fume is specified by ASTM C 1240 and AASHTO M 307.

Silica fume is non-crystal fine powder of high-purity silicon dioxide (SiO2) obtainable in collected dust from Ferro Silicon, Silicon Metal and Fuzed Zirconia productions.

Silica fume, is a by-product of producing silicon metal or ferrosilicon alloys in electric furnaces.
The fumes emitted from furnace operation are collected and sold as Silica fume.
Silica fume's used to provide an increase in durability in concrete.

Silica fume 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.
Silica fume 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 is a byproduct of producing silicon metal or ferrosilicon alloys.
One of the most beneficial uses for Silica fume is in concrete.

Because of its chemical and physical properties, Silica fume is a very reactive pozzolan.
Concrete containing Silica fume can have very high strength and can be very durable.

Silica fume, a co-product from the production of silicon or ferrosilicon metal, is an amorphous silicon dioxide - SiO2 which is generated as a gas in submerged electrical arc furnaces during the reduction of very pure quartz.
This gas vapor is condensed in bag house collectors as very fine powder of spherical particles that average 0.1 to 0.3 microns in diameter with a surface area of 17 to 30 m2/g.
Silica fume is used in a variety of cementitious (concrete, grouts and mortars), refractory, elastomer and polymer applications.

Silica fume, as the by-product of metallic silicon production, is collected and processed by a special trapping device from the soot that escapes with the exhaust gases in the process of melting industrial silicon and ferrosilicon in industrial electric furnaces at high temperatures.

As a new raw material, Silica fume is commonly used in concrete, oil well, mortar and refractory castable.
Silica fume can fill the pores between cement particles, and at the same time generate gel with hydration products and react with alkaline material MgO to generate gel.

Mixing appropriate amount of Silica fume can significantly improve the performance of concrete, in particular its compressive strength, bond strength, and abrasion resistance.
Addition of Silica fume also reduces the permeability of concrete to chloride ions, which protects the reinforcing steel of concrete from corrosion, especially in chloride-rich environments such as coastal regions and those of humid continental roadways and runways (because of the use of deicing salts) and saltwater bridges.

With stable PH and low impurity level, Silica fume is perfectly adapted to the requirements of refractory castables.
Silica fume enhances the flowability, hot strength and durability of refractory materials.

Usually the density of Silica fume used in refractory materials is around 0.28-0.40 tonne/m3 and in concrete is around 0.55-0.70 tonne/m3, and it is usually transported in tonnage bags.
Silica fume is produced according to ASTM C1240-20 Standard Specification for Silica fume Used in Cementitious Mixtures.

Silica fume is produced by ferroalloys in smelting ferrosilicon and industrial silicon (silicon metal), and a large amount of highly volatile SiO2 and Si gas is produced in the mineral-heated electric furnace, which is precipitated by rapid oxidation and condensation with air after the gas is discharged.
Silica fume is a by-product in large industrial smelting, and the whole process needs to be recovered with dust removal and environmental protection equipment, and because of the small density, it also needs to be encrypted with encryption equipment.

Silica fume is an ultrafine, amorphous form of silicon dioxide (SiO₂) generated as a by-product during the production of silicon metal and ferrosilicon alloys in electric arc furnaces.
During this high-temperature process, silicon monoxide vapor is oxidized and condensed into extremely fine, spherical particles, typically with diameters in the sub-micron range.
This results in a material with an exceptionally high specific surface area and a very high silica content, usually exceeding 90 percent, which gives Silica fume its unique chemical and physical reactivity.

In cementitious systems, Silica fume acts as a highly reactive pozzolanic material.
Silica fume reacts with calcium hydroxide released during cement hydration to form additional calcium silicate hydrate (C-S-H), the primary binding phase responsible for strength development in concrete.

This reaction leads to significant improvements in compressive strength, tensile strength, and abrasion resistance, while also reducing porosity and permeability.
As a result, concrete containing Silica fume exhibits enhanced durability, improved resistance to chloride penetration, sulfate attack, and alkali–silica reaction, making it particularly suitable for high-performance concrete, marine structures, bridges, and infrastructure exposed to aggressive environments.

Beyond construction applications, Silica fume is used as a functional additive and reinforcing filler in a wide range of industrial products.
In refractories and ceramics, Silica fume improves packing density, thermal stability, and mechanical strength.

In rubber, plastics, coatings, and sealants, Silica fume enhances reinforcement, rheological control, and surface properties.
Owing to its ultrafine particle size and high surface activity, Silica fume also serves as a flow modifier, anti-settling agent, and performance enhancer in specialty chemical formulations.

Silica fume is non-flammable, chemically stable under normal conditions, and environmentally inert when properly handled.
Silica fume is typically supplied in densified powder, slurry, or pelletized form to facilitate handling and reduce dust formation.

Applications of Silica Fume:
Silica fume is primarily used as a high-performance mineral admixture in concrete and cement-based materials to enhance strength, durability, and impermeability.
Silica fume is widely applied in high-performance and ultra-high-performance concrete, precast concrete elements, bridges, tunnels, marine structures, and infrastructure exposed to aggressive chemical or environmental conditions.

In refractory and ceramic applications, Silica fume is used to improve packing density, thermal resistance, and mechanical strength.
Silica fume is also utilized as a functional filler in rubber, plastics, coatings, sealants, and adhesives, where it enhances reinforcement, rheology control, and surface properties.
Additionally, Silica fume is employed in grouts, mortars, repair materials, and specialty construction products, as well as in selected chemical and industrial formulations requiring fine particle size and high silica reactivity.

Concrete:
Because of its extreme fineness and high silica content, Silica fume is a very effective pozzolanic material.
Standard specifications for Silica fume used in cementitious mixtures are ASTM C1240, EN 13263.

Silica fume is added to Portland cement concrete to improve its properties, in particular its compressive strength, bond strength, and abrasion resistance.
These improvements stem from both the mechanical improvements resulting from addition of a very fine powder to the cement paste mix as well as from the pozzolanic reactions between the Silica fume and free calcium hydroxide in the paste.

Addition of Silica fume also reduces the permeability of concrete to chloride ions, which protects the reinforcing steel of concrete from corrosion, especially in chloride-rich environments such as coastal regions and those of humid continental roadways and runways (because of the use of deicing salts) and saltwater bridges.
Furthermore, Silica fumes has important uses in oil and gas operations.
Silica fume can be used for a primary placement of grout as a hydraulic seal in the well bore, or secondary applications such as remedial operations including leak repairs, splits, and closing of depleted zones.

Prior to the mid-1970s, nearly all Silica fume was discharged into the atmosphere.
After environmental concerns necessitated the collection and landfilling of Silica fume, it became economically viable to use Silica fume in various applications, in particular high-performance concrete.

Effects of Silica fume on different properties of fresh and hardened concrete include:

Workability:
With the addition of Silica fume, the slump loss with time is directly proportional to increase in the Silica fume content due to the introduction of large surface area in the concrete mix by its addition.
Although the slump decreases, the mix remains highly cohesive.

Segregation and bleeding:
Silica fume reduces bleeding significantly because the free water is consumed in wetting of the large surface area of the Silica fume and hence the free water left in the mix for bleeding also decreases.
Silica fume also blocks the pores in the fresh concrete so water within the concrete is not allowed to come to the surface.

Silicon carbide:
The Silica fumes, as byproduct, may be used to produce silicon carbide.

Silica fume for Cement:
Silica fume can fill the pores between cement particles.
Silica fume 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.

For Refractory Materials:
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.

Benefits of Silica Fume:
Silica fume provides significant improvements in mechanical strength by enhancing the formation of calcium silicate hydrate in cementitious systems, resulting in higher compressive, tensile, and abrasion resistance.
Silica fume markedly reduces porosity and permeability, which improves durability and resistance to chloride penetration, sulfate attack, and other aggressive chemical environments.

The ultrafine particle size of Silica fume improves particle packing and microstructure refinement, leading to denser and more uniform materials.
In addition to construction benefits, Silica fume enhances reinforcement, rheological control, and thermal stability in industrial applications such as refractories, polymers, coatings, and ceramics.
Silica fume is non-flammable, chemically stable, and environmentally inert, offering long-term performance improvements without compromising safety or compatibility.

Advantages of Silica Fume:
Silica fume is a by-product of silicon or ferrosilicon production and consists of ultra-fine (sub-micron), amorphous, non-porous, perfectly spherical silicon dioxide (SiO₂) particles with purity levels ranging from 85–99%.
The primary effect of these ultra-fine particles is a significant improvement in particle packing and the generation of a highly pozzolanic reaction in concrete and other cementitious binder systems, where silicon dioxide reacts with calcium hydroxide; this leads to enhanced performance levels, strength, and durability of these materials.

In addition, due to its amorphous (non-crystalline) structure and high specific surface area, Silica fume provides beneficial properties in various applications, including improved sintering capability of refractory castables and enhanced high-temperature performance, resulting from reactions between Silica fume and other components of refractory mixes.
In summary, Silica fume has two key characteristics that form the basis of its high performance, either individually or in combination:

An efficient “filler effect” arising from the size of Silica fume's spherical, non-porous primary particles, which are approximately 0.15 μm (150 nm) in diameter.
A “chemical effect” resulting from Silica fume's extremely high reactivity due to its amorphous structure and high specific surface area.

Properties of Silica Fume:
Silica fume is an ultrafine material with spherical particles less than 1 μm in diameter, the average being about 0.15 μm.
This makes Silica fume approximately 100 times smaller than the average cement particle.

The bulk density of Silica fume depends on the degree of densification in the silo and varies from 130 (undensified) to 600 kg/m3.
The specific gravity of Silica fume is generally in the range of 2.2 to 2.3.

The specific surface area of Silica fume can be measured with the BET method or nitrogen adsorption method.
Silica fume typically ranges from 15,000 to 30,000 m2/kg.

Production of Silica Fume:
Silica fume is a byproduct in the carbothermic reduction of high-purity quartz with carbonaceous materials like coal, coke, wood-chips, in electric arc furnaces in the production of silicon and ferrosilicon alloys.

Silica fume is generated as a by-product during the production of silicon metal and ferrosilicon alloys in electric arc furnaces.
In this process, high-purity quartz is reduced with carbon-based materials at temperatures exceeding 2 000 °C, forming silicon or ferrosilicon and releasing silicon monoxide (SiO) vapor.
As the hot exhaust gases leave the furnace and encounter oxygen, the silicon monoxide rapidly oxidizes and condenses into extremely fine, amorphous silica particles.

These ultrafine particles are captured from the furnace off-gases using high-efficiency collection systems such as baghouse filters or electrostatic precipitators.
After collection, the Silica fume may undergo further processing, including densification or pelletization, to improve bulk density, reduce dust formation, and facilitate handling and transportation.
Depending on application requirements, Silica fume is supplied as undensified powder, densified powder, pellets, or as an aqueous slurry.

History of Silica Fume:
The first testing of Silica fume in Portland-cement-based concretes was carried out in 1952.
The biggest drawback to exploring the properties of Silica fume was a lack of material with which to experiment.

Early research used an expensive additive called fumed silica, an amorphous form of silica made by combustion of silicon tetrachloride in a hydrogen-oxygen flame.
Silica fume on the other hand, is a very fine pozzolanic, amorphous material, a by-product of the production of elemental silicon or ferrosilicon alloys in electric arc furnaces.
Before the late 1960s in Europe and the mid-1970s in the United States, Silica fumes were simply vented into the atmosphere.

With the implementation of tougher environmental laws during the mid-1970s, silicon smelters began to collect the Silica fume and search for its applications.
The early work done in Norway received most of the attention, since it had shown that Portland cement-based-concretes containing Silica fumes had very high strengths and low porosities.
Since then the research and development of Silica fume made it one of the world's most valuable and versatile admixtures for concrete and cementitious products.

Stability and Reactivity of Silica Fume:

Chemical Stability:
Silica fume is chemically stable under normal ambient temperatures and recommended storage conditions and does not undergo hazardous decomposition during routine handling.

Reactivity:
Silica fume is largely inert and shows very low chemical reactivity.
However, due to its very high specific surface area, Silica fume may exhibit increased reactivity toward aggressive chemicals under extreme conditions.

Conditions to Avoid:
Avoid excessive dust generation, contact with molten strong acids or bases, and exposure to extremely high temperatures in the presence of reactive chemicals.

Incompatible Materials:
Hydrofluoric acid, strong alkalis at elevated temperatures, strong fluorinating agents, and reactive metals should be avoided.

Hazardous Decomposition Products:
No hazardous decomposition products are expected under normal use; at very high temperatures or in contact with incompatible materials, silicon tetrafluoride or other corrosive vapors may form.

Handling and Storage of Silica Fume:

Safe Handling:
Handle Silica fume in well-ventilated areas to minimize airborne dust.
Use enclosed systems or controlled transfer methods where possible and avoid generating fine dust clouds that may cause respiratory irritation.

Hygiene Measures:
Wash hands and exposed skin thoroughly after handling.
Avoid eating, drinking, or smoking in areas where Silica fume is handled.
Remove contaminated clothing before entering clean areas.

Storage Requirements:
Store in tightly closed containers made of moisture-resistant materials in a cool, dry, and well-ventilated environment.

Packaging Integrity:
Keep containers sealed when not in use to prevent moisture uptake and dust release.

Shelf Stability:
Silica fume remains stable for long periods when stored dry at room temperature; moisture absorption may affect handling properties but not chemical identity.

First Aid Measures of Silica Fume:

Inhalation:
Move the exposed person to fresh air immediately and keep at rest.
If coughing, throat irritation, or breathing difficulty persists, seek medical attention.

Skin Contact:
Wash skin with water to remove dust.
Silica fume is not chemically irritating but may cause mechanical dryness; apply skin moisturizer if needed.
Seek medical attention if irritation persists.

Eye Contact:
Rinse eyes gently with clean water for several minutes while holding eyelids open.
Remove contact lenses if present and easy to do.
Seek medical attention if discomfort continues.

Ingestion:
Rinse mouth with water.
Ingestion of small amounts is not expected to be harmful.
Seek medical advice if large quantities are swallowed or discomfort occurs.

Firefighting Measures of Silica Fume:

Flammability:
Silica fume is nonflammable, noncombustible, and does not support combustion.

Suitable Extinguishing Media:
Use extinguishing media appropriate for surrounding materials, such as water spray, foam, dry chemical, or carbon dioxide.

Hazardous Combustion Products:
Silica fume itself does not decompose under fire conditions; surrounding materials may release hazardous fumes.

Special Protective Equipment for Firefighters:
Firefighters should wear self-contained breathing apparatus and full protective gear when fires involve surrounding materials.

Specific Hazards:
Airborne dust may reduce visibility during firefighting; avoid dispersing dust with high-pressure water jets.

Accidental Release Measures of Silica Fume:

Personal Precautions:
Avoid breathing dust and ensure adequate ventilation.
Wear appropriate personal protective equipment to prevent inhalation and eye contact.

Environmental Precautions:
Silica fume is insoluble and environmentally inert; prevent formation of large airborne dust clouds near air intakes or sensitive equipment.

Cleanup Methods:
Collect material using gentle sweeping, HEPA-filter industrial vacuuming, or damp methods to minimize dust formation.
Do not use compressed air.

Additional Advice:
Dispose of collected material in accordance with local regulations; Silica fume is generally classified as non-hazardous waste.

Exposure Controls / Personal Protective Equipment of Silica Fume:

Engineering Controls:
Use local exhaust ventilation, enclosed transfer systems, and dust collection units with HEPA filtration to control airborne particles.

Respiratory Protection:
Use particulate respirators such as N95 or P2 when dust concentrations exceed recommended occupational exposure limits.

Hand Protection:
Wear protective gloves to prevent mechanical dryness; nitrile, latex, or PVC gloves are suitable.

Eye Protection:
Use safety glasses or goggles to protect against airborne dust.

Skin and Body Protection:
Wear lightweight protective clothing to minimize skin contact and contamination.

Environmental Exposure Controls:
Implement dust containment and filtration systems in processes generating high particulate loads.

Identifiers of Silica Fume:
Product Name: Silica fume
Chemical Name: Amorphous Silicon Dioxide
Common Name: Microsilica
CAS Number: 69012-64-2
EC / EINECS Number: 273-761-1
Molecular Formula: SiO₂
Molecular Weight: 60.08 g/mol
Chemical Family: Inorganic Oxides – Amorphous Silica
Product Type: Pozzolanic Mineral Admixture, Functional Filler

Properties of Silica Fume:
Physical State: Solid powder
Appearance: Very fine, gray to dark gray powder (may appear light gray to nearly black depending on carbon content)
Odor: Odorless
SiO₂ Content: Typically ≥ 90 percent (may exceed 95 percent for high-purity grades)
Particle Size: Ultrafine; average particle diameter approximately 0.1–0.3 µm
Specific Surface Area: Very high, typically 15–30 m²/g (BET)
Bulk Density: Low in undensified form; higher in densified or pelletized grades
Solubility in Water: Insoluble
Solubility in Organic Solvents: Insoluble
Crystallinity: Amorphous (non-crystalline)
Pozzolanic Activity: Very high
Thermal Stability: Stable at high temperatures; does not melt or decompose under normal processing conditions