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What is the introduction of silica fume?

May. 13, 2024

Silica Fume Introduction - Advanced Cement Technologies

Silica Fume Introduction
Silica Fume
Supplementary Cementitious Material

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The American Concrete Institute (ACI) defines silica fume as "very fine non-crystalline silica produced in electric arc furnaces as a by-product of the production of elemental silicon or alloys containing silicon" (ACI 116R). It is usually a gray colored powder, somewhat similar to portland cement or some fly ashes.

Silica fume is typically classified as a supplementary cementitious material. This term refers to materials used in concrete alongside portland cement.

Silica fume is frequently referred to by other names:

Condensed silica fume
Microsilica
Volatilized silica

Silica fume is a by-product of producing silicon metal or ferrosilicon alloys in smelters utilizing electric arc furnaces. These metals are utilized in various industrial applications, including aluminum and steel production, computer chip fabrication, and production of silicones, which are widely used in lubricants and sealants. Though these materials are invaluable, the by-product silica fume holds greater importance for the concrete industry.

Densified and Undensified Silica Fume

Historically, silica fume has been available in three basic product forms: undensified, slurried, and densified. After extensive testing over the years, no data suggests that any one form performs better in a concrete mixture than the others.

Slurried silica fume is no longer available in the U.S. market. Undensified silica fume is available but is not commonly used in ready-mixed or precast concrete. This form is primarily utilized in pre-bagged products, such as grouts or repair mortars.

Densified silica fume is produced by treating undensified silica fume to increase its bulk density to a maximum of about 400 to 720 kg/m3. This bulk density increase is typically achieved by tumbling the silica-fume particles in a silo, allowing surface charges to build up and draw the particles together to form weak agglomerates. The increased bulk density makes this material more economical for truck transportation.

Densified silica fume performs well in concrete. However, adequate mixing is crucial to break up particle agglomerations. In applications like dry mix shotcrete, roof tiles, or other uses where coarse aggregate is absent, mixing may not be sufficient to break up the agglomerates. In these situations, undensified silica fume might be more suitable. Contact the Silica Fume Association for assistance in these specific applications.

REFERENCES

Information provided by US DEPARTMENT OF TRANSPORTATION – Federal Highway Administration – Silica Fume Association - April 2005

Silica fume

Silicon dioxide nanoparticles

Not to be confused with fumed silica

Silica fume particles viewed in a transmission electron microscope

Silica fume, also known as microsilica, (CAS number 69012-64-2, EINECS number 273-761-1) is an amorphous (non-crystalline) polymorph of silicon dioxide, silica. It 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.

It is sometimes confused with fumed silica (also known as pyrogenic silica, CAS number 112945-52-5). However, the production process, particle characteristics, and fields of application of fumed silica are all different from those of silica fume.

History

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 the 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.

Properties

Silica fume is an ultrafine material with spherical particles less than 1 µm in diameter, the average being about 0.15 µm. This makes it 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. It typically ranges from 15,000 to 30,000 m2/kg.

Components of cement:
comparison of chemical and physical characteristics:

Property	Portland cement	Siliceous fly ash	Calcareous fly ash	Slag cement	Silica fume
Proportion by mass (%)	21.9	52	35	35	85-97
SiO2	21.9	52	35	35	85-97
Al2O3	6.9	23	18	12	—
Fe2O3	3	11	6	1	—
CaO	63	5	21	40	< 1
MgO	2.5	—	—	—	—
SO3	1.7	—	—	—	—
Specific surface (m2/kg)	370	420	420	400	15,000–30,000
Specific gravity	3.15	2.38	2.65	2.94	2.22
General purpose	Primary binder	Cement replacement	Cement replacement	Cement replacement	Property enhancer

Production

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.

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Applications

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, particularly its compressive strength, bond strength, and abrasion resistance. These improvements stem from both the mechanical enhancements resulting from the addition of a very fine powder to the cement paste mix and 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 humid continental roadways and runways (due to 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, particularly high-performance concrete. The 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 the increase in silica fume content due to the introduction of a large surface area in the concrete mix by its addition. Although the slump decreases, the mix remains highly cohesive.
Segregation and bleeding: Silica fume significantly reduces bleeding as the free water is consumed in wetting the large surface area of the silica fume. Consequently, the free water left in the mix for bleeding decreases. Silica fume also blocks the pores in the fresh concrete, preventing water within the concrete from coming to the surface.