When silica fume is added to concrete, initially it remains inert. Once portland cement and water in the mix start reacting with each other (hydrating), primary chemical reactions produce two chemical compounds: Calcium Silicate Hydrate (CSH), which is the strength producing crystallization, and Calcium Hydroxide (CH), a by-product also called free lime which is responsible for nothing much other than lining available pores within concrete as a filler or leaching out of inferior concrete. Pozzolanic reaction occurs between silica fume and the CH, producing additional CSH in many of the voids around hydrated cement particles. This additional CSH provides the concrete with not only improved compressive, flexural and bond- strength but also a much denser matrix, mostly in areas that would have remained as small voids subject to possible ingress of deleterious materials.
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The word pozzolanic is derived from the name of a town in Italy, Pozzuoli. It was there that the ancient Romans combined ground lime stone with volcanic ash to produce mortars to bind large stones together. The durability of the resulting structures can still be witnessed today, two thousand years later.
The transport properties through the silica fume concrete medium are dramatically curtailed, i.e. liquid compounds and even electrical currents experience a diminished capability to migrate, resulting in very low permeability and high electrical resistivity. Silica fume&#;s benefits are already evident in the fresh concrete state before it begins to harden. Its small particle size which is 100 times finer than ordinary portland cement complements the finess modulus of concrete and provides a ball-bearing effect, which improves thixotropic behavior, in effect modifying concrete viscosity. Because of the high surface area of silica fume particles affecting the mobility of water within concrete, segregation and bleeding of concrete are virtually eliminated. Rheological benefits inherent in silica fume concrete allow for custom-tailoring concrete placement methods, such as very high cohesive workability, ability of fluid concrete to hold slope,and/ or long distance pumping of concrete.
Not to be confused with fumed silica
Silica fume particles viewed in a transmission electron microscopeSilica fume, also known as microsilica, (CAS number -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 -52-5). However, the production process, particle characteristics and fields of application of fumed silica are all different from those of silica fume.
History
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The first testing of silica fume in Portland-cement-based concretes was carried out in . 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 s in Europe and the mid-s in the United States, silica fumes were simply vented into the atmosphere.
With the implementation of tougher environmental laws during the mid-s, 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
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Components of cement:Proportion by mass (%)
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)[d] 370 420 420 400 15,000Values shown are approximate: those of a specific material may vary.
ASTM C618 Class F
ASTM C618 Class C
Specific surface measurements for silica fume by nitrogen adsorption (BET) method, others by air permeability method (Blaine).
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.[4] 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.[5]
Production
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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.
Applications
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Concrete
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Because of its extreme fineness and high silica content, silica fume is a very effective pozzolanic material.[6][7] Standard specifications for silica fume used in cementitious mixtures are ASTM C,[8] EN .[9]
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.[10]
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.[11] 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. [12]
Prior to the mid-s, 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.[13] Effects of silica fume on different properties of fresh and hardened concrete include:
Silicon carbide
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The silica fumes, as byproduct, may be used to produce silicon carbide.
See also
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References
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Further reading
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