The terms microsilica, condensed silica fume, silica fume are often used to describe by-products extracted from the exhaust gases of ferrosilicon, silicon and other metal alloy smelting furnaces. However the terms silica fume and microsilica are used to define those condensed silica fumes that are of high quality for using in the cement and concrete industry. The term of silica fume has been also used in the European standard.
Silica fume was first obtained in Norway in 1947, when the environmental controls made the filtering of the exhaust gases from furnaces. The main portion of theses fumes was a much fine composed of a high percentage of silicone dioxide. As the pozzolanic reactivity for silicon dioxide was well known, vast researches have done about it. There are over 3000 publications now available which detail work on silica fume and silica fume concrete.
Silica fume conforming to AASHTO M 307 or ASTM C 1240 can be utilized as a supplementary cementations material for increasing strength and durability. Based on Florida DOT (2004), the quantity of cement replacement with silica fume should be between 7% and 9% by mass of cementation materials.
Silica fume consist of much fine particles whit specific surface about six times of cement so that it is very much finer than cement particles. So, it has been found that if silica fume mixes with concrete the minute pore spaces can decrease resulting in high strength concrete. Silica fume is pozzolanic, i.e., it is reactive, like volcanic ash. Its effects are relative to the strength, modulus, ductility, sound absorption, vibration damping capacity, abrasion resistance, air void content, bonding strength with reinforcing steel, shrinkage, permeability, chemical attack resistance, alkali-silica reactivity reduction, creep rate, corrosion resistance of embedded steel reinforcement, freeze-thaw durability, coefficient of thermal expansion (CTE), specific heat, defect dynamics, thermal conductivity, dielectric constant, and degree of fiber dispersion in mixes containing short microfibers. Also, silica fume addition decreases the workability of the mix.
Silica fume can handle problems, due to very loose bulk density and much fine particles. It can also create other problem such as stickiness and bridging in storage silos and clogging of the pneumatic transport equipment.
2. Silica fume source
It is much fine noncrystalline silica manufactured by electric arc furnaces as a by-product of the production of metallic silicon or ferrosilicon alloys. The raw materials are coal, quartz, and woodchips. The smoke that results from furnace operation is stored and sold as silica fume, rather than being landfilled.
Figure1. Package of silica fume in factory
3. Physical properties
The properties of silica fume depend on the type of the product produced and the process used for its manufacture. Anyway it is in form of spherical particle shape. It is a powder with particles having diameters 100 times smaller than Portland cement particles. Silica fume comes in three forms of powder, condensed and slurry.
The color of silica fume varies from light to dark grey. The color depends upon the process for manufacturing, i.e., wood chip composition, furnace temperature, ratio of wood chip to the coal used, exhaust temperature and type of metal produced (William Andrew, 1996).
Figure 2. Scanning electron microscopy of condensed silica fume (Chai Jaturapitakkul, 2003)
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Table 1. Physical properties of OPC, MK and SF (H. Abdul Razak, H.S. Wong, 2004)
Figure 3. Silica fume in form of powder
4. Chemical properties
As a brief, properties of cement and silica fume will be compared in the following table.
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Table 2 - Properties of cement and silica fume (Fuat Ko¨ksal, Fatih Altun, et al, 2007)
5. Production and extraction
Silica fume is produced during a high-temperature reduction of quartz in an electric arc furnace when the most important product is silicon or ferrosilicon. Due to the large amount of electricity needed, theses arc furnaces are located in countries with well-provided electrical capacity including Scandinavia, Europe, Canada, the USA, South Africa and Australia.
The chemistry of process is complex and depends on temperature. The SiC formed, initially plays important intermediate roles.
At temperatures > 1520 ÌŠ C
SiO2 + 3C = SiC+2CO
At temperatures > 1800 ÌŠ C
3SiO2+2SiC = Si+4SiO+2CO
The unstable gas goes up in the furnace where it reacts with oxygen to give the silicon dioxide
4SiO+2O2 = 4SiO2
6. Workability
With addition of the silica fume the slump loss with time is proportional increase in concrete mix. Due to the large surface area of silica fume particles in the concrete mix, workability and consistency decrease (Collins, 1999).
These are restrains against suitable utilization of silica fume concrete. However, the consistency of silica fume mortar is greatly increased by either using silane treated silica fume, i.e., silica fume which has been coated by a silane coupling agent prior to incorporation in the mix, or utilizing silane as an additional admixture (Idem, 2000).
7. Vibration damping capacity
Vibration reduction is useful for structural stability, hazard mitigation, and structural performance improvement.
Effective vibration reduction requires both stiffness and damping capacity. Silica fume is effective for increasing both damping capacity and stiffness (Idem, 2000).
8. Sound absorption
Sound or noise absorption is helpful for numerous structures, such as noise barriers and pavement overlays. The
addition of silica fume to concrete increases the sound absorption ability ( M. Tamai and M. Tanaka, 1994).
9. Health and safety
The same as all fine powder, there are potential for health risks, particularly in relation to silicon dioxide and the lung disease silicosis.
10. Available forms of silica fume
As the silica fume powder particles are a hundred times finer than ordinary Portland cement, there are dispensing consideration and transportation and storage to be taken in to account.
To provide some of these difficulties the material is commercially in various forms. The differences between these forms are related to the size particles shape and do not greatly affect the chemical make-up or reaction of material.
These differences will influence the areas of use. So careful thinking should be given to the type of silica fume chosen for a specific application.
11. Undensified
Bulk density is in range of 200-350 kg/m3. Due to the low bulk density this form is considered impractical to utilize in normal concrete production.
The important area of use is in refractory products and formulated bagged material such as mortars, grouts, protective coatings and concrete repairs system.
12. Densified
Bulk density is in range of 500 -650 kg/m3.In the densification process the ultra fine particles becomes loosely agglomerated making the size of particles larger. This makes the powder easier to handle, with less dust in compare to the untensified forms.
Areas where this material is used are in those processes that utilize high shear mixing facilities such as concrete rooftile works, pre-cast works and readymixed concrete plants with wet mixing units.
13. Effects on setting and hardening concrete
AS the concrete sets and hardens the pozzolanic action of the silica fume starts from the physical effects, the silica fume has reaction with the calcium hydroxide to produce calcium silicate and aluminates hydrates.
Theses both increase the strength and decrease the permeability by densifying the matrix of the concrete.
According to the literature review, silica fume having a greater surface area and higher silicon dioxide content is much more reactive than pfa or ggbs (Regourd, 1983).
This increased reactivity is caused an increase to the rate of hydration of C3S fraction of the cement in the first instance (Andrija, 1986), thus creating more calcium hydroxide, but settles down to more normal rates further away two days.
14. Effect of silica fume on compressive strength of concrete
High compressive is normally the first property associated with silica fume concrete.
A lot of reports (Sellevold and Radjy, 1983; Loland and Hustad,1981) have shown that the addition of silica fume to a concrete mix will increase the strength of that mix by between 30% and 100% dependent on the type of cement, type of mix, use of plasticizers, amount of silica fume, aggregates type and curing regimes.
Generally, for an equal strength, an increase will be achieved in the w/c ratio while for a given w/c ratio an increased strength will result (Shellevold and Radjy, 1983).
Figure 4. Reaction between compressive strength and age for concrete incorporating various percentages of silica fume as an addition to cement (John brian Newman, B. S. choo, 2003)
15. Effect of silica fume on tensile and flexural strength of concrete
The relationship between tensile, flexural and compressive strengths in silica fume concrete is the same as those for ordinary strength concrete. An increase in compressive strength using silica fume will result in the same relative increase in the tensile and flexural strength. This plays an important role when silica fume concrete is used in bridging, flooring, and roadway projects. The increased tensile strength causes a possible reduction in slab thickness, maintaining high compressive strengths, thus reducing overall slab weight and cost.
16. Effect of silica fume on brittleness and modulus of elasticity
The stronger concrete is more brittle and silica fume concrete is no exception to this rule. E modulus does not follow the pattern of tensile strength and only displays slight increase in comparison to compressive strength. So, high and ultra-high strength concrete can be used for tall structures without loss of ductility (Larrard, 1987).
17. Effect of silica fume on bonding
Silica fume concrete has a much finer phase in compare to ordinary concrete and the good bonding to substrates. Research has shown (Carles-Gibergues, 1982) that the aggregate-cement interface changes when silica fume is present. Bonding to fibres is greatly increased (Krenchle and Shah, 1985). This is particularly useful in the steel fibre/silica fume modified shotcrete which is used in Scandinavia.
18. Effect of silica fume on creep rate
The addition of untreated silica fume to cement paste reduces the compressive creep rate at 200-C from 1.3-10−5 to 2.4-10−6 min−1 (Fu and Chung, 1999).
19. Effect of silica fume on shrinkage
Shrinkage in cement pastes is increased when silica fume is used. In concrete the shrinkage is related to the aggregate quality and aggregate volume and low w/c ratio concrete. Properly designed and produced and finished silica fume concrete normally gets very low rating.
20. Effect of silica fume on carbonation
Based on literature review (Johansen, 1981) for equal strengths and any concretes strengths below 40 MPa carbonation is higher in silica fume concretes. Concretes above 40 MPa give a reduction in carbonation rate but, theses concretes are influenced by attack and damage if there is reinforcement present. A silica fume concrete is normally utilized where the compressive strengths are above 40 MPa .It is an issue as to whether carbonation is a serious risk. Concrete curing procedures are necessary to make sure about optimum performance of the silica fume concrete.
21. Effect of silica fume on efflorescence
Efflorescence not only increases the aesthetic quality of the structures but can also give an increase in permeability, porosity and ultimately a weaker and less durable concrete. Research has shown (Samuelsson, 1982) that addition of silica fume will decrease efflorescence due to refined pore structure and increased consumption of the calcium hydroxide.
22. Effect of silica fume on frost resistance - freeze-thaw durability
The durability of silica fume concrete to freeze thaw is normally satisfactory at silica fume content of less than 20%. Freeze-thaw durability is relative to the ability to withstand changes between temperatures above 0-C and those below0 -C. Due to the presence of water, which undergoes freezing and thawing and also in turn cause changes in volume, concrete shows a tendency to decrease upon such temperature cycling. Air voids which are called air entrainment are utilized as cushions to accommodate the changes in volume, thereby improving the freeze-thaw durability. The addition of silica fume to mortar enhances the freeze-thaw durability, in spite of the poor air void system. Anyway, the use of air entrainment is still recommended (Lachemi, 1998).
23. Effect of silica fume on permeability
The permeability of chloride ions in concrete reduces by the addition of untreated silica fume. In related to this effect, there is a reduction in the water absorbance. Both effects are due to the microscopic pore structure due to the calcium silicate hydrate (CSH) formed upon the pozzolanic reaction of silica fume with free lime within the hydration of concrete (Poon and Wang, 1999).
24. Effect of silica fume on steel bar corrosion resistance and chemical attack resistance
The addition of untreated silica fume to steel reinforced concrete improves the corrosion resistance of the reinforcing steel and also increases the chemical attack resistance, whether the chemical is acid, chloride, sulfate, etc. Theses are related to the reduction in the permeability (Hou and Chung, 2000).
25. Effect of silica fume on fiber dispersion
Short microfibers, such as glass, carbon, polypropylene, steel and other fibers are used as an admixture in concrete to improve the tensile and flexural properties and reduce the drying shrinkage.
Effective use of the fibers, which are consumed in very small quantities such as 0.5% by weight of cement in the case of carbon fibers, needs good dispersion of the fibers. The addition of untreated silica fume to microfiber reinforced cement enhances the degree of fiber dispersion, due to the fine silica fume particles helping to the mixing of the microfibers. Silica fume also increases the structure of the fiber-matrix interface, decreases the weakness of the interfacial zone and also the number and size of cracks (Park and Yoon, 1999).
26. Effect of silica fume on thermal conductivity
Concrete of low thermal conductivity is helpful for the thermal insulation of buildings. On the other hand, concrete of high thermal conductivity is useful for decreasing temperature gradients in structures.
The thermal stresses which result from temperature gradients may cause mechanical property reduction in the structure. Bridges are among structures that tend to be faced with temperature differentials between their top and bottom surfaces. In contrast to buildings, which also is faced temperature differentials; bridges do not require thermal insulation. Hence, concrete of high thermal conductivity is in demand for bridges and related structures.
The thermal conductivity reduced by the addition of untreated or silane treated silica fume (Idem, 2000), due to the interface between silica fume particles and cement acting as a obstacle against heat conduction. However, the thermal conductivity is improved by the use of silane and untreated silica fume as two admixtures due to the network of covalent coupling improvement heat conduction through phonons.
27. Effect of silica fume on bleeding
Silica fume decreases bleeding remarkably, because the free water is used in wetting of the large surface area of the silica fume and hence the free water left in the mix water for bleeding also decrease. Silica fume also blocks the pores in the fresh concrete; hence water within the concrete cannot come to the surface.
