Silica fume is a form of silicon dioxide in amorphous or non-crystalline form. Silica fume is produced as a by-product in carbothermic reactions of silicon and ferrosilicon alloys.
It is greatly used as a pozzolanic admixture nowadays in the construction industry to enhance the properties of cement concrete-like strength, durability, resistance to corrosion, sea attack, chloride attack, etc.
What Is Silica Fume?
Silica fume is a by-product when the alloy of silicon and ferrosilicon is produced. It is an ultrafine powder having spherical particles of size about 150 mm in diameter. Silica fume is a very reactive pozzolana owing to its chemical and physical properties.
The word silica fume came into existence from the word- Pozzuoli- a town in Italy. In this village, the Romans in ancient times mixed limestone after grinding with volcanic ash to produce the mortar. In addition, this mortar was used to bind the stones together. Moreover, the construction was quite durable that lasted for thousands of years.
Silica Fume and Fumed Silica – Are they One?
Silica fume is often confused with fumed silica for its resemblance in names. But, these two are entirely different from each other. Right from the production process to the characteristics of particles- everything is quite different. Even the application of these two particles is not the same.
Silica Fume and Fly Ash – How are Both of Them Different?
Some of the key differences between silica fume and fly ash are given below:
Because of the small particle size of silica, silica fume is more reactive. In the earlier stages, its reactivity is especially more due to the presence of silicon dioxide.
Silica fume is produced as a by-product from the alloys of the silicon industry whereas fly ash is produced as a residue on the burning of coal.
The composition of silica fume is mostly composed of silicon dioxide- silicon dioxide is present to about 85 %. While in fly ash, the composition varies.
History of Silica Fume
The first examination of silica fume was done in 1952 on Portland Cement Concrete. The earlier research of silica fume proved quite expensive, as the material with which the silica fume was to be tested was fumed silica. Fumed silica is formed from the combustion of silicon tetrachloride in the hydrogen-oxygen flame.
Until the 1960s, the silica fumes were released into the atmosphere for this reason in the United States and Europe.
In the 1970s silica fumes started to be collected by silicon smelters and research on the applications of silica fume began in full fledge.
Silica Fume Properties
The properties of the silica fume are described below
The dimension of silica fume is much less than that of cement particles. The particles of silica fume are less than 1 micrometre in diameter. The average diameter of the particles is about 0.15 micrometre. Thus, these particles are about 100 times a minute than the cement particles.
The bulk density of the silica fume is about 130 to 600 kg/m3. 130 kg/m3 value is for unidentified silica fumes. Bulk density is dependent on the degree of densification in the silo.
The specific gravity of silica fume is in the range of 2.2 to 2.3.
Upon addition of silica fume to concrete, it does not start reacting instantly. For a small-time, it remains inert. As the Portland cement and water are added, the mixing starts which is termed hydration.
Two primary chemical compounds are produced through this hydration, calcium silicate hydrate, and calcium hydroxide. Calcium silicate hydrate aids in strength production while calcium hydroxide does not have many functions.
Availability of Silica Fume
The suppliers of the concrete admixtures also distribute the silica fume. Thus, the admixture supplier is also a silica fumes supplier. It is simply added to the concrete at the time of mixing.
Electron furnaces producing silicon and alloys of silicon produce silica fume as a by-product. Quartz, coal, and wood chips are used as raw materials and when these release smoke during the furnace operation, smoke is produced. This smoke is collected and sold as silica fume.
The quality specifications of silica fume are mentioned in the following- ASTM C 1240 and AASHTO M 307.
Advantages of Silica Fume
Following are some of the advantages listed down for silica fume:
- It gives high compressive strength earlier
- It also gives high flexural strength
- It increases the value of modulus of elasticity
- It reduces the permeability of concrete to chloride and water intrusion considerably.
- It increases the durability of the concrete
- It increases the toughness of the concrete
- It gives improved abrasion resistance to decks, overlays, floors, and marine structures
- It gives great resistance to chemicals like chlorides, nitrates, acids, and sulphates
- It reduces the cost of the concreting in its life by reducing the maintenance cost
- It gives increased bond strength to concrete
- It fills in voids present between the cement particles as its size is much smaller comparatively
- It increases the electrical resistance of concrete
- It decreases the permeability of concrete to water
Application of Silica Fume In Concrete
Silica fume is used as a pozzolanic material in the concrete because of its fine size and high silica content. However, when silica fume is added to the concrete, special attention should be paid to the placing, finishing as well as curing of concrete. The specifications for the same are specified by the American Concrete Institute abbreviated as ACI.
Upon addition of silica fume to concrete, the properties of concrete like compressive strength, bond strength as well as abrasion resistance are increased. Silica fume also helps in reducing the attack of chloride ions; hence, the steel reinforcement can be protected from corrosion. Hence, silica fume is useful in coastal areas. Thus, silica fume helps in enhancing the properties of high performance concrete.
High performance concrete containing silica fume is extensively used in the infrastructure development of the nation. Besides giving high strength and durability to the high performance concrete, silica fume also gives toughness, and resistance to various deteriorating characteristic such as abrasion, corrosion, and chemicals causing the detrimental effect. It also helps in making the structure cost-effective in the long run.
Some of the infrastructures that are subjected to deterioration constantly because of rebar corrosion current; abrasion and chemical attack are highway bridges, parking decks, bridge decks, and marine structures. Silica fume renders protection to concrete against sea attack, traffic, as well as heavy impact. As concrete deterioration is almost none because of silica fume, the maintenance cost is considerably reduced.
The increased strength of the concrete also gives the architect and designers design flexibility. Also, a larger usable area is available as the concrete with silica fume permits smaller columns.
Apart from high strength concrete and ultra high strength concrete, silica fume is also used while mass concreting. It is normally used by 7-8 %. It can be used along with fly ash too. Silica fume helps in increasing the tensile strength of concrete. This reduction helps in overcoming thermal cracking. Thermal crack results when stress in concrete is greater than its tensile strength.
Another concreting application of silica fume is self-compacting concrete. Adding silica fume reduces the slump value of concrete used in self-compacting concrete. Silica fume can be helpful if used in vertical elements of structure like columns, walls, etc. Silica fume increases the viscosity and hence, concrete can be cast into these elements without the fear of segregation.
The specific gravity of cement is 2.2. Thus, it is lighter than cement. Thus, silica fume can be used to produce lightweight concrete that will give the same or even higher strength as when cement was added. It also increases viscosity, which prevents the floating of some of the lightweight aggregates.
The concreting for high rise buildings can also be done with the addition of silica fume. Silica fume along with fly ash increases pumpability and reduces the segregation of concrete. This is due to the fact that the viscosity of concrete is increased.
Silica fume also affects the mechanical properties of fresh concrete as described below:
Workability- As silica fume is added to the concrete, the surface area is increased. And slump loss is directly proportional to the slump loss. Even with the loss in the sump, the cohesive properties are maintained.
Segregation and bleeding- Segregation and bleeding should not occur in the concrete. Both are detereminous for quality construction. Silica fume reduces both segregation and bleeding as it provides a large surface area. To wet this large surface area, more amount of water is required and thus a large amount of water is consumed. Thus, water is not available for bleeding. Moreover, the pores are blocked by the silica fume, so the concrete cannot travel up to the surface.
As the silica fume is introduced into the concrete, the microstructure of the concrete undergoes change as millions of fine minute particles are added to the concrete. These minute particles fill up the voids between the cement. It is the same concept as that of fine aggregates filling the space between coarse aggregates and cement paste filling pores between the fine aggregate. This process of filling the voids between the cement particles is known as particle packing or micro filling too.
Application of Silica Fume
Following are some of the fields enlisted for which or in which the silica fume finds its application-
- High performance concrete
- Self compacting concrete
- Mass concreting
- High strength and low permeability concrete
- Flooring of industries
- Water tanks
- Chemical storage tanks
- Pipelines for sewerage
- Refractory mix
- Tunnels, etc.
Precautions while using Silica Fume
Silica fume can prove hazardous when inhaled inside the body.
Silica fume can also irritate the eyes if it comes in contact with the eye. And if the exposure of the silica fume to the eyes is repeated multiple times, eyes can be damaged too.
If a person is exposed to silica fumes for a long time, then flu-like infections like headache, fever, aches, chest tightness and cough.
Silica Fume Price
Silica fume is available in powdered form in the retail market. It is filled in the HDPE bag at a weight of 25 kg. Its price ranges from 20 to 25 Rs. Per kg.
Its colour is either grey or greyish blue.
The silicon content present in the particular bag is mentioned on the bag in its technical specifications. Silicon content should be more than 85 %.
Silica Fume at a Glance
Silica fume is a fine powder formed while producing the alloy of silicon and ferrosilicon in an amorphous or non-crystalline form. It gains reactive pozzolanic properties because of its physical and chemical properties which include dimension, bulk density, specific gravity, and hydration.
Silica fume should not be confused with fly ash. Silica fume is more reactive than fly ash, especially in earlier stages. Silica fume and fumed silica- both are different too. Their manufacturing process, properties, and even their applications are different.
The particles of silicon dioxide are about 100 times smaller than that of cement. Thus, they help in filling up the voids between cement particles as cement fills the voids between aggregates. As silica fume is added to concrete, it remains inert initially, however, afterwards on the addition of cement and water, the reaction starts which is called hydration. Calcium silicate hydrate produced as a product of hydration of silica fume imparts strength to the concrete.
Silica fume is formed from electric furnaces and collected. It is distributed by the suppliers of the admixtures used in concrete. ASTM C 1240 and AASHTO M 307 gives their specifications.
Silica fume is used in concrete to increase its strength and durability. It also increases compressive strength, bond strength and abrasion resistance. It helps in preventing corrosion of the reinforcing steel too.
Silica fume is applied in the following types of concrete:
- High performance concrete
- Mass concreting
- Self compacting concrete
Care should be taken that silica fume is not inhaled by the body. Long exposure to silica fume may lead to flu-like infections like headache, fever, aches, chest tightness, cough, etc.
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