Silica fume in concrete : what is silica fume, properties, advantages, disadvantages and application

What is silica fume?

Silica fume is an artificial pozzolanic material also called 'micro silica' or 'condensed silica fume'. It is a non-crystalline polymorph of silicon dioxide.

Silica fume is ultrafine power which is a by-product resulting from the reduction of high purity quartz with the coal in an electric arc furnace in the manufacturing of silicon or ferrosilicon alloy. 

It consists of microspherical particles.

Micro silica fume
96 % Densified silica fume Image credit-
 http://www.microsilica-fume.com/product/96-silica-fume.html

Production

The reduction of high purity quartz to silicon with the temperature up to 2000℃ makes silicon dioxide vapors. 

which oxidized, and condensed in the low-temperature zone and then collected in the cloth bags. 

This condensed silica fume consists of 90 % of non-crystalline spherical ultrathin particles of silicon dioxide.

The average diameter of these particles is around 0.1 micron, which is 100 times smaller than cement particles. 

Properties of silica fume

Physical properties


Silica fume particles can be treated as nanoparticles. As most of the particles are below 100 nanometers. 

It is available in premium white and standard white colors.

It has a specific surface area of 20,000 square meters per kg.
The specific gravity of the silica fume is 2.2.

The average bulk density of silica fume is 586 kg per meter cube.

The size distribution of silica fume are provided as 99 % below 0.50 micrometer, 95 % below 0.20 micrometer, 70 % below 0.10 micrometer, 20% below 0.10 micrometer. 

Chemical properties


Silica fume contains more than 90 % of Silicon oxide which means silicon oxide is the main constituent.

Other constituents in silica fume are oxide of aluminum, iron, calcium, magnesium, potassium, sodium, carbon, chlorine, Sulpher. 

The constituents of the silica fume can be seen from the below figure.


Chemical properties of silica fume
Chemical composition of silica fume

Silica fume in concrete


Silica fume is very efficient in the concrete. It is available in powder form and slurry form. 

When working with very fine particles, it can cause some health issues by breathing. 

A slurry form of silica fume should be adopted to avoid health issues. 

From 5 % to 15 % of silica fumes are normally use to replace the portland cement.

Silica fume confirming to code ASTM C1240, the maximum percentage of silica fume replacing the cement by weight is 10%.

Silica fumes can easily fill the space between cement particles. As result, a dense microstructure of the cement is obtained which helps to achieve high strength concrete.
The pozzolanic reaction is as follow,

 Pozzolan + Calcium hydroxide + water ⟶ Calcium silicate hydrate

Microsilica is very much reactive pozzolana compare to others. by measuring the amount of calcium hydroxide in the cement paste, the reactivity of pozzolana can be quantified. 

When silica fume is added to concrete, it initially acts as inert. silica fume is very reactive and reacts with CH and water and forms secondary C-S-H. 

Silica consumes a large amount of water and C-S-H fills the capillary voids with the pozzolanic reaction.

The standard codes for silica fume used in cementitious material are BS EN 13263 and ASTM C1240.

The available form of silica fume

  • Undensified forms with a bulk density of 200 to 300 kg/㎥.
  • Densifies forms with a bulk density of 500 to 600 kg/㎥.
  • Micro-pelletized forms with a bulk density of 600 to 800 kg/㎥. 
  • Slurry forms, with a density of 1400 kg/㎥.
  • With the surface area from 15000 to 20,000 kg/㎥.
  • Standard grade slurry with a pH value of 4.7, specific gravity of 1.3 to 1.4, and dry content of micro silica from 48 to 52 %.

Properties of fresh silica fume concrete

Workability of concrete


The silica fume is more efficient in the high water-cement ratio of concrete. 

Silica fume consumes more water, thus, the workability of silica fume concrete is reduced.

Using silica fume in concrete, the mix becomes cohesive, and the slump value decreases as compared to normal concrete.

Segregation and bleeding of concrete


As the silica fume is ultrathin particles as compare to cement particles, these silica fume particles become filler between cement particles and result, bleeding of concrete is reduced.

As discussed earlier, the silica fume concrete is more cohesive as compare to the normal concrete mix, segregation is reduced in silica fume concrete.

Plastic shrinkage and setting time of concrete


Due to no bleeding in silica fume concrete, the plastic shrinkage of the concrete will be increased.

The initial setting time and final setting time of the concrete mix will not be greatly affected in silica fume concrete. it may increase up to 30 minutes.

Properties of hardened silica fume concrete


The high strength of the concrete can be achieved by using silica fume in the concrete. silica fume concrete can achieve the strength of 62 to 80 Mpa easily. 

With the silica fume in concrete, the thickness of the transition zone decreases, and accumulating water under the aggregate particles is reduced. As result, strength is increased in the concrete. 

The thickness of the transition zone decreased, the porosity in the transition zone is also reduced and hence, the permeability of the concrete is also reduced.

The creep of the silica fume concrete is lower than the normal concrete.

Using silica fume in concrete, the high modulus of elasticity can be achieved.

Silica fume concrete is more durable as compared to traditional concrete.

Advantages of silica fume concrete

  • High strength can be achieved.
  • The permeability, porosity can be increased.
  • It makes the structure more durable.
  • Due to the finer particle, the bond strength also increased.
  • Low bleeding and segregation.
  • It can control alkali-aggregation reaction.
  • Improves the consistency of the mix.
  • Enhances the fluidity of the mix.

Disadvantages of silica fume concrete

  • The cost of silica fume is high.
  • Require more water to maintain a constant slump.
  • Poor durability against freeze-thaw-attack.
  • Availability issues.
  • SF is too viscous and difficult to apply.

Application of SF in concrete

  • High-rise building.
  • Dam structures.
  • Bridge structures.
  • Girder.
  • Where resistance against chemical attack is required.
  • Precast concrete
  • Shotcrete.
  • Parking.

Conclusion

From the above discussion, it is clear that silica fume is much more efficient in concrete. 

It is very useful to achieve high-performance concrete. It influences the properties of the fresh concrete as well as hardened concrete. some of the precautions required while using this material.

As the construction industries demand more study in all the construction material, further study or research needs to be done.

Comment below for the discussion on this topic.  








 



  
 

 

  

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