What is Silica Fume?

What is it?

Silica fume, also known as microsilica, is an ultrafine pozzolanic material. It’s essentially a very fine powder, much like the consistency of ash, composed mainly of amorphous silicon dioxide (SiO2).

How is it made?

Silica fume is a byproduct of the production of silicon metal and ferrosilicon alloys. During the smelting process, high-purity quartz is reduced with coal in electric arc furnaces. The intense heat vaporizes some of the silica, which then condenses into ultra-fine particles as it cools down. These particles are collected and processed to create silica fume.

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What are its properties?

Silica fume has several unique properties that make it valuable in various applications:

  • Extremely small particle size: The particles in silica fume are about 100 to 150 times smaller than the average cement particle. This allows for denser packing in concrete mixtures, leading to increased strength and durability.
  • High pozzolanic activity: Silica fume reacts chemically with calcium hydroxide, a byproduct of cement hydration, to form additional calcium silicate hydrate (CSH) gel. CSH gel is the main binding agent in concrete, and its increased formation due to silica fume enhances the strength and other performance characteristics of concrete.
  • Low permeability: The dense microstructure of concrete containing silica fume makes it less permeable to water, chlorides, and other harmful substances. This improves the concrete’s resistance to corrosion and chemical attack, making it ideal for use in marine environments, bridges, and other structures exposed to harsh conditions.

Physical characteristics:

Color: gray or gray white powder
Refractoriness: >1600℃
Bulk density: 250-700 kg/m3
Chemical composition:

ItemsSiO2Al2O3Fe2O3MgOCaONa2OPH average value
content75～98%1.0±0.2%0.9±0.3%0.7±0.1%0.3±0.1%1.3±0.2%neutral

What are its uses?

Silica fume is primarily used as a mineral admixture in high-performance concrete. It’s particularly beneficial in applications where:

  • High strength and durability are required: This includes structures like bridges, dams, and high-rise buildings.
  • Resistance to corrosion and chemical attack is crucial: For example, in marine structures, parking garages, and industrial facilities exposed to chemicals.
  • Low permeability is essential: This is important for preventing water ingress and protecting reinforcing steel from corrosion.

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