Silica Fume or Fumed Silica: Comparison

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Silica Fume or Fumed Silica: Comparison.

Silica fume and fumed silica are two important materials in the field of industrial production, especially in the construction and chemical industries. Although both products are used as additives in various applications, they have different properties that make them suited for different uses. In this article, we will compare silica fume and fumed silica, explaining their differences and highlighting their respective advantages and disadvantages.

What is Silica Fume?

Silica fume, also known as microsilica, is a byproduct of the production of silicon and ferrosilicon alloys. It is a highly reactive and pozzolanic material that consists of ultrafine particles of amorphous silica. These particles are so small (around 100 times smaller than cement particles) that they can fill the voids between cement grains, enhancing the strength, durability, and workability of concrete.

Silica fume also has a high surface area and a strongly negative charge, which means it can attract and bind positively charged ions such as calcium, sodium, and potassium, reducing their mobility in the concrete mix. This can reduce the risk of efflorescence, shrinkage, and corrosion, and improve the resistance to chemical attack and abrasion.

What is Fumed Silica?

Fumed silica, also known as pyrogenic silica, is a synthetic and amorphous form of silica that is produced by the oxidation of silicon tetrachloride or other silicon compounds in a flame of hydrogen and oxygen. The result is a powder of nanoparticles that have a high surface area (up to 400 m2/g) and a strong tendency to form agglomerates due to their high surface energy.

Fumed silica is used as a reinforcing and thickening agent in various applications, such as adhesives, coatings, paints, and sealants, due to its high surface area and ability to improve the rheological properties of the system. It can also act as a drying agent due to its hydrophilic nature, absorbing moisture and preventing the growth of microorganisms.

Comparison of Silica Fume and Fumed Silica.

Silica fume and fumed silica have some similarities but also many differences that make them suited for different applications. Here are some of the main differences:

Particle size: Silica fume particles are much coarser than fumed silica particles, with a median size of 0.1-0.2 µm compared to 7-40 nm for fumed silica. This makes silica fume better suited for filling voids in concrete, while fumed silica is better suited for forming a network of fine particles in coatings.

Surface area: Fumed silica has a much higher surface area than silica fume, with a typical range of 100-400 m2/g compared to 15-27 m2/g for silica fume. This makes fumed silica more effective as a thickening agent and adsorbent, but also more prone to agglomeration and difficult to disperse.

Chemical reactivity: Silica fume is a highly reactive and pozzolanic material that can react with calcium hydroxide in cement to form calcium silicate hydrate (C-S-H), which is the main binder in concrete. Fumed silica, on the other hand, is chemically inert and does not react with other materials.

Conclusion.

In conclusion, silica fume and fumed silica are two important materials with different properties and applications. Silica fume is a pozzolanic material that improves the strength and durability of concrete, while fumed silica is a thickening agent and adsorbent that improves the rheological properties of coatings. Understanding the differences between these materials is essential for selecting the right product for each application and optimizing their performance.

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