silica fume concrete

Why Adding Silica Fume to Concrete is Good?

Silica fume is added to concrete during the mixing process a long with the normal components such as cement, water, fine sand and coarse aggregate. Adding silica fume to concrete significantly enhances the properties of concrete both in the fresh and hardened states.

Fig. 1: Silica fume

Named a property of concrete? Yes, probably silica fume will enhance that property! Before diving into more details, let’s know first what is silica fume? and have a look at some of the properties and composition of silica fume.

What is Silica Fume?  

Silica fume (or micro silica) is a pozzolanic material that has a light to dark grey color. Pozzolanic material means that it reacts with the calcium hydroxide existing normally in concrete and forms what is called “calcium-silicate-hydrate” which is responsible for the good properties of concrete.

Composition of Silica Fume

  • At least 85% of silica fume is ultra fine, amorphous glassy spherical silicon dioxide (SiO2) particles. It comes as a by-product during the manufacturing process of silicon metal or ferrosilicon alloys.
silica fume sem
Fig 2. Silica fume under the microscope
  • How about the other 15% or less? the rest is impurities such as sulfur, aluminum oxide etc.

Physical Properties of Silica Fume

  • Particles are super fine, nearly about 1/50 of the average size of portland cement (0.3 µm).
  • Large surface area (between 13,000 and 30,000 m2/kg)
  • Highly reactive with concrete compounds
  • The bulk density of the as-produced silica fume ranges from 200 to 300 kg/m3, while densified silica fume has a bulk density that can range from 500 to 600 and in the form of micro pellets with bulk density of 600 to 800 kg/m3
  • The specific gravity of silica fume is 2.2

Effect of Silica Fume on Concrete

Silica fume affects concrete by two primary mechanisms:

  1. Pozzolanic reaction: Silica fume reacts with calcium hydroxide which is a normal product from the hydration of cement. The result of reaction is more C-S-H gel that fills the existing pores of concrete, which lead to an increase in strength and durability.
  2. Micro filler effect: The extreme fineness of silica fume allows it to fill the microscopic voids between cement particles. The net effect is a great decrease in permeability and improved paste to aggregate bond, and ultimate strength of concrete. 

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