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.
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.
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What is Silica Fume? Â
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.
- 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:
- 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.
- 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.Â
Fresh Properties of Concrete + Silica Fume
- Low workability: Silica fume reduces the slump as its content increases due to its large surface area
- The mix is highly cohesive
- Adding superplasticizer can enhance the workability of silica fume concrete
- Silica fume reduces bleeding significantly as it reduces the free water left in the mixture for bleeding.
- Early curing is required to prevent plastic shrinkage
- Need to finish the concrete as soon as possible after placing and compacting to prevent the surface from drying out
- For small amounts of silica fume, setting time is not significantly affected. However, it decreases the setting time for higher amounts
Hardened Properties of Concrete + Silica Fume
- Strength, either compressive, flexural, or splitting, of concrete made with silica fume is higher than ordinary concrete. Also, modulus of elasticity is way higher for the silica fume concrete.
- Penetrability of silica fume concrete is low; thus, it is more effective at preventing the detrimental substances from the environment to get inside concrete.
Advantages of Silica Fume Concrete
- Silica fume decreases the segregation and bleeding.
- High strength and durability propertiesÂ
- Improve bond strength to steel
Disadvantages
- Low slump but can be alleviated by using superplasticizersÂ
- Higher initial cost but life cycle cost is better
- Longer mixing time
- May negatively affect the setting timeÂ
Applications
Silica fume can be used in combination with any concrete but often used in special projects due to higher initial cost including high rise buildings, dams, towers, etc.
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