Silica fume is a highly pozzolanic material that is used to improve the performance of concrete and other cementitious materials. It is a byproduct of the ferrosilicon and silicon metal industries, and consists of very fine spherical particles of amorphous silica.
Silica fume has a number of performance characteristics that make it a valuable addition to cementitious materials:
- High pozzolanic activity: Silica fume reacts with calcium hydroxide (CH), a byproduct of cement hydration, to form calcium silicate hydrate (CSH), which is the main strength-giving phase in concrete. This reaction reduces the amount of CH in the concrete, which can improve its durability and resistance to chemical attack.
- High surface area: Silica fume particles have a very high surface area, which gives them a strong ability to adsorb water. This can help to improve the workability of concrete and reduce its segregation.
- Filler effect: Silica fume particles fill the voids between the cement particles and aggregates, which can improve the density and strength of concrete.
The addition of silica fume to concrete can improve its performance in the following ways:
- Increased strength: Silica fume can significantly increase the compressive, tensile, and flexural strength of concrete. For example, a 10% addition of silica fume can increase the compressive strength of concrete by 20-30%.
- Improved durability: Silica fume can improve the durability of concrete by reducing its permeability to chloride ions and other aggressive chemicals. It can also improve the resistance of concrete to abrasion and freeze-thaw damage.
- Reduced water permeability: Silica fume can reduce the water permeability of concrete, which can improve its resistance to water ingress and leaching. This can be beneficial for applications such as dams, reservoirs, and other structures that are exposed to water.
Silica fume is typically used in concrete at replacement levels of 5-15% by weight of cement. However, it can be used at higher levels in some applications, such as high-performance concrete and self-compacting concrete.
In addition to concrete, silica fume can also be used to improve the performance of other cementitious materials, such as mortar, grout, and precast concrete products. It is also used in a variety of other applications, such as refractory materials, dental cements, and oil well cements.
Overall, silica fume is a versatile and high-performance material that can be used to improve the performance of a wide range of cementitious materials. It is an important component of high-performance concrete and other specialized cementitious products.