Silica fume is a by-product obtained by producing silicon metal or else ferro silicon alloys. One of the major applications of silica fumes is in the building and construction industries. Silica fume or Micro silica is a fine powder that has particles even more tiny than cement particles.
Why does Silica Fume need to be added to concrete?
Silica fume in concrete helps in increasing the strength and durability of the concrete. While making concrete mixtures to obtain fluidity water is added to the concrete.
Because of the addition of this water to the cement slurry, thinning it and reducing the binding force of the cement, excess water introduced into concrete forms blisters and water channels.
When the concrete hardens, the water inside the cement concrete evaporates and the pores remain inside the concrete.
These pores reduce the strength of concrete and when the concrete is stressed concentration is likely to form around the pores.
The surface of the micro silica powder particles is hydrophilic and has a wide surface area. This means that the amount of water needed to cover the vast area of the surface is considerably large.
In the mixture when the quantity of silica increases, the concrete mixture reaches the same slump, and the demand for water or water binder ratio increases accordingly.
When the water consumption or water binder ratio does not change, the silica fume content in concrete increases, and the concrete becomes very sticky.
To increase the strength and impermeability of the concrete without increasing the water binder ratio to obtain good impermeability silica fume is used with water-reducing agent.
The silica fume has a very strong binding force which makes it difficult to make the silica fume separate from the concrete.
In low fume silica content with the presence of silica fume in less than 5%, it is possible to reduce the viscosity of the concrete mixture.
The particle range of the silica fume plays a leading role in spherical particle ball lubrication and exceeds the wetting water requirement effect of a high specific area.
The low-content silica fume increases the cohesive force & fluidity of the concrete mixture and reduces the pumping pressure. It is highly recommended for the preparation of high-workability or else self-compacting concrete.
In what way silica fume is used in concrete?
Before knowing the advantages and disadvantages of the silica fume it is necessary to know about its chemical reactions when the silica fume is added to the concrete.
When the silica fume is added to the concrete it remains inert. When the water is added to the concrete along with the silica fume the chemical reaction gets started and basically two different types of chemical compounds are formed.
The types of chemical compounds are basically:
- Calcium silicate hydrate
- Calcium hydroxide
- Amorphous Silica
- Pozzolanic compound
- Silicate Phases
- Alkali silica reaction
Details about the information about the chemical reaction:
- Calcium silicate hydrate: Calcium silicate hydrate (CSH) is basically a crystalline compound responsible for increasing the strength of the concrete.
- Calcium hydroxide: Calcium hydroxide (CH) It is basically a by-product that fills the pores inside the concrete. The concrete, when mixed with cement, sand & water still has some pores inside it which requires a thin crystalline material to be mixed inside the concrete for which silica fume is best suited material to be mixed inside the concrete.
Thus, the concrete helps develop flexural, compressive, and bond strength inside the concrete.
- Amorphous Silica: Amorphous silica is widely used in cement hydration. This material increases the strength and durability of concrete.
- Pozzolanic compound: Additional cementitious material is formed when the silica fume reacts with calcium hydroxide.
- Silicate Phases: Due to pozzolanic reaction, complex silicate compounds are formed thus increasing the strength and durability of the concrete.
- Alkali Silica reaction: Silica fume helps to reduce the alkali silica reaction tendencies by consuming free alkalis in the system, preventing the formation of the expansive Alkali silica gels.
Effects of adding silica fume inside the concrete:
The Filling effect:
Research shows that if 15% of silica fume is added to the concrete, in that case, the ratio of the silica fume to the cement particles is 2 million silica fumes against the one cement particle. Thus, the silica fume exerts an important influence on the filling of pores and voids within the concrete.
The silica fume used in high-strength concrete is 100 times smaller than the cement particles. Silica fumes will affect the same compared with the particles of cement filling.
The fine aggregates fill in the voids of the coarse aggregates. The effect of silica fumes is filled with the increased high density of concrete.
Improve bleeding of the concrete:
Inside the high-strength concrete, the silica fume is filled with cement paste which helps to reduce the pores inside the concrete and improves the homogeneity in the concrete whereas the total porosity remains unchanged.
The silica fume helps to reduces the bleeding inside the concrete and also avoids the moisture accumulation below the aggregates. Silica fumes make the transition zone of aggregate interface like the micro-structure of cement.
This, in effect, shall increase the compactness of the interface filtration area and reduce the thickness of the interface transition zone.
Improving the strength of the concrete:
It can improve the adhesion between the cement particles upon addition of silica fume to the cement concrete. This material reduces the bonding strength of cement paste and aggregates in high-strength concrete which has the characteristics of composite materials.
Aggregate particles in concrete play a reinforcing role, not just an inert filler. Silica fumes can even improve the strength of the concrete without the mixing of aggregates.
Silica fume contributes less to strengthening aggregate-free cement paste, but it can render the strength of concrete with the same water-to-binder ratio rather obvious, higher than that of its pure paste matrix.
Benefits of Silica Fume in concrete:
- Increase in compressive strength of the concrete: Silica fume increases the compressive strength of concrete manifold through increased packing density and forming additional C-S-H gel.
- Helps in reducing permeability: The silica fumes can fill the tiny pores present inside the concrete. It helps to minimize the concrete permeability. It increases the resistance to water and chemical ingress, hence increasing durability.
- Improves durability of concrete: By mixing silica fumes to the cement concrete the resistance to concrete gets improved to aggressive environments such as the one containing Chlorides, sulphates and acids.
- Resistance to abrasion & erosion: Wherever the abrasion and erosion is a concern the Silica Fumes increases the resistance to wear and tear of the concrete.
- Reduces alkali silica reaction: It reduces the effect of alkali silica reaction which causes deterioration of the concrete quality by binding alkalis in pore solution.
- High performance concrete: Especially when the stronger structures are required to be built along with higher durability and low permeability. It is suitable for high performance and ultra-high performance concrete strength to be gained.
- Marine Structures: Lower permeability and improved durability make it more suitable for marine exposure, where it will protect against chloride-induced corrosion.
- Industrial Floors: Silica fume helps in gaining high abrasion and impact resistance. It is suitable for storing heavy load objects on the floor and preventing the formation of cracks.
- Bridges & Highways: Silica fumes are beneficial for the construction of infrastructure projects wherein structures are exposed to deicing salts and free thaw cycles.
- Pre-Cast Concrete: The mechanical property of the structure gets improved like the beams, panels & columns.
- Repairs & Rehabilitation: This material can be used in concrete mortars is beneficial for increasing the strength and reducing permeability, helping in increasing life of repair structures.
Disadvantages of the micro silica in cement concrete:
In case of hot temperatures, there are chances that the concrete may cause cracks in the structures when micro silica is added to the concrete.
Otherwise, if silica fume is added to the concrete, it will gain strength. The hydration of the concrete dissipates fast, and by doing this, the concrete temperatures increase.
Accordingly, it exposes the material to high-temperature stress and concentrates at the top of dry shrinkage cracks. The cracks may go deeper with the use of silica fume thus leading to transfixion cracks.
Why does silica fume concrete easily crack?
Silica fumes are basically prone to cracks very easily. If the characteristics of the silica fume are not known properly or else the control of properties is not adjusted properly, the maintenance quality is not good enough.
Silica fumes are generally used in concrete wherein high-performance requirements such as high strength, high durability, and low permeability. The silica fume concrete has low water binding and a huge amount of cementitious material.
Its properties like auto-shrinkage as well as rise in temperature that’s why to a chance to increase in the sensitivity of cracks while silica may lead to early auto-shrinkage.
The well-known micro silica provider
Sakshi Chem Sciences Pvt Ltd is a popular to manufacture silica fumes. The product’s name Fumecrete 90 which is a grey-colored powder that is available in 25 kg packs. The product is of excellent quality and is best recommended for increasing the strength of the concrete.
The product is useful for the increase of compressive strength of concrete, flexural, and bonding strength. This material is manufactured with the help of very fine glassy spheres of silicon dioxide.
Conclusion:
Thus, the Silica fume is one of the very important materials in construction which helps in increasing the strength of the concrete.
As the particles of the Silica Fume are very tiny compared to cement. It is beneficial to be added inside the concrete so that they can fill the pores and gaps present inside the concrete.
It helps in building high flexural, compressive, and bonding strength inside the concrete. Careful use of the material should be made as in other cases the material has got some drawbacks as well such as cracks developing inside the concrete in case of improper use of the material.
The accumulation of moisture inside the concrete gets reduced and the bleeding as well gets reduced. Thus, to gain the best results for the concrete we recommend the use of Fumecrete 90 to be added in cement concrete for the best quality results while constructing the structures.