Have you come across silica fume? It is a wonder construction material that has revolutionized concrete technology. Microsilica fume is a mineral admixture that is a by-product of silicon and ferrosilicon alloy production. Because of its physical and chemical properties, Silica fume is a very reactive pozzolan. By incorporating silica fume into concrete mixes, construction engineers and contractors can develop high-performance concrete suitable for structures requiring high strength and extreme durability, such as bridges, marine structures, high-rise buildings, and industrial facilities. It is also sought for architectural decorative concrete due to its smooth finishing and anti-bleeding properties. In this article, we will explore how micro silica fume concrete works, its background, its benefits, mechanisms, applications,
Background History of Silica Fume (SF)
The first use of silica fume in Portland cement dates back to 1952. It was first used in concrete in 1971 at the Fiskaa metallurgical plant in Norway. By 1978, the Norwegian government had adopted standards for using silica fume in concrete. The practice spread to other countries, with Canada approving its use in 1981. Since then, silica fume has been widely used in the construction industry to improve the properties of concrete, particularly in high-performance and self-compacting concrete (SCC).
The global production of silica fume stands around 1.5 million of which China takes the lion’s share of about 50%.
What is Micro Silica Fume?
Micro silica fume is an ultrafine powder byproduct obtained from the silicon and ferrosilicon manufacturing processes. Silicon metal is used to produce silicon conductors in the semiconductor industry. Ferrosilicon is an alloy of steel and silicon produced in metallurgical plants.
Silica fume consists primarily of silicon dioxide (SiO₂) particles, which are spherical and have an average diameter of less than 1 micron. The material is highly reactive due to its large surface area and amorphous structure, making it suitable for use as a supplementary cementitious material in concrete mixes.
Silica fume is often added to concrete at a dosage of 5% to 15% by weight of cement, depending on the desired properties of the final product. This material is also available in various forms, including dry powder, slurry, and densified silica fume.
How Does Micro Silica Fume Enhance Concrete?
Micro silica fume contributes to the strength, durability, and overall performance of concrete through two main mechanisms: pozzolanic reaction and micro filler effect.
A. Pozzolanic Reaction:
The pozzolanic reaction is a chemical process that occurs between the silica fume and calcium hydroxide (CH), a byproduct of cement hydration. The silica fume exhibits high reactivity due to its high surface area and amorphous structure which boosts the pozzolanic reaction, When water is added to cement, calcium silicate hydrate (C-S-H) gel and calcium hydroxide are formed. While C-S-H contributes to the strength and binding properties of concrete, calcium hydroxide is considered weak and porous, which can negatively impact durability.
Silica fume reacts with calcium hydroxide to form additional C-S-H gel through the following equation:
\[
\text{SiO₂} + \text{Ca(OH)₂} → \text{C-S-H}
\]
This secondary reaction results in a denser and more durable concrete structure by reducing the amount of calcium hydroxide and increasing the quantity of C-S-H, the primary binding component.
B. Microfiller Effect:
The extremely fine particles of micro silica fume which are about 100 times smaller than cement particles fill the voids between cement particles, effectively acting as a microfiller. This reduces the porosity of the concrete and minimizes the amount of free water within the mix. As a result, the concrete becomes more compact, impermeable, and resistant to external elements such as water, chemicals, and environmental pollutants.
The microfiller effect, combined with the pozzolanic reaction, helps produce highly durable, high-strength concrete that can withstand harsh conditions.
Applications of Micro Silica Fume Concrete
Micro silica fume concrete is commonly used in various applications that require enhanced strength, durability, and longevity. Micro silica fume can be added to concrete in dry or wet forms. 6000 psi concrete was considered high strength some time ago, however with silica fumes 20000 psi(140Mpa) can be reached which can be used in high-rise buildings.
Some of the key applications include:
-Silica fume concrete is ideal for high-performance concrete (HPC) structures, which require superior mechanical properties and resilience. Bridges, high-rise buildings, and long-span structures benefit from the increased compressive strength and reduced permeability of silica fume concrete.
-It is used for self-compacted concrete, due to its high surface area it has sticky and easy-flowing properties and prevents bleeding and separation
– When used for Shotcreting, it reduces spring back, increases thickness, and better scour resistance for offshore construction, it can eliminate the use of mesh if used with fibers
-It is suitable for Sulfate resistant and chemical-resistant concrete because of its low permeability and chemical resistance
-It is Corrosion resistant and hence suitable for structures exposed to deicing salts and seawater ports, docks, local groundwater, and onshore buildings with chlorides in the soil and air.
– it has high abrasion and wear resistance hence suitable for ground, pavement, and spillways
A. Precast industry
In architectural decorative precast works, the addition of silica fume results in a smooth and dense surface which can be given various finishes to form an appealing product. The use of Silica fume in HPC makes it suitable for load-bearing precast elements and infrastructure attributed to its superior strength, durability, and resistance to aggressive environments
B.Infrastructure
Concrete infrastructure like highway bridges, parking structures, marine facilities, and bridge deck overlays face ongoing deterioration from multiple sources. The primary causes include corrosion of steel reinforcement bars (rebar), mechanical wear from traffic, and chemical degradation. However, incorporating silica fume into concrete mixtures provides exceptional protection against these destructive forces. This supplementary material creates a denser, more impermeable concrete that resists damage from deicing salts, seawater intrusion, vehicle traffic, and heavy impacts. By significantly reducing both rebar corrosion and concrete deterioration, silica fume concrete dramatically lowers long-term maintenance costs for these critical structures.
C. Marine Structures:
Concrete used in marine environments is exposed to aggressive saltwater conditions, which can cause corrosion of reinforcement and degradation of the structure over time. Micro silica fume concrete, with its low permeability and resistance to chloride ingress, helps extend the lifespan of marine structures such as piers, docks, and sea walls.
D. Industrial Floors:
Industrial floors are subjected to heavy loads, abrasive materials, and chemical spills, all of which can lead to rapid wear and tear. Silica fume concrete provides enhanced durability and abrasion resistance, making it suitable for use in industrial flooring applications.
E. Repair and Rehabilitation:
Due to its high bond strength and low shrinkage properties, silica fume concrete is frequently used in the repair and rehabilitation of damaged concrete structures. It ensures a strong bond between the new and old concrete, helping to restore the structural integrity of deteriorated elements.
F. Tunnels and Dams:
Tunnels, dams, and other infrastructure exposed to water pressure and harsh environmental conditions benefit from silica fume concrete’s impermeability and resistance to sulfate attack, which can lead to concrete deterioration.
Advantages and Disadvantages of Micro Silica Fume Concrete
While micro silica fume concrete offers numerous benefits, it also has certain limitations. Below are the main advantages and disadvantages associated with this material:
Advantages:
Improved Mechanical Properties: Silica fume significantly enhances the compressive strength, flexural strength, and modulus of elasticity of concrete.
Increased Cohesion: Silica fume improves the cohesiveness of fresh concrete, reducing bleeding and segregation.
Environmental Benefits: Using silica fume as a supplementary cementitious material reduces the amount of cement needed, thereby reducing the carbon footprint associated with cement production.
– Reduced Permeability: The microfiller effect and pozzolanic reaction contribute to lower permeability, preventing water and chemical ingress.
– Enhanced Durability: Silica fume concrete exhibits superior resistance to abrasion, chemical attack, and corrosion.
– Improved Bond Strength: due to the fine particle size and reactivity of silica fume, the material creates a stronger bond between the cement matrix and aggregates.
– Resistance to Chlorides and Sulfates: The material’s low permeability protects it from chloride ingress and sulfate attack, making it suitable for marine and underground environments.
-Generates low heat at high strength, hence possible for use in hot weather concreting and mass concrete
-Serves as a waterproofing method to use in water-prone areas such as underground parks
Disadvantages:
Reduced Workability: Silica fume can reduce the workability of concrete, making it more difficult to handle and place. This often necessitates the use of superplasticizers to maintain workability.
Increased Water Demand: The high surface area of silica fume particles can increase the water demand of the concrete mix, which may require adjustments in the mix design.
Cost: Silica fume can be more expensive than other supplementary cementitious materials, which may increase the overall cost of the concrete mix.
Handling and Health Concerns: Silica fume is a very fine powder, which can pose health risks if inhaled. Proper handling and safety measures are required during its use.
– Curing Requirements: due to its rapid strength gain, silica fume concrete may require additional curing measures to prevent cracking from early-age shrinkage.
– Costly and difficult handling of material: it requires special equipment for transportation and mixing to prevent the formation of dust. Hence it is rarely used in normal applications except in special applications such as high-strength concrete construction.
-Resistance to carbonation and it can cause corrosion of black steel reinforcement.
Mechanical properties of Silica fume concrete
Silica fume concrete can reach a compressive strength of 20000 psi or 140 MPa. Hence it makes it possible to design high-rise and large infrastructures that demand high compressive strength.
The modulus of elasticity can be a high modulus of elasticity exceeding 7 million psi (40,000 Mpa). Hence it can be used for structures that need high stiffness or high bending resistance.
It can provide high flexural strengths of up to 2,000 psi (14 MPa). This is important to achieve longer spans, larger open spaces, and thinner sections and performs better for dynamic loads, such as bridges and industrial facilities
It makes high early strength possible to speed up construction and precast works.
Mix Design Constituents for Silica Fume in Concrete
When incorporating silica fume into concrete, the mix design must be carefully adjusted to account for its effects on workability, strength, and durability. The typical constituents of a concrete mix with silica fume include:
Cement: The primary binder in the mix. The amount of cement may be reduced when silica fume is used, as it acts as a supplementary cementitious material.
Silica Fume: Typically added in proportions ranging from 5% to 15% by weight of cement. The exact percentage depends on the desired properties of the concrete.
Water: The water-to-cement (w/c) ratio is a critical factor in the mix design. Silica fume can increase the water demand, so the w/c ratio may need to be adjusted, often with the help of superplasticizers.
Superplasticizer: Essential for maintaining workability when silica fume is used. Superplasticizers reduce the water demand and improve the flowability of the concrete.
Aggregates: Both coarse and fine aggregates are used in the mix. The gradation and quality of aggregates must be carefully controlled to ensure good workability and strength.
Supplementary Cementitious Materials (SCMs): Other SCMs like fly ash (FA) or ground granulated blast furnace slag (GGBFS) may also be used in combination with silica fume to further enhance the properties of the concrete.
Example Mix Design procedures with Silica Fume for your silica fume projects
Here is an example of a typical mix design for self-compacting concrete (SCC) with silica fume:
Cement: 400 kg/m³
Silica Fume: 5% to 15% of cement weight (e.g., 20 to 60 kg/m³)
Water: 180 to 200 kg/m³ (adjusted based on w/c ratio and superplasticizer dosage)
Superplasticizer: 1% to 3% of cement weight (e.g., 4 to 12 kg/m³)
Coarse Aggregate: 800 to 1000 kg/m³
Fine Aggregate: 800 to 900 kg/m³
Water-to-Cement Ratio (w/c): Typically between 0.35 to 0.45, depending on the desired workability and strength.
Silica fume is a valuable material in the production of high-performance and self-compacting concrete. It offers numerous advantages, including improved mechanical properties, reduced permeability, and enhanced durability. However, its use also presents challenges, such as reduced workability and increased water demand, which must be carefully managed through proper mix design and the use of superplasticizers. By understanding the properties and effects of silica fume, engineers can optimize concrete mixes to achieve the desired performance characteristics.Its applications are varied, from high-rise buildings to marine structures and industrial floors, making it a valuable material for modern construction. Despite its higher cost and workability challenges, the long-term performance benefits make it an essential component in high-performance concrete mixes.
Frequently Asked Questions (FAQs)
1: What is the typical dosage of micro silica fume in concrete?
The typical dosage of micro silica fume in concrete ranges from 5% to 15% by weight of cement, depending on the desired performance characteristics. Higher dosages may be used for ultra-high-strength concrete applications.
2: How does silica fume affect the workability of concrete?
Silica fume reduces the workability of concrete due to its fine particle size and high surface area. To compensate, superplasticizers or high-range water reducers are often added to the mix to improve workability.
3: Can silica fume concrete be used for residential construction?
While silica fume concrete is generally used for high-performance and specialized applications, it can be used in residential construction where enhanced durability or strength is needed, such as in foundations, driveways, and basement walls.
4: Does silica fume concrete require special curing methods?
Yes, due to its rapid strength gain and potential for early-age shrinkage, silica fume concrete may require additional curing measures such as continuous water curing or the application of curing compounds to prevent cracking.
5: How does silica fume concrete improve resistance to chloride ingress?
The fine particles of silica fume fill the voids in the concrete matrix, reducing permeability and preventing the ingress of chloride ions, which can cause corrosion of reinforcing steel.
6: Is silica fume environmentally friendly?
Silica fume is a byproduct of industrial processes and its use in concrete helps reduce waste. However, its production requires energy, and the handling of fine particles must be managed to minimize environmental impact.
References:
– Mehta, P. K., & Monteiro, P. J. M. (2014). *Concrete: Microstructure, Properties, and Materials. McGraw-Hill Education.
– Aïtcin, P.-C. (1998). High-Performance Concrete. CRC Press.
– Malhotra, V. M. (1993). Supplementary Cementing Materials for Concrete. CANMET.
– Mindess, S., Young, J. F., & Darwin, D. (2003). Concrete. Prentice Hall.
Effect of Silica Fume and Super-Plasticizer on Mechanical Properties of Self-Compacting Concrete: A Review
Silica fume concrete project report – HSA Microsilica
Silica Fume Applications in Concrete
JL-94-September-October_Guide_to_Using_Silica_Fume_in_Precast_Prestressed_Concrete_Products.pdf