Micro silica, also called silica fume, is a byproduct of silicon production. When added to concrete at 5–10% of cement weight, it fills the microscopic voids between cement particles, dramatically increasing compressive strength, reducing permeability, and improving long-term durability. It is specified in high-performance concrete for metro foundations, tunnel linings, marine structures, and bridge piers where strength and durability are non-negotiable.
What is micro silica and where does it come from?
Micro silica is an industrial byproduct. It forms when silicon metal or ferrosilicon alloys are produced in electric arc furnaces at temperatures above 2,000°C. During that process, silicon dioxide vapour rises from the furnace and oxidises. It then condenses into extremely fine spherical particles with an average diameter of 0.10–0.15 micrometres, roughly 100 times smaller than a cement particle.
That size is not incidental. It is what makes micro silica useful in concrete. The particles are small enough to slip into the spaces cement cannot reach.
Most of it used in Indian infrastructure projects is sourced from domestic ferro-alloy producers or imported. WG Micro Silica from WallGrip India is a densified, high-reactivity grade. Densified micro silica has a bulk density of 500–700 kg/m³, compared to 200–350 kg/m³ for undensified silica fume. This makes it significantly easier to handle, transport, and dose accurately on site.
Is micro silica the same as silica fume?
Yes. Micro silica and silica fume are the same material. The two names are used interchangeably across specifications, standards, and supplier datasheets.
In Indian standards, the material is referred to as silica fume and is covered under IS 15388:2003, published by the Bureau of Indian Standards (BIS). In international project specifications and supplier literature, micro silica is the more common commercial term.
If a mix design says silica fume and the TDS says micro silica, they are referring to the same product. What matters is whether the material meets the purity, fineness, and reactivity requirements of IS 15388, not which name appears on the bag.
How does micro silica improve concrete compressive strength?
Micro silica improves concrete strength through two separate mechanisms working at the same time.
The first is physical. Micro silica particles fill the voids between cement grains that water would otherwise occupy. This produces a denser, less porous matrix from the start.
The second is chemical. Micro silica is highly reactive with calcium hydroxide, which is a weak byproduct released when cement hydrates. In a standard concrete mix, calcium hydroxide sits in the paste contributing little to long-term strength. Micro silica reacts with it to form additional calcium silicate hydrate (C-S-H), the same compound responsible for concrete strength.
The result is a concrete that is stronger, less permeable, and more resistant to chemical attack than a plain cement mix at the same water-cement ratio.
As noted in research on Delhi Metro underground concrete quality control, silica fume is essential to produce high-strength, abrasion-resistant concrete for underground infrastructure. In high-performance mixes, micro silica addition at 8–10% of cement weight can increase 28-day compressive strength by 20–30%. For metro foundation piles and tunnel linings where M50 or M60 grade concrete is specified, this is standard practice, not an optional enhancement.
What is the correct dosage of micro silica in concrete mixes?
The standard dosage range is 5–10% of cement weight.
Within that range, the right figure depends on the application:
For general high-performance concrete, 5–7% is typically sufficient. This improves strength and reduces permeability without significantly affecting workability.
For aggressive environments, marine exposure, chemical attack, or underground structures, 8–10% is more commonly specified. At this level, the permeability reduction is significant enough to extend service life by decades in corrosive conditions.
Going beyond 10% without adjusting the mix design is not recommended. Micro silica significantly increases water demand. At higher dosages without a compatible superplasticiser, workability drops sharply and the mix becomes difficult to place and compact.
WG Micro Silica is specified at 5–10% of cement weight. TDS documentation includes mix design guidance and superplasticiser compatibility notes.
What Indian standard covers micro silica in concrete?
IS 15388:2003 is the relevant Indian standard, published by the Bureau of Indian Standards and reaffirmed in 2022. It covers silica fume for use in concrete and sets minimum requirements for:
- Silicon dioxide content (SiO2): minimum 85%
- Specific surface area: minimum 15 m²/g
- Loss on ignition: maximum 4%
- 28-day strength activity index: minimum 85%
- Moisture content and bulk density for both densified and undensified grades
Any micro silica specified on a PSU or EPC project in India should comply with IS 15388. A compliant TDS will reference this standard explicitly.
WG Micro Silica from WallGrip India complies with IS 15388. The SiO2 content is a minimum 85% and specific surface area falls within the 15–35 m²/g range specified in the standard. TDS and MSDS are available on request.
Can micro silica be used in piling and foundation concrete?
Yes. Piling and foundation concrete is one of the primary applications for micro silica in India.
Foundation piles for metro projects, elevated corridors, and bridge piers are exposed to groundwater, sulphates, and chlorides throughout their service life. Standard concrete, even at M40 grade, is permeable enough to allow ingress over time. Once chlorides reach the reinforcement, corrosion begins and structural integrity is progressively compromised.
Micro silica dramatically reduces this permeability. In a well-designed mix at 8% dosage, water permeability can drop by more than 50% compared to a plain cement mix of equivalent grade.
Delhi Metro Phase IV design specifications require high-performance concrete for underground and foundation structures. Bangalore Metro Blue Line foundation and pile works have similarly required silica fume in the concrete specification for durability-critical elements.
WG Micro Silica has been supplied for metro foundation and piling applications. TDS documentation is available for project-specific mix design review.
What documentation do PSU and EPC teams need when specifying micro silica?
PSU procurement and EPC evaluation teams typically require three documents before approving a micro silica product:
Technical Data Sheet (TDS): Confirms product specification, IS 15388 compliance, dosage range, and mix design guidance. The TDS should explicitly state SiO2 content, specific surface area, bulk density, and activity index.
Material Safety Data Sheet (MSDS): Required for site safety compliance and storage protocols. Micro silica is a fine respirable powder and must be handled with appropriate PPE including dust masks and eye protection. As noted in IS 15388:2003, amorphous silica fume is less hazardous than crystalline silica but inhalation precautions still apply.
Test Reports: Third-party lab reports confirming SiO2 content and activity index are standard for PSU approval processes.
WallGrip provides TDS and MSDS for WG Micro Silica on request. For projects requiring third-party test reports, contact the WallGrip team directly.
How Does Micro Silica Compare to Fly Ash and GGBFS?
| Property | Micro Silica | Fly Ash | GGBFS |
|---|---|---|---|
| Typical dosage | 5–10% of cement weight | 15–35% of cement weight | 25–70% of cement weight |
| Fineness (specific surface) | Ultra-fine (IS 15388, measured in m²/g) | Min. 320 m²/kg (IS 3812 Part 1, Blaine method) | Min. 225 m²/kg (IS 455) |
| Governing Indian Standard | IS 15388:2003 | IS 1489 (Part 1) / IS 456:2000 | IS 455 |
FAQs about Micro Silica
Question: What is micro silica used for?
Answer: Micro silica is used to improve the strength, durability, and impermeability of concrete. It is commonly specified for high-performance applications including metro foundations, bridge piers, tunnel linings, marine structures, and industrial flooring where standard concrete is not sufficient. It is also used in specialised mortar formulations for repair and overlay systems in infrastructure applications.
Question: How to use micro silica in concrete?
Answer: Micro silica is added to the concrete mix at 5–10% of cement weight. It is introduced at the batching plant along with cement and aggregates. A compatible superplasticiser is essential to maintain workability, as micro silica increases water demand significantly. Densified micro silica, such as WG Micro Silica, is easier to handle and dose accurately on site than undensified silica fume, which is light, dusty, and difficult to batch consistently.
Question: How is micro silica produced?
Answer: Micro silica is produced as a byproduct of silicon metal and ferrosilicon alloy manufacturing. During smelting in electric arc furnaces at temperatures above 2,000°C, silicon dioxide vapour rises, oxidises, and condenses into ultrafine spherical particles with an average diameter of 0.10–0.15 micrometres. These particles are collected using bag filters and processed into either undensified powder or densified pellets for commercial supply. The process is described in detail in IS 15388:2003 published by the Bureau of Indian Standards.
Question: What are the three types of silica?
Answer: In construction materials, three commonly referenced forms are relevant. Crystalline silica is found in sand, quartz, and rock. Its dust form is a recognised health hazard and is regulated under site safety guidelines. Amorphous silica is the form found in micro silica and silica fume. It is reactive, pozzolanic, and safe when handled with appropriate PPE. Precipitated silica is a manufactured form used primarily in rubber, tyres, and personal care products, not in structural concrete. Micro silica used in concrete is amorphous silica and does not carry the same long-term health risk as crystalline silica dust, though dust precautions remain necessary on site.
Request TDS and MSDS for WG Micro Silica
WG Micro Silica is a densified, high-reactivity silica fume compliant with IS 15388:2003. It is suitable for metro foundation piles, tunnel linings, bridge piers, raft foundations, and any high-performance concrete application where strength and durability are specified.
Available in 25 kg bags and bulk road tanker supply. TDS and MSDS are available for PSU and EPC evaluation teams on request.
Contact WallGrip India to request documentation or discuss mix design requirements for your project.