What project owners need to know.
Silica fume, also called microsilica, is an ultra-fine, amorphous silicon dioxide powder collected from the off-gas of electric arc furnaces that make silicon metal or ferrosilicon. It forms when silicon monoxide (SiO) gas escaping the furnace meets air, oxidises to SiO₂ and condenses into spheres around 0.1–0.15 µm across. For a silicon smelter it is partly silicon that never reached the metal, and for the construction industry it is a high-performance pozzolan, specified in India by IS 15388:2003.
What silica fume is
Silica fume is the smoke of a silicon furnace, captured and bagged. India's standard for the material, IS 15388:2003 from the Bureau of Indian Standards (BIS), defines it as a very fine pozzolanic material made mostly of amorphous silica, produced by electric arc furnaces as a by-product of making elemental silicon or ferrosilicon alloys. The standard's foreword describes the particles as ultra-fine, glassy spheres with an average diameter of 0.10–0.15 µm.
The same material goes by several names: silica fume, condensed silica fume and microsilica. All three describe the same thing. It should not be confused with fumed silica, a different, deliberately manufactured product, or with crystalline quartz dust; silica fume is amorphous.
- Also called microsilica or condensed silica fume
- Amorphous (glassy) SiO₂, not crystalline quartz
- Collected from the off-gas filters of silicon and ferrosilicon arc furnaces
How it forms inside a silicon furnace
Silicon metal is made by reducing quartz with carbon in a submerged arc furnace. Part of the silicon travels through the charge as silicon monoxide (SiO) gas. IS 15388 describes the furnaces as running above 2,000 °C and explains that when this SiO gas mixes with oxygen, it oxidises to SiO₂ and condenses into the spherical particles that make up most of the furnace smoke.
A 2023 NTNU and SINTEF study in Norway calls silica fume an important by-product of silicon production, used mainly in concrete and refractories, and shows that its fineness depends on conditions at the furnace top. In small-scale tests, particles formed at a 1,650 °C holding temperature had a specific surface area of 32.9 m²/g versus 22.3 m²/g at 1,700 °C. The authors conclude that temperature and SiO concentration during combustion matter more than whether the surrounding gas is oxygen or carbon dioxide.
- Quartz + carbon → silicon, with SiO gas formed along the way
- SiO that escapes the charge burns above it and condenses as SiO₂ fume
- Furnace-top temperature and SiO concentration shape the fume's fineness
A yield loss and a by-product at the same time
Every kilogram of fume is silicon that left as gas instead of being tapped as metal, so smelters work to keep SiO reacting inside the charge. The fume also continues after the furnace. A 2013 NTNU doctoral thesis by Mari Næss measured fume from industrial refining of metallurgical-grade silicon, when liquid metal is in contact with air, at about 0.7–1.8 kg of SiO₂ per tonne of silicon produced. It found that the exposed surface area of the melt is a main controlling factor, and that fume makes up 40–80% of a plant's fugitive emissions during tapping, refining and casting.
That refining figure covers one stage only and is not the furnace's total fume output, which is mostly captured in off-gas filters. The thesis also notes that the fume may be harmful to plant workers, which is why capture and handling are part of operating a silicon plant, not an afterthought.
- Fume = silicon lost as SiO rather than tapped as metal
- Refining alone: ~0.7–1.8 kg SiO₂ per tonne Si (NTNU, 2013)
- 40–80% of fugitive emissions during tapping, refining and casting (NTNU, 2013)
What India's IS 15388 specifies
IS 15388:2003 covers the chemical and physical requirements of silica fume for concrete and other systems containing hydraulic cement, and was drafted with reference to ASTM C 1240 and EN 197-1. Chemically it requires at least 85.0% SiO₂, no more than 3.0% moisture and no more than 4.0% loss on ignition, with an alkali limit of 1.5% (as Na₂O) applying when the concrete contains reactive aggregate. The committee also asked manufacturers to declare chloride content.
Physically, the standard calls for a specific surface of at least 15 m²/g, or alternatively no more than 10% retained on a 45 µm sieve, plus a seven-day compressive strength of at least 85% of a control sample. It recognises three supply forms by bulk density: natural (about 150–350 kg/m³), densified (above 500 kg/m³) and slurry (typically 50% solids). In August 2025 BIS circulated the grant of the first all-India licence for silica fume under IS 15388:2003.
- SiO₂ ≥ 85.0%; moisture ≤ 3.0%; loss on ignition ≤ 4.0%
- Specific surface ≥ 15 m²/g, or ≤ 10% retained on 45 µm sieve
- Forms: natural, densified and slurry; first all-India BIS licence circulated August 2025
Why it matters for India's silicon chain
USGS's Mineral Commodity Summaries 2026 lists India as a ferrosilicon producer but shows no silicon-metal output, so India's silica fume today comes from ferroalloy furnaces rather than a silicon-metal industry. As India discusses building upstream solar capacity from quartz to polysilicon, any new silicon-metal smelter will produce fume alongside its metal.
That has practical consequences for planning. Off-gas capture and filtering are part of the plant, and fume quality depends on the alloy made and on furnace conditions, so it should not be assumed to meet a construction-grade specification without testing. Silica fume will not decide whether a silicon project works, but a plan that ignores it has missed part of what a silicon furnace produces.
- India: ferrosilicon producer, no silicon metal listed (USGS MCS 2026)
- New silicon-metal smelters would also be silica-fume producers
- Fume grade varies with alloy and furnace practice; test against IS 15388
How JRST supports this requirement
JRST's team works across India's upstream silicon chain, from quartz and crucibles to ingot growth and wafering, and helps project teams understand each stage before they commit to it. This guide summarises public standards and research on silica fume for awareness only. Fume quality, quantities and handling depend on the alloy, the furnace and its operation, and must be validated by testing with qualified engineers.
Discuss your requirement
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As Co-Founder and designated partner of JRST Technology LLP, Mohammed Saif Zaveri works across industrial growth, strategic partnerships, client conversations, equipment strategy, and project pathways for silicon, solar, semiconductor, and advanced-manufacturing opportunities.
This knowledge page is part of JRST's public industrial knowledge base, designed to help buyers move from search terms and early research toward a structured technical-commercial discussion.
View Mohammed Saif Zaveri's profileConnect on LinkedInFrequently asked questions
What is silica fume?
Silica fume is an ultra-fine, amorphous silicon dioxide powder collected from the off-gas of electric arc furnaces that make silicon metal or ferrosilicon. Its particles average about 0.10–0.15 µm across, according to the foreword of IS 15388:2003.
Is silica fume the same as microsilica?
Yes. Silica fume, microsilica and condensed silica fume are names for the same material. It is different from fumed silica, which is a separately manufactured product, and from crystalline quartz dust.
How is silica fume formed?
Silicon monoxide (SiO) gas forms during the reduction of quartz with carbon. When SiO escaping the charge meets air at the top of the furnace, it oxidises to SiO₂ and condenses into tiny glassy spheres that are collected in the off-gas filters.
What is the Indian standard for silica fume?
IS 15388:2003, Silica Fume – Specification, from the Bureau of Indian Standards. It requires at least 85.0% SiO₂, no more than 3.0% moisture and 4.0% loss on ignition, and a specific surface of at least 15 m²/g or no more than 10% retained on a 45 µm sieve, among other requirements.
Why is silica fume a loss for silicon producers?
The silicon in the fume left the furnace as SiO gas instead of being tapped as metal. NTNU research measured 0.7–1.8 kg of SiO₂ fume per tonne of silicon during industrial refining alone, with the exposed melt surface as a main controlling factor.
What is silica fume used for?
Mainly as a pozzolanic admixture in concrete and in refractories. The 2023 NTNU study names concrete and refractory applications as its main uses.
Does India produce silica fume?
India's silica fume comes from ferroalloy production: USGS lists India as a ferrosilicon producer but shows no silicon-metal output. BIS circulated the first all-India licence for silica fume under IS 15388:2003 in August 2025.
How can JRST help?
JRST works across India's upstream silicon chain, from quartz to crucibles, ingot growth and wafering, and helps project teams frame what a silicon-metal step involves, including its by-products. It does not produce or trade silica fume.
Primary sources and further reading
Last reviewed 2026-10-16. Technical scope, policy eligibility, availability, and commercial terms should be independently confirmed for each project.