Silica fume (CAS#69012-64-2)
Silica fume, also known as microsilica, is an ultrafine, amorphous (non-crystalline) polymorph of silicon dioxide collected as a by-product of the production of elemental silicon and ferrosilicon alloys in electric arc furnaces. It consists of spherical particles with an average primary diameter of approximately 150 nm and a BET specific surface area typically in the range of 15–35 m²/g, making it one of the finest mineral admixtures available. The material is supplied as a grey to light grey, odorless powder with a true specific gravity of 2.1–2.3 and a melting point of 1550–1570 °C. It is insoluble in water and organic solvents but dissolves in hydrofluoric acid and hot concentrated alkali solutions.
The amorphous structure is the critical feature that distinguishes silica fume from crystalline silica polymorphs such as quartz. The SiO₂ content typically ranges from 85% to 97%, with minor constituents including iron oxide, aluminum oxide, calcium oxide, magnesium oxide, and alkali oxides, the exact composition depending on the alloy being produced and the furnace charge. Because it is produced at high temperature and quenched rapidly in the off-gas stream, silica fume is thermodynamically metastable and highly reactive, a property that underpins its principal commercial value as a pozzolan.
Applications
The dominant application of silica fume is as a highly reactive pozzolanic admixture in high-performance concrete and cementitious systems. In the alkaline environment of hydrating Portland cement, the amorphous silica reacts with calcium hydroxide (portlandite) to form additional calcium silicate hydrate (C-S-H) gel, the primary binding phase responsible for strength development. This pozzolanic reaction, combined with the physical filler effect of the ultrafine particles, refines the pore structure of the cement paste, particularly in the interfacial transition zone between paste and aggregate, leading to marked improvements in compressive strength, impermeability, and durability against chloride ingress and sulfate attack. Silica fume is consequently specified in high-strength concrete, marine and bridge deck concrete, shotcrete, precast elements, and oil-well grouting where enhanced density and chemical resistance are required.
In refractory materials, silica fume functions as a functional additive in low-cement and ultra-low-cement castables. It promotes flow and workability in the fresh mix, contributes to mullite formation at elevated temperatures in alumina-silica systems, and enhances packing density, hot strength, and thermal shock resistance in refractory castables, precast shapes, and gunning mixes.
As a functional filler in rubber, plastics, and polymer composites, the fine particle size and amorphous surface chemistry of silica fume improve mechanical reinforcement, abrasion resistance, and thermal stability. It is used in natural and regenerated rubber compounds, unsaturated polyester resins, and specialty coatings, where it also serves as an anti-caking agent and pellet binder. In paints, coatings, and corrosion-protection systems, silica fume acts as a rheology modifier and reinforcing filler that contributes to film integrity and barrier performance. Additional applications include use in ceramic bodies, insulation materials, and as a source of reactive silica in chemical synthesis.
Preparation Method
Silica fume is not synthesized as a primary product; it is recovered as a by-product of the silicon and ferrosilicon industries. In a submerged electric arc furnace, quartz (SiO₂) is reduced by carbonaceous materials at temperatures exceeding 2000 °C to produce molten silicon or ferrosilicon alloy. A portion of the silicon monoxide vapor generated in the high-temperature reaction zone escapes the furnace with the off-gas and is oxidized and condensed in the cooler upper regions of the furnace and in the off-gas ductwork, forming ultrafine amorphous SiO₂ particles.
The material is collected from the furnace off-gas using baghouse filter systems and is subsequently processed to meet commercial specifications. The as-collected fume is extremely low in bulk density and difficult to handle and transport; therefore, it is commonly subjected to densification, a mechanical process that uses compressed air or mechanical agitation to agglomerate the ultrafine particles into larger, more easily handled granules, increasing the bulk density from approximately 280–450 kg/m³ to 500–700 kg/m³ while preserving the primary particle size and pozzolanic reactivity. Product quality is governed by standards such as ASTM C1240 and EN 13263, which specify limits for SiO₂ content, moisture, loss on ignition, and pozzolanic activity index.
Safety Information
The hazard classification of silica fume is grade- and jurisdiction-dependent, largely reflecting the potential presence of trace crystalline silica in the product and the physical form of the dust. Many suppliers classify silica fume under GHS with the signal word Danger, citing H372 (Causes damage to respiratory organs through prolonged or repeated exposure) — Specific Target Organ Toxicity, Repeated Exposure, Category 1 — and H350i (May cause cancer by inhalation) — Carcinogenicity, Category 1A. These classifications derive from the known health hazards of respirable crystalline silica, of which silica fume may contain small amounts (typically below 1–2%, and the respirable crystalline fraction is generally below 0.1%). Other regulatory assessments, including some EU CLP evaluations, conclude that the substance does not meet the criteria for hazard classification where the crystalline silica content is controlled below the relevant thresholds.
The primary occupational health concern is inhalation of respirable dust. Prolonged overexposure to amorphous silica dust may lead to respiratory tract irritation, and extended exposure has been associated with a risk of fibrosis in some assessments. Finely divided dust may also cause mechanical irritation and dehydration of mucous membranes. The occupational exposure limit for respirable amorphous silica is commonly set at 2 mg/m³ as a TLV-TWA. In the presence of hydrofluoric acid, silica fume can generate toxic silicon tetrafluoride (SiF₄) gas, and contact with HF must be rigorously avoided.
For safe handling, use local exhaust ventilation or dust extraction at points of transfer, weighing, and mixing. Personal protective equipment should include respiratory protection where dust concentrations may exceed occupational limits, safety glasses or goggles, and protective gloves and clothing. Avoid breathing dust and wash exposed skin thoroughly after handling. In case of eye contact, flush with water; for skin contact, wash with soap and water. Store in a cool, dry, well-ventilated area with containers kept closed and protected from moisture, which can cause caking and loss of flowability. Spills should be cleaned using dust-suppression methods (water spray or vacuum with HEPA filtration) and disposed of in accordance with local regulations; release to drains and watercourses should be avoided.
