Silica Fume (Microsilica) in Concrete: Dosage, Strength and Low Permeability

Silica fume (microsilica) is an ultrafine amorphous silicon dioxide, collected as a co-product of silicon and ferrosilicon production, that replaces 5 to 10 % of the cement by mass to make concrete stronger and denser. The same material is sold as condensed silica fume, and its spherical particles of 20 to 500 nm (Xu et al., 2016) fill the spaces between cement grains. Why does such a small dose matter? The particles react with calcium hydroxide and pack the gaps between cement grains at once.

Three national specifications set the acceptance values for silica fume: ASTM C1240-20 in the US, EN 13263-1:2005+A1:2009 in Europe and IS 15388:2003 in India. In the US, 26 of 34 responding state DOTs (76 %) allow it (ADOT SPR-780 survey, 2025), which places it among the concrete additives that state specifications already name.

The Silica Fume Association (SFA) manual gives typical values of below 1,000 coulombs and above 50 MPa (7,250 psi) at 28 days for 5 to 10 % silica fume, against above 3,000 coulombs and 35 MPa (5,080 psi) without it. Its starting mixtures carry 24 to 47 kg/m³ (40 to 79 lb/yd³) of silica fume at a w/cm of 0.24 to 0.45, always with a water reducer or high-range water reducer. The fume raises air-entraining admixture demand by about 50 to 100 %, needs a tighter evaporation limit of 0.5 kg/m²/h (0.1 lb/ft²/h), and was priced at USD 496 per tonne (USD 450 per US ton) in a 2025 Tennessee DOT study.

The table below summarizes silica fume as a concrete ingredient, from material to hazard line.

Each value is traced to its source: standards and codes to their issuing body, product values to the data sheet named with them, prices to the dated source beside them (methodology and fact-checking).

What Is Silica Fume?#

Silica fume is a very fine pozzolan of at least 85 % amorphous SiO2, with spherical particles of about 20 to 500 nm, that ASTM C1240-20, EN 13263-1:2005+A1:2009 and IS 15388:2003 specify as a cementitious ingredient of concrete. The 85 % floor is the ASTM C1240-20 and EN 13263-1 Class 1 minimum; EN Class 2 accepts 80 %.

Its chemical identity is Fumes, silica (silica fume), CAS 69012-64-2, EC 273-761-1, registered under REACH (ECHA, checked 2026-10-23). In concrete it works as a pozzolanic type II addition (European Guidelines for Self-Compacting Concrete, 2005; EN 13263-1 scope), so it is batched as part of the binder and counted in the water-cementitious materials ratio.

That binder role places silica fume beside fly ash, slag cement, metakaolin and calcined clay, the family covered under supplementary cementitious materials (SCMs).

How is silica fume made, and what is it made of?#

Silica fume is made as a co-product of silicon and ferrosilicon production, and it consists of amorphous silicon dioxide at a purity of 85 to 99 % SiO2 (Elkem). The fume is collected as a powder of spherical particles of 20 to 500 nm (Xu et al., 2016), with a BET specific surface of 15,000 to 30,000 m²/kg that raises the water demand of any mix it enters (SFA manual). The material carries three names, silica fume, microsilica and condensed silica fume, for one substance. It is not fumed silica: fumed silica and nano-silica are synthetic amorphous silica with CAS 7631-86-9 and EC 231-545-4, while silica fume has CAS 69012-64-2.

Densified, undensified and slurry forms#

Silica fume is supplied in three forms: densified powder in bulk and bags, undensified powder, and slurry, which is no longer available in the US (SFA manual). Densified fume in bulk is stored and dispensed like other cementitious materials (SFA manual). ASTM C1240-20 tests slurried or densified silica fume on the raw fume it was made from (scope 1.2), so every form answers to the same requirements.

Undensified fume is the form for mixes where densified fume may not break down. The SFA manual states that densified fume may not disperse in dry-mix shotcrete or in mixes without coarse aggregate, where undensified fume is used instead, and that undispersed lumps can cause freeze-thaw popouts. Silica fume in oil-well cements, refractory castables and pre-bagged repair products belongs to other industries and is not covered here. The dry-process case belongs with shotcrete mixes.

Regional names: microsilica, condensed silica fume and IS 15388#

The material is called silica fume in US practice, where it is the ACI term, and microsilica in the UK, Europe and India, with "micro silica" and "silica fume" both used under IS 15388. Condensed silica fume is a further name for the same powder.

How Does Silica Fume Work in Concrete?#

Silica fume works in two ways at once: it reacts with the calcium hydroxide released by cement hydration to form more calcium silicate hydrate, and its sub-micron particles fill the spaces between cement grains as a microfiller. The chemical part is the pozzolanic reaction; the physical part is particle packing.

Both effects come from the fineness of the fume. A specific surface of 15,000 to 30,000 m²/kg (BET) gives fast reaction and close packing, but the same surface must be wetted, so water demand rises and silica fume mixes are always used with a water reducer or high-range water reducer (SFA manual).

Reaction with calcium hydroxide and the microfiller effect#

The reaction converts calcium hydroxide into additional C-S-H, while the microfiller effect refines the pore structure and densifies the paste-aggregate interface (SFA manual). The fume also stops bleeding: the SFA manual reports no bleeding in most mixtures above about 5 % silica fume. That effect changes how silica fume concrete is finished and protected, as the water demand section below explains.

What Does Silica Fume Change in Fresh and Hardened Concrete?#

Silica fume changes four things: it raises strength, cuts chloride penetration to below 1,000 coulombs at 5 to 10 %, raises water demand while removing bleed water, and adds as much heat as the cement it replaces. PCA EB547 puts the heat of silica fume at 100 to 120 % of the heat of the cement it replaces at 5 to 10 %, and the Gajda (2006) rule in PCA EB547 counts silica fume as 1.25 times cement in the adiabatic rise estimate. Silica fume therefore does not reduce heat in mass concrete (PCA EB547), which matters wherever heat of hydration controls the pour.

Strength gain by silica fume content#

Strength rises with silica fume content, from 35 MPa (5,080 psi) at 0 % to above 50 MPa (7,250 psi) at 5 to 10 % (SFA manual Table 5.2, typical 28-day values). The same table gives above 65 MPa (9,430 psi) above 10 % and above 85 MPa (12,330 psi) above 15 %. The top band applies at 475 kg/m³ (801 lb/yd³) of binder and a w/cm below 0.35.

Ultra-high-performance concrete shows the upper end. The Ductal composition in FHWA-HRT-13-060 (Table 1) carries 231 kg/m³ (390 lb/yd³) of silica fume, 9.3 % of the mix mass, with 712 kg/m³ (1,200 lb/yd³) of cement. The non-proprietary NS1-SF mix of the Tennessee DOT study RES2024-04 uses 113 to 116 kg/m³ (190 to 195 lb/yd³) of silica fume, 10 % of cementitious materials, at a w/cm of 0.18. In that study, silica fume with NS1 sand reached 119 MPa (17,300 psi) at 28 days, and with MS sand 136.9 MPa (19,860 psi).

Low permeability: coulomb values by silica fume content#

Silica fume concrete at 5 to 10 % passes below 1,000 coulombs in the ASTM C1202 test, the "very low" class, against above 3,000 coulombs for concrete without silica fume (SFA manual Table 5.2, typical values). ASTM C1202-25 is the active edition; its charge-passed classes, as reproduced by NRMCA (2018), are above 4,000 high, 2,000 to 4,000 moderate, 1,000 to 2,000 low, 100 to 1,000 very low and below 100 negligible coulombs. The test indicates resistance to chloride ion penetration and is not a direct measure of the permeability of concrete.

The table below sets the SFA typical values for strength and charge passed against the C1202 classes.

SFA manual Table 5.2, typical values; classes as reproduced by NRMCA.

Two studies confirm the direction with measured numbers. Xu et al. (2016), at w/b 0.30 and 560 kg/m³ (944 lb/yd³) of binder, measured 529 coulombs for the control, 493 at 5 % silica fume, 212 at 10 %, 198 at 15 % and 189 at 20 %, so the largest drop came between 5 and 10 %. In NRMCA research by Obla, at w/cm 0.39 and 56 days, portland cement alone passed 1,722 coulombs and a mix with 7 % silica fume passed 299 coulombs.

Higher water demand and no bleeding#

Silica fume raises water demand because its specific surface of 15,000 to 30,000 m²/kg must be wetted, and above about 5 % it stops bleeding in most mixtures (SFA manual). Water demand is met with a water reducer or high-range water reducer rather than extra water, so the w/cm stays low.

The missing bleed water changes protection on site. With no bleed water reaching the surface, the SFA guidance sets an evaporation limit of 0.5 kg/m²/h (0.1 lb/ft²/h) for silica fume concrete instead of the ACI 1.0 kg/m²/h (about 0.2 lb/ft²/h). The lower evaporation limit is the silica fume rule against plastic shrinkage cracking.

Silica Fume Dosage: Ranges, Basis and Batching#

Silica fume is dosed at 5 to 10 % by mass of cement in structural concrete (IS 456:2000 cl. 5.2.1.2; Concrete Society). The SFA starting mixtures carry 24 to 47 kg/m³ (40 to 79 lb/yd³) at a w/cm of 0.24 to 0.45. UHPC uses far more, and deck binders combine silica fume with fly ash or slag cement at lower shares.

Dose by application, with a worked batch#

A silica fume dose is stated on one of two bases, % by mass of cement or % by mass of total cementitious material, or as kg/m³ of concrete, and the basis changes the number for the same batch. IS 456:2000 and the Concrete Society give 5 to 10 % by mass of cement, while NYSDOT and INDOT state silica fume as a share of total cementitious materials. The table below compares published doses by source and basis.

Convert doses between bases with the admixture dosage calculator.

The worked example below uses three labeled inputs: 420 kg/m³ of total cementitious material, 8 % silica fume by mass of cementitious material and a w/cm of 0.35. The four lines are a calculated example.

1. Calculated: 420 kg/m³ (708 lb/yd³) of total cementitious material at 8 % silica fume by mass of cementitious material = 33.6 kg/m³ (56.6 lb/yd³) of silica fume and 386.4 kg/m³ (651 lb/yd³) of cement.
2. Calculated: the silica fume equals 8.7 % of the cement mass, inside the 5 to 10 % band, and 33.6 kg/m³ sits inside the SFA 24 to 47 kg/m³ starting range.
3. Calculated: that is about 1.5 bags of 22.7 kg (50 lb) per m³.
4. Calculated: at w/cm 0.35 the water is 147 kg/m³ (248 lb/yd³); the full proportioning method is covered under concrete mix design.

Why silica fume mixes need a superplasticizer#

Silica fume mixes need a water reducer or high-range water reducer because the fume's surface area of 15,000 to 30,000 m²/kg raises water demand (SFA manual). The SFA starting mixtures run at a w/cm of 0.24 to 0.45, so the water reducer, not extra water, supplies the workability.

The dose of that dispersant climbs as the w/cm falls. The SFA manual reports that the high-range water reducer dose for very low w/cm can reach twice the recommended dose, and more in UHPC; the TDOT study's UHPC mixes ran at a w/cm of 0.18. Dose by chemistry and slump life are covered under superplasticizers.

Batching densified silica fume in truck mixers#

Densified silica fume is never batched alone or first, because it packs at the head of the drum and forms balls instead of dispersing (SFA manual). Mixing has to break densified fume down, which takes load control, revolutions and the right slump. The five truck-mixing steps of the SFA manual are listed below.

1. Load the truck only to its rated mixing capacity, 63 % of drum volume.
2. Add the densified silica fume together with the other cementitious materials, never alone or first.
3. Mix at least 100 revolutions at mixing speed.
4. Mix at a slump of 50 to 100 mm (2 to 4 in.).
5. Add 30 extra revolutions after each admixture addition; haul and dosing practice is covered under admixtures in ready-mix concrete.

Which Standards Specify Silica Fume?#

Three national specifications set the requirements for silica fume: ASTM C1240-20 in the US, EN 13263-1:2005+A1:2009 in Europe with Classes 1 and 2, and IS 15388:2003 in India. ASTM and EN Class 1 both require at least 85 % SiO2. The table below compares the three, with the European concrete standard and the ACI guide.

The US specification and the ACI guide#

ASTM C1240-20, "Standard Specification for Silica Fume Used in Cementitious Mixtures", is the active US specification, approved 2020-01-15 and the only active edition on the ASTM store on 2026-10-23. First approved in 1993, it replaced C1240-15 and is maintained by ASTM Subcommittee C09.24. Its mandatory limits, as reproduced in the SFA manual, are SiO2 at least 85 %, moisture at most 3.0 %, loss on ignition at most 6.0 %, at most 10 % retained on the 45 µm (No. 325) sieve with a variation of at most 5 points from the average, an accelerated pozzolanic activity index of at least 105 % of control at 7 days, and a BET specific surface of at least 15 m²/g. Optional requirements cover air-entraining admixture uniformity (variation at most 20 %), an 80 % reduction of mortar-bar expansion from alkali reactivity at 14 days, and sulfate expansion; bulk density, density and total alkalies are reported only.

ACI PRC-234-26, "Use of Silica Fume in Concrete: Guide" (2026, 112 pages), replaces ACI PRC-234-06 (reapproved 2012) as the ACI guide.

The European standard: Classes 1 and 2#

EN 13263-1:2005+A1:2009 sets two classes of silica fume for concrete, Class 1 with at least 85 % SiO2 and Class 2 with at least 80 %, under a harmonised standard with AVCP system 1+. Both classes share the other limits: SO3 at most 2.0 %, chloride at most 0.3 % and declared if above 0.10 %, free CaO at most 1.0 %, free Si at most 0.4 %, loss on ignition at most 4.0 %, BET 15 to 35 m²/g and an activity index of at least 100 % at 28 days (as summarized in the SFA manual). Amendment A1, approved 2009-02-08, added Class 2, and the A1 edition replaced EN 13263-1:2005 in the Official Journal list with coexistence from 2010-01-01 to 2011-01-01. CEN confirmed the standard at stage 90.93 with the review completed 2026-10-21, and no successor is listed.

The concrete itself is specified under EN 206-1:2026, approved by CEN on 2026-01-19 and dated March 2026, which supersedes EN 206:2013+A2:2021. In UK EN 206 practice, silica fume to EN 13263-1 may carry up to 0.30 % chloride, declared above 0.10 %, against 0.10 % for cement, fly ash and GGBS (BRMCA, 2019). The rules for counting additions sit in the EN 206 concrete specification.

Silica fume under IS 15388 and IS 456#

IS 15388:2003, "Specification for Silica Fume", is the Indian standard, active, reaffirmed in 2022 and carrying one amendment (BIS, checked 2026-10-23). IS 456:2000 clause 5.2.1.2 calls for silica fume to IS 15388 as part replacement of cement with uniform blending and gives 5 to 10 % of the cement content as the usual proportion. The IS 15388 requirement values are not reproduced here, because no verified copy of the amended text was available.

How Does Silica Fume Interact with Cement, Admixtures and Other SCMs?#

Silica fume interacts with three parts of the mix: the water reducer it depends on, the air-entraining admixture whose demand it raises, and the other SCMs it is blended with.

Why silica fume mixes need more air entrainer#

Silica fume mixes need more air entrainer because the fume raises air-entraining admixture demand by about 50 to 100 % (SFA manual). The SFA manual also allows the air content to be 1 percentage point lower when f'c is above 35 MPa (5,000 psi). Dispersion matters too: undispersed lumps of densified fume can cause freeze-thaw popouts. Dose ranges and the air-void system are covered under air-entraining admixtures.

Ternary blends with other SCMs#

Silica fume is combined with fly ash or slag cement in ternary blends, and three published blends are listed below.

• NYSDOT deck binder: 20 % Class F fly ash in concrete plus 6 % silica fume by mass of cementitious materials, used for bridge decks since 1998 (agency practice, as reported by the National Academies).
• Slag ternary against ASR: 50 % slag, or 25 % slag plus 3.8 to 4 % silica fume, were effective against ASR (Hooton et al. 2013, via the National Academies); see slag cement (GGBS).
• INDOT internally cured HPC deck concept: below 1,500 coulombs with silica fume 3 to 7 %, fly ash 20 to 25 % or slag 15 to 20 %, at w/cm 0.36 to 0.43 (INDOT, presented by Weiss, Purdue, 2014).

When Should You Specify Silica Fume?#

Specify silica fume where the job needs very low chloride penetration, high strength or a dense UHPC matrix: bridge decks, parking structures and marine elements in the first group, high-strength members and UHPC in the second. The SFA manual records US use in parking structures from the early 1980s, and its typical values place 5 to 10 % silica fume in the very low coulomb class. The table below matches applications to what silica fume must do in each.

When silica fume is the wrong choice#

Silica fume is the wrong choice in three cases: heat reduction in mass concrete, architectural or UHPC work that needs light color or more flow, and members where carbonation governs. The fume gives 100 to 120 % of the heat of the cement it replaces (PCA EB547), so it adds no heat relief. In the TDOT RES2024-04 study, silica fume with NS1 sand reached 514 mm (20.25 in.) of slump flow against a 533 mm (21 in.) target, while metakaolin reached 648 mm (25.5 in.) and 131 to 132 MPa (19,060 to 19,150 psi); metakaolin also suits light-colored architectural concrete. At 25 % replacement, silica fume raises the carbonation coefficient by a factor of about 2.3 against portland cement (RILEM TC 281-CCC). For light color and flow, the alternative pozzolan is metakaolin.

Which Tests Prove Silica Fume Performance?#

Two sets of tests prove silica fume performance: material acceptance against ASTM C1240-20 or EN 13263-1:2005+A1:2009 on every lot, and concrete tests on the mix, led by ASTM C1202-25 charge passed, slump by ASTM C143/C143M-26b and compressive strength. The table below lists each test with its standard and what it shows.

How Does Silica Fume Change Durability?#

Silica fume improves most durability properties through the dense, fine-pored paste it makes, but it raises autogenous shrinkage and does not improve carbonation resistance. The seven effects below run from the main benefit to the main limits.

1. Chloride: silica fume improves resistance at early ages and slightly reduces chloride binding (NRMCA, 2018); see chloride ingress and rebar corrosion.
2. ASR: AASHTO PP 65-11 (as reproduced in FHWA-HIF-13-019) sets minimums of 2.0, 2.5, 3.0 and 4.0 times KGA for levels W to Z, where KGA is the cement content in kg/m³ times the cement alkali in % divided by 100 (US form 1.2, 1.5, 1.8 and 2.4 times LBA), never below 7 % as the sole measure; see alkali-silica reaction (ASR).
3. Sulfate: up to 15 % silica fume performed well in sodium sulfate but poorly in magnesium sulfate (Whittaker et al. 2015, study result); see sulfate attack on concrete.
4. Freeze-thaw: the air-entraining admixture demand rises by about 50 to 100 %, and undispersed lumps can cause popouts (SFA manual); see freeze-thaw damage.
5. Autogenous shrinkage: silica fume refines pores and raises self-desiccation shrinkage (Bentz and Jensen 2004), with chemical shrinkage up to 0.22 mL/g against about 0.07 mL/g for portland cement (NISTIR 7765, 2011); see autogenous shrinkage.
6. Carbonation: at 25 % replacement the carbonation coefficient rises by a factor of about 2.3 against portland cement (RILEM TC 281-CCC); see carbonation of concrete.
7. Fire spalling: where fck is 70 MPa (10,150 psi) or more or silica fume is 6 % or more by weight of cement, EN 1992-1-2:2023 requires a specific spalling assessment or polypropylene fibers, with kpp = 2.0 kg/m³ (3.4 lb/yd³) of monofilament fibers of 50 µm diameter or less unless the National Annex differs (as reproduced in Hormigón y Acero, 2023); see polypropylene microfibers.

Silica Fume Safety and Handling#

Silica fume (Fumes, silica, CAS 69012-64-2) has no harmonised CLP classification: 93.44 % of ECHA C&L notifications record it as not classified, and the product SDS governs handling of the fine dust. STOT RE 2 H373 is also notified by a minority (6.56 %) of notifiers (ECHA C&L, checked 2026-10-23). The table below lists the hazard line and the limits that apply to crystalline silica.

Silica fume itself is amorphous; the crystalline silica limits above govern any crystalline silica a product or a jobsite process releases, not silica fume as such.

What Does Silica Fume Cost?#

Two dated public records price silica fume in the US: USD 496 per tonne (USD 450 per US ton) as of April 2025 in a Tennessee DOT study, and USD 661 per tonne (USD 600 per ton) in July 2003. The TDOT RES2024-04 value is a study input price for UHPC mixes, and the FHWA report 03-047 value describes 2003 only, never today's market.

The SFA manual expresses cost as an index instead: silica-fume concrete mixes cost 128 to 179 % of the control mix in materials per m³ (US, 2022), an index and not a price. A quote compares only when it names the standard and class, the form (densified or undensified), the quantity, the packaging and the delivery point, and is stated per tonne delivered.

Silica Fume Grades, Packaging and Sourcing#

Silica fume is sold densified in bulk tankers of about 20 t, in 22.7 kg (50 lb) and 11.4 kg (25 lb) bags and in big bags, and buyers compare lots on the standard class and test report. It is available from suppliers of concrete admixtures and is added during concrete production (SFA). Contracts, delivery forms and acceptance across the SCM family are described under how SCM supply works.

Global output is at most 1 to 1.5 million tonnes a year (literature cited in Xu et al. 2016), and Grand View Research, a commercial publisher, estimates the silica fume market at USD 1.5 billion in 2025. The five items to request with every offer are listed below.

• Standard and class: ASTM C1240-20, EN 13263-1:2005+A1:2009 Class 1 or Class 2, or IS 15388:2003, with the edition named.
• Test report per lot: SiO2, loss on ignition, 45 µm residue, BET surface and activity index against the class limits.
• Form: densified or undensified, chosen for the batching plant and the mix type.
• Safety data sheet: the current SDS with the product's crystalline silica content.
• Delivery and storage: bulk tanker, 22.7 kg (50 lb) or 11.4 kg (25 lb) bags or big bags, matched to silo or bag-handling capacity.

Producers and distributors are compared under concrete admixture manufacturers and suppliers.

Where Silica Fume Fits Among Supplementary Cementitious Materials#

Silica fume is a sub-micron pozzolan dosed at a fraction of the fly ash or slag cement replacement levels: 5 to 10 % by mass of cement. Class F fly ash runs at 15 to 25 % (FHWA citing ACI) and slag cement at 20 to 80 % (Slag Cement Association), and silica fume pairs with both in ternary blends such as 25 % slag plus 3.8 to 4 % silica fume.

Alternatives differ in fineness and identity. Metakaolin is the light-colored pozzolan, while synthetic amorphous silica with CAS 7631-86-9 is a different product, with particles finer than 100 nm, sold as nano-silica (colloidal silica).

Silica Fume FAQs#

Is silica fume harmful?#

Silica fume is not classified in 93.44 % of ECHA C&L notifications, but it is a fine dust handled by the product SDS. Any crystalline silica a product contains is the regulated hazard, with an OSHA PEL of 50 µg/m³ as an 8-hour TWA.

Is microsilica the same as silica fume?#

Yes: microsilica, condensed silica fume and micro silica are regional names for the same material that ACI and ASTM call silica fume.

Is silica fume the same as fumed silica?#

No: fumed silica (CAS 7631-86-9) is a synthetic amorphous silica, while silica fume (CAS 69012-64-2) is collected as a co-product of silicon and ferrosilicon production. Nano-silica shares the fumed silica identity, CAS 7631-86-9.

What is silica fume used for?#

In concrete, silica fume is used for high-strength and low-permeability mixes: US parking structures from the early 1980s, an Ohio bridge deck overlay in 1990 and the first US UHPC bridge deck overlay in Buchanan County, Iowa, in 2016.

What is Elkem Microsilica?#

Elkem Microsilica is the name under which Elkem markets its silica fume. Like any silica fume, a lot is judged by its test report against ASTM C1240-20 or EN 13263-1:2005+A1:2009.