Supplementary Cementitious Materials: Fly Ash, Slag, Silica Fume, Metakaolin and Calcined Clay

Supplementary cementitious materials (SCMs) are fine mineral powders, such as fly ash, slag cement, silica fume, metakaolin and calcined clay, that replace part of the portland cement in concrete and react to form more binder. Replacement runs from 5 to 10 % by mass of cement for silica fume (Concrete Society) to 20 to 80 % by mass of total cementitious material for slag cement (Slag Cement Association). Which SCM fits a mix, how much cement it replaces and which standard proves it?

The same materials are also called supplementary cementing materials, and British practice writes GGBS for ground granulated blast-furnace slag, PFA for pulverized fuel ash and microsilica for silica fume. SCMs are not chemical admixtures in the ACI sense; they form their own group of concrete additives, next to chemical admixtures, fibers and pigments.

Class F fly ash cuts the heat of hydration about 50 % (PCA EB547), silica fume at 5 to 10 % brings the ASTM C1202 charge from over 3,000 to under 1,000 coulombs (Silica Fume Association typical values), and the NRMCA Industry-Wide EPD v4 (2026) lists 330.17 kg CO2e/m³ for 27.6 MPa (4,000 psi) concrete without SCMs against 215.07 with 20 % fly ash plus 30 % slag. The trade-off is carbonation, about 2.3 times faster at 25 % fly ash (RILEM TC 281-CCC, 2025). ACI CODE-318-25 removed the exposure class F3 caps on SCM content.

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 Are Supplementary Cementitious Materials?#

A supplementary cementitious material is a pozzolanic or latent hydraulic mineral powder that is batched with portland cement and reacts with the calcium hydroxide or the alkalis that cement releases, adding binder and refining pores.

SCMs come from three origins: industrial by-products such as coal fly ash, blast-furnace slag and silica fume; calcined minerals such as metakaolin and calcined clay; and natural deposits such as volcanic ash. Replacing cement with them also dilutes its C3A and alkalis (NASEM, 2025).

What is a pozzolan?#

A pozzolan is a siliceous or siliceous and aluminous material with little or no cementitious value of its own that reacts with calcium hydroxide in the presence of moisture to form cementitious compounds. The ACI wording reads: "a siliceous or siliceous and aluminous material that in itself possesses little or no cementitious value but will, in finely divided form and in the presence of moisture, chemically react with calcium hydroxide at ordinary temperatures to form compounds having cementitious properties" (ACI FAQ 688).

In UK practice, pozzolana names natural pozzolans: BS 8615-1:2019 covers natural pozzolana and natural calcined pozzolana for use with portland cement of class 42,5N or higher. Pozzolans are used to replace part of the portland cement in concrete.

SCMs, mineral admixtures and additions by region#

The same materials carry three names: supplementary cementitious materials in the US, additions in European standards and mineral admixtures in India. EN 206-1:2026 (clause 3.2.1) defines an addition as a finely divided inorganic constituent used in concrete to improve certain properties or to achieve special properties, and EN 450-1:2012, EN 15167-1:2006 and EN 13263-1:2005+A1:2009 cover siliceous fly ash, GGBS and silica fume as type II additions for EN 206 concrete.

Mineral admixture is the older ACI term and the current Indian term for SCMs. ACI CT-25 defines a chemical admixture as a liquid or dispersible powder that improves economy or properties, so SCMs and fibers are not admixtures in the ACI sense. The three words admixture, additive and addition are compared in admixture vs additive vs addition.

How Do Supplementary Cementitious Materials React in Concrete?#

SCMs react later than portland cement: pozzolans consume the calcium hydroxide that cement hydration releases, latent hydraulic materials hydrate once cement alkalis activate them, and fillers act as packing and nucleation sites. This lag slows set and early strength in fly ash and slag mixes and raises later strength (FHWA; Slag Cement Association).

The pozzolanic reaction: from calcium hydroxide to C-S-H#

The pozzolanic reaction combines the reactive silica of a pozzolan with calcium hydroxide and water to form additional calcium silicate hydrate (C-S-H). Class F fly ash, natural pozzolans, silica fume, metakaolin, calcined clay, rice husk ash and ground glass react this way (NASEM, 2025); metakaolin also forms alumina-bearing hydrates.

The extra C-S-H refines the pore structure, which improves later strength and resistance to ASR and sulfate attack. Silica fume also acts as a microfiller that densifies the paste and the paste-aggregate interface (Silica Fume Association).

Latent hydraulic reaction, filler and nucleation effects#

Slag cement is latent hydraulic: its calcium aluminosilicate glass, made by water-quenching iron blast-furnace slag, hydrates to C-S-H once portland cement alkalis and calcium hydroxide activate it. Slag also consumes calcium hydroxide, has negligible C3A and binds alkalis (Slag Cement Association), and Class C fly ash is partly hydraulic.

Ground limestone works physically: its fine particles improve packing and act as nucleation sites for C-S-H, with minor carboaluminate formation that increases when metakaolin, calcined clay or fly ash supply alumina. Nano-silica combines both effects: its very high surface area gives a fast pozzolanic reaction and C-S-H nucleation (review, Khan et al. 2022).

Reactivity testing with ASTM C1897 (R3)#

ASTM C1897-20 measures SCM reactivity by isothermal calorimetry or bound water at 40 °C (104 °F) after 3 and 7 days, the R3 test. It does not separate hydraulic from pozzolanic reactivity and does not replace concrete tests, so a reactive material still needs its product standard and trial batches.

Types of SCMs: Pozzolanic, Latent Hydraulic and Filler Materials#

SCMs fall into three reaction groups: pozzolans such as Class F fly ash, silica fume, metakaolin and calcined clay; latent hydraulic materials such as slag cement; and fillers such as ground limestone. Product standards follow in Table 5.

Table 1 compares the reaction groups of SCMs by origin and reaction route.

Pozzolans from coal combustion, silicon smelting and calcined minerals#

Pozzolans come from coal combustion, silicon smelting and heated or natural minerals. The seven examples of pozzolanic materials below give each one's identity and one standard or dose fact.

• Fly ash: glassy spherical SiO2-Al2O3 particles from coal combustion. ASTM has used "coal ash" as the umbrella term since C618-22/23, and since C618-25 excludes ash from fluidized-bed combustion, co-burning of municipal solid waste and incinerators.
• Silica fume: an amorphous co-product of silicon and ferrosilicon smelting. ASTM C1240-20, the only active edition on 2026-10-23, requires at least 85 % SiO2 and an accelerated pozzolanic activity index of at least 105 % at 7 days.
• Metakaolin: dehydroxylated kaolinite with 50 to 55 % SiO2 and 40 to 45 % Al2O3, commercial since the mid-1990s (ADOT SPR-780).
• Calcined clay: clay calcined in rotary kilns at 650 to 900 °C (1,202 to 1,652 °F) (Concrete Centre). In VTRC 26-R52 (2026), a calcined clay meeting the ASTM C618 Class N requirements at 30 % of a Type IL cement performed comparably to or better than 30 % Class F coal ash.
• Natural pozzolans: volcanic ash, pumice and calcined shale. ASTM C1945-26b (first edition C1945-24) covers natural pozzolans historically referred to as Class N in C618, and C618-26 still covers raw or calcined natural pozzolan.
• Rice husk ash: amorphous silica, highest when husks burn at 500 to 700 °C (932 to 1,292 °F) (study review), with no dedicated ASTM, EN or IS product standard.
• Ground-glass pozzolan: ASTM C1866/C1866M-25, active since 2025-08-01, covers container, plate or E-glass as Type GS (soda-lime) and Type GE (E-glass).

Latent hydraulic slag and high-calcium ash#

Slag cement is a latent hydraulic SCM that replaces portland cement 1:1 by mass at 20 to 80 % of the total cementitious material (SCA Info Sheet 2). ASTM C989/C989M-26 sorts it into Grades 80, 100 and 120 by a 28-day slag activity index of at least 75, 95 and 115 % (values reported for C989-18). Class C fly ash, with more than 18 % CaO under ASTM C618-26, sits between the groups. Fly ash and slag cement differ in dose range, carbon content and cold-weather set, and the full comparison is in fly ash vs slag cement.

Ground limestone and nanoscale silica#

Ground limestone is the filler among the SCMs, covered by EN 18136:2026 "Ground limestone for use in concrete, mortar and grout" (published 2026-05-12) and ASTM C1797-24, which excludes ground calcium carbonate fillers from sulfate exposure unless mitigated.

Nano-silica is SiO2 finer than 100 nm, supplied as powder or colloidal dispersion, with an optimum of 2 to 3 % by mass of cement and 0.5 to 4 % reported effective (review, Khan et al. 2022); it has no ASTM, EN or IS product standard.

What Do SCMs Change in Fresh and Hardened Concrete?#

SCMs shift concrete properties in known directions: fly ash and slag lower water demand and heat, silica fume raises water demand, and fly ash, slag and silica fume at adequate replacement lower chloride penetration and alkali-silica reaction risk.

Table 2 compares the effect of SCMs on nine properties, with the type of source behind each value.

Water demand, workability and entrained air#

Fly ash lowers water demand about 10 % at about 20 % replacement (FHWA), and slag cement lowers it up to 5 % (SCA). Silica fume, at 15,000 to 30,000 m²/kg, raises water demand, so a water reducer or HRWR is always used, at up to twice the normal HRWR dose at very low w/cm (Silica Fume Association); metakaolin at 15 % cut slump about 30 % in studies reported by ADOT SPR-780.

Unburned carbon in fly ash adsorbs air-entraining admixture, so air is tested at every delivery, while silica fume raises the air-entraining dose 50 to 100 % (Silica Fume Association). ASTM C618-26 Note 1 adds that finely divided materials may reduce entrained air. Silica fume and metakaolin mixes rely on high-range water reducers, set out under superplasticizers (high-range water reducers).

Setting, early strength and temperature rise#

Class F fly ash can delay finishing by 1 to 2 hours, and at 1:1 replacement Class F concrete can gain strength more slowly for 60 to 90 days, then exceed plain concrete (FHWA-RD-97-148). Slag concrete is weaker to about 7 to 14 days and stronger later (Slag Cement Association).

Class F fly ash cuts the heat of hydration about 50 % at all ages, while silica fume gives 100 to 120 % and metakaolin 100 to 125 % of the cement heat at 5 to 10 % and 5 to 15 % replacement (PCA EB547). The Gajda rule in PCA EB547 puts the adiabatic rise at about 0.13 °C per kg/m³ (0.14 °F per lb/yd³) of equivalent cement, with equivalents of 0.5 for Class F fly ash, 0.8 for Class C, 1.25 for silica fume and metakaolin, 0.9 for slag at 50 % and 0.8 at 75 %. Worked example (calculated): 100 kg/m³ (169 lb/yd³) of Class F fly ash counts as 50 kg/m³ of equivalent cement, about 6.5 °C (11.7 °F) of rise against 13 °C (23.4 °F) for the same mass of cement.

Pore refinement, chloride resistance and ASR control#

SCMs slow chloride ingress by refining pores: in Silica Fume Association typical values (Table 5.2), concrete passes over 3,000 coulombs in ASTM C1202 without silica fume and under 1,000 coulombs with 5 to 10 %. The Life-365 v2.2.3 manual models three mixes at w/cm 0.40 with the same 28-day diffusion coefficient of 79 × 10⁻¹³ m²/s; at 25 years it is 25 × 10⁻¹³ m²/s for portland cement, 9.3 for 30 % slag and 3.9 for 40 % fly ash (model).

For alkali-silica reaction, AASHTO PP 65-11, as reproduced in FHWA-HIF-13-019 (2013), sets minimum levels by prevention level W, X, Y and Z: fly ash with CaO at most 18 % and Na2Oe at most 3.0 % at 15, 20, 25 and 35 %, and slag with Na2Oe at most 1.0 % at 25, 35, 50 and 65 % by mass of cementitious material. A combination also qualifies when ASTM C1567 mortar bars expand at most 0.10 % after 14 days in 1 M NaOH; C1567-25 is the active edition.

Carbonation and sulfate resistance#

SCMs speed up carbonation: at 25 % replacement the carbonation coefficient rises against portland cement by a factor of about 2.3 for fly ash and silica fume, 1.9 for limestone and 1.3 for slag (RILEM TC 281-CCC, 2025). With less portlandite, CO2 binds in C-S-H, which coarsens pores.

Sulfate resistance moves the other way. Under ACI CODE-318-25, SCMs that improve sulfate resistance by service record or ASTM C1012 results are permitted besides the listed cement types, and S3 Option 1 combines Type V or HS cement with pozzolan or slag cement at w/cm 0.45 and 31.0 MPa (4,500 psi) (Table 19.3.2.1, NRMCA 2PE004-26). The Slag Cement Association gives 15 to 50 % slag for moderate and 35 to 50 % for high sulfate resistance, by ASTM C1012.

SCM Replacement Levels: Typical Ranges and Dose Basis#

SCMs are dosed as a replacement level by mass: 5 to 10 % of the cement for silica fume, 15 to 35 % of the total cementitious material for fly ash and 20 to 80 % for slag cement. The sources are the Concrete Society, NASEM (2025) and the Slag Cement Association. High-volume fly ash means more than 37 % fly ash by mass of cementitious material (ACI definition, as reported by NASEM).

Table 3 compares the replacement levels of SCMs with the mass basis and source type of each range.

Natural pozzolans, calcined clay and limestone have no general replacement range in these sources; trial batches set them.

Worked example (calculated): 400 kg/m³ (674 lb/yd³) of total cementitious material at 25 % Class F fly ash splits into the quantities below.

• Portland cement: 400 × 0.75 = 300 kg/m³ (506 lb/yd³), calculated.
• Class F fly ash: 400 × 0.25 = 100 kg/m³ (169 lb/yd³), calculated.
• Cement counted under the EN 206 k-value concept: 300 + 0.4 × 100 = 340 kg/m³ (573 lb/yd³), calculated, at a fly ash to cement ratio of 0.33.

Water, aggregate and admixture weights for this batch are worked through in concrete mix design with SCMs.

Replacement by mass, by volume and the 1:1 rule#

Slag cement replaces portland cement 1:1 by mass (SCA), and fly ash replaces it 1:1 to 1.5:1 by mass (FHWA Fly Ash Facts, 2017). At 1.5:1, 100 kg of cement removed is replaced by 150 kg of fly ash (calculated).

Equal mass does not mean equal volume: slag has a specific gravity of 2.85 to 2.94 against 3.15 for portland cement (Slag Cement Association). Worked example (calculated): 100 kg of portland cement occupies 31.7 L (8.4 gal), and 100 kg of slag at a specific gravity of 2.90 occupies 34.5 L (9.1 gal), about 9 % more paste volume.

Binary and ternary blends (ASTM C1697)#

ASTM C1697-26 covers blends of two or three SCMs that each conform to ASTM C618, C989/C989M or C1240, blended or interground. One SCM besides cement makes a binary mix, two make a ternary mix.

New York State DOT bridge decks have used 20 % Class F fly ash plus 6 % silica fume by mass of cementitious material since 1998, and 50 % slag, or 25 % slag with 3.8 to 4 % silica fume, controlled ASR (Hooton et al. 2013); both are reported by NASEM.

Batching and storing SCMs at the plant#

Fly ash and slag cement arrive in bulk, go to a separate silo, are weighed as cementitious material and count in the w/cm (NASEM, 2025). The Silica Fume Association manual adds that densified silica fume is never batched alone or first, because it balls, and that silica-fume concrete bleeds little, so its evaporation limit is 0.5 kg/m²/h (0.1 lb/ft²/h), half the general 1.0 kg/m²/h.

Dry handling of SCM powders can release respirable crystalline silica, for which OSHA sets a PEL of 50 µg/m³ as an 8-hour TWA and an action level of 25 µg/m³ (29 CFR 1926.1153 and 1910.1053, effective 2016-06-23); the EU limit is 0.1 mg/m³ (Directive (EU) 2017/2398). Each product's safety data sheet gives its hazards.

Selecting SCMs by Application, Exposure and Climate#

SCMs are selected by element need: slag cement and Class F fly ash for heat, ASR and later strength, silica fume and metakaolin for low permeability and high strength, and high-volume fly ash or slag for lower CO2.

Table 4 compares the role of SCMs across applications, exposures and climates.

SCMs at high and low concrete temperatures#

SCMs slow down in cold concrete and help in hot concrete: the fly ash reaction is strongly reduced below 4.4 °C (40 °F) (FHWA), and fly ash and slag lower the heat that hot placements add. Slag concrete sets later below 29 °C (85 °F) (Slag Cement Association). In thick sections the temperature rise in mass concrete is planned under temperature rise in mass concrete.

Hot and heat-cured elements also face delayed ettringite formation, which ACI CT-25 describes as not likely unless the curing temperature reaches 70 °C (158 °F) and less likely with pozzolan or slag cement.

SCMs for lower-CO2 mixes#

SCMs lower the global warming potential (GWP) of concrete, and the NRMCA Industry-Wide EPD v4 (version 2026-10-09) quantifies it for 27.6 MPa (4,000 psi) normal-weight non-air-entrained concrete, A1 to A3. Its industry averages are 330.17 kg CO2e/m³ (252.4 per yd³) at 0 % SCM, 261.74 (200.1) at 30 % slag, 239.26 (182.9) at 40 % fly ash, 216.12 (165.2) at 50 % slag and 215.07 (164.4) at 20 % fly ash plus 30 % slag, 35 % below the plain mix (calculated).

NRMCA advises about 30 % fly ash or slag where high early strength is needed and up to 70 % in footings, basement walls and some columns and shear walls tested at 56 or 90 days. The RILEM carbonation factor of about 2.3 for 25 % fly ash means carbonation exposure must be checked when GWP falls; targets for sustainable mixes are compared under low-carbon concrete.

ACI 318 SCM limits: from the F3 caps to ACI CODE-318-25#

ACI 318-19 Table 26.4.2.2(b) capped SCMs only in exposure class F3, and ACI CODE-318-25 removed class F3 and those caps (NRMCA, 2026). The caps limited fly ash or other pozzolans to 25 %, slag cement to 50 %, silica fume to 10 %, fly ash plus silica fume to 35 % and all three to 50 % of total cementitious material by mass, and NRMCA advises not to specify them for other concrete.

Other SCM rules stay in force. ACI CODE-318-25 Table 19.3.2.1 counts SCM mass in the chloride limit only up to the mass of cement, and FHWA-HIF-17-008, citing ACI 201.2R-16, limits fly ash to 25 % and slag to 50 % on hand-finished surfaces exposed to deicers, not on formed or machine-finished ones.

Which Standards Classify SCMs? ASTM, EN and IS Side by Side#

The main SCMs have product standards in each region: ASTM C618-26, C989/C989M-26 and C1240-20 in the US, EN 450-1:2012, EN 15167-1:2006 and EN 13263-1:2005+A1:2009 in Europe, and IS 3812, IS 16714, IS 15388 and IS 16354 in India. Newer documents add ASTM C1945-26b (natural pozzolans), C1866/C1866M-25 (ground glass), C1797-24 (ground calcium carbonate fillers) and EN 18136:2026 (ground limestone).

The three EN product standards are harmonised under Regulation (EU) No 305/2011 with assessment system AVCP 1+. ACI guides add practice: ACI PRC-234-26, Use of Silica Fume in Concrete: Guide (2026), and ACI 233R-17, Guide to the Use of Slag Cement in Concrete and Mortar.

Table 5 compares the ASTM, EN and IS standards for SCMs by edition, status and key requirement.

European additions and the k-value rule#

The EN 206 k-value concept counts part of an addition as cement: fly ash to EN 450-1 counts with k = 0.4 up to a fly ash to CEM I ratio of 0.33 by mass (Concrete Society). In the worked example above (calculated), 300 kg/m³ of CEM I with 100 kg/m³ of fly ash counts as 340 kg/m³ (573 lb/yd³) of cement. The UK uses the equivalent-combination (EPCC) route of BS 8500-2 instead.

The k-value clauses of EN 206-1:2026 and the k-values for silica fume and GGBS are not reproduced: the 2026 clauses are not in the public preview, and the other values are not verified against the EN text. How the k-value enters the equivalent water-cement ratio is shown under water-cement ratio (w/c and w/cm).

Indian standards for mineral admixtures#

India specifies its mineral admixtures in IS 3812 (Parts 1 and 2):2013 for fly ash, IS 16714:2018 for GGBS, IS 15388:2003 for silica fume and IS 16354:2015 for metakaolin. On the BIS portal (checked 2026-10-23), IS 3812 is reaffirmed 2022, IS 16714:2018 is active with 1 amendment, IS 15388:2003 is active, reaffirmed 2022, with 1 amendment, and IS 16354:2015 is active, reaffirmed 2020, with 1 amendment. Their requirement values are not reproduced because they are not yet matched to the current editions.

DOT acceptance of SCMs in the US#

Of 34 US transportation agencies answering the ADOT SPR-780 survey (2025), 26 (76 %) accept silica fume and 26 (76 %) Class F fly ash. Slag follows with 22 (65 %), then natural pozzolans with 12 (35 %), ground limestone with 9 (26 %) and calcined clay with 4 (12 %). Ground glass to ASTM C1866 is used by at least 4 states.

Complete List of Supplementary Cementitious Materials#

The complete list runs from fly ash, of which US concrete used 13.2 million t (14.6 million short tons) in 2024 (ACAA), to ground-glass pozzolan and nano-silica, each with its own page.

Table 6 compares the supplementary cementitious materials by group, standard, replacement level and hazard line.

The hazard lines are for the dry powder as notified at ECHA (checked 2026-10-23), and none is a harmonised classification. Dry handling can release respirable crystalline silica: see each product's safety data sheet.

US slag cement shipments passed 4 million t in 2025 (Slag Cement Association). Haul distance and terminal logistics dominate the delivered cost of SCMs, and harvested or beneficiated ash, ground slag and densified silica fume add processing cost; bulk supply is covered under fly ash, slag cement and silica fume suppliers.

When Should SCM Content Be Limited?#

SCM content is limited on hand-finished slabs exposed to deicers, at 25 % fly ash or 50 % slag (FHWA-HIF-17-008), and is checked for cold placements, early-strength schedules and chloride calculations. Cold matters because the fly ash reaction is strongly reduced below 4.4 °C (40 °F) (FHWA), and schedules because Class F concrete at 1:1 can gain strength more slowly for 60 to 90 days (FHWA-RD-97-148).

In chloride calculations, ACI CODE-318-25 Table 19.3.2.1 counts SCM mass only up to the mass of cement, so extra SCM does not dilute the chloride content on paper beyond that point.

What is SCMs in cement?#

SCMs reach concrete in two ways: batched separately at the concrete plant, or blended or interground with cement before delivery. Blended cements that contain SCMs as a cement constituent, such as ASTM C595 types, EN 197-1 cements, LC3 and Indian portland pozzolana and slag cements, are cement products and are not covered here.

Is pozzolan the same as coal ash?#

No: coal ash is one pozzolan among several. Fly ash is pozzolanic as Class F and partly hydraulic as Class C, split at 18 % CaO in ASTM C618-26, which has used "coal ash" as its umbrella term since C618-22/23. Silica fume, metakaolin and natural pozzolans do not come from coal.

Is portland cement a pozzolan?#

No: portland cement is hydraulic and hardens with water alone, while a pozzolan has little or no cementitious value until it reacts with calcium hydroxide.

Is pozzolana still used today?#

Yes: natural pozzolans have had their own ASTM specification, C1945, since C1945-24, and 12 of 34 responding US DOTs accept them (ADOT SPR-780, 2025). Roman concrete already combined volcanic-ash pozzolan with lime (study, Seymour et al., Science Advances 2023).

Are SCMs the same as chemical admixtures?#

No: SCMs replace part of the cement and react as binder, while chemical admixtures are liquids or dispersible powders dosed to improve economy or properties (ACI CT-25). The additive families of concrete are listed below.

• Chemical admixtures, such as water reducers, retarders and accelerators: see concrete admixtures.
• Supplementary cementitious materials, such as fly ash, slag cement and silica fume: this page.
• Steel, synthetic, glass and basalt fibers for fiber-reinforced concrete.
• Iron oxides and other integral colors among concrete pigments.