Fly Ash in Concrete: Class F vs Class C, Replacement Rates, Strength and Durability

Fly ash replaces 15 to 35 % of the total cementitious material in concrete by mass in its normal range (National Academies, 2025). ASTM C618-26 sorts it into Class F, with calcium oxide (CaO) at most 18 %, and Class C, with more than 18 %. British and Irish specifications call the same powder pulverised fuel ash (PFA), and Europe specifies it as siliceous fly ash under EN 450-1:2012.

Coal fly ash reacts with the calcium hydroxide that portland cement releases, so it adds later strength, refines pores and lowers permeability, while it slows set and early strength. It mitigates alkali-silica reaction and sulfate attack at the right dose, and US concrete used 14.6 million short tons of it in 2024 (American Coal Ash Association, 2025). These effects place fly ash among the concrete additives that a producer weighs into a batch instead of part of the cement.

FHWA, citing ACI, puts Class F at 15 to 25 % and Class C at 20 to 35 % by mass of total cementitious material, at a substitution ratio of 1:1 to 1.5:1 by mass (FHWA Fly Ash Facts, 2017 update). Water demand falls about 10 % at about 20 % fly ash, and high-volume fly ash concrete starts above 37 %. Against ASR, AASHTO PP 65-11 asks for 15 to 40 % depending on prevention level and ash alkali, and the 2024 US supply came with a 72 % recycling rate for coal combustion products.

The table summarizes fly ash as a concrete constituent, with the source of each value.

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 Fly Ash?#

Fly ash is the fine, mainly glassy and spherical ash of coal burned in power plants (National Academies, 2025), weighed into concrete as a supplementary cementitious material in place of part of the portland cement. Its glass consists of silica, alumina and iron oxide.

Fly ash comes from coal-fired power plants and, as a harvested material, from sites where it was landfilled or ponded (ADAA, 2022). Calcium content splits it into the two ASTM classes, and the unburned carbon it carries is measured as loss on ignition.

ACI does not call it an admixture: ACI CT-25 defines a chemical admixture as a liquid or dispersible powder, and ACI PRC-212.3-16 leaves fly ash to ACI 232.2R and ACI 232.1R. "Mineral admixture" survives as the Indian term. Fly ash belongs with slag cement and silica fume among the supplementary cementitious materials (SCMs).

Coal ash, harvested ash and bottom ash#

"Coal ash" is the ASTM C618 umbrella term since the C618-22/-23 revision, and it covers fly ash from the power plant, harvested fly ash and bottom ash. Harvested fly ash comes from a site where ash was landfilled or ponded, not directly from the power plant (Ash Development Association of Australia, 2022). Harvested ash and bottom ash comply only when processed to meet the specification, and the certificate must disclose the material type.

Since C618-25 (scope 1.2, retained in C618-26), the specification excludes three residues. These are residue from fluidized-bed combustion, residue from burning municipal solid waste or other refuse with coal, and residue from incinerators. An ash from one of those sources cannot meet C618, whatever its chemistry.

Regional names: PFA, siliceous fly ash and Classes F and C#

Fly ash carries a different name in each specification region. The US uses Class F and Class C under ASTM C618 and AASHTO M 295. The UK and Ireland say PFA, and BS 8500 labels a fly ash combination type V, with the prefix C for combinations (Concrete Society). Europe uses siliceous fly ash as a type II addition under EN 206 and V in EN 197-1. India specifies pulverized fuel ash to IS 3812, Part 1 as pozzolana and Part 2 as admixture, and Canada splits Class C into CI and CH (ADOT SPR-780).

Chemical composition of fly ash#

Fly ash is a glass of SiO2, Al2O3 and Fe2O3 with variable CaO and unburned carbon, and three composition values decide its class and category. These three values are listed below.

• Oxide sum (SiO2 + Al2O3 + Fe2O3): at least 50 % for both ASTM C618 classes (since C618-19), at least 70 % under EN 450-1:2012.
• Calcium oxide: the 18 % CaO boundary separates Class F from Class C in ASTM C618; EN 450-1:2012 caps reactive CaO at 10.0 %.
• Carbon: unburned carbon, measured as LOI, at most 6.0 % under C618 as reported for C618-23 (National Academies, 2025), and in Categories A, B or C under EN 450-1:2012.

How Does Fly Ash React in Concrete?#

Fly ash reacts pozzolanically: its glass combines with the calcium hydroxide released by portland cement hydration to form more calcium silicate hydrate (C-S-H), so its reaction starts only after the cement hydrates. The reaction is slower than cement hydration, and that lag shapes every property on this page (National Academies, 2025).

What follows from the lag? Set and early strength slow down, while later strength, pore refinement and permeability improve. The spherical particles improve packing and lower water demand. Fly ash also binds alkalis and dilutes C3A and calcium hydroxide, which is why it mitigates ASR and raises sulfate resistance. Its unburned carbon adsorbs air-entraining admixture.

Why Class C fly ash also hardens on its own#

Class C fly ash is partly hydraulic, so its calcium-rich glass hydrates with water as well as reacting with calcium hydroxide. PCA EB547 states that heat behavior follows CaO: a low-CaO Class C behaves like Class F and a high-CaO Class C behaves like cement. Without portland cement, fly ash needs an alkaline activator, and the Concrete Society lists it as a precursor for geopolymer and alkali-activated concrete.

What Does Fly Ash Do to Fresh and Hardened Concrete?#

Fly ash changes concrete in five ways: it lowers water demand, slows set and early strength, raises later strength, cuts heat release and can darken the surface when its carbon content is high. The FHWA Fly Ash Facts put the water saving at about 10 % at about 20 % fly ash of total cementitious material. The five effects are described in the order a producer meets them, from the fresh mix to the finished surface.

Water demand, workability and bleeding#

Fly ash lowers water demand by about 10 % at about 20 % of total cementitious material (FHWA Fly Ash Facts), with larger reductions at higher contents. The spherical shape improves workability and packing (National Academies, 2025).

The National Precast Concrete Association states that fly ash is effective at reducing bleed rates. Lower bleeding has a cost on flatwork: NRMCA states that plastic shrinkage cracking becomes a risk when the surface drying rate exceeds the bleeding rate.

Set and early strength#

Class F fly ash mixes set slower, and finishing may be delayed 1 to 2 hours depending on temperature (FHWA-RD-97-148). The delay grows as the concrete cools, because the pozzolanic reaction waits on cement hydration. Trial batches at the job temperature measure the setting time of concrete with the ash that will supply the job.

The FHWA Fly Ash Facts state that the pozzolanic reaction is significantly reduced below 4.4 °C (40 °F). Early strength at low temperature is therefore the first property to verify on cold placements.

Later strength and the substitution ratio#

At 1:1 substitution, Class F concrete may gain strength more slowly for 60 to 90 days and then exceeds plain concrete (FHWA-RD-97-148). At a substitution ratio of 1.1:1 to 1.5:1 by mass, the 28-day strength is about equal to the plain mix.

The FHWA Fly Ash Facts (2017 update) describe the practice: 15 to 30 % of the portland cement is replaced, more in mass concrete, at 1:1 to 1.5:1 by mass. A 1.25:1 ratio means 125 kg of fly ash for each 100 kg of cement removed. NRMCA lists up to 70 % fly ash or slag in footings, basement walls and some columns and shear walls tested at 56 or 90 days, and about 30 % where high early strength is needed.

Heat release in large placements#

Class F fly ash reduces heat of hydration about 50 % at all ages, and PCA EB547 gives 35 to 50 % of cementitious material for mass placements. Gajda (2006), in PCA EB547, gives a rule of thumb for placements over 2 m (6 ft): an adiabatic rise of 0.13 °C per kg/m³ (0.14 °F per lb/yd³) of equivalent cement, counting Class F at 0.5 and Class C at 0.8. That equivalency is the core of the heat of hydration calculation for SCM mixes.

Calculated from that rule, a 350 kg/m³ binder with 20 % Class F gives the two rises listed below.

• Equivalent cement: 280 + 0.5 × 70 = 315 kg/m³, giving an adiabatic rise of 41.0 °C (73.7 °F).
• Cement alone: 350 kg/m³, giving 45.5 °C (81.9 °F).

Color and surface appearance#

Fly ash with very high carbon content can cause dark surface streaks during finishing (FHWA-RD-97-148). For that reason some states restrict fly ash in white concrete items such as curbs and sidewalks.

Fly Ash Replacement Rates: Ranges, Basis and Batching#

Fly ash replaces 15 to 25 % of the total cementitious material by mass when it is Class F and 20 to 35 % when it is Class C (FHWA-RD-97-148, citing ACI). The basis, by mass of total cementitious material, travels with every rate on this page. A rate quoted without its basis can mean a share of the cement or of the binder, and the two differ by the fly ash itself.

Typical replacement by class and application#

Replacement rates depend on class, application and exposure, and each source states its own basis. The table compares the published rates with their basis and source.

Rates are by mass of total cementitious material unless the row states another basis.

Every rate in the table is a mass share, never a volume share. The ACI 301-10 floor limits are dated, as reported by NRMCA SIP 1, and the 25 % deicer cap of FHWA-HIF-17-008 applies only to hand-finished surfaces.

High-volume fly ash concrete (HVFA)#

High-volume fly ash concrete contains more than 37 % fly ash by mass of cementitious material (ACI definition, as reported by the National Academies), and its fly ash content may exceed 50 %. It sits at the top of the 15 to 35 % normal range and overlaps the 40 to 70 % that some DOTs use in mass concrete (National Academies, 2025). HVFA mixes carry the effects of fly ash to their limit, with slower early strength and lower heat. They suit elements tested at 56 or 90 days, such as footings and basement walls in the NRMCA list, rather than elements that need early stripping or loading.

Worked example: 20 % Class F in a 350 kg/m³ binder#

Worked example, calculated from the FHWA Class F range and the EN 206:2013 k-value rule: a 350 kg/m³ binder with 20 % Class F holds 70 kg/m³ of fly ash. The inputs are 350 kg/m³ (590.0 lb/yd³) of cementitious material, 20 % Class F by mass of total cementitious material and 157.5 kg/m³ (265.5 lb/yd³) of water.

• Fly ash: 0.20 × 350 = 70 kg/m³ (118.0 lb/yd³); cement: 280 kg/m³ (472.0 lb/yd³).
• Water-cementitious materials ratio: 157.5 / 350 = 0.45.
• EN 206:2013 equivalent ratio (fly ash/cement 70 / 280 = 0.25, within 0.33): 157.5 / (280 + 0.4 × 70) = 0.51.

The two ratios differ because the two systems count fly ash differently. ACI counts fly ash in full as a cementitious material, so the w/cm is 0.45. EN 206:2013 counts only 0.4 of its mass as cement, so the same batch has an equivalent w/c of 0.51. A mix written to a 0.45 water-cement ratio limit in one system therefore needs recalculation before it is offered under the other, which is the core of the water-cement ratio and k-value concept.

Batching, storage and commingled sources#

Fly ash is batched as a dry cementitious powder, and four steps keep it consistent from delivery to the mixer. These steps follow the National Academies (2025) account of practice.

1. Deliver the ash dry, in bulk by pneumatic tanker or rail.
2. Store it in its own silo, separate from the cement.
3. Weigh it as a cementitious material.
4. Count it in the water-cementitious materials ratio.

Caltrans specifies AASHTO M 295 Class F with available alkali at most 1.5 % or total alkali at most 5.0 %, in categories FA (CaO at most 10 %) and FB (CaO 10 to 15 %). Caltrans allows commingled sources only within 0.25 relative density and 1 % LOI of each other.

Which Standards Classify Fly Ash?#

Three specifications classify fly ash for concrete: ASTM C618-26 as Class F or C coal ash, EN 450-1:2012 by fineness Category N or S and LOI Category A, B or C, and IS 3812:2013 in India. The tables compare their status and their published limits.

The limits below compare the US and European requirements; ASTM C618 values are as reported for C618-23 (National Academies, 2025), not as C618-26 requirements.

Class F and Class C under the US specification#

ASTM C618-26 is active, last updated 2026-08-11, supersedes C618-25a and is titled "Standard Specification for Coal Ash and Raw or Calcined Natural Pozzolan for Use in Concrete". Since C618-19, CaO separates the classes: Class F at most 18 %, Class C more than 18 %, both with the oxide sum at least 50 %. Before C618-19, the oxide sum was at least 70.0 % for Class F and 50.0 % for Class C (NRMCA SIP 4, 2015).

The National Academies (2025) report the C618-23 limits as LOI at most 6.0 %, at most 34 % retained on the 45 µm (No. 325) sieve, a strength activity index of at least 75 % of control at 7 or 28 days and moisture at most 3.0 %. AASHTO M 295 follows ASTM C618 with LOI at most 5 %, a 56-day strength activity index permitted and additional optional ASR requirements (Caltrans 2021, FHWA 2017).

Categories N, S and A, B, C under the European standard#

EN 450-1:2012 is the harmonised European standard for siliceous fly ash as a type II addition in EN 206 concrete and in mortars, under the Construction Products Regulation 305/2011 with AVCP 1+. It replaced EN 450-1:2005+A1:2007, with coexistence from 2013-05-01 to 2014-05-01, and BS EN 450-1:2012 is current in the UK.

Fineness Category N allows at most 40 % retained on 45 µm, within ±10 points of the declared value, and Category S at most 12 %. LOI Categories A, B and C allow at most 5.0, 7.0 and 9.0 % by mass. The activity index, measured on 25 % fly ash with 75 % test cement, must reach 75 % of the control at 28 days and 85 % at 90 days. Processed ash is allowed, and co-combustion ash qualifies only with coal at least 60 % of the dry mass (50 % with green wood only) and co-combustion ash at most 30 %. Free CaO above 1.5 % requires a soundness test, and municipal and industrial incineration ash is excluded.

IS 3812 Parts 1 and 2 in India#

India specifies fly ash under IS 3812 (Part 1):2013, as pozzolana for cement, mortar and concrete, and IS 3812 (Part 2):2013, as admixture for cement mortar and concrete; both are active and reaffirmed in 2022 (BIS). IS 3812:1981 is withdrawn. BIS certifies Part 1 under product manual PM/IS 3812 (Part 1) of May 2020, which licenses the Standard Mark. The IS 3812 requirement values are not reproduced here because no copy of the current edition has been checked against the BIS record.

Counting fly ash as cement: the k-value and code limits#

EN 206:2013 counts fly ash to EN 450-1 as cement with k = 0.4, up to a fly ash/cement ratio of 0.33 by mass with CEM I and 0.25 with CEM II/A; any excess counts as aggregate. In the Concrete Society example, 300 kg/m³ (505.7 lb/yd³) of CEM I allows 100 kg/m³ (168.6 lb/yd³) of fly ash, of which 40 kg/m³ (67.4 lb/yd³) counts as cement. The UK uses the equivalent performance of combinations route of BS 8500-2 (4.2.3.2) instead. EN 206-1:2026 has superseded EN 206:2013+A2:2021, and its k-value clauses are not reproduced here because they are not in the public preview of the EN 206 concrete specification.

ACI 318-19 Table 26.4.2.2(b) limited fly ash or other pozzolans in Exposure Class F3 to 25 %, with silica fume to 35 % and with slag cement and silica fume to 50 % of total cementitious materials by mass. ACI CODE-318-25 removed Exposure Class F3 and these caps, and no maximum SCM limits remain in the code (NRMCA 2PE004-26, March 2026).

How Does Fly Ash Interact with Admixtures and Other SCMs?#

Fly ash works with the standard chemical admixtures, but its unburned carbon changes the air-entraining admixture dose, and its slow reaction makes temperature, cement type and accelerators part of the mix decision. Each of the three interactions has its own control.

Carbon, LOI and air-entraining admixture demand#

Unburned carbon in fly ash adsorbs air-entraining admixture, and the same LOI from two sources can need different doses (NRMCA SIP 4). ASTM C618-26 Note 1 adds that finely divided materials may reduce entrained air. Air is therefore checked at every delivery, and the dose is set for each pairing of ash source and air-entraining admixtures.

FHWA (1997) reported ready-mix fly ash LOI usually below 3.0 %, with many states capping LOI at 3 or 4 %. Those state caps sit below the 6.0 % C618 limit and the 5 % AASHTO limit, yet the same LOI from two sources can still need different doses.

Ternary blends with a second SCM#

Fly ash is combined with a second SCM in ternary blends, and the NYSDOT bridge deck binder of 20 % Class F plus 6 % silica fume by mass of cementitious materials has been agency practice since 1998 (National Academies). In NYSDOT work by Streeter (1996), cited by the National Academies, that blend cut chloride penetration 20 to 30 % against a no-SCM control. The combined caps of ACI 318-19 Class F3 (fly ash plus silica fume 35 %, with slag 50 %) are history since ACI CODE-318-25.

Accelerators, cement type and low temperatures#

The pozzolanic reaction of fly ash is significantly reduced below 4.4 °C (40 °F) (FHWA Fly Ash Facts), so cold mixes need an offset. The National Academies (2025) list four offsets: accelerators, Type III cement, heated materials or a lower replacement where early strength at low temperature governs. The first offset is a dose of accelerating admixtures, chosen with the reinforcement and the chloride limit in mind.

Class F or Class C: How Do You Choose Fly Ash?#

Choose Class F where the mix must resist ASR or sulfates or keep heat low, Class C where its calcium is no durability risk, and a lower rate or another SCM where low-temperature early strength governs. The table matches each application to the class and the evidence.

Class C is partly hydraulic, and its chemistry varies more, including SO3 content; and ADOT SPR-780 (2025) advises ASTM C1567 testing when a producer switches from Class F to Class C or between Class C sources.

When is fly ash the wrong choice?#

Fly ash is the wrong choice, or needs a lower rate, where early strength, set time, air control or carbonation governs. The four downsides are listed below, and white concrete items add a fifth where states restrict dark-streaking ash.

• Low-temperature reaction loss: the pozzolanic reaction is significantly reduced below 4.4 °C (40 °F) (FHWA), a limit for cold-weather concreting without offsets.
• Delayed finishing: Class F mixes may delay finishing 1 to 2 hours depending on temperature (FHWA-RD-97-148).
• Air-content variability: carbon changes the air-entraining admixture demand from lot to lot (NRMCA SIP 4).
• Faster carbonation: at 25 % replacement the carbonation coefficient is about 2.3 times that of portland cement concrete (RILEM TC 281-CCC), relevant to carbonation of concrete in dry, sheltered exposure.

Which Tests Show That a Fly Ash Mix Works?#

Four tests and one certificate show whether a fly ash mix works: the C618-26 or EN 450-1:2012 certificate, air content by ASTM C231/C231M-26, ASTM C1567-25 for ASR and ASTM C1012/C1012M-24a for sulfates. ASTM C1202-25 adds an index of chloride resistance, and ASTM C1897-20 screens SCM reactivity.

The certificate proves the ash, and the concrete tests prove the combination. A C1567 result for one cement, ash and aggregate does not carry over to another source of any of the three.

How Does Fly Ash Change Durability?#

Fly ash improves durability in three exposures and costs it in one: at the replacement levels below, it mitigates ASR, raises sulfate resistance and slows chloride ingress over time, while it speeds carbonation. Each exposure has its own evidence and its own dose.

ASR prevention levels#

AASHTO PP 65-11, as reproduced in FHWA-HIF-13-019, sets minimum fly ash contents by prevention level W, X, Y and Z, and the ash alkali content decides which row applies. The table lists the minimums for fly ash with CaO at most 18 %.

Minimum fly ash, % by mass of cementitious material (AASHTO PP 65-11 as reproduced in FHWA-HIF-13-019).

Thomas and Folliard (2007), cited in FHWA-HIF-13-019, report 20 to 30 % of cementitious material for low-calcium fly ash (CaO below 8 %), 25 to 35 % for moderate-calcium ash (8 to 20 % CaO) and 40 to 60 % for high-calcium ash (CaO above 20 %). The National Academies literature review (2025) puts Class C at about 35 to 45 %. ADOT SPR-780 limits alkali loading to 2.1 kg/m³ (3.5 lb/yd³) when designing for alkali-silica reaction (ASR).

The performance route replaces the tables with a test: ASTM C1567-25 expansion at most 0.10 % after 14 days of immersion (16 days) shows an effective combination, valid only for the materials tested.

Sulfate exposure#

About 20 % fly ash is needed for sulfate resistance, and calcium-rich fly ash may perform poorly (Dunstan 1980, in Whittaker and Black 2015). The same study cites a screening factor R = (CaO − 5)/Fe2O3, with R below 1.5 satisfactory. A calcium-rich ash with R at 1.5 or above is the case to test before use.

ACI 318-19 Table 26.4.2.2(c) qualifies sulfate-resistant combinations by ASTM C1012: S1 at most 0.10 % at 6 months; S2 at most 0.05 % at 6 months or 0.10 % at 12 months; S3 option 1 at most 0.10 % at 18 months. ASTM C618 also carries an optional sulfate limit of 0.10 % (moderate) or 0.05 % (high) at 6 months (reported for C618-22).

Chloride resistance and carbonation#

The Life-365 v2.2.3 manual example shows fly ash lowering chloride diffusion over time: at w/cm 0.40 with 40 % fly ash, the 25-year coefficient is 3.9 × 10⁻¹³ m²/s against 25 × 10⁻¹³ m²/s for portland cement concrete. Both mixes start at the same 28-day value of 79 × 10⁻¹³ m²/s, and the difference comes from the ageing factor m of 0.52 against 0.20. NRMCA (2018) states that fly ash and slag give larger chloride-resistance gains at later ages than silica fume and metakaolin, a pattern central to chloride ingress and rebar corrosion.

In an NRMCA study by Obla, the 56-day charge passed was 1,557 coulombs with 15 % fly ash at w/cm 0.39 against 1,722 coulombs for portland cement concrete at 0.39, and 4,012 coulombs with 30 % fly ash at 0.62. ASTM C1202-25 is an index, not a permeability test. Carbonation runs the other way: RILEM TC 281-CCC reports a carbonation coefficient about 2.3 times that of portland cement concrete at 25 % fly ash replacement.

Frost and deicing-salt exposure#

FHWA-HIF-17-008, citing ACI 201.2R-16, limits fly ash to 25 % on hand-finished surfaces exposed to deicers and does not apply that limit to formed or machine-finished surfaces. Frost resistance comes from entrained air supplied by an air-entraining admixture, not from the fly ash. The ash only changes how much admixture that air requires.

Fly Ash Safety and Handling#

Coal fly ash has no CLP Annex VI entry: it is REACH registered as Ashes (residues), coal (EC 931-322-8), and 100 % of its ECHA C&L notifications are not classified (checked 2026-10-23). Its dust can contain respirable crystalline silica, which OSHA limits to 50 µg/m³.

"Not classified" applies to the named registration only, not to dust exposure in a silo or at a tanker coupling, and fresh concrete with any binder is caustic. EPA evaluated the encapsulated use of coal ash in concrete and found releases comparable to or lower than those of non-coal-ash products, and EPA announced a proposal to revise the beneficial-use definition on 2026-04-09.

What Does Fly Ash Cost?#

FHWA report 03-047 priced fly ash at USD 28 to 33 per tonne (USD 25 to 30 per ton) in July 2003, against USD 88 per tonne (USD 80 per ton) for Type I cement. These are US values of July 2003, a historic reference and not a price for today.

A quote for fly ash is built from four factors, listed below.

• Substitution ratio: at 1:1 to 1.5:1 by mass (FHWA Fly Ash Facts, 2017 update), the ash mass per cubic meter can exceed the cement mass it removes.
• Class and LOI: a tighter LOI range or a specific class narrows the eligible sources.
• Harvested processing: drying, de-agglomeration and removal of the coarse fraction add processing to landfilled or ponded ash.
• Haul: pneumatic tanker or rail delivery to a separate silo, priced by distance.

Fly Ash Grades, Supply and Sourcing#

Fly ash is delivered as a dry powder by pneumatic tanker or rail and stored in its own silo, and buyers compare sources on class, certificate, LOI and its variation, and whether the ash is fresh or harvested. State DOTs publish source lists: NYSDOT keeps Approved List 711-10 of fly ash suppliers, updated 2026-06-10, and 26 of 34 agencies (76 %) accept Class F fly ash (ADOT SPR-780 survey, 2025). Those agency lists sit alongside the commercial routes described for fly ash, slag cement and silica fume suppliers.

The five documents and values to request from a fly ash source are listed below.

1. The class or category and the standard edition on the test certificate (ASTM C618-26 or EN 450-1:2012).
2. The LOI and its range across recent lots.
3. A declaration of whether the ash is fresh, harvested or bottom ash.
4. The SDS, with the REACH identity (EC 931-322-8) or the CAS number it uses.
5. A DOT source-list entry or an EU Declaration of Performance, compared across concrete admixture manufacturers and suppliers.

US supply, harvesting and EPD carbon accounting#

US concrete used 13.24 million t (14.6 million short tons) of coal fly ash in 2024, up from 10.80 million t (11.9 million short tons) in 2023 (ACAA, 2025-12). The same survey counts 57.70 million t (63.6 million short tons) of coal combustion products in 2024, 72 % recycled, with coal at 16 % of US electricity and several million tons of previously disposed ash used. In 2022 the US produced more than 25.40 million t (28 million short tons) of fly ash and used 9.98 million t (11 million short tons, 39 %) in concrete, with production 36 % below 2015 (National Academies).

ADOT SPR-780 describes harvesting: the ash is dried to at most 3.0 % moisture, de-agglomerated and stripped of the fraction coarser than 45 µm. Its risks are moisture, LOI, free lime, alkalis or sulfur, contaminants, heterogeneity and lower reactivity. Harvested ash is typically Class F-like (National Academies, 2025), with a strength activity index slightly lower than, higher than or comparable to fresh ash. The NRMCA industry-wide EPD v4 (2026) assigns fly ash no production burden (transport only) and 75 % reactivity. For 27.6 MPa (4,000 psi) non-air-entrained concrete it lists 330.17 kg CO2e per m³ (252.4 per yd³) with no SCM, 287.11 (219.5) with 20 % fly ash and 239.26 (182.9) with 40 % fly ash, values that feed the embodied carbon of concrete.

Where Fly Ash Fits Among the Other SCMs#

Fly ash shares the SCM role with slag cement, which is latent hydraulic, silica fume, a finer pozzolan used at 5 to 10 % by mass of cement, and calcined clay and natural pozzolans. The four alternatives and one comparison are listed below.

• Slag cement (GGBS): latent hydraulic plus pozzolanic, while Class F fly ash is pozzolanic only (ADOT SPR-780).
• Silica fume (microsilica): 5 to 10 % by mass of cement (Concrete Society), and the ternary partner of fly ash in NYSDOT decks.
• Calcined clay and LC3: 30 % calcined clay meeting Class N matched or beat 30 % Class F (VTRC 26-R52, 2026).
• Natural pozzolans: raw or calcined pozzolans, still within the ASTM C618-26 scope.
• Comparison: fly ash vs slag cement sets the two SCMs side by side.

Fly ash interground or blended into Type IP, CEM II/B-V and PPC cements is a cement product and is not covered here.

Fly Ash FAQs#

Where does fly ash come from?#

Fly ash comes from coal burned in power plants, and since the C618-22/-23 revision it may also come from harvesting sites where ash was landfilled or ponded. US concrete used 14.6 million short tons of it in 2024 (ACAA).

What are the key differences between fly ash and cement?#

Portland cement hydrates on its own, while fly ash needs the calcium hydroxide that cement releases, so EN 206:2013 counts fly ash at 0.4 of its mass as cement. For heat, a large placement counts Class F at 0.5 (PCA EB547), and the NRMCA EPD v4 assigns fly ash no production burden.

Is fly ash in concrete hazardous?#

Coal fly ash is not classified in ECHA C&L notifications, but its dust can carry respirable crystalline silica, listed under California Proposition 65 since 1988-10-01. Fresh concrete with any binder is caustic, and EPA found encapsulated use comparable to non-coal-ash products.

Which type of fly ash is best?#

No single fly ash suits every mix: Class F is generally more effective against ASR than Class C (NRMCA SIP 4), while Class C is partly hydraulic and varies more in chemistry. The right ash is the one whose certificate meets the specified class.

What can I use instead of fly ash in concrete?#

Slag cement, calcined clay, natural pozzolans, ground glass, silica fume and limestone filler replace fly ash in concrete (National Academies, 2025). A 2026 Virginia study (VTRC 26-R52) found 30 % calcined clay meeting Class N comparable to or better than 30 % Class F.