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Fly ash, slag, and calcined clay can each improve concrete, but none is best for every project. Class F fly ash is often the first candidate when heat control, alkali–silica reaction (ASR) mitigation, or sulfate resistance matters and a consistent supply is available. Slag cement is a strong option for low permeability and higher replacement levels if the mix can accommodate slower early strength and longer curing. Calcined clay can help where fly ash is scarce or early reactivity is valuable, but its high surface area may increase water and admixture demand. The right choice depends on exposure, construction schedule, local supply, and performance in trials using the actual project materials.
At a glance
| Name | Best for | Key spec | Checked |
|---|---|---|---|
| Class F fly ash | Heat control, ASR mitigation, and sulfate exposure when supply is secure | Commonly 15–35% of total cementitious material; high-volume mixes can exceed 37% and 50% | November 2026 |
| Slag cement (GGBFS) | Low permeability, later-age strength, and higher replacement levels | Traditional replacement range: 30–50%; early strength, curing, and air entrainment need attention | November 2026 |
| Calcined clay / metakaolin | Fly-ash supply resilience or a need for a more reactive pozzolan | Metakaolin is commonly used at up to 10% in NSW guidance; high surface area can reduce workability | November 2026 |
These percentages are reported ranges and guidance examples, not universal mix limits. Specifications, source characteristics, exposure, and trial results govern the allowable replacement for a project.
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What does an SCM change in concrete?
A supplementary cementitious material (SCM) replaces some Portland cement in a concrete mixture and contributes to its performance through chemical reaction, physical effects, or both. Fly ash and calcined clay are pozzolans: they react with water and calcium hydroxide to form additional calcium-silicate-hydrate. Slag cement is a hydraulic material that can also contribute to strength and durability. In each case, the material interacts with the project cement, aggregates, water, admixtures, and curing conditions; its name alone does not predict how a particular mix will behave.
That interaction matters because the project may need several things at once: manageable heat in a large placement, early strength for form removal or traffic, later-age strength, resistance to chloride ingress or sulfate exposure, a stable air-void system, and a workable placing window. A mixture that improves one property can complicate another. The U.S. Department of Defense’s UFC 3-250-04 puts the decision plainly: “For a particular application, some supplementary cementitious materials are better than others, and some may be completely inappropriate.”
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How do fly ash, slag, and calcined clay differ?
| Decision factor | Class F fly ash | Slag cement / GGBFS | Calcined clay / metakaolin |
|---|---|---|---|
| Heat and early strength | Usually lowers heat and delays early strength and set | Generally lowers heat and early-age strength; high contents need planning | Can decrease heat while increasing early strength, depending on source and mixture |
| Later-age performance | Often matches or exceeds reference strength at 28–56 days; can reduce permeability | Can support later-age strength and reduce permeability and diffusivity | Can increase early and long-term strength; study results depend on the clay and mix |
| Durability uses | ASR mitigation and potential sulfate-resistance benefits; confirm by testing | Can contribute to sulfate resistance; low permeability is a common objective | Reported chemical-resistance and ASR benefits; verify performance for the intended exposure |
| Fresh-concrete concerns | Set delay and early strength may affect schedule; confirm air and admixture compatibility | Fineness can reduce air content and increase air-entraining admixture demand | High surface area and porous micromorphology can sharply reduce workability and raise admixture demand |
| Supply and carbon considerations | U.S. supply has declined; transport and source consistency matter | Regional availability and transport distance determine feasibility and project carbon outcome | Can diversify supply where suitable clay is local; calcining and processing energy matter |
When is fly ash the better fit?
Class F fly ash is often worth evaluating first when the project prioritizes lower heat, ASR mitigation, or sulfate exposure, provided a qualified source can supply consistent material. Its pozzolanic reaction is slower than the initial hydration of Portland cement, so the mix may set later and develop less strength in the first several days. This can be an advantage in a placement where heat buildup is a concern, but a constraint when early opening, stripping, or loading controls the schedule.
The National Academies’ 2025 review reports that Class F replacement delays initial set, lowers 3–7-day strength, and produces significantly lower heat. It also reports that fly-ash mixtures usually match or exceed strength at 28–56 days and provide significant ASR mitigation. The American Concrete Institute notes that fly ash can increase long-term strength, decrease permeability, improve durability, and reduce alkali-aggregate expansion; the ASR benefit should be investigated by testing rather than assumed for every combination of ash and aggregate.
Fly ash commonly replaces 15–35% of total cementitious material. High-volume applications can exceed 37% and may go beyond 50%, but those figures describe applications, not a general prescription. At higher replacement, early-age performance and curing become especially important to check against the project’s construction sequence.
What is the main risk with fly ash?
Supply is a strategic risk as well as a mix-design issue. The National Academies reports that more than 28 million short tons of coal fly ash were produced in the United States in 2022, of which 11 million tons (39%) were used in concrete and concrete products. It also reports that 2022 production was down 36% from 2015 and concrete use was down 31%. Those U.S. figures do not predict supply in every region, but they make it unwise to assume that a source will remain available and unchanged through a long project.
Qualify the actual source and agree on how production, processing, handling, or delivery changes will be reviewed. The National Academies emphasizes availability, consistency, cost, and proximity to processing and distribution when assessing SCM feasibility. A nominally suitable ash is not a dependable choice if its supply chain cannot meet project needs.
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When is slag cement the better fit?
Slag cement, also called ground-granulated blast-furnace slag (GGBFS), is a strong candidate when low permeability, later-age strength, or relatively high replacement is a priority. It can lower heat of hydration and reduce permeability and diffusivity, and it can contribute to sulfate resistance. The trade-off is that early strength may develop more slowly, particularly at higher slag contents, so the schedule and curing plan must allow the concrete to reach the required performance.
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Plan for longer curing and evaluate cold-weather effects when using high slag contents. Confirm setting and finishing behavior with trial batches. Air entrainment deserves particular attention: the American Concrete Institute notes that increasing GGBFS fineness can decrease air content and may require substantially more air-entraining admixture. A change in fineness or source can therefore affect a property that is essential to the specified concrete.
When is calcined clay the better fit?
Calcined clay can be a useful alternative when fly-ash supply is constrained, when early reactivity is valuable, or when a local clay source could make the supply chain more resilient. Calcination heats suitable clay to roughly 650–850°C; metakaolin is the most common commercial form identified in NSW guidance. Reported benefits include decreased heat, increased early and long-term strength, improved chemical resistance, and ASR mitigation.
There is promising but source-specific evidence. The Virginia Transportation Research Council’s report 26-R52, updated July 28, 2026, found calcined clay substantially more reactive than Class F fly ash and more comparable with slag. The report also found that the clay’s porous micromorphology and high surface area significantly reduced workability, making admixture selection and dosage critical. In that study, concrete with 30% Class N calcined clay performed comparably with or better than concrete with 30% Class F fly ash, and its air-entrainment behavior was similar to a Type IL control. That result supports evaluation of calcined clay; it does not guarantee equivalent performance from every clay source or in every mixture.
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Workability is a practical design concern, not merely a laboratory detail. High fineness can increase water demand, while adding water without accounting for the specified mixture can undermine the intended concrete performance. Optimize admixtures and verify workability retention, air content, and placing behavior in trials using project materials.
NSW guidance describes metakaolin as typically used at up to 10% replacement in its recommendations. It estimates calcined clay at about 16–24% of the embodied carbon of ordinary Portland cement (OPC), but also warns that high replacement can affect carbonation resistance and may require additional cover or a coating. Assess carbonation exposure and the relevant design provisions before selecting a high replacement level.
Which option has the lowest embodied carbon?
There is no region-independent winner. Replacing clinker can lower embodied carbon, but the result depends on the amount of cement displaced, processing energy, grinding, transport distance, local availability, and the project’s concrete requirements. The material with the lowest reported production footprint may not produce the lowest whole-project result if it must travel much farther or requires additional processing.
NSW Government’s 2025 guidance lists illustrative embodied-carbon savings of 10–25% for traditional fly-ash blends and 25–40% for traditional slag blends. It gives calcined clay at approximately 16–24% of OPC embodied carbon. These figures use different descriptions and should not be read as a direct, like-for-like ranking: the calcined-clay figure is a share of OPC’s embodied carbon, while the fly-ash and slag figures are savings for traditional blends. They are guidance values, not project-specific declarations.
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For a defensible comparison, use the same project boundary and calculation method for each candidate, include the actual replacement level and transport route, and use product- or supplier-specific environmental data where available. Then compare only mixtures that meet the same performance requirements. A lower-carbon option that cannot meet strength, durability, curing, or supply requirements is not a viable substitute.
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How should you qualify an SCM for a project?
Choose based on the actual source and concrete mixture, not the generic label. A disciplined qualification process reduces the risk of a mix that looks suitable on paper but sets too slowly, loses workability, misses its air target, or fails to meet durability requirements.
- Define exposure and construction priorities. Identify heat limits, ASR and sulfate concerns, chloride ingress, required early strength, finish, curing window, and carbonation exposure. Establish which requirements are mandatory and which are optimization goals.
- Check the governing standards and project limits. ASTM C618 covers coal ash and raw or calcined natural pozzolans; ASTM C989 covers slag cement; ASTM C1709 provides an evaluation framework for alternative SCMs. Confirm the edition and acceptance criteria that the project specification or jurisdiction requires.
- Qualify the actual source. Review chemistry, fineness, loss on ignition, reactivity, and variability. A material name or past approval does not establish that a new source, production condition, or handling process will behave the same way.
- Run trial batches with project materials. Use the intended cement, aggregate, admixtures, water, temperature range, and air-entrainment target. The UFC warns that complex SCM/admixture combinations can lead to early stiffening, delayed set, shrinkage cracking, or poor air-void systems.
- Measure fresh and hardened performance. Check early and later strength, setting, workability retention, air system, permeability or chloride indicators, shrinkage, and carbonation risk. Evaluate ASR or sulfate performance where relevant to the exposure and specification.
- Secure supply and change control. Include delivery lead times, inspection documents, and procedures for reviewing changes in source or production in the concrete specification. Requalify when production or handling changes could affect consistency.
For complex combinations, compare single-SCM and ternary options. UFC notes that a ternary mixture may be the best alternative for some applications, but compatibility still requires testing. A second SCM can offset a weakness in the first, yet it also adds interactions that must be checked rather than presumed beneficial.
Independent reader supportYour contribution helps us test, update, and keep practical guides available for everyone.Which SCM should you choose for common project conditions?
| Project priority | First candidate to evaluate | What must be confirmed |
|---|---|---|
| Mass concrete or heat-sensitive placement | Class F fly ash; slag may also be suitable | Heat profile, set, early strength, placement sequence, and curing plan |
| ASR concern | Class F fly ash; calcined clay is also a possible candidate | Performance with the actual aggregate and cement by appropriate testing |
| Sulfate exposure | Class F fly ash or slag, depending on source and design requirements | Specified sulfate performance and exposure-specific test evidence |
| Low permeability or later-age strength | Slag; fly ash is also worth evaluating | Strength at required ages, curing duration, and permeability or chloride indicators |
| Early reactivity with limited fly-ash supply | Calcined clay / metakaolin | Workability, admixture demand, early strength, air system, and carbonation exposure |
| High replacement target | Slag or a qualified high-volume fly-ash or blended system | Specification limits, early performance, curing, supply continuity, and trial results |
| Lowest project carbon | No universal winner | Comparable project-specific carbon data, including processing and transport |
These are starting points for evaluation, not prescriptive mix designs. UFC and the other cited guidance all support a project-specific decision: performance depends on source, combination, exposure, and construction conditions.
What should you put in the concrete specification?
Specify acceptance and control of the material and mixture, not just a preferred SCM name or replacement percentage. Identify the applicable standard and edition, the approved source or qualification procedure, required documentation, and the project performance tests. State the replacement range or limits, curing expectations, required early and later strength ages, fresh-concrete targets, durability indicators, and procedures for source changes.
Also establish how the producer and project team will handle supply interruptions. Fly ash availability can change; slag and calcined clay also depend on regional distribution, processing, and consistent source characteristics. If a substitute or ternary blend is proposed, require review and qualification against the original performance requirements rather than accepting a material swap solely because it is also an SCM.
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Finally, align testing and construction timing. A mixture that achieves its specified later-age strength but misses an early opening date may not fit the project. Likewise, a durable laboratory mix is not a safe choice if the field operation cannot maintain the required curing. The qualification program should reflect the temperatures, placement scale, finish, and schedule the concrete will actually face.
Frequently Asked Questions
Is metakaolin the same thing as calcined clay?
Metakaolin is the most common commercial form of calcined clay identified in the NSW guidance, but calcined clays can vary in mineralogy and processing. Treat performance data for one clay or metakaolin product as evidence about that source, not as a guarantee for every calcined clay.
Does a higher SCM replacement percentage always mean lower-carbon concrete?
No. Replacement can reduce clinker use, but processing energy, grinding, transport, supply, and the concrete performance required all affect the project result. Compare product- or supplier-specific data using the same accounting boundary rather than ranking materials by a single generic percentage.
Does Class F fly ash automatically prevent ASR?
No. Fly ash can mitigate alkali–aggregate expansion, but the benefit depends on the materials and mixture. The American Concrete Institute advises investigating the effect by testing.
When should an approved SCM source be requalified?
Requalification is prudent when production or handling changes may affect chemistry, fineness, reactivity, or consistency, or when the supply source changes. The project specification should define the documentation and review process before a change occurs.
