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Choose a supplementary cementitious material (SCM) by starting with the concrete’s required performance, then checking exposure, schedule, curing, local supply and the governing specification. Fly ash, slag cement, silica fume, natural pozzolans and ground-glass pozzolans are not interchangeable. The right choice is the material—or blend—that meets the project’s acceptance tests with the actual cement, aggregates, admixtures, placement and curing plan.
Work with the concrete producer on candidate materials and trial batches. Published replacement percentages are useful context, not recipes: state transportation departments report widely different practices, while current guidance recommends performance-oriented requirements that account for constructability and local availability. (NRMCA, 2026; National Academies, 2025)
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What an SCM does in concrete
An SCM partially replaces portland cement or is used with it to change concrete performance. Some SCMs react chemically with cement hydration products (pozzolanic reaction); slag cement is hydraulic as well as supplementary. The resulting binder can improve later-age strength, reduce permeability and help control heat evolution, but reaction rate, water demand, setting and curing sensitivity vary by material and source.
That variation is why there is no universally best SCM percentage. A mixture that performs well in a mass placement may be unsuitable for a fast-track pavement, and a silica-fume mixture designed for very low permeability may require admixture and finishing controls that are unnecessary elsewhere. FHWA’s technical brief describes these construction effects, while NRMCA’s March 2026 specification guide emphasizes matching requirements to performance, constructability and local availability. (FHWA; NRMCA)
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1. Define what the concrete must do
Write a short performance brief before comparing materials. Give the producer and engineer the same requirements so a trial mixture can be judged against the project rather than against a generic percentage.
Strength and acceptance age
- State the specified compressive or flexural strength and the age at which it will be accepted (for example, an early opening strength, 28-day strength or another contract age).
- Identify whether later-age strength gain is useful or whether rapid strength is essential to stripping, traffic opening or sequencing.
- List any modulus, shrinkage or other mechanical criteria in the project specification.
Exposure and durability
- Identify chloride exposure, sulfate exposure, freeze-thaw conditions, abrasion, carbonation or other durability demands.
- Set permeability or chloride-penetration limits when the specification uses them.
- Assess alkali-silica reaction (ASR) risk from the actual aggregate and alkali-bearing materials. FHWA notes that SCM effectiveness depends on material characteristics, alkali content, dosage and aggregate reactivity; an ASR claim must therefore be supported by tests using the materials proposed for the job. (FHWA ASR guidance; FHWA interim recommendations)
Heat, placement and finish
- For mass concrete, define the allowable temperature rise, maximum internal temperature and temperature differential limits.
- Describe placement method, section thickness, pumping distance, reinforcement congestion, finishing method and the required setting window.
- Record whether the crew can maintain the specified curing temperature and duration, including in cool weather.
2. Confirm the specification and the materials you can actually obtain
Identify the governing building code, contract specification, transportation-agency provisions and material standards before selecting an SCM. Use the current edition required by the contract; standards and agency provisions can change.
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Ask the ready-mixed concrete producer which SCMs are stocked, how consistently they are supplied and which combinations the plant routinely handles. NRMCA cautions that producers generally do not stock many SCM types. Requiring an unavailable material can add haul distance, cost, supply risk and unfamiliar batching or quality-control procedures. Local availability is part of technical suitability, not an afterthought. (NRMCA, 2026)
Supply risk is especially relevant for fly ash. In the National Academies’ 2025 survey, 28 of 33 responding state departments of transportation had experienced or expected fly-ash shortages. The same report received responses from 43 DOTs, an 83% response rate, so its figures describe broad practice but remain a dated survey snapshot rather than a guarantee for your market. (National Academies state DOT practices)
3. Compare the main SCM options
Fly ash (coal ash)
Fly ash is common but variable. Class, chemistry, dosage, cement, aggregate and the performance target all affect setting, strength development and durability. A suitable source can improve later-age strength, workability and resistance to fluid or ion ingress, but a slow-reacting source may delay early strength or setting. Confirm the actual source and class rather than relying on the word “fly ash.”
Availability needs explicit checking: the National Academies survey found that 40 of 43 responding DOTs used Class F coal ash and 28 used Class C fly ash, but use does not mean either class is available for every project or region. (National Academies)
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Slag cement is hydraulic and typically reacts more slowly than portland cement. FHWA identifies longer setting and the need for adequate curing, particularly in cool conditions. The slower reaction and lower heat evolution can be valuable in mass concrete, where temperature control is a primary design constraint. FHWA describes mixtures with up to 80% replacement in mass concrete as an example of practice; that figure is not a universal recommendation or a default limit for other work. (FHWA)
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Plan the curing regime and finishing sequence around the actual temperature and setting behavior. If the schedule requires early opening or cold-weather placement, verify early-age performance in a trial mixture instead of assuming that a familiar slag percentage will behave the same way with different cement, aggregates or admixtures.
Silica fume
Silica fume consists of extremely fine, highly pozzolanic particles. It is selected when high strength, very low permeability, ASR mitigation or sulfate-resistance goals justify its handling and finishing demands. The fine particles increase water demand, so a high-range water reducer is often needed. Dispersion is critical: clumps can damage performance.
NRMCA notes that high quantities can make concrete sticky and contribute to early-age cracking when construction precautions are inadequate. Confirm dosage, admixture compatibility, mixing energy, finishing practice and curing before approving a silica-fume mixture. (NRMCA, 2026)
Natural pozzolans
Natural pozzolans can provide a useful alternative where a consistent local source exists and the specification permits it. Their chemistry and reaction rate differ by deposit and processing method, so a source qualification and project-specific performance evidence are important. NRMCA identifies ASTM C1945 as the material standard to check for natural pozzolans. (NRMCA, 2026)
Ground-glass pozzolans and other alternative SCMs
Ground-glass pozzolan and other materials outside established project specifications may help where conventional SCM supply is constrained. They should not be accepted solely because a laboratory property resembles a listed SCM. ASTM C1709-22 calls for evaluation of alternative SCM properties and concrete performance suited to the intended end use. NRMCA lists ASTM C1866 for ground-glass pozzolans and ASTM C1912 as a performance-oriented specification for certain materials outside other SCM specifications. (ASTM C1709-22; NRMCA, 2026)
Ternary and other blended mixtures
A ternary mixture combines two SCMs with portland cement. Combining materials can balance reaction rate, heat, permeability and workability, but the combination still has to pass the project’s acceptance tests. The National Academies reported that 29 of 33 responding DOTs allowed ternary mixtures; permission does not eliminate the need for a trial mixture with the project materials. (National Academies state DOT practices)
Comparison at a glance
| SCM | Characteristics that may help | Risks or controls to plan |
|---|---|---|
| Fly ash | Later-age strength and durability benefits; widely used in many DOT programs | Source and class variability; possible slower early strength or setting; supply shortages in some markets |
| Slag cement | Lower heat evolution can suit mass concrete; later-age performance | Typically slower reaction and longer setting; curing is especially important in cool conditions |
| Silica fume | High strength, low permeability and potential ASR or sulfate-resistance benefits | Higher water demand; usually needs high-range water reducer; dispersion, stickiness and early-age cracking require control |
| Natural pozzolan | Potential local alternative where a consistent source is available | Deposit and processing chemistry vary; specification status and project track record must be verified |
| Ground-glass or other alternative SCM | Can expand supply options when qualified for the intended use | Requires comprehensive, end-use-specific evaluation when performance history or specification coverage is limited |
4. Use published percentages as context, not a recipe
The National Academies’ 2025 DOT survey reported common pavement practices of approximately 20% Class F fly ash, 50% slag cement and less than 10% silica fume among responding agencies. It also found that 40 of 43 responding DOTs used Class F coal ash, 37 used slag cement, 35 used silica fume and 28 used Class C fly ash. These are reported practices, not universally approved limits or predictions of strength, set or durability for your mixture. (National Academies, 2025; state DOT practices)
| Survey item | Reported value | How to use it |
|---|---|---|
| Class F coal ash use | 40 of 43 responding DOTs | Shows common agency use, not local availability or approval for a specific job |
| Slag cement use | 37 of 43 responding DOTs | Shows widespread use, not a required replacement level |
| Silica fume use | 35 of 43 responding DOTs | Shows adoption, while dosage and handling remain project-specific |
| Class C fly ash use | 28 of 43 responding DOTs | Indicates reported use among respondents, not a supply guarantee |
| Pavement practice: Class F fly ash | Approximately 20% | Survey context only; verify the actual project mixture by testing |
| Pavement practice: slag cement | Approximately 50% | Survey context only; confirm setting, curing and acceptance performance |
| Pavement practice: silica fume | Less than 10% | Survey context only; evaluate water demand, dispersion and finishing |
| Ternary mixtures allowed | 29 of 33 responding DOTs | Agency permission still requires a compliant, tested combination |
NRMCA’s 2026 guidance cautions against unnecessary SCM quantity caps when performance requirements can govern. A limit may still be justified for constructability, schedule or finishing, but it should be agreed with the supplier and contractor and supported by the project’s needs. (NRMCA, 2026)
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5. Trial-batch the actual mixture
Do not approve an SCM from a datasheet percentage alone. Use the cement, SCM source, aggregates, water, admixtures, air system, batching sequence and curing plan proposed for construction.
- Collect material information. Record the SCM class or product designation, source, recent quality data, cement chemistry, aggregate reactivity information and admixture brands and dosages. Confirm continuous supply for the placement schedule.
- Set measurable acceptance criteria. Include strength at every required age, air content, slump or spread, temperature, unit weight, setting time, permeability or chloride-penetration measure, heat limits and any ASR or sulfate-related test required by the specification.
- Run trial mixtures. Compare the candidate SCM or blends at practical dosages using the actual aggregate gradation and admixture system. For alternative SCMs, follow an evaluation approach suited to the intended end use as described by ASTM C1709-22. (ASTM C1709-22)
- Reproduce construction conditions. Include expected ambient temperature, haul time, placement rate, consolidation, finishing and curing. Check cold-weather behavior for slow-reacting materials and hot-weather setting for mixtures that may stiffen rapidly.
- Review fresh-concrete behavior. Measure water demand, slump retention, air stability, pumpability, finishability, bleeding, setting window and early cracking indicators. A mixture that meets strength but cannot be placed or finished on schedule is not a successful selection.
- Verify hardened performance. Test the specified strength ages and durability indicators. For ASR mitigation, use the applicable test method and the actual aggregate, alkali level, cement and SCM dosage; FHWA warns that effectiveness depends on all of these factors. (FHWA ASR guidance)
- Document the approved mixture and controls. Identify approved sources, dosage tolerances, admixture adjustments, mixing sequence, curing duration, temperature limits and actions if a shipment or test result falls outside the acceptance criteria.
6. Adjust the choice to the project type
Mass concrete
Prioritize heat evolution, temperature differential control and thermal-cracking risk. Slag cement or a blend may be useful because slower reaction and lower heat can ease temperature management. Confirm that the slower setting will not conflict with placement lifts, finishing or formwork removal, and provide the curing and temperature-monitoring plan needed by the specification.
Pavements
Balance opening strength, setting, finishability, freeze-thaw air management, permeability and long-term durability. Survey percentages can provide a starting discussion, but pavement acceptance ages and local agency provisions control. Test the proposed mixture for the actual weather and paving sequence.
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Bridge decks and overlays
Chloride exposure, permeability, shrinkage, early cracking, finishing window and traffic-opening requirements often compete. Silica fume may be appropriate for a low-permeability target, but its water demand and stickiness require a compatible high-range water reducer and disciplined finishing and curing. Other SCMs or a ternary blend may better balance constructability and durability when the trial results support them.
General structural concrete
Start with the specified strength age, exposure class, placing method and available curing. Select the simplest locally reliable SCM or blend that meets every acceptance criterion; a more complex material is not automatically better.
Independent reader supportYour contribution helps us test, update, and keep practical guides available for everyone.7. Check the governing material standards
NRMCA’s March 2026 guide identifies these standards as relevant references: ASTM C618 for coal ash, C989 for slag cement, C1240 for silica fume, C1945 for natural pozzolans, C1697 for blended SCMs, C1866 for ground-glass pozzolans and C1912 as a performance-oriented specification for certain materials outside other SCM specifications. Confirm the contract-required editions and any agency modifications before writing a submittal or mix requirement. (NRMCA, 2026)
ASTM C1709-22 provides a guide for evaluating alternative SCM properties and concrete performance, with the evaluation matched to the intended end use. It is especially relevant when a material lacks a substantial performance record under the project’s exposure and construction conditions. (ASTM C1709-22)
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8. Troubleshoot common selection problems
The specified SCM is unavailable
Ask the producer for a locally stocked equivalent category or a ternary alternative, then obtain engineer approval and repeat the required trial testing. Do not substitute by name alone: chemistry, fineness, reaction rate and dosage can change fresh and hardened performance.
Strength is late
Check whether the SCM is slow-reacting, whether curing temperature was lower than assumed, and whether the acceptance age matches the mixture’s development. Review cement, aggregate, water-reducer dosage and temperature records before changing the percentage. A higher cement content or different SCM may solve the schedule problem, but only a tested mixture should be adopted.
Concrete sets too slowly
Review SCM type and dosage, concrete temperature, retarding admixture, haul time and curing conditions. Slag cement and some fly ashes can extend setting, especially in cool weather. Coordinate any admixture or dosage adjustment with the producer and verify the revised setting window in a trial.
Concrete is sticky or difficult to finish
Silica fume’s fine particles can increase water demand and stickiness. Check dispersion, high-range water-reducer compatibility, total paste volume and finishing timing. Do not add water at the jobsite to recover workability; adjust the approved mixture and retest.
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Investigate evaporation, restraint, temperature gradients, shrinkage, finishing practices, curing and the SCM’s influence on setting. NRMCA specifically notes early-age cracking risk with high silica-fume quantities when construction precautions are inadequate. Correct the curing and construction process as well as the mixture, rather than assuming that changing SCM percentage alone will solve the problem. (NRMCA, 2026)
ASR mitigation is uncertain
Confirm aggregate reactivity, cement alkali content, SCM characteristics and dosage, then use the applicable ASR test program with the actual combination. FHWA’s guidance does not support treating one SCM class or percentage as universally protective. (FHWA ASR guidance)
Decision checklist for the project team
- Performance requirements and acceptance ages are written down.
- Exposure, permeability, ASR, sulfate and heat-control needs are identified.
- Placement, finishing, setting window and curing conditions are defined.
- The governing specification and current material standards are confirmed.
- The concrete producer has verified local SCM availability, consistency and delivery capacity.
- At least one candidate mixture has been trial-batched with the actual project materials.
- Fresh, hardened, durability and temperature results meet the contract criteria.
- Any alternative SCM has been evaluated for its intended end use and approved by the responsible engineer.
- The approved mixture includes handling, admixture, curing, temperature-monitoring and contingency instructions.
Final recommendation
Select the SCM that demonstrably satisfies the project’s durability, strength, heat, constructability and supply requirements—not the one with the highest replacement percentage or the most favorable reputation. Treat survey ranges as background, qualify unfamiliar materials under an end-use-specific test plan, and make the final decision with the producer, engineer and contractor using a trial mixture and the governing acceptance tests.
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