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Supplementary cementitious materials (SCMs) are not interchangeable “cement replacements.” Fly ash, slag cement, silica fume, natural pozzolans and newer materials can alter setting, strength development, heat, permeability and durability in opposite ways. The sound engineering approach is to define the concrete performance you need, identify materials the producer can supply consistently, then qualify the actual combination through mixture testing and the governing specification.
What SCMs are and why they matter
SCMs are finely divided materials used with portland cement or blended cement. Some are pozzolanic: they react with calcium hydroxide in the presence of water to form additional cementitious products. Others, such as slag cement, have latent hydraulic behavior and can react directly when activated in a cementitious system. In practice, the distinction matters less than the resulting concrete performance.
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SCMs can reduce clinker demand and change the chemistry and pore structure of concrete, but a material name or type designation does not guarantee a particular result. Mineralogy, fineness, particle shape, loss on ignition, glass content, alkali content, replacement level, cement chemistry, aggregate, water-to-cementitious-materials ratio, curing and temperature all influence the outcome. Treat each source and proposed proportion as a mixture-design decision, not as a generic product substitution.
Which SCMs are used in US concrete?
Fly ash (coal ash)
Class F and Class C fly ash are both used in US transportation practice. Their chemistry and reactivity differ, so neither class is universally suitable for every exposure or schedule. Class F commonly develops strength more slowly and can delay initial set; Class C can also delay set, although its strength and setting behavior depend on the source and mixture. Evaluate the actual ash against the project’s strength-age, finishing and durability requirements.
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Slag cement
Slag cement is covered by ASTM C989. Its grades are based on relative mortar strength activity index; the NRMCA’s 2026 guide notes that US slag cements are generally Grade 100 or Grade 120. Grade alone does not predict every field result: cement compatibility, curing temperature, replacement percentage and early-age requirements still require verification.
Silica fume
ASTM C1240 covers silica fume. Its extremely fine particles can produce high strength and very low permeability when properly dispersed. That fineness also raises water demand and can make concrete sticky, complicate finishing and increase early-age cracking risk if shrinkage, curing or restraint are not controlled. Silica fume must be dispersed throughout the paste; undispersed clumps can act adversely in alkali-silica-reaction (ASR) situations rather than providing the intended mitigation.
Natural pozzolans
Natural pozzolans include calcined clay, metakaolin and calcined shale. Available sources and specifications vary substantially by region. ASTM C1945 addresses raw or calcined natural pozzolans. Calcined materials can be reactive and useful where other SCMs are scarce, but their water demand, color, setting behavior and strength development should be established with the local cement and aggregate.
Ground-glass and other alternative materials
Current US guidance also addresses ground-glass pozzolan (ASTM C1866), blending of conforming SCMs (ASTM C1697), and certain pozzolanic or cementitious materials outside other SCM specifications (ASTM C1912). ASTM C1709-22 provides an evaluation approach for alternative SCMs. Confirm the current edition and whether the owner, DOT or building-code provisions have adopted it before using a standard as a contract requirement.
How SCMs affect concrete performance
The following are tendencies reported in an FHWA technical brief, not mix-design prescriptions. The direction and magnitude can change with material source, dosage, cement, aggregate, water content, curing and temperature.
| Property | Class F fly ash | Class C fly ash | Slag cement | Silica fume |
|---|---|---|---|---|
| Initial set | Often delayed | Often delayed | Can be delayed; source and temperature matter | Can change with dosage and admixture compatibility |
| Early strength | Often slower | Variable; verify at the required age | Often slower at low temperatures or high replacement | Can increase strength when well dispersed and proportioned |
| Heat of hydration | Generally lower | Generally lower | Generally lower | Dosage- and mixture-dependent |
| Permeability | Generally reduced | Generally reduced | Generally reduced | Can be substantially reduced |
| Fresh workability | Often improved, but source-dependent | Source- and dosage-dependent | Usually workable, with possible finishing changes | Often stickier and more water-demanding |
| ASR mitigation | Can be effective at an appropriate dosage | Can be effective, but verify with the aggregate and chemistry | Can reduce ASR risk | Can reduce ASR risk only with adequate dispersion |
Setting, finishing and schedule
Delayed set can be beneficial for hot-weather transport and large placements, yet it can postpone finishing, saw-cutting and form removal. Slow strength gain is especially important for cold-weather construction, prestressed operations, opening-to-traffic dates and any work tied to early-age test results. Conversely, a very fine or reactive SCM may accelerate water demand or stiffening. Use trial batches at representative temperatures and measure setting time, slump or spread retention, air content and finishing response.
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Strength at different ages
Specify the age that controls acceptance. A mixture that misses a three-day target but exceeds a 56-day target may be appropriate for a mass foundation and unacceptable for a fast-track deck. Do not infer strength development from an SCM category alone. Test the proposed cement, SCM, aggregate, admixtures and curing regime together, and use maturity or other approved methods only after their relationship to project concrete has been established.
Heat and thermal cracking
Fly ash and slag commonly lower heat of hydration, which can help mass concrete and restrained placements. Lower heat does not automatically eliminate thermal cracking: temperature differentials, restraint, ambient conditions, placement size and curing determine the actual risk. Establish a thermal control plan with maximum concrete temperature and differential limits where the contract requires them.
Permeability and transport
Many SCM mixtures refine capillary pores and reduce permeability, improving resistance to ingress of water, chlorides and other aggressive agents. The benefit depends on curing and sufficient reaction; a poorly cured mixture may not realize its potential. Use the transport or durability test required for the exposure rather than treating a lower water-to-cementitious-materials ratio or an SCM label as proof of performance.
Durability: ASR, sulfates, corrosion and freeze-thaw
Alkali-silica reaction
SCMs are a common ASR-control strategy. They can lower permeability, alter pore-solution alkalinity and bind alkalis. Effectiveness varies with aggregate reactivity, available alkalis, SCM composition and dosage. A percentage that worked with one aggregate or cement may not control a more reactive combination. Follow the applicable project specification and test the actual aggregate and materials using the accepted ASR approach; never present one replacement percentage as universally effective.
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Appropriate SCMs can improve resistance to sulfate attack and reduce the movement of water and chloride ions that contribute to reinforcement corrosion. The result is exposure-specific and depends on cement chemistry, curing, cracking and permeability. Confirm the governing exposure class, sulfate limits, air-entrainment requirements and durability tests in the contract documents.
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Freeze-thaw
Freeze-thaw durability is controlled primarily by a sound air-void system, saturation history, concrete quality and curing. SCMs may help by reducing permeability, but they do not replace air-entrainment or verification of the specified freeze-thaw performance.
How to choose an SCM for a concrete mix
Use this sequence before fixing a replacement percentage.
- Define performance and exposure. Record required strength ages, permeability or durability criteria, heat limits, ASR or sulfate provisions, air content, finishing needs and the construction schedule.
- List viable local materials. Ask the concrete producer for current source information, specification compliance, typical test data, seasonal capacity and experience with the proposed cement and admixtures. Local availability is a design constraint, not merely a purchasing issue.
- Screen mixture interactions. Review water demand, slump retention, air-entraining response, setting, bleed water, finishing, early strength, later strength and temperature rise. Include expected placement and curing temperatures.
- Set an initial proportioning range. Use the project specification and producer experience to select trial replacement levels or a ternary blend. A blend can combine useful attributes and offset drawbacks, but it still needs its own qualification.
- Run representative trials. Batch with production-scale materials where possible. Test fresh properties, setting, strength at acceptance and later ages, heat where relevant, permeability or other durability indicators, and cracking-related behavior when the risk warrants it.
- Document acceptance criteria. Tie approval to demonstrated performance and constructability, not simply to a material name, class or nominal percentage.
Qualifying an alternative SCM
For a material outside an established specification, ASTM C1709-22 provides an evaluation framework. ASTM International states in section 4.1: “Performance of SCM in specific concrete mixtures is commonly verified through preconstruction testing.” That principle is important: a laboratory result on the powder alone cannot establish performance in the project’s concrete.
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Information to obtain from the supplier
- Source, processing method and production controls.
- Chemical and physical characterization relevant to the applicable specification.
- Consistency data over time, including fineness, moisture and reactivity indicators.
- Recommended storage, handling and batching procedures.
- Existing concrete test results that identify cement, aggregate, admixtures, proportions, curing and test ages.
Project-specific test program
Build the program around the failure modes that matter. At minimum, compare a control mixture and the proposed mixture at the intended water-to-cementitious-materials ratio and admixture dosage. Measure workability and retention, air, setting, bleeding, strength development and the durability or transport tests named by the project. Add heat measurements for mass or thermally restrained placements and ASR or sulfate testing when those exposures govern. Repeat batches when source variability is a concern.
Define what happens if a test fails: adjust dosage, admixtures, curing or the SCM source, or reject the combination. Record batch temperature, curing temperature, specimen age and test method so that results can be compared with production concrete.
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Standards, specifications and agency practice
ASTM designations identify test methods or material specifications; they do not by themselves authorize a material on every project. Check the contract, adopted building code, DOT standard specifications and special provisions for the current edition and permitted materials. An owner may impose limits on type, quantity, strength age, heat, chloride exposure or ASR control that are more restrictive than a general industry guide.
NRMCA’s March 2026 guide cautions that restrictive type and quantity limits can constrain performance and emphasizes local availability and producer experience. Treat that document as industry guidance, then verify the requirements actually adopted for the project jurisdiction.
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A National Academies of Sciences, Engineering, and Medicine survey published in 2025 received responses from 43 of 52 state-level DOT jurisdictions, an 83% response rate. All 43 responding DOTs allowed SCM use. Among those respondents, 40 used Class F fly ash, 37 used slag cement, 35 used silica fume and 28 used Class C fly ash.
Supply planning is essential. Twenty-eight of 33 responding DOTs had experienced or expected fly ash shortages. Some DOTs reported SCM dosages of 50% to 70% by mass of total cementitious material for bridge structures; those figures describe practices reported by particular agencies and applications, not a universal recommendation.
Before approving a mixture, identify backup sources, storage capacity, delivery lead times and the change-control process if the source changes. A substitute SCM can alter setting, strength, air entrainment and durability enough to require a new qualification batch.
Common mistakes and practical corrections
- Choosing by category alone: Replace the assumption with source-specific data and trial batches.
- Copying a replacement percentage: Use the percentage only as a starting point; verify it against the actual aggregate, cement, exposure and schedule.
- Ignoring cold weather: Test setting and early strength at expected low temperatures and plan curing accordingly.
- Assuming low permeability means complete durability protection: Confirm air system, cracking control, curing and the exposure-specific test.
- Underestimating silica-fume handling: Check dispersion, water demand, slump retention, finishing and early-age cracking controls.
- Approving a new source without change control: Define what source, processing or test-data change triggers requalification.
- Relying on laboratory powder tests alone: Verify performance in the complete concrete mixture before construction.
A field-ready approval checklist
- Exposure classes, durability mechanisms and acceptance ages are identified.
- Current ASTM, DOT and contract requirements are confirmed for the project location.
- The producer has documented source, consistency and supply information.
- Trial batches use the actual cement, SCM, aggregate, water and admixtures.
- Fresh properties, setting, strength, heat and required durability tests meet defined limits.
- Curing, temperature control, finishing and cold-weather procedures are compatible with the mixture.
- Alternative sources and a formal requalification trigger are documented.
- Approval records identify the exact material source and mixture proportions.
Bottom line for engineers
SCMs can lower heat and permeability, improve resistance to ASR and other exposures, and help meet performance goals, but they also can delay setting, slow early strength, increase stickiness or create supply risk. Select them against the project’s exposure and schedule, qualify the actual mixture before construction, and keep the specification flexible enough to use reliable local materials without weakening performance requirements.
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