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Supplementary cementitious materials (SCMs) are mineral materials used with portland or blended cement to make concrete. They react hydraulically, pozzolanically, or through both pathways, and can change workability, setting, strength development, permeability, heat generation, cracking risk, and cement demand. Common SCMs include fly ash, slag cement, silica fume, metakaolin, calcined clay, calcined shale, and ground-glass pozzolan.
The important qualification is that an SCM is a category, not a guaranteed performance upgrade. The material’s chemistry, processing, replacement level, aggregate, water-to-cementitious-materials ratio, curing, temperature, placement method, and exposure determine the result. The right selection therefore starts with the project’s performance requirements and current specifications, not with a universal “best” percentage.
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What are supplementary cementitious materials?
SCMs are finely divided mineral materials that contribute to concrete’s cementitious system when used alongside portland cement or blended cement. The Federal Highway Administration describes them as materials used as a partial replacement for portland cement to improve fresh and hardened concrete properties. They may be batched as a separate powder at the concrete plant or supplied already incorporated into a blended cement.
SCMs are not all chemically alike. Some are industrial byproducts, some are manufactured by calcining or otherwise processing natural minerals, and some are recovered or specially processed glass-based materials. Their source, chemistry, fineness, processing history, quality-control record, and local supply all affect how they behave in a mixture.
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Replacing part of the portland cement changes more than the cement percentage. It changes the balance of reacting compounds, the amount and timing of heat release, the pore structure that develops during curing, and the way the fresh mixture responds to water and finishing. Those changes can solve one design problem while creating another, so SCM selection is a mixture-design decision rather than a simple substitution.
How do hydraulic and pozzolanic SCM reactions differ?
A hydraulic material can react with water and form cementitious compounds on its own. Portland cement is the familiar hydraulic binder; some SCMs also have hydraulic activity. A pozzolan generally needs moisture and calcium hydroxide, which is produced as portland cement hydrates. It reacts with that calcium hydroxide to form additional cementitious compounds. Some materials show both hydraulic and pozzolanic behavior.
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The FHWA overview of SCM use and the National Ready Mixed Concrete Association’s CIP 30 guide both emphasize that performance must be evaluated in the actual mixture rather than inferred from the material name alone.
Which SCM families are used in concrete?
| SCM family | Reaction and form | Potential contribution | Design cautions |
|---|---|---|---|
| Coal fly ash (Class C or Class F) | Fine ash from coal combustion; behavior varies with class and source chemistry. | Can improve later-age strength, workability, permeability, heat control, and resistance to some durability mechanisms in suitable mixtures. | Early strength and setting may be slower, especially with higher replacement, cool temperatures, or inadequate curing. Source consistency must be verified. |
| Slag cement | Granulated blast-furnace slag ground for use as a cementitious material; commonly exhibits hydraulic activity. | Can contribute to strength development, reduced permeability, and lower heat in appropriate proportions. | Early-age behavior, color, setting, and availability vary by grade, replacement level, temperature, and curing. |
| Silica fume | Extremely fine, silica-rich material with strong pozzolanic activity. | Can produce very low permeability and high later-age performance when proportioned and cured correctly. | Mixtures can be cohesive or sticky, with reduced bleeding and greater finishing sensitivity. Water demand, dispersion, curing, and shrinkage control require attention. |
| Natural pozzolans, including metakaolin and calcined clay | Natural minerals or clays processed to develop pozzolanic reactivity; calcined shale is another recognized family. | Can supply pozzolanic reaction and, depending on material, support strength, permeability, and durability objectives. | Processing energy, color, water demand, setting, and supply differ substantially among sources. Performance data must be material-specific. |
| Ground-glass pozzolan and other alternative SCMs | Processed glass or other materials evaluated for pozzolanic contribution. | May broaden SCM supply and provide useful performance when a qualified material is available. | Alternative materials may not fit an existing prescriptive specification; project-specific qualification and testing are important. |
The table describes possible tendencies, not guaranteed outcomes. The same family can behave differently when its chemistry, fineness, moisture condition, or dosage changes. Blended SCM systems can also interact, making the performance of the combined system different from either material used alone.
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How can SCMs change fresh concrete?
SCMs can alter consistency, water demand, bleeding, cohesion, color, setting, and finishing. Some mixtures become easier to place or pump because the particles improve packing and cohesion. Others require more water-reducing admixture or a different paste volume to reach the target slump without increasing the water-to-cementitious-materials ratio.
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- Workability and cohesion: Fine particles can improve suspension and reduce segregation, but very fine materials—especially silica fume—can make concrete feel sticky and difficult to finish if the admixture and paste balance are not adjusted.
- Bleeding: Reduced bleeding can help limit channels and surface water, but it also removes a natural source of surface moisture. A slab that is finished too early or left without prompt curing can become more vulnerable to plastic shrinkage.
- Setting: Some SCMs delay setting, particularly at higher replacement levels or in cool conditions. The effect depends on cement chemistry, admixtures, temperature, and moisture, so a nominal dosage cannot predict the exact finishing window.
- Color and appearance: SCMs can change paste color and the appearance of formed or finished surfaces. If appearance is critical, approve a representative mixture and curing method rather than relying on a material description.
- Admixture interaction: The same water reducer, accelerator, air-entraining system, or set-control admixture may behave differently after the cementitious blend changes. Trial batches and supplier data should address compatibility.
For flatwork, the practical response is to plan finishing and curing around the actual set and bleeding behavior. Do not add water on site to recover slump without considering strength, permeability, and finishing consequences.
What happens to setting time and strength development?
Many SCM mixtures develop strength more slowly at early ages, although silica fume is a notable exception in the FHWA’s general overview. The delay is most noticeable with higher replacement, low concrete temperature, inadequate curing, or a cementitious blend whose reaction is intrinsically slower. Longer-term strength can equal or exceed a portland-cement-only reference when the mixture is properly proportioned and cured.
Early-opening work, precast stripping, post-tensioning, and winter placement therefore need an age-specific strength plan. Specify the required strength at the age that matters for the operation—not only the conventional 28-day value—and confirm that the proposed mixture can reach it under the project’s curing conditions. The FHWA reports that its Mobile Concrete Technology Center observed only a slight delay to open-to-traffic strength in its field experience, but that observation is not a universal prediction for every SCM, climate, or pavement.
Temperature management is especially important. Cold conditions slow both portland-cement hydration and many SCM reactions. The response may include changing the blend, adjusting admixtures, protecting the placement, extending curing, or delaying loading. Those choices should be made with the concrete producer and engineer because a faster set can affect heat, finishing, air entrainment, and later durability.
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Can SCMs improve durability and control heat?
SCMs can reduce permeability by developing additional cementitious products and refining the hardened paste’s pore network. Lower permeability can limit the movement of water and aggressive dissolved substances, but it is not automatic: poor curing, cracking, high water content, or an unsuitable SCM can erase the expected benefit.
FHWA guidance identifies possible mitigation of alkali-silica reaction (ASR), reduced peak hydration temperature, and lower thermal-cracking risk as important reasons to use SCMs. ASR control depends on the reactivity of the aggregate, the alkali contribution of the entire mixture, exposure to moisture, and the chemistry and amount of the SCM. A generic “ASR-safe” replacement percentage is not a design prescription. The FHWA ASR chapter explains why mitigation must be matched to the aggregate and mixture.
Lower heat can help in thick footings, dams, bridge elements, and other placements where temperature differences create tensile stress. The result still depends on placement size, ambient conditions, insulation, curing, restraint, and the total heat of the blend. An SCM cannot by itself prevent thermal cracking if the temperature-control plan is inadequate.
Do SCMs make concrete lower-carbon?
Using less portland cement can reduce cement demand and may lower concrete’s embodied carbon. The size of the reduction cannot be stated as a universal percentage because it depends on the SCM’s processing energy, transport distance, replacement level, cement source, batching, curing, and the rest of the mixture.
Traditional industrial byproduct SCMs may have limited or changing availability. Manufactured pozzolans can expand supply, but manufacturing them consumes energy. The Precast/Prestressed Concrete Institute’s sustainability FAQs specifically caution that metakaolin requires energy to manufacture and that replacing portland cement with it does not automatically reduce environmental impact.
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A credible low-carbon specification should therefore define performance and permit suitable qualified materials rather than mandate a single product or replacement rate. Compare the full mixture’s life-cycle result, service life, transport, curing needs, and expected durability—not only the mass of cement removed at the batch plant.
How should a project select and specify an SCM?
Start with the exposure and construction requirement, then choose a cementitious system that can meet them. The U.S. Environmental Protection Agency’s cement and concrete specifications guidance recommends established consensus specifications for common fly ash, natural pozzolan, slag, and silica-fume applications and encourages allowing these materials where they meet performance and price objectives.
- Define the controlling outcomes. Identify required early strength or opening age, durability exposures, ASR risk, maximum temperature, permeability, finish, color, pumping distance, and curing conditions.
- Check local supply. Ask producers which qualified SCM sources, grades, and blended cements are reliably available for the project’s delivery period. Availability can change, and a nominally ideal material is not useful if replacement deliveries are uncertain.
- Review documentation. Obtain current certificates, test data, relevant specification compliance, source history, and information about variability. Confirm that the material and proposed proportion are permitted by the governing structural, pavement, or jurisdictional requirements.
- Design the complete mixture. Evaluate cement, SCMs, aggregates, water, admixtures, air content, and target strength together. Check fresh properties, setting, early and later strength, shrinkage where relevant, permeability or transport indicators, heat, and durability tests tied to the exposure.
- Verify production and curing. A laboratory result does not replace field controls. Plan batching accuracy, moisture corrections, temperature protection, finishing, curing duration, and acceptance testing before placement.
- Document the approved mixture. Record the material source, cementitious proportions, admixture system, water limits, required performance ages, curing method, and procedures for a source change.
For an SCM outside an established prescriptive specification, ASTM C1709-22 provides a framework for evaluating alternative SCMs. The guide notes that performance in the specific mixture is commonly verified before construction. The current edition of the applicable ASTM, AASHTO, building code, pavement specification, and local authority requirements governs the project; this article cannot substitute for those documents.
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- Treating all SCMs as interchangeable: Fly ash, slag, silica fume, and calcined clay have different reaction pathways and fresh-concrete effects.
- Copying a replacement percentage: A percentage that worked for one cement, aggregate, climate, or exposure may fail another mixture.
- Ignoring curing: Slower-reacting systems need adequate moisture and temperature for their intended properties to develop.
- Designing only for 28-day strength: Early opening, stripping, loading, and thermal limits may control the mixture.
- Assuming lower bleeding is always beneficial: Less bleed water can increase plastic-shrinkage and finishing risk if curing is delayed.
- Assuming a carbon benefit without a life-cycle check: Processing and transport can materially change the result, especially for manufactured pozzolans.
- Failing to plan for source changes: A new supplier or production lot can alter color, setting, air entrainment, or strength development and may require renewed qualification.
What does the FHWA experience show about SCM use?
The FHWA’s Mobile Concrete Technology Center reported testing concrete in 35 states over the preceding 12 years and found only two states not using SCMs. That is a report of the center’s observations, not a current national census and not a claim that every project in those states uses SCMs. It does illustrate how widely SCMs are incorporated into contemporary concrete practice.
What is the practical takeaway?
SCMs are tools for engineering the cementitious system. They can reduce portland-cement demand and improve selected fresh, hardened, durability, and heat-related properties, but each benefit comes with mixture-specific conditions. Select the material and proportion from the project’s exposure, schedule, curing plan, supply, specification, and verified performance, then control production closely enough for the intended reaction to occur.
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Frequently Asked Questions
Can SCM concrete be placed in cold weather?
Yes, but the mixture may set and gain strength more slowly, extending protection and curing needs. The contractor and engineer should establish temperature limits, maturity or strength criteria, and any accelerator or blend changes before placement rather than relying on a standard schedule.
Are fly ash and slag cement interchangeable?
No. They differ in chemistry, reaction behavior, early-age response, heat development, color, and source availability. A substitution should be treated as a new mixture requiring compatibility and performance verification.
What information should a concrete supplier provide for an SCM?
Request the material designation, current compliance certificates, source and variability information, recommended handling, admixture-compatibility data, and performance results in a comparable mixture. Also confirm delivery reliability and the procedure for notifying the project of a source change.
Does using an SCM always lower concrete cost?
No. Cost depends on local supply, transport, processing, dosage, admixture demand, curing, schedule effects, and the value of the durability or heat-control benefit. A material that reduces cement quantity can still increase other project costs or require a different construction sequence.
