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Concrete mix design with industrial cement-replacement materials means qualifying supplementary cementitious materials (SCMs) in the complete concrete mixture—not choosing a universal replacement percentage. Fly ash, slag cement, silica fume, natural and calcined pozzolans, and ground-glass pozzolan can be considered when they meet applicable specifications and the proposed mix satisfies the project’s fresh-concrete, strength, durability, exposure, and cost requirements. The right proportions depend on the material source, the rest of the mix, the project specification, and documented testing. This guide sets out a practical qualification process; it does not prescribe a dosage for an unspecified project.
What are industrial cement-replacement materials?
In concrete mix design, “cement replacement” commonly refers to using a supplementary cementitious material (SCM) as part of the cementitious system. The relevant question is not simply how much Portland cement can be removed. It is whether the actual combination of cement, SCM, aggregate, water, and admixtures will meet the job’s requirements when produced and placed under the specified conditions.
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SCMs used in concrete include fly ash (also called coal ash in some guidance), slag cement or ground granulated blast-furnace slag (GGBFS), silica fume, natural pozzolans, calcined clays, and ground-glass pozzolan. These are distinct material families, not interchangeable products. Their specifications and evidence requirements differ. In the United States, examples include ASTM C618 for fly ash and natural pozzolans, C989 for slag cement, C1240 for silica fume, C1866 for ground-glass pozzolan, and C1697 for blended SCMs. The Portland Cement Association’s Design and Control of Concrete Mixtures discusses SCM families and the applicable standards; ASTM C1697-25a describes blended SCMs made from qualifying materials including slag cement, natural pozzolans, coal ash, silica fume, and ground-glass pozzolan.
Those references are U.S. standards. Confirm which edition and requirements the contract, jurisdiction, and project have adopted before specifying or accepting a material. A material’s compliance with an applicable standard is a qualification about the material; it does not, by itself, demonstrate that a particular concrete mixture will meet the project’s needs.
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Why is there no universal replacement percentage?
There is no defensible single percentage for every SCM and concrete application. A replacement level that appears suitable in one mixture may not deliver the required setting, early strength, durability, or placement behavior in another, because performance depends on the particular SCM and the other constituents.
ASTM notes that SCMs can significantly change both fresh and hardened concrete properties, and that performance in a specific mixture is commonly verified by preconstruction testing. ACI likewise notes that fly ash performance depends on the other mixture constituents. Its education bulletin advises re-evaluating fly ash with trial batches before commercial production when mixture components change. These cautions apply to practical mix approval: do not copy a percentage from a different source, project, or generic chart and treat it as a project-specific design.
Instead, start with specified performance and test a proposed mixture made with the actual materials. Any proposed proportion is conditional on the governing specification, exposure, material properties, placement requirements, and test results. Where an exact limit is necessary, the responsible design professional and project documents must establish it for the job.
How do the main SCM families differ?
The materials below are recognized SCM families, but the cited guidance does not rank them or establish that they are substitutes for one another. Compare each candidate against the requirements of the intended concrete and the evidence available for its source.
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| Material family | What the guidance establishes | Questions for the proposed mix |
|---|---|---|
| Fly ash / coal ash | A recognized SCM family covered by ASTM C618; it may be used as a partial replacement or addition. ACI emphasizes that its performance depends on the complete mixture. | Does the actual mix meet the specified setting, early-strength, durability, and placement requirements? |
| Slag cement / GGBFS | A recognized SCM covered by ASTM C989 and included in EPA procurement guidance on partial cement substitution. | Does the proposed combination achieve required strength development and durability at an acceptable project cost? |
| Silica fume | A recognized SCM covered by ASTM C1240 and included in ASTM’s blended-SCM specification. | Does the complete mix satisfy the required fresh and hardened properties for this application? |
| Natural or calcined pozzolans, including calcined clays | Included among SCM families in the PCA guide and in ASTM C1697 references. | Is the specific material covered by the relevant standard, and what test evidence supports it in this project’s mix? |
| Ground-glass pozzolan | Included in ASTM C1697-25a’s described blend materials and in the PCA guide’s SCM families. | Does the source satisfy the project’s specification, and has its actual mixture performance been demonstrated? |
| Alternative SCMs outside existing specifications | ASTM C1709-22 provides an evaluation approach and advises comprehensive evaluation for materials without a significant performance record. | What project-specific testing and field evidence are necessary before use or broader deployment? |
The table is a screening framework, not a substitution chart. One material may have stronger project-specific evidence, a better fit for the exposure, or a more reliable supply than another. The answer must come from applicable specifications and measured performance of the proposed mix, not from the material name alone.
How should you qualify an SCM concrete mix?
Use a performance-based workflow: define the job, identify the materials, proportion the full cementitious system, test the proposed mix, and release it only against the contract requirements. A material certificate or generic recipe is not a replacement for this sequence.
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- Define the job requirements. Record specified compressive strength and the age at which it must be achieved; exposure and durability requirements; fresh-concrete and placement needs; heat constraints, if relevant; cost targets; and governing codes and specifications. Identify any project-specific limits or acceptance criteria. EPA recommends tying restrictions on cement or concrete to reasonable performance requirements or documented technical reasons.
- Check whether the specification allows the candidate material. Identify the jurisdiction and adopted standard, the project’s concrete specification, and any exposure-related conditions. Confirm the applicable material standard and edition for the SCM. If a specification names a particular material or imposes a limit, do not assume that a different SCM or proportion is permitted without approval.
- Document the actual sources. Record the cement and SCM sources and the other constituents that affect the proposed mixture. A material family or specification designation alone does not capture the performance of a particular source in combination with a particular cement, aggregate, or admixture. The source identity matters when assessing whether existing test evidence is relevant.
- Proportion the complete cementitious system. Consider Portland cement together with the proposed SCM contribution rather than using a Portland-cement-only ratio that hides the SCM’s role. EPA’s guidance discusses treating Portland cement plus fly ash or slag as the total cementitious component. This is guidance for considering the system, not a universal mix recipe or a direction to use any particular amount.
- Trial the proposed combination. Evaluate concrete made from the actual combination of materials against the project specification. ASTM C1709-22 describes preconstruction testing as a common way to verify SCM performance in specific mixtures. For alternatives not covered by an established specification, the guide’s evaluation approach supports a more comprehensive assessment, particularly where there is little performance history.
- Review both fresh and hardened performance. Check the fresh-concrete and placement properties required by the project, then assess the specified hardened properties and the relevant test ages. Include setting and early strength when schedule or construction sequencing depends on them; include durability and exposure-specific criteria when required. Consider heat where it is a project constraint. Do not infer a passing result for a property that was not evaluated.
- Re-evaluate changes before production. If the cement, SCM, aggregate, or admixture changes, determine whether the previous qualification remains applicable. ACI specifically cautions that fly ash performance depends on other mixture constituents and advises trial batches before commercial production when components change. A change in source can therefore be a mix-design issue, not merely a purchasing substitution.
- Release against documented criteria. Keep the approved materials, proportions, test results, acceptance criteria, and any required approvals together. Confirm that the released mix satisfies the governing documents and project performance requirements. If results fall short, revise and test the mixture rather than assuming a higher or lower replacement level will solve the problem.
Which performance requirements should the mix address?
Set the criteria before selecting the SCM. A mix that meets one strength target may still fail a placement, setting, durability, or exposure-related requirement that matters to the project.
- Workability and placement: Identify the fresh-concrete behavior and placement needs required by the job. Assess them in the proposed mixture, since SCMs can alter fresh properties.
- Setting and strength development: State the required strength and test age, and consider early strength and setting where they affect construction. ACI cautions that maintaining some Portland cement may be needed to meet certain early-strength and setting obligations in fly-ash mixtures. That is not a universal minimum; the actual requirement depends on the project and the trial results.
- Durability and exposure: Translate the exposure class and durability obligations into the applicable project acceptance criteria. For fly-ash mixtures, ACI notes that some freeze-thaw and deicer-scaling obligations may mean some Portland cement must be maintained. Do not assume an SCM automatically satisfies an exposure requirement.
- Heat, where relevant: If heat is a project constraint, make it an explicit comparison criterion and evaluate the proposed mixture accordingly. The available data does not provide a universal heat result for any one SCM family.
- Cost and supply: Compare the cost of an approved, deliverable mixture—not just the unit price of a binder component. Supply and source changes matter because the qualification is tied to the actual mix and materials.
The point of a performance-based specification is to state what the concrete must do and how that will be accepted. It should not inadvertently rule out a qualified cementitious system through language that measures only Portland cement or treats water limits without considering the whole system. EPA warns that minimum-cement or maximum-water provisions can discriminate against fly ash or slag when they fail to account appropriately for the total cementitious system. Its procurement guidance on cement and concrete specifications recommends reviewing performance standards and justifying restrictions on a job-by-job basis.
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What should be compared before choosing an SCM?
Compare candidates against the same project criteria, using evidence for the proposed materials and mixture. A useful decision record includes:
- Specification fit: Does the material meet the relevant standard, and does the project allow its use?
- Fresh performance: Do the trial results meet placement and workability requirements?
- Strength and setting: Does the mix meet the specified strength at the required age and any early-strength or setting needs?
- Durability and exposure: Is the evidence relevant to the project’s exposure obligations, including any freeze-thaw or deicer-scaling criteria that apply?
- Heat behavior: If heat matters, is the option supported by project-relevant evaluation?
- Cost and availability: Can the qualified materials be supplied for the work at an acceptable project cost?
- Evidence depth: Is the candidate covered by a recognized standard and supported by relevant mixture results, or does it need more comprehensive evaluation?
This method avoids declaring a generic winner. Fly ash, slag cement, silica fume, pozzolans, ground-glass pozzolan, and alternative SCMs have different standards and different bodies of project-specific evidence. Choose the option that meets the specified requirements with adequate qualification and supply—not the one associated with a broad claim about replacement or sustainability.
How should project specifications treat cement replacement?
Specifications should make room for qualified cement replacement where project performance and price objectives permit, while retaining the acceptance criteria necessary for the job. That means defining the required performance and applicable standards clearly, rather than relying on a blanket rule that may exclude a suitable SCM system without a technical reason.
EPA’s guidance specifically warns that cement minimums or maximum-water provisions may inadvertently restrict fly ash or slag if they do not treat the total cementitious system appropriately. It recommends reviewing those requirements and justifying a restriction on a job-by-job basis. This does not mean every material or mixture must be accepted: project specifications and exposure obligations still govern. It means limits should be evaluated in context, with the cement-plus-SCM system and documented performance in view.
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For designers and owners, useful specification language identifies permitted material standards, the performance criteria, the test and acceptance requirements, and the process for approving a proposed mixture. For producers and contractors, the practical step is to submit the source information and supporting mixture results required by the contract before production. Where the documents are silent or conflicting, resolve that issue with the responsible project authority rather than assuming that a material standard overrides the concrete specification.
What can the published sustainability figures tell you?
Use sustainability figures only for the material and context they describe. The National Ready Mixed Concrete Association’s 2021 guide says Type IL Portland limestone cement can reduce embodied carbon by approximately 10% for projects with a sustainability goal. That is an approximate figure for Type IL cement, not an estimate for every SCM, replacement rate, or concrete mix. The guide does not establish the embodied-carbon reduction for an unspecified project’s SCM mixture.
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The PCA guide also cites a 2000 reference stating that SCM use was present in at least 60% of ready-mixed concrete. That is a historical figure attributed by the guide, not a current market statistic. Neither figure replaces project-specific mixture qualification or a project-specific environmental assessment.
Independent reader supportYour contribution helps us test, update, and keep practical guides available for everyone.What are the common mix-design mistakes?
- Choosing a percentage before defining the application. A number without the exposure, strength age, project specification, and material source is not a complete recommendation. Define those conditions first.
- Treating material compliance as mix approval. Meeting an SCM standard does not prove that a particular concrete mixture meets its requirements. Test the actual proposed combination.
- Ignoring interactions with other constituents. Changing cement, aggregate, admixture, or SCM source can change the relevance of prior results. Reassess and trial-batch when components change, particularly in fly-ash mixtures.
- Using a Portland-cement-only lens. A minimum cement or water rule may fail to account for the complete cementitious system. Review how the contract defines and evaluates the mixture, and seek clarification where necessary.
- Checking only one strength age or property. A mixture may need to satisfy placement, setting, early strength, durability, and exposure-specific obligations as well as the specified strength. Identify the required criteria and test evidence before approval.
- Assuming all SCMs have the same evidence base. Standards and performance records vary. An alternative material without a significant performance record may require comprehensive evaluation under the ASTM C1709-22 framework.
- Carrying a recipe across jurisdictions or editions. The cited standards are U.S. references, and adopted requirements can differ or change. Verify the project’s governing documents and current editions rather than importing a dosage or approval from elsewhere.
Which standards and guides are relevant?
For U.S. work, the available data identifies ASTM C618 for fly ash and natural pozzolans, C989 for slag cement, C1240 for silica fume, C1866 for ground-glass pozzolan, and C1697 for blended SCMs. Check the current edition and whether the project has adopted it. ASTM C1697-25a describes blended SCMs; ASTM C1709-22 is a guide for evaluating alternative SCMs, including approaches for materials outside existing specifications. These are references for qualification, not substitutes for the project’s concrete mixture acceptance criteria.
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The ACI Cementitious Materials for Concrete education bulletin addresses SCM use and the dependence of fly ash performance on mixture constituents. The PCA’s Design and Control of Concrete Mixtures guide provides broader concrete and SCM context. The NRMCA Guide to Improving Specifications for Ready Mixed Concrete includes the cited Type IL sustainability figure and specification context. Always confirm current editions and project adoption before relying on any standard or guide.
Frequently Asked Questions
Does an ASTM-compliant SCM automatically qualify for my concrete mix?
No. Material compliance establishes that the material meets its applicable specification; it does not establish that the concrete made with it will meet your project requirements. The project must still evaluate the proposed mixture against its acceptance criteria.
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A total cementitious-system view can prevent the SCM contribution from being ignored, but it is not a complete proportioning method by itself. The approved mixture still needs to account for project requirements and be supported by performance testing.
Can I use the same SCM mix design in another state or country?
Not automatically. The cited standards are U.S. references, and another jurisdiction or contract may adopt different editions, material requirements, exposure rules, or acceptance criteria. Confirm those governing documents and re-establish that the mixture evidence applies.
Do sustainability claims for one cement or SCM apply to my project’s concrete?
Not without project-specific context. Embodied-carbon results depend on what material and mixture the claim covers; for example, the cited approximate 10% figure concerns Type IL cement, not every SCM mixture.
