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Use cement-replacement materials as supplementary cementitious materials (SCMs), not as an unmeasured one-for-one powder swap. Combine the selected SCM with Portland cement as total cementitious material, set the water-to-cementitious-material ratio, and validate the blend with trial batches made from the actual local materials. Check workability, air, temperature, setting, strength at the specified ages, curing, and exposure durability before placing production concrete.
What cement-replacement materials do in concrete
SCMs react with water, calcium hydroxide, or both to contribute to the hardened cement paste. They can reduce permeability, control alkali-silica reaction (ASR), moderate heat, use recovered or naturally occurring materials, and lower the Portland-cement portion of a mixture. They do not all react at the same rate or produce the same fresh-concrete behavior.
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The useful design quantity is total cementitious material: Portland cement plus the measured mass of every SCM. A replacement percentage is normally calculated against that total, not against Portland cement alone. For example, a 400 kg/m³ total cementitious content with 25% fly ash contains 100 kg/m³ fly ash and 300 kg/m³ Portland cement. The water-to-cementitious-material ratio (w/cm) is the mixing water divided by the 400 kg/m³ total, not divided only by the 300 kg/m³ cement.
The U.S. Environmental Protection Agency states that cement ratios may be retained when they reflect the cementitious characteristics that coal fly ash or ground-granulated blast-furnace (GGBF) slag contribute to the mixture. This approach keeps strength and durability calculations tied to the material that actually hardens in the concrete.
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Choose the SCM for the performance requirement
Fly ash: Class F and Class C
Class F fly ash is generally lower in calcium and often gains strength more slowly. Federal Highway Administration guidance describes strength development that can continue for 60 to 90 days. Class C fly ash commonly reacts faster, although the actual result depends on its chemistry, fineness, cement, aggregates, temperature, curing, and admixtures.
Typical starting ranges are 15–25% of total cementitious material for Class F and 20–35% for Class C in structural or pavement mixtures. FHWA also reports overall fly-ash use commonly in the 15–30% range and a fly-ash-to-cement substitution ratio of about 1:1 to 1.5:1 by weight, depending on the ash and the performance target. These are trial ranges, not universal limits.
Fly ash can reduce permeability and heat of hydration, but some ashes reduce the effectiveness of air-entraining admixtures. Air content must therefore be measured in the fresh concrete rather than inferred from the admixture dose.
GGBF slag
GGBF slag is often used at higher percentages than fly ash. FHWA reports that Grade 120 slag replacing up to 50% of cement can generally produce equivalent or higher compressive strength from seven days onward when the mixture and curing are suitable. Slag can lower permeability and improve sulfate resistance, and it is frequently selected for ASR-control or other durability objectives.
Slag mixtures may have slower early strength, slower setting, or greater sensitivity to cool weather and inadequate curing. The trade-off can be favorable when the specification allows later-age strength or when reduced permeability is more important than rapid form stripping.
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Silica fume
Silica fume is an extremely fine, highly pozzolanic SCM covered by ASTM C1240. Its small particles can greatly increase water demand and make the mixture more sensitive to high-range water-reducer dosage. Dispersion is essential: clumps can contribute to poor workability and may contribute to ASR-related problems if the product is not properly incorporated.
Silica fume is normally used at a project-specific, relatively low percentage rather than by applying a general replacement number. Establish the dosage with the supplier’s product data, ASTM C1240 compliance, and concrete trials. Do not assume that an unlabeled “silica” powder is equivalent; FHWA cautions that other amorphous-silica products require performance verification in concrete.
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Natural pozzolans and calcined clay or shale can contribute to the cementitious system when they meet the applicable specification and demonstrate performance with the proposed cement, aggregates, water, and admixtures. Their color, fineness, water demand, setting behavior, and strength development can vary substantially by source, so a generic percentage is not reliable.
Typical starting ranges and what they imply
| SCM | Common starting proportion | Likely benefits | Design cautions |
|---|---|---|---|
| Class F fly ash | 15–25% of total cementitious material; FHWA reports 15–30% as a common overall fly-ash range | Lower permeability, lower heat, later strength gain, ASR mitigation when chemistry and dosage are suitable | Slower early strength; may require longer curing; can reduce air-entraining-admixture effectiveness |
| Class C fly ash | 20–35% of total cementitious material | Often faster reaction than Class F, reduced permeability and heat compared with a cement-only mixture | Reaction and ASR-control performance depend on calcium, alkali, aggregate, and project testing |
| Grade 120 GGBF slag | 25–50% is commonly evaluated for durability or ASR objectives; FHWA reports up to 50% can generally achieve equivalent or higher strength from seven days onward under suitable conditions | Lower permeability, sulfate resistance, ASR control, reduced heat | Early strength and setting can be slower; curing is especially important |
| Silica fume | Project-specific low-percentage dosage governed by ASTM C1240 and trials | Very low permeability and strong pozzolanic reaction; useful where transport of water or aggressive ions must be restricted | High water demand, high-range-water-reducer sensitivity, dispersion and finishing challenges |
| Natural or calcined pozzolan | Not stated as a universal percentage; determine by specification and trials | Can reduce cement demand and permeability when locally suitable | Source-to-source variability; qualification with project materials is required |
Set requirements before calculating proportions
Write the performance requirements before choosing a replacement level. Include:
- Required compressive or flexural strength and the age at which it must be demonstrated.
- Exposure conditions, including freezing and thawing, sulfate, chlorides, deicing salts, wetting and drying, and possible reactive aggregate.
- Slump, slump flow, pumping distance, placement method, finishing needs, and allowable temperature.
- Required entrained-air range where freeze-thaw exposure applies.
- Limits on heat of hydration, setting time, bleeding, plastic-shrinkage risk, permeability, or electrical resistivity.
- Available cement, SCM source and grade, aggregates, water, and chemical admixtures.
- Construction schedule, including stripping, loading, saw-cutting, and opening-to-traffic ages.
Do not select an SCM solely because it has the highest replacement percentage or the lowest purchase price. The suitable blend is the lowest-cost mixture that satisfies every specified fresh, hardened, durability, and scheduling requirement.
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Proportion a cement-SCM mixture step by step
- Define the governing strength and durability criteria. Establish the design strength, test age, exposure class, air range, workability, temperature, and any permeability, sulfate, or ASR test required by the project.
- Identify the failure mechanism you need to control. For reactive aggregate, evaluate total alkali, cement alkali, SCM alkali, aggregate reactivity, and SCM chemistry. Silica fume can be effective at a lower percentage, while slag or higher-calcium fly ash may require a higher replacement level. No SCM dosage controls ASR independently of the aggregate and alkali system.
- Choose a total cementitious content. Start from the strength, durability, paste-volume, and workability requirements. Divide that total between Portland cement and SCM; do not reduce the total binder merely because a percentage of it is replaced.
- Calculate the replacement. For a 400 kg/m³ total cementitious content at 30% Class F fly ash, use 120 kg/m³ fly ash and 280 kg/m³ Portland cement. If the specification expresses a substitution ratio instead, document the ratio and the resulting total binder so that the w/cm calculation remains unambiguous.
- Set the water-to-cementitious-material ratio. Calculate w/cm using all Portland cement and SCM. Do not add water at the truck or mixer to recover slump without recalculating the ratio; extra water can reduce strength and durability. Adjust aggregate grading, paste volume, or chemical-admixture dosage instead.
- Proportion aggregates and admixtures. Use the actual aggregate moisture, absorption, grading, and shape. Account for the higher water demand of silica fume and for any change in air-entraining-admixture response caused by fly ash. High-range water reducers are often needed with silica fume.
- Make several trial mixtures. For fly ash investigations, FHWA commonly references trials in 5-percentage-point increments. For example, compare 15%, 20%, 25%, and 30% rather than assuming one replacement level will work. Keep the target total cementitious content and w/cm controlled while changing the SCM proportion, unless the test plan deliberately studies those variables.
- Test fresh and hardened concrete. Measure slump or flow, air, temperature, setting, bleeding, finishability, early strength, and later strength. Add project-specific permeability, sulfate, or ASR testing when those exposures govern.
- Select and document the production mixture. Record material sources, batch quantities, moisture corrections, admixture sequence, mixing time, test results, curing conditions, acceptance ages, and allowable field adjustments.
How replacement changes water, strength, and setting
Water and workability
A lower Portland-cement percentage does not automatically mean a lower water demand. Particle shape, fineness, carbon content, and surface area vary among fly ashes. Silica fume has an especially high surface area and often requires a high-range water reducer. Maintain the specified w/cm while obtaining slump through aggregate optimization and admixtures rather than uncontrolled water.
Early and later strength
Class F fly ash and slag commonly develop strength more slowly at early ages, particularly in cool weather or with poor curing. A mixture that misses a seven-day stripping requirement may still meet a 28-day or later requirement, but that schedule must be agreed before placement. Class F concrete may continue gaining strength for 60–90 days. Class C fly ash often reacts faster, and FHWA reports suitable Grade 120 slag mixtures can produce equivalent or higher compressive strength from seven days onward; neither statement replaces testing with the actual materials.
Heat and durability
Fly ash and slag generally reduce permeability and heat compared with a comparable cement-only mixture. Silica fume can produce very low permeability but demands careful water-reducer control and dispersion. For sulfate or ASR exposure, evaluate the complete cementitious system and aggregate rather than relying on an SCM label alone.
Trial-batch testing and acceptance
Use the proposed construction materials in the mix design. FHWA’s guidance references ACI 211.1 trial proportioning and 28-day moist-cured strength testing. A useful trial program includes a cement-only reference and several SCM levels, with at least duplicate batches when variability is a concern.
Record each batch’s cement and SCM masses, aggregate moisture corrections, water, admixture dosages, mixing sequence, ambient and concrete temperature, and elapsed time to testing. Test:
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- Slump or slump flow immediately after mixing and at the expected delivery interval.
- Entrained-air content and unit weight, especially when fly ash is present.
- Concrete temperature, initial and final setting, bleeding, segregation, and finishability.
- Compressive or flexural strength at the specified acceptance age plus earlier and later diagnostic ages.
- Permeability, electrical resistivity, sulfate expansion, or ASR-related tests where exposure requires them.
Compare results against the specification, not just against the cement-only reference. A mixture with lower early strength can be acceptable when its later-age strength, durability, and construction schedule all comply.
Independent reader supportYour contribution helps us test, update, and keep practical guides available for everyone.Curing SCM concrete correctly
Moist-cure test specimens and the placed concrete for the duration required by the project specification. Fly-ash mix design commonly uses strength after 28 days of moist curing, and Class F mixtures may continue gaining strength for 60–90 days. If the field concrete dries early, its performance will not match a laboratory cylinder cured under ideal conditions.
Slag concrete is particularly vulnerable to inadequate moist curing. FHWA notes that poor curing can worsen setting problems and plastic-shrinkage risk. Begin curing as soon as finishing permits, maintain continuous moisture or an approved curing system, and protect the surface from wind, sun, freezing, and rapid temperature changes. Do not shorten curing solely because the concrete contains an SCM; use measured strength and the governing specification.
Quality control from delivery through placement
Verify the incoming materials
Require certificates and current test data for every delivered SCM. EPA guidance identifies ASTM C618 for fly ash or natural pozzolans, ASTM C311 for sampling and testing, and ASTM C1240 for silica fume. Confirm the grade, source, lot identification, moisture condition, and storage requirements before unloading.
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- Keep Portland cement, fly ash, slag, and silica fume in separate silos or clearly identified containers.
- Calibrate scales, weigh hoppers, and automated dispensers; verify that the batch ticket shows each cementitious component.
- Correct aggregate batch water for current moisture and absorption before adding water.
- Follow the specified admixture sequence and mixing time, particularly for silica fume dispersion.
- Measure air at the point required by the specification and adjust admixture dosage only through the approved procedure.
Control field changes
Do not add water to a delivered load unless the approved quality plan permits it, the added quantity is measured, the resulting w/cm remains within the limit, and the load is remixed for the required time. If slump is low, first check elapsed time, concrete temperature, aggregate moisture, admixture calibration, and mixing energy. A field adjustment that fixes slump but violates w/cm can create a later strength or durability failure.
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Common problems and corrective actions
| Symptom | Likely SCM-related causes | Checks and corrective action |
|---|---|---|
| Low early strength | High Class F or slag replacement, cool concrete, insufficient curing, or excessive w/cm | Verify temperature, curing, batch water, and strength-age requirement; reduce replacement or adjust the schedule only after new trials |
| Slump loss or sticky concrete | Silica-fume surface area, incompatible admixture, excessive fines, or poor dispersion | Review mixing sequence and high-range-water-reducer compatibility; do not add unaccounted water |
| Air content below target | Fly ash consuming or adsorbing air-entraining admixture, changed carbon content, or dosage error | Run an admixture demand test with the delivered ash, recalibrate dosage, and recheck air after transport and placement |
| Delayed setting or difficult finishing | High slag or Class F content, low temperature, or inadequate curing | Measure setting time and temperature; protect and moist-cure the surface, and confirm that the construction schedule allows the selected blend |
| Plastic shrinkage or surface cracking | Rapid evaporation, delayed setting, poor curing, or finishing while bleed water is present | Control wind, sun, and temperature; start curing promptly; never finish bleed water into the surface |
| Unexpected ASR concern | Reactive aggregate, insufficient or unsuitable SCM dosage, or unverified silica product | Check aggregate reactivity and total alkali; use specified ASTM testing and a qualified SCM blend rather than assuming any silica-containing powder is equivalent |
Safety, storage, and handling
Follow the supplier’s safety data sheet for respirable-dust controls, protective equipment, ventilation, spill response, and loading. Fine SCM powders can become airborne during pneumatic transfer, silo filling, bag handling, or cleanup. Use the specified dust collection and respiratory protection procedures.
Keep materials dry and prevent cross-contamination. Label silos, conveying lines, bags, and batch compartments so that a cement or SCM cannot be loaded into the wrong storage location. Moisture, hardened lumps, or an incorrect grade can change both the batch quantity and the concrete performance.
How to decide whether an SCM blend is ready for production
Release a mixture only when the evidence answers all of these questions:
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- Does the measured strength meet the required age and acceptance criteria with the proposed materials?
- Does the fresh concrete maintain slump or flow, air, temperature, and setting limits through the expected delivery and placement time?
- Does the mixture meet the specified permeability, sulfate, ASR, freeze-thaw, or other durability criteria?
- Can the batch plant store, weigh, and load every cementitious component without confusion or contamination?
- Can the contractor place, finish, and cure the concrete within the setting and weather limits?
- Are certificates, sampling, testing, curing, and field-adjustment procedures written into the quality plan?
The practical rule is simple: count Portland cement and SCMs together as cementitious material, control w/cm, and prove the complete mixture with the actual materials and curing conditions. Replacement percentages are useful starting points, but performance testing determines whether a particular blend is suitable for a particular structure.
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