Preventing cracks in cement-free concrete starts with qualifying the complete alkali-activated mix, then controlling shrinkage, curing, restraint, and aggregate reactivity during construction. “Cement-free” is not one recipe: precursor chemistry, alkaline activator, aggregate, admixtures, fibers, curing, thickness, weather, and exposure all change cracking risk. Use a supplier- or engineer-qualified formulation and performance testing rather than transferring a Portland-cement or unrelated geopolymer recipe to the job.
What cement-free concrete is—and why it cracks
In construction literature, cement-free concrete commonly refers to alkali-activated concrete (AAC), including geopolymer concrete. Instead of Portland clinker, the binder is made by reacting a reactive aluminosilicate precursor—such as a slag-, fly-ash-, or other industrial-mineral source—with an alkaline activator. The exact precursor blend, activator chemistry, concentration or modulus, water content, aggregate grading, admixtures, and curing regime determine how the paste reacts and deforms.
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That variation is why there is no universal anti-cracking dosage or curing schedule. A 2020 review examined 184 published studies and assembled 1,756 AAC mixes from 111 of those studies. The database demonstrates how broad the formulation range is; it is not a set of independently validated field recipes.
| Cracking pathway | What drives it | Primary prevention focus |
|---|---|---|
| Drying and autogenous shrinkage | Moisture loss, reaction-related pore changes, paste volume, restraint, and the particular precursor/activator system | Qualify proportions, manage water and admixtures, optimize curing, and detail or stage work to reduce restraint |
| Early thermal or setting movement | Heat and reaction rate, temperature gradients, rapid surface drying, and timing of finishing or form removal | Follow the qualified setting and curing procedure and protect the surface from abrupt environmental change |
| Tensile cracking under restraint or load | Concrete tensile capacity is exceeded by shrinkage, imposed movement, or structural stress | Use structural detailing, joints, reinforcement and, where qualified, fibers to improve crack distribution and post-crack behavior |
| Alkali-aggregate reaction (AAR) | Reactive aggregate, sufficient alkali, and moisture; ASR and ACR are distinct reactions | Evaluate aggregate reactivity and apply the project’s prescriptive or performance-based mitigation |
Shrinkage can produce uneven deformation and harmful cracks in AAC, but the severity depends on the material system and exposure conditions. A crack diagnosis should therefore identify the mechanism before selecting an additive or fiber.
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Qualify the actual project mix before placing concrete
Record the variables that control cracking before approving a batch. The word “cement-free” alone does not identify a workable or durable formulation.
- Binder: identify every precursor, its source and composition, and the proportions in the blend.
- Activator: document the chemical type, concentration or modulus, liquid-to-binder ratio, solids, and batch temperature.
- Aggregate: record grading, absorption, moisture condition, petrographic information, and reactivity test results.
- Admixtures and fibers: verify chemistry, dosage basis, sequence of addition, and compatibility with the activator.
- Construction conditions: define member thickness, reinforcement and restraint, placement rate, finishing window, formwork, ambient temperature, humidity, wind, and expected moisture exposure.
- Performance targets: set workability retention, setting time, strength, shrinkage, permeability or durability, and crack-width criteria appropriate to the use.
Make trial batches with the materials intended for the job. Measure fresh behavior and hardened deformation under representative curing and restraint, not just compressive strength. A formulation that works with one precursor or activator can set, shrink, or lose workability differently when a source or concentration changes.
Reduce shrinkage through mix design
Control activator concentration and modulus
Activator concentration or modulus changes reaction rate, gel chemistry, pore structure, water demand, and deformation. The 2022 review of shrinkage-reduction methods identifies reducing activator modulus or concentration as one reported approach. That does not mean the lowest possible concentration is best: lowering it can alter strength development, setting, and durability. Select the level through trial batches that meet the project’s performance requirements.
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Source-material composition is a major variable in AAC. Two materials sold under the same broad precursor name may differ in reactive glass, calcium content, fineness, and contaminants. Those differences affect both early reaction and long-term shrinkage. Use a consistent, characterized supply, establish acceptance limits for incoming material, and requalify the mix when the source or blend changes materially.
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Use admixtures only after compatibility testing
Mineral admixtures, chemical additives, shrinkage-reducing admixtures, expansive agents, superabsorbent polymers, and nanoparticles have all been investigated. Their effects depend on activator chemistry, pore structure, water availability, dosage, and curing. An admixture that reduces measured shrinkage in one laboratory formulation can increase setting problems or reduce workability in another. Confirm fresh properties, strength, durability, and dimensional stability together.
| Approach reported in the 2022 review | Review-synthesized result | How to use the evidence |
|---|---|---|
| Nanoparticles | About 30% lower shrinkage with 3% nanoparticles in particular studies | Study-specific observation; do not treat 3% as a universal dosage or product guarantee |
| Expansive agents | About 45% lower shrinkage in reported formulations | Requires compatibility and restraint evaluation; expansion can create other stresses if misapplied |
| Shrinkage-reducing admixtures | About 55% lower shrinkage in reported formulations | Verify compatibility with the alkaline solution, setting, strength, and durability |
| Superabsorbent polymers | About 70% lower shrinkage with 0.3% in a reported study | Study-specific result; account for water release, pores, workability, and strength |
These percentages are review-reported outcomes from selected studies, not predictions for an unknown job mix. They should guide a test program, not replace one.
Consider porous aggregates and internal moisture
Porous aggregates can provide internal moisture and are included among reported shrinkage-mitigation approaches. Their absorption, strength, grading, and moisture conditioning must be controlled. A saturated porous aggregate may change the effective water balance and workability; a dry one may draw liquid from the paste. Establish aggregate moisture corrections and verify the resulting strength and deformation.
Make curing a mix-specific control, not an afterthought
Curing regime is one of the critical AAC variables identified in the mix-design literature, and curing optimization is a reported way to reduce shrinkage. There is no universally valid temperature, humidity, or duration for an unspecified cement-free concrete. A schedule suitable for one precursor and activator may produce different reaction products or cracking in another.
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- Use the qualified curing procedure. State the required temperature, humidity, duration, enclosure, and sequence in the method statement supplied by the mix designer or engineer.
- Protect moisture immediately. Prevent wind, direct sun, heated surfaces, and premature drying during the period when the surface is vulnerable.
- Control transitions. Avoid abrupt changes from sealed or heated curing to cool, dry air. Gradual equalization limits differential movement.
- Monitor the member, not only the room. Record concrete temperature and surface condition at representative locations, especially in thick sections and exposed edges.
- Coordinate form removal and finishing. Removing forms, applying water, or opening a surface to air at the wrong time can create a moisture gradient and restraint.
Document curing deviations. If the concrete experienced unexpected heat, drying, or delay, obtain engineering advice before accepting the member or applying a repair.
Use fibers for tensile-crack control only when the mix supports them
Fibers can improve tensile and flexural characteristics and may distribute cracks more finely. They do not eliminate shrinkage, prevent AAR, or guarantee a crack-free surface. The 2026 RILEM Technical Committee 294-MPA chapter describes fiber use for tensile cracking in AAC as an ongoing research topic.
The chapter identifies steel, synthetic, carbon, and organic fibers and discusses type, orientation, dosage, and geometry. It also cautions that fibers can compromise mixing, setting time, and workability. A fiber dosage selected for ordinary concrete may not disperse or wet properly in an alkaline-activated matrix.
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- Confirm fiber chemical resistance and compatibility with the activator and any admixtures.
- Define whether fibers are intended for plastic-shrinkage control, residual tensile capacity, impact resistance, or structural design; these are different objectives.
- Validate feeding and dispersion at production scale. Clumping, balling, or blocked pumps can create weak zones.
- Measure slump or flow, air, setting time, finishability, and hardened residual strength after adding fibers.
- Have the supplier or project engineer approve type, geometry, and dosage for the qualified binder system.
Screen aggregates for alkali-aggregate reaction
Shrinkage control does not address alkali-aggregate reaction. ASTM C1778 covers the risk of deleterious AAR, including alkali-silica reaction (ASR) and alkali-carbonate reaction (ACR). Its guidance states: “In this guide, the level of prevention required is a function of the reactivity of the aggregate, the nature of the exposure conditions (especially availability of moisture), the criticality of the structure, and the availability of alkali in the concrete.”
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Evaluate the actual aggregate and exposure for the project. Use the standardized tests and acceptance approach required by the governing specification. For ASR-reactive aggregate, mitigation may be prescriptive or performance-based. For ACR, the guide identifies avoidance of the reactive aggregate as the preventive measure. ASTM’s page identifies C1778-25 as active; verify the edition adopted by the project jurisdiction.
- Obtain petrographic and reactivity information for each aggregate source.
- Consider whether the structure will remain wet, undergo wetting and drying, or be exposed to deicing salts or other moisture sources.
- Increase the level of verification for critical structural members, restrained elements, and repairs that are difficult to replace.
- Do not assume that a low-shrinkage admixture or fiber package mitigates AAR.
Control placement, restraint, and early-age exposure
A sound mix can still crack if the member is restrained while it contracts. Coordinate reinforcement, joints, penetrations, embeds, and connections with the expected movement. Provide movement joints and crack-control reinforcement where the design requires them; do not cut joints arbitrarily after cracking has begun.
- Plan the pour sequence so that long, thin panels are not locked to older concrete before they have developed sufficient capacity.
- Keep substrates and forms in the moisture and temperature condition required by the qualified procedure.
- Do not add unapproved water or activator at the truck, mixer, or pump to recover workability.
- Finish only within the tested window. Reworking bleed water or closing a rapidly drying surface can trap weak material or hide plastic cracking.
- Protect corners, edges, and penetrations, where drying and restraint are concentrated.
- Keep records of batch time, placement time, ambient conditions, curing start, and any interruption.
Diagnose cracks before choosing a repair
Map cracks by time of appearance, width, depth, orientation, location, and relation to joints or reinforcement. Photograph a scale, mark changes, and record moisture and temperature. A crack that appears minutes after finishing has a different likely cause from one that develops weeks later.
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| Observation | Questions to answer | Immediate action |
|---|---|---|
| Many short, shallow cracks soon after finishing | Was the surface exposed to wind, sun, heat, or rapid humidity loss? Was finishing delayed? | Protect remaining work, preserve records, and obtain an early-age crack assessment |
| Long cracks at restraints, openings, or re-entrant corners | Was movement restrained? Are joints and reinforcement detailed for the expected strain? | Have the engineer assess structural and serviceability implications before patching |
| Map or random cracking that grows with moisture exposure | Are aggregates potentially reactive? Is the member repeatedly wet? | Begin AAR evaluation; shrinkage-only repairs will not remove the reaction |
| Cracking accompanied by poor flow, delayed set, or fiber clumps | Did an admixture, fiber, or activator change from the qualified batch? | Quarantine the affected batch and compare production records with the approved mix |
Do not rout, seal, inject, or overlay until the cause and movement are understood. A cosmetic repair can conceal an active mechanism and make later diagnosis harder.
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Codes, testing, and acceptance
Requirements depend on jurisdiction, structural use, exposure, and whether the concrete is precast, cast in place, or a repair material. A 2023 review identified codes and constructability as barriers to wider geopolymer implementation and called for flexible, performance-based specifications and testing. ACI’s resources include ACI PRC-242-22, Alkali-Activated Cements—Report, along with other alternative-cement practice material. These references do not automatically govern every project.
Before production, identify the adopted building code, concrete specification, testing standards, and authority having jurisdiction. Agree on acceptance criteria for fresh properties, setting, strength, shrinkage or expansion, durability, and crack width. For structural work, the qualified engineer should approve the mix, curing method, reinforcement or fiber design, test plan, and disposition of nonconforming results.
A practical crack-prevention workflow
- Classify the binder. Confirm that the material is AAC or geopolymer and list all precursors and activator constituents.
- Define exposure and restraint. Record moisture, temperature cycling, member thickness, joints, reinforcement, and structural criticality.
- Qualify materials. Characterize precursor consistency, activator properties, aggregate absorption and reactivity, and admixture or fiber compatibility.
- Run representative trials. Test workability retention, setting, strength, dimensional change, and curing response using production materials and realistic geometry.
- Select mitigation. Adjust proportions, activator, admixtures, internal moisture, fibers, joints, or reinforcement only when the trial data support the change.
- Write the construction method. Include batching sequence, no-water-addition rule, placement limits, finishing window, curing, monitoring, and contingency actions.
- Verify every pour. Check batch tickets, fresh properties, temperatures, curing start, and weather; retain specimens and records.
- Investigate deviations. Map cracks and compare them with timing, restraint, curing, and material records before approving repairs.
This process treats cracking prevention as a system-control problem rather than a search for one miracle additive. It also makes the limits of available evidence explicit: published percentages and laboratory outcomes cannot predict a result for a mix whose chemistry, geometry, and exposure have not been tested.
Frequently Asked Questions
Does geopolymer concrete always shrink?
No. Shrinkage varies with precursor, activator, proportions, pore structure, curing, and exposure. Some AAC systems show substantial shrinkage, while others behave differently; the project mix must be measured under representative conditions.
Can fibers stop cracks in alkali-activated concrete?
Fibers can improve tensile and flexural behavior and distribute some cracks, but they do not remove shrinkage or prevent aggregate-reaction cracking. They also can reduce workability or change setting, so type and dosage require mix-specific qualification.
What curing temperature is correct for cement-free concrete?
There is no single temperature, humidity, or duration that applies to every AAC formulation. Use the curing regime qualified for the actual precursor and activator system and verify it on the member.
How is alkali-aggregate reaction different from shrinkage cracking?
Shrinkage is a contraction and restraint problem; AAR is an expansion-related reaction involving susceptible aggregate, alkali, and moisture. Aggregate testing and AAR mitigation are required even when shrinkage has been controlled.
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