Yes, carbonated concrete waste can form a cement-free concrete product, but only through a controlled carbonation-curing process—not by replacing cement with ordinary recycled aggregate. The clearest published demonstration used 0–4 mm recycled concrete fines, blended them with 25% stainless-steel slag, compacted the mix, and cured the molded blocks with carbon dioxide. The laboratory hollow blocks reached 15.4 MPa compressive strength without hydraulic cement.
A second, more established route uses carbonated recycled aggregate or powder in a conventional cement mix. That approach can reduce virgin aggregate demand, lower absorption, and sometimes permit substantial aggregate replacement, but it remains a low-cement or conventional-concrete route. Treat the fully cement-free method as a product-development process that requires feedstock testing, controlled gas curing, and qualification against the standard for its intended use.
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What “cement-free” means in this application
Concrete normally gains its strength from hydration of Portland cement or another hydraulic binder. A cement-free carbonated product uses a different mechanism: calcium-bearing particles react with carbon dioxide in the presence of moisture and form solid carbonate phases that bind the compacted particles together.
That distinction matters because “carbonated concrete waste” can describe several different materials:
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- Coarse recycled aggregate: crushed concrete particles used in place of natural stone.
- Recycled concrete fines: a 0–4 mm fraction containing crushed aggregate, adhered mortar, and cement paste.
- Recycled concrete powder: a finer, more reactive fraction used as a partial binder or mineral addition.
- Carbonated material: any of those fractions after controlled exposure to CO2, which changes the calcium-bearing phases and pore structure.
Only the fines-based route has the directly demonstrated zero-cement formulation described here. Carbonating coarse aggregate alone does not create a cement-free concrete; it improves the aggregate and may allow replacement in a mix that still contains cement.
The demonstrated cement-free formulation
Feedstock and blend
The 2024 laboratory demonstration by Jef Bergmans and colleagues used recycled concrete fines between 0 and 4 mm. The formulation also contained 25% stainless-steel slag as a calcium-bearing co-material. The compacted mixture was then carbonation-cured rather than hydrated with a hydraulic binder.
The 25% slag proportion belongs to that tested formulation. It is not a universal recipe for every demolition stream. Stainless-steel slag chemistry, free lime content, particle size, aging history, and leachable constituents vary by source. A different slag or a different recycled-concrete feedstock can require a different proportion—or may be unsuitable altogether.
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The reported product was a hollow block made in the laboratory. Its compressive strength reached 15.4 MPa. That number describes the tested blocks and curing method; it is not a blanket structural-concrete rating and should not be transferred to walls, foundations, pavements, or load-bearing units without product-specific qualification.
The result nevertheless establishes an important engineering point: a compacted blend of concrete-derived fines and a calcium-bearing industrial co-material can gain useful strength through mineral carbonation without Portland cement.
How mineral carbonation creates the binder
Concrete waste contains residual cement paste and hydrated calcium phases. When carbon dioxide reaches those phases under suitable moisture conditions, calcium can be converted into calcium carbonate. The newly formed carbonate fills pores, densifies particle surfaces, and creates contact points between compacted grains.
Carbonation is not simply “drying with CO2.” The reaction needs enough water to dissolve and transport ions, but excessive water blocks gas pathways and can leave the interior under-reacted. Particle grading controls both packing and the distance gas must travel. Compaction controls density and contact area, while the chamber controls how evenly carbon dioxide reaches the product.
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Step-by-step process for a cement-free carbonated mix
1. Define the intended product
Choose the product before selecting proportions. A hollow block, solid masonry unit, paver, panel, or non-structural fill element will have different strength, absorption, dimensional-stability, and durability requirements. Identify the applicable local product or structural standard and its test methods first. The target determines compaction pressure, geometry, curing schedule, and acceptance criteria.
2. Select and sort the concrete waste
Use separately processed concrete waste rather than unsorted mixed demolition debris. Remove wood, gypsum, plastics, soil, insulation, metals, and other contaminants. The published demonstrations concern concrete-derived fractions, not a generic mixed waste pile.
Record the source and processing history. Adhered mortar content, original concrete composition, carbonation history, and contamination all affect calcium availability, water demand, gas transport, and leaching behavior.
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3. Crush and grade to the required fraction
For the directly demonstrated route, produce a 0–4 mm recycled-concrete-fines fraction. Measure the grading curve rather than relying on a crusher setting alone. A change in the amount of material below a few hundred micrometres can alter surface area, moisture demand, packing density, and carbonation rate.
Keep coarse particles, fines, and powder separate during processing. Blending them later allows you to study whether a denser skeleton improves compaction or instead restricts gas access. Do not assume that a grading used in one plant will behave identically at another plant.
4. Characterize the calcium-bearing co-material
If following the published cement-free concept, evaluate stainless-steel slag as the co-material. Determine its chemical composition, free lime or other reactive calcium phases, particle-size distribution, moisture, expansion potential, and leachable constituents. Age or pre-treat the slag if the relevant standard or supplier specification requires it.
Start with the documented 25% proportion only as a laboratory reference point. Prepare neighboring trial proportions so that strength, absorption, expansion, and leaching can be compared rather than assuming the reference blend is optimal.
5. Set and verify moisture
Precondition the fines to a known moisture content before mixing. A 2026 recycled-fines study selected approximately 20 wt% initial moisture as favorable for surface area and reactivity. The wettest condition in that work absorbed the most CO2, yet its reactivity was lower, illustrating why maximum gas uptake is not the same as maximum binding performance.
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Measure moisture on representative samples, account for absorption by old mortar, and mix long enough to distribute water uniformly. If the outside of a molded piece is damp while its core is dry, carbonation will be uneven even when the batch-average moisture appears correct.
6. Blend without hydraulic cement
Combine the graded recycled fines and calcium-bearing co-material uniformly. Avoid adding Portland cement, slag cement, or another hydraulic binder if the objective is a genuinely cement-free formulation. A small amount of unreported binder would change the mechanism and invalidate comparisons with the zero-cement demonstration.
Document batch mass, moisture corrections, mixing time, and the order in which dry solids and water are introduced. These records are essential when a strength result cannot be reproduced.
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7. Compact the fresh mixture
Form the intended block or molded product before carbonation curing. Control mold filling, vibration or pressing, density, and dimensions. Under-compacted zones contain more voids and can remain weak; over-compaction can close gas pathways and slow reaction in the interior.
Measure the green density of each trial batch. Density is a practical check that the compaction process, moisture, and particle grading have remained consistent.
8. Carbonation-cure in a controlled chamber
Place the compacted products in a chamber that can regulate carbon dioxide concentration, gas flow, humidity, temperature, exposure time, and exhaust. The cited cement-free work used atmospheric pressure and a low carbon dioxide concentration of about 10%. It also immobilized carbon dioxide in the product rather than treating the gas exposure as a simple surface treatment.
Arrange pieces so gas can reach all exposed faces. Record inlet and outlet conditions, chamber concentration, residence time, and product mass before and after curing. A chamber reading alone does not prove that the center of a dense block reached the same reaction conditions as its surface.
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After carbonation, condition specimens according to the intended product standard and test them at the specified age. At minimum, measure compressive strength and water absorption. Add dimensional stability, freeze–thaw or wet–dry cycling, chloride or other transport indicators, abrasion, and fire performance when the intended application requires them.
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Test leachable contaminants from both the recycled fines and the slag. A strong block is not acceptable if it fails environmental release limits or changes volume after installation.
Carbon dioxide curing: variables that control the result
| Variable | Why it matters | What to control |
|---|---|---|
| Moisture | Water enables ion transport, while excess water blocks gas diffusion. | Initial moisture, uniform distribution, and moisture loss during handling. |
| Particle size | Fine particles provide reactive surface; excessive fines increase water demand and can restrict gas movement. | Full grading curve, powder content, and adhered-mortar fraction. |
| CO2 concentration | Controls the driving force and reaction rate. | Inlet concentration, chamber uniformity, and concentration over time; the cited block work used about 10% at atmospheric pressure. |
| Gas flow and exposure time | Determine whether the product core receives enough carbon dioxide. | Flow rate, loading pattern, residence time, and outlet concentration. |
| Temperature and humidity | Influence reaction kinetics, drying, and gas transport. | Stable chamber conditions and product temperature, not only room readings. |
| Compaction | Sets strength and pore connectivity. | Pressing or vibration energy, green density, mold fill, and uniformity. |
| Product geometry | Thick sections create longer diffusion paths. | Maximum thickness, exposed area, and orientation in the chamber. |
There is no single curing schedule that can be copied safely from the published block experiment to every product. A thicker unit, tighter grading, different slag, or different moisture level changes the path carbon dioxide must travel.
What the published performance numbers actually show
| Result | Qualification | Engineering meaning |
|---|---|---|
| 15.4 MPa compressive strength | Laboratory hollow blocks made by the 2024 cement-free fines-and-slag route. | Shows that carbonation can provide useful binding without hydraulic cement in a molded unit; not a universal structural rating. |
| 25% stainless-steel slag | Proportion in the tested cement-free formulation. | A starting point for trials, not a general replacement rule. |
| 3.0% water absorption | Lowest reported absorption for recycled aggregate in the cited 2024 work. | Indicates that carbonation can densify calcium-bearing surfaces and reduce uptake; it does not guarantee that value for another feedstock. |
| 50% carbonated recycled-aggregate replacement | One study reported no significant compressive-strength reduction at this replacement level. | Supports a substantial aggregate-replacement route in conventional concrete, not a cement-free mix. |
| Approximately 20 wt% initial moisture | Selected as favorable for reactivity in a 2026 recycled-fines study. | Useful as a trial condition to verify, not a universal optimum. |
| 13.4% higher 28-day strength | Selected carbonated fines compared with a limestone reference in the 2026 study. | Shows that carbonated fines can outperform a mineral reference in a particular formulation. |
| Up to about 33% lower CO2 emissions, 28% lower carbon intensity, and 25% lower production cost | Values reported by Chen and colleagues in 2026 for their carbonated recycled-concrete configuration. | Potential environmental and economic benefit, dependent on the study’s feedstock, energy, transport, and curing assumptions. |
These results should be read as evidence of feasibility and potential, not as guaranteed performance ranges. Carbonation efficiency, electricity source, aggregate transport, chamber utilization, and waste-processing energy can change the life-cycle result.
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Carbonated aggregate in conventional concrete: the more established option
If a project does not require zero cement, carbonated recycled aggregate is generally the lower-risk development path. Carbonation can densify calcium-bearing phases and reduce the absorption of recycled aggregate. The cited work reported absorption as low as 3.0% and a case in which replacing 50% of aggregate with carbonated recycled aggregate caused no significant compressive-strength reduction.
In this route, cement still hydrates and supplies the primary binder. The carbonated aggregate functions as a quality-improved recycled aggregate. Mix water, admixture dosage, paste volume, and grading must still be adjusted for the actual absorption and surface texture of the treated aggregate.
Carbonated recycled powder is another intermediate option. Reviews identify recycled powder as a partial cement-replacement material. That lowers cement content but is not cement-free unless carbonation curing supplies the binding mechanism and no hydraulic binder is used.
Independent reader supportYour contribution helps us test, update, and keep practical guides available for everyone.Choosing between the three practical routes
| Route | Binder mechanism | Typical development difficulty | Best use of current evidence |
|---|---|---|---|
| Carbonated aggregate in ordinary concrete | Hydration of cement plus improved recycled aggregate. | Lower than a zero-cement product, but still requires concrete mix and durability testing. | Projects seeking recycled aggregate, lower absorption, and reduced virgin stone use. |
| Carbonated powder replacing part of cement | Hydraulic cement remains the main binder; powder contributes as a mineral addition. | Moderate; compatibility, water demand, and strength development must be established. | Low-cement designs where existing concrete production equipment is retained. |
| Carbonation-cured fines and calcium-bearing slag | Mineral carbonation of compacted calcium-bearing particles. | Highest; product geometry, chamber curing, gas transport, leaching, and standards all require qualification. | Molded blocks or other controlled products designed specifically for cement-free manufacture. |
Quality-control plan before scale-up
Feedstock acceptance
- Verify that the load is concrete-derived and document demolition source.
- Screen for wood, gypsum, soil, plastics, metals, and other contaminants.
- Measure grading, moisture, density, adhered mortar, and chemical composition.
- Check expansion-related phases and leachable constituents in the slag.
Process checks
- Confirm batch moisture after correction, not just the water added at the mixer.
- Record green density and mold dimensions for every trial series.
- Map chamber carbon dioxide concentration and humidity at several locations.
- Compare inlet and outlet gas conditions to identify under-cured loads.
- Track product mass, dimensions, and visible cracking before and after curing.
Performance checks
- Compressive strength at the age and geometry required by the applicable standard.
- Water absorption and accessible porosity.
- Dimensional change, expansion, and shrinkage.
- Freeze–thaw, wet–dry, abrasion, chloride, or other transport tests required by exposure.
- Leaching tests for metals and other regulated constituents.
- Fire and thermal tests where the product will be used in a building assembly.
Do not approve a formulation from one cube or one block. Use replicate specimens and include intentionally varied moisture, density, and curing conditions so the process window—not only the best result—is known.
Common failure modes and corrections
Strong surface, weak core
This usually indicates limited carbon dioxide penetration, excessive density, a wet outer layer, or insufficient exposure time. Check gas flow through the loaded chamber, reduce product thickness or loading density for trials, and verify core strength separately from surface samples.
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High CO2 uptake but low strength
Excess water can increase measured uptake while reducing reaction efficiency and gas transport. Recheck moisture distribution and compare a drier trial near the approximately 20 wt% condition identified as favorable in the 2026 fines study.
Variable strength between batches
Look first for changes in demolition source, adhered mortar, grading, moisture, green density, or slag chemistry. A crusher output that looks visually identical can still contain a different fine-particle fraction and therefore require a new moisture and curing setting.
Cracking or dimensional instability
Investigate reactive slag phases, uneven drying, over-compaction, and thermal or moisture gradients. Test the slag and the complete blend for expansion before increasing production volume.
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Carbonation can produce a strong laboratory specimen that still has excessive connected porosity or poor resistance to environmental cycling. Add the exposure-specific tests required for the intended location instead of using compressive strength as the sole acceptance criterion.
Environmental and cost interpretation
Mineral carbonation can permanently bind carbon dioxide in a solid product while diverting concrete waste from disposal. Bergmans and colleagues described it as a route to increase concrete-waste recycling in added-value products and support circular-economy principles.
Chen and colleagues reported, for their 2026 carbonated recycled-concrete configuration, up to approximately 33% lower carbon dioxide emissions, 28% lower carbon intensity, and 25% lower production cost. Those are configuration-specific results. A plant should recalculate them using its own haul distances, crushing energy, slag supply, chamber electricity, carbon-dioxide source, curing throughput, rejected-product rate, and end-of-life assumptions.
Carbon dioxide is not automatically a free or low-impact input. Compression, purification, transport, chamber operation, and exhaust treatment can offset part of the benefit. The credible comparison is the complete product system against the conventional product it replaces.
Regulatory and construction-use limits
Published laboratory proportions are not construction specifications. Building codes and product standards may assume a hydraulic binder, prescribe constituent limits, or lack a classification for carbonation-bound products. Confirm whether the proposed unit can be certified under an existing standard or needs an engineering assessment and a new product-qualification route.
For structural use, require independent review of load capacity, durability, fire behavior, reinforcement compatibility, and long-term dimensional stability. For non-structural use, environmental release limits and surface durability may still control acceptance. Keep the zero-cement claim precise: it should mean no hydraulic cement in the tested formulation, not merely a lower cement content.
A practical decision framework
- Need a cement-free product? Start a dedicated fines-and-calcium-bearing-co-material program and plan for chamber curing and product certification.
- Need recycled content with existing ready-mix equipment? Evaluate carbonated aggregate first, because the concrete retains a conventional cement binder.
- Have a large powder stream but no carbonation chamber? Study partial cement replacement, recognizing that this is a low-cement route rather than a cement-free one.
- Cannot control feedstock contamination or moisture? Do not scale the process; improve sorting and conditioning before comparing strength results.
- Cannot demonstrate uniform curing through the product thickness? Redesign geometry, loading, compaction, or gas delivery before approving the formulation.
The central engineering choice is therefore not simply how much carbonated waste to add. It is whether the project can control the calcium-bearing feedstock, moisture, particle packing, compaction, carbon-dioxide delivery, and qualification testing tightly enough for the intended product.
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