Concrete waste is everywhere. Old buildings are demolished, damaged concrete elements are removed, and thousands of test specimens are crushed after laboratory testing.
Much of this material can be processed into recycled concrete aggregate, or RCA, and used again in new concrete. However, there is one major problem: recycled aggregate normally carries a layer of old cement mortar on its surface.
That old mortar contains pores and small cracks. As a result, recycled aggregate usually absorbs more water, has lower density, and creates a weaker bond with the new cement paste. These weaknesses often reduce the strength, stiffness, and durability of recycled aggregate concrete.
A recent study investigated a practical way to improve RCA before using it in high-strength concrete. The researchers combined carbon dioxide curing with an alkaline slurry made from fly ash and calcium carbide residue. The results showed that treated recycled aggregate could perform much better than untreated material—and, in some areas, approach concrete made with natural stone.
Why Is Recycled Concrete Aggregate Usually Weaker?
Natural coarse aggregate, such as crushed limestone, is generally hard, dense, and relatively non-porous.
Recycled concrete aggregate is different. It consists of the original stone plus old cement mortar attached to its surface. This mortar may contain:
- Small cracks created during demolition and crushing
- Capillary pores that absorb water
- Weak areas between the old and new cement paste
- Irregular and rough surfaces
In the study, natural aggregate had a water absorption of only 0.61%, while the untreated recycled aggregate had an absorption of 5.23%. This difference shows how much more porous the recycled material was before treatment.
The problem is not that recycled aggregate cannot be reused. The problem is that its weak outer layer needs improvement.
The Idea: Repair the Aggregate Before Using It
Instead of accepting the poor surface quality of RCA, the researchers treated the aggregate before mixing it into concrete.
They studied three methods:
1. Carbon dioxide treatment
The recycled aggregate was placed in a chamber containing a controlled concentration of carbon dioxide.
The CO₂ reacts with calcium hydroxide in the old cement mortar and forms calcium carbonate. This new material fills part of the pores and makes the surface denser.
2. Alkaline slurry followed by CO₂ curing
In the second method, the aggregate was first soaked in a slurry containing ground calcium carbide residue. It was then treated with carbon dioxide.
The calcium carbide residue supplied additional calcium hydroxide, which supported further carbonation.
3. Fly ash and calcium carbide slurry followed by CO₂ curing
The most successful treatment combined:
- Ground calcium carbide residue
- High-calcium fly ash
- Water
- Carbon dioxide curing
The slurry had a high alkalinity, with a reported pH of approximately 13. This environment encouraged both carbonation and pozzolanic reactions around the recycled aggregate.
In simple terms, the treatment did three things:
- Carbonation filled pores with calcium carbonate.
- Pozzolanic reactions formed additional cement-like products.
- Fine fly ash particles coated and filled defects in the old mortar.
Why Was Seven Days the Best Treatment Period?
The researchers tested treatment periods of one, three, and seven days.
In general, the longer treatment gave better results. After seven days, the treated aggregates had their highest density, lowest water absorption, and best resistance to abrasion.
For the best-performing treatment, the results were:
- Specific gravity: increased to 2.55
- Water absorption: decreased to 3.85%
- Los Angeles abrasion loss: decreased to 24.10%
The untreated recycled aggregate originally had a water absorption of 5.23%. Therefore, the best treatment reduced absorption by approximately 26.4%.
Lower absorption matters because porous aggregate can draw water from the concrete mix. It can also increase shrinkage and allow moisture or harmful chemicals to move through the hardened concrete more easily.
The abrasion result is also important. A lower abrasion loss means the aggregate became more resistant to breaking and surface wear during handling and use.
Did the Treatment Improve Concrete Strength?
Yes.
The researchers produced high-strength concrete using:
- Natural coarse aggregate
- Untreated recycled aggregate
- CO₂-treated recycled aggregate
- Slurry-treated recycled aggregate
- Fly ash–slurry-treated recycled aggregate
Concrete made with untreated RCA had lower strength than concrete made with natural aggregate. At 28 days, the natural aggregate concrete reached 88.0 MPa, while the untreated recycled aggregate concrete reached 75.6 MPa.
At 90 days, the values were:
This reduction occurred because the old mortar and weak interface around the recycled particles created easier paths for cracking.
However, treatment significantly improved the results.
The concrete made with RCA treated using the combined fly ash, calcium carbide residue, and CO₂ method reached strengths close to the natural aggregate concrete at 28 and 90 days.
This is one of the most important findings of the study. It suggests that recycled aggregate does not always have to remain a lower-quality material. With proper treatment, it may become suitable for demanding concrete applications.
Strength Improved, but Stiffness Still Remained Lower
Strength and stiffness are related, but they are not the same.
Compressive strength measures how much load concrete can resist before failure. The modulus of elasticity describes how much the concrete deforms under load.
Although the treatment improved stiffness, the concrete made with treated RCA still had a lower modulus than concrete made with natural aggregate.
At similar compressive strengths of approximately 86–88 MPa:
- Natural aggregate concrete had a modulus of about 37.0–39.7 GPa
- Treated RCA concrete had a modulus of about 32.4–34.0 GPa
The reason is that the recycled particle still contains old cement mortar, which is less stiff than natural crushed limestone. Surface treatment can densify the material, but it does not completely change the internal nature of the aggregate.
This difference matters in structural design because lower stiffness can lead to greater deflection and time-dependent deformation.
The Treatment Reduced Concrete Shrinkage
Shrinkage occurs as concrete loses moisture and changes volume over time. Excessive shrinkage may contribute to cracking, especially when movement is restrained.
After 91 days, the measured total shrinkage was:
- Natural aggregate concrete: 454 microstrain
- Untreated RCA concrete: 684 microstrain
- CO₂-treated RCA concrete: 562 microstrain
- Calcium carbide slurry and CO₂ concrete: 517 microstrain
- Fly ash slurry and CO₂ concrete: 505 microstrain
Therefore, the best-performing treatment reduced shrinkage by 25.9% compared with the untreated recycled aggregate concrete.
The treatment reduced shrinkage because it made the old mortar denser and reduced its water absorption. With fewer open pores, less water could move through and leave the recycled aggregate during drying.
What About Long-Term Deformation?
Concrete continues to deform slowly when it carries a sustained load. Engineers call this behaviour creep.
Untreated recycled aggregate concrete showed the highest creep because the recycled particles were more porous and less stiff.
The measured specific creep values were:
- Natural aggregate concrete: 15.45 × 10⁻⁶/MPa
- Untreated RCA concrete: 28.10 × 10⁻⁶/MPa
- Best treated RCA concrete: 21.82 × 10⁻⁶/MPa
The treatment reduced specific creep by approximately 22.4% compared with untreated RCA concrete.
The treated material still did not fully match natural aggregate concrete. However, the improvement was clear and could be valuable where long-term deflection matters.
Did the Concrete Resist Chloride Penetration?
Chlorides are a major concern in reinforced concrete because they can reach the steel and initiate corrosion.
This is especially important in:
- Coastal buildings
- Bridges
- Marine structures
- Parking structures
- Roads exposed to de-icing salts
The study measured rapid chloride migration coefficients for three concrete types:
- Natural aggregate concrete: 2.07 × 10⁻¹² m²/s
- Best treated RCA concrete: 2.29 × 10⁻¹² m²/s
- Untreated RCA concrete: 2.71 × 10⁻¹² m²/s
A lower value indicates better resistance to chloride movement.
The treated recycled aggregate concrete did not quite equal the natural aggregate concrete, but it performed noticeably better than the untreated RCA mixture.
This improvement was mainly linked to the denser surface, reduced absorption, and improved bond around the treated particles.
What Did the Microscope Images Show?
The scanning electron microscope images helped explain the test results.
The untreated recycled aggregate had an uneven surface with visible pores and a weaker interfacial transition zone. This transition zone is the thin region where the aggregate meets the new cement paste.
After treatment, the interface appeared denser and more continuous. The researchers also reported evidence of calcium carbonate and additional cementitious gels forming around the treated particles.
This microscopic improvement explains why the treated material showed:
- Higher compressive strength
- Lower water absorption
- Better abrasion resistance
- Reduced shrinkage
- Lower creep
- Improved chloride resistance
The treatment did not simply coat the aggregate. It changed and densified the weak old mortar attached to its surface.
Why This Research Matters
Recycled aggregate offers a way to reduce construction waste and decrease demand for newly quarried stone.
However, engineers cannot select recycled materials based on environmental benefits alone. The concrete must still meet structural and durability requirements.
This study is valuable because it tested 100% recycled coarse aggregate in high-strength concrete, rather than using only a small replacement percentage in normal-strength concrete. The target strength was at least 55 MPa, with a low water-to-binder ratio of 0.27.
The results suggest that proper pre-treatment may allow recycled concrete aggregate to enter applications that were previously considered too demanding.
Possible future applications could include:
- Structural concrete
- Precast elements
- High-rise construction
- Industrial floors
- Pavements
- Infrastructure projects
However, engineers would still need to verify performance for each material source and project.
Does This Mean Treated RCA Can Replace Natural Aggregate Everywhere?
Not yet.
The results are promising, but this was a controlled laboratory study. The recycled aggregate came from failed concrete test cylinders with original strengths of about 30–40 MPa. Real demolition waste can be much more variable.
Recycled material from an actual building may contain:
- Brick
- Plaster
- Asphalt
- Wood
- Glass
- Soil
- Chloride-contaminated concrete
- Different original aggregate types
In addition, the study used controlled CO₂ curing conditions and carefully prepared slurry treatments. Applying the same process at an industrial scale would require further evaluation.
Before widespread use, further work should examine:
- Treatment cost
- Energy requirements
- CO₂ consumption
- Processing time
- Large-scale production
- Aggregate quality control
- Different sources of demolition waste
- Field performance
- Environmental impact of the full treatment process
Therefore, the study should be seen as evidence of technical potential rather than immediate approval for unrestricted site use.
The Main Takeaway
Recycled concrete aggregate is often weaker because old porous mortar remains attached to its surface. However, this study shows that the weak layer can be improved.
The best treatment combined high-calcium fly ash, ground calcium carbide residue, and carbon dioxide curing. After seven days, it:
- Reduced aggregate water absorption by about 26%
- Improved abrasion resistance
- Raised concrete strength close to natural aggregate concrete
- Reduced shrinkage by 25.9%
- Reduced specific creep by 22.4%
- Improved resistance to chloride migration
The treated concrete still had lower stiffness than natural aggregate concrete, and more work is needed before large-scale application. Nevertheless, the results show that recycled aggregate can become much more than crushed construction waste.
With the right treatment, it may become a reliable material for stronger, more durable, and more sustainable concrete.
Frequently Asked Questions
What is recycled concrete aggregate?
Recycled concrete aggregate is produced by crushing old or rejected concrete and processing it into particles that can replace natural stone in new concrete.
Why does recycled aggregate absorb more water?
Old cement mortar remains attached to the aggregate. This mortar contains pores and small cracks that absorb more water than most natural aggregates.
How does CO₂ treatment improve recycled aggregate?
Carbon dioxide reacts with calcium compounds in the old mortar and forms calcium carbonate. This fills some pores and creates a denser surface.
Can recycled aggregate be used in high-strength concrete?
The study showed that treated recycled aggregate could produce concrete exceeding 55 MPa and achieve strength close to natural aggregate concrete. However, performance depends on the aggregate source, treatment method, and concrete mix.
Does treated RCA perform exactly like natural aggregate?
Not completely. The treated concrete achieved similar compressive strength, but its modulus of elasticity remained lower because recycled particles still contain old cement mortar.
Is treated recycled aggregate suitable for structural concrete?
It may be suitable when testing confirms that strength, stiffness, shrinkage, creep, and durability meet the project requirements. Engineers should not assume that all recycled aggregate will perform in the same way.
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