Fine recycled concrete aggregates sourced from construction waste and treated with a crystallizing agent can successfully replace standard sand in mortar, improving compressive strength while maintaining workability, according to a new study published in Materials.
Study: Modified Fine Recycled Concrete Aggregates with a Crystallizing Agent as Standard Sand Replacement in Mortar. Image Credit: Achira22/Shutterstock.com
The research tested four different mortar series with sand replacement levels ranging from 25 % to 100 %. The recycled sand was derived from processed concrete waste and treated with a crystallizing waterproofing agent to enhance performance. Despite long-standing concerns about the mechanical limitations of recycled aggregates, the study found that in several cases, mortars incorporating recycled sand outperformed those made with traditional standardized sand (SS), particularly in compressive strength.
Why Recycled Aggregates Typically Fall Short
Recycled aggregates (RAs) are often seen as environmentally favorable alternatives to natural aggregates, but they tend to underperform mechanically. This is largely due to their physical and chemical limitations: high porosity, elevated water absorption, microcracks in the interfacial transition zones (ITZ), and inconsistent quality due to contamination or residual cement paste.
To overcome these challenges, researchers have explored a range of improvement strategies. These fall into two categories:
- “Improve by removing” techniques, such as chemical or thermal treatment, aim to eliminate old mortar residue.
- “Improve by adding” approaches use admixtures, carbonation, self-healing compounds, or coating and permeation agents to strengthen or seal the aggregate structure.
Among the latter, crystallizing agents, commonly used for waterproofing concrete, have shown promise for enhancing RA properties by penetrating the pore structure and promoting internal densification.
Experimental Setup: Matching Particle Size, Varying Replacement Levels
In the study, recycled sand (RS) was produced from fine RAs that were modified to match the particle size distribution of standard sand. Four types of sand (grouped as CON) were prepared for the tests, each within a specified size range, and used to create mortar mixes with 25 %, 50 %, 75 %, and 100 % RS content.
To assess the impact of the crystallizing treatment, RS samples were further divided into three categories:
- OPC-X: The crystallizing agent Admixplus (AD) was added directly during mortar mixing.
- OPC-Y: RA was pre-soaked in AD for 15 days.
- OPC-Z: RA was pre-soaked in AD for 45 days.
The materials were subjected to multiple analyses:
- Pycnometry for density
- Thermogravimetric analysis (up to 1000 °C) for thermal behavior
- FE-SEM to study crystallization effects and microstructure
- X-ray diffraction (XRD) and X-ray fluorescence (XRF) to evaluate structure and chemical composition
Each mortar series was tested using three cylindrical specimens (50 mm × 95 mm) and three cubic specimens (100 mm). The study evaluated mechanical performance via three-point bending and compressive strength tests, as well as durability through rapid chloride permeability and water penetration resistance.
Results: Strength Gains Without Extra Admixture—But Durability Still a Challenge
The mechanical results were clear: replacing standard sand with RS improved compressive strength, particularly at 25 % and 50 % replacement levels. These mortars achieved up to 15 % higher compressive strength than control samples made entirely with SS, without requiring more superplasticizer than standard limits.
However, mortars made with RS treated using the crystallizing agent required increased superplasticizer dosages to maintain the same workability. This higher demand reduced practical applicability, especially for large-scale mixes.
Importantly, the crystallizing agent did not improve compressive strength. However, it had a selective impact on flexural strength:
- OPC-X and OPC-Y showed improved 7-day flexural strength.
- OPC-Z, which underwent longer pre-soaking in AD, displayed delayed gains, with higher flexural strength observed at 28 days.
Despite these benefits, durability results were disappointing. No significant improvements were observed in water penetration resistance or chloride ion permeability. In some cases, performance worsened. For instance, OPC-Z samples, despite containing standard sand, showed deeper water penetration when AD was used, likely due to a weakened interface between the crystallizing agent and the cement paste.
Conclusion
This study provides compelling evidence that fine recycled aggregates can enhance the mechanical performance of mortar and be used at up to 100 % replacement levels when particle size is properly controlled. Even without chemical treatment, RS improved compressive strength significantly, offering a viable alternative to standard sand in structural and non-structural applications.
However, the use of crystallizing agents like Admixplus remains limited. While it can improve early or late flexural strength depending on application method, it does not enhance compressive strength or durability, and its interaction with superplasticizers may complicate mix design.
Still, the broader implications are promising. By leveraging properly processed recycled aggregates, the construction industry could significantly reduce its dependence on natural sand, a non-renewable resource under increasing environmental pressure.
Journal Reference
Suarez-Riera, D., et al. (2025). Modified Fine Recycled Concrete Aggregates with a Crystallizing Agent as Standard Sand Replacement in Mortar. Materials, 18(17), 4208. DOI: 10.3390/ma18174208. https://www.mdpi.com/1996-1944/18/17/4208
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