By Nidhi DhullReviewed by Susha Cheriyedath, M.Sc.Jun 20 2024Concrete is a widely used building material, with aggregates comprising about 75 % of its total volume, significantly contributing to environmental degradation. Additionally, a substantial amount of construction waste (CW) is generated during construction activities.1 According to the World Green Building Council, the building sector is responsible for over 50 % of global material extraction and 35 % of landfill waste.2
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Thus, incorporating recycled CW in concrete production can promote sustainability and a circular economy by effectively managing and repurposing substantial volumes of CW. Moreover, substituting natural aggregates with recycled CW in concrete can reduce project expenses by 10-20 % and potentially enhance concrete performance.1 This article explores the recycling of CW and its application in concrete production.
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The Recycling Process for Construction Waste
Buildings of different types and materials across various regions generate a diverse range of CW. This waste is transformed into smaller, reusable fractions through recycling, either using mobile machinery or at fixed plants.
Mobile recycling machinery processes CW directly on-site, allowing the quick use of recycled materials without the need for additional transportation. However, the quality of these aggregates often falls short, containing various types of partially processed waste materials, which frequently end up in landfills.
The mobile recycling process involves three main steps: initial pre-sorting to separate unprocessable or contaminated waste, with usable materials directed to a feed port; crushing and magnetic separation to reduce aggregate size and remove ferrous materials; and finally, a thorough screening to classify aggregates by size for different applications.
In contrast, fixed recycling plants offer more robust processing capabilities, resulting in higher-quality aggregates due to extensive sorting, crushing, and separation processes. These aggregates have lower impurity levels, better soundness, and more closely meet particle size requirements.
The operation of a fixed recycling plant follows four detailed steps:
- Preliminary treatment includes feed port processing, soil removal, crushing, magnetic separation, and the removal of small and ferrous elements.
- Aggregate size reduction: Manual sorting to eliminate large chunks of metal, wood, plastic, or paper.
- Secondary crushing is used to reduce the aggregate size further and extract the remaining ferrous materials.
- Sorting of recycled aggregates by size for various applications.1
Properties of Recycled Construction Waste
The multiple processes of crushing, sorting, and eliminating contamination are critical in determining the properties of recycled aggregates. These aggregates, produced through various crushing stages, often differ significantly in shape (with fewer needle-like flakes) compared to natural aggregates.
Recycled aggregates typically have a rough and irregular surface with sharp corners and may contain numerous internal microcracks due to mechanical crushing processes involving compression and collision. Additionally, these microcracks can also originate from alkali-aggregate reactions in the original waste before dismantlement.
Due to their porous structure, the density of recycled materials is lower than that of natural aggregates. The density of recycled aggregates also depends on factors such as the strength, composition, age, and usage environment of the original concrete. The complex internal structure of recycled aggregates, which includes pores, adhered mortar, interface transition zones, and cracks, leads to high water absorption.1
The Use of Recycled Construction Waste in Concrete
Recycled CW is increasingly recognized as a viable alternative to natural materials in concrete production. Its use not only reduces the environmental impacts associated with CW and natural resource extraction but also fosters the development of innovative materials.
Designing concrete mixes requires the careful optimization of parameters like water-cement ratio, binder content, and aggregate particle size distribution. This optimization enhances concrete properties such as workability, strength, durability, and sustainability.
However, the workability of fresh concrete decreases with higher substitution rates of recycled aggregates due to their high water absorption and lower density. This challenge can be addressed with the use of superplasticizers or through meticulous mix design to maintain the desired workability.2
The strength properties of concrete containing recycled aggregates, both compressive and flexural, show a nonlinear relationship with substitution ratios. This issue can be mitigated by incorporating fibers to enhance durability, which is essential for the broader adoption of sustainable concrete solutions.1
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Challenges and Solutions
Recycled aggregates typically exhibit lower specific gravity, increased porosity, higher water absorption, and poorer interfacial transition zones compared to conventional aggregates, primarily due to their origins from crushed concrete blocks. Despite multiple processing stages, the original mortar often remains adhered to the aggregate surfaces, affecting their water absorption and density. To economically reduce the impact of this residual mortar, innovative methods are required.2
A recent study in Construction and Building Materials introduced a heat treatment method to improve the quality of recycled aggregates. This treatment not only removed old mortar but also enhanced the physical and mechanical properties of the aggregates. The study suggested using a blend of heat-treated recycled aggregates and heat-activated recycled cement to produce high-quality, fully recycled concrete.3
Other modification techniques, such as mild acetic acid treatments, nano-silica sprays, bio-cement treatments, carbonation, dissociation agent additions, and silica fume treatments, can further enhance the properties of recycled materials. Physical methods like rubbing, grinding, milling, and bio-treatments are also effective in removing adhered mortar.1
Conclusion and Future Prospects
The use of recycled construction waste in concrete is not just a technical innovation—it is a step toward a more sustainable and economically sensible construction industry. Research has shown that when CW is skillfully mixed with specific additives, it matches the strength of traditional materials while cutting down costs significantly.4
Yet, the road ahead is not without its bumps. The varied and complex nature of CW often complicates its integration into new concrete, affecting the final product's performance.2 To smooth out these challenges, there is a pressing need for universally accepted standards. Establishing these standards would bring much-needed uniformity and reliability to recycled materials, encouraging more widespread use and acceptance in the construction sector.
Embracing these changes could dramatically shrink the industry's environmental impact and foster innovation, making sustainable construction not just an option but a norm for the future.
References and Further Reading
1. Luo, H., Aguiar, J., Wan, X., Wang, Y., Cunha, S., & Jia, Z. (2024). Application of Aggregates from Construction and Demolition Wastes in Concrete: Review. Sustainability, 16(10), 4277. https://doi.org/10.3390/su16104277
2. Joseph, H. S., Pachiappan, T., Avudaiappan, S., Maureira-Carsalade, N., Roco-Videla, Á., Guindos, P., & Parra, P. F. (2023). A Comprehensive Review on Recycling of Construction Demolition Waste in Concrete. Sustainability, 15(6), 4932. https://doi.org/10.3390/su15064932
3. Wu, H., Liang, C., Zhang, Z., Yao, P., Wang, C., & Ma, Z. (2023). Utilizing heat treatment for making low-quality recycled aggregate into enhanced recycled aggregate, recycled cement and their fully recycled concrete. Construction and Building Materials, 394, 132126. https://doi.org/10.1016/j.conbuildmat.2023.132126
4. Yadav, N., & Kumar, R. (2024). Performance and Economic Analysis of the Utilization of Construction and Demolition Waste as Recycled Concrete Aggregates. International Journal of Engineering, 37(3), 460-467. https://doi.org/10.5829/ije.2024.37.03c.02
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