Concrete is the second most used material after water. In the current global scenario with a strong focus on sustainability, recycling resources are essential to protect the environment. Recycling concrete leads to a reduction in the utilization of natural resources and transformation costs and minimizes the level of waste in landfills. This article will discuss how concrete recycling works, including major challenges and opportunities.
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The Basic Steps of Concrete Recycling
Concrete recycling is possible through mechanical breakdown and passing the concrete through screens that divide it into fine and coarse parts. This is followed by magnetic separation to remove steel and flotation to remove other unwanted things. As a result of the recycling process, concrete can be utilized to create hardcore sub-base layers for new structures, gravel for paths or driveways, or even aggregate for more concrete.
It can be difficult to recycle concrete with numerous contaminants. Recycling concrete has environmental and financial benefits. Furthermore, it is estimated that recycling one ton of concrete saves about 6,182 liters of water and reduces about 900 kg of CO2 emissions.1
Methods of Concrete Recycling
Two innovative technologies for fully recycling end-of-life (EOL) concrete are advanced dry recovery (ADR) and heating air and classification systems (HAS).
ADR is a mechanical system that extracts fine particles from moist, crushed concrete. It uses kinetic energy to break the water bonds with these particles. The system has two main parts: the rotor and the air sifter. The rotor breaks the water bonds, separating the particles into different sizes and compositions.
ADR processes the crushed concrete waste (0–12 mm) into three products: coarse (4–12 mm), fine (1–4 mm), and another fine product (<1 mm). The air sifter separates the lighter contaminants, like wood and plastics, from the coarse material.
The HAS system works alongside ADR. It processes the fine materials (1–4 mm and 0–1 mm) from ADR. HAS uses hot gas to dry the fine particles and remove unwanted contaminants like wood and plastic. This process takes place in a fluidized reactor, where air heats and sorts the fine particles by size.
The hot air also helps activate the ultrafine particles, mainly made of hydrated cement. The contact between hot air and the particles is maximized by tubes within the vertical structure of the HAS setup.2
Major Challenges: Concrete Recycling Faces Major Setbacks
Concrete recycling is hindered by a lack of understanding of its technical aspects and standards.
Current project guidelines and procedures limit the concept to designers and builders, leaving cases that deviate from the norm without clear standards. Unique methods may be needed to maintain structural integrity due to the rapid dismantling of structural components.
During demolition, contractors often choose the quickest and easiest method without methodically separating structural components. Abandoned buildings are usually entirely demolished if a new project is planned. Time constraints often justify this quick approach.
Private and academic research in construction has been minimal. Despite the availability of resources and the global distribution of materials, the public is largely unaware of the potential for reuse and the value of existing materials. Industry experts say that most people do not realize that materials can be reused in their original state.
Reusing components is challenging due to a lack of standards and approvals. Many governments are cautious about recycled components or even ban them for structural or energy-efficiency reasons. Several legislations, programs, and goals address minimizing construction and demolition waste. A comprehensive document that includes all relevant environmental construction policies would benefit individuals.3
A Novel Eco-Sustainable Recycling Method of Cement Kiln Dust (CKD)
Cement kiln dust (CKD) is a major environmental issue in cement manufacturing. CKD is a fine powder produced at high temperatures, collected in dust collectors, and then usually sent to landfills. However, CKD has useful chemical and physical properties that make it valuable for various industrial uses.
Researchers have developed eco-friendly solutions by combining CKD with copper tailings in cemented paste backfill mixtures. This approach transforms waste into valuable materials used in mining applications, providing proven benefits.4
The recent study focusing on eco-sustainable methods uses three types of CKD materials: YCKD, ACKD, and UCKD. Researchers prepared and tested 60 different mixtures. They found that CKD could replace up to 15% of cement in the cemented paste backfill (CPB) mixture. This substitution increased the density of the samples compared to the pristine samples and improved long-term strength in binary and ternary mixtures with silica sand. However, when copper tailings were used in ternary mixtures, the lime saturation factor (LSF) of CKD greatly influenced the maximum strength achieved.
Recycling CKD and copper tailings in the mining industry offers many benefits. It can reduce the cost of mine backfill operations, decrease the amount of CKD and copper tailings sent to landfills, conserve natural resources used in the cement industry, save energy, and promote sustainability. The positive results from using CKD and copper tailings in CPB applications suggest that further research is warranted to explore additional parameters.
What is the Cement Sustainability Initiative (CSI)?
The Cement Sustainability Initiative (CSI) promotes concrete recycling as part of sustainable business practices. Concrete's unique properties mean that its recovery often does not fit neatly into standard definitions of reuse or recycling. Typically, concrete is broken down into aggregate for new uses.5
The CSI has highlighted that maximizing concrete recycling depends on how building codes and green rating schemes recognize it. Generally, there are few legal restrictions on using recycled concrete as aggregate in projects like filling, sub-base, asphalt, and outdoor landscaping. However, there are often limits on how much can be used in structural concrete. These limits are usually due to public misconceptions about the quality of recycled concrete or a lack of awareness about its potential uses. Green building schemes and rating systems can help change these perceptions by addressing concrete recycling.
The CSI aims for "zero landfill" of concrete. However, cement producers play an indirect role in supporting this goal. The CSI believes increasing awareness about concrete recycling will promote discussions and encourage all stakeholders to recycle concrete. Cement producers can contribute through their concrete, aggregate, and construction subsidiaries.
Electric Recycling Process for Cement: A Major Breakthrough
Researchers have developed a method to produce very low-emission concrete at scale, which could significantly aid the transition to net zero. This innovative method uses electrically powered arc furnaces, typically employed in steel recycling, to recycle cement.6
The research indicates that recovered cement paste can be reprocessed as a partial substitute for the lime-dolomite flux currently used in steel recycling. The resulting slag meets the specifications for Portland clinker and can be blended with calcined clay and limestone. The novel process and its parameters affect the quantity of the silica content present in the cement paste obtained after the recycling process, and by altering the parameters of the reaction, silica and alumina content can be readily adjusted. The proposed method is potentially economically competitive and, if powered by emissions-free electricity, can produce zero-emissions cement while reducing the emissions from steel recycling by lowering lime flux requirements.7
In short, many developments are being observed in recycling cement, and experts are devising novel ways to incorporate recycled concrete into sustainable construction. With each passing day, concrete recycling is becoming decarbonized and contributing positively to emission-free sustainability objectives.
References and Further Reading
1. Bentley, P. (2022). Can we recycle concrete? (Online). BBC Science Focus. Available at: https://www.sciencefocus.com/science/can-we-recycle-concrete
[Accessed on 24 May, 2024].
2. Gebremariam, A. et. al. (2020). Innovative technologies for recycling End-of-Life concrete waste in the built environment. Resources, Conservation and Recycling, 163, 104911. Available at: https://doi.org/10.1016/j.resconrec.2020.104911
3. Badraddin, A. et. al. (2021). Main Challenges to Concrete Recycling in Practice. Sustainability. 13(19):11077. Available at: https://doi.org/10.3390/su131911077
4. Al-Bakri, A. et. al. (2023). Eco-Sustainable Recycling of Cement Kiln Dust (CKD) and Copper Tailings (CT) in the Cemented Paste Backfill. Sustainability. 15(4). 3229. Available at: https://doi.org/10.3390/su15043229
5. The Cement Sustainability Initiative (2023). Department of Economic and Social Affairs Sustainable Development. (Online). Available at: https://sdgs.un.org/partnerships/cement-sustainability-initiative
[Accessed on May 28, 2024].
6. University of Cambridge (2024). Cement recycling method could help solve one of the world's biggest climate challenges. (Online). Available at: https://www.sciencedaily.com/releases/2024/05/240522130434.htm
[Accessed on May 29, 2024].
7. Dunant, C. et al. (2024). Electric recycling of Portland cement at scale. Nature 629, 1055–1061. Available at: https://doi.org/10.1038/s41586-024-07338-8
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