Concrete Mix Design of Recycled Concrete Aggregate (RCA): Analysis of Review Papers, Characteristics, Research Trends, and Underexplored Topics

Abstract

Recycled concrete aggregate (RCA) has been widely adopted in construction and emerged as a sustainable alternative to conventional natural aggregates in the construction industry. However, the study of holistic perspectives in recent literature is lacking. This review paper aims to provide a comprehensive analysis of RCA, highlighting its properties, applications, and overall sustainability benefits to facilitate the comprehensive points of view of technology, ecology, and economics. This paper explores the manufacturing process of RCA, examines its mechanical and durability characteristics, and investigates its environmental impacts. Furthermore, it delves into the various applications of RCA, such as road construction materials, pavement bases, and concrete materials, considering their life cycle performance and economic considerations. This review reveals that there is a need for systemic data collection that could enable automated concrete mix design. The findings concerning various mix concrete designs suggest that increasing the 1% replacement level reduces the compressive strength by 0.1913% for coarse RCA and 0.2418% for fine RCA. The current critical research gaps are the durability of RCA concrete, feasibility analyses, and the implementation of treatment methods for RCA improvement. An effective life cycle assessment tool and digitalization technologies can be applied to enhance the circular economy, aligning with the United Nations’ sustainable development goals (UN-SDGs). The equivalent mortar volume method used to calculate the RCA concrete mix design, which can contain chemical additives, metakaolin, and fibers, needs further assessment.

1. Introduction

1.1. Background

The concept of a circular economy in the construction industry aims to minimize waste and maximize resource efficiency throughout the entire life cycle of a building [1]. It involves designing, constructing, and managing buildings in a way that promotes the reuse, recycling, and recovery of materials, reducing the reliance on raw materials and minimizing the environmental impact of the construction sector. The construction sector is already known as a major consumer of natural resources. Estimates suggest that the sector consumes about 50% of global steel production, 25% of global water resources, and 40% of global energy [2,3,4]. In a circular economy approach, buildings are seen as repositories of valuable resources that can be harvested and reused rather than being treated as disposable structures [5]. This means that instead of demolishing buildings and sending the materials to landfills, components and materials are carefully deconstructed and salvaged for reuse or recycling. The circular economy encourages the use of renewable and sustainable materials, promotes modular and flexible design, and emphasizes efficient resource management practices. The Ellen MacArthur Foundation [6] estimates that by adopting circular principles, the global construction industry could generate savings of $630 billion annually by 2025, and the European Commission estimated that the circular economy could create around 1.2 million additional jobs in the European Union’s construction sector by 2030 [7]. The Royal BAM Group [8] assessed the feasibility of their construction projects using circular economy principles and reported that a 96% recycling rate could potentially be achieved for their projects in 2020.

However, despite the potential benefits, the implementation of circular economy principles in the construction industry faces several challenges, including a lack of technical solutions [9]. One of the key obstacles is the fragmentation of the construction sector, where numerous stakeholders, including architects, engineers, contractors, and suppliers, operate independently and often with limited communication and collaboration. This lack of integration hampers the adoption of circular practices as it requires coordination among various actors. Previous studies [10,11] have shown that the current reuse and recycling rates in the construction industry are relatively low. In some countries, such as Iran, less than 10% of construction and demolition waste (CDW) is recycled or reused [12].

Another challenge lies in the technical complexity of deconstruction and material recovery processes [13,14,15]. Traditional construction methods and materials often make it difficult to disassemble and separate components for reuse or recycling. Many existing buildings were not designed with circularity in mind, and their structural systems and material compositions can pose challenges for efficient resource recovery. Additionally, there is a lack of standardized systems and protocols for material identification, sorting, and quality control [16]. For example, the European Union reports recycling rates ranging from 30% to 90% for CDW across member states, demonstrating inconsistencies in material handling and quality control [17,18]. Without clear guidelines and industry-wide standards, it becomes harder to ensure the quality and safety of recycled or reused materials. The absence of robust certification systems for recycled construction products further hampers market acceptance and consumer confidence. Furthermore, the economic viability of circular practices can be a barrier [7,19]. At present, the cost of implementing circular economy strategies such as deconstruction, sorting, and reprocessing can be higher than conventional construction methods. Demolition enterprises frequently salvage historical brandings that are deemed valuable and can be sold at a premium compared to items lacking such perceived worth [20]. The initial investments required for specialized equipment, skilled labor, and advanced technologies often deter stakeholders from embracing circularity. Ma et al. [21] also advocated that recycled concrete was priced at a premium of 0–10% in comparison to normal concrete. In Spain, the RCA cost varies between 3 and 6 € per ton (equivalent to US $3.65–7.30 per ton), whereas the NA cost ranges from 5 to 7.5 € per ton (equivalent to US $6.08–9.12 per ton) [22,23].

The global construction industry faces significant challenges in the form of resource scarcity and environmental concerns. Traditional construction practices heavily rely on the extraction of natural aggregates, such as gravel, limestone, and sand, which leads to the depletion of finite resources and disruption of ecosystems [24]. A recent study determined taxing strategies using natural aggregate (NA) in Sweden, Denmark, and the United Kingdom [25]. The implementation of such a strategy may entail negative consequences in terms of inadequate incentives and restricted policy legitimacy. Additionally, the disposal of CDW, particularly concrete waste, contributes to the growing problem of landfill congestion. According to a report by the World Economic Forum, CDW accounts for around 40% of the total solid waste generated worldwide [26], and RCA accounts for approximately 35% of all CDW generated in the United States [27]. In this context, the concept of recycled concrete aggregate (RCA) has gained considerable attention as a sustainable solution to address these challenges.

The production of RCA involves collecting and processing concrete waste generated from demolition sites or construction projects. The applicability of RCA is greatly influenced by the nature and composition of the source concrete. To avoid compromising the quality and function of the produced RCA, care must be taken to ensure that the source concrete does not include pollutants or excessive concentrations of dangerous elements. With the rapid economic growth of megacities and built environments, the per capita consumption of concrete materials is soaring day by day. Concrete is mainly composed of coarse aggregate (stone) and fine aggregate (sand). These two aggregates account for about 75% of the total concrete [28]. The applications of RCA to concrete are thus instrumental in enabling a pathway for circular economy. The utilization of RCA in construction projects offers a range of opportunities, such as road construction [29], pavement base [30], and concrete production [31], demonstrating its versatility and potential to replace NA. This systematic review paper aims to provide a comprehensive analysis of RCA from a state-of-the-art review and meta-analysis articles, focusing on their mix designs. It explores the production process of RCA, including the factors influencing its quality and characteristics. The mechanical and durability properties of RCA concrete are examined, shedding light on its performance compared to conventional concrete. The environmental impact of RCA production and use is evaluated, considering factors such as energy consumption, greenhouse gas emissions, and waste reduction. Moreover, the various applications of RCA in construction are explored, along with the sustainability and circular economy considerations associated with its implementation.

1.2. Problem Statement

Recently, there has been a growing trend in technical and review literature toward the integration of RCA in cementitious materials. The technical publications underscored the utilization of RCA as a substitute, either in part or holistically, for conventional normal-weight coarse and fine aggregates. Hence, a systemic (umbrella) review publication summarizing the overall aspects of technical review publications is neither fully presented nor discussed due to the difficulties and constraints in various aspects, including their concrete mix designs. Hence, this systemic review paper aims to accumulate, summarize, and brief the pertinent review publications of RCA material technology in construction, offering superior-quality technical evidence. By consolidating the existing knowledge and research on RCA, this review paper aims to highlight the potential of RCA as a sustainable alternative to NA to facilitate the comprehensive perspectives of technology, ecology, and economics. It emphasizes the need for further research, standardization, and awareness to promote the widespread adoption of RCA in the construction industry. The findings presented in this review paper can guide practitioners, policymakers, and researchers in making informed decisions regarding the implementation of RCA in construction projects, ultimately contributing to a more sustainable and resource-efficient construction sector.

The scope of this review consists of concrete and mortar mixtures containing RCA that were designed using ordinary Portland cement (OPC) and geopolymer, and other mix proportions entail fRCA, chemical and mineral by-products, silica fume (SF), rubber, polymer, and fiber. Concrete mixtures containing these discussed additives and admixtures aim to develop special concretes such as self-consolidating concrete (SCC), 3D concrete printing (3DCP), pervious concrete, and ultra-high-performance concrete (UHPC). The performance, workability, mechanical properties, microstructure, and durability have been analyzed and discussed. The processes of quality improvement of RCA in concrete are elaborated. Moreover, up-to-date environmental studies focusing on life cycle assessment and the circular economy have been reported. This systemic review article can offer knowledge that has already been understood in the field and determine what research should be assessed for future technical issues.

1.3. Significance of the Review

A systematic review paper of review publications, referred to as an umbrella review, is a type of systematic review that summarizes and evaluates existing systematic reviews and meta-analyses on a certain research area [32]. Papatheodorou [33] asserted that this type of review had the potential to offer superior-quality evidence. In comparison to individual studies or narrative reviews, the umbrella review offers a more in-depth degree of synthesis and analysis and serves as a thorough summary of the data that are currently available on the concrete mix design containing the RCA topic. The contribution of this review work is to summarize and synthesize the current evidence on RCA concrete mix design themes that have been obtained from trustworthy and certified sources. The information gaps and contradictions were investigated as a thorough blueprint for future research. It aids in making educated judgments and guiding future research efforts for academics, policymakers, designers, and technical professionals.

2. Methodology

To determine the state of the art on this subject, systematic quantitative literature reviews were conducted and reported. A systematic review and meta-analysis of all relevant publications on concrete mix design containing recycled aggregate were conducted. Google Scholar, Scopus, and Emerald databases were searched. This search for systematic reviews was conducted in publications that included the term “review” in their titles, abstracts, and/or keywords, with no constraints on date or language. The terms searched herein were “recycled concrete aggregate” and “mix design”, as given in

Table 1. Search strategy for electronic databases.

Study Type	TITLE-ABS-KEY (Review)
	AND
Recycled concrete aggregate	TITLE-ABS-KEY (recycle * AND aggregate)
	OR TITLE-ABS-KEY (recycle * AND aggregate AND concrete)
	OR TITLE-ABS-KEY (recycle * AND concrete)
	OR TITLE-ABS-KEY (reclaime * AND aggregate)
	AND
Mix design	TITLE-ABS-KEY (mix * AND design)
	OR TITLE-ABS-KEY (mix * AND proportion *)

Note: * represents a loose phrase.

.

As depicted in Figure 1, this systematic review has been conducted using the Preferred Reporting Items for Systematic Reviews and Meta-Analyses (PRISMA) reporting checklist [34]. Initial searches of the aforementioned databases yielded 108 publications. After deleting duplicates, limiting the publishing time, including just English, and incorporating the mix design content, the number of relevant publications decreased to 103. When analyzing the title and abstract, some papers were focused on asphalt applications, outside the topic of the study, or the full-text version could not be downloaded, and low-quality conference review papers were sorted out; 32 publications were valid. Finally, after a full-text review and quality paper check, 23 important reviewed papers that had been published were analyzed. A comprehensive literature search was conducted to identify systematic review publications for this investigation. Specifically, these journals included Science of the Total Environment, Polymers, Cement and Concrete Composites, Sustainability, Crystals, Recycling, Journal of Building Engineering, Construction, and Building Materials, Journal of Cleaner Production, Infrastructures, Materials, Construction Innovation, Clean Technologies, and Environmental Policy, and Jurnal Teknologi. Twenty-three journals were subsequently considered.

During the publication analysis, the title, abstract, keywords, authors’ names and affiliations, journal name, and publication year of the detected records were exported to a Microsoft Excel spreadsheet. Two independent reviewers separately examined the titles and abstracts of the records. Then, the two reviewers individually screened the entire texts of the remaining papers to assess their eligibility. During this step, reviewer differences were discussed and resolved through consensus. In the event that no consensus could be reached, the opinions of a third reviewer were considered. We included all review articles that demonstrated, to some extent, that the processes used to discover and choose the literature were explicit, replicable, and devoid of a priori assumptions regarding the relevance of the selected literature [35]. Specifically, we used reviews that identified and chose papers by scanning journal databases for preselected keywords. We also eliminated studies that examined publications published in a single journal. In contrast to systematic literature reviews, narrative literature reviews identify and select literature based on the authors’ assessment, often without disclosing the search criteria [36].

We acknowledge that several authors of the articles discovered and incorporated in this research did not label their reviews as “systematic”. The exclusion criteria are given below. Therefore, as all chosen papers utilized keywords and databases/journals for literature identification rather than subjective methods, they may be classified as systematic reviews. This study enriches our comprehension of systematic reviews by analyzing “grey reviews”, hence increasing the validity of the results drawn from such studies and offering recommendations for performing systematic reviews.

• Exclusion criteria:
  • Non-technical review papers.
  • General arguments.
  • Papers based on another study.
  • Conference review papers.
  • Investigations lacking adequate information regarding the mix design.

3. Qualitative Analysis

After defining the relevant review articles within a 10-year period, as shown in Figure 2a, we determined that over the past five years, research on recycled aggregates has increased year by year, especially in terms of mechanical properties and environmental impact assessments. As technology and development have evolved, it is important to highlight that this study focuses solely on review papers published within this decade. Prior publications that are involved may lead to erroneous analyses, and review articles that are of poor quality may present impartial and unwarranted outcomes. The quality of the review articles was then evaluated, as shown in Figure 2b. The Scopus database Q1, Q2, and Q3 paper classifications were established as A, B, and C papers, respectively. The conference review articles and Q4 review papers were excluded here. Approximately three-fourths of the articles published were of A quality.

Additionally, Appendix B presents the quantity of papers analyzed in each review article and their corresponding review classification. The narrative review methodology lacks a pre-defined prototype and provides a broader overview of concrete mix design incorporating the topic of recycled concrete aggregate. On the other hand, the systematic review methodology adheres to a pre-defined protocol such as PICO and utilizes data synthesis methods to minimize bias, following formal guidelines such as PRISMA [37]. In contrast, the meta-analysis adheres to a structured protocol for synthesizing data and employs statistical techniques to evaluate information gathered from a variety of empirical studies [38]. The number of papers scrutinized for each review article varies between 30 and 253 papers. The mean value is 135.5 documents. In the realm of review classification, this narrative review paper has been the subject of the majority of studies, accounting for 56.5% of the total. This is followed by meta-analyses at 34.8% and systematic reviews at 8.7%. An overview of the published timeline and review classification is exhibited in Figure 3. The PRISMA checklist is shown in Appendix C.

4. Recycle Concrete Aggregate

Cement-based materials are one of the most widely used man-made materials worldwide [39]. Every year, a substantial number of varieties of cement-based materials are manufactured. Due to the widespread application of this material, numerous researchers are investigating its nano-, micro-, and macroscale engineering properties [40,41]. However, cement-based materials are among those that use the most natural resources and leave the largest carbon footprint on our planet. It is reported that in 2017, cement and concrete production accounted for approximately 5–8% of greenhouse gas emissions worldwide, with an annual CO₂ output of 2.2 Gt [42,43]. Therefore, material scientists, chemists, and civil and construction engineers made numerous attempts to reuse and recycle this material by incorporating agricultural and industrial byproducts into their mix designs. To achieve the objective of a carbon footprint of less than 0.8 Gt/year in the cement sector by 2050, it is plausible to consider the recycling of CDW a viable and economical alternative [44]. This approach has the potential to significantly reduce global CO₂ emissions by up to 1.3 Gt/year [45]. Investigation programs have been implemented by the CDW in the concrete industry over a period of time, specifically RCA, as a binder, filler, or aggregate in civil engineering, architectural, and construction applications.

4.1. Definition of RCA

CWD is produced on a global scale, consisting of RCA, brick, plastering, gypsum, glass, asphalt, stone, steel, fiber, and polymeric materials. A considerable amount (about 80 percent) of CWD is comprised of recycled concrete [46]. On the basis of particle size, RCA may be divided into three categories: recycled concrete fine, which is less than 0.15 mm; fRCA, which is smaller than 4.75 mm; and cRCA, which is greater than 4.75 mm. This categorization has been proposed in prior scholarly works. Images of RCA and concrete containing RCA with different magnification levels are shown in Figure 4.

RCA has evolved as a viable alternative to natural fine and coarse aggregates during the past decade. This strategy is regarded as an environmentally viable method for disposing of CDW in landfills, with the potential to minimize CO₂ emissions and nonrenewable energy consumption by up to 65% and 58%, respectively [47].

4.2. Production Process of RCA

The production process of RCA, as illustrated in Figure 5, involves the transformation of waste concrete into usable aggregate for various construction applications. The process typically begins with the collection and transportation of concrete waste from demolition sites or construction projects. Then, the waste is separated into each type. The quality and composition of the source concrete play a crucial role in determining the potential of RCA. It is important to ensure that the source concrete does not contain contaminants or excessive amounts of hazardous materials that could affect the properties of the resulting RCA. Most of these processes are performed onsite.

Once the concrete waste is separated and collected, it undergoes a series of crushing, screening, and separation procedures to separate the aggregate from other materials, such as reinforcement steel or debris. It is common that the price of reinforcement steel is comparatively higher when it is usually sold to a recycling plant. These processes can be performed at the factory or onsite using mobile equipment. Initially, the concrete is crushed using specialized equipment, such as jaw crushers or impact crushers, to reduce the size of the waste material. The crushed concrete is then further processed through a screening mechanism to separate it into various grades based on desired aggregate sizes.

In some cases, pre-processing treatments may be employed to enhance the quality of RCA. These treatments can include removing surface coatings or adhered mortar from the crushed concrete particles to improve their cleanliness and minimize the presence of attached cement paste, as shown in Figure 6. The adhered (old) mortar generally affects the interfacial transition zone (ITZ), which then results in lower concrete performance. Surface treatments like washing or scrubbing may be utilized to achieve this purpose.

The resulting RCA is typically classified into different gradations based on particle size distribution, following the standards and specifications set by relevant regulatory bodies or construction guidelines. The gradations are often categorized as cRCA, fRCA, or a combination of both, depending on the intended applications. It is worth noting that the production process of RCA can vary depending on regional practices, available technologies, and specific project requirements. Different crushing techniques, such as impact crushing, cone crushing, or even mechanical grinding, may be employed to achieve desired aggregate characteristics and gradations.

Overall, the production process of RCA involves the careful collection, crushing, screening, and separation of concrete waste to produce recycled aggregate with specific gradations. The quality and properties of RCA depend on factors such as the quality of the source concrete, the crushing techniques employed, and any pre-processing treatments applied. McGinnis, et al. [49] recommended that preprocessing techniques like washing to remove fine and organic matters, which primarily involve assessing the deteriorated content and absorption, can significantly reduce these drops. Moreover, concrete incorporating RCA may even exhibit strength enhancements. By effectively managing the production process, the construction industry can obtain high-quality RCA that can be used as a sustainable alternative to natural aggregates.

4.3. Mechanical Properties of RCA

The mechanical and durability properties of RCA play a crucial role in determining its suitability as a replacement for natural aggregates in concrete production. This section presents a comprehensive analysis of the existing studies that have evaluated the mechanical and durability characteristics of concrete incorporating RCA. The focus is on key properties such as compressive strength, tensile strength, modulus of elasticity, shrinkage, and creep behavior. Furthermore, the influence of factors such as the water–cement ratio, replacement levels of RCA, and curing conditions on the performance of RCA concrete is thoroughly examined.

Compressive strength is one of the most important mechanical properties of concrete and serves as a fundamental parameter in structural design. Numerous studies have investigated the compressive strength of RCA concrete and compared it with conventional concrete. The findings suggest that the incorporation of RCA can lead to a reduction in compressive strength compared to natural aggregate concrete. However, the extent of this reduction depends on various factors, including the quality of the RCA, replacement levels, and curing conditions. Proper quality control measures and optimized mix design can help mitigate the reduction in compressive strength and ensure the acceptable performance of RCA concrete. This analysis reviewed 41 papers that studied the influence of the replacement levels of cRCA and fRCA on compressive strength. It is noted that the concretes that incorporate RCA are not used in equivalent applications. Consequently, these mixtures were evaluated independently to determine their impact on 28-day compressive strength. The calculation of the normalized 28-day compressive strength values was conducted by taking into account the values of ${\displaystyle \frac{f_c^{\prime }}{f_{c\_control}^{\prime }}}$, with the aim of mitigating the potential impact of material-related variables deriving from diverse origins. Equation (1) depicts the model for simple regression of the concrete mixtures containing different RCA.

$${\displaystyle \frac{f_c^{\prime }}{f_{c\_control}^{\prime }}}=a+mx$$

(1)

where x is an independent variable for prediction in the regression model, which is defined as the normalized 28-day compressive strength, a is an intercept value from the linear regression model, and m is a slope value from the model. It is crucial to note that the purpose of the regression model is to demonstrate the trend of the dependent variable, specifically the normalized compressive strength, rather than providing an exact prediction.

A meta-analysis of the 28-day compressive strength of concrete mix design containing cRCA and fRCA was conducted, as shown in Figure 7. The results indicate that the normalized 28-day compressive strength of concrete containing both cRCA and fRCA linearly decreases as a function of replacement level. In Figure 7a, the majority of the recorded values fall within the 90% confidence interval, which is visually depicted through the use of grey shading. The reduction in concrete strength is observed to be proportional to the replacement of aggregates with cRCA, resulting in a linear decrease in compressive strength. This is also confirmed by the similar patterns of the concrete containing fRCA, as exhibited in Figure 7b.

After conducting the regression model analysis as given in Equation (1), the results exhibited the effects of replacement-level concrete containing cRCA and fRCA on the normalized 28-day compressive strength as given in

Table 2. Summary of regression model of effects of concrete containing cRCA and fRCA on normalized compressive strength.

Parameter	Number of Observations	a	M
cRCA	42	0.871657	−0.001913
fRCA	54	0.969544	−0.002418

. The regression model of concrete mixtures containing cRCA and fRCA results contains the number of observations, intercept (a), and slope (m) of 28-day compressive strength and replacement parameters. For the cRCA parameter, the results indicate that when the replacement level of the concrete mixture increases by 1%, the normalized 28-day compressive strength decreases by 0.001913 (or 0.1913%). On the other hand, for the fRCA parameter, increasing the replacement level of concrete mixtures by 1% results in a 0.002418 (or 0.2418%) reduction in the normalized 28-day compressive strength. The present analysis avoids interpreting the intercept (a) due to the normalization of the compressive strength data. The slope (m) of the concrete mixture incorporating fRCA exhibits an increase of approximately 26.4% in comparison to the mixture containing cRCA. Meaningfully, for the slope, varying the fRCA replacement of concrete has a greater negative effect on the 28-day compressive strength than that of the cRCA. This assertion is supported by established scientific knowledge, which indicates that cRCA concrete outperforms fRCA concrete owing to its lesser amount of adhered mortar.

In addition to compressive strength, the tensile strength of RCA concrete has also been extensively studied. It has been observed that the presence of RCA can slightly decrease the tensile strength of concrete. However, with appropriate mix design adjustments, including the use of supplementary cementitious materials or chemical admixtures, the tensile strength of RCA concrete can be improved. The modulus of elasticity, which relates to the stiffness of the material, is another important mechanical property. Studies have shown that the incorporation of RCA generally leads to a reduction in the modulus of elasticity, but this reduction can be minimized by adjusting the water–cement ratio and utilizing high-quality RCA.

Durability is a crucial aspect of concrete performance, especially in harsh environmental conditions. Shrinkage and creep are two key durability characteristics evaluated in relation to RCA concrete. Shrinkage refers to the volume change in concrete due to drying and chemical reactions, while creep refers to the time-dependent deformation under sustained loads. Research on RCA concrete has indicated that it may exhibit slightly higher shrinkage and creep compared to conventional concrete. However, the magnitude of these effects is influenced by various factors, including the quality of the RCA, curing conditions, and mix proportions. Proper mix design adjustments, such as incorporating shrinkage-reducing admixtures or using a lower water–cement ratio, can help mitigate these effects.

The water–cement ratio is a critical factor affecting the mechanical properties of RCA concrete. Studies have shown that a higher water–cement ratio can lead to reduced strength and increased permeability in RCA concrete. Therefore, it is essential to carefully optimize the water–cement ratio to achieve the desired performance while considering the specific characteristics of the RCA used.

Furthermore, the replacement levels of RCA in concrete significantly influence its properties. Higher replacement levels generally lead to a decrease in mechanical strength, but the effect can be mitigated through appropriate mix design adjustments and optimization. Several studies have compensated by adding more cement [50,51,52]. However, the extra material cost and sustainability seem to be imbalanced. It is crucial to strike a balance between maximizing the use of RCA and maintaining the required performance criteria for the specific application.

Curing conditions, including temperature, moisture, and duration, also have a notable impact on the properties of RCA concrete. Adequate curing is essential to promoting hydration and achieving optimal strength and durability. Studies have shown that extended curing periods can compensate for the potential strength reduction associated with RCA incorporation [53,54,55,56].

In conclusion, the mechanical and durability characteristics of RCA concrete are affected by several factors, including the water–cement ratio, replacement levels of RCA, and curing conditions. While the incorporation of RCA may lead to a reduction in compressive and tensile strengths, proper mix design adjustments and optimization can help mitigate these effects. After analyzing all review articles, the four macro-topics, including performance, sustainability, mix design, and special admixtures and additives, can be distributed and are shown in Figure 8. The related key topics of each macro-topic were also provided. It is found that the numbers of review articles on the macro-topics of performance, sustainability, and mix design are similar. However, the articles reviewed on special admixtures and additives showed a smaller number.

5. Mix Proportions

The performance of concrete containing RCA depends significantly on the mortar content of the recycled aggregate, and the mortar content depends on the strength of the original concrete recovered from the recycled aggregate. The amount of mortar adhering to RCAs depends on their crushing processes and the water–cement ratio of the original concrete.

5.1. General Mix Design

Figure 9 shows the number of review publications on concrete mix design topics of concrete incorporating RCA. Our analysis indicates several involved topics entailing by-products, durability, circular economy, geopolymer, Life Cycle Assessment (LCA), fRCA, SCC, methods for quality improvement, steel fiber, pavement, rubber material, 3DCP, microstructural analysis, polymeric material, and pervious concrete. The topics that were heavily focused on in review papers (>65%) include by-products, durability, circular economy, and LCA. These topics seemingly focus on sustainability and long-term performance, while concrete mix design with special ingredients was reviewed to a lesser extent. The baseline of review publications determining the field of concrete mix design containing RCA is tabulated in Appendix A. The following sections provide technical knowledge and discussion related to the mentioned topics.

In terms of concrete mix designs incorporating RCA, their special proportioning process should be performed. Most RCA concrete mix designs classically replace NA with the equivalent weight or volume of RA, which are called direct weight replacement and direct volume replacement methods [57]. Such replacements only contribute to the environmental impact difference in altering the aggregate type, without paying attention to the porosity, water absorption, and residual mortar content of RCA. Advanced concrete mix designs are advised to synchronize the technical properties of concrete containing NA and RCA; then the proportion of cement in RCA concrete is changed. Another selection of RCA concrete mix design can be achieved through the utilization of the equivalent mortar volume method. This method operates on the premise that RCA is a composite material consisting of two distinct phases, namely adhered mortar and NA. The method of calculating total mortar volume involves the addition of residual and fresh mortar volumes in concrete that incorporates RCA.

This approach is widely accepted in the concrete industry, but practical implementation may be limited. Empirical evidence has confirmed that the mechanical characteristics have undergone experimental validation [58]. Previous research conducted a comparison between the environmental impact of concrete designed using the equivalent mortar volume method and the direct weight replacement or direct volume replacement methods. The results indicated that RCA concrete designed using the equivalent mortar volume method exhibited superior environmental performance. An effective method for calculating the mix design of RCA concrete was proposed by [59]. The classical direct weight replacement or direct volume replacement methods were not suitable for RCA concrete, as confirmed by [59]. The attached mortar on RCA should be assessed beforehand. Various techniques have been suggested for the assessment of the AM, including the freeze-and-thaw procedure using 26%wt sodium sulfate solution, heat treatment at 500 °C for 24 h, and 0.1 mol/L hydrochloric acid treatment for 24 h [59].

Additional techniques during material preparation and the mixing process have been documented to improve the quality of RCA and its concrete results. These methods include pre-moistening the RCA before mixing with cement, modifying the water–binder ratio in concrete blends, incorporating superplasticizers, utilizing an alternative mixing methodology, and introducing SCM [59,60]. Several discussed methods have been proposed to mitigate this major adverse factor. This factor pertains to the adhered mortar present in RCA, which exhibits a porous nature and has the ability to absorb water [61]. The presence of high porosity in the material has been observed to have a negative impact on the mechanical properties and long-term performance of the resulting concrete.

5.2. RCA Concrete with By-Products

Related review papers on RCA concrete with a by-product were conducted by several research teams [58,59,61,62]. Many by-products were utilized in RCA concrete, including several kinds of supplementary cementing materials (SCM), paper sludge ash, dust polymer waste, and rubber waste. The use of by-products in RCA concrete has significant benefits. The utilization of RCA in the construction industry may not inherently qualify as a sustainable solution, given the lack of apparent direct advantages associated with its implementation.

Xing et al. [63] asserted that the utilization of SCM in conjunction with RCA concrete was found to yield superior sustainability outcomes in comparison to concrete that solely replaces NA. According to González-Fonteboa and Martínez-Abella [64], the compressive strength of RCA concrete was similar to that of NA concrete when an additional 8% SF or 6.2% cement was incorporated. However, regarding fly ash (FA), Faella, et al. [65] claimed that the presence of a significant amount of FA in concrete was mostly found in several studies to have an adverse impact on its strength after 28 days, despite its positive effect on the durability of RCA concrete. It is noteworthy that the strength of RCA concrete exhibited a decrease as the amount of FA replacement increased, while fewer studies found an increase. Bottom ash (BA) is another substance that is produced as a result of the operation of coal-fired power plants. Efforts have been made to incorporate its addition into RCA concrete [66,67,68]. Nakararoj, et al. [69] claimed that it is possible to substitute 50% of cement with BA while maintaining comparable compressive strength to that of RCA concrete, which can reach up to 95 MPa. Nonetheless, the concern at hand pertains not to the substance per se, but rather to the means by which effective dispersion of its particles can be attained. Oruji, et al. [70] proposed a three-stage mixing process to enhance dispersion, resulting in a 12% increase in compressive strength by augmenting the presence of C-S-H in the ITZ region.

Metakaolin (MK) is a widely utilized SCM in concrete applications. Its incorporation into RCA concrete results in an enhancement of its quality. The addition of MK to both NA and RCA concrete results in an improvement in their strength properties and durability [71]. It is recommended to employ a combination of 15% MK and 100% recycled coarse aggregate in order to achieve comparable performance to that of natural aggregate concrete [72,73]. MK utilization in 100% RCA concrete had the potential to produce concrete of M35 grade, which is suitable for practical construction applications [74].

5.3. Wastes

Recently, an extensive variety of unconventional solid waste materials have been investigated and harnessed for the production of concrete. The utilization of waste glass, sugarcane ash, rice husk ash (RHA), and palm leaf ashes as a partial cement replacement has been investigated by researchers in concrete production, as reported in recent studies [75,76,77,78,79]. Various forms of solid waste, including ceramic tile waste, red mud, and waste glass, have been incorporated into concrete blends to enhance their mechanical properties and durability, as reported by Amin, et al. [80], Ismail, et al. [81], and Xu, et al. [82]. However, research regarding the utilization of waste materials in concrete remains constrained. The objective of this review is to examine the impact of waste materials on the endurance and microstructure of concrete, while also examining the feasibility of substituting conventional materials with waste materials to improve their sustainability and decrease the cost of end products. Ultimately, this review offers direction for the development and formulation of environmentally sustainable concrete materials.

The mining sector generates substantial quantities of solid waste [83]. Mine tailings refer to the residual material that remains after the extraction of valuable minerals from ore. This substance is characterized by its high toxicity and mud-like consistency, necessitating its storage in impounding lakes as a slurry in an isolated environment [84]. The creation and upkeep of these reservoirs for storage purposes incur significant costs. As a result, the recycling of this waste material is a crucial matter of both environmental and financial importance. The extraction activities of mines and quarries make a noteworthy contribution to the overall quantity of industrial waste generated globally. According to Silva, et al. [85], Eurostat reported in 2009 that industrial waste accounts for up to 55% of the total waste generated in Europe. The proper management of waste materials through recycling and reuse is a significant environmental issue that requires attention. The production of geopolymers was carried out by Silva, Castro-Gomes, and Albuquerque [85] using tungsten mine waste sourced from a prominent tungsten mine in Europe, with experimentation conducted under diverse conditions. Other waste types from mining include waste from metallic ore deposits, phosphate ores, coal seams, oil shale, and mineral sands [86].

The utilization of rubber waste in concrete presents undeniable benefits and is a crucial consideration for the construction sector. The utilization of rubber waste as a substitute for natural aggregates and the potential application of rubber waste as RCA in upcoming times can be observed [87]. The efficient introduction of diversity into rubber waste applications can be achieved through the appropriate development of rubber waste surface treatment and concrete mix design optimization for each specific type of rubber waste application in concrete, with the aim of facilitating potential field applications. The range of optimal replacement of rubber waste, as determined by considerations of mechanical, physical, and durability properties, may differ depending on whether the replacement is for fine or coarse aggregates. Specifically, the replacement of fine aggregates may fall within the range of 10–20%, while the replacement of coarse aggregates may be limited to 5% [88]. The study validated other waste polymers such as nylon, expanded polystyrene (EPS) beads, polyurethane (PU), and polyethylene terephthalate (PET) [89,90,91,92].

Furthermore, the impact of waste paper sludge ash on the characteristics of RCA was examined in Bui et al.’s [93] study. The findings indicated that the inclusion of waste paper sludge ash resulted in a significant enhancement of the mechanical properties of RCA concrete during the initial stages of development. Furthermore, the incorporation of waste paper sludge ash led to a substantial improvement in the durability of RCA concrete against acid and sulfate attacks.

5.4. Fine-RCA

fRCA constitutes a major constituent of RCA, characterized by a substantial proportion of attached mortar. The use of adhered mortar in conjunction with a fresh concrete mixture results in increased water absorption and porosity, which negatively impact the ITZ region. Concrete was produced by incorporating fRCA with a particle size of less than 5 mm as a substitute for natural fine aggregates. The results indicated a decrease in compressive strength but an increase in strength development after 28 days. Additionally, the concrete exhibited higher shrinkage compared to conventional concrete. Studies conducted by Aytekin and Mardani-Aghabaglou [94] and Sim and Park [95] claimed the degradation of various fresh, mechanical, and long-term properties, such as water absorption, resistance to chloride ion penetration, water sorptivity, and water penetration under pressure. Furthermore, the particle shape and size of the fRCA can play a key role in the fresh and hardened performance of the resulting concrete. The irregular morphology of the fRCA particles can exacerbate the viscosity, resulting in a 20% increase in viscosity due to increased internal friction among the constituents of the mixture [96]. The limit content of fRCA was reported at 30% as the structure of the modulus of elasticity, drying shrinkage, and sorptivity properties limited the maximum percentages of recycled aggregate [97]. Therefore, the utilization of fRCA in concrete has typically been restricted to products with lower grades. One potential application of controlled low-strength materials is the integration of fRCA, which is attributed to its low strength requirements [98,99,100].

However, the benefit of using fRCA in concrete when the water sorptivity and porosity are high is that it can offer an internal curing benefit. Internal curing of the mechanically treated fRCA or simply pre-soaked fRCA can be based on the theory of the capillary mechanism. The internal curing benefit demonstrated remarkable efficacy in mitigating chloride penetration, as evidenced by the significant reduction in values across all replacement percentages, water–cement ratios (w/c), and RCA particle morphology [101]. The results also exhibited a significant reduction in drying shrinkage strain values in comparison to their respective reference mixtures but were less influenced by flexural strength [102,103].

5.5. Geopolymer

Several review works have been performed on geopolymers incorporated with RCA [4,83,104,105]. As shown in Figure 8, the research on mix design has extensively investigated RCA geopolymer concrete. Geopolymers are type of inorganic material that possess the properties of being non-combustible, heat-resistant, and capable of rapidly transforming and adopting their shape at low temperatures and are synthesized through a chemical reaction between alumino-silicate oxides as a precursor and an alkaline solution, resulting in the formation of a three-dimensional network of polymeric Si-O-Al bonds [83,106,107]. The precursors may have a natural origin, such as clay or kaolinite, or may be derived from by-products like fly ash, MK, and slag [108,109,110,111,112,113]. These materials are no longer considered waste due to their extensive and effective use in the concrete industry as distinct pozzolanic substances. This information is supported by [16]. These precursor substances have the potential to be utilized either individually or in combination with one another. Consequently, there is a growing focus on exploring alternative resources like RCA that can be effectively utilized in geopolymers to alleviate the substantial reliance on by-products.

Geopolymer concrete containing RCA has been extensively studied with the goal of minimizing environmental impacts. Several types of RCA can be replaced with NA with some addition of additives and admixtures [114,115,116,117,118]. Their results revealed that the inclusion of 100% fRCA in geopolymers resulted in increased yield stresses, plastic viscosity, and rigidity in comparison to natural sand and slag geopolymers. However, this also led to higher thixotropy, which may have a significant impact on improved constructability during the initial stages of geopolymerization [119]. Furthermore, using RCA could improve the mechanical characteristics of geopolymer concrete. This is due to the increased number of nucleation sites for geopolymerization reactions. In terms of the durability of RCA geopolymer, Nikmehr and Al-Ameri [105] reported that RCA geopolymer is better than NA geopolymer in terms of freeze–thaw resistance. Although its permeability, acid attack, and chloride ion penetration were higher, these could be mitigated by increasing NaOH molarity and the ratio of the alkali activator to the binder [117,120].

5.6. Self-Consolidating Concrete

RCA has been adopted in SCC to increase sustainability. According to the review of Revilla-Cuesta, et al. [121], when incorporating RCA, SCC had lower flowability, which can be compensated by adding more water. Nonetheless, there is no observable pattern of RCA leading to a reduction in compressive strength or mechanical properties. Uncertainty persists with respect to the durability of the results. However, Liu, et al. [122] reported that the challenge of producing SCC products lay in the high water absorption, resulting in high porosity, which weakened the performance and durability. According to Martínez-García, et al. [123], the structural viability of SCC with RCA was found to be fully satisfactory based on EFNRARC standards. The fresh properties of this type of concrete were also reported to meet the required criteria for structural applications. However, it is necessary to have restricted content for each combination in order to regulate the quality of the final SCC product [81].

Several research investigations have incorporated additives as a means of mitigating the impact of inhomogeneity in SCC that contains RCA [124,125,126,127]. In a recent study, Sharma [124] investigated the efficacy of incorporating Wollastonite microfiber in SCC that contained RCA. The findings suggested that the use of microfiber had the potential to serve as a viable approach for incorporating RCA with elevated levels in SCC. The use of FA has been shown to enhance the workability and hardened characteristics of SCC when compared to the use of NA [125]. In addition, SCC with RCA and limestone powder as a filler was investigated [126]. The results indicated that the mixtures were feasible to utilize SCC containing a maximum of 50% fRCA for structural purposes. However, it is advised to restrict the RCA content to 25% due to service requirements related to deformability. The inclusion of fRCA in SCC mixtures resulted in a decline in both compressive strength and permeability characteristics. The addition of 10% by weight of SF to cement proved effective in mitigating the loss of permeability properties, even when substituting 50% of both fRCA and cRCA [126].

The study conducted by Omrane, et al. [127] involved the development of SCC utilizing a combination of cRCA and fRCA in equal proportions of 50%, alongside a natural pozzolan. The study assessed two durability attributes, namely the capacity to withstand chloride ion penetration under full immersion and the ability to resist H₂SO₄ attack. The results indicate that the concrete specimens containing elevated proportions of RCA and natural pozzolan exhibited reduced levels of ion penetration in both tests. Moreover, Boudali, et al. [128] assessed the impact of sulphate attack on concrete specimens. The results of the conducted tests indicate that the SCC with RCA exhibited superior performance in comparison to the SCC used as a reference. The observed enhancement can be attributed to the sustained progress of the matrix hydration reaction, facilitated by the presence of non-hydrated cement that adhered to the aggregate. This phenomenon resulted in the formation of obstructive barriers that impeded the ingress of extraneous agents. The compressive strength was enhanced by the hydrated cement, which filled the voids, notwithstanding the impact of said agents.

5.7. Other Mix Designs with RCA for 3D Concrete Printing, Pervious Concrete, and Ultra-High-Performance Concrete

Other mix designs incorporating RCA have been studied and reviewed including 3DCP, pervious concrete, and UHPC. The analysis derived from the review indicates that the aforementioned topics have been addressed to some extent, albeit not extensively. There is still scope for further investigation.

The utilization of automated systems in 3DCP is an innovative construction technique aimed at addressing the issue of labor shortages [129]. Several studies have been conducted to develop a concrete mix design that is suitable for printing and laying, as well as to determine appropriate design and construction methods. Its mix design showed varieties of binders and additives such as calcium aluminate cement, calcium sulfoaluminate cement, retarder, superplasticizer, FA, slag, MK, and SF [130,131,132]. Research has been identified that specifically examines the use of 3DCP in conjunction with RCA. The analysis conducted in the review indicates that printability and the interlayer bond are crucial factors to consider when examining the properties of 3DCP products. According to Mechtcherine et al. [133], it is recommended to incorporate cRCA with a diameter greater than 8 or 15 mm into mix designs. Nonetheless, the dimensions were restricted by the diameter of the nozzle. According to Ivanova Ivanova and Mechtcherine [133] and Mechtcherine, et al. [134], various research studies have indicated that the content, surface area, and morphology of RCA have a significant impact on the extrudability and buildability of the final product in 3DCP. Wu et al. [135] determined the time-dependent behavior of the yield stress and shear modulus of 3DCP with RCA. The yield stress exhibited an exponential increase with time, whereas the shear modulus displayed a linear increase over time. During the initial 15 min printing period, the buildability of RCA concrete exhibited an upward trend as the RCA replacement rate increased.

Only one review publication has been conducted by Singh, et al. [136]. Pervious concrete is composed of materials that are comparable to those found in traditional concrete, with the exception that the inclusion of fine aggregates is restricted or eliminated to produce a material that is highly permeable, thereby facilitating the infiltration of stormwater. Further information regarding the significance of aggregate gradation, type, and size, cementitious materials and their ratios, and admixture types and quantities in pervious concrete mix design can be found elsewhere [137,138]. Furthermore, it is imperative that potable water be utilized in the production of pervious concrete mixtures. Insufficient water content results in the formation of rigid blends that exhibit poor workability. Conversely, excessive water content causes the cement paste to flow off the surface of the aggregates. Bonicelli, et al. [139] developed pervious concrete with fine rubber as fRCA. The results indicated that the increase in concrete density was accompanied by a reduction in permeability and indirect tensile stresses when fRCA was utilized. The acquisition of fRCA led to the augmentation of indirect tensile stresses and resulted in an appropriate level of density.

UHPC has been extensively utilized in specific civil engineering contexts, including skyscrapers, bridges, and infrastructure, for some time. Due to the high cement content required in its mix proportion relative to normal concrete, it is commonly believed that UHPC is not sustainable. There have been multiple efforts to incorporate RCA into UHPC blends with the aim of mitigating adverse environmental effects. However, additional techniques for improvement should be devised [140]. The durability of UHPC based on solid waste can be significantly enhanced through the process of grinding and activation when the waste is utilized as a binder. The incorporation of RCA in UHPC has the potential to yield favorable outcomes, including improved performance attributed to the coarse texture, possible reactivity, and internal curing properties of RCA. These benefits are derived from the rough surface and water retention capacity of the RCA [141,142]. Due to its compact microstructure, it is capable of efficiently encapsulating hazardous elements, such as heavy metal ions, from solid waste [143].

6. Technical Performance Aspect

6.1. Durability

The performance of concrete is significantly impacted by its durability, particularly in adverse environmental circumstances [144]. It is known that adding RCA to concrete lowers its long-term performance because of its high porosity and wider ITZ region. The durability attributes of RCA concrete are commonly assessed through the examination of shrinkage and creep. The process of shrinkage pertains to the change in the volume of concrete as a result of dehydration and chemical reactions, whereas creep denotes the deformation that occurs over time when subjected to sustained loads. In a study conducted by Yang and Lee [145], conventional RCA concrete exhibited a drying shrinkage deformation of 1204 mm/m after 41 days, indicating a 42% increase compared to that of NA concrete. Meanwhile, the 100% cRCA concrete had 20.5–76.9% higher than NA concrete, and this is reportedly due to the adhered mortar of RCA particles [146,147]. Research on RCA concrete has indicated that it may exhibit higher shrinkage and creep compared to conventional concrete. However, the magnitude of these effects is influenced by various factors, including the quality of the RCA, curing conditions, and mix proportions.

Common techniques for reducing drying shrinkage have been identified. The implementation of appropriate mix design modifications, such as the integration of shrinkage-reducing admixtures and a small quantity of nanomaterial, the utilization of coarse aggregate with a larger size, or the application of surface coating, can be feasibly accomplished [148,149]. There are three viable techniques for reducing drying shrinkage, namely, implementing a controlled moist-curing period, utilizing a surfactant with a concentration of less than 3% to decrease surface tension, and incorporating an expansive additive with a concentration of less than 3% to counteract volume shrinkage [150]. Maltese, et al. [151] evaluated the integrated utilization of a calcium oxide-based expanding agent and a propyleneglycol ether-based shrinkage-reducing admixture and their facilitation of the production of mortars that exhibit reduced sensitivity to drying. The combined impact of both additives exhibits a synergistic effect that can effectively operate collectively.

Although there are other qualities of durability like wear, erosion, and corrosion, when RCA is present, they are usually found to have detrimental effects. The adhered mortars in RCA cause the final RCA to have a wider ITZ and higher porosity. These make RCA concrete more vulnerable to surface abrasion, as well as acid or salt attack. Therefore, the limitation of using RCA as structural concrete should be addressed, such as the limit of RCA concrete in coastal areas and the top surface of solid pavement [152,153].

6.2. Methods to Improve RCA Quality and Reliability

The section mainly presents treatment methods to improve the RCA quality. A recent narrative review article from Liu et al. [154] and a meta-analysis from Zhang et al. [155] demonstrated that various treatment techniques could be employed to decrease the porosity and sorptivity of RCA and its resultant concrete product. Various techniques can be employed to achieve the pre-treatment of RCA surfaces, including coating with sodium silicate, cement paste, polymer, wax, bio-deposition, and concentrated acid solution [156,157,158,159,160,161,162,163]. Additionally, mechanical grinding and thermal or microwave heating of the adhered mortar of RCA, as well as CO₂ mineralization methods such as carbonation at high CO₂ concentration conditions, gas–solid carbonation, and liquid–solid accelerated carbonation, are also viable options [164].

The utilization of CO₂ mineralization techniques by RCA has drawn significant interest as a means of advancing the sustainable and circular economy within the construction sector. The numerical findings of the study suggest that the process of chemical carbonation has a notable impact on various properties of RCA, including water absorption, permeable void volume, workability, and compressive strength of the cementitious system [165,166,167]. The process of carbonation in cement paste that is adhered to mortar results in the formation of calcite and an amorphous alumina-silica gel. However, the quality of the resulting outcome is influenced by various factors, such as the CO₂ mineralization method, the surrounding environment, and the RCA itself.

7. Environmental Impact Aspect

The environmental impact of RCA production and use is a critical aspect to consider when assessing its sustainability. This section delves into the various environmental factors associated with RCA, focusing on LCA and carbon footprint analyses that evaluate the environmental benefits of utilizing RCA as a substitute for NA. The circular economy, which is the main topic relating to sustainability in any industry, is elaborated on next.

7.1. Life Cycle Assessment

LCA provides a comprehensive evaluation of the environmental impact of a product or process throughout its entire life cycle, from raw material extraction to disposal. Several studies have employed LCA to compare the environmental performance of RCA concrete with conventional concrete. These assessments consider various environmental indicators, including energy consumption, resource depletion, greenhouse gas emissions, cost, and waste generation. Ref. [168] assessed the cost and potential reduction in embodied CO₂ (ECO₂) emissions achieved by incorporating recycled aggregates in four different types of buildings, specifically focusing on apartments with 31 distinct characteristics. The ECO₂ can be reduced by 5–10% in the applied buildings.

One of the significant environmental benefits of RCA is the reduction in energy consumption. The use of RCA eliminates the need for virgin aggregate extraction and processing, which are energy-intensive processes when considering the overall life cycle environmental impacts. By recycling concrete, agricultural, and industrial wastes and incorporating RCA into concrete production, a substantial amount of energy can be saved, contributing to a more sustainable construction industry [18]. The utilization of prefabricated slabs can result in a reduction of approximately 40% in embodied energy, despite an increase in transportation requirements for the concrete structure [169].

Greenhouse gas emissions are another crucial aspect of the environmental impact. The production of cement, a primary component of concrete, is associated with CO₂ emissions. By reducing the demand for virgin aggregates and cement through RCA utilization, the carbon footprint of concrete can be significantly lowered. Several studies have indicated that incorporating RCA into concrete can lead to notable reductions in CO₂ emissions, primarily due to the avoided emissions from aggregate extraction and processing. Using RCA in concrete production can lead to a 20% to 50% reduction in CO₂ emissions compared to using natural aggregates [170].

Landfill space reduction is another key environmental benefit of RCA adoption. The disposal of CDW, including concrete, contributes to landfill congestion. By diverting concrete waste from landfills and recycling it into RCA, the volume of waste requiring landfill disposal is significantly reduced. This helps mitigate the strain on landfill capacity and reduces the associated environmental risks and costs. For instance, in New York City, the landfilled waste or RCA used in ready-mixed concrete might not affect carbon footprint emissions. However, the indicators for acidification and smog formation show, in lieu, reductions of 16% and 17%, respectively [171].

It is important to note that the environmental benefits of RCA can vary depending on various factors, such as the quality of the source concrete, the recycling process, and transportation distances. To maximize the environmental advantages, it is crucial to establish efficient recycling systems and optimize the logistics involved in the collection and processing of concrete waste. Knoeri, et al. [172] found that RCA concrete decreased environmental impact by 30%. RCA concrete transportation of 15 km with 22–40 kg/m³ cement has the same environmental impact as NA concrete manufacturing. Cradle-to-cradle LCA for entirely recyclable concrete [173] showed 66–70% and 7–35% reductions in global warming potential for high- and medium-strength entirely recyclable concrete, respectively. The LCA database can be procured from the Bill of Quantities (BOQ) and the distance from the production plant to the site. The eCO₂ can calculated from the RCA volume using the ISO14001 standard [174], for which commercially available software like SigmaPro, Open LCA, and EcoChain can be employed. Concerning the CO₂ emissions from transportation, the estimated distance and type of vehicle transporting can be computed to the carbon footprint. Both eCO₂ from the material itself and transporting CO₂ should be combined.

While RCA offers substantial environmental benefits, it is essential to consider potential environmental challenges as well. Contamination in the source concrete can pose environmental risks if not properly identified and addressed. Contaminants such as hazardous materials or pollutants can leach into the environment if RCA is not carefully produced and used. Therefore, quality control measures and strict monitoring are necessary to ensure the suitability of RCA for various applications and prevent any potential environmental hazards. Quantitative analysis can involve testing and measuring the concentrations of contaminants in the source concrete. This analysis may include laboratory testing methods, such as chemical analysis and spectroscopy, to identify and quantify hazardous materials or pollutants present in the concrete.

The initial step in this process involves the development of an estimation methodology for the quantification of CO₂ emissions resulting from the generation of RCA and other waste materials. The aforementioned value can be linked to the concepts of digital twins and building information modeling (BIM) in order to provide carbon credit, carbon trading, and taxation [164,175,176]. Furthermore, there are several digital technologies, including machine learning (e.g., neuron networks, forest tree, K-nearest neighbor, support vector machine, decision tree, gradient boosting, and gene expression programming) and blockchain (e.g., cryptocurrency and game simulation platform), that are presently being scrutinized in this regard [15,177,178,179,180,181,182,183,184]. These can simulate and predict their performance and interesting aspects with high efficiency of the computational time in seconds and accuracy as high as 99% of R².

The qualitative LCA has also been assessed, especially for risks and uncertainty levels, at different stages of life cycles. Streamlined LCA has revealed that the use phase often causes significant risks to the ability to reuse, repurpose, and recycle the CWD at the end-of-life phase. The most common actions during the use phase of the life cycle are those that potentially cause ettringite, disturb asbestos, break down microplastics, and/or contaminate the constituent materials. These have significantly raised the danger to public health and safety and natural ecosystems stemming from reuse, repurpose, and recycling activities [185].

In conclusion, the environmental impact of RCA is a crucial aspect to consider in promoting circular economy construction practices. LCA and carbon footprint analyses have shown that RCA utilization can lead to significant reductions in energy consumption, greenhouse gas emissions, and landfill space. By diverting concrete waste from landfills and incorporating RCA into concrete production, the construction industry can contribute to resource conservation and mitigate environmental degradation. However, proper quality control measures and monitoring are necessary to address potential contaminants and ensure the environmental suitability of RCA.

7.2. Circular Economy

As previously mentioned, there has been significant research on the efficacy of cement systems that incorporate RCA. Based on a review by Silva, et al. [186], circular economy construction initiatives across the globe have incorporated RCA into their concrete mix designs, with several projects employing it at a full replacement rate of 100%. The successful application of this material in civil engineering applications has been verified to yield advantageous results. Presently, there is a growing global emphasis on environmental issues, with a focus on sustainable development for the goal of achieving a circular economy. These concerns will be the primary focus of attention from the present day until 2050. These developments are crucial development objectives for every industry, including the civil engineering and construction industries.

In contemporary industry, it is imperative for manufacturers and service providers across all sectors to furnish Environmental Product Declarations (EPDs) in order to ensure that their products and services are subjected to a thorough evaluation of their environmental impacts [187,188,189]. This includes the products and services in the building and construction industries, which are followed per ISO 21930 [190] and EN 15804 [191] standards.

The International Federation for Structural Concrete (FIB) begins by developing a “design for sustainability” code. The implementation of this “design for sustainability” in concrete structural codes remains a distant goal. The FIB Model Code for Concrete Structures has incorporated sustainability as a performance criterion in the design of concrete structures, in addition to structural safety and serviceability [192,193].

Kadawo, et al. [194] conducted a calculation of the circular index for RCA concrete produced in Sweden. The results indicated that the circularity values for concrete with a 100% RCA replacement level and a 50% RCA replacement level were determined to be 0.5 and 0.6, respectively. Nonetheless, it has been contended that the transportation of RCA to the location and its supply chain constitute the principal factors that are considered to be risks of its circularity [195]. The instruments utilized for assessing LCA and the circular index ought to concentrate on a meticulous examination of the logistical, equipment, and handling procedures. Yu, et al. [196] studied the circular economy RCA industry in Dutch and found that the context of RCA circular economy businesses required supply chain management, enabling companies to effectively implement and enhance their circular economy business strategies. Additionally, it establishes a solid foundation for governmental entities to customize circular economy policies based on scientific principles before taxation can be implemented. The circular economy of RCA business has many facets to develop, and it will take some time before an effective implementation can be achieved.

The discussion of each macro-topic and research gap is detailed in this section. Figure 10 outlines the important research trends of the macro-topics and their explanations. For the topic related to RCA performance, the current research trend pertaining to the performance of RCA reveals a significant extent of literature that focuses on the evaluation of fresh properties and mechanical properties. The durability of the material presents ambiguous results with regard to practical implementations and the experimental assessment associated with microstructural features such as the ITZ, which can lead to micro- and macro-cracking. Lu et al. [197] asserted that new mortar-bonded mortar exhibited worse mechanical characteristics, increased widths, and more pronounced interface effects, yielding crack development. Several studies in the literature also found evidence of this behavior [198,199]. The quality method of CO₂ mineralization still requires refinement and implementation in both production and commercial contexts.

The validation of the sustainability of RCA is comprised of two distinct components, namely LCA and the circular economy. The present methodology for assessing the quantity of CO₂ emissions associated with individual stages of a product’s life cycle is deemed to be suboptimal. The present launch of EPD in construction is massively impacted by the stakeholders. Hence, there is a requirement for an improved prognostic instrument for LCA techniques. Digital Twin and BIM are contemporary technological advancements that can facilitate the evaluation of carbon credit, carbon trade, and carbon taxation at both national and global levels once implemented. The integration of machine learning algorithms and blockchain technologies has the potential to facilitate the digitalization shift aimed at promoting the sustainability of RCA.

Concerning the concrete mix design incorporating RCA, studies have revealed that the normal calculation of the concrete mix design by directly replacing RCA in concrete mixtures is not totally satisfactory. In lieu of this, the new calculation of its mix design based on the equivalent mortar volume method, of which the adhered mortar is concerned, should be used for the assessment of RCA mix proportions. Normal RCA concrete, geopolymers, and SCC are known research topics in this area. Future work should be evaluated regarding innovative concrete technologies such as 3DCP, pervious concrete, and UHPC. The 3DCP and pervious concrete may require lower workability. This can be approximated to employ RCA since it has high water absorption [200,201,202,203,204,205].

Lastly, there is a relatively limited body of research on the macro-topic of special admixtures and additives, whereas a significant amount of research has been conducted on the utilization of fRCA in various applications. This highlights the importance of fully leveraging the potential of RCA in any given context. One potential opportunity for future research could involve an examination of the efficacy of incorporating MK in order to enhance the quality of the resultant RCA products. It is recommended that research be directed toward the incorporation of steel, glass, and cellulose into both natural and synthetic fibers in order to enhance product quality.

8. Discussions and Implications

The results of this study carry significant weight for enhancing sustainable construction methods and fostering the circular economy in the sector, as given in Figure 11. Incorporating RCA into construction initiatives allows the industry to tackle significant environmental issues, including resource depletion, waste buildup, and carbon emissions, aligning with the United Nations’ Sustainable Development Goals (UN-SDGs). The investigation underscores RCA as a practical substitute for natural aggregates, stressing its capacity to considerably lessen the environmental impact of concrete manufacturing while facilitating efficient waste management for UN-SDG 11 (sustainable cities and communities) [206]. This method is in harmony with worldwide sustainability objectives and promotes resource efficiency throughout the construction process.

One important consideration is the optimization of concrete mix designs to improve the technical performance and durability of RCA-based concrete. The investigation highlights the equivalent mortar volume method as a viable alternative to traditional replacement techniques, promoting enhanced compatibility between RCA and other mix components. Although it seems not practical industry-wise since the quality control of the product can be more intricate, this approach considers the distinct features of RCA, including its porosity and adhered mortar, yielding effective mix design and facilitating the creation of high-performance concrete. Furthermore, the integration of sophisticated treatment techniques, such as surface coatings and CO₂ mineralization, is crucial for enhancing the quality and dependability of RCA across various applications. Akin to the equivalent mortar volume method, the treatment methods stated earlier can be cost-inhibited and corrected at the lab scale [77,207,208].

The study highlights the critical role of developing policies and establishing standards. Creating definitive guidelines (like cradle-to-grave LCA for eCO₂), certifications (like the LEED rating system), and quality control protocols for RCA production and their applications can tackle inconsistencies across the industry and encourage wider acceptance [209,210]. When combined with economic incentives and subsidies like the green loan for mega-project (like One Bangkok in Thailand and Anzalduas Land Port in Texas, USA), these measures can enhance the attractiveness of RCA-based construction to stakeholders for more investment in particular green projects [211,212,213]. Moreover, the incorporation of digital technologies [214,215,216] and nanotechnologies [217,218,219,220,221] like BIM, digital twins, blockchain, and machine learning, as well as nano and quantum dot materials, presents fresh avenues to improve mix design optimization, track sustainability metrics, and support carbon credit accounting and the cryptocurrency market, fostering data-driven advancements for smart and sustainable construction.

Collaboration among various parties—designers, builders, project managers, and regulatory authorities—is essential to addressing the technical and logistical challenges associated with the adoption of RCA as a local agency can be adopted. The double-handing from the demolition site to the concrete factory and, finally, to the new construction constitutes a challenge, where logistic costs and transporting CO₂ can be doubled. The mobile crusher and mobile plant can mitigate the double-handing challenges [222], but storage and a large site area are mandatory. The storage for RCA can be costly due to land rental or land acquisition, yielding increased OPEX and CAPEX, respectively. It should also be noted that the context for each nation for using CDW is varied. Many governments support the amelioration of financing for waste products, while many do not, especially for developing nations where natural resources are still abundant [223,224,225,226,227]. Therefore, without financial support, the implementation of waste can be cost-competitive compared to the virgin one. This investigation emphasizes the necessity for collaborative initiatives to create groundbreaking solutions and execute RCA in extensive construction endeavors to international policy like EPD, international trade, and sister-city investment [228,229,230,231,232,233,234,235,236]. Ultimately, this study acts as a driving force for subsequent investigations, promoting deeper inquiry into sophisticated RCA applications in a circular economy. By tackling these gaps in knowledge and utilizing the industrial capabilities of RCA, the construction sector can move toward more sustainable and resource-efficient methods.

9. Conclusions

This systematic review study highlights the considerable potential of RCA as a sustainable substitute for natural materials in concrete manufacturing. RCA integration tackles significant environmental issues, such as resource depletion, waste production, and greenhouse gas emissions. This investigation demonstrates the viability of RCA in several applications, including traditional concrete, geopolymers, SCC, and innovative technologies such as 3DCP and UHPC. Important findings encompass:

• Performance and durability: RCA concrete demonstrates marginal decreases in mechanical qualities (e.g., compressive and tensile strength) and durability relative to natural aggregate concrete. Increasing the 1% replacement level reduces the compressive strength by 0.1913% for cRCA and 0.2418% for fRCA (equivalent to a 26.4% difference). Nevertheless, these deficiencies can be alleviated by optimal mix designs, enhanced treatment techniques (e.g., CO₂ mineralization and surface coatings), and the use of SCMs such as metakaolin and silica fume.
• Environmental impact: LCA indicates that RCA markedly decreases the carbon footprint, energy use, and landfill demands associated with concrete manufacturing. The ideals of a circular economy are further reinforced by the efficient incorporation of RCA into the building value chain.
• Innovative applications: Research trends underscore the potential of RCA in specific applications such as SCC, pervious concrete, and geopolymer systems. The implementation of modern digital technologies such as BIM, digital twins, and machine learning can enhance the optimization of RCA consumption and carbon credit accounting.

This research highlights essential domains for more investigation, including the formulation of special binders and standardized mix design techniques (e.g., equivalent mortar volume approach), durability evaluations in extreme conditions, and the extensive validation of novel RCA applications.