Role of Mineral Processing in Enhancing Recycled Concrete Aggregate Quality—A Critical Review

Abstract

Mineral processing may decisively influence recycled aggregate (RA) production, yet it is systematically underreported. This critical review screened 338 Scopus-indexed publications (2004–2024) and retained 204 studies after eligibility assessment. Reporting on comminution was limited: ~52% (105 studies) of studies did not explicitly mention crushing, while ~26% (53 studies) identified the crusher type, and only about 1% (two articles) reported operating conditions, which undermines reproducibility and cross-study comparability. RA quality is application-/market-dependent. The literature was classified into cement-based materials (46.1%), pavement applications (44.6%), and fundamental studies without application (9.3%). For cement-based materials, water absorption and compressive strength were the most frequently reported primary and secondary properties, respectively. For pavement applications, particle-size distribution and optimum moisture content predominated. Overall, mineral processing directly governs the primary attributes of recycled aggregates (RAs) and indirectly influences their secondary performance outcomes. The main gap identified in the literature is the lack of clear recommendations for processing procedures, which limits the reproducibility and comparability of reported results. To address this limitation, this article proposes a mineral-processing framework intended to standardize both RA processing and reporting practices, thereby improving crosslink study comparability, experimental reproducibility, and evidence-based specification according to end-use requirements.

1. Introduction

Construction and demolition waste (CDW) primarily comprises materials such as concrete, mortar, ceramic, bricks, wood, plastic, metal, and waste glass [1]. The basic composition of concrete material includes natural aggregates (constituted by multiple minerals such as quartz, feldspars, amphibole, pyroxene, mica, clay, shale, calcite, dolomite, iron sulfides, gypsum, and anhydrite) and cement-based phases (constituted by multiple phases such as CH, ettringite, C-S-H, C₃S, and C₂S) [2,3]. Mineral composition varies by construction site and is related to the natural aggregates available in the area and local market.

Comminution, encompassing both crushing and grinding, is crucial in the production of RCAs. Size reduction and phase liberation, the process of separating natural aggregates from adhered cement paste (CP), are results of the comminution process used to produce RCAs. The equipment used in this process, such as crushers and mills, functions based on fundamental breakage principles, including loading mechanisms, breakage mechanisms, and breakage modes. These principles govern how particles respond to the stresses applied during comminution, ultimately influencing the efficiency and quality of the resulting RCAs [4,5].

Which considerations during comminution does the literature suggest for producing high quality RCAs? Cement paste content can significantly influence various normalized indices that assess the quality and suitability of RCA applications. For instance, it can affect key properties such as water absorption, density, and porosity, which subsequently impact the performance of new material applications, whether in cement-based products or pavement construction [3,6,7,8].

Using an appropriate comminution process flowsheet is the simplest way to produce high-quality RCAs. Müller and Martins [9] emphasized that both the type of material being processed and the selected processing technology and flowsheet can significantly influence the quality of the RCAs. Despite that, comminution equipment selection often prioritizes financial considerations over characteristics of the comminuted material, mechanisms of the equipment, and operational capabilities. Without controlling mineral processing to produce RCAs, how can the research identify the best procedure for RCA applications?

This critical review aims to elucidate the significance of comminution within the context of recycled concrete aggregates (RCAs) through a two-stage approach. First, it presents a comprehensive review of the existing literature addressing comminution processes related to RCA production. Second, it conducts a systematic assessment of the quality parameters and evaluation criteria established in the literature over the past two decades. By examining these aspects, the review seeks to assess the attention devoted to mineral processing in the field and, where necessary, propose guidelines for its further improvement. The importance of mineral processing lies in its direct influence on the physical and mechanical performance of RCAs, as well as on the overall sustainability and efficiency of construction material recycling.

Published RCA Processing Reviews

Several prior works have addressed RCA processing from different perspectives. However, none of them systematically evaluate reporting transparency or define minimum reproducible processing information across literature.

Pepe et al. (2014) [10] focus on an experimental processing route for structural RCA. The study details jaw crushing followed by autogenous cleaning in a rotating drum or mill under controlled operational conditions. While it provides high reproducibility for the specific route tested, it does not generalize its findings across the broader literature. It also does not evaluate how processing is reported in other studies or propose a minimum reporting standard. Its contribution is therefore process-specific rather than methodological or comparative.

Mistri et al. (2020) [11] present a review compiling several treatment and improvement methods for RA/RCA, including mechanical grinding and autogenous cleaning routes. The review is primarily descriptive. It does not quantify reporting gaps, define a minimum reproducible information set, or systematically link processing descriptions to reporting quality. As a result, it offers breadth of techniques but limited analytical assessment of methodological transparency.

Silva et al. (2017) [12] discuss the CDW recycling supply chain and provide a conceptual comparison of crushing stages and plant configurations, including jaw, impact, and gyratory crushers as well as multi-stage arrangements. The emphasis is operational and logistical rather than methodological. The study does not assess whether published works report sufficient processing parameters and does not propose standardized criteria for reproducibility or reporting adequacy.

Busari et al. (2019) [13] review the use of RCA in pavement applications and describe processing in generic plant-flow terms such as sorting, crushing, and sieving. Processing is treated superficially, without differentiation of equipment types, operational parameters, or completeness of reporting. Consequently, the review provides application-oriented context but does not address methodological rigor or reproducibility of processing data.

In contrast, the present study adopts a critical review combined with bibliometric mapping covering 2004–2024. It explicitly positions comminution and processing description as central determinants of material quality and performance. Rather than only listing processing routes, this work quantitatively evaluates the presence or absence of essential methodological information, such as equipment type and operational parameters, across a large dataset of publications. Its main contribution is therefore not merely the compilation of techniques, but the diagnostic assessment of reporting processing quality and the proposal of a simplified framework defining minimum reproducible processing information considering main application context. This approach directly addresses the transparency and reproducibility gaps left unexamined in prior reviews and provides evidence linked structure intended to improve consistency and comparability in future studies.

2. Mineral Processing

2.1. Breakage Fundamentals

Mineral processing routes are designed based on the intrinsic properties of the minerals and the desired characteristics of the final product. The design of mineral processing for RCAs and natural aggregates follows equivalent principles, provided that specific product properties are respected. Comminution, which includes crushing and grinding, constitutes the first stage of recycling processing. The selection of optimal comminution equipment or equipment sets (primary, secondary, and tertiary crushing) is crucial to efficiently separate valuable minerals from gangue minerals, or in the case of construction and demolition waste (CDW), to detach natural aggregates from cement paste [4].

Particle breakage is influenced by the sequence of comminution equipment, which determines the repetition and intensity of breakage events. Two main strategies are applied to optimize size reduction. The first involves selecting available comminution equipment according to the material’s properties (e.g., texture), whereas the second incorporates the material’s texture into the design process of new comminution equipment. In the context of CDW comminution, the literature predominantly addresses only the first strategy [5].

The fundamentals of particle breakage encompass three key criteria: loading mechanisms, breakage mechanisms, and breakage modes. Loading mechanisms involve the application of physical forces or momentum to a particle assembly. In mineral comminution literature, the terminology associated with loading mechanisms includes “compression,” “impact,” “attrition,” and “strike” (see Figure 1) [5,14].

The breakage mechanism involves understanding how materials respond to stress. A particle fractures when the applied force exceeds the atomic bonding strength. The potential breakage energy in brittle materials represents the minimum energy required to rupture atomic bonds at the minimum separation distance. The cohesive stress (σ_c) needed to achieve the potential breakage energy can be calculated using the Young’s modulus (E_γ) and the minimum distance (X₀). However, σ_c is typically overestimated because the equation does not account for weakened planes within the particles, such as microcracks and microdefects. Since the breakage mechanism results from particle failure, it is therefore essential to understand the particle’s behavior under loading to properly characterize the mechanism of breakage [5,15,16].

Breakage mechanisms can be classified into four categories: chipping, abrasion, cleavage, and shattering (see Figure 2). Chipping is a low-energy mechanism in which the application of force to an irregular grain causes the corners and edges to break off. Abrasion involves the localized application of both normal and shear forces, also at low energy intensity, resulting in the fracture of small portions of the grain and the production of fine residues. Cleavage, or the compression mechanism, occurs when the contact duration of the compressive force exceeds the transit time of the elastic wave through the grain. The energy applied induces fractures at a limited number of points within the grain, generating particles of similar size to the original. Finally, shattering, or the impact mechanism, employs high-intensity energy to produce overloading and extensive breakage, resulting in particles with comparatively large grain sizes [15,17].

The degree and behavior of breakage mechanisms are influenced by various factors related to the particles, including their shape, size, and texture, as well as by environmental conditions at the microscale and nanoscale levels. At the microscale level, important material properties are based on grain size, shape, composition, and the presence of microcracks. The interfacial transition zone (ITZ) between the aggregate and cement paste is considered the weakest part of concrete. The presence of microcracks in cementitious materials can be classified into large-scale cracking in three-dimensional volume (“ITZ failure”) or bidimensional interface between aggregate–ITZ (“ITZ detaching”). The influence of the microcracking in the breakage of cementitious materials can be described according to the ratio of the ITZ and mortar strength (σ_ITZ/σ_Mortar). Higher σ_ITZ/σ_Mortar propagates the microcracks along the aggregates in the direction of the loading vector. The first life of concrete is composed of the formation of a single ITZ; however, recycled concrete is composed of three different types of ITZ: ITZ¹-original aggregate and old cement paste; ITZ²- old mortar and new mortar, and ITZ³-new mortar and original aggregate. In other words, many heterogeneous ITZs of recycled concrete may favor the production of RCAs for a second life. However, producing a third-life RCA will be a challenge, due to the exponential increase in ITZ. Thus, the number of RCA life cycles is determined by the efficiency with which mineral processing removes adhered cement paste. Most of the recycled material processed is not used in its original application in the first life cycle. Even so, mineral processing is the key for high-quality material [3,18,19].

The breakage mode of a particle depends on the stress applied to it. Guldris Leon, Hogmalm, and Bengtsson [20] emphasize that understanding how particles fracture is fundamental to understanding phase liberation. According to Fuerstenau and Han [17], particles composed of multiple phases exhibit breakage behavior that reflects the distinct mechanical properties and textures of their constituent phases. This concept can be extended to concrete materials, as demonstrated by Hoffmann Sampaio et al. [21]. These authors investigated the influence of the coarse aggregate liberation degree in recycled concrete aggregates obtained through the comminution of concretes with different compressive strengths (16 MPa, 54 MPa, and 85 MPa). They observed a tendency toward random breakage in concretes with higher compressive strengths (54 MPa and 85 MPa) due to the increased hardness of the cement paste. In contrast, concretes with lower compressive strengths (16 MPa) exhibited a greater degree of aggregate liberation from the adhered cement paste. Breakage can therefore be classified as either random or non-random (see Figure 3) [22,23].

Random breakage, also known as transgranular liberation [15] generates mixed particles [17]. In this type of breakage, particle fracture is not influenced by the contour, texture, or mineralogical and mechanical properties of the material [20,23]. Consequently, a greater degree of size reduction is required to improve the level of liberation [17]. As a result, a higher proportion of locked particles is produced, as illustrated in Figure 3, where random breakage produces fractured particles composed of mixed phases (gray and black).

In non-random liberation, also referred to as intergranular or selective breakage [15], fracture propagation occurs along grain boundaries, resulting in the preferential liberation of phases [15,17]. The ideal model for concrete comminution is selective liberation, in which the natural aggregate and cement paste are separated along these boundaries. As shown in Figure 3, non-random breakage promotes the selective separation of the gray matrix from the black phase. King [24] classified non-random breakage into six distinct fracture effects. Little et al. [25] later reorganized this classification and simplified it into two primary non-random breakage modes, summarized in

Table 1. Classification of non-random breakage.

King (2001) [24] Classification	Little et al. (2016) [25] Classification	Definition
Selective breakage	Preferential breakage	Fractures occur more frequently in one of the phases [24,25].
Differential breakage
Preferential breakage
Phase-boundary fracture	Phase-boundary fracture	Breakage occurs preferentially along a specific phase boundary rather than across the other phases [24,25].
Liberation by detachment
Boundary-region fracture

.

Understanding the microstructural characteristics of a material that influence its response to breakage mechanisms under applied physical forces is essential for achieving selective comminution. Moreover, these characteristics are valuable for selecting appropriate equipment and defining optimal operational parameters during the comminution process. Hesse, Popov, and Lieberwirth [26] highlighted the importance of quantifying the following characteristics:

• Volume percentages of the interest phase: used to assess the efficiency of selective comminution.
• Texture: grain size indicates the potential for selective comminution; grain shape affects how the material responds to the applied loading mechanism; and surface roughness influences the strength of the grain, particularly its shear strength and fracture toughness.
• Structure: information regarding the spatial orientation and distribution of the target grains helps determine the appropriate size fraction; additionally, the degree of space filling can affect the overall material strength.

The particle breakage mode is closely related to the degree of liberation. Concrete is primarily composed of cement, coarse aggregates, and fine aggregates. The breakage mode should aim to promote the liberation of aggregates from the hardened cement paste. Gaudin and Kelly and Spottiswood [15,22] defined the liberation degree as a quantitative parameter expressed as the percentage of a specific mineral or phase that is free, relative to the total (free plus locked) particles of that mineral or phase, as shown in Equation (1). Although natural aggregates do not constitute a distinct phase or mineral, this concept can be adjusted to RCAs to estimate the liberation degree of natural aggregates from the cement paste. Ulsen et al. [27] demonstrated this concept in a study illustrating the liberation behavior of fine recycled aggregates through practical examples, in addition to including a dedicated section focused on definitions related to liberation.

$$Liberation degree (\%)={\displaystyle \frac{Phase of interest \left(free\right)}{Total of phase of interest (free+locked)}}\times 100$$

(1)

The degree of liberation can be expressed in terms of area or perimeter, and this distinction is important depending on the mineral application. Particles that are not liberated along their perimeter hinder the flotation process, as the incomplete exposure of the mineral surface reduces the efficiency of organic molecule adsorption. For RCAs, since the adhered cement paste typically surrounds the aggregate surface, it is more appropriate to express the degree of liberation based on the perimeter [22,28,29].

2.2. Comminution Equipment

Comminution, a pivotal stage in mining, is responsible for size reduction and phase or mineral liberation [30] and includes crushing and grinding stages. Gaudin [22] defined crushing as the process of reducing material size into smaller fragments. Since crushers perform this operation, their operational parameters are fundamental to promoting effective particle liberation. Crushers can be classified as follows [31]:

• Primary crushers: generally large machines with a reduction ratio of approximately 8:1, including jaw crushers, gyratory crushers, horizontal impact crushers, and rotary breakers.
• Secondary crushers: designed to further reduce particle size prior to grinding, with a reduction ratio between 6:1 and 8:1; these typically include cone crushers, horizontal impact crushers, and high-pressure grinding rolls.
• Tertiary crushers: characterized by a reduction ratio between 4:1 and 6:1; these typically include cone crushers and high-pressure grinding rolls.
• Quaternary crushers: also considered a coarse grinding stage, depending on the particle size range, typically including vertical shaft impact crushers (VSI).

Jaw, impact, and cone crushers are the most common equipment used in the production of RCAs. Additionally, a grinding stage, such as ball milling, can be employed to produce fine RCAs or remove superficial adhered cement paste [32]. Appendix A summarizes the different equipment used in the comminution stages, the corresponding breakage mechanisms, and their influence on the resulting products.

Comminution equipment can significantly influence the physical characteristics of the resulting products, such as particle size and shape. In jaw crushers, the open and closed settings (discharge conditions) determine the particle size distribution and generally produce a low concentration of fine particles [15].

In contrast, impact crushers generate particles with a cubic shape and a well-graded size distribution [15]. The potential for optimization in impact crushers is considerable, as modifications to components such as the blow bar material, the rotor speed, and the shape or angle of the impact plates can all affect the crushing force and the resulting particle shape [33].

Particles produced by cone crushers exhibit an intermediate angularity compared to those generated by horizontal and vertical impact crushers. The motion pattern of the cone crusher can also lead to the formation of elongated and flaky particles [34]. Roll crushers, on the other hand, produce fewer fine particles; however, their relatively low capacity restricts their widespread use in industry [15,35].

2.3. Advanced Separation Technologies

Effective comminution and liberation depend on the intrinsic properties of the materials and the associations among their constituent minerals or phases [36]. At this stage, our focus shifts from a macroscopic perspective to micro- and nanoscale interactions. Materials are composed of distinct phases, each comprising specific elements. These elements are predominantly arranged in crystalline structures, particularly in natural aggregates. Conversely, the main hydrated phase of the cement paste (C–S–H) exhibits an amorphous structure [2,3]. Separation technologies comprise a set of techniques selected based on differences in the physical or chemical properties of material phases, aiming to concentrate the desired phase, commonly by exploiting contrasts in magnetic susceptibility or density.

In mineral processing, gravity concentration refers to processes that exploit differences in particle density. In these methods, minerals with distinct specific gravities are segregated according to their relative motion under the influence of gravity and additional forces commonly the resistance imposed by a viscous medium such as water or air. These operations are designed to keep particles slightly apart, enabling differential movement and the formation of layers composed of heavy and light minerals [37,38].

While gravity concentration methods rely on differences in particle density to achieve mineral separation, magnetic separation techniques operate on differences in magnetic properties. Minerals that exhibit natural magnetism, or those in which magnetic properties are artificially induced by polarization, can be effectively separated from their gangue constituents when subjected to appropriate magnetic forces. The efficiency of this process depends primarily on three factors: the strength of the applied magnetic field, the magnetic susceptibility of the mineral, and the gradient or variation in the field strength. The magnetic behavior of materials, which originates from differences in their electronic structures, can be categorized into three main types: paramagnetism, diamagnetism, and ferromagnetism [39,40]. Appendix B provides examples of physical separation technologies commonly applied in the mineral processing industry.

In concrete, these properties are associated with the minerals that compose both the aggregates and the cement paste. Minerals such as quartz, calcite, and feldspar, commonly used as aggregates, have diamagnetic properties. These minerals are considered non-magnetic at 1.70 A and can be repelled by magnetic fields with stronger currents [41,42,43]. Cement, on the other hand, is composed of phases such as alite, belite, aluminate, and ferrite. Among these, ferrite is ferromagnetic and is therefore strongly attracted to magnetic fields [2,41]. These contrasting properties can be exploited to distinguish RCAs from cementitious material, resulting in higher-quality aggregates [44].

Carriço et al. [43] investigated phase liberation and separation processes to obtain a purified cementitious waste fraction and cleaner fine RCAs. Phase liberation was achieved during the comminution stage using a jaw crusher followed by a roller press, with 90% of particles within the 125 µm to 1.00 mm size range. A dry high-intensity magnetic separator was then applied to three particle size fractions: 125–250 µm, 250–500 µm, and 500–1000 µm. The reduction in particle size enhanced phase liberation, which in turn increased the magnetic recovery rate in fine products. Therefore, particles in a size range of 150–500 µm exhibited the most favorable response to magnetic separation. However, the finest particles (125–150 µm) were affected by agglomeration and deviations in trajectory caused by air flow, leading to limited phase differentiation. The magnetic fractions were primarily composed of cement paste, while a noticeable reduction in cement paste content was observed in the non-magnetic fractions across all particle sizes, demonstrating the effectiveness of the liberation process.

Ulsen et al. [44] investigated the potential of density and magnetic separation methods to separate recycled fine aggregates from cement paste, obtaining promising results. Fine recycled concrete aggregates were produced in a private recycling plant and classified by wet screening into different particle size fractions (2.0, 1.2, 0.6, 0.3, and 0.15 mm). These fractions were subjected to two different density separation methods using heavy liquids with three different densities (2.2, 2.5, and 2.6 g/cm³), elutriation with a density cut at 2.4 g/cm³, and magnetic separation using a Frantz barrier model with magnetic field intensities of 1.1, 4.0, and 8.9 kG. All separation tests yielded positive results, confirming the potential of these methods. Both density and magnetic separations demonstrated significant reductions in cement paste content (a lighter and magnetically responsive material), thereby improving the quality of the recycled sand. Notably, the heavy products obtained from density separation achieved a higher mass recovery (80%) than the non-magnetic products (60%).

Due to the density contrast between aggregates and cement paste, liberated particles can exhibit substantial variations in density, reflecting compositional differences. This variation allows for the application of gravimetric concentration techniques [21]. Equipment such as air sifters, floaters, jigs, and spirals can be used to obtain high-purity fractions of RCAs [45].

Angulo et al. [46] utilized density differences to produce high-quality RCAs from mixed construction and demolition waste through sink-float separation. This process enabled the separation and classification of materials based on their porosity. RCAs with lower porosity exhibited improvements, including reduced water absorption. Consequently, the authors concluded that the porosity of RCAs directly influences the compressive strength of the resulting concrete.

Malysz, Dal Molin, and Masuero [47] reported that RCAs processed through air jigging showed lower water absorption than aggregates that were only crushed. Furthermore, concrete produced using the sorted RCAs demonstrated enhanced compressive strength. These findings reinforce the importance of understanding the intrinsic characteristics of recycled materials to design more efficient processing routes and improve the overall performance of recycled concrete.

2.4. Comminution Process Flowsheets for RA Production

Building upon the abovementioned concepts regarding mineral processing and its importance for the quality of recycled aggregates, several authors have described the comminution process of transforming CDW into RAs. Appendix C summarizes data from the literature on the processing methods used to produce RAs, presenting the origin of the material, the water absorption of the recycled products, and their potential applications after recycling. Water absorption is one of the aggregate properties most significantly affected by the presence of adhered cement paste. Inadequate processing often cannot remove this cement paste effectively, leading to undesirable effects on water absorption. These data clarify the possible relationship between selected processing methods and the resulting RA properties, which in turn influence the suitability of materials for specific applications.

Özalp et al. [48] crushed and sieved CDW to produce RCAs in different particle size ranges. They analyzed the physical properties of the resulting material according to granulometric intervals and found that aggregates larger than 5 mm exhibited lower water absorption than those smaller than 5 mm. Since high water absorption in RCAs can negatively affect concrete strength, they concluded that aggregates larger than 5 mm are of superior quality. Omary, Ghorbel, and Wardeh [49] also reported that finer fractions (<4 mm) exhibited higher water absorption.

Ultrafine particle size fractions (under 0.15 mm) contain a high concentration of cement paste and a low concentration of natural aggregates, whereas larger particle size fractions contain greater amounts of natural aggregates and less cement paste [50]. Therefore, separating ultrafine particles rich in cement paste from other size fractions can enhance RCA quality.

According to Fan et al. [51], reducing RCA granulation produces materials with lower water absorption. This is likely due to the increase in release between the aggregates and the cement paste resulting from particle size reduction. The authors also separated the small fraction (0.15–4.75 mm) from the ultrafine fraction (<0.15 mm) after processing. Ulsen et al. [52] produced fine RCAs using two- and three-stage crushing, with tertiary crushing performed using vertical shaft impactors (VSI) operating at different rotational speeds. The results indicated that the fine RAs produced were less porous and more spherical than both the original CDW particles and those generated by secondary crushing. Moreover, differences in rotational speed had a measurable influence on RCA properties.

Gomes et al. [53] analyzed processing routes and observed that the use of a jaw crusher reduced water absorption by 5% compared to pre-fragmentation alone. The incorporation of a grinding stage further decreased water absorption by 41%.

Ogawa and Nawa [54] investigated fine RCAs produced through three different combinations of comminution methods. They observed that repeating each treatment step increased the density and decreased the water absorption of RCAs. Additionally, fine RCAs produced by ball milling exhibited superior quality, as the particles had a regular and rounded morphology.

Martínez et al. [55] noted that the use of RAs from various origin types in mortars is less frequently addressed in the literature than its use in concrete. They therefore produced RAs from ceramic, concrete, and mixed wastes for application in masonry mortars. Their findings showed that mortars made with these three types of RAs outperformed those made with natural aggregates, which they attributed to the uniform granulometric distribution of RAs and the lower quality of the natural aggregates currently available in Havana.

Saiz Martínez et al. [56] also used fine RAs from CDW treatment plants in the Madrid region and examined the potential to incorporate the resulting products into masonry mortars. They found that RAs with a water absorption rate between 5% and 10% were suitable for use in recycled mortars. Similarly, Miranda et al. [57] highlighted that the production of masonry mortars requires a substantial amount of natural sand. Since such mortars are non-structural and not exposed to moisture, substituting natural sand with fine RAs has proven to be a viable alternative.

Gong et al. [58] combined mechanical grinding with microbe-induced carbonate precipitation to process RAs. They used approximately 25-year-old CDW composed of concrete and bricks, which underwent jaw crushing followed by mechanical grinding, using a concrete mixer and steel balls at 45 rpm at different times. They found that longer grinding times reduced aggregate water absorption and increased apparent density. Although grinding effectively removed the mortar adhered to the original aggregates, it also created microfractures within the RAs, compromising their strength. To mitigate this, microbes were employed to induce carbonate precipitation, filling the fractures and pores in the RAs and improving their surface quality. This study demonstrated the effectiveness of combining mechanical and biological methods.

Ma et al. [59] explored the potential of producing fine RCAs by crushing coarse aggregates. The raw materials were categorized according to their mortar content. Recycled sand obtained from coarse aggregates with high mortar content had a low apparent density (2306 kg/m³) and high-water absorption (9.4%). After a second crushing, the same material showed improvement, with a bulk density of 2504 kg/m³ and a water absorption rate of 4.06%. Coarse aggregates with a low mortar content produced even better results, yielding a bulk density of 2706 kg/m³ and a water absorption of 1.69%. These results indicate that secondary crushing effectively reduces the amount of old mortar.

Kumar Vaishnav and Kumar Trivedi [60] conducted experiments on CDW using two different processing methods. One route employed a hammer and drill followed by sieving through a 4.75 mm sieve, while the other included additional jaw crushing and grinding. As expected, the first method produced aggregates with high water absorption (11.40%) and low apparent density (1280 kg/m³). In contrast, the fine RCAs subjected to crushing and grinding showed significant improvements, with water absorption reduced to 4.80% and apparent density increased to 1650 kg/m³. These improvements were attributed to the crushing and grinding process, which removed adhered mortar and reduced porosity.

Rakesh Kumar Reddy, Yaragal, and Sanjay [61] performed grinding tests on coarse RCAs using a ball mill, varying both the grinding time and the weight of the grinding media. They found that increasing both parameters decreased the water absorption of RCAs.

Gebremariam et al. [62] employed two innovative types of equipment for RCA processing: an air-based dry recovery system and a heating air classification system. The first is a mechanical process that separates fine fractions from wet crushed concrete aggregates using kinetic energy to break bonds between moisture and fine particles. The second exposes fine aggregates to hot gas, drying them and removing small contaminants such as wood and plastic fragments.

Some authors have conducted studies focusing on concrete with specific structural components. For example, Sainz-Aja et al. [63] processed waste from concrete sleepers using jaw crushing and screening, resulting in RCAs with relatively low water absorption, likely due to the properties and strength of the original concrete. Similarly, Venkrbec and Klanšek [64] worked with concrete waste from various building elements; however, the water absorption rate values they reported were comparable to those found in several other previous studies.

Other studies have shown that combining thermal and mechanical treatments can effectively produce high-quality fine RCAs [65,66,67]. In this approach, concrete waste is subjected to high temperatures, reducing its resistance to fragmentation and promoting the release of cement paste. However, this process has been found to be inefficient due to high energy consumption and CO₂ emissions [68,69].

Alternative processing methods, such as sonic and high-voltage impulse fragmentation, have also been explored to enhance RCA quality. Linß and Mueller [70] applied high-performance sonic impulses to crush materials, achieving better liberation of old aggregates with minimal damage and less adhered cement paste compared to conventional mechanical methods such as jaw or impact crushing. Hentges et al. [71] introduced electrodynamic fragmentation as a novel approach to concrete comminution, using different pulse intensities, and reported positive fragmentation results. The authors concluded that this method facilitated concrete fragmentation. Similarly, Dong et al. [72] employed high-voltage pulse processing of recycled coarse aggregates, which resulted in selective breakage between the old aggregates and cement paste. They also observed that the electrohydraulic effect improved the cleaning of RCA surfaces.

Appendix C highlights the relationship between water absorption rates and various factors such as particle size, waste type, and the processing method used in the production of RAs. Figure 4 presents the correlation of mineral processing method and water absorption by particle size (coarse and fine fraction). Does the poor data correlation suggest that mineral processing has no significant influence on the quality of RAs? The question cannot be readily answered without a critical evaluation of the minimum processing parameters provided by the evaluated articles.

3. Methodology—Bibliometric Research

This study is structured as a critical review, with the primary objective of providing an analytical and interpretative discussion of the literature. To ensure transparency and methodological rigor in the identification of relevant studies, a systematic literature search was conducted as an integral component of the review of the influence of mineral processing on RCA quality by application. This systematic stage was designed in accordance with the core principles of the PRISMA (Preferred Reporting Items for Systematic Reviews and Meta-Analyses) guidelines, including the definition of databases (Scopus), identification, eligibility criteria, and screening process.

Nevertheless, the present study does not aim to perform a comprehensive evidence synthesis or meta-analysis, nor does it seek to exhaustively capture all available studies within a predefined protocol. The systematic approach was therefore applied specifically to support the organization, selection, and traceability of the literature, while the subsequent analysis was predominantly qualitative and critical. For this reason, the full PRISMA checklist is not provided, as several items are not applicable to the scope and objectives of a critical review. The PRISMA framework was employed selectively to enhance transparency in the search and selection phases, without constraining the interpretative and conceptual focus of the review.

3.1. Identification

The literature search was conducted in the Scopus database on 22 July 2024, seeking studies published between 2004 and 2024. The search strategy was based on predefined query strings combining keywords related to recycled concrete aggregates and processing. An initial search was performed in the TITLE-ABS-KEY fields using the terms “recycled concrete aggregates” AND “applications”, combined using the Boolean operator AND. his search was subsequently refined by applying the truncation term “crush*” across all fields to capture variations related to crushing and comminution processes. A publication year filter (2004–2024) was then applied. The complete set of search strings, fields, filters, and the number of records retrieved at each step is summarized in

Table 2. Search strings and filters used in the Scopus database.

Step	Database	Search Field(s)	Search String	Filters Applied	Records Retrieved
1	Scopus	TITLE-ABS-KEY	“recycled concrete aggregates” AND applications	None	637
2	Scopus	ALL FIELDS	“crush*”	Applied as refinement of Step 1	342
3	Scopus	—	—	Publication year: 2004–2024	338
4	Scopus	—	—	Data export format: Excel	338

. All retrieved records were exported in Excel format for subsequent screening and analysis.

3.2. Screening, Eligibility, and Inclusion

The initial screening of the 338 retrieved records was performed based on title and document type. The inclusion criteria comprised original research articles published in English. The exclusion criteria included conference papers, books and book chapters, as well as publications not indexed in Scopus or lacking a DOI, resulting in the exclusion of 82 records. After this screening stage, 256 articles were deemed eligible for full-text assessment (see Figure 5). This screening process was conducted using a Microsoft Excel spreadsheet, with careful manual review supported by data-filtering functions to ensure consistent application of the inclusion and exclusion criteria.

The selected papers were then subjected to quantitative and qualitative analysis of their approaches to mineral processing to understand the process of comminution and which parameters were used to evaluate RA quality for the studied applications.

• Does the paper mention crushing and/or grinding (e.g., jaw crusher, impact crusher, ball mill)?
• What type of crusher and/or grinding was used?
• What is the parameter of the equipment described (e.g., discharge conditions, rotor speed, number of balls)?
• Does the paper mention separation techniques?
• What is the application of the recycled concrete aggregates?
• What primary properties (aggregate properties) were analyzed for quality control?
• What secondary properties (product properties) were analyzed for evaluation quality?

Of the 256 papers selected for full-text assessment, 52 did not provide sufficient information to address the guiding research questions and were therefore considered out of scope. Among these, 29 were review papers, 22 did not report any information related to mineral processing, and one paper was unavailable. For example, the study by Wijayasundara, Mendis, and Crawford [73], in which the authors compared the use of recycled concrete aggregates (RCA) with natural aggregates, focusing primarily on the financial implications. Another example is the work of Nataraja et al. [74], in which recycled concrete aggregates are mentioned in the abstract, but crushed vitrified tiles were used as the recycled material in the experimental investigation. As a result, a total of 204 papers (approximately 80% of the initially eligible documents) were retained and examined to quantify and analyze information related to RCA mineral processing [75,76,77,78,79,80,81,82,83,84,85,86,87,88,89,90,91,92,93,94,95,96,97,98,99,100,101,102,103,104,105,106,107,108,109,110,111,112,113,114,115,116,117,118,119,120,121,122,123,124,125,126,127,128,129,130,131,132,133,134,135,136,137,138,139,140,141,142,143,144,145,146,147,148,149,150,151,152,153,154,155,156,157,158,159,160,161,162,163,164,165,166,167,168,169,170,171,172,173,174,175,176,177,178,179,180,181,182,183,184,185,186,187,188,189,190,191,192,193,194,195,196,197,198,199,200,201,202,203,204,205,206,207,208,209,210,211,212,213,214,215,216,217,218,219,220,221,222,223,224,225,226,227,228,229,230,231,232,233,234,235,236,237,238,239,240,241,242,243,244,245,246,247,248,249,250,251,252,253,254,255,256,257,258,259,260,261,262,263,264,265,266,267,268,269,270,271,272,273,274,275,276,277,278].

4. Results and Discussion

4.1. Crushing Methods

About 48.5% of the analyzed documents mention crushing and/or grinding (see Figure 6A). The term “crush*” is found mostly in introduction and reference sections. The data highlights an absence of information regarding the mineral processing methodology for producing RCAs. How can producers ensure product quality in the absence of a standardized method? In the analyzed data, 51.5% of the authors did not report the information related to the term “crush*” in scientific articles.

For example, Saberian, Li, and Cameron [214] published a paper in which the term “crusher” appears in the title, but their study provides no details regarding the production process for the RCAs. Similarly, Pavlu et al. [174] investigated the use of RCAs in structural concrete but did not report any information on the crushing process. This omission highlights a broader absence of emphasis on optimizing the quality of RCAs for critical structural applications.

Among the 51.5% of articles that do not mention the crushing process, most studies (44.6%) apply RCAs in pavement construction. In theory, this application requires less stringent control of material parameters and is considered less dependent on processing conditions. However, the mechanical performance of pavement layers still depends directly on the consistency and quality of the RCAs. Primary properties such as gradation, angularity, and water absorption strongly influence secondary performance parameters, including compaction efficiency, load-bearing capacity, and long-term durability. The use of RCAs in sub-base layers is generally less restrictive than in base layers, where higher mechanical strength and stability are required. Nevertheless, even for sub-base applications, proper qualification and documentation of aggregate processing are essential to ensure adequate performance and reproducibility of results.

The lack of information regarding processing methodologies for producing RCAs remains a concern. Although some studies mention the use of crushing as a processing technique, approximately 46.5% of the papers do not specify the type of equipment or methodology employed (see Figure 6B).

Among the 53.5% of the analyzed studies (53 articles) that report the type of equipment used, some authors describe more than one crushing stage. For example, Sim and Park reported the use of jaw, cone, and roll crushers [201]. The reported data are presented as percentages, as shown in Figure 6C. Jaw crusher is the most frequently cited, followed by impact crusher. In certain cases, authors have even reported the use of manual crushing, often performed with a hammer, or basic milling procedures, revealing a non-structured approach to material processing. Such inconsistencies suggest that the choice of crushing technique is often guided more by equipment availability and cost considerations than by process efficiency or control of aggregate properties. Therefore, the systematic reporting of processing parameters, such as crusher type, operational settings, and number of crushing stages, should be considered essential for ensuring methodological rigor and advancing the scientific understanding of RCA performance.

Among the papers that discuss the equipment and/or processing methodologies employed, 3.8% provide specific settings or parameters used in their equipment, which corresponds to two studies (see Figure 7 and

Table 3. Overview of bibliometric analysis.

Screening	Bibliometric Analysis	Number
1	Scopus search	338 *
2	English language, journal articles, indexed publication	256 **
3	Bibliometric quantified papers	204
4	RCA applications? (204 papers)
		Number	Percentage (%)
	Concrete	94	46.1
	Pavements	91	44.6
	No application	19	9.3
5	Paper mentions crushing and/or grinding? (204 papers)
		Number	Percentage (%)
	Yes	99	48.5
	No	105	51.5
6	What type of crusher and/or grinding was used? (99 papers)
		Number	Percentage (%)
	Reported	53	53.5
	Not reported	46	46.5
7	Are the equipment parameters described? (53 papers)
		Number	Percentage (%)
	Yes	2	3.77
	No	51	96.2
8	Total number of articles reporting minimum processing parameters	Number	Percentage (%)
		2	0.98

*: Excluding 82 papers: books, non-indexed publications, conference papers, and non-English language. **: Excluding 52 non-accessible papers: out of scope, no access, reviews.

) detailed below. In other words, among the 53 articles that mention the type of equipment, only two report any corresponding operational parameters. Amin et al. [223] utilized a manual technique involving a hammer as a primary crusher to obtain coarse recycled concrete aggregates. The authors specify the number of balls used in the milling process, but do not mention the rotation speed.

This brings us back to the question: Does the poor data correlation suggest that mineral processing has no significant influence on the quality of RAs? This question cannot be answered reliably without a critical evaluation of the minimum processing parameters reported in the evaluated articles, because the available evidence is largely incomplete. This reporting gap undermines reproducibility and makes meaningful cross-study comparisons impracticable, as key attributes that control performance, such as gradation, angularity, and water absorption, are directly affected by processing procedures and, in turn, govern secondary outcomes including compaction efficiency, load bearing capacity, and long-term durability, even in pavement applications, which are often considered less restrictive. The overview of bibliometric analysis was presented at Table 3.

Grabois et al. [75] utilized RCAs previously produced by Pepe et al. [10]. CDW was processed using a jaw crusher, followed by particle size classification. Coarse RCAs, specifically gravel in two different sizes, underwent autogenous grinding in a tumbling mill. The authors provided details on the model of the jaw crusher, the dimensions of the autogenous mill, and critical settings such as rotations per minute and solid rate. This level of reporting represents an important benchmark for the minimum information necessary to achieve experimental reproducibility.

Crushers operate based on specific breakage mechanisms that determine how materials are broken apart. Each type of crusher has a unique configuration, which can result in varying breakage outcomes and can influence particle characteristics such as size distribution and shape. Additionally, different crushers have different size reduction capabilities. The settings of a crusher also impact particle size. For example, in jaw crushers, the open and closed side aperture affect particle size; in impact crushers, factors like the number and shape of the impact bars, shaft orientation (vertical or horizontal), and the rotor peripherical speed are crucial; and in cone crushers, the height of the cone in relation to the mantle plays a significant role [15].

Although rare, some studies present comprehensive descriptions of the processing route, setting a valuable benchmark for subsequent investigations. Diógenes et al. [279] noted that the crushing process and characteristics of the material significantly affect the properties of manufactured aggregates. For example, using a cone crusher typically produces material with higher sphericity and lower angularity. In contrast, products from a jaw crusher usually have particles with lower sphericity and higher angularity compared to those produced by impact and cone crushers. Fladvad and Onnela [280] analyzed how the parameters of a jaw crusher affect the characteristics of manufactured aggregates. They found that variations in the crushing process significantly influence particle shape. Additionally, the configurations of the crusher are directly linked to the maximum size of the produced aggregates. A reduction in feed rate also affects the final product, leading to less well-graded particles.

In several cases, data reported in scientific studies are conflated with application procedures, revealing a lack of methodological rigor that compromises data interpretation and reproducibility. While large-scale applications may justify the use of simplified processing methods due to economic and operational constraints, scientific research demands a higher level of scrutiny. Experimental reproducibility and critical evaluation require more comprehensive methodological descriptions and deeper intellectual engagement with the processing variables involved. Without such rigor, it becomes difficult to distinguish empirical practices driven by convenience from scientifically validated procedures capable of advancing the understanding of recycled aggregate behavior and performance.

A significant and well-structured improvement in crushing methods are often sufficient to ensure the quality of RCAs, even with minimal methodological complexity. When processing conditions are adequately controlled and documented, researchers and practitioners can develop the discernment needed to determine whether the application of advanced separation technologies is truly necessary.

4.2. Separation Technology

Approximately eight (3.92%) of the assessed documents mention separation as part of the processing of RCAs. The most frequently cited physical separation method is manual sorting or picking, which is commonly employed in the construction and demolition waste industry to remove materials such as wood, paper, plastic, and some metals [91,142,161,247]. Another highly cited separation method is magnetic separation, which is commonly responsible for metal material extraction and can be incorporated into all stages of CDW processing [119,161,246,247]. Both methods reveal more about practical applications than scientific contributions.

Hu et al. [148] addressed physical separation methods, employing air jigging technology to separate concrete from brick particles within the 5–10 mm size range, achieving a RCA product with 95 wt% purity. Jigging involves the vibrating fluidization of a vertical bed of particles. The continuous flow of expansion and compression facilitates particle stratification, resulting in separation based on material density [38]. Hu et al. [180] presented an innovative method for bricks-concrete separation based on particle shape and rotational surface treatment to reduce brick content (about 45–55% fewer bricks) and detach the adhered cement paste (about 25–30% less cement paste). They validated the proposed upgrading process under real industrial conditions by implementing it in an operating CDW recycling plant.

RCA applications have consistently steered the direction of scientific research; however, economic viability remains the prevailing factor in balancing environmental, technical, and scientific considerations. This imbalance endures largely because RCA research is predominantly disseminated through engineering-oriented journals, while no specialized journals devoted to the fundamental sciences of RCAs currently exist. Despite this context, there is an urgent need to broaden the horizons of discussion, avoiding the restriction of research perspectives to a single dimension, as such a tendency inherently favors financial bias over comprehensive scientific understanding.

Ulsen et al. [44] provide an example of utilizing separation techniques to enhance the quality of RCAs. In their study, laboratory-scale density and magnetic separations effectively reduced the presence of cement paste and red ceramic particles. Density separation (for products with d > 2.5 in heavy liquids and heavy elutriation fractions) achieved a mass recovery of 80%, while magnetic separation (non-magnetic at 8.9 kG) resulted in a mass recovery of 60%. Dry magnetic separation not only yielded positive results but also serves as a more sustainable alternative [44].

4.3. Applications and Measured Properties of Recycled Concrete Aggregates

The systematic bibliometric review examined the crushing methods used to produce RCAs, as well as their applications and evaluated properties. Which key properties are used to assess RCA quality?

Studies of these applications were divided into two main categories: pavement materials and cement-based materials, which include self-compacting concrete, conventional concrete, and mortar (see Figure 8).

The application of RCAs serves as a practical criterion for evaluating their primary and secondary properties. Primary properties refer to intrinsic characteristics of RCAs obtained after CDW comminution, whereas secondary properties are related to the application efficiency of the material.

4.4. Primary Properties

The primary properties evaluated in cement-based materials and pavements are quite similar. Figure 9 and Figure 10 display word clouds created from these properties. For both types of materials, particle size distribution and water absorption are key attributes. The size distribution of the aggregates is linked to the performance of the pavement layers [281] and can impact workability, cement-water ratio, and ultimately, the costs involved in concrete production [3,7].

Water absorption, defined as the increase in mass percentage due to the water content present in aggregates with saturated pores, is a critical property that can impact the resilience and durability of concrete [3,282]. High water absorption can negatively affect the workability and strength of RCAs, compromising the properties of the concrete in which they are utilized. This is largely due to the CP that adheres to the RCAs, resulting in higher water absorption than that of natural aggregates [6,283,284]. Additionally, water absorption serves as a parameter to indicate the amount of asphalt absorbed by aggregates in pavement applications [7].

Specific gravity is a crucial property to analyze, particularly for aggregates used in concrete production. Like water absorption, the specific gravity of aggregates can be affected by the presence of CP. When the concentration of CP in RCAs increases, the density decreases. This reduction in density impacts the hydration process of the cement paste in the new concrete [8].

The Los Angeles abrasion test, also known as the aggregate abrasion value, measures the resistance of particles to degradation caused by abrasion and impact forces [7,282]. A high degradation value, determined by the amount of material that passes through a sieve, can indicate greater potential for material degradation and dust generation during construction [7]. Additionally, abrasion resistance can impact the lifecycle of road pavement [285].

All these primary properties may be influenced by an appropriate processing methodology that promotes selective crushing and phase separation. Such an approach may minimize the amount of CP and, consequently, reduce water absorption, while potentially increasing specific gravity and enhancing the abrasion resistance of RCAs [163].

Table 4. Standard limits for recycled aggregate applications in concrete.

Country	Standard	RA Type	Particle Density (kg/m3)	Water Absorption (%)
Germany	DIN 4226-100 [286]	Concrete rubble	≥2000	≤10
		Demolition debris		≤15
		Brick rubble	≥1800	≤20
		Coated rubble	≥1500	-
Brazil	NBR 15116 [287]	Concrete	-	Coarse: ≤7 Fine: ≤12
		Mixed	-	Coarse: ≤12 Fine: ≤17
International	RILEM 1994 [288]	Masonry rubble	≥1500	≤20
		Concrete rubble	≥2000	≤10
		Mixed	≥2400	≤3

summarizes standard limits for particle density and water absorption as key parameters that define the quality and suitability of RAs for use in concrete. According to Standard DIN 4226-100 [286] (Germany), RAs made of concrete rubble should exhibit a density above 2000 kg/m³ and water absorption below 10%, while RAs made of brick rubble may reach values up to 20%. The Brazilian Standard NBR 15116 [287] establishes higher permissible absorption limits, especially for mixed aggregates, whereas RILEM guidelines specify minimum densities between 1500 and 2400 kg/m³ and maximum absorptions ranging from 3% to 20%, depending on the aggregate’s origin. These standards provide essential benchmarks for evaluating RAs and guiding their appropriate application in construction materials [286,287,288]. Most standards only select RA type by application and not by particle size distribution, with only Brazilian standards splitting it into fine and coarse [287].

One study of Alberte and Handro (2021) [289] provides a comparative analysis of international technical standards governing the use of recycled aggregates across different applications in the construction sector. Most standards emphasize coarse RAs and their use in pavements or base layers, while specific guidelines for mortars and fine fractions are still lacking. In Brazil, despite alignment with international standards, regulations are outdated and require revision to reflect current advances in material performance and sustainability. The absence of detailed procedural guidance covering production, quality control, and practical applications limits broader and more effective adoption of recycled aggregates in construction [289].

The size of the particles is highly connected to the type of crusher (and consequently the crushing mechanism) chosen, influencing not only the size of the aggregate but also its shape [282]. Water absorption is one of the most relevant primary properties of RCAs. It is indirectly related to the CP adhered to the particles. However, the standardized procedure for determining water absorption remains rudimentary, relying on surface drying with absorbent paper for coarse RCAs and on air or heat flux (like a hair dryer) for fine RCAs. Water absorption, being the key indicator of RCA quality, is subject to considerable experimental uncertainty. However, the examined articles do not comment on the merit or limitations of this imprecision, despite its centrality to the qualification of RCAs according to current standards.

Particle size distribution is an important parameter that influences a material’s suitability for different applications and market categories. It affects packing, workability, and mechanical behavior, helping to define processing efficiency and compliance with technical specifications [290].

Specific gravity is measured by water pycnometer, with measurements performed by Le Chatelier’s principle (Chapman flask). Methods such as gas pycnometry governed by Boyle’s Law can replace water pycnometry, increasing the degree of trust and reliability by decreasing the influence of the operators. The procedure consists of drying the aggregate sample in an oven at 110 ± 5 °C to constant mass, cooling it, and then immersing it in water for 24 ± 4 h (or 72 ± 4 h for lightweight aggregates). After immersion, the sample is surface-dried with an absorbent cloth to remove visible water films, weighed in the saturated surface-dry (SSD) condition, submerged in water to determine its apparent mass, and finally oven-dried again to obtain the dry mass. However, this procedure is highly sensitive to operator influence, since variations in drying technique and handling time can alter the measured SSD condition. Moreover, the presence of air bubbles trapped within the aggregate pores and the possible reactivity of fine cementitious residues agglomerated on the coarse aggregate surface in contact with water can significantly affect the accuracy of both absorption and specific gravity measurements [291,292].

4.5. Secondary Properties

The secondary properties of materials containing RCAs can be directly influenced by their primary properties. Typically, the most evaluated properties of cement-based materials involve their resistance to the application of different forces (see Figure 11).

The properties of aggregates can significantly impact the strength of cement-based materials [293,294]. In addition to particle size distribution, shape, surface texture, and mineral composition, factors such as porosity and the ITZ also play a crucial role in how concrete responds to stress. Aggregates with high angularity can affect the water-to-cement ratio, which in turn influences the workability of the concrete [3,295,296]. The CP is most commonly reported as being responsible for the poor application of RAs; however, the reactive aggregates can also be considered an inherent problem. The RA comminution process can generate new faces in reactive aggregates, thereby exposing fresh silica surfaces, or induce microcracking that facilitates the ingress of alkalis and promotes alkali-silica reaction (ASR) [297,298,299]. The use of supplementary cementitious materials, as well as lithium nitrate and calcium nitrate, has been shown to be effective in mitigating ASR [300,301].

The surface texture and shape of aggregates also influence the water/cement ratio, which in turn affects the bond between the aggregates and CP [295]. The concrete with RCAs exhibits two types of ITZ: one related to adhered cement paste and a second between RCAs and the new cement paste [302]. This region can become strength limiting in concretes. By correctly choosing processing methods, it is possible to deliver RCAs with characteristics that minimize the effects of cement-based materials [3].

Secondary parameters for pavements are influenced by the characteristics of components such as aggregates. Most of the evaluated studies focus on the use of RCAs in the base and sub-base layers of pavements. The properties shown in Figure 12 are related to the soil’s compaction capacity. Dry density (TAP density) is one indicator of soil compaction and its load-carrying capacity. Higher densities lead to greater resistance, lower permeability, and minimized volume changes due to weathering [7]. At low moisture contents, cohesion between particles can increase, enhancing strength in the base and subbase layers. Conversely, at high moisture levels, the rise in pore pressure reduces effective stress, decreasing resistance [281].

The characteristics of RCAs are primarily influenced by the cement paste content, which can affect properties such as dry density, moisture content, and compaction capacity [303,304,305,306]. RCAs typically exhibit low-density, high-water absorption, and increased porosity, which in turn influence the minimum dry density and optimal moisture content. Additionally, their lower strength and higher deformation capacity can increase the potential for particle breakage during compaction [282,307].

Globally, the use of RCAs remains predominantly concentrated in road engineering applications, particularly in base and sub-base layers, where performance requirements are moderate and large volumes of material are required. In the United States, legislation permits the use of RCAs in pavements; in the city of San Francisco, the scope of use is extended to general non-structural concrete works such as sidewalks, curbs, and urban infrastructure elements [308].

In Hong Kong, Germany, and Denmark, regulations allow the use of up to 100% concrete-based or mixed RCAs in non-structural concretes with compressive strengths of approximately 20 MPa, provided that the aggregates meet basic limits on contaminants and particle cleanliness. In this context, the quality demand is intentionally low, as the priority is high-volume utilization at the lowest possible cost, making pavement layers the most economically rational application [308].

For ordinary structural concrete, such as columns, solid slabs, and concrete pavements within the strength range of 20–30 MPa, partial replacement is generally recommended. The United Kingdom permits the use of approximately 20% RCAs in concretes between C20/25 and C40/50, provided they are not exposed to chloride attack. Although Portugal permits up to approximately 25% RCAs in concretes of 20–35 MPa, most RCAs in the country are still directed to backfilling rather than structural concrete [308].

Some countries formally allow the use of RCAs in medium to higher strength concretes above 40 MPa. However, robust certification chains and strict quality assurance procedures limit the practical implementation of such applications. Germany stands out in this regard, as its guidelines provide graduated substitution limits that correlate the allowable RCA content with the required compressive strength class, offering one of the most technically coherent regulatory frameworks currently in operation [308].

Although national standards and guidance documents define quality parameters for RCAs, they tend to organize applications primarily based on the end-use category, such as structural or non-structural, rather than on performance-based qualification of the RCAs themselves. As a result, the suitability of RCAs continues to be judged mainly by application class rather than by processing route and material performance. Improving RCA quality standards, particularly by incorporating processing requirements and performance-based classification, represents an important turning point. This improvement would enable more precise selection of RCAs for specific concrete applications, support confidence in their structural use, and contribute to the reduction in natural aggregate extraction without compromising functionality or durability.

5. Best Practices in Mineral Processing for RAs

The identified gap in both scientific literature and regulatory frameworks motivated the development of this study. As previously discussed, mineral processing is often regarded as a secondary concern in RA production, despite being the only reliable means of producing a specified quality of RAs. In current practice, mineral processing is typically evaluated from an energy or economic cost perspective, rather than as a technical pathway for quality enhancement.

Over the years, Ulsen has developed a series of studies that apply mineral processing principles to construction and demolition waste (CDW) recycling, providing a consistent example of how structured processing approaches can be used to produce RAs with improved quality. Research addressing the degree of liberation and phase associations in recycled sand has been shown to be essential for improving the quality of fine recycled aggregates and for guiding the design of recycling processes [27]. Comparative studies involving different crusher types for coarse RA production [309], the application of vertical shaft impact crushers to obtain higher-quality recycled sand [52], and laboratory-scale investigations of density and magnetic separation [44] further demonstrate how processing choices directly influence aggregate quality. In addition, an industrial-scale study on the production of high-quality recycled sand showed that pre-operational tests were fundamental for defining appropriate operating conditions [310]. Collectively, these studies support the relevance of adopting a structured and systematic processing guideline similar to those used in mineral processing as a basis for improving the quality and consistency of RAs.

A simplified mineral processing model was developed to address this limitation and provide clearer guidance for decision-making (see Figure 13). This model establishes a set of minimum reproducible requirements for RA treatment and aligns these requirements with main RA applications. In this way, it enables the systematic selection of processing routes according to the target performance level, contributing to their practical and technically grounded use in concrete.

A compilation of relevant mineral processing information was selected (

Table 5. Minimum processing information to be reported.

Process Condition	Critical Condition
	High	Medium	Low
Type of equipment	X
Number of stages	X
Screen size	X
Separation methods	X
Top size	X
Material recirculation			X
Feed			X
Moisture condition			X
Equipment operational conditions	X
Sampling methodology		X
Test standards	X

), and the level of importance of each parameter to be reported was specified, aiming at minimum mineral processing reproducibility. The processing information that must be reported includes the type of equipment, number of processing stages, screen size, separation methods, and top size, because these parameters directly determine the comminution/classification and separation pathway that controls RA quality. In addition, equipment operational conditions (e.g., operating settings) and the test standards used are highly critical items, since they govern process intensity and how performance is measured, respectively. In the absence of these details, independent replication is not feasible, and cross-study comparisons of mineral processing outcomes are inherently unreliable.

All conditions related to the equipment used for processing RAs are of high importance. The type of equipment employed, whether crushers or grinding devices, as well as their operational parameters, is critical to ensuring reproducibility. In addition, the number of processing stages, screen sizes, and top-size settings are equally important, since particle size is one of the primary properties to be controlled. Separation methods and their operational conditions are also highly critical, as they can selectively separate materials according to physical characteristics, such as reducing the cement paste content. Furthermore, the test standards adopted must be explicitly cited, as this information directly supports the reproducibility and comparability of the study.

Sampling is another key aspect, due to the inherent heterogeneity of recycled materials. Properly specifying the sampling methodology increases reproducibility and allows the representativeness of the samples studied to be demonstrated. However, the number of samples and the minimum sample mass required to achieve representativeness with acceptable variability and error depend on the specific characteristics of the material.

The parameters classified as less critical for reporting are mainly related to operating logistics and feed condition specifically material recirculation, feed description, and moisture condition because although they may influence short-term performance, they do not define the core processing route to the same extent as equipment selection, stage configuration, and separation and classification settings.

Information related to feeding conditions and material recirculation in closed-circuit systems is considered less important. Feeding conditions generally exert a limited influence on crushing outcomes. Similarly, material recirculation is not regarded as a highly critical parameter, provided that the top size and/or screen size is specified, as these parameters already govern the final particle size distribution. Moreover, the moisture condition of the material is not considered a relevant parameter, since the processing is conducted under dry conditions.

6. Future Research Perspectives

Research efforts are encouraged to prioritize the definition and reporting of a minimum, standardized set of processing descriptors that enables meaningful replication and benchmarking, including crusher type and configuration, number of crushing stages, screen cut sizes and classification strategy, top size, recirculation strategy, applied separation steps, key operational settings when available, moisture condition, and the testing standards adopted. In parallel, experimental programs are encouraged to isolate and quantify processing effects through sufficiently large datasets that support robust statistical analyses, enabling the individual and combined contributions of each unit operation to be evaluated across both simple processing routes, such as comminution, screening, and fines management, and advanced routes, such as magnetic separation, density-based separations, thermal treatments, sensor based sorting, and related upgrading technologies. Such unit operation level evidence is essential to distinguish genuine process-driven improvements from variability associated with feed composition, to identify the most cost-effective interventions, and to define technically grounded pathways for achieving target quality classes.

Another priority is the adoption of well-structured sampling strategies tailored to the inherent heterogeneity of recycled aggregates. Future work is encouraged to treat sampling as a core methodological component rather than a procedural detail, recognizing that batch-to-batch heterogeneity originates at the construction site and is driven by variability in source materials, demolition practices, contamination, and on-site segregation. Accordingly, studies are encouraged to explicitly define lots, justify sample mass and number of increments, and ensure adequate spatial and temporal coverage and replication, so that laboratory outcomes are demonstrably representative of plant scale production rather than isolated measurements. Sampling protocols should align with the intended application and with the variability of the incoming waste stream and should be reported in sufficient detail to support error and uncertainty assessment, including the rationale for minimum sample mass, number of samples, and acceptable limits for variability.

In addition, future research is encouraged to systematically explore pre-concentration approaches and advanced separation strategies as mechanisms to simplify downstream mineral processing and its efficiency. Early removal of adhered CP and targeted concentration of mineral phases can stabilize feed quality, reduce unnecessary comminution and classification effort, and enable more predictable compliance with specification limits. Comparative studies are encouraged to evaluate the effectiveness of different pre-concentration and separation routes in terms of both quality outcomes and operational impacts, including yield, energy demand, equipment loading, fines generation, and sensitivity to moisture and contamination. This should include direct, side-by-side evaluations of conventional flowsheets against hybrid configurations that integrate advanced separations to determine when added complexity produces a measurable performance benefit.

Finally, future work is encouraged to develop standards specific to RA processing as a prerequisite for consistent quality and broader market adoption. Research can support standardization by proposing minimum processing requirements, defining critical control parameters and acceptable operating windows, establishing quality classification thresholds linked to end use performance, and aligning test methods and reporting formats. Standardization would reduce interplant variability and improve traceability, enabling designers, contractors, regulators, and asset owners to specify recycled aggregates with greater confidence. By connecting verified processing controls to predictable performance in real applications, such standards can increase market trust, strengthen procurement and compliance pathways, and accelerate the technical maturation of recycled aggregate processing from case specific practice to reproducible, comparable, and performance-based engineering.

7. Conclusions

This review demonstrates that, although RCAs have been extensively studied over the past two decades, mineral processing has remained critically underexplored and underestimated. Operational parameters are rarely reported in the analyzed studies (only 2 studies, less than 1%), making it impossible to establish meaningful correlations between processing parameters and aggregate quality. As a result, the scientific discourse tends to focus on primary and secondary property classifications without addressing the fundamental factor that defines both: the processing route itself.

The distinction identified between primary properties (aggregate-focused, low robustness, operator-dependent) and secondary properties (concrete-focused, standardized, and widely applied) highlights a structural imbalance in how recycled aggregates are evaluated, specified, and ultimately used. Although application-based classifications for pavements and new concrete are well established, the lack of reproducible and practical guidelines for mineral processing remains a major barrier to improving material performance and expanding structural applications.

Therefore, the central contribution of this work is the proposal of a simplified mineral processing framework that defines minimum reproducible conditions and application-oriented pathways. Beyond consolidating current knowledge, this best-practices guideline serves as a structured reference for future research, supporting the systematic design, comparison, and optimization of processing routes for recycled aggregates. The proposed model provides a practical basis for the quality-oriented selection and treatment of recycled aggregates, promotes more consistent performance outcomes, and contributes to reducing dependence on natural aggregates. By addressing this gap, the framework establishes a foundation for more rational, efficient, and technically grounded use of recycled aggregates in concrete, while also guiding future investigations toward more reproducible and performance-driven approaches.