Refuse-derived fuel in cement manufacturing: a systematic review of research trends, gaps and future research directions

Cement is the world's most widely used construction material, forming the backbone of modern infrastructure and the built environment. Its indispensable role has led to sustained growth in global demand (Uratani and Griffiths, 2023). However, cement manufacturing is also one of the most carbon-intensive industries, accounting for an estimated 8% of total annual anthropogenic carbon dioxide (CO₂) emissions (Moumin et al., 2020) and ranking as the third-largest source of global CO₂ emissions (Andrew, 2018, 2019). This significant carbon footprint arises primarily from two sources: the release of CO₂ during the calcination of limestone in clinker production and the extensive combustion of fossil fuels required to heat rotary kilns to high temperatures (Benhelal et al., 2013; Bosoaga et al., 2009; Gao et al., 2015; Wojtacha-Rychter et al., 2021). Traditionally, cement kilns have relied on fossil fuels, notably coal and petroleum coke, as their primary sources of thermal energy, making them significant contributors to the industry's greenhouse gas emissions (Belbute and Pereira, 2020; Benhelal et al., 2013). As the urgency to address climate change intensifies, reducing emissions from cement manufacturing has become a critical sustainability priority.

Among various decarbonisation strategies, including carbon capture and storage, process optimisation and clinker substitution (Amiri and Vaseghi, 2015; Atmaca and Atmaca, 2022; Çimen, 2021; van Deventer et al., 2021; Poerner and Rimpel, 2017), fuel substitution using alternative fuels, particularly refuse-derived fuel (RDF), has emerged as a practical and scalable pathway (Bień, 2021; Güereca et al., 2015). Within the broader cement manufacturing process, the simultaneous use of RDF alongside conventional fossil fuels during clinker production is referred to as co-processing (Long et al., 2022). RDF co-processing offers dual benefits: reducing fossil fuel dependence and diverting waste from landfill, thereby supporting climate mitigation, sustainable waste management and circular economy (CE) objectives (Chandrasekhar and Pandey, 2020; Imbabi et al., 2012). In this context, RDF represents a critical interface between waste management systems and industrial decarbonisation efforts.

Despite the growing interest in RDF as an alternative fuel, its co-processing in cement manufacturing remains underexplored within sector-specific literature reviews. Existing reviews have largely focused on RDF production systems or technology-specific feasibility, with limited attention to its application within cement manufacturing as an integrated system. For instance, Tahir et al. (2024) reviewed sustainable RDF production in waste-processing facilities, focusing on municipal solid waste (MSW) treatment systems rather than end-use applications in cement kilns. Similarly, Chavando et al. (2022) offered a broad overview of RDF characteristics and applications, while Shehata et al. (2022) examined RDF in the context of CE and the Sustainable Development Goals, without addressing cement-sector-specific operational dynamics. More targeted reviews, such as Sharma et al. (2022) and Tihin et al. (2023), remain predominantly technology-oriented, emphasising combustion processes, feedstock preparation and kiln compatibility.

As a result, the existing literature lacks a comprehensive, sector-specific synthesis that integrates the conceptual, regulatory and sustainability dimensions of RDF co-processing in cement manufacturing. In particular, inconsistencies remain in how RDF and solid recovered fuel (SRF) are conceptualised within the literature. While SRF is frequently discussed alongside RDF in cement-kiln fuel substitution studies and is commonly treated as a waste-derived fuel (Chavando et al., 2022; De Gisi et al., 2018), variations in terminology and regulatory interpretation contribute to an inconsistent understanding of the field. Although RDF is widely recognised as a sustainable waste-to-energy (WtE) solution that supports CE objectives (Shehata et al., 2022), existing studies have not explicitly positioned RDF within the combined context of CE principles and the waste management hierarchy (WMH), particularly in relation to cement-kiln co-processing. This limitation is important because RDF utilisation in cement kilns is influenced not only by fuel properties and technological feasibility, but also by operational requirements, policy frameworks and broader sustainability considerations. Without a more integrated perspective, current knowledge remains insufficient to support coherent academic understanding and effective large-scale implementation within the cement sector. To position the contribution of the present study, Table 1 summarises the key characteristics, contributions and limitations of major review studies identified in the literature.

As illustrated in Table 1, existing review studies have provided valuable insights into RDF production systems, combustion technologies, sustainability implications and waste-management applications (Chavando et al., 2022; Shehata et al., 2022). However, existing reviews remain fragmented in scope and predominantly technology-oriented, with limited attention given to the conceptual distinction between RDF and SRF, regulatory frameworks governing RDF utilisation and the positioning of RDF within CE and WMH frameworks specifically in the context of cement manufacturing. Moreover, current reviews have not developed a comprehensive sector-specific research agenda that integrates these dimensions within a unified analytical framework. Therefore, there remains a clear need for a comprehensive, sector-specific systematic review that integrates the conceptual, regulatory, sustainability and research and development dimensions of RDF co-processing in cement manufacturing. Addressing these gaps is particularly important for supporting the sustainable integration of RDF within cement manufacturing systems and for guiding future research, policy development and industrial implementation.

Accordingly, the aim of this study is to systematically review and synthesise the current state of knowledge on RDF co-processing in cement manufacturing, with particular attention to RDF/SRF conceptualisation, regulatory frameworks, CE positioning and future research directions.

Within this context, the following research questions are proposed:

The study, therefore, seeks to advance a more coherent understanding of RDF co-processing by addressing conceptual fragmentation and developing an integrated, sector-specific research agenda for cement manufacturing. The remainder of the paper is structured as follows. Section 2 outlines the methodology adopted for the systematic literature review (SLR). Section 3 presents the findings and discussion of the review. Section 4 discusses future research directions and develops a research agenda. Finally, Section 5 concludes the study.

This study conducted an SLR, a legitimate method for conducting an extensive review of the literature to frame a research area (Azarian et al., 2023). An SLR provides a structured framework for evaluating, classifying, selecting and critically assessing prior studies to create new knowledge (Azarian et al., 2023; Khaw et al., 2024; Yildizbasi et al., 2025). Since SLRs are based on a strict set of guidelines, the process followed is scientific, transparent and replicable (Tranfield et al., 2003). Accordingly, this SLR intends to capture scattered knowledge on the use of RDF in cement manufacturing to generate new insights. This review follows the five-step approach for conducting SLRs proposed by Denyer and Tranfield (2009) (see Figure 1), which has been used successfully in previous SLRs (Ali et al., 2017; Tennakoon et al., 2022, 2023).

The subsequent sections provide a comprehensive explanation of each step.

A well-formulated review question is fundamental to shaping the direction of an SLR, as it determines the inclusion criteria, guides the development of the search strategy and defines the scope of data extraction (Denyer and Tranfield, 2009). Accordingly, the present study is guided by four review questions, previously introduced in Section 1 and reiterated here for clarity: RQ1: What is the current state of the art of research on RDF co-processing in cement manufacturing?, RQ2: How are RDF and SRF conceptualised and regulated within the context of cement manufacturing?, RQ3: How is RDF positioned within circular economy and waste management frameworks in cement manufacturing? and RQ4: What are the future research directions for advancing RDF use in cement manufacturing?.

The process of locating relevant studies began with selecting appropriate databases and keywords. Several academic databases, including Emerald Insight, Web of Science (WoS), Scopus, ScienceDirect and Taylor and Francis, were initially evaluated for their relevance and coverage. Among them, Scopus and WoS yielded the highest number of relevant results and are widely recognised for their multidisciplinary scope and inclusion of peer-reviewed literature (Zhu and Liu, 2020). Ultimately, WoS was selected as the primary database for this study due to its rigorous inclusion criteria and emphasis on academic quality (Li et al., 2018).

The search string was developed through an initial scoping review of studies related to RDF, SRF, waste-to-energy (WtE) and cement manufacturing to ensure alignment with the research questions. The search terms were structured around three main concept groups: (1) RDF/SRF terminology, (2) energy and waste recovery terminology and (3) cement-sector terminology. Although this review is primarily framed around RDF, the scope also includes SRF because it is frequently discussed alongside RDF in cement-kiln fuel substitution studies and is commonly regarded as a more standardised form of waste-derived fuel (Chavando et al., 2022; De Gisi et al., 2018). Accordingly, both “solid recovered fuel” and “SRF” were included as keywords to ensure comprehensive coverage of relevant studies.

The term “co-processing” was not included as a mandatory standalone search term. Preliminary scoping indicated that relevant studies were already captured through combinations of cement-sector and energy-related keywords. Furthermore, many studies examining RDF utilisation in cement kilns refer more broadly to alternative fuels, energy recovery, clinker production, or fuel substitution rather than explicitly using the term “co-processing” (Genon and Brizio, 2008; Hemidat et al., 2019; Patel and Chauhan, 2014). Therefore, broader cement manufacturing and energy-recovery terminology was adopted to ensure comprehensive coverage of studies related to RDF utilisation within cement manufacturing and cement-kiln fuel substitution. The final search string used in the database is presented as follows:

The search string above is presented exactly as entered in the database to improve transparency and reproducibility. In line with bibliometric evidence from Sarquah et al. (2022), which indicates that RDF-related literature began emerging around 1991, the search period was set from 1991 to July 2025 to ensure comprehensive coverage of the research landscape.

The preferred reporting items for systematic reviews and meta-analyses (PRISMA) was used as a guideline for documenting the study selection process. It assists in planning the study in a way to improve the transparency, accuracy, quality and completeness of the review (Moher et al., 2009). Figure 2 presents a simplified depiction of the PRISMA flow diagram, which is used in this study.

“ALL” fields from the WoS database were searched using the search string established earlier. Accordingly, the initial search resulted in 218 articles. Before proceeding to the screening, seven records written in languages other than English were removed. Therefore, 211 English-language records published between 1991 and July 2025 were retained for descriptive analysis. At the screening stage, titles, abstracts and conclusions were systematically reviewed to assess relevance to the review questions and the cement manufacturing context (Davey et al., 2011). Studies were retained if they explicitly addressed RDF or SRF utilisation, energy-recovery applications, or cement-kiln-related processes. Records were excluded at this stage if they focused on unrelated sectors, did not involve cement-based applications, or lacked sufficient information to determine relevance. This stage served as a preliminary filtering step prior to full-text assessment.

A comprehensive screening process was conducted on 211 records to select articles for content analysis. 18 inaccessible articles and 28 review or book chapter articles were excluded to minimise overlap and potential bias in the full-text synthesis. A further 126 articles were removed as they did not meet the inclusion criteria related to RDF use in cement manufacturing. Specifically, studies were excluded if they: (1) focused on RDF production, waste management, or energy recovery without reference to cement kiln applications, (2) examined RDF utilisation in non-cement sectors such as agriculture, general manufacturing, or power generation or (3) did not provide sufficient evidence of RDF integration within cement manufacturing processes. Relevance was determined based on whether the study explicitly addressed RDF or SRF use in cement kilns, clinker production, or cement-related energy substitution. This process resulted in 39 articles being selected for full-text review.

To enhance the completeness of the dataset, a secondary search was conducted using Google Scholar to identify potentially relevant studies not captured in the WoS database, particularly recent publications and non-indexed academic sources. Google Scholar was selected due to its broader indexing coverage, including grey literature and early-access publications that may not yet be indexed in WoS, thereby reducing the risk of database bias. This approach is consistent with prior systematic review studies by Kabirifar et al. (2020) and Tennakoon et al. (2023). In parallel, a snowballing technique was employed through backward and forward citation tracking to identify additional relevant sources (Erdiaw-Kwasie et al., 2024). This approach was particularly useful for identifying regulatory and policy-related documents referenced within the reviewed studies (Evans et al., 2012; O'Connor et al., 2024). Accordingly, seven regulatory documents were incorporated as grey literature to capture sector-specific regulatory perspectives that are not typically represented in peer-reviewed journal databases. Grey literature refers to materials produced outside traditional academic publishing and indexing systems, including policy documents, industry reports and other non-indexed scholarly outputs (Benzies et al., 2006; El jaouhari et al., 2025; Mahood et al., 2014). Such sources are particularly valuable in applied fields, as they provide critical insights into regulatory frameworks and policy actions that are often underrepresented in academic literature. The inclusion of grey literature therefore enhances the contextual relevance of the review. The combined process resulted in a final dataset of 48 records for full-text review and subsequent content analysis (refer to Figure 2).

Informed by Grant and Booth (2009), the results of this SLR are presented in two categories: descriptive analysis and content analysis. The descriptive analysis of the articles offers a solid foundation for subsequent content analysis (Vaismoradi et al., 2013). The articles were organised by year of publication, geographic contribution and keyword frequency to address RQ1. In the content analysis, the selected articles were synthesised into distinct themes using an inductive approach to address RQ2 and RQ4.

Strategies should be implemented to enhance the study’s trustworthiness and analytical rigour (Thisakya and Dissanayake, 2026). Potential bias in study selection and reporting was minimised by involving four researchers. Initially, the first researcher selected, evaluated and analysed the review findings. Any ambiguities that arose in this process were resolved with the involvement of the other three researchers.

Findings from the descriptive and content analyses are presented in this section.

The distribution of publications based on the year (Section 3.1.1), geographical location (Section 3.1.2) and co-occurrences of keywords (Section 3.1.3) was analysed under the descriptive analysis to discern trends in research on the use of RDF.

Figure 3 illustrates the annual distribution of publications related to RDF in cement manufacturing. Although previous studies have identified RDF-related publications as early as 1991 (Sarquah et al., 2022), no publications from1991 that met the scope and inclusion criteria of the present review were identified. Accordingly, the earliest relevant publication included in this review was published in 1992. Research output remained relatively low and sporadic until a noticeable rise after 2013. Since then, publication numbers have fluctuated, with dips in 2017, 2019 and 2022, yet the overall trajectory has been upward, culminating in a peak of 24 publications in 2023. Over 80% of the reviewed articles (f = 173) were published within the past 11 years, demonstrating strong and sustained research growth despite short-term fluctuations. This trend is consistent with previous studies highlighting increasing global pressure on the cement industry to reduce greenhouse gas emissions and transition toward alternative fuel systems (Benhelal et al., 2013; van Deventer et al., 2021; Farfan et al., 2019). With eight articles already published in 2025, this trend suggests continued academic and industrial interest in sustainable practices within the cement manufacturing sector.

Figure 4 presents the global distribution of RDF-related research in cement manufacturing, categorised by colour gradient to indicate publication frequency (1–29). China leads with the highest number of publications (f = 29), followed by Germany (f = 21), Poland (f = 19) and Brazil (f = 12). Europe emerges as a central hub, with substantial contributions from the UK, Italy, the Netherlands and Austria. The strong European contribution may reflect the region's long-standing policy emphasis on waste recovery, landfill diversion, CE implementation and alternative fuel utilisation within energy-intensive industries (Deja et al., 2010; European Union, 2010; Garg et al., 2007). In contrast, research output from Africa and parts of the Middle East remains minimal. This disparity highlights an imbalance in global research efforts, with developed nations dominating the discourse while underrepresented regions, despite their potential for RDF adoption, remain largely overlooked. Co-occurrences of keywords.

Keyword co-occurrence analysis is an effective method for uncovering core themes, research trends and focal areas within a body of literature (Grames et al., 2019). Using VOSviewer, co-occurrence analysis was performed on 211 articles, generating a network map based on 59 keywords with a minimum threshold of five occurrences. The resulting map includes 749 links and a total link strength of 1,506, as illustrated in Figure 5.

The co-occurrence map is divided into four main clusters, highlighted by colour: red, green, blue and yellow, representing the key research themes identified in the literature-namely MSW management and CE (cluster 1-red), combustion technologies and thermal processing (cluster 2-blue), alternative fuels in the cement manufacturing industry (cluster 3-green) and cement kiln efficiency and emission control (cluster 4-yellow).

Figure 6 illustrates the interrelationships of the keyword “refuse-derived fuel” from Cluster 1 with other clusters, as well as the keyword “solid recovered fuel” from Cluster 3 and its connections to other clusters.

As shown in Figures 6a and 6b are positioned centrally within the network of co-occurring keywords, with numerous direct connections to other important concepts, including waste, combustion, energy, emissions, alternative fuels, CE, sustainability and the cement industry. This central position suggests that much of the academic and industrial discussion centres on these two keywords, focusing on concepts of CE and sustainable waste management. This knowledge sets the stage for a detailed analysis of RDF and SRF, their unique features and their role as enablers in advancing CE and sustainable waste management goals. Consequently, this provides a strong foundation for the following sections, in which these themes are integrated into a comprehensive narrative, contributing to a deeper understanding and paving the way for future research.

This section presents insights derived from 48 publications retrieved through a comprehensive review of the full-text literature, grey literature and Google Scholar search outputs (refer to Appendix A). Although the review is framed primarily around RDF, SRF is included within the analytical scope because it represents a standardised subset of RDF and is frequently discussed alongside RDF in studies on cement-kiln fuel substitution. The section is organised to represent a holistic overview of RDF co-processing in cement manufacturing, with the identified themes divided into the following sections: (1) the difference between RDF and SRF (Section 3.2.1), (2) legal regulations related to RDF/SRF (Section 3.2.2) and (3) RDF as an enabler of CE and sustainable waste management (Section 3.2.3).

In cement industry literature, the term RDF is more commonly used than SRF when discussing its uptake. This trend is evident in studies by Patel and Chauhan (2014), Hemidat et al. (2019), Tihin et al. (2023) and Paramita et al. (2018). Many high-calorific waste materials are labelled as RDF; however, their compositional quality and environmental characteristics often remain unclear (De Gisi et al., 2018). Although previous research has acknowledged the confusion caused by the interchangeable use of RDF and SRF (Chavando et al., 2022; De Gisi et al., 2018), the rational response is yet to be discovered.

Both RDF and SRF offer advantages over untreated waste, including higher calorific values, reduced moisture content and improved particle uniformity with lower metal contamination. However, SRF is primarily distinguished by its compliance with specific international quality standards, such as ISO 21640:2021, whereas RDF typically lacks such standardisation (De Gisi et al., 2018; Psaltis and Komilis, 2019). As such, applying these standards remains a practical approach to differentiate between the two fuel types in both academic and industrial contexts.

Standardisation refers to the development of specifications for key product attributes to ensure consistent quality and performance (Jones and Hudson, 1996). As a fundamental component of quality management, effective standardisation builds trust among suppliers, end-users and regulatory authorities (Central Public Health and Environmental Engineering Organisation, 2018). This is particularly important for waste-derived fuels such as RDF, which originate from heterogeneous MSW and are therefore subject to significant compositional variability. The most discussed standards in the literature are identified below.

1. ISO 21640:2021 “Solid recovered fuels–Specifications and classes”

Among the most widely cited standards in the literature is ISO 21640:2021 “Solid Recovered Fuels–Specifications and Classes.” This international standard offers a comprehensive framework for classifying SRF. It is considered a global extension of EN 15359:2011, the earlier European standard developed by the European Committee for Standardisation (CEN/TC 343) (Central Public Health and Environmental Engineering Organisation, 2018). Both standards categorise SRF into five classes based on three primary fuel properties: net calorific value, chlorine (Cl) content and mercury (Hg) content. However, ISO 21640:2021 is broader in scope, placing greater emphasis on mercury limitations and incorporating additional parameters such as particle size, ash content, moisture and pollutant levels (e.g. halogens and heavy metals), making it more applicable to contemporary regulatory and environmental demands (European Investment Bank, 2024; Del Zotto et al., 2015). While both standards provide a foundational basis for SRF classification, ISO 21640:2021 is generally viewed as more robust due to its global applicability, enhanced environmental criteria and expanded classification metrics. The classification system defined under ISO 21640:2021 is summarised in Table 2.

Classes 1, 2 and 3 of the ISO 21640:2021 standard represent the most desirable SRFs for use in cement plants, offering optimal performance in terms of economic efficiency (LHV >15 MJ/kg), technological compatibility (Cl ≤ 1.0%) and environmental safety (Hg 80th percentile <0.10 mg/MJ). In contrast, fuels classified under classes 4 and 5 are generally considered of lower value for such applications (European Investment Bank, 2024; Kahawalage et al., 2023; Santos et al., 2023; Sarquah et al., 2022a, b).

Despite the clarity offered by ISO 21640:2021 and its predecessor EN 15359:2011, the classification of SRF remains a source of confusion in the literature. Several studies have incorrectly referred to these standards as RDF classifications, even though they explicitly define specifications for SRF (Bień, 2021). This terminological ambiguity extends further, with some researchers referring to SRF classes as “RDF classes” or broadly categorising all waste-derived fuels as SRF without formal classification. Moreover, fuels that meet ISO 21640:2021 requirements are sometimes mislabelled as RDF, despite the standard classifying them as SRF (Gałko et al., 2023; Mateus et al., 2023; Sarquah et al., 2022a, b). In some cases, the use of “RDF” as a general term is deliberate, intended to simplify communication, since SRF is technically a refined subset of RDF (Sakri et al., 2021; Sarquah et al., 2022a, b). This conceptual relationship among waste, RDF and SRF is illustrated in Figure 7.

It is important to note that SRF and RDF differ in terms of classification and quality standards; however, the choice of which to use for cement co-processing ultimately lies with the end-user. Both RDF and SRF are suitable for this application, depending on specific operational requirements and regulatory contexts. This study, therefore, considers SRF within the broader RDF framework, recognising it as a standardised subset rather than a separate category.

In addition to classification standards, various legal regulations govern the use of RDF and these differ by country or region, reflecting the legislative authority of local or supranational bodies. Notably, the EU has implemented several key directives and regulations to formally address the use and management of RDF. For cement manufacturing, these regulations are particularly important because they shape fuel acceptance criteria, emissions compliance, cross-border movement of waste-derived fuels and the permitting environment within which cement kilns operate. Table 3 summarises some of the most relevant EU legal instruments applicable to RDF. These regulatory frameworks are particularly significant for cement manufacturing, as they directly govern the conditions under which RDF can be utilised as an alternative fuel in kiln operations.

Beyond the EU, several countries have established national legislation to govern the production, classification and utilisation of RDF in various applications. In Germany, stricter regulatory frameworks distinguish RDF from SRF, reflecting rigorous emission control and energy recovery requirements (Gałko et al., 2023). In the USA, RDF is regulated under the Clean Air Act and the Resource Conservation and Recovery Act, both of which prioritise emissions management and the safe handling of potentially hazardous waste materials (Shehata et al., 2022). Indonesia has adopted policy incentives such as feed-in tariffs to support RDF-based WtE initiatives, alongside reinforced regulations for MSW management (Mateus et al., 2023). In Portugal, Decree-Law No. 103/2015 establishes legal guidelines for RDF classification and energy recovery, aligning with the country's national waste management objectives (Berardi et al., 2020). Similarly, Brazil incorporates RDF into its industrial energy policy, promoting the use of alternative fuels to reduce landfill dependency and enhance energy efficiency (de Lorena Diniz Chaves et al., 2021). In Poland, RDF is legally classified as waste, meaning that its handling, transportation and storage are subject to waste licensing requirements under the amended Waste Act of 2012 (Gałko et al., 2023). Taken together, these regulatory arrangements directly influence whether and how RDF producers and cement manufacturers can produce, procure, transport and co-process RDF/SRF. In particular, they determine fuel acceptance criteria, emissions compliance thresholds, permitting requirements and cross-border waste movement, all of which are critical to the operational feasibility of RDF co-processing in cement kilns.

The WMH, formalised under the EU Waste Framework Directive (2008/98/EC), provides a globally recognised framework for ranking waste treatment strategies in order of environmental preference (Ewijk and Stegemann, 2016). The hierarchy comprises five tiers, arranged from most to least desirable: waste prevention, reuse, recycling, other recovery and disposal (Cole et al., 2019; Ewijk and Stegemann, 2016). Within this structure, the EU Waste Framework Directive classifies recycling, other recovery and disposal as the final stages of waste treatment (European Council, 2013). Although the term “other recovery” is not explicitly defined, it is exemplified through energy recovery. Building on this, the UK Department for Environment Food and Rural Affairs (2011) interprets other recovery as encompassing both energy and material recovery processes aimed at extracting value from waste that cannot be reused or recycled through conventional methods. Lombardi et al. (2015) argue that while reuse and recycling primarily target the recovery of materials, energy recovery serves as a necessary alternative when material recovery is technically or economically unfeasible. Therefore, it can be argued that, within the WMH framework, other recoveries can be effectively interpreted as energy recovery. In the context of cement manufacturing, this framework is particularly relevant, as cement kilns offer a viable industrial pathway for energy recovery via the co-processing of non-recyclable waste-derived fuels, such as RDF (see Figure 8).

Emphasising these higher-order strategies within the WMH framework contributes to a broader conceptual shift, repositioning waste not as a terminal by-product but as a resource with potential value. This perspective closely aligns with the core principles of the CE, which aims to optimise resource efficiency, extend material lifecycles and minimise environmental impact by promoting the continual circulation of resources within the economic system (Ghisellini and Ulgiati, 2020; Johansson and Henriksson, 2020).

CE promotes a closed-loop system aimed at minimising waste and reusing resources, thereby replacing the traditional linear economy model, where resources are extracted, consumed and discarded (Ghisellini and Ulgiati, 2020; Johansson and Henriksson, 2020; Mancheri et al., 2018). This transition is extensive and supported by evolving strategies. Originally founded on the 3 Rs (Reduce, Reuse and Recycle), CE has expanded into the 10 R framework: Refuse, Rethink, Reduce, Reuse, Repair, Refurbish, Remanufacture, Repurpose, Recycle and Recover (Potting et al., 2017). In an ideal CE, recycled materials maintain their original quality for use in equivalent products, thus eliminating the need for additional natural resources (Potting et al., 2017; Romero-Hernández and Romero, 2018). Although this form of perfect circularity, where materials can be endlessly cycled, is conceptually desirable, it is rarely achievable in practice (Genovese et al., 2017; Potting et al., 2017). Therefore, a pragmatic CE approach focuses on prolonging material lifespan and reducing waste generation (Neves and Marques, 2022). From a cement manufacturing perspective, this positioning highlights the role of RDF co-processing as a practical mechanism for integrating waste management and industrial production systems, thereby contributing to both resource efficiency and emission reduction targets.

In practical application, both WMH and CE converge in later stages, particularly in recycling and recovery. While recovery represents the penultimate phase in CE, WMH concludes with disposal when recovery is no longer feasible. Thus, when waste is no longer viable for reuse or recycling, recycling and recovery remain the most appropriate strategies, in accordance with CE principles, to avoid landfilling.

Certain waste materials present technical or economic challenges for recycling, such as plastic bags and films (Dey et al., 2021), blended fabrics (Kahoush and Kadi, 2022; Loo et al., 2023; Subramanian et al., 2022) and treated wood (Van Acker et al., 2023; Townsend and Anshassi, 2023). These materials can be evaluated for their calorific value and processed into energy, either directly (as primary sources) or as processed fuels like RDF/SRF (secondary sources) (Bendjedia et al., 2018; Ferdan et al., 2018; Forman et al., 2016; Fumo and Chamra, 2010; Kaur et al., 2021; Serrano et al., 2024). In the specific context of cement manufacturing, RDF serves as a practical bridge between waste recovery and industrial decarbonisation by substituting fossil fuels in clinker production. This positions cement kilns as key enablers of CE implementation, particularly for waste streams that are not suitable for recycling. Accordingly, RDF co-processing represents a sector-specific application of both WMH and CE principles, translating abstract sustainability concepts into operational practices within the cement industry. Figure 9, therefore, positions RDF within the fourth tier (“other recovery”) of the WMH and as the tenth strategy (“recover”) in the 10 R CE framework.

As illustrated in Figure 10, the research gap and future research agenda mapping diagram establishes explicit linkages between the findings of the content analysis and the proposed research directions, thereby providing a structured pathway to support the sustainable adoption of RDF in cement manufacturing. Importantly, the future research directions are systematically derived from the core themes identified in the content analysis. Specifically, the conceptual ambiguity between RDF and SRF identified in Section 3.2.1 highlights the need for clearer classification frameworks and standardisation mechanisms. The regulatory fragmentation discussed in Section 3.2.2 underscores the need for harmonised policy frameworks and cross-jurisdictional alignment. Similarly, the sectoral positioning of RDF within CE and waste management frameworks (Section 3.2.3) reveals gaps in understanding how RDF contributes to broader sustainability transitions, thereby motivating further research into its role in integrated resource recovery systems.

Although these themes have broader relevance, they are interpreted in this study within the specific operational, environmental and policy context of cement-kiln co-processing. Accordingly, the diagram comprises two core components: (1) identified research gaps emerging directly from the findings and (2) corresponding research directions that address these gaps. This structured mapping ensures a clear and transparent connection between the evidence generated through the SLR and the proposed future research agenda.

The literature highlights persistent confusion between RDF and SRF, which complicates regulatory compliance and contributes to inconsistencies in fuel quality and operational performance (Chavando et al., 2022; De Gisi et al., 2018). In cement manufacturing, this ambiguity is not merely conceptual: it has direct implications for kiln feed acceptance, emissions assurance, procurement contracts and communication between RDF producers and cement manufacturers. While this review addresses the theoretical distinctions between the two, there remains a need for greater practical clarity to support industry understanding and application. Enhancing awareness among practitioners can promote accurate classification, improve regulatory alignment and guide informed procurement decisions for cement plants. Ultimately, this would contribute to the development of a harmonised terminology and quality standards, fostering stakeholder confidence and enabling more effective policy implementation and market integration within the cement sector.

The ongoing variability in the compositional quality of RDF/SRF, particularly in terms of calorific value, chlorine content and mercury content, remains a significant barrier to their broader adoption in cement manufacturing. While the introduction of ISO 21640:2021 has been recognised as an essential step toward building trust and consistency in the fuel market (Central Public Health and Environmental Engineering Organisation, 2018), its practical implementation is still limited. There is a notable lack of documented awareness and application of this standard in the current peer-reviewed academic literature. The efficient operation of a cement kiln and the quality of the final cement product are highly dependent on the use of fuels that meet strict compositional requirements (Genon and Brizio, 2008). This underscores the critical importance of ensuring that only RDF/SRF of appropriate quality is used as kiln feedstock. Given these challenges, future research should prioritise the investigation of effective implementation models for ISO standardisation across diverse regional contexts. In addition, there is a pressing need to develop robust quality control protocols that can ensure consistent compliance with these standards throughout the entire supply chain. Such efforts will be essential for supporting the reliable and sustainable integration of RDF/SRF in cement manufacturing.

In the EU regulations, RDF is governed by a comprehensive suite of waste management directives that shape operational requirements for industrial users, including cement plants. These regulations help define the conditions under which RDF can be transported, classified, stored and combusted in cement kilns. While the EU's regulatory approach to RDF is well documented and widely adopted among member states, there remains a notable lack of detailed peer-reviewed analysis regarding the specific policy and regulatory frameworks governing RDF in other regions. Countries such as Indonesia, the USA and Brazil do have national regulations that address RDF to varying extents (de Lorena Diniz Chaves et al., 2021; Mateus et al., 2023; Shehata et al., 2022), but the academic literature rarely provides in-depth comparative discussion of how these frameworks affect cement-sector uptake.

As highlighted in Table 2 of this review, key EU regulations generally refer to RDF in broad terms, without always making an explicit distinction between RDF and SRF. This is understandable, as SRF is produced from RDF and therefore falls under the broader regulatory umbrella. Within this context, ISO 21640:2021 stands out as the main international standard that formally differentiates SRF from RDF. Future research should therefore undertake comparative investigations of the regulatory frameworks governing RDF and SRF across different countries and regions, with explicit attention to the permitting, emissions and supply-chain implications for cement manufacturing. Such studies can generate context-specific insights for policymakers and support the design of regulatory settings that facilitate responsible cement-kiln adoption.

A thorough evaluation of the environmental and economic impacts associated with the use of RDF/SRF is essential for advancing sustainable decision-making in the cement manufacturing industry. Although lifecycle assessment studies have provided initial insights into the environmental advantages and potential trade-offs of substituting conventional fossil fuels with RDF or SRF (Anasstasia et al., 2020; Grzesik and Malinowski, 2016), there remains a notable gap in analyses that are specifically tailored to the context of cement manufacturing. Such work is essential for capturing the full spectrum of environmental implications and for understanding how local policy frameworks and market dynamics influence the feasibility and attractiveness of RDF/SRF adoption in the cement manufacturing sector.

This study contributes to the RDF literature by providing a sector-specific synthesis of RDF utilisation within cement manufacturing. A key contribution of the study is the clarification of the conceptual relationship between RDF and its standardised subset, SRF and the positioning of these within the context of cement kiln co-processing, CE and the WMH. In doing so, it addresses conceptual ambiguity identified in the literature and responds to the need for integrated, sector-specific analyses. Consistent with Makadok et al. (2018), the study contributes theoretically by refining key constructs, structuring fragmented knowledge and identifying new research questions that extend the scope of existing RDF discourse. The study also identifies several important research gaps that warrant further investigation. The findings suggest a need for greater standardisation of RDF classification systems and fuel quality assessment approaches, particularly across different regulatory contexts. Existing variations in national standards and operational requirements continue to hinder direct comparisons between studies and limit broader implementation. Furthermore, while environmental benefits associated with RDF utilisation are widely reported, additional research is required to evaluate long-term environmental, economic and social sustainability outcomes under different geographical and industrial conditions. Comparative studies examining RDF utilisation across multiple sectors and regions may further strengthen understanding of best practices and implementation pathways. Finally, the study develops a structured future research agenda derived directly from the identified knowledge gaps. By systematically linking conceptual ambiguities, regulatory fragmentation and sustainability challenges with future research priorities, the study provides a foundation for advancing both theoretical and empirical knowledge relating to RDF utilisation in cement manufacturing.

The findings of this study provide several practical implications for industry stakeholders involved in RDF production, utilisation and regulation. Cement manufacturers can use the findings of the study to better align RDF/SRF selection with kiln operational requirements, emissions compliance and fuel quality standards, thereby reducing operational risks associated with inconsistent fuel characteristics. As highlighted by Genon and Brizio (2008), maintaining appropriate fuel quality characteristics is important for minimising operational disruptions and ensuring stable kiln performance. Similarly, improving RDF/SRF standardisation and quality assurance practices may enhance supply-chain reliability and stakeholder confidence, particularly in relation to calorific consistency and contaminant control (Central Public Health and Environmental Engineering Organisation, 2018; Kahawalage et al., 2023).

For policymakers and regulatory agencies, the review highlights the importance of coherent and regionally adapted regulatory frameworks to support RDF co-processing in cement manufacturing. By identifying gaps in policy alignment and regulatory consistency, the findings provide a basis for designing policies that facilitate safe, efficient and scalable RDF utilisation, contributing to both waste diversion and industrial decarbonisation objectives.

From a broader sustainability perspective, the findings reinforce the potential role of RDF co-processing in supporting CE objectives by reducing reliance on landfill and substituting for fossil fuel consumption in cement manufacturing systems. As discussed by Ghisellini and Ulgiati (2020), CE approaches contribute to improved resource efficiency and more sustainable industrial systems. Therefore, wider adoption of RDF co-processing may contribute to both environmental sustainability and long-term industrial transition goals.

This study synthesised the current state of knowledge on the use of RDF in cement manufacturing, with a particular focus on its future research avenues. An SLR of 211 scholarly articles published between 1991 and July 2025 was subjected to descriptive analysis, and 48 records, including seven regulatory records, were employed for the content analysis. The results indicate that scholarly interest in RDF as a sustainable fuel alternative has increased significantly over the past decade, with more than 80% of publications emerging since 2014. This trend reflects a heightened global emphasis on reducing carbon emissions from hard-to-abate industries, such as cement manufacturing.

The analysis also reveals persistent confusion in the literature between the terms “RDF” and “SRF”, as many studies use these terms interchangeably. This lack of terminological clarity poses challenges for quality control and regulatory compliance, particularly at the national level. In terms of functional role, RDF is found to be an important enabler in both the sustainable waste management and CE frameworks. Positioned in the “other recovery” tier of the WMH and corresponding to the “recover” strategy in the 10 R CE framework, RDF provides a critical solution for managing non-recyclable, combustible waste fractions.

Further, several limitations should also be acknowledged. First, the exclusive reliance on the WoS database may have limited the potential to overlook relevant studies from other databases. Thus, future studies should including other major academic databases such as Scopus, PubMed, Google Scholar and specialised or regional databases. This widens the coverage and increases the likelihood of capturing relevant studies that may not be indexed in WoS. Second, the analysis was limited to English-language publications, which could have excluded critical regional studies, particularly from non-English-speaking countries. Future work should consider including non-English literature to enrich the understanding, especially regarding region-specific practices and innovations. Third, the study deliberately focused on the cement manufacturing sector to enable an in-depth, sector-specific analysis. This targeted approach enabled a detailed examination of technical, environmental and policy factors unique to cement co-processing. However, it inherently excludes potentially transferable insights from other industries that utilise RDF, such as WtE plants or district heating networks. While this focus ensures depth and contextual relevance, future comparative analyses across sectors could yield broader generalisability and policy-relevant insights. Overall, this study contributes to a foundational understanding of RDF uptake in cement co-processing, positioning it as both an environmental and resource-efficiency solution aligned with global sustainability objectives.

The supplementary material for this article can be found online.