A Scientific Review of Recycling Practices and Challenges for Autoclaved Aerated Concrete in Sustainable Construction

Abstract

Autoclaved Aerated Concrete (AAC) is a lightweight, thermally insulating, and fire-resistant building material that has become prominent in sustainable construction due to its reduced production energy demands and minimal environmental impact. As an increasing number of AAC-based structures reach end-of-life, the effective recycling and reuse of AAC waste present both challenges and opportunities within the context of sustainable building practices and circular economy frameworks. This study presents a scientometric review of AAC recycling research published between 2014 and 2024, using the Web of Science database and bibliometric tools such as CiteSpace. Key trends, techniques, and knowledge gaps in AAC recycling are identified, highlighting issues such as high energy consumption, limited practical implementation, and the absence of standardized recovery protocols. The study also outlines emerging research pathways, including detailed material characterization, development of recycling standards, innovative reuse techniques, hybrid material systems, and the integration of recycled AAC in new construction. These insights provide a foundation for advancing sustainable building material strategies and inform policy and practice in construction waste management.

1. Introduction

Autoclaved Aerated Concrete (AAC) is a versatile concrete-based building material that has gained widespread use globally due to its advantageous properties. Distinguished by its lightweight nature compared to traditional concrete blocks, AAC offers several key characteristics that make it an ideal material for modern construction. These include high compressive strength, excellent thermal insulation, and fire resistance, all of which contribute to its broad adoption in both residential and commercial construction projects [1,2]. The core composition of AAC consists of cement, water, fine aggregate, and aluminum powder, which undergo a unique autoclaving process that results in the material’s distinctive cellular structure [3,4], This structure not only reduces the overall weight of AAC but also enhances their insulating properties, providing long-term energy savings in buildings constructed with this material [5].

According to Sherin and Saurabh [6], AAC blocks are increasingly regarded as an environmentally friendly alternative to traditional clay bricks. One of the critical factors contributing to this perception is that AAC production does not require the high-temperature firing process used in clay brick manufacturing, thereby saving significant amounts of energy [7]. Moreover, the absence of firing means that AAC production avoids the release of harmful by-products such as smoke and fly ash, which are typically generated during the clay brick manufacturing process [8]. This reduction in emissions positions AAC as a sustainable choice, aligning with global efforts to minimise the environmental impact of construction materials. Since its invention nearly a century ago, AAC has become a staple in the construction of numerous buildings and infrastructure projects worldwide, with notable adoption in regions where environmental considerations and energy efficiency are prioritized [9].

China, as the largest producer of AAC, exemplifies the material’s significant role in contemporary construction. In 2018, China’s AAC production reached an impressive 177 million cubic metres, representing a substantial share of the global output [10]. This high production volume reflects the growing demand for AAC, driven by its favourable properties and alignment with sustainable building practices [11]. However, despite its many advantages, AAC construction faces a critical challenge as many buildings that utilise AAC are approaching the end of their service lives. The resulting decommissioning of these structures will generate large quantities of AAC construction waste, necessitating effective and sustainable waste management solutions.

In modern construction, AAC has established itself as a versatile and innovative material with a wide range of applications. Due to its lightweight yet durable nature, AAC is extensively used for building walls, partitions, floors, and roofs in both residential and commercial projects [12]. Its excellent thermal insulation properties make it particularly suitable for energy-efficient buildings, helping to reduce heating and cooling demands and thereby lowering operational costs over a building’s lifetime [13]. Moreover, the inherent fire resistance and sound insulation further enhance its suitability for high-density urban environments where safety and comfort are critical considerations. Beyond these functional advantages, its ease of handling and workability contribute to faster construction times and reduced labour costs, offering tangible benefits to contractors and developers. As sustainable construction practices gain prominence globally, AAC has reduced environmental footprint, stemming from its lower raw material consumption and energy-efficient manufacturing process, positions it as a key material in achieving green building certifications and meeting regulatory standards for sustainable development [13,14]. This combination of performance, sustainability, and economic efficiency underscores the growing relevance in addressing the challenges of modern construction.

Currently, landfilling remains the predominant method for disposing of AAC construction waste [15]. This approach, however, presents significant drawbacks. Beyond the considerable waste of resources, landfilling AAC waste poses environmental risks, including the potential for secondary pollution. The growing volume of AAC waste highlights an urgent need for more sustainable disposal methods that align with the principles of a circular economy [16]. In this context, the recycling and reuse of AAC offer a promising avenue for reducing the environmental impact of construction waste. Not only does AAC recycling hold the potential to conserve valuable raw materials, but it also contributes to reducing carbon emissions and energy consumption associated with the production of new building materials.

The construction industry is one of the largest consumers of natural resources and a major contributor to global carbon emissions, making sustainability a pressing priority for the sector. Recycling and reusing construction materials are vital strategies in reducing the environmental impact of building activities, as they help conserve finite resources, minimize waste sent to landfills, and lower the energy demands associated with producing new materials. Incorporating recycled materials also supports the principles of a circular economy, where resource loops are closed, and materials are kept in use for as long as possible. Within this context, AAC stands out as a material with significant potential for sustainable end-of-life management. While AAC is already valued for its low operational carbon footprint due to its thermal performance, the ability to recycle AAC waste will further enhances its sustainability credentials [17,18,19]. By diverting AAC construction and demolition waste from landfills and reintegrating it into the production cycle or other applications, the environmental footprint of buildings can be substantially reduced. Expanding research and innovation in AAC recycling therefore aligns closely with broader industry goals to create more sustainable, resource-efficient, and environmentally responsible construction practices.

In recent years, the economic recycling of construction waste has garnered increasing attention from researchers and industry professionals alike. Nevertheless, existing research on AAC recycling remains insufficient in providing practical solutions that meet the recycling goals for aerated concrete [20,21]. Current recycling methods face several challenges, including low rates of implementation, high energy consumption, and inefficiencies that conflict with the environmental objective of energy conservation [22,23]. These limitations underscore the need for innovative approaches that can overcome the technical and economic barriers to AAC recycling.

This research seeks to address these challenges by reviewing recent advancements in AAC recycling and reuse. The study aims to identify more sustainable and efficient recycling processes, exploring new possibilities for the recovery of AAC. By synthesising the latest developments in this field, this study will offer a comprehensive summary of current AAC recycling practices and propose new directions for the advancement of AAC recycling. The findings of this research are expected to contribute significantly to the construction and engineering fields by promoting the adoption of more sustainable practices in AAC waste management. Ultimately, this project aims to facilitate the transition towards a more circular economy in the construction industry, reducing the environmental footprint of AAC and enhancing the economic viability of recycling initiatives.

2. Research Method

This study undertakes an extensive scientific review of research efforts focused on the recycling processes of Autoclaved Aerated Concrete (AAC) over the decade spanning from 2014 to 2024, with a particular emphasis on identifying emerging opportunities and innovative applications for recycling. The research was organised into five distinct steps: (1) conducting keyword searches to identify relevant literature, (2) employing scientometric methods to analyse the data, (3) categorising the various research topics that have emerged in the literature, (4) providing a detailed analysis of each identified subtopic, and (5) discussing the analytical results, highlighting the existing research gaps, and outlining potential directions for future research. To facilitate a thorough and reliable literature review, the study utilised the Web of Science (WoS) database as the primary tool for sourcing academic articles. WoS is recognized as a leading database in the engineering and scientific fields, renowned for its extensive coverage, stability, and rigorous indexing standards [24]. The use of WoS ensures a comprehensive retrieval of relevant literature, contributing to the credibility and thoroughness of the review process.

Furthermore, this study employed the bibliometric analysis tool CiteSpace, which is widely recognized for its capacity to conduct scientometric analysis. CiteSpace was used to create clusters of literature on various topics within the AAC recycling domain, thereby facilitating the construction of a coherent and comprehensive knowledge framework. The tool aids in the visualisation and analysis of complex research networks, making it possible to clearly identify patterns, trends, and key areas of focus within the field. By leveraging CiteSpace, this study was able to provide a more systematic and structured examination of AAC recycling research, which is critical for understanding the evolution and current state of the field. The following Figure 1 is a flow chart that provides a intuitive display of the methodology.

2.1. Keyword Search

The literature search employed a keyword-driven approach to ensure a comprehensive capture of relevant studies within the scope of AAC recycling. Given that the primary focus of this study was on the recycling and reuse of aerated concrete, an array of keywords and their variations were employed to maximise the retrieval of pertinent literature. The search field was set to encompass terms related to both “recycling” or “reuse” in combination with “aerated concrete,” “aerated blocks,” or “autoclaved concrete,” which are common terminologies used in the field. The temporal scope of the search was confined to publications from 2014 to 2024, ensuring that the review reflected the most recent developments and emerging trends in AAC recycling research. Moreover, to maintain academic rigour, the document type filter was restricted to peer-reviewed journal articles or review articles, which underwent more stringent evaluation processes compared to conference papers [25]. This decision is critical as peer-reviewed articles are generally considered to provide more reliable and validated research findings. Additionally, the language filter was set to English to ensure consistency in the reviewed literature. Despite the growing interest in sustainable construction materials, research on AAC recycling is still in its formative stages, and as a result, the number of directly relevant articles obtained through these keyword searches remains limited. As of 21 December 2024, a total of 156 articles were retrieved from the Web of Science database, reflecting the current state of academic inquiry into AAC recycling. Figure 2 illustrates the annual distribution of papers retrieved from WoS between 2014 and 2024, along with the corresponding citation data for those papers. Although the annual distribution trend of the number of articles is slightly tortuous, it is generally on an upward trend, and this upward trend is more prominent in the annual distribution of citations.

2.2. Data Visualisation

In order to gain deeper insights into the AAC recycling research landscape, this study utilised CiteSpace to generate several types of bibliometric networks. Specifically, the study focused on creating a term co-occurrence network, a document co-citation network, and a reference co-citation network. These networks are instrumental in revealing the structural and dynamic properties of research domains, providing a comprehensive overview of the intellectual landscape. Term co-occurrence networks, for example, allow for the identification of key concepts and themes that frequently appear together in the literature, offering insights into the primary focus areas within AAC recycling research. On the other hand, document co-citation networks highlight influential papers that are frequently cited together, thereby uncovering foundational works and significant research clusters within the field. Similarly, reference co-citation networks help to identify core references that serve as primary sources of knowledge in the domain, reflecting the interdisciplinary nature of the research [26].

The use of these networks enables a more nuanced understanding of how AAC recycling research has evolved over time, as well as the key areas where knowledge has been concentrated. This visualisation serves as a critical tool for tracking the growth of scholarly output in AAC recycling research and gauging the impact of this body of work. By analysing these temporal trends, the study was able to identify periods of significant research activity and pinpoint emerging trends and gaps in the literature. The application of such advanced scientometric methods ensures that the review provides not only a summary of past research efforts but also a forward-looking perspective on the future directions of AAC recycling research.

3. Literature Analysis Based on Scientometrics

3.1. Key Research Topics

The co-occurrence network illustrated in Figure 3 comprises 270 distinct nodes interconnected by 1169 links. Each major cluster is marked with # and a number in order from largest to smallest cluster density. Each link represents the simultaneous appearance of two terms within at least one scholarly article. The intensity of each link is quantified by the cosine similarity coefficient, which measures the relevance of the term pairs. This coefficient highlights pairs that are more closely related compared to others. The size of the rhombic frame in Figure 3 signifies the frequency of term occurrences. The larger the diamond, the more frequent and concentrated the term appears.

Table 1. Top 20 frequently cited labels.

No.	Frequency	Label	Centrality
1	48	Autoclaved aerated concrete	0.32
2	31	Cement	0.16
3	31	Compressive strength	0.08
4	30	Strength	0.14
5	29	Microstructure	0.13
6	29	Fly ash	0.11
7	21	Waste	0.13
8	19	Performance	0.08
9	19	Mechanical property	0.04
10	18	Aggregate	0.12
11	16	Concrete	0.18
12	16	Ash	0.06
13	15	Aerated concrete	0.16
14	14	Slag	0.07
15	13	Construction	0.08
16	13	Hydration	0.03
17	12	Fly ash	0.11
18	10	Behaviour	0.08
19	10	Powder	0.01
20	8	Recycled aggregate	0.04

enumerates the top 20 most cited terms, which indicate prevalent themes in AAC recycling research. Notably, 6 out of these 20 terms (compressive strength, strength, microstructure, performance, mechanical property, behaviour) pertain to characteristics of recycled materials. Meanwhile, 5 terms are related to the replacement of recycled AAC for other materials, namely Cement, Aggregate, Concrete, Powder and Recycled aggregate. This suggests that the feasibility and practice of using AAC waste to produce new concrete is a significant focus of research. The network diagram in Figure 3 also reveals strong associations between autoclaved aerated concrete and mechanical property, compressive strength and recycled aggregate, underscoring concerns related to the structural safety implications of using recycled AAC materials. Additionally, a related cluster encompasses terms associated with the construction phase, comprehensive review, and fly ash, underscoring the prominence of recycling of AAC in evaluating the sustainability of construction practices. Recent years have seen a surge in research interest in terms like recycling concrete, durability properties, mechanical performance, and geopolymer technologies, indicating a notable increase in scholarly attention on these topics.

3.2. Document Co-Citation Network

Document co-citation analysis serves as a powerful method for uncovering the underlying intellectual framework within a particular domain. This technique maps out the citation and co-citation patterns found in scientific literature, thereby revealing connections between scholarly works [27]. Within each identified cluster of this network, two sets of articles are linked: the group of citing articles and the group of cited references. In this research, the document co-citation network was constructed from articles published between 2014 and 2024. To enhance the clarity of the network, the pathfinder algorithm was used to prune it by identifying the most significant co-citation links.

As illustrated in Figure 4, the constructed network is composed of 262 nodes and 1349 links, representing the interconnectedness of various elements within the dataset. A number with # are used to sort the clusters from largest to smallest. The network’s modularity score of 0.4833 is relatively low, indicating that the network does not clearly differentiate between distinct research areas or specialties. This low modularity implies a lack of well-defined boundaries between clusters, suggesting that the research topics are interrelated rather than isolated. Additionally, the network’s weighted mean silhouette score is 0.83, which is substantially higher than the commonly accepted threshold of 0.5. While a higher silhouette score typically indicates strong internal consistency within clusters, in this case, it paradoxically suggests that the connections between the clusters are highly intertwined, rather than distinct. Therefore, it is difficult to analyse clusters in the network independently, and it is necessary to combine the main nodes in different networks to better form a knowledge framework.

This interdependence is further evidenced by the dense pattern of links between clusters, and no node in the co-citation network exists in isolation. The significant overlap in citation articles across different clusters demonstrates that the major works cited within these clusters cover multiple areas of research, reflecting a broad focus among the authors. The presence of high coverage across clusters highlights the tendency of researchers in these groups to explore similar aspects of AAC recycling, often from comparable perspectives. Furthermore, this network structure indicates that no single cluster appears to be disproportionately shaped by the research interests of an individual author. Rather, the findings suggest a more collective effort, with a diversity of contributors influencing the discourse within each cluster, pointing to a collaborative rather than author-driven research environment.

3.3. Reference Co-Occurrence Network

Reference co-citation network uses a similar analysis method to the document co-citation network, but it uses references as nodes instead of cited journals. In CiteSpace, the process of extracting cluster labels involves analysing the titles, abstracts, or keywords from citation data using one of three distinct methods: log-likelihood ratio (LLR), Latent Semantic Indexing (LSI), and mutual information (MI). These methods each offer unique advantages for identifying significant terms that best represent the content of each cluster. To determine the most effective approach, this study conducted a comparative analysis of all possible combinations. Following a thorough evaluation, the top-ranked terms generated through LSI were selected as the most appropriate cluster labels. The decision to utilize LSI was based on its ability to produce labels that are not only highly representative of the clusters but also capable of conveying more meaningful and relevant information about the underlying research themes within the network. This choice reflects the strengths of LSI in capturing the semantic relationships between terms, which ultimately enhances the clarity and interpretability of the cluster labels. The generated network is shown in Figure 5 and the clusters are labelled with # and numbers according to their size. It consists of the six largest clusters, and they are summarized as

Table 2. Summary of the major clusters.

Cluster ID	Cluster Size	Cluster (LSI)	Silhouette	Label (Mutual Information Index)
1	30	Synthetic aggregate	0.811	Raw material (0.83)
2	29	Recycling option	0.811	Waste (1.4)
3	27	Compressive strength	0.993	Partial replacement (0.11)
4	23	Sustainable lightweight concrete	0.915	Foamed concrete (0.15)
5	16	Concrete properties	0.996	Physical properties (0.03)
6	16	Economic cost	0.971	Energy-efficient material (0.34)

below.

The Silhouette column provides insight into the degree of homogeneity within each cluster. This measure is particularly useful for assessing the consistency of cluster membership, especially when the clusters being compared are of comparable size. A higher silhouette score generally indicates that the members of a cluster are more closely related to one another, suggesting greater internal consistency [28]. In Table 2, the silhouette scores for all clusters exceed 0.8, signifying a high degree of internal cohesion and similarity among the members of each cluster. This indicates that the elements within each cluster are well integrated and share common characteristics, reflecting a strong consistency in the grouping. Notably, the silhouette scores for clusters 3 and 5 approach the maximum value of 1.00, suggesting that these clusters exhibit an exceptionally high level of internal homogeneity. In these cases, the nearly perfect silhouette scores imply that the members within clusters 3 and 5 are highly similar to one another and are well separated from members of other clusters. However, this metric becomes less meaningful when applied to smaller clusters. This high score is likely since all references within the cluster are citations from the same author or closely related works. Consequently, cluster 3 and 5 may primarily reflect the citation practices or preferences associated with a single paper, rather than representing a broader or more diverse area of research. In such cases, the high silhouette score does not necessarily indicate that the cluster is representative of a larger scholarly trend. In general, although the internal consistency between different clusters is high, it is still necessary to integrate the clusters together for research content analysis rather than analysing the clusters independently due to the limited sample size and high cluster correlation.

In the timeline visualization depicted in Figure 6, clusters are displayed horizontally along a time axis, while vertically, they are ordered by cluster size with # and larger clusters positioned higher. Each node in this view corresponds to a specific paper that has been cited by others within the network. The curved lines connecting these nodes represent co-citation relationships, with their colour indicating both the year of the first co-citation and the number of co-citations. Although 4 out of the 6 clusters eventually disappearing at various points, there are also clusters that have emerged in recent years and remain active such as clusters 2 and 4. In contrast, clusters 1, 3, 5, and 6 concluded their activity prior to 2021. Despite the cessation of these clusters, their influence persists in ongoing research, as evidenced by the cross-cluster co-citation links. The presence of these connections indicates that the work produced within clusters 1, 3, 5, and 6 continues to shape research in other, more current clusters. Clusters 2 and 4 have gained prominence and can be identified as central to contemporary research efforts, reflecting the current focus of scholarly attention within the field.

4. Analysis Results

The development of the knowledge framework surrounding AAC recycling can be understood as a process unfolding in two distinct stages. Initially, through the journal co-citation analysis, it becomes evident that all identified clusters can be categorized into two overarching thematic areas: recycled product performance and testing and evaluation of recycling methods. This initial division helps in organizing the research into broad domains. In the second stage, a more granular analysis is conducted by examining the keywords and abstracts of the primary members across the six clusters. From this detailed review, three major research areas related to AAC recycling are identified, based on the strength of the connections among the members within each cluster. A knowledge framework can be constructed as shown in Figure 7. These research areas represent different dimensions of AAC recycling, with each area reflecting a unique facet of the overarching themes. The names of these research areas are derived from key research directions identified through term co-occurrence network analysis. Importantly, the research areas span multiple clusters rather than confined to a single cluster. This cross-cluster membership underscores the interconnected nature of the research areas and highlights the multifaceted exploration of AAC recycling within the broader academic discourse.

Presently, two predominant approaches are being utilized for the recycling of AAC. The first method focuses on transforming AAC waste into powder form, which can then be used as a cementitious material in concrete, effectively replacing a portion of the cement typically required in concrete mixtures. The second method involves breaking down AAC waste into sand-sized particles, which are then used as a lightweight aggregate in concrete, substituting for either natural or artificial sand. Currently, it is impossible or economically expensive to recover AAC in its intact block or panel form. Figure 8 is a flow chart briefly showing the current recycling path of AAC. Moreover, a number of researchers are actively exploring innovative recycling methods that diverge from conventional approaches currently in use. These pioneering studies aim to introduce novel techniques for AAC recycling that challenge traditional paradigms. Although these investigations have yielded promising preliminary findings, they have yet to be fully implemented within the industry. The gap between research and industrial application suggests that while these innovative methods hold potential, further refinement, validation, and testing are necessary before they can be adopted on a broader scale in practical recycling operations.

4.1. Recycling Method in Powder Form

In recent years, the use of AAC waste in a powdered form as a filler material in concrete has garnered increasing attention. However, this practice often leads to a reduction in concrete strength, as noted in several studies [30]. Despite this drawback, there is evidence suggesting that finely ground AAC powder can contribute positively to concrete properties [31]. Specifically, when AAC powder is refined to a sufficiently small particle size, it can serve as a supplementary cementitious material that enhances concrete’s strength and durability [32]. This highlights the importance of achieving a high degree of refinement when processing AAC waste for use as a cement substitute. Failure to do so may result in lower strength properties and limited rehydration capabilities of the AAC particles, rendering them ineffective in enhancing concrete performance. Experimental results indicate that AAC powder with a particle size of less than 0.09 mm is the most suitable for achieving positive outcomes [33]. In contrast, when the particle size exceeds this threshold, the addition of AAC powder may not provide the desired benefits.

To achieve the necessary level of refinement, a variety of techniques can be employed, with grinding and wet milling being the most commonly used methods. Dry grinding, while a viable option, presents certain challenges, such as particle interactions and agglomeration, which can hinder the production of sufficiently fine and well-dispersed powders [34]. Wet milling, on the other hand, has been shown to significantly improve the fineness of solid waste particles, making it a more effective method for processing AAC waste [35]. In wet milling, water acts as a grinding aid, promoting more uniform grinding and increasing the speed of mixing [36]. Additionally, wet milling helps to prevent particle agglomeration, resulting in a narrower particle size distribution—up to one-quarter the size of particles produced through conventional grinding methods [37]. Moreover, studies have demonstrated that wet milling enhances the performance of cementitious materials, improving their microstructure, mechanical properties, and hydration reaction rates [38,39]. Consequently, wet milling is currently considered the most effective method for producing AAC waste powder, offering superior results compared to other available techniques.

Beyond particle size refinement, research has also explored the impact of various processing conditions on the performance of AAC waste powder. For example, Yang et al. [10,40] conducted an extensive analysis of the particle size distribution and processing time of AAC waste under wet-milling conditions, examining key properties such as hydrothermal reactivity, electrical conductivity, and water absorption. These findings provide valuable insights into the potential applications of AAC waste powder as a cement substitute. Liu et al. [20,41] similarly conducted a comprehensive series of usability tests on mortars produced from AAC waste powder, assessing properties such as workability, physical and mechanical characteristics, shrinkage, and microstructural features. The results of these studies collectively underscore the viability of replacing cement with AAC waste powder, and this recycling method has already been successfully implemented in some industrial settings.

4.2. Recycling Method in Sand Form

Among the recycling methods for AAC, the conversion of AAC waste into lightweight aggregates is the most widely adopted and generates the largest volume of recycled material. Due to its high porosity, AAC sand is not suitable for use as coarse aggregate and is instead applied as a lightweight aggregate [24]. Research has shown that low-density aggregates tend to reduce the strength of concrete, with this reduction becoming more pronounced as the replacement rate increases [42]. However, reducing the size of the aggregates can mitigate the negative impact on concrete strength [43]. Given the extensive use of mortar in construction and the depletion of natural river sand resources [44], utilizing AAC waste as a source of recycled aggregate offers an environmentally sustainable alternative to traditional materials.

Depending on the specific application, AAC-derived lightweight aggregates can be categorized into three primary types: load-bearing aggregates, non-load-bearing aggregates, and no-fines concrete aggregates. Both load-bearing and non-load-bearing applications have been shown to achieve 100% replacement rates [29,45]. In load-bearing applications, AAC waste is incorporated into the concrete mix as a lightweight aggregate, producing lightweight concrete with specific structural properties. For non-load-bearing applications, AAC waste aggregates are used in the production of materials such as vertical coverings, including tiles and wall panels. Over 30 different mixtures have been developed for these applications, each tailored to meet specific needs and tested according to the European Concrete Standard [46,47]. Factories can then select the appropriate mixture based on their production requirements [48].

No-fines concrete, a lightweight material primarily capable of withstanding compressive loads but with limited tensile capacity, represents an efficient use of AAC waste aggregates when saturated slurry is not required. This type of concrete is frequently employed in the construction of formwork, free-standing walls, and decorative elements [49]. The application of no-fines concrete reduces the need for conventional aggregates, providing a viable solution for recycling AAC waste on a larger scale and promoting sustainable construction practices.

Further research has been conducted to explore the properties of AAC-derived aggregates and the performance of the finished products. For instance, Liu et al. [50] investigated the effect of AAC sand particle size on mortar properties, using recycled AAC sand of varying sizes to examine its potential applications in construction. This study provides valuable quantitative data on the use of AAC sand in mortar production and offers insights into the impact of different particle sizes on the properties of the resulting mortars. Similarly, Murthi et al. [7,51] explored the influence of AAC sand on the service life and mechanical performance of concrete, providing critical information on the long-term behaviour of concrete containing AAC waste. Zou et al. [42,52] examined optimal replacement rates for AAC sand in concrete production, identifying the strength levels achievable with different replacement ratios. His research delved into the underlying mechanisms involved in the incorporation of AAC waste of varying particle sizes and substitution rates in mortar mixtures. He assessed the density, water absorption, void volume, and mechanical strength of the resulting mortars and evaluated these properties from both a macroscopic and microstructural perspective. These findings underscore the potential of converting AAC waste into sand for use in concrete production.

The process of converting AAC waste into recycled aggregate generally involves mechanical crushing and screening. Crushing is typically performed in multiple stages, with different types of crushers utilized to achieve the desired aggregate size. Jaw crushers are commonly employed for the initial breakdown of large AAC blocks into smaller pieces, making them suitable for secondary crushing. Impact crushers are often preferred for secondary crushing due to their ability to produce aggregates with minimal mortar adhesion, resulting in higher-quality recycled aggregates [53,54]. The choice of crushing equipment depends on factors such as the size of the feed material and the specific requirements for the final particle size [55,56]. Additional processing steps, such as washing and sorting, may also be required to ensure that the recycled aggregates meet the necessary quality standards for use in concrete production.

Overall, converting AAC waste into sand for lightweight aggregate production is a viable method that has already demonstrated its potential in real-world applications. By integrating AAC waste into concrete as a replacement for natural aggregates, it is possible to reduce the environmental impact of construction activities and promote the circular economy within the construction industry.

4.3. Evaluation of Current Recycling Methods

It is technically feasible to transform AAC waste into a fine powder and recycle it as a supplementary cementitious material in concrete mixes. However, this process presents a number of significant limitations. A primary concern is that AAC powder cannot serve as a complete replacement for cement. Current studies have only explored replacement ratios ranging from 10% to 30%, which falls short of expectations. As a result, cement remains the predominant component of the mixture, and this method does not substantially address the large-scale recycling needs for AAC materials. Moreover, the addition of AAC powder leads to alterations in mortar properties, such as reduced flowability and increased shrinkage [50,57]. At this time, no studies have successfully identified the underlying reaction mechanisms responsible for these changes, nor have they offered effective solutions for mitigating these effects in practical applications. Furthermore, several critical properties of the final product, including long-term durability, have yet to be thoroughly investigated. This lack of comprehensive understanding about the performance characteristics of the recycled product continues to limit the widespread adoption of this recycling method. Additionally, according to Lam [58,59], factories in Vietnam that produce this recycled material are achieving actual replacement ratios as low as 5%, which is far below the optimal rates suggested by academic research. Addressing this gap will require more extensive research efforts and the development of systematic guidelines to better inform industrial practices. Another issue is the significant energy consumption associated with the mechanical processes involved, particularly crushing and wet grinding, with the latter being especially demanding in terms of process control and energy requirements. Jeswiet and Szekeres [60] evaluated the energy needs of shredding operations and concluded that they performed poorly in terms of efficiency. Therefore, reducing energy consumption during the grinding phase is a critical area for future research efforts.

When comparing the aforementioned method to the alternative approach of recycling AAC waste by replacing natural sand, the latter option offers several advantages. This technique not only achieves a higher rate of material utilization but also allows for a wider array of potential applications for the resulting product. Furthermore, this method circumvents the need for wet milling, which is both energy-intensive and process-heavy. Nevertheless, this method is not without its limitations. AAC’s low density inherently makes it a suboptimal choice as an aggregate material, limiting its utility in the production of high-strength concrete. Although some studies have indicated that AAC waste can replace up to 100% of natural sand, the compressive strength of the resulting product declines rapidly as the replacement ratio increases. Research has demonstrated that within a specific range of particle sizes, replacement ratios of up to 50% can be achieved without significantly compromising the compressive strength of the mortar [42,61]. However, even under these conditions, Lam [59] has recommended a more conservative optimal replacement rate of 25%. Current research, however, tends to focus on narrowly defined conditions—such as fixed replacement ratios, specific particle sizes, or particular types of cement. There is a clear need for more integrative studies that examine the combined effects of various materials and conditions to provide the industry with more practical and comprehensive guidelines. Lastly, although this method eliminates the need for wet milling, the conversion of AAC waste into compliant sand still requires a substantial amount of energy. For example, research on industrial by-products like recycled concrete pavement has shown that renewable materials only become more energy-efficient than natural aggregates when the latter must be transported over four times the distance of the recycled material [62]. Given the immense volume of AAC waste that must be recycled, minimizing energy consumption in this recycling process remains a key challenge for achieving sustainability on a larger scale.

The distinctive microstructural and chemical characteristics of AAC critically influence its behaviour when used as a secondary material in cementitious systems. AAC is characterized by high porosity, typically exceeding 70%, and its specific mineralogical composition (characterized by the presence of tobermorite, residual portlandite, and unreacted quartz) determine its water absorption capacity, shrinkage behaviour, reactivity, and compatibility with other constituents in concrete and mortar [63,64].

The high open porosity of AAC significantly increases its water absorption compared to conventional aggregates. Zou et al. [42] and Liu et al. [50] reported that recycled AAC aggregates and powders exhibited water absorption rates up to three times higher than those of natural sand. This elevated absorption can reduce the effective water-to-cement ratio during mixing, impairing workability and potentially leading to insufficient hydration if not properly accounted for. Moreover, the high porosity contributes to increased drying shrinkage in mortars and concretes incorporating AAC waste, as observed by Liu et al. [41], which can compromise dimensional stability and lead to cracking.

From a chemical perspective, the reactivity of AAC powder as a supplementary cementitious material is largely influenced by its tobermorite content and residual portlandite. Finely ground AAC particles have been shown to participate in pozzolanic reactions, consuming calcium hydroxide and forming additional calcium silicate hydrates (C–S–H), thereby contributing to strength development and densification of the matrix [32,35]. However, the degree of reactivity depends strongly on particle fineness, as coarser particles display limited pozzolanic activity and may act primarily as inert fillers. He et al. [35] demonstrated that AAC powder with particle sizes below 90 µm achieved improved reactivity and compressive strength contributions, while coarser fractions yielded negligible benefits.

Despite these potential benefits, the chemical composition of AAC waste can introduce compatibility challenges. The residual alkalinity of AAC, attributable to unreacted portlandite and alkali-bearing phases, can raise the pH of pore solutions and contribute to alkali–silica reactions when combined with reactive aggregates in the matrix [56]. Furthermore, carbonation of AAC waste, common during storage or demolition, alters its surface chemistry, reducing its reactivity and increasing water absorption [55]. These factors underscore the need for careful material characterization and potential pretreatment to mitigate adverse effects.

In summary, the high porosity and specific chemical phases present in AAC waste confer both opportunities and limitations when recycled as powder or aggregate. While the porous structure and mineralogical composition enable water retention, internal curing, and partial pozzolanic reactivity, they also increase shrinkage, reduce workability, and may pose durability risks. These findings highlight the necessity of optimizing processing parameters, such as particle size refinement and moisture conditioning, and controlling replacement ratios to fully exploit the benefits of recycled AAC while maintaining acceptable performance standards in concrete and mortar.

4.4. Possible Reuse Methods

4.4.1. An Adsorbent for Treating Contaminated Water

In recent years, several studies have highlighted the potential of AAC crushed particles as an effective material for treating heavy metals in wastewater. Researchers have focused on the material’s properties, specifically its low density and high porosity, which confer a strong adsorption capacity. For instance, Tran et al. [65] conducted heavy metal adsorption experiments using highly concentrated arsenic-contaminated wastewater in southern Hanoi, Vietnam. Their findings demonstrated that pulverized AAC particles achieved a 100% removal rate in column filtration experiments. Furthermore, a series of experiments conducted by the American Society of Civil Engineers investigated AAC’s metal adsorption capacity. These experiments revealed that AAC exhibits a consistent adsorption capacity for heavy metals under various conditions, with the maximum adsorption capacity being minimally affected by particle size [66,67]. Remarkably, AAC often outperforms or matches the adsorption efficiency of other adsorbents used in previous studies. This suggests that AAC particles represent a cost-effective and practical material for heavy metal adsorption, requiring only minimal pre-treatment before use. The research also identified a specific adsorption hierarchy for AAC particles, where Pb²⁺ > Cu²⁺ > Ni²⁺ > Cd²⁺ > Zn²⁺, with the adsorption capacities for lead and copper being particularly prominent [66,68,69]. Additionally, it has been shown that the chemical components of AAC significantly contribute to the removal of phosphorus and the regulation of pH levels in wastewater [70,71].

However, there remain several challenges that need to be addressed before AAC can be widely applied in practical wastewater treatment scenarios. First, the variability in wastewater composition, as well as differences in AAC material properties, can impact its adsorption performance [72]. Therefore, further research using real wastewater samples is crucial to establish a more comprehensive understanding of the conditions under which this adsorbent material can be most effectively employed. Second, there has been little discussion in the literature regarding the post-adsorption management of AAC materials. If this issue is not adequately addressed, there is a risk that these materials, after adsorbing heavy metals, could still end up in landfills, thus undermining their environmental benefits.

Despite these challenges, there is no doubt that utilizing recycled AAC waste for sewage treatment is a valuable and feasible approach. AAC waste is relatively inexpensive and can be used for pollutant adsorption with minimal processing. In contrast, the cost of commonly used commercial adsorbents, such as graphene and activated carbon, is significantly higher [73]. Additionally, these commercial adsorbents often face challenges related to their recyclability, which places them at a disadvantage compared to AAC waste [74]. Furthermore, it is widely recognized that no single method can effectively remove all pollutants or heavy metals from wastewater, necessitating the use of multiple treatment methods or adsorbents in combination [75]. Given AAC’s demonstrated effectiveness in adsorbing certain heavy metals, it could be integrated with other treatment methods to enhance the overall adsorption effect. Future research should focus on identifying the optimal synergistic combinations of AAC with other adsorbents and treatment techniques to maximize its potential in wastewater management.

4.4.2. Mixed Subgrade Materials

AAC granules, owing to their low strength, are generally not considered suitable materials for road construction. One of the main reasons for this is that AAC recycled aggregate is particularly vulnerable to certain environmental pollutants, such as sulfates and carbonates, which can significantly deteriorate the concrete over time [76]. These contaminants chemically react with components in AAC, leading to the formation of calcium sulphate and ettringite. The presence of these substances can cause various structural issues, including deformation, cracking, and general damage to roads [77]. As a result, AAC waste is currently not utilized in road construction projects.

However, some recent studies have begun to explore the potential benefits of using mixed materials that incorporate AAC waste for road construction. Altun et al. [78] investigated the effects of combining recycled concrete with AAC particles and clay brick aggregates, focusing specifically on gas transport parameters. Their findings indicated that the addition of AAC particles could contribute to the development of a permeable pavement system, which is advantageous for road infrastructure. Additionally, Cong et al. [79] explored the water absorption and antifreeze properties of AAC, using various chemical additives to enhance its performance. They found that the use of foam stabilizers significantly improved the compressive strength of AAC. In their study, nine sets of AAC samples, each with different chemical additives, were subjected to 50 freeze–thaw cycles, and all samples remained structurally intact. The excellent frost resistance demonstrated by AAC in these tests suggests that it could potentially serve as a viable material for roadbed construction. Similarly, Thai et al. [80] examined the thermal conductivity of a mixture containing concrete, clay bricks, and AAC recycled aggregates. Their research concluded that the thermal properties of this mixture are well suited for road materials and could even help mitigate the effects of urban heat islands.

In summary, while AAC recycled aggregate has not traditionally been used in road construction due to its susceptibility to pollutants and low strength, emerging research suggests that it may be feasible to incorporate AAC into road construction materials, particularly when used in combination with other aggregates and chemical additives. Further research is necessary to fully assess the performance of mixtures containing AAC recycled aggregates and to determine the optimal mixing ratios for road construction applications.

4.4.3. Biological Filter

A biological filter is essentially a reactor designed to develop a biofilm on a microbial carrier, enabling microbial reactions that facilitate the filtration of pollutants [81]. These filters are particularly effective in addressing water pollution, especially organic pollution issues such as the eutrophication of water bodies. Currently, various filter media, such as ceramsite and polyethylene plastics, have been developed for this purpose. To function effectively as a biofilter, the carrier material must exhibit certain characteristics: it should have high porosity and a large specific surface area, allowing for the accommodation of substantial active biomass and diverse microbial populations [81,82]. Coincidentally, AAC particles possess these crucial attributes.

Research has indicated that AAC particles meet the essential criteria for an effective biofilter carrier. AAC particles have a porosity exceeding 80%, making them highly suitable for this role [83]. In addition to their high porosity, AAC particles have open pores and a surface roughness that is conducive to the immobilization of microorganisms on their surface [84]. When compared with commercially available ceramsite (CAC), AAC particles demonstrate more pronounced and favourable characteristics in terms of porosity and surface area, and density (

Table 3. Comparative characteristics of AAC particles and CAC [81,85].

Characteristics	CAC	AAC
Silt carrying capacity (Cs/%)	<1	3.34
Void fraction (v/%)	>42	69.10
Specific surface area (Sw/cm2/g)	>2 × 104	8.1 × 105
Piled density (Pp/g/cm3)	<1.0	0.53
Apparent density (Pap/g/cm3)	1.4–1.8	1.71
Porosity (P/%)	53%	89.21%
Removal rate of TOC within 3.5 h	68%	66.8%
Removal rate of NH3-N within 3.5 h	49.4%	95.55%
Removal rate of TN within 3.5 h	25.6%	41.56%
Removal rate of PO43− within 3.5 h	42.8%	57.55%

). Table 3 also presents a comparison of the removal capacity of AAC and CAC for specific target pollutants, which are the total organic carbon (TOC), total nitrogen (TN), ammonia nitrogen (NH3-N) and phosphorous (PO43-). These are standard indicator substances for contaminant testing. Compared to the conventional biofilter on the market, which is CAC, AAC has a higher removal rate for most standard pollutants within a specific time. For TN and NH3-N especially, the removal efficiency of AAC is more than twice that of CAC.

Overall, AAC proves to be an effective biological reaction medium and, in certain aspects, outperforms CAC. This suggests that recycling AAC material holds significant potential to replace CAC as a biofilter carrier for the filtration of organic matter, thereby contributing to the resolution of water eutrophication issues. Furthermore, some studies have proposed that AAC particles could serve as carriers for methane-oxidizing bacteria [86,87]. This concept involves using AAC particles as an outer layer in domestic landfills, where the methane emitted by the landfill could be oxidized by the bacteria into carbon dioxide and water [86,88]. This research opens up the possibility for AAC to be utilized in diverse environmental applications, particularly in enhancing sustainable waste management and water treatment practices.

4.5. Secondary Material Recipient

AAC, as a cementitious composite material, has been extensively studied for its potential to incorporate secondary materials. Numerous investigations have explored the integration of recycled constituents into AAC mixtures, aiming to enhance material circularity while maintaining or improving mechanical and thermal performance. In this context, AAC serves as a receptive matrix for recycled secondary materials, facilitating their reintegration into the construction industry and contributing to resource conservation and waste minimization [89,90].

Table 4. Past innovations of using secondary material to replace AAC base materials.

References	Secondary Materials	Replacement	Achievement
[91]	Waste aluminium dust	Foaming agent	Every 15.6 g of aluminium dust can replace 1 g of aluminium powder.
[92]	Municipal solid waste incineration ash	Sand and fly ash	20% sand replacement rate, 10% fly ash replacement rate.
[93]	Recycled wood fibre	Additive	Improvement of brittleness and low fracture toughness.
[94]	Agriculture and industrial solid wastes	Cement	Higher mechanical properties appear in the addition of blast furnace slag.
[95]	Recycled shale gas drilling cuttings	Cement	40% fly ash replacement rate.
[96,97]	Rice husk ash	Quartz sand	27 °C reduction in autoclave temperature, 22% increase in strength.
[98]	Copper tailings	Quartz sand	40% sand replacement rate.
[99]	Lead-Zinc tailings	Siliceous materials	Lead-Zinc tailings incorporation of AAC at 62% mass ratio.
[100]	Coal gangue and iron ore tailings	Sand	Optimal calcination temperature was approximately 600 °C.
[101]	Gold tailings	Siliceous materials	Gold tailings incorporation of AAC at 62% mass ratio.
[102]	Basic oxygen furnace slag	Sand	15 wt.% replacement using the basic oxygen furnace slag.
[103]	Desulfurization slag	Sand	Using 0.17 M NaOH(aq) to replace water or calcining the De-S slag in advance both improved the foaming of mortars.
[104,105]	Iron ore tailings	Siliceous materials	Iron ore tailings incorporation of AAC at 62% mass ratio.
[106]	Ceramic polishing slag	Sand	The best weight ratio of slag paste, cement, and lime is 14.02:1.13:1.17.
[107]	Desulfurization wastewater and sludge	Water	40% water-saving rate.
[108]	High-volume calcium coal gangue waste	Sand	58% incorporation of high-volume calcium coal gangue waste; 40% cost savings.
[109,110]	Crushed waste oyster shell	Natural sand	30% natural sand replacement rate.

shows the previous studies that have achieved results using waste materials to replace AAC base materials.

While these studies highlight the feasibility of incorporating recycled materials into AAC, several critical challenges remain. The variability in secondary material composition can lead to inconsistencies in AAC performance, necessitating rigorous quality control and standardisation. Furthermore, some waste materials, such as municipal solid waste incineration (MSWI) ash and tailings from mining activities, may contain heavy metals or hazardous elements, raising concerns about long-term environmental and health impacts. Strategies for material pretreatment and stabilisation are crucial to mitigating these risks.

A closer examination of the findings in Table 4 reveals that different secondary materials offer distinctive advantages and operational benefits. For example, rice husk ash not only improves strength but also reduces autoclaving temperature, yielding significant energy savings, while wood fibres enhance toughness by mitigating brittleness. Substitutions with high calcium or siliceous industrial by-products, such as slag or gangue, often improve mechanical performance and reduce production costs. At the same time, the high variability in substitution rates, treatment requirements, and resulting property enhancements underscores the complexity of achieving consistent quality and scalable production. Importantly, not all substitutions are equally advantageous; some may require extensive pretreatment, introduce durability concerns, or offset cost savings due to higher processing demands.

These findings collectively highlight a critical trade-off between sustainability, economic viability, and technical performance in AAC recycling practices. While certain substitutions, such as desulfurization wastewater or calcium-rich coal gangue, demonstrate clear environmental and cost benefits, others raise questions about long-term structural integrity or environmental safety. This suggests that future research should prioritise systematic investigations into the durability of AAC with high proportions of recycled materials under real-world conditions. Furthermore, comprehensive lifecycle assessments and cost–benefit analyses are essential to evaluate the true environmental and economic impacts of these substitutions over the entire service life of AAC-based products. Developing robust guidelines and standardised protocols for incorporating secondary materials can help overcome technical and regulatory barriers, ensuring that AAC recycling innovations align with circular economy principles while maintaining practical feasibility in construction.

By synthesising these insights, it becomes evident that AAC holds substantial potential as a sustainable and resource-efficient construction material when paired with effective recycling strategies. Addressing the identified gaps through interdisciplinary research and industry collaboration will be key to unlocking this potential and advancing the role of AAC in meeting the growing demand for environmentally responsible building materials.

5. Discussion and Future Research Directions

The literature review on AAC recycling categorizes the current research into three primary themes: filling or supplementary material, aggregate use, and innovative recycling options. Each theme has been critically assessed. Additionally, this review identifies knowledge gaps that require further investigation. The findings reveal that recycling technologies for AAC are still in their infancy, with development largely remaining in the early stages. While some technologies and related studies have been identified, many are still confined to the laboratory, lacking large-scale application or commercialization. Based on these observations, the study outlines several key directions for future research aimed at advancing the recycling of AAC and overcoming the current technological limitations.

Table 5. Future research directions in AAC recycling.

Research Directions	Previous Research	Limitations	Research Gaps
Comprehensive Characteristic Study	Studies have examined the characteristics of recycled AAC products, including some mechanical and thermal properties.	Long-term durability; intrinsic reaction mechanisms (e.g., fluidity variations, shrinkage) remain poorly understood.	In-depth investigations into long-term performance, microstructural changes, and behaviour under varied environmental conditions to support reliable, high-performance recycled AAC products.
Integrated Production Standards	Research often explores specific parameters (e.g., fixed replacement rates, particle sizes, or cement types) for recycled AAC production.	Approaches are fragmented, overlooking the complex interplay of materials and conditions, and lacking holistic guidance for industry.	Systematic studies considering broader variables and conditions, and development of clear, adaptable production standards to enable scalable, industrial implementation.
Enhancement of Recycling Techniques	Current recycling processes rely on high-energy crushing and grinding; technologies like process modelling and minimum quantity lubrication are known.	High energy consumption increases costs and undermines environmental benefits, limiting competitiveness of AAC recycling.	Need to refine existing technologies and develop novel, low-energy processes to improve efficiency, lower emissions, and make AAC recycling more sustainable and economically viable.
Operational Standards for Pollutant Management	Laboratory research has explored recycled AAC as a heavy metal adsorbent and as biofilters for water eutrophication and methane emissions.	Applications remain at experimental stage; no industrial-scale implementation; operational parameters and protocols are undefined.	Need to establish detailed operational standards and evaluate regeneration, performance, and economic feasibility under real-world conditions to enable pollutant management applications at scale.
Hybrid Materials for Road Construction	Mixed materials incorporating AAC waste have shown benefits like improved frost resistance and breathability.	Standalone AAC particles are unsuitable due to low strength and chemical instability; existing studies assess only individual properties, not holistic performance.	Comprehensive evaluation of mechanical, chemical, and environmental behaviour of hybrid road materials, and optimisation of mixing ratios and application parameters for durable, cost-effective solutions.
Sustainable AAC with Recycled Materials	Significant efforts have introduced secondary materials into AAC to improve sustainability, reduce costs, and enhance resource efficiency.	Challenges include material variability, contamination risks, and maintaining durability; quality control is difficult.	Need to optimise material formulations, assess environmental risks, and ensure compliance with standards to facilitate broader adoption of sustainable AAC products in commercial construction.

below summarises the key research directions in AAC recycling, highlighting current research efforts, identified limitations, and corresponding research gaps that need to be addressed to advance sustainable and practical applications of recycled AAC in the construction industry.

6. Conclusions

As a widely used primary construction material, AAC is produced in volumes reaching hundreds of millions of cubic meters annually. As buildings composed of AAC near the end of their useful lives, the necessity for recycling these materials becomes increasingly urgent. When AAC cannot be recycled, it must be disposed of in landfills, which imposes significant environmental burdens. Conversely, recycling AAC waste offers substantial economic advantages and helps conserve valuable resources. This study presents a thorough review of current recycling techniques for AAC waste. While existing methods are generally effective, there are notable discrepancies between theoretical research and practical application. Various studies address different facets of the recycling process, yet no single study provides a holistic overview or systematic production guidelines. Additionally, unresolved uncertainties impede the commercialization of these recycling technologies. Current methods are also criticized for their high energy consumption, which limits their environmental efficacy. Consequently, there is a pressing need for the development of more efficient and straightforward recycling techniques.

The study delves into current recycling technologies and methodologies for AAC and identifies new opportunities for recycling innovations. To address the gaps in existing research and further the advancement of AAC waste recycling, this paper proposes several critical areas for future exploration: (1) in-depth analysis of material properties; (2) the formulation of unified production standards; (3) advancements in recycling techniques; (4) the development of a comprehensive pollutant management method; (5) investigation into the use of hybrid materials in road construction; and (6) integration as a secondary material recipient. This overview aims to inform both researchers and industry professionals, providing a foundation for future research initiatives and practical applications in the field of AAC recycling.