*3.3. Keywords Analysis*

Keywords are concise summaries of the research content covered. We can identify the main hotspots and central trends in the field of C&D waste and the sustainable development of resources through bibliometric analysis.

### 3.3.1. Keywords Co-Occurrence Analysis

As shown in Figure 6, the co-occurrence network with 528 keywords that have a frequency of more than 10 was presented by VOSviewer. Each node represents a keyword, and the frequency of keywords is expressed by the size of the nodes. The thickness of the connection lines between nodes intuitively reflects the intensity of the connection between keywords. These keywords are mainly distributed in the first three clusters, respectively by three colors (red, green and blue).

As can be seen from Figure 6, except for the keywords "sustainability", "construction" and "waste" searched in this research, "performance" (*n* = 566), "concrete" (*n* = 516) and "mechanical properties" (*n* = 492) are the top 3 keywords frequently used. It demonstrates that the study of materials' mechanical properties is especially crucial in the field of C&D waste and the sustainable development of resources in order to expand the market for recycled products, enhance participants' confidence in the use of recycled products in construction projects (especially recycled concrete products), and ensure their bearing capacity and durability within the design period. How to improve the quality of recycled products has gradually become a hot point in this field. Some scholars have analyzed the physical properties of concrete containing recycled aggregate [52]. Other scholars have studied the mechanical, permeable and physical properties of recycled aggregate by summarizing previous publications, and finally put forward the basic performance classification mainly used for concrete, which provides a practical method for measuring the quality of recycled aggregate [53]. As self-compacting concrete has become extremely popular in recent decades, Aslani et al. evaluated the optimal mix design of recycled concrete and crumb rubber aggregates in self-compacting concrete to optimize performance [54]. These are followed by "fly ash" (*n* = 408), "life cycle assessment" (*n* = 401) and "management" (*n* = 339), indicating that in addition to concentrating on waste treatment technology and treatment goals, it is equally necessary to explore and innovate new waste management methods to alleviate the tremendous burden of natural resource consumption and environmental degradation. Advanced management tools and methods play an important role in promoting the economic and environmental benefits of the construction industry. The introduction of the life cycle assessment (LCA) management method reflects that current research on construction waste is not just limited to waste reduction and recycling in the construction stage; BIM, big data, and other technical tools can be used for detailed planning, error correction, and waste management during the planning and design period to reduce C&D waste. At the purchasing phase, more green and recycled materials can be ordered to reduce the environmental impact of construction waste. Hossain et al. compared the environmental impact of producing recycled aggregate from C&D waste with that of producing natural aggregate from raw materials using a life cycle assessment approach [55].

Lu et al. established performance benchmarks for the management of construction waste in different project categories using big data technology to promote better management of construction waste [56]. Figure 6 also shows newer high-frequency keywords, such as "circular economy" (*n* = 285) and "microstructure" (*n* = 151). Studies have shown that about three quarters of the solid waste generated by the construction industry has residual value [57]. As awareness of sustainable development and resource management improved, many nations started exploring new models, and researchers gradually paid more attention to waste reduction, so as to minimize the negative impact of construction waste on the environment and further realize the sustainable development of resources. This also makes the circular economy model continue to attract the attention of researchers [58,59].

#### 3.3.2. Keyword Evolution Analysis

In order to more intuitively understand the research work of C&D waste and sustainable development, the evolution of keywords in this field was analyzed using Citespace, as shown in Figures 7 and 8. The time axis in Figure 7 indicates the time point when the keyword first appeared. The red section in Figure 8 represents a sudden increase in interest in keywords during this period.

As can be seen, "emission" and "sustainable development" appeared the earliest, demonstrating that harmful gases produced by the construction industry, particularly CO2, are responsible for the deterioration of the ecological environment. It is the gradual exposure of ecological and social issues that promote the sustainable development trend of the construction industry. Although the issue of emissions was brought up in 2002, little research has been carried out on it, and it has only recently become active. Some researchers have started to reduce CO2 emissions by studying environmentally-friendly and cost-competitive geopolymer concrete to gradually replace ordinary Portland concrete (OPC) that is energydependent and environmentally damaging [60]. There are also scholars who calculated the carbon emissions of geopolymer recycled aggregate concrete and studied its physical properties, such as slump and compressive strength, in order to prove its feasibility to replace OPC-based concretes [61]. From Figure 7, it is also clear that many keywords related to materials, such as "energy", "concrete", "cement", and "aggregate", occurred between 2004 and 2006. Among them, "energy" has the highest centrality (centrality = 0.13) and functions as a bridge. The energy consumption and pollution of the construction industry is enormous, so scholars have begun to pay attention to the environmental performance of the construction industry and its materials [62]. The keywords "environmental impact" and "design" appeared in 2006 and also reflect that researchers have gradually shifted their focus to other links besides the construction phase, in order to optimize the final disposal process of waste materials [63]. In the following years, most efforts were devoted to the development and performance optimization of various recycled materials. Since 2018, the emergence of "green concrete" and "circular economy" indicates that the construction industry has been guided by new theories, such as green building and the circular economy [64,65]. It is necessary to explore new and effective management models to promote resource utilization of construction waste and the sustainable development of construction resources.

With the development of C&D waste and the sustainable development of resources, although the introduction of new topics has slowed down after 2018, there are still many new opportunities and challenges. Combined with Figure 8, it can be seen that the keywords "optimization", "implementation" and "strategy" have again received great attention from academia in the recent period (2019–2022). Their respective citation bursts also rank among the top three. Therefore, these keywords can be identified as recent hot topics in this field. Identifying and implementing standards for green building materials (GBM) and assessing their sustainability, breaking down barriers to promoting a circular economy (CE), integrating life cycle sustainability assessment (LCSA) into the design phase to optimize building performance, and designing waste minimization strategies are expected to be future research directions.

**Figure 7.** Keywords timezone view.


**Figure 8.** Top 25 keywords with the strongest citation bursts.

#### *3.4. Documents Co-Citation Analysis*

In this paper, a cluster analysis of references, which reflect the knowledge bases and research frontiers, was carried out using the g-index operation in Citespace (the scale factor k = 25), and each cluster was labeled by keyword terms. Finally, 13 main clusters were generated (Cluster 0 was the search keyword, cluster 8 and 21 had little relationship with the research topic, and cluster 9 was repeated, so 4 clusters were manually deleted [66]). Modularity and silhouette values are 0.7877 and 0.8974, indicating the high reliability of the clustering structure. The result is shown in Figure 9 and Table 2.

**Figure 9.** Cluster analysis of documents co-citation.



In Table 2, the cluster size represents the number of co-cited references in the cluster. The larger the value, the more popular it is in a certain field. The average year can reflect the development trend of cluster. It can be seen from the results that there were many studies on the definition, classification and treatment of C&D waste in the early stages, and then the research direction turned to life cycle assessment and performance research of recycled aggregate concrete. In recent years, most scholars have begun to discuss the sustainable development of C&D waste based on some emerging theories, materials and technologies, such as the circular economy, geopolymer concrete, 3D printing technology

and prefabricated buildings, in order to further explore the sustainable development mode of C&D waste resources.

To further understand the recent development status of this research area, the three clusters with the highest average years are listed in Table 3. Meanwhile, the three most active cited articles and the three most active citing articles related to the cluster are listed in order to identify the intellectual bases and research frontiers in this field [67]. Details are discussed below.


**Table 3.** The three most active citing and cited papers in the latest three clusters.

Cluster 1 is labeled "circular economy", which has been a hot topic in the last five years. The three most active cited papers mainly focus on defining the circular economy and identifying obstacles to its implementation. Kirchherr et al. [68] summarized 114 definitions of CE and finally outlined the definition, which refers to an economic system that replaces the "end-of-life" of waste with reducing, reusing and recycling materials and recycling in the production/distribution and consumption process. Geissdoerfer et al. [69] provide conceptual clarity by illustrating similarities and differences between the terms "circular economy" and "sustainability" to improve the efficiency of the use of these approaches in research and practice. Mahpour [70], on the other hand, provided direction for further research by identifying barriers to the transition to CE in C&D waste management. Therefore, on the basis of intellectual bases, the three papers most actively cited are related to the promotion of CE. Norouzi et al. [59] further clarified the development process of CE through bibliometric analysis, and put forward future research topics, such as exploring the relationship between the smart city and CE, developing the business model of CE, etc. Yu et al. [71] found the lack of a decision-making framework in CE mainly from the perspective of policy formulation, and proposed a bi-directional policy-making mechanism based on two policy models. Rahla et al. [72] proposed strategies to promote the circular economy in three respects: resource management, architectural design methods, and digitalization of the construction industry.

Cluster 13, labeled "3D concrete printing", reflects that researchers have recently paid particular attention to the combination of waste disposal with 3D printing technology. In the intellectual bases, Wong et al. [73] found that brick powder is the most practical form of recycled brick, but it can only be used sparingly as concrete aggregate because it cannot significantly improve concrete performance. Meng et al. [74] reviewed the published literature on the use of various wastes in the production of concrete blocks, demonstrating the good potential of incorporating C&D waste into concrete blocks as aggregates. Habert et al. [75] provided medium-term and long-term solutions to environmental problems in concrete production. In the most active citing papers, Ahmed et al. [76] reviewed the existing 3D concrete printing technology and studied the different application technologies for structural reinforcement. Sahin et al. [77] showed that the use of geopolymers, recycled aggregates and waste in the mix design of 3D printed concrete (3DPC) can contribute to the sustainability of 3DPC. Qian et al. [78] also suggested that using recycled products as 3D

concrete printing materials can reduce carbon emissions and technology costs, which have high economic and environmental benefits. In general, 3D concrete printing technology is worth promoting in the field of C&D waste and the sustainable development of resources.

Cluster 23 is labeled "Prefabricated Residential Building" (PRB), so the top three cited papers all focus on the topic of prefabricated buildings. Hong et al. [79] established a framework for cost performance analysis to investigate the basic cost composition of prefabricated buildings, and assess the impact of adopting prefabricated technologies on the total cost of actual construction projects. Teng et al. [80] systematically examined evidence of reducing building life cycle carbon through prefabricated technologies. It showed that prefabrication resulted in a 15.6% reduction in embodied carbon and a 3.2% reduction in operational carbon. Hao et al. [81] developed a BIM-based approach to evaluating carbon emission reduction of a prefabricated building project, which showed that prefabrication has less negative environmental impact compared to traditional building technology. In the three citing papers, Lopez-Guerrero et al. [82] considered that the sustainability of industrial building systems (IBS) had previously only been examined in terms of environmental aspects and through qualitative indicators. To fill the gap, IBS sustainability was assessed using quantitative and qualitative indicators based on economic, social and environmental aspects. Based on evolutionary game theory, Yuan et al. [83] discussed the evolutionary decision-making behavior and stabilization strategy of the government, real estate developers and homebuyers in the PRB industry. They also proposed a promotion mechanism to help China's construction industry achieve orderly and sustainable development of the PRB. Zhang et al. [84] assessed the environmental impact of prefabricated building policies in Hong Kong SAR and Singapore by comparing these policies. This shows that researchers are beginning to notice the impact of prefabricated buildings on environmental benefits and sustainable development.

In general, Cluster 1 discusses the resourceful and sustainable treatment of construction waste from the perspective of circular economy management methods. Cluster 13 focuses on 3DCP technology for the reuse of construction waste. Cluster 23, on the other hand, focuses more on prefabricated building policies and regulations, and takes them as a new engine for reducing waste and carbon footprint and promoting resourceful utilization of construction waste.

#### **4. Discussion**

The research hotspots in the field of C&D waste and sustainable development of resource can be concluded based on the findings of keyword co-occurrence and cluster analysis. According to Figure 6, "performance", "concrete", "mechanical-properties", "flyash", "management", "life-cycle assessment", "circular economy" and "microstructure" have high frequency. At present, concrete is the material most often considered among recycled products. Geopolymer concrete, steel fiber recycled aggregate concrete, and other new types of recycled concrete are being developed. Research on the compressive strength, flexural strength, slump, durability and other mechanical properties of recycled concrete are also enduring topics in this field. In recent years, construction waste management methods have been developed and some achievements have been made. Academia has given the life cycle assessment (LCA) approach a lot of attention, showing that relevant research on reducing the environmental footprint is a hot topic in this area, as shown in Table 2. The application of LCA in the resource management of C&D waste places an emphasis on preventing the overall environmental impact of waste throughout the entire process. It changes the previous idea of focusing only on economic benefits or technological development, optimizes the specific steps of some promising technologies, and provides a new direction for the management of construction waste. There are also proposals to incorporate Life Cycle Sustainability Assessment (LCSA) into building design. In addition, the related research on CE and reduction has also become the focus in this field, so as to further improve the level of resourceful and harmless utilization of construction waste.

Combined with Figure 8 and Table 3, research frontiers in C&D waste and resource sustainability can be identified. Although the circular economy has been extensively promoted in recent years, many countries still face difficulties in implementing it due to a variety of problems. The development of 3D concrete printing technology conforms to the current trend of construction informatization. It can realize the recycling of resources when choosing construction waste as printing materials. Prefabricated buildings can make full use of their labor-saving and energy-saving advantages in the selection of prefabricated materials, production process and construction process, but the technical bottleneck of using construction waste in prefabricated buildings has always existed. Therefore, breaking the restriction of CE promotion and effective implementation, optimizing the performance of recycled products to meet the material selection standards of emerging technologies, such as 3D printing technology and prefabricated technology, using evolutionary game theory and other methods to study the behavior factors of various stakeholders and making policy suggestions to the government to encourage the use of the above management methods and technologies can be regarded as the current research frontiers.

Trends in C&D waste and sustainable development are summarized in Figures 7 and 8 and Table 2. The construction industry has contributed significantly to energy consumption and environmental damage while advancing economic development, such as resource shortages, greenhouse gas emissions, land loss, and other issues. The contradiction between them is becoming increasingly acute. As a result, many countries are gradually realizing that if construction waste continues to rise in line with the current situation, it will lead to huge losses. Therefore, the concept of resource sustainability is gradually integrated into construction waste considering the high residual value of construction waste. First, scholars started to develop and study recycled products to value construction waste and expand the market, such as recycled aggregate, recycled blocks and recycled bricks. After that, scholars began to emphasize the reduction in construction waste, and pay attention to the economic and environmental benefits of resource recovery to prove its sustainability, such as life cycle assessment, performance optimization of recycled products, the establishment of an environmental benefit assessment model, cost compensation model research, etc. BIM, GIS and big data technology are also constantly evolving. The circular economy and green building materials have been the focus of C&D waste and sustainable development in recent years, as the aim is to explore a new management model and fully integrate the idea of resource sustainability. In general, the evolution process can be summarized as recycling– reduction–sustainability. It is worth emphasizing that the stages in the evolution process is not completely separate from each other. For example, in the past five years, in addition to focusing on sustainability, reduction technologies such as 3D printing technology and prefabricated construction are constantly advancing.

Through the discussion of the above research hotspots, research frontiers and development trends, future research directions are also proposed:


and prefabricated construction, are still only used in projects with simple structure and small scale. How to combine existing mature information technologies or find new ones to solve the limitations of emerging technologies deserves further consideration.

