*3.2. Co-Author Analysis*

Co-author analysis can identify authors who have made significant contributions to this field, and their changes in research interests also reflect research trends to some extent. At the same time, the regional or global development of this field can be grasped in a timely manner by revealing the close relationship between them.

### 3.2.1. Co-Authorship Analysis

In this bibliometric study, 11,795 authors were included in the research on C&D waste and the sustainable development of resources. As shown in Figure 2, the network view and density view of 149 authors with more than 5 articles were drawn by VOSviewer. In the network view, a node represents an author, and its size is proportional to the number of articles by each author. The links between nodes intuitively demonstrate the cooperative relationship between authors. The thicker the line, the closer the connection between them. The distance between nodes indicates the affinity between them, and different colors represent different clusters of author collaboration. Accordingly, in the density view, the more nodes around a node, the brighter the color and the closer the cooperation.

**Figure 2.** Co-author network.

As can be seen from Figure 2, 149 authors were labeled as 11 research clusters according to their degree of cooperation, and they can be roughly divided into five research communities. With Vivian W.Y. Tam, an author from the University of Western Sydney, as the central author, the most connected research community is formed with Jian Zuo from the University of South Australia and Xiangyu Wang from Curtin University, Australia, etc. They focused primarily on recycled aggregate concrete and sustainable performance assessment, while recent research has focused mainly on quality improvement of recycled concrete and lean construction management. Another closely connected research community consists of Chi Sun Poon, Lei Wang and Md Uzzal Hossain et al. They mainly worked on the environmental friendliness and sustainable management of recycled concrete products.

#### 3.2.2. Co-Institution Analysis

To discover the important research organizations in the field of C&D waste and the sustainable development of resources, the collaborative network of institutions with more than 10 published articles is shown in Figure 3. In this part, 115 nodes represent 115 institutions, most of which are universities. The size of the node depends on the number of publications, and the degree of collaboration between the two institutions is indicated by the thickness of the lines. Colors of institutions represent the clusters to which they belong. As can be seen from Figure 3, Hong Kong Polytechnic University is the largest node, which is most closely related to the purple cluster and the blue cluster, such as Tongji University, Hong Kong University and City University of Hong Kong in the purple cluster, while in the blue cluster, the China Academy of Science and Shanghai Jiao Tong University are the main partners, indicating that there are close cooperations between top scientific research institutions and top universities in China. There are also orange clusters and red clusters with frequent cooperation. Malaysia Petroleum University is the central institution that maintains close cooperation with Chongqing University, Melbourne Institute of Technology, Milan Institute of Technology, Curtin University and Delft University of Technology, etc. It also shows that global institutions attach greater importance to the areas of C&D waste and the sustainable development of resources, and are actively expanding cooperation and exchange of research experiences.

**Figure 3.** Co-institution network.

#### 3.2.3. Co-Country Analysis

The research situation of countries/territories was analyzed using VOSviewer. A total of 117 countries/territories have contributed to this field, but only 69 countries/territories with more than or equal to 10 papers are presented in Figure 4. Similarly, the larger the node, the more articles the country or region has published, and the thicker the line, the more frequently the two countries/territories communicate. China is the most productive country (*n* = 828, 23.32%), followed by India (*n* = 339, 9.55%), Australia (*n* = 299, 8.42%), the United States (*n* = 285, 8.03%) and England (*n* = 258, 7.27%).

**Figure 4.** Co-country network.

With the development of the construction industry, countries pay more attention to construction waste. It can be observed that China, which publishes the most articles, has very frequent cooperation with Australia, the United States and England. These countries have many partners and have made rapid progress in C&D waste and the sustainable development of resources through global communication, cooperation and information sharing. As can be seen from Figure 1, despite India publishing research articles later than expected, the number of articles on construction waste has increased rapidly in recent years, indicating that India has gradually realized the importance of the sustainable development of construction resources.

Analyzing the research cooperation network among individuals, institutions and countries from the perspective of temporal evolution can better reveal the overall development trend of this field in recent years [51]. Starting from the year (2003) when the number of papers was more than 10, longitudinal change charts of nodes, links, and density are illustrated using Citespace. Among them, the time interval is set to 5 years, nodes represent authors, links represent cooperation among authors, and the density is the actual number of relationships in the network divided by the theoretical maximum coefficient. Figure 5 shows the results. Three line charts show changes in relevant parameters for individuals, institutions and countries, respectively. The horizontal axis represents time, the left side of the vertical axis represents the counts of nodes and links, while the right side represents density value.

It can be seen from Figure 5 that the blue line representing the number of authors continues to grow, indicating that new researchers (individuals, institutions, and nations) have joined and contributed to the field of C&D waste and sustainable development as a result of the current environmental deterioration, and the increase has gradually accelerated since 2018. Another set of green lines indicates a change in the number of links among researchers, which is also on the rise in general. Although the number of collaborations between individuals and institutions decreased slightly from 2003 to 2007, they all resumed their enthusiasm for cooperation in 2008 and continued to increase later, while cooperation between countries kept increasing throughout the period. The strength of the nodes' collaboration is shown by the purple line. According to Figure 5, research on C&D waste and sustainable development was still in its infancy between 2003 and 2017. Many new researchers were still in the exploratory phase and had not started active external communication, which led to a downward trend in the overall density line. With the continuous deepening of research, this field has achieved great results. Therefore,

researchers have engaged in active experience sharing and academic cooperation to seek new breakthroughs in the field, making the density curve rise again.

(**c**)

**Figure 5.** (**a**) Longitudinal analysis of co-author networks. (**b**) Longitudinal analysis of co-institution networks. (**c**) Longitudinal analysis of co-country networks.
