Research trends on ash stabilization in the pavement during 2002–2021

Incineration ash stabilization in the pavement is an effective approach to reduce environmental impact and proper disposal. This study aimed to achieve a bibliometric analysis of “ash stabilization in pavement” research during 2002–2021, considering language, chronological trends, source types, subject areas, document types, affiliations, journals, countries, and author keywords. Also, social network analysis (SNA) was used for trends mapping and global collaboration determination among countries. The results exposed that the number of publications has been significantly increased by more than 14-fold over the studied period. Engineering (42%), material science (17%), and environmental science (10%) were three major subject areas. The USA, by 383 publications, was the leading country, followed by India (370) and China (288). The most independent rate of the publications belonged to India (93%), while Australia ranked 1st in cooperator publications (63.4%). The “Construction and Building Materials” published the most related articles, followed by “Journal of Materials in Civil Engineering” and “Road Materials and Pavement Design.” Among the top ten productive institutes and organizations, four affiliations were from China, and three institutes were from the USA. Edil, T.B. (34) from the USA, Arulrajah, A. (29) from Australia, and Horpibulsuk, S. (24) from Thailand were the most productive authors. “Fly ash,” “compressive strength,” “durability,” “geopolymer,” “strength,” and “resilient modulus” had the most growth rate in recent years, based on the author keywords analysis. Besides, the increase of emerging keywords, “pervious concrete,” “reclaimed asphalt pavement,” and “lateritic soil” are noteworthy in the second half period.


Ash stabilization in the pavement
The global municipal solid waste (MSW) generation is expected to increase to 2.2 billion tonnes by 2025 (Hoornweg and Bhada-Tata 2012). Incineration is of the most common methods of waste disposal (Rahman et al. 2020). The incineration reduces the MSW mass and volume by 70% and 90%, respectively (Becquart et al. 2009;Silva et al. 2019).
This method generates two main kinds of ash, named bottom ashes (BA) and fly ashes (FA) (Lam et al. 2010). Incineration ashes, particularly fly ash, need to be disposed of properly because of their substantial amounts of toxic materials and leachable heavy metals (Wang et al. 2019). Three treatment approaches have been applied and may be categorized as (1) separation processes, (2) solidification/stabilization, and (3) thermal methods to utilize ash and the environmental impact reduction (Reijnders 2005;Quina et al. 2008).
Most developed countries tend to reduce landfill waste because of scarcity of landfill sites, high landfilling costs, and tight environmental policies (Banar and Özkan 2008;Lam et al. 2010). Stabilization in the pavement is the effective approach in which ashes can be efficiently stabilized in road pavement as a common and straight method of ash reusing (Abukhettala 2016;Joshi et al. 2019).
The considerable number of publications and various discussions have motivated many researchers globally to stabilize incineration ashes in the pavement. Xue et al. replaced incineration ash with part of the pavement filler. All samples from fly ash performed better and met environmental standards in the TCLP test (Xue et al. 2009). In 2014, Vizcarra et al. stabilized fly ash at a ratio of 20-40% based on the pavement. Applying fly ash reduced expansion and increased the California bearing ratio (CBR) and resilient modulus (Vizcarra et al. 2014). Tang et al. stabilized Changshu's incineration fly ash along with cement between the base and subbase layers and then performed resistance tests on the compound. The results showed that placing the layer of fly ash as a layer of pavement material between a foundation and substructure helped prolong the fatigue life and reduce the asphalt rutting (Tang et al. 2017). Romeo et al. investigated fly ash, washed fly ash, and limestone filler in three sample models. Creep compliance, resilient modulus, and tensile strength were performed on them. Finally, due to the leaching rate of heavy metals, the use of washed fly ash is recommended, which also samples containing it had a higher resistance than the other two ashes (Romeo et al. 2018). In 2022, Li et al. stabilized incineration ash in asphalt, along with varying amounts of cement. According to the results, when the ash content was higher, the stability of heavy metals could be achieved by adjusting the cement dose parameter. Finally, fly ash utilization in road pavement has been reported to be technologically and environmentally safe (Li et al. 2022). Rahman et al. (Rahman et al. 2020), Lu et al. (Lu et al. 2020, and Vishnu et al. (Vishnu and Singh 2021) also compared different recycling methods for various types of waste, including ash stabilization in the pavement. Everyone declared this method safe and environmentally friendly.

Bibliometric analysis
The bibliometric method for organizing research trends and categorizing issues is used to investigate the literature in various terms, based on publications history in different literature characteristics (Ye et al. 2014;Wu et al. 2015;Qi et al. 2019;Sabour et al. 2021b). This method reflects global developments in specific research fields (Sabour et al. 2021a). The mandatory bibliometric data could be derived from various databases (Zhao et al. 2019). In this research, Scopus was selected as the most widely used database, the most advanced database of abstracts and citations in the literature, for research and download data. (Borthakur and Govind 2018;Sharma and Kaur 2020;Fakher et al. 2021). SNA maps have also been used in this study to show proper visualization results of derived bibliometric data (Zhuang et al. 2013). VOSviewer software has been applied to display maps of bibliometric data in the appropriate illustrations (Velmurugan and Ramasamy 2021). This software is one of the software originated by Van Eck and Waltman from the University of Leiden in the Netherlands (Meng et al. 2020). It can also demonstrate maps with many data, while most computer programs used for bibliographic illustrating do not have this characteristic (van Eck and Waltman 2010). This application is freely available to bibliometric researchers (www. vosvi ewer. com). It could be used to exhibit maps in different forms such as network, overlay, and density visualization based on the relationship of publication components, time of published research, and intensity and number of elements (Mostafa Hatami et al. 2022).
Despite numerous research studies on ash stabilization in the pavement, no bibliometric analysis has been done in this field. Therefore, all parts of this systematic analysis of ash stabilization in the pavement were conducted at K. N. Toosi University of Technology from January 2022 to March 2022.

Data collection and processing
In this research, Scopus has been considered due to the wideranging statistical supplies and extensive usage in academic study (Gao et al. 2016;Effendi et al. 2021). Furthermore, Scopus statistics could be exported in comma-separated values (CSV) format, and in this study, the mentioned format was used to assess data on numerous categorizations systematically.
The research was achieved on publications that enclosed "pavement" and "ash" in the title, abstract, and keywords from 2002 to 2021. Language, source type, subject area, document type, affiliation, country, and keywords were considered from 2002 to 2021. More than 95% (1693) of the publications were in English, followed by 4.2% (74) Chinese and 2.3% (4) Japanese. All of the document types, including journal articles, conference papers, book series, trade journals, and books, were studied in the research.
SNA has been utilized to emphasize the relationship among different nodes as the countries or the author keywords in other clusters of the network structure (Wang et al. 2016a). In this research, VOSviewer 1.6.6, one of the most helpful visualization software for the execution of SNA, was applied for forming bibliometric maps of analysis of countries' cooperation and analysis of the author keywords.
Also, the impact factor (IF), h-index, subject area, country, and publisher of each journal were evaluated from Scopus and SCImago Journal Rank (SJR) in this study (SCImago Journal Rank 2022; Scopus 2022).

Results and discussion
Chronological trends and source types Figure 1 shows the yearly number of related publications in two decades. The publications are in the field of pavement and ash, which are briefly shown as "PAVE + ASH" in this research henceforth. Studies have increased from 17 in 2002 to 247 in 2021, which has grown significantly. A significant percentage (83.9%) is related to journal articles and conference papers. The 1778 publications studied involved 1032 journal articles (58.0%), 460 conference papers (25.9%), 238 series of books (13.4%), 37 trade journals (2.1%), and 11 books (0.6%).

Subject categories
PAVE + ASH publications have been widely distributed in 21 areas. Also, most publications are categorized into several subject areas (for example, finite-element modeling of the pavement containing incineration ash test section using PLAXIS 3D (Khan et al. 2022), categorized in engineering, material science, environmental science, and computer science). Incineration has many environmental and technical aspects. Besides, pavement is highly concerned with engineering, material science, and geology. So, according to Fig. 2, the divisions of engineering (42%) and material science (17%) are conspicuous. Two 10%-portions are related to environmental science and earth and planetary sciences, and the following areas are social sciences (4%), physics and astronomy (3%), computer science (3%), and energy (3%).
While the rest (Agricultural and Biological Sciences, Chemical Engineering, Business, Management and Accounting, Chemistry, Multidisciplinary, Economics, Econometrics and Finance, Medicine, Biochemistry, Genetics and Molecular Biology, Mathematics, Arts and Humanities, Immunology and Microbiology, Pharmacology, Toxicology and Pharmaceutics, and Decision Sciences) provided the residual 9%. Since this study is a general review and no filter has been used other than the original Scopus settings (searched keywords and applied period), it is normal for some studies unrelated to the article's primary purpose.

Countries
The studies have been published from 81 countries in the studied period, while twelve countries had only one, nine countries had two PAVE + ASH publications, and 109 publications had no author's country information.
The top ten most productive countries in the field of PAVE + ASH are listed in Table 1. The USA, with 383 publications, ranked 1st, followed by India (370) and China (288). In the survey, the top three countries (the USA, India, and China) in the field of ash utilization in the pavement were in the list of top ten countries in the field of incinerationrelated studies (Mostafa Hatami et al. 2021). More than 75%   Fig. 2 The most major disciplines (1338) of the PAVE + ASH publications are related to these ten countries. Independent publications exhibit that all their authors are from the same country. In contrast, the cooperator publications identify individuals whose authors are not from a single country. According to Table 1, 76.8% of Americans, 93.0% of Indians, and 78.5% of Chineses were exclusively published from these countries. With 63.4% of Australian PAVE + ASH publications, Australia ranked 1st in cooperator publications. It could be a reasonable reason that Australian students transfer to various foreign countries.
As shown in Fig. 3, the nodes displayed in network visualization specified the countries (Wang et al. 2016b), while the nodes' size demonstrated the activity of that country, and each link exposed a relation between two countries (Mostafa Hatami et al. 2022).
The USA, India, and China have contributed many publications in this field. The USA and China had the most substantial number of cooperations with the other countries, with 32 and 24 links.
The USA was also the most influential country based on the number of citations (4890), followed by India (2507), China (2148), Australia (1920), Thailand (1389), Turkey (1237), and Canada (1036). Besides, regarding "total link strength" analysis, the USA (92), China (64), and Australia  Table 2 shows the ten most productive sources with their respective IFs, h-indexes, subject areas, countries, publishers, and the number of publications. IF is the indicator to measure the journal's value (Amin and Mabe 2000). Also, h-index is specified as the h number of publications with h minimum number of citations each (Hirsch 2005). The specifications of the journals, conferences, and book series shown in Table 2 have been extracted from "Scopus" and "SJR" (Scopus 2022;SCImago Journal Rank 2022).

Source titles
As shown in Table 2, 575 publications (32.3% of all) are published in the ten sources, among which, there are four journals, four book series, and two conferences and proceedings.
The UK with four and the USA with three sources were the productive countries in this classification. All these ten sources involve engineering in their subject areas. At the same time, the American Society of Civil Engineers (ASCE), Trans Tech Publications, and Taylor and Francis Ltd. have been the most productive publisher in the field. In addition, "Construction and Building Materials" ranked 1st in the number of publications (105), IF (6.141), and h-index (170) among the journals. Besides, "Geotechnical Special Publication" was the effective source in the conferences and proceedings category, with 79 papers. Table 3 shows the top ten most productive affiliations in the field of PAVE + ASH. More than 17% (307) of the PAVE + ASH publications are related to these ten affiliations. "Ministry of Education of China" with 57 publications was the most productive organization in the field of "PAVE + ASH," followed by the University of Wisconsin-Madison (USA), Chang'an University (China), and Swinburne University of Technology (Australia), with 44, 40, and 32 publications, respectively. It was worth noting that four institutes were from China among the top ten affiliations, and three ones were from the USA. Table 4 shows the top ten authors who have been most active in this field, their respective countries, affiliations, h-indexes, subject areas, the number of citations, and publications. Edil   Table 4, and the most related subject area was engineering.

Author keywords
A total of 1797 author keywords during 2002-2021 were surveyed in this bibliometric research. A total of 1362 (75.8%) keywords appeared only once, and 202 (11.2%) keywords occurred twice. Each node represents a specified author keyword. Also, the bigger the size of the nodes, the more the occurrence of the keywords (Wang et al. 2016b).
Depending on the software settings, each number of related keywords form a cluster that was characterized by an indicated color. Figure 4 shows the studied keywords, which are classified into six clusters. Each cluster accommodates at least 200 keywords, while the most significant cluster was cluster 1 with 494, and the smallest one was cluster 6 with 218 keywords.
Cluster 1 (red) was approximately related to the publications pointing strength and stiffness characteristics by covering keywords such as "resilient modulus," "unconfined compressive strength," "CBR," and "California bearing ratio." Cluster 2 (green) belonged to recycling studies, by keywords "recycling," "cold recycling," and "recycled concrete." Some strength characteristics related keywords can be seen in cluster 3 (blue), such as "compressive strength," "flexural strength," "elastic modulus," and "stiffness modulus." "Durability," "strength," "mechanical properties," "water absorption," "porosity," and "waste materials" could be found obviously among the most-repeated keywords of cluster 4 (yellow). Cluster 5 (purple) involves "fly ash" with the most occurrence among the existing keywords of all groups, "pavement," and "sustainability." Cluster 6 (pale blue) belongs to soil-related publications by covering the keywords "black cotton soil," "lateritic soil," and "soil stabilization." The network visualization map of frequently used keywords in three time periods of 10 years (2002-2011 and 2012-2021) is shown in Fig. 5. Keywords with at least one occurrence are displayed on the maps in both periods. The keywords "pavement," "pavements," and "fly ash" were  The minimum number of words in each cluster was considered to be 100 in the first period and 250 in the second period. A total of 682 keywords are visible in four clusters in the first period, while 2334 keywords are visible in six clusters in the second period. In addition to the increase in the number of keywords, the increase in the number of clusters could also be considered. The keyword "compressive strength" ranks first in the second period with 67 repetitions, while "durability" ranks first in the first period with 19 repetitions. In contrast, "durability" ranks second in the second period with 56 repetitions, and "compressive strength" ranks second in the first period with 17 repetitions. Comparing the trend of using keywords in the two periods, the increase of "Stabilization," "Sustainability," and "microstructure" could be mentioned, which increased from 7, 3, and 2 occurrences in the period 2002-2011 to 54 and 29 and 24 occurrences in the period 2012-2021.
Regarding the growth of emerging keywords, "geopolymer," "pervious concrete," "reclaimed asphalt pavement," and "lateritic soil" are noteworthy that none of which existed in the first period; while have reached 49, 25, 24, and 17 repetitions in the second period, respectively.
Pavement testing keywords have also grown significantly. The keywords "CBR," "unconfined compressive strength," "California bearing ratio," and "UCS" were repeated 6, 4, 1, and 0 in the first period, and 44, 43, 38, and 19 repetitions in the second period. Finally, the reduction in the keyword "recycling" is considerable, from 19 to 15 repetitions.
On the other hand, by investigating the CSV output of VOSviewer software, the growth of some specific keywords could be significantly remarkable, representing the growth Fig. 4 The author keywords overlay visualization network map of related research in recent years. Among these keywords, "sustainability," "environmental sustainability," "life cycle assessment," and "circular economy" could be mentioned strictly, which were not detectable at the VOSviewer maps. It is envisaged that the mentioned keywords will also reach the stage of significant emergence in VOSviewer maps in the next few years.

Conclusion
Based on ash stabilization in pavement research retrieved from the Scopus database, the systematical analysis was conducted from 2002 to 2021. The number of mentioned publications has increased significantly in the past two decades, from 17 in 2002 to 247 in 2021. A total of 1778 publications were distributed in 21 subject categories, while more than half were published in engineering and material science areas. More than 95% (1693) of the publications were in English, while the majority of them belonged to journal articles (58.0%) and conference papers (25.9%). The "Construction and Building Materials," "Journal of Materials in Civil Engineering," "Road Materials and Pavement Design," and "International Journal of Pavement Engineering" were the most effective journals. "Geotechnical Special Publication" and "IOP Conference Series Materials Science and Engineering" were the most productive conferences and proceedings. The USA had the most growth rate and was the most effective country with the most publications (383), followed by India (370) and China (226). These three countries were in the list of top ten countries in the field of incineration-related studies, too.
The independent rate of these three countries' publications was 76.8%, 93.0%, and 78.5%, respectively. Based on the "citations," the USA (4890), India (2507), and China (2148) also ranked 1st to 3rd, respectively. The three most productive affiliations were "Ministry of Education of China" (China), the "University of Wisconsin-Madison" (USA), and "Chang'an University" (China), with 57, 44, and 40 publications. Among the top ten effective affiliations, four institutes were from China, and three were from the USA. According to the analysis of the author keywords, "fly ash," "compressive strength," "durability," "geopolymer," "strength," and "resilient modulus" were much more prevalent than other keywords.
Edil, T.B. (34) from the USA, Arulrajah, A. (29) from Australia, and Horpibulsuk, S. (24) from Thailand have the most publications, while the highest citations belonged to Benson,C.H. (12,488) from the USA. In the list of top ten productive authors, three authors were from the USA, and three were from India.
The increase of "geopolymer," "pervious concrete," "reclaimed asphalt pavement," and "lateritic soil" in the second half period is considerable, none of which occurred in the first ten years. Pavement testing keywords such as "CBR," "unconfined compressive strength," "California bearing ratio," and "UCS," besides "sustainability," "environmental sustainability," "life cycle assessment," and "circular economy" had the most increasing rate in recent years. These consequences could support researchers to improve understanding of the current situation, overall development, and the trends of ash stabilization in pavement research. The results will also provide a basis for future research in this area.