Bibliometric Analysis of Steelmaking Slag-Related Studies for Research Trends and Future Directions

Abstract

Although steel slag has been used over the past years, the data-based evidence cannot even be found to indicate the development of steel slag-related technologies. To clarify the leading regions, reveal the research trends, and propose future research directions, a bibliometric and visualization analysis was conducted based on the Web of Science (WOS) core databases. China, the USA, and Italy are the top three eminent countries, and the institution of Tsinghua University and Wuhan University of Technology tied for the first place with an H-index of 27. The research focus of steel slag has changed from mechanical properties, such as durability and strength, to functional applications, pretreatment technology, carbon sequestration, and environment-friendly products. Finally, several research directions were proposed for future investigation. This was the first time steel slag was investigated from a bibliometric perspective. The information obtained could provide scholars with the current recycling situation and the following academic steps.

1. Introduction

Steel slag is an inevitable by-product generated by the steel manufacturing industry. There are four types of slags during the whole steelmaking process, including blast furnace slag (BFS) produced in the iron-making procedure and basic oxygen furnace slag (BOFS), electric arc furnace slag (EAFS), and ladle furnace slag (LFS) produced during converting pig iron to steel. Although the characteristics of slag vary universally depending on the chemical impurities of iron ore, quality of flux, environmental conditions of the furnace, etc., the primary chemical composition of steel slag are Ca-, Fe-, and Mg- silicates. Furthermore, it was found that slags have the potential heavy metal leaching risk for low-end use, such as landfilling and dumping [1,2,3]. Around 100 million and 20 million tons of slags are produced annually in China and Europe, respectively. The large quantities of solid waste slags cause severe concerns about natural environment and ecological sustainability [4,5,6].

In order to lower the environmental concerns including heavy metal leaching risk and land wastage caused by massive accumulation of steel slag, thereby ensuring the sustainable development of the steelmaking industry, steel slag has been increasingly used in various aspects, including road construction, soil amendments, hydraulic engineering, fertilizer production, iron reclamation, and CO₂ fixation [7,8,9]. More than 98% of steel slag was recycled in Japan and the relative ranking of aspects according to the recycling rate placed road construction (32.4%) as the largest, with civil engineering (30.9%) and metallurgy (20.8%) following behind [10]. Europe generated 16.3 million tons of steel slag in 2018, of which 11.8 million tons (72.4%) were recycled. Road construction is the main recycling method of steel slag, accounting for 70.6% of the total amount of recycling, followed by metallurgical use (13.1%), cement concrete (5.4%), and fertilizer (4.5%) [11]. In addition, the recycling rates of steel slag in America and Australia also exceeded 70% [12,13]. However, the regional imbalance of recycling steel slag can be found especially in developing countries. For instance, the recycling rate for steel slag in China is extremely lower compared to the above developed countries, with over 70% of steel slag being stacked and landfilled [10]. The development of steel slag-related technologies could decrease the consumption of natural resources, relieve environmental stress, reduce economic costs, and convert solid waste into ecological environment material [14].

Although steel slag has been used in many fields over the past years, there are still some remaining uncertainties, such as the statistical analysis of publications and the development of research hotspots. According to the Web of Science database, researches related to steel slag cover the categories of Materials Science, Construction Building Technology, Civil Engineering, and Environmental Sciences. For this multidisciplinary research field, the data-based evidence cannot be even found to indicate the productive scholars, influential journals, predominant, and significant topics. In order to obtain a comprehensive recognition and latest hotspots of this research field, a bibliometric analysis is needed to be carried out.

Bibliometric analysis is a statistical approach to evaluate the development tendency in certain research areas over time by extracting basic information from a large number of publications. It can be used to identify productive countries, influential journals, cooperative relationships, citation networks, and hot topics in this field. For the moment, bibliometric analysis has been employed in various fields, such as plastic pollution [15], alternative energy [16], security of pipeline transportation [17], and household carbon emissions [18]. However, there is currently no bibliometric analysis of publications related to steel slag, causing a lack of comprehensive understanding in this research area.

For filling the knowledge gaps regarding the lack of quantitative insights into the topic “steel slag” systematically and statistically, a bibliometric technique was used to analyze publications about steel slag derived from the Web of Science (WoS) Core Collection database in this study, aiming to (1) clarify the predominant countries, leading institutions, and influential journals; (2) reveal the research trends and current hot topics of steel slag; (3) propose the future research directions in steel slag domain in steel slag domain. This was the first time that publications related to steel slags were analyzed using the bibliometric method. The results of this study could provide relevant researchers with a comprehensive overview and the following academic steps.

2. Methodology

2.1. Data Sources

The publications involved in this study were collected from the WoS Core Collection database of Thomson Reuters. It is considered one of the most influential and high-quality databases, including Science Citation Index Expanded (SCI-EXPANDED), Social Sciences Citation Index (SSCI), and Conference Proceedings Citation Index-Science (CPCI-S), for retrieving peer-reviewed literature in multidisciplinary research fields. The “steel slag” was defined as the search topic and the timespan of publications was determined within the period from 1 January 2000 to 1 August 2022. The retrieval was carried out on one day of 4 August 2022 to avoid the changes in data due to the WoS update.

A total of 3595 relevant documents were obtained from the WoS Core Collection database. There are three main types of documents including journal articles, proceedings papers, and reviews. Journal articles account for 80.3% of all publications, followed by proceedings papers (16.3%) and review articles (4.5%). Other publication types, including early access, meeting abstracts, editorial material, and corrections, were less than 2%. In addition, publications in English account for 97.7% of the total, indicating that most scholarly research utilized this language.

2.2. Analysis Method

Firstly, this study counted the top WoS categories and the total number of published articles on steel slag from 2000 to 2022 to describe the growth trends of publication. Secondly, the leading countries, institutions, and journals on steel slag research were analyzed and determined by using the number of publications, the number of citations, citations of per publication (CPP), Hirsch index (H-index), and impact factor (IF). The CPP is the result of dividing the total number of citations by the number of publications. It indicates the general average citations of a country, an institution, or a journal. The definition of H-index is that h publications from a certain country and institution, have been cited at least h times. Therefore, the value of H-index must be an integer according to this concept. This index demonstrates the general productivity and influence of publications from different countries and institutions. Thirdly, the current hot topics, research trends, and future research directions of steel slag were discussed. The VOSviewer (version 1.6.18) developed by Leiden University was used to conduct bibliometric network analysis. The visualization network can be constructed and drawn accurately to display the connection of bibliometric attributes in terms of clusters, timelines, and item density.

3. Basic Bibliometric Discussions

3.1. Publication Outputs

There were a total of 3595 steel slag-related publications from 2000 to 2022 and studies contributing to the steel slag research field referring to 106 types of subject categories. Figure 1 describes the distribution of the main categories of these publications according to the definition of WoS. It can be seen from the data in Figure 1 that the Materials Science Multidisciplinary group, with a percentage of 35.5%, reported significantly more publications than the other categories. This indicates that steel slag has been emphatically explored from the perspective of materials science, including synthesis and processing, composition and structure, and property and performance. Since steel slag is a solid waste that may pollute the environment, the category with the second highest number of publications is Environmental Science, with a percentage of 21.3%. In addition, the functions of CO₂ sequestration, wastewater treatment and gamma radiation realized by steel slag were explained in several articles, which are also categorized under Environmental Science [19]. According to the number of publications, the following five categories are all related to engineering, such as Engineering Civil (21.3%), Metallurgy Metallurgical Engineering (16.8%) and Engineering Chemical (8.0%). This discovery demonstrates that a considerable number of studies related to steel slag were devoted to finding solutions to multiple engineering problems. As sustainable development of economics and ecology has received increasing attention, some studies related to the recycling of steel slag have been carried out, resulting in Green Sustainable Science Technology also occupying a noticeable proportion of 7.5%.

The annual numbers of publications of the top eight related subject categories and their total are shown in Figure 2 to illustrate research trends of steel slag. As Figure 2 presents, the overall number of publications relating to steel slag shows an increasing trend year by year. In the first eleven years (2000–2010), slow growth with fluctuations and continued volatility was observed over time. Subsequently, in the latest eleven years (2011–2021), a significant increase in the number of publications was found compared with the past ten years and more than 85% of articles were published during this period. The annual number of publications reached 550 in 2021 which is over six times that of 2010, indicating that the steel slag has received abundant attention in the recent research literature. On the other hand, the trends in publications within each category are similar to the total number of publications. A considerable increase in publications can be observed for the category of Materials Science Multidisciplinary since 2018. In addition, there are almost no publications in the category of Green Sustainable Science Technology before 2015. However, the number of published articles in 2016 increased sharply from 5 to 23, exceeding the sum of the previously published articles. The reason for this situation has something to do with the recognition of recycling steel slag in sustainable development.

3.2. Analysis of Countries

A total of 87 countries are extracted in the publications related to steel slag. Figure 3 illustrates the regional distribution of H-index, CPP, number of citations, and publications of the top 15 countries by publication volume. The number of publications is displayed using gradually varied colors. The darker the color, the larger the article volume. As can be seen from the figure, the publications were mainly distributed in Eurasia and North America. The publication volume indicates the activity of a country in a certain research field. The ranking of the top ten productive countries places China as the first (1616), followed by the United States (253), South Korea (180), India (171), Spain (144), Italy (130), Japan (126), England (105), Brazil (103), Australia (96), Malaysia (88), and Canada (88). These top ten countries account for more than 80% of the total number of publications. In terms of citation frequency, China (20,607), the USA (5616), and Canada (3936) are the top three countries. The number of citations represents influence and how much attention a country has received. Interestingly, although Canada only ranks tenth in the number of publications, it has the third most citations, demonstrating that the research quality of Canada is generally high and that their research topics are of concern to relevant scholars.

The top three countries in terms of CPP values are Canada, England, and Italy. The CPP explains the average quality of a single publication in a certain country. Consistent with the previous analysis, the CPP of Canada ranked first in the world with a value of 44.73 because of its high citations and small number of publications. Although China has the highest number of citations, its number of publications is also extremely large, which makes its CPP not ranked in the top ten in the world. Especially, several countries have high CPP rankings due to less than 50 publications, such as Saudi Arabia and Egypt, with 39 and 27 publications, respectively. Therefore, the overall academic level of a country cannot be judged only by sole index such as the number of publications, citations, or CPP.

As regards to the H-index, China ranked first in the world with an H-index of 61, followed by the USA (38), Italy (32), England (31), and Spain (30). This index represents both influence and output of publications from various countries. For instance, the H-index of China is 61, which means that there are 61 publications, each of which has been cited more than 61 times. The great concern reflected by the large number of citations and high activity represented by the large quantity of publications are both necessary conditions for a country to obtain a high H-index. The H-index is labeled as a logical and reasonable indicator to characterize the overall academic level and research capacities. Therefore, China, the USA, Italy, England, and Spain are regarded as eminent countries in the research field of steel slag. Meanwhile, China (100 million tons/year), Europe (20 million tons/year), and the USA (10 million tons/year) have been identified as the top three regions with the highest steel slag generation. These regions with the highest steel slag generation are consistent with the leading countries in the research field of steel slag, revealing that the research level of a country, to a great extent, is related to steel slag generation.

The countries with more than 60 publications were screened to establish a cooperation network in the research field of steel slag by VOSviewer, as presented in Figure 4. The size of node represents the total link strength of each country, the thickness of connecting lines between nodes reflects link strength and cooperative relationship between the two countries, and the gradient colors indicate average year of publications. All the connecting lines of a country make up its total link strength and this index manifests the total strength of the cooperative links of a given country with other countries. Furthermore, the distance between nodes also indicates the relationship. The closer the distance between the nodes, the closer the cooperation between the two countries. It can be concluded from the figure that China (234 total link strength) and the USA (129 total link strength) play the most crucial role in the international cooperation network. Next, the countries ranked third to fifth in cooperation activity are Australia, England, and South Korea, with a total link strength of 75, 66, and 57, respectively. However, the rest of the countries are not quite active in the cooperative network. For instance, India and Italy have a relatively small total link strength, with values of 32, even though they have large numbers of publications and H-index, respectively. Therefore, international cooperation and academic exchanges on steel slag need further development. Furthermore, the colors of the nodes demonstrate that the USA, England, Canada, and Sweden have a longer research history and originally established basics in the research field of steel slag, while Australia, India, and South Korea have more recent average publication years. This result could be explained by the fact that the massive production and accumulation of steel slag waste has caused noticeable concern about the ecological environment, social economy, and sustainable development. Scholars from more regions are exploring new technologies to use steel slag as a valuable resource.

3.3. Analysis of Institutions

A total of 2423 institutions from 87 countries were identified in the research field of steel slag. Figure 5 and Figure 6 show the number of publications, number of citations, CPP, and H-index of the top 15 productive institutions. According to the number of publications from large to small, the organizations in the figures are arranged from left to right. As regards to the number of publications, University of Science Technology Beijing and Wuhan University of Technology rank first and second, with values of 221 and 126, respectively, while the numbers of publications in other institutions are all less than the top two. Remarkably, nine of the top ten productive institutions are from China. The incomparable activity and motivation of China in the field of steel slag can be observed, which is consistent with previous analyses of countries. Another interesting finding is that although the USA is the second most productive country, none of the top 15 institutions belong to the USA. This phenomenon can be partly explained by the fact that the experiments and researches conducted by the USA are universally distributed among different institutions. In addition, it can be seen from Figure 5 that although some institutions are not ranked at the forefront regarding the number of publications, they have a large number of citations, such as Ku Leuven and University of Padua. This shows that the studies of these institutions reflect the research concerns and received considerable attention.

Figure 6 manifests that the H-index and CPP have a consistent pattern for most countries, i.e., high CPP corresponds to a large H-index. The Ku Leuven ranked first in terms of CPP (28.4), followed by Tsinghua University (27.7), University of Padua (23.3), Tongji University (20.3), and Wuhan University of Technology (19.6). They exhibit the outstanding average quality of a single publication. In regards to the H-index, Tsinghua University and Wuhan University of Technology tied for first place with a value of 27. It is evident that both of them are extremely influential and eminent institutions in the research field of steel slag. This information guides research emphasis at the institutional level and establishes the foundation for cooperation analysis in the recent two decades.

A total of 20 institutions with at least 25 publications were selected to draw the cooperative network diagram, as shown in Figure 7. The size of the node represents the total link strength and cooperative relationship among different institutions and the color indicates the cluster to which the institution belongs. The clusters of institutions are classified by the VOS clustering technique and there are four clusters in the institution cooperation network [20,21]. The green, yellow, red, and blue clusters were dominated by the University of Science Technology Beijing, Northeastern University China, Chinese Academy of Sciences, and Wuhan University of Technology, respectively. Significantly, although Chinese Academy of Sciences has only a small number of publications and link strength, it connects the other three leading institutions in the collaborative network and plays a prominent role in the collaborative network due to its highest authority in Chinese academia.

To more intuitively characterize the contribution of each institution in the cooperation network, the density maps based on the total link strength are drawn, as shown in Figure 8. The larger the number of total link strengths for an institution, the closer the color of the institution is to red. It was found from Figure 8 that University of Science Technology Beijing ranked first in terms of the total link strength, with a value of 33, followed by Chinese Academy of Sciences, Northeastern University China, and Wuhan University of Technology. The co-authoring publication is an effective tool to develop an inventive research direction and facilitate academic exchange. Close cooperation can give full play to the advantages and characteristics of various institutions, such as huge databases, complete experimental platforms, or abundant theoretical basis, thus promoting in-depth exploration of more valuable and meaningful issues and research. Consequently, the cooperative work among authors from different countries and institutions are encouraged to expand. These bibliometric analyses of institutions can guide scholars to execute the following academic steps, such as choosing future work institutions or looking for research partners.

3.4. Analysis of Journals

There are 1016 journals with steel slag-related publications in the recent two decades.

Table 1. The top 10 productive journals in the field of steel slag.

Journal	Publication	Citation	CPP	H-Index	IF	Subject Category
Construction and Building Materials (CBM)	357	9596	26.9	51	7.693	Construction and Building Technology (Q1, 6/68); Engineering, Civil (Q1, 5/138); Materials Science, Multidisciplinary (Q1, 77/345);
Journal of Cleaner Production (JCP)	129	3233	25.0	31	11.072	Engineering, Environmental (Q1, 9/54); Environmental Sciences (Q1, 24/279); Green and Sustainable Science and Technology (Q1, 5/47);
Materials	108	870	8.1	15	3.748	Metallurgy and Metallurgical Engineering (Q1, 18/79); Chemistry, Physical (Q3, 84/163); Materials Science, Multidisciplinary (Q3, 177/345); Physics, Applied (Q2, 56/161); Physics, Condensed Matter (Q2, 28/69);
Advanced Materials Research	99	185	1.9	6	-	-
Journal of Materials in Civil Engineering	65	1301	20.0	18	3.651	Construction and Building Technology (Q2, 26/68); Engineering, Civil (Q2, 52/138); Materials Science, Multidisciplinary (Q3, 181/345);
Metallurgical and Materials Transactions B Process Metallurgy and Materials Processing Science	65	1082	16.7	18	2.872	Metallurgy and Metallurgical Engineering (Q2, 23/79); Materials Science, Multidisciplinary (Q3, 215/345);
Environmental Science and Pollution Research	59	498	8.4	14	5.190	Environmental Sciences (Q2, 87/279);
Journal of Hazardous Materials (JHM)	57	3396	59.6	29	14.224	Engineering, Environmental (Q1, 3/54); Environmental Sciences (Q1, 9/279);
ISIJ International	51	949	18.6	17	1.864	Metallurgy and Metallurgical Engineering (Q3, 40/79);
Steel Research International	43	389	9.1	12	2.126	Metallurgy and Metallurgical Engineering (Q2, 32/79).

shows the top 10 productive journals in the field of steel slag. However, these journals only account for 30% of all publications, demonstrating the great diversity of journals in the steel slag domain. Construction and Building Materials ranked first with 357 articles published, followed by Journal of Cleaner Production with 129 articles. The fourth most productive source is Advanced Materials Research with 99 publications. Notably, Advanced Materials Research is the book series used to collect the proceedings papers, which results in the low number of citations and the inexistence of subject category and IF in JCR. Materials and Journal of Materials in Civil Engineering ranked third and fifth with 108 and 65 papers, respectively.

Among these prominent journals, Journal of Hazardous Materials (IF: 14.224) has the highest reputation and quality of the publications in the steel slag domain when IF is used as the evaluation index, followed by Journal of Cleaner Production (11.072) and Construction and Building Materials (7.693). In addition to the IF, with regard to the other three citation indexes including the number of citations, CPP and H-index, Construction and Building Materials, Journal of Hazardous Materials, and Journal of Cleaner Production are all in the top three among productive journals. This phenomenon manifests the prominent place of these three journals in the research field of steel slag. On the other hand, Table 1 displays that the top 10 productive journals refer to multiple disciplines and cover 10 subject categories of JCR. It was found that Metallurgy and Metallurgical Engineering and Materials Science Multidisciplinary emerged as the most high-frequency categories indicated by 4 out of 10 journals. In addition, the category of Environmental Sciences belonging to three journals also expresses the research direction of steel slag.

The citation density diagram was drawn to characterize the citation networks among the productive journals, as presented in Figure 9. There are 32 journals selected with a minimum of 20 publications. The larger the number of citations to other journals in the graph for a certain journal, the closer the color of the journal is to red. In this case, it can be seen that Construction and Building Materials is identified as the journal that received the most concern from other leading journals, with a total link strength of 3048, followed by Journal of Cleaner Production (1421) and Journal of Hazardous Materials (1081). Furthermore, although Resources, Conservation & Recycling (RCR) is not involved in Table 1, it ranks fourth and receives considerable attention from other productive journals, with a link strength of 738. The Engineering Environmental and Environmental Sciences are shared categories for JHM, JCP, and RCR, while CBM focuses on the Construction and Civil Engineering issues. The categories of these four leading journals in the citation density diagram manifest the research directions and hotspots concerned in the steel slag domain. The bibliometric analysis regarding journals can provide evidence for the scholars to choose the target journal for their work, and guide the researchers to efficiently follow the latest findings in the steel slag domain.

4. Hot Topics and Future Directions

4.1. Current Hot Topics

The publications with high citation numbers can reveal the research directions that have received extensive attention.

Table 2. The top 10 most cited publications in steel slag domain.

No.	Author and Year	Title	Journal	Country	Institution	Citations
1	Huijgen, WJJ 2005 [22]	Mineral CO2 sequestration by steel slag carbonation	Environmental Science & Technology	The Netherlands	ERCN	473
2	Shi, CJ 2000 [23]	High performance cementing materials from industrial slags—A review	Resources Conservation and Recycling	Canada	CJST	389
3	Shi, CJ 2004 [24]	Steel slag—Its production, processing, characteristics, and cementitious properties	Journal of Materials in Civil Engineering	Canada	CJST	374
4	Yi, H 2012 [25]	An overview of utilization of steel slag	Proceedings Paper	China	SWSEP	360
5	Huang, Y 2007 [26]	A review of the use of recycled solid waste materials in asphalt pavements	Resources Conservation and Recycling	England	NU	359
6	Tsakiridis, PE 2008 [27]	Utilization of steel slag for Portland cement clinker production	Journal of Hazardous Materials	Greece	NTUA	350
7	Bobicki, ER 2012 [28]	Carbon capture and storage using alkaline industrial wastes	Progress in Energy and Combustion Science	Canada	UA	336
8	Piatak, NM 2015 [29]	Characteristics and environmental aspects of slag: A review	Applied Geochemistry	USA	GS	300
9	Ahmedzade, P 2009 [30]	Evaluation of steel slag coarse aggregate in hot mix asphalt concrete	Journal of Hazardous Materials	Turkey	EU	276
10	Kourounis, S 2007 [31]	Properties and hydration of blended cements with steelmaking slag	Cement and Concrete Research	Greece	NTUA	257

ERCN: Energy Research Centre of The Netherlands; CJST: CJS Technology Inc.; NU: Newcastle University; NTUA: National Technical University of Athens; SWSEP: Sinosteel Wuhan Safety & Environmental Protection Research Institute; UA: University of Alberta; GS: Geological Survey; EU: Ege University.

provides the 10 most frequently cited papers in the steel slag domain with the following information: first author, publication year, title, journal, country, first institution, and a total number of citations during 2000–2022. There are 6 review papers among these frequently cited publications, ranking 2nd, 3rd, 4th, 5th, 7th, and 8th, respectively. Reviews could give a comprehensive evaluation and intensive analysis of certain research fields. Focusing on reviews can also help scholars understand the latest developments. Therefore, it is reasonable for the review papers to obtain many citations and make up more than half of the top ten. Among them, Yi provided a comprehensive overview of steel slag as raw material in various aspects, including road construction, metallurgy, wastewater treatment, cementitious additive, and fertilizer production. Two review papers summarized the pozzolanic and cementitious properties of steel slag that are used as a replacement for Portland cement. The other three review papers focused on recycling steel slag as an aggregate in asphalt pavement, carbon sequestration using steel slag, and the toxic elements and leaching consequences of steel slag.

Research papers are inclined to explore a specific topic in a field of research. It can be found that the research paper titled “Mineral CO₂ sequestration by steel slag carbonation” has the largest number of citations, authored by Huijgen, WJJ in Environmental Science & Technology, with 473 times. This study used steel slag as an alternative feedstock for CO₂ sequestration to reduce carbon emission and the effect of particle size, CO₂ pressure, and reaction time on the carbonation rate was evaluated. There are two research papers exploring the feasibility of adding steel slag to raw material for cement clinker production and investigating the cementitious properties. The remaining paper is titled “Evaluation of steel slag coarse aggregate in hot mix asphalt concrete” recycled steel slag as coarse aggregate in asphalt mixture. According to the information of the frequently cited papers, the hottest topics and application methods for steel slag are CO₂ sequestration and recycling as the cementing component and aggregate in cement or asphalt concrete.

However, the earlier a paper is published, the more favorable it is to obtain more citations. As expected in Table 2, nine of the ten papers were published before 2010. In addition, due to the differences in disciplines, there is some irrationality in sorting articles according to the number of citations, without considering the research field and publication time. To address this issue, ESI proposes the concept of highly cited papers which are published in the past ten years and received enough citations to place them in the top 1% of the certain academic field in ESI. The number of ESI highly cited papers is one of the most essential indicators to evaluate the academic level and influence of a country/institution in a certain field. There are 21 ESI highly cited papers in the steel slag domain and the statistical analysis of these papers is shown in Figure 10, with respect to the academic field, countries, and journals.

It can be found from Figure 10 that the top five academic fields in terms of the number of highly cited papers are Engineering, Environmental Sciences Ecology, Science Technology Other Topics, Construction Building Technology, and Materials Science. Half of the papers belong to the academic field of engineering with a value of 12, which manifests that most of the hot topics papers are solving engineering problems with the help of steel slag. Highly cited papers come from eight countries. There are 12 in China, two each in England and Canada, and one in each of the other countries. The top three journals in terms of the number of highly cited papers are Journal of Cleaner Production, Construction and Building Materials, and Applied Geochemistry, with values of 3, 3, and 2, respectively.

Noticeably, Nature (IF = 69.504) and Progress in Energy and Combustion Science (IF = 35.339) have significantly higher IF than other journals. The highly cited papers published in these two journals all involve the use of alkaline silicate contained in steel slag, mainly calcium silicates, to realize carbon capture and storage, thus mitigating anthropogenic climate change, relieving the need for new mining and energy-intensive treatments [28,32]. Among these 21 highly cited papers, two other similar studies that investigated the feasibility of utilizing industrial solid wastes to reduce the emission of waste gas, such as SO₂ and CO₂ [33,34]. Eight papers focus on using steel slag as cementitious materials or granular aggregate in cement concrete. It is recognized by some scholars that the utilization of slag for value-added purposes can be achieved in cement and concrete materials. In general, lower lifecycle cost and higher strength of concrete can be achieved by adding steel slag aggregate or powder, while volume expansion and workability need to be concerned [35,36,37,38,39,40,41,42]. Three papers focused on the environmental impacts resulting from heavy metal leaching produced by rainwater ingress [29,43,44] and two papers used steel slag as silicon fertilizers [45,46]. Furthermore, the research topics of recycling as an aggregate in asphalt mixture, wastewater treatment, application case analysis, and preliminary treatment technology have one paper each [10,47,48,49].

In summary, the top three hot topics of steel slag at present are carbon sequestration to mitigate greenhouse gas emissions, using as cementitious materials or quarried particles in concrete, and the leaching behavior of heavy metal ions.

4.2. Research Trends of Steel Slag

A comprehensive understanding of the research trend is the premise of giving reasonable future directions for steel slag. The keyword of the paper can manifest the core focus and research patterns of each study. Therefore, research trends over time in steel slag domain can be recognized by the frequency of keywords in different periods.

Table 3. The top 20 keywords in frequency.

Rank	2000–2009	2010–2014	2015–2018	2019–2022
	277 Publications	647 Publications	931 Publications	1740 Publications
1	Slag (21)	Slag (41)	Compressive strength (33)	Compressive strength (87)
2	Concrete (9)	Concrete (31)	Slag (33)	Microstructure (66)
3	Recycling (8)	Compressive strength (24)	Recycling (28)	Carbonation (53)
4	Adsorption (6)	Microstructure (23)	Stainless steel slag (28)	Mechanical properties (46)
5	Steel (6)	Adsorption (22)	Concrete (26)	Slag (45)
6	Steel slag powder (6)	Carbonation (19)	Adsorption (23)	Adsorption (37)
7	Aggregate (5)	Durability (19)	Carbonation (23)	Concrete (37)
8	Durability (5)	Leaching (19)	Microstructure (23)	Durability (36)
9	Mechanical properties (5)	Hydration (17)	Fly ash (22)	Fly ash (35)
10	Carbon dioxide (4)	Strength (16)	Durability (20)	Electric arc furnace slag (34)
11	Carbonation (4)	Cement (15)	Asphalt mixture (19)	Recycling (34)
12	Cement (4)	Fly Ash (15)	Mechanical properties (19)	Steelmaking slag (32)
13	Chromium (4)	Recycling (15)	Mineral carbonation (18)	Sustainability (29)
14	Compressive strength (4)	Mineral carbonation (14)	Steelmaking slag (17)	Hydration (28)
15	Fly ash (4)	Mechanical properties (13)	Strength (17)	Leaching (28)
16	Hydration (4)	Stainless steel slag (13)	Blast furnace slag (16)	Asphalt mixture (27)
17	Leaching (4)	Steel slag powder (13)	CO2 sequestration (16)	Stainless steel slag (27)
18	Modeling (4)	BOF slag (11)	Leaching (16)	CO2 sequestration (24)
19	Physical properties (4)	Blast furnace slag (10)	Phosphorus (16)	Mineral carbonation (23)
20	Strength (4)	Permanent deformation (9)	Kinetics (15)	Blast furnace slag (23)

summarizes the top 20 keywords by occurrence in four time frames (2000–2009, 2010–2014, 2015–2018, and 2019–2022) during the studied period. It was found that studies before 2009 focused on recycling steel slag aggregate and steel slag powder in concrete and exploring its durability. The number of keywords in this stage is smaller than that in other periods. The research on steel slag is in the initial stage, and only steel slag-related materials partially replace the original materials in concrete. In the second period, compressive strength has moved from 14th to 3rd, indicating a great deal of attention has been devoted to the effect of steel slag on the compressive properties of concrete. Meanwhile, the microstructure is an emerging word ranking fourth compared to the previous stage, resulting from the advances in material testing technology. Microscopic test equipment such as scanning electron microscopes, electron probe microanalyzer, and X-ray photoelectron spectroscopy are more accessible to scientific institutions and universities.

During 2015–2018, several new keywords including “asphalt mixture”, “CO₂ sequestration”, and “phosphorus” appear for the first time, ranking 11th, 17th, and 19th, respectively. This indicates the burgeoning research areas of functional applications such as carbon fixation and wastewater treatment with respect to steel slag. In addition, a technology of recycling steel slag as an aggregate in asphalt mixture, which can digest a large amount of steel slag solid waste, has been gradually studied. In the fourth period, the top ten terms did not change significantly compared to the third stage. The “compressive strength” ranked first with the occurrence of 87, followed by “microstructure” (66) and “carbonation” (53). The compressive strength has a certain logical relationship with carbonation since the compressive strength of cement concrete can be enhanced by carbonation pretreatment of steel slag when it is used as binding material [50,51,52]. Therefore, the application of steel slag in cement concrete and the exploration of microscopic properties were most frequently carried out by the latest research. This result is consistent with the analysis of highly cited papers in the previous chapter, that is, more than one-third of ESI highly cited papers focus on using steel slag as cementitious materials or granular aggregate in cement concrete. The new word, “sustainability” first appeared in the fourth stage, due to the massive consumption of resources and the improvement of social environmental awareness. Furthermore, steel slag used as the adsorbent for wastewater treatment has always been a hot topic since the keyword of adsorption is in the top ten for all four stages.

However, Table 3 does not contain the keywords that appeared recently with a low number of frequencies. These fresh keywords cannot be ignored when analyzing novel research directions. To deal with this problem, the keywords co-occurrences network was established using the keywords of publications with a frequency of not less than 10 times, as presented in Figure 11. The “steel slag” in the keyword results was eliminated since it was defined as the topic during retrieval in WoS, and a total of 153 items were obtained. Keywords have a great number of occurrences represented by a node with a larger size and the thickness of the lines indicates the correlation strength between the two connected keywords.

To combine keywords with the timeline, the color of a node in Figure 11 was represented by gradient color on behalf of the average publication year. Some new words with recent publication time can be found in the figure, such as “self-healing”, “induction heating”, “microwave heating”, “geopolymer”, “the lifecycle assessment”. Therefore, the functional applications in asphalt pavement and carbon storage are also a research tendency in recent years. In summary, the research focus of steel slag has changed from the mechanical properties, such as durability and strength, to the functional applications, pretreatment technology, microstructural properties, and environment-friendly products.

4.3. Pathways for the Future of Steel Slag

Combining the bibliometric information, research trends, and current hot topics of steel slag, several future research directions (RD) were proposed:

RD1: The construction industry plays the most important role in the large-scale utilization of steel slag. However, the volume instability due to the f-CaO is a tremendous obstacle to recycling as construction materials. Although there are some technologies to eliminate the active components in steel slag, they are all aimed at cooled solid steel slag, and the treatment effect varies [53,54]. The new equipment and treatment technology need to be further developed and modified for molten steel slag. Based on the heat carried by molten steel slag, the active components in steel slag can be eliminated by adding external additives before steel slag leaves the steelmaking plant. This method can effectively improve the application efficiency of steel slag in the construction industry and save the energy consumption of subsequent pretreatment, thus forming a new environmental-friendly treatment technology.

RD2: Although the components of calcium silicate in steel slag endows it with cementitious properties, steel slag power exhibits insufficient cementitious activities due to the different crystal structure and components’ content with cement. About 25% RO phase in steel slag is inactive. Several approaches, including mechanical grinding, rapid cooling, and incorporation of activators, were used to improve the reactivity [55,56,57]. However, the chemical composition of steel slag varies from the steelmaking process and heat treatment method of molten slag, and replacing all cementitious materials with steel slag is still in its infancy. In the future, mineral separation technology applicable to steel slag powder should be investigated to fundamentally solve the problems of component differences and low content of active substances in steel slag.

RD3: The use of steel slag as an aggregate in cement or asphalt pavement has been a wide concern. Researchers hope to improve the mechanical properties of pavement with the help of the high hardness, wear resistance, and density of steel slag. However, with the rapid development of the transportation industry, the requirements for pavement are not limited to the normal running of vehicles, and the demand for road functionalization is also increasing. Future research can further develop the functional application of steel slag-based pavement materials using the unique physicochemical properties of steel slag. For example, the results in Figure 11 show that it is the latest research trend to realize the self-healing function of steel slag-based asphalt pavement by microwave radiation with the help of iron and Fe₃O₄ contained in steel slag.

RD4: Steel slag has been used as a potential economic feedstock of CO₂ sequestration to reduce emissions of greenhouse gases and it exhibits great capacity due to its intrinsic alkalinity and high reactivity. The effect of slag particle size, CO₂ pressure, temperature, and reaction time on carbon sequestration efficiency was integrally explored. However, the carbon capture process will also consume energy and cause carbon emissions. Meanwhile, the foreseeable cost-saving of utilization of steel slag for CO₂ sequestration has not been distinctly quantified. Therefore, the environmental and economic effect of recycling steel slag for carbon storage should be systematically quantified by lifecycle assessment (LCA) and lifecycle cost analysis (LCCA) in future studies.

RD5: Due to the addition of natural minerals such as magnetite and dolomite during steelmaking, heavy metal elements such as Cu, Cr, Pb, and Zn may exist in steel slag. The leaching risk needs to be evaluated no matter where the steel slag is applied. Most research proved the security of short-term leaching characteristics, but there is considerable concern about long-term leaching behavior. For example, Xie found that the cumulative concentration of Cd exceeded the critical value safety limit after 15 years [58]. The envelopment of asphalt can only tentatively prevent leaching behavior. In the future, the method of removing heavy metal ions before recycling needs to be explored and put forward after the long-term leaching law of steel slag is clarified.

RD6: The physicochemical properties of steel slag are significantly affected by many parameters such as cooling methods, type of iron ore, quality of flux, and gas environment. Uncertain physicochemical properties are one of the main reasons that limit the large-scale application of steel slag. Steel slag with different physical properties and chemical composition corresponds with the most reasonable recycling ways. Therefore, to realize the efficient utilization of steel slag, a comprehensive database of steel slag including material data and process data of steelmaking, data of heat treatment procedure, physical properties, mechanical properties, chemical composition, and phase types of each steel mill needs to be established in the future.

RD7: The results of the cooperative analysis show that international communication has been neglected to some extent. For instance, India and Italy have relatively low activity in the cooperative network even though they have large numbers of publications and H-index. The cooperation research and academic exchanges among authors from different countries and institutions are encouraged to expand close cooperation which can give full play to the advantages and characteristics of various countries and institutions, such as huge databases, complete experimental platforms, or abundant theoretical basis, thus promoting in-depth exploration of more valuable and meaningful issues and research regarding steel slag.

5. Conclusions

This study provided a comprehensive bibliometric analysis of papers related to steel slag to clarify the leading countries, institutions, and journals, reveal the research trends, and propose future research directions in the steel slag domain. The crucial findings were as follows:

(1) China, the USA, Italy, Spain, and England are regarded as the top five eminent countries in the research field of steel slag. The research level of a country, to a great extent, is related to steel slag generation. Tsinghua University and the Wuhan University of Technology tied for first place with an H-index of 27, indicating the extremely influential and eminent position in the research field of steel slag. The cooperation research and academic exchanges among authors from different countries and institutions are encouraged to expand. With regard to the IF, the number of citations, CPP, and H-index, Construction and Building Materials, Journal of Hazardous Materials, and Journal of Cleaner Production are all in the top three among productive journals, manifesting the prominent place of these three journals in the research field of steel slag.

(2) The studies before 2009 focused on recycling steel slag aggregate and steel slag powder in concrete and exploring its durability. In the period from 2010 to 2014, a great deal of attention has been devoted to the effect of steel slag on the compressive properties of concrete. Meanwhile, the microstructure of steel slag is gradually concerned due to the advances in material testing technology. During 2015–2018, the frequency of keywords such as “CO₂ sequestration” and “phosphorus” increased significantly, indicating the burgeoning research areas of functional applications such as carbon fixation and wastewater treatment. After 2018, the new word, “sustainability” first appeared in the top 20 keywords list due to the improvement of social environmental awareness. In summary, the research focus of steel slag has changed from the mechanical properties, such as durability and strength, to the functional applications, pretreatment technology, microstructural properties, and environment-friendly products.

(3) The bibliometric information and research trends reveal some directions for future research: (1) equipment and pretreatment technology for molten steel slag to eliminate active components before steel slag leaves the steelmaking plant; (2) mineral separation technology applicable to steel slag powder should be investigated to fundamentally solve the problems of component differences and low content of active substances in steel slag; (3) functional application of steel slag-based pavement materials using the unique physicochemical properties of steel slag; (4) quantification of the environmental and economic effect of carbon storage by LCA and LCCA; (5) the method of removing heavy metal ions before recycling needs to be explored and put forward after the long-term leaching law of steel slag is clarified; (6) a comprehensive database containing data of steelmaking and steel slag characteristics needs to be established.

This was the first time steel slag was investigated from a bibliometric perspective. The conclusions obtained in this study could provide scholars with current research hotspots and the following academic steps, including following the latest findings, selecting future work directions, looking for research partners, and choosing the target journal for their work in the steel slag domain. This study can serve as the reference basis to promote safer and more efficient recycling of steel slag in the future.