Conventional cement-based composites, such as concrete and mortar, are ranked as the most used construction materials in the world ^[1][2][1,2]. Concrete and mortar are mainly produced with ordinary Portland cement (OPC), coarse and fine aggregates, water, and additives. The widespread use of OPC by the construction industry is attributed to its impressive performance, affordability, availability, standardization, and compatibility with different types of materials and admixtures. Due to the increase in human population and the exponential increase in construction work, it is predicted that the OPC demand will reach 3.7–4.4 billion tons in 2050 [3]. In addition to the need for a large number of natural resources for the manufacture of OPC, it is estimated that this process emits around 6 to 9% of the total greenhouse gas emissions globally, which mainly result from the combustion of fossil fuels for the kiln, the heating limestone, and the consumption of electrical power [4]. Accordingly, the production of 1 ton of OPC emits 1 ton of CO₂ and consumes 1.5 tons of natural resources [5]. In addition, OPC is ranked as the third most energy-intensive material, after steel and aluminum, with a percentage of global energy consumption of 7% ^[5][6][7][5,6,7]. These environmental concerns created a need to find alternative materials to partially or fully replace it.

Geopolymers and alkali-activated materials are cement-free composites formulated by activating alumino-silicate materials, such as fly ash, metakaolin, and silica fume, and calcium-rich materials, such as blast furnace slag or ladle slag, respectively, with a sodium, potassium, or carbonate hydroxide-based alkaline solution. In 1978, Prof. Joseph Davidovits introduced geopolymer materials in construction [8]. During the geopolymerization reaction, the dissolution of silicate and aluminate compounds creates oligomers that condensate to form an amorphous and partially crystalline structure of polymers ^[9][10][9,10]. In the last two decades, the performance of geopolymers has been extensively studied ^{[11][12][13][14][15][16]}[11,12,13,14,15,16]. Since then, these cement-free materials have shown superior mechanical performance and a higher ability to maintain their properties under harsh environments to cement-based materials, such as high temperatures, salt and acid attacks ^{[17][18][19][20][21]}[17,18,19,20,21]. Furthermore, the use of geopolymer mortar in various construction applications has been examined, such as corrosion-resistant materials ^[22][23][24][22,23,24], fire protection materials ^[17][18][25][17,18,25], repairing, strengthening, and retrofitting old structures ^{[26][27][28][29][30][31][32]}[26,27,28,29,30,31,32], grouting ^[33][34][33,34], bonding ^[27][35][27,35], coating ^[22][24][22,24], masonry materials [36], and for underwater placing ^[15][37][15,37].

The number of scientific publications has been exponentially increasing. As such, it is becoming nearly unfeasible to cover all studies within a specific topic. Nowadays, scholars are using different approaches for conducting quantitative and qualitative literature reviews. The scientometric study has been introduced as an essential tool that gives an objective, reliable, transparent, and systematic review that covers scientific activities and publications describing a specific topic [38]. Based on the data extracted from publications (authors, total publications and citations, affiliations, countries, etc.), a scientometric analysis can display a structure analysis for a large number of data, show trends of paper publications and keywords over time, detect the most productive authors, countries, affiliations, and journals, infer gaps or shifts within a specific topic, and analyze the connections of the extracted data plotted in the form of mapping and clustering networks ^[39][40][41][39,40,41]. The scientometric analysis was described by Boquera et al. [42] as “a way to elucidate the past, present, and future within the different areas of knowledge, reporting the main research interest and future trends”. The deep analysis of keywords used by scholars to describe and summarize their research content is crucial to describe the current trending topics and their evolutions and to highlight gaps that can be covered in future work ^[39][41][43][39,41,43].

Lately, scientometric reviews have been used in different areas, such as cryptocurrency and stock markets [44], concrete as a thermal energy storage material [42], self-healing concrete [41], construction demolition waste management [45], biological water treatment [46], business and management [47], and sports [48], among others. Similarly, geopolymer and alkali-activated composites have been introduced in scientometric studies in recent years. Yang et al. [2] have found that the acceptance of geopolymer concrete by the industry is not achieved yet due to the lack of long-term performance testing. In addition, future work was suggested to examine the microstructure of the geopolymer matrix. Another study performed by Tian et al. [49] on fly ash-based geopolymers highlighted the high contribution and impact of China and Professor Davidovits as a prolific reference in the investigation and the development of fly ash-based geopolymer properties. According to Elmesalami and Celik [50], more studies should be carried out to evaluate the effect of steel polyethylene, glass basalt polypropylene, and natural microfibers on the properties of the engineered geopolymer composites. In addition, Ji and Pei [51] highlighted the efficiency of geopolymer composites in immobilizing heavy metals. Based on the literature, numerous articles have been published to investigate the use of geopolymers in different applications. Hence, there is a pressing need to determine the research trends and gaps for the use of geopolymers and alkali-activated materials in various construction applications.

2. Keyword Analysis

Figure 1 illustrates the evolution of investigating different types of applications of geopolymer mortars and composites between 2000 and 2022. Fire protection, corrosion resistance, coating, repair, and masonry were the top five applications with a cumulative number of publications of 79, 51, 45, 37, and 37 in 2022, respectively. Generally, the trend of this graph is similar to that of the total publications, where most applications have been investigated since 2013, except fire protection and corrosion resistance, which have been studies since 2003 and 2004, respectively. The early investigation of using geopolymer composites for fire protection can be related to their impressive ability to maintain their weight and strength under high-temperature exposure [25]. In addition, the paper published in 1997 by Lyon et al. [18] was used as a benchmark to advance the comprehension of geopolymers prepared with different precursors under high-temperature exposure. Similarly, the ability of geopolymer mortars and composites to protect steel reinforcement against corrosion has been assessed extensively. With a decrease in the matrix permeability caused by the formation of the dense N-A-S-H and C-A-S-H gels, lower amounts of chloride or carbon dioxide accessed the steel reinforcement ^[23][52][23,73]. Furthermore, over the last few years, several articles investigated the use of geopolymer composites in repair, strengthening, and rehabilitation applications, owing to their excellent bonding behavior ^{[5][6][23][53][54]}[5,6,23,54,56]. Meanwhile, it was difficult to differentiate the use of geopolymers in masonry blocks and plastering applications. Thus, the term masonry in Figure 1 refers to both applications. Similar to repair, strengthening, and rehabilitation, geopolymer materials were only assessed in masonry production from 2013. Using geopolymer mortars is particularly beneficial for masonry applications to accelerate construction operations and eliminate water curing that is generally required for the strength gain of cement-based masonry materials ^[36][55][56][36,74,75]. New applications for geopolymer mortars have been explored in the last 5 years, including sewage lining, wastewater treatment, 3D printing, and grouting. The performance of geopolymer mortars in a sewage environment has been assessed in past studies ^{[57][58][59][60]}[76,77,78,79]. A fly ash geopolymer mortar could be a sustainable alternative for a sulfate-resistant Portland cement-based mortar, owing to its superior ability to maintain its mass and greater depth of neutralization under a sewage environment ^[57][58][76,77]. In other work, Bogdan et al. suggested a geopolymer material for wastewater treatment applications. Results showed that geopolymer mortars limited the growth of microorganisms on the surfaces of concrete samples to a better extent than their plain Portland cement and calcium aluminate cement-based counterparts ^[61][80]. Lately, there has been an increasing interest in developing the thixotropic, mechanical, and bonding properties of fly ash and slag geopolymer mortars reinforced with different types of reinforcement for 3D printing applications ^{[54][62][63][64][65]}[56,57,81,82,83]. Research findings noted that 3D printing parameters and material strength development mostly affected the interlayer bond strength ^[54][62][56,57]. Meanwhile, the addition of slag to a fly ash-based geopolymer 3D printed mortar required a faster printing time to counter the fast setting of slag-based geopolymers and to ensure proper bonding between the 3D-printed layers ^[63][81]. Furthermore, reinforcing 3D geopolymer composites with steel cables achieved 290% higher flexural strength than their plain counterparts ^[65][83]. For the production of geopolymer mortar as a grouting material, Gullu et al. ^[66][84] produced a fly ash-based geopolymer mortar with feasible rheological properties compared to a native cement grouting material. Research gaps could be highlighted through the analysis of the trend of applications identified in Figure 1. Geopolymer mortars and composites have displayed impressive performances when used as fire and corrosion protection materials, coating and masonry materials, or for repair, strengthening, and rehabilitation applications. Yet, their adoption in such construction applications requires further assessment in terms of serviceability. Thus, future research entails focusing on examining the lifecycle and economic impact of geopolymer mortar and composites in such applications. Other construction applications for geopolymer mortars are yet to receive adequate attention, such as tunnel and pipe lining or underwater placing. Also, despite their aptitude to adsorb and immobilize heavy metals, as highlighted in [11], limited studies have been carried out in research field. Similarly, more work is needed in the recently explored applications, including 3D printing, wastewater treatment, sewage and tunnel lining, and as grouting materials. Such a demand for more research is to provide critical scientific evidence that could promote the adoption of geopolymer composites by relevant industries.

3. Keyword Co-Occurrence

Bibliometrix was used to produce the authors’ keyword co-occurrence network. A threshold value of fifty nodes and minimum edges equal to two were set to include the keywords most repeatedly used. Figure 2 shows the cluster map of the authors’ keywords. The box size represents the keyword occurrence by the authors in their research works, while the thickness of the connecting lines signifies the intensity of the interconnection between the nodes. The word “geopolymer” is noted to be the most commonly used keyword. In turn, “fly ash”, “metakaolin”, “silica fume”, and “slag” are the primary aluminosilicate precursors employed in producing geopolymer composites. In addition, keywords such as “elevated temperature”, “high temperature”, “fire resistance”, and “thermal conductivity” show that geopolymer composites were mostly assessed for fire and high-temperature resistance applications. Furthermore, the co-occurrence network shown in Figure 2 was divided into four clusters. The red cluster is related to the main precursors used in the production of geopolymer composites while focusing on the main properties that have been tested. The green cluster represents the correlation between durability testing, especially thermal and high-temperature resistance, and each of mechanical properties and microstructure analysis. While the purple cluster highlights the applications related to bonding, strengthening, and protection of old and damaged structural elements, and the blue cluster is related to corrosion. The red cluster was represented by one primary keyword, “geopolymer”. Conversely, the other clusters were characterized by a group of keywords. The red cluster mainly highlighted the precursors used in the production of geopolymer mortars and the main properties that have been evaluated, such as compressive strength, water absorption, and flexural strength. However, terms related to the fresh mortar properties, such as flow, setting time, plastic viscosity, and yield stress, were not found in the network. This indicates that more work should be carried out to assess the fresh properties of geopolymer materials. Moreover, for the green cluster, the main keywords were microstructure, durability, and mechanical properties, providing evidence to the need to conduct microstructure analysis when mechanical and durability testing were performed in the research work. Other keywords have also been found to be interconnected. For instance, the keyword porosity was connected to mechanical properties. This may be due to the direct relationship between the porosity, i.e., volume of pores in the mortar matrix, and the mechanical properties of geopolymer composites ^[26][67][26,85]. In the meantime, the purple cluster highlighted the keywords geopolymers, metakaolin, masonry, cultural heritage, coating, and strengthening. This cluster seemed to be focused on the use of geopolymers in specific construction applications. Lastly, the blue cluster was characterized by different keywords, including corrosion, chloride, and carbonation. Therefore, it is inferred that the blue cluster is primarily associated with the use of geopolymer composites as a protective layer for steel against corrosion. In fact, chloride ingress and carbonation are the two main reasons for corrosion initiation and rust creation ^{[24][52][68][69]}[24,73,86,87].

4. Word Cloud

A word cloud is another tool to highlight research interests and trend developments. Figure 3 presents word clouds illustrating the most used author keywords within three time intervals: 1996–2017, 2018–2020, and 2021–2023, corresponding to 237, 261, and 291 articles, respectively. The time intervals were divided in a way to obtain a similar number of publications in each to facilitate the comparison. As anticipated, “geopolymer” was the most occurring keyword in all time slots, with a total occurrence of 244. Meanwhile, other terms, such as alkali-activated material, alkali-activated fly ash, and alkali-activated mortar were not as frequently used. As for the precursor binders, fly ash and metakaolin were mostly employed in past research. Yet, slag (also referred to as blast furnace slag and GGBS) became more prominently utilized after 2018. This could be due to the superior performance of blended geopolymer mortars made with fly ash and slag compared to counterparts comprising one of the two binders. Other materials, such as red mud and silica fume, also became more apparent in the latest time interval, indicating the exploration of new alternative materials in the past few years. Nevertheless, while geopolymer mortar is cement-free, cement was a commonly used keyword between 2018 and 2023. This is probably owed to the comparison of geopolymer mortar with cement-based mortar in terms of performance and microstructure. Indeed, the evidence emerging in the last five years on the superiority of geopolymers has been promoting its adoption by the construction industry. The word clouds also highlight several material properties. For example, a focus on the compressive strength and mechanical properties of the geopolymer mortar was evident, while less attention to durability and microstructure was observed. Though no significant differences were noted in the second time period, more properties were addressed, including bond strength, shrinkage, and water absorption. Bond strength is significant to several applications of geopolymer mortar, such as repair and strengthening. As these two applications became more prominent in this time period, their corresponding characterization tests were more frequently employed. In the last time period, greater attention was paid to durability and microstructure than compressive strength and mechanical properties. This indicates a more profound knowledge of the material under study, with a need to develop a better understanding of its durability performance and microstructure. Based on the word cloud analysis, several research gaps can be depicted. Despite the introduction of new aluminosilicate binding materials in the production of geopolymer composites, fly ash has remained the most used precursor. However, owing to the depletion in its quantities over the past years, researchers are required to search for other industrial materials that possess similar chemical compositions to fly ash to serve as a suitable replacement. Another reason to find other alternatives to fly ash is that the level of toxicity varies between one region and another, where Russian fly ash showed less toxicity level with a pH near neutral compared to fly ash exported from other countries ^[70][88]. In addition, the complete or partial replacement of natural fine aggregates with recycled counterparts in producing geopolymer mortars has not received adequate attention ^[71][89]. Also, it seems that, other than workability, the fresh properties and rheology of geopolymers have not been investigated thoroughly.