Foamed geopolymers as low carbon materials for fire-resistant and lightweight applications in construction: A review

3.1 Research development on FGPs

For research development evaluation, pertinent academic disciplines were identified through the Scopus analyzer. Figure 2 illustrates the disciplinary distribution of research on FGPs, with 36% of the studies originating from Materials Science, reflecting the field’s focus on developing and characterizing innovative materials. Engineering (27%) follows, emphasizing structural applications, while Physics and Astronomy (10%) contribute theoretical frameworks. Chemical engineering accounts for 7%, highlighting its role in advancing geopolymerization processes and optimizing chemical formulations. Environmental Science (5%) and Chemistry (4%) underline the aspects of sustainability and chemical synthesis, respectively. The remaining 11% labeled as “other” highlights interdisciplinary contributions, showcasing the broad applicability of FGPs across scientific domains. Figure 3 categorizes the types of publications contributing to FGP research. It shows that 78% of the publications are journal articles, highlighting the dominance of peer-reviewed studies in advancing the field. Conference papers (14%) and reviews (5%) provide additional insights and summaries of current trends, while a smaller proportion of conference reviews (3%) reflect limited discussions on the topic in conference settings. This analysis underscores the advancement of the discipline, as evidenced by the number of published research and its dependence on academic journals to disseminate significant findings. The first document on FGP research was found in 2005. From 2005 to 2023, the yearly total of publications in the field of FGP research is depicted in Figure 4. Until 2011, there were very few publications on FGP study. However, the publication counts increased in 2012 but decreased in 2013. The number of publications was almost equal from 2014 to 2016. Then, the number of documents increased gradually from 2017 to 2020, increasing from 27 to 52. Again, there is a slight decrease in 2021 to a number of 44. Inconsistent progress was discovered in the investigation, and no period of consistent growth was observed during that time. Thereafter, annual publication rates increased from 2022 to 2023, with an annual number of publications equal to 58 and 64, respectively. The evaluation suggested that the development of FGP over the past 7 years has shifted the attention of researchers to FGP research.

Figure 2

Subject areas that are relevant and featured articles on FGP studies.

Figure 3

Document types pertaining to FGP studies.

Figure 4

Publications on GPS research from 2005 to 2023.

3.2 Science mapping of publication outlets

On the basis of bibliographical data, publishing sources (conferences/journals) were assessed using VOSviewer. A mere 16 out of 134 sources were found to have published a minimum of five articles pertaining to FGP research. The sources comprising a minimum of five articles on FGP studies as of 2023 are listed in Table 1, along with the citations record. The publishers that published the most articles overall were identified as “Construction and building materials,” with 46 articles, “Ceramics International,” with 27 articles, and “Materials,” with 22 articles. Additionally, as of 2023, “Construction and building materials” with 1,941 citations; “Ceramics International,” with 1,248 citations; and “Cement and concrete composites,” with 962 citations in terms of the number of citations obtained were the three most prominent journals. More specifically, the results obtained from this inquiry would establish the basis for subsequent scientometric assessments of FGP research. Standard review studies that were previously conducted were incapable of furnishing such information.

The mapping of publication sources containing a minimum of five FGP publications is illustrated in Figure 5. In Figure 5(a), the frame size indicates the relative significance of each source based on article quantity. For instance, “Construction and Building Materials” has a more substantial influence than other publications, rendering it a more significant source within the discipline. The map exhibits four discrete groupings, each of which is symbolized by a unique color: green, red, blue, and yellow. Groups are established based on the frequency of co-citations in articles or the scope of the research source [33]. In the case of VOSviewer, the sources were categorized according to their co-citation frequencies in articles. The green cluster, for instance, comprises six articles that have been co-cited on multiple occasions in publications of a similar nature. The strength of the links between adjacent frames is greater in a group than between frames that are far apart. Four clusters of journals were noted from the map. The green cluster, led by “Construction and Building Materials,” includes journals emphasizing materials science and sustainability, such as “Journal of Cleaner Production,” “Cement and Concrete Composites,” and “Materials,” highlighting the structural and environmental aspects of FGPs. The red cluster, centered around “Ceramics International,” focuses on ceramics and composite materials, reflecting research on optimizing FGP properties like porosity and mechanical strength. The blue cluster represents conference proceedings (e.g., “IOP Conference Series: Materials Science and Engineering”), which highlight ongoing discussions and emerging ideas. Finally, the yellow cluster features journals like the “Journal of Building Engineering,” emphasizing practical engineering applications. This network illustrates the interdisciplinary nature of FGP research, spanning materials science, engineering, and sustainability domains.

Figure 5

Publication sources that published a minimum of five documents: (a) network visualization and (b) density visualization.

As illustrated in Figure 5(b), different hues correspond to distinct densities of concentration at an outflow. As one progresses from red to yellow, green, and blue, the color intensity diminishes. The reddish-yellow hues observed in prestigious periodicals, including “Construction and Building Materials,” “Ceramics International,” and “Materials,” indicate a greater commitment to FGP studies.

3.3 Science mapping of keywords

Keywords are essential elements of the research process as they function to differentiate and highlight the central topic of the study [34]. An analysis revealed that 92 out of 2,785 keywords contained at least 10 repetitions. Table 2 presents the 32 terms that are most frequently encountered in scholarly works. The five most regularly utilized keywords in FGP research articles are inorganic polymers, geopolymers, geopolymers, compressive strength, and thermal conductivity. According to the results of the keyword analysis, the majority of FGP research has been devoted to its application for thermal insulation and fire resistance material applications. Keywords such as “compressive strength” and “thermal conductivity” appear prominently, reflecting a significant emphasis on the mechanical and thermal performance of FGPs. This indicates that much of the research is directed toward evaluating and optimizing these critical properties to meet the demands of construction applications, particularly in thermal insulation and lightweight structural materials. Also, using FGP is a viable approach for reducing the carbon footprints of the construction industry.

Figure 6 presents a systematic map of keywords, illustrating their interrelationships, density, and connections. The frequency of a keyword frame’s appearance in articles is denoted by its magnitude in Figure 6(a), while the categories of content in which it appeared together are indicated by its position. In contrast to the remaining keywords, the most prominent ones exhibit more expansive frameworks on the map, indicating their significance in the FGP study. The figure highlights clusters to illustrate the frequency with which they occur in conjunction across multiple sources. The keywords are color-coded according to the frequency with which they co-occur in the articles. Five clusters of diverse colors are illustrated in Figure 6(a). Cluster 1 (red) contains 26 items, cluster 2 (green) contains 21 items, cluster 3 (blue) contains 19 items, cluster 4 (yellow) contains 15 items, and cluster 5 (purple) contain 11 items. Keywords like “thermal conductivity” and “fire resistance” (yellow cluster) highlight a focus on energy-efficient and fireproof construction materials. The red cluster emphasizes mechanical properties such as “compression strength” and “porous materials,” crucial for lightweight and structural applications. The green cluster reflects environmental applications, with terms like “adsorption” and “heavy metals,” showcasing FGPs’ potential in remediation and carbon reduction. Meanwhile, the blue cluster compares FGPs with conventional materials like Portland cement, addressing sustainability advantages, and the purple cluster focuses on the fundamental chemical used for FGP production, such as “silicates,” “sodium,” and “silicon compounds.” These clusters collectively underline the multidisciplinary nature of FGP research, addressing both performance optimization and environmental impact.

Figure 6

Visualization of keywords: (a) network and (b) density.

As illustrated in Figure 6(b), a spectrum of colors can be utilized to visually represent the density of keywords. The terms inorganic polymers, geopolymers, geopolymers, and other pertinent information are visually represented in red or yellow to indicate a higher frequency of usage. These results will support scholars in the process of choosing suitable keywords that will streamline the lookup for pertinent scholarly articles.

3.4 Science mapping of authors

The extent of one’s scholarly impact can be gauged by the frequency with which their work is referenced [35]. Out of the 1,244 academicians who were examined, only 37 fulfilled the criterion of having a minimum of five publications. The authors of the most prolific FGP research are enumerated in Table 3. The average citations of individual authors were calculated using the ratio of the total citations and total articles. Assessing the productivity of a researcher becomes a complex task when every pertinent metric is taken into account, including the number of published papers, average citations, and cumulative citations. Total publications reflect the productivity of a researcher, while average citations per publication indicate the impact and quality of their work. Cumulative citations provide a broader measure of their overall influence in the field. Conversely, the scientist’s performance shall be assessed impartially in accordance with all criteria. Based on statistical data, Colombo, Paolo emerges as the most prolific author, having authored 16 articles, while Bai, Chengying, and Rossignol, S. each contribute 12 papers. With a total of 1,139 citations, Zhang, Zuhua is the foremost authority in the field of FGP research, followed by Wang, Hao (1,072 citations) and Colombo, Paolo (1,045 citations). Conversely, with an average of 179 citations, Wang, Hao might rank first, Zhang, Zuhua might rank second with 163 citations, and Peyratout, C., Smith, A. might rank third with 86 citations. These prolific authors and their highly cited works have shaped key research directions in FGPs. For instance, their studies have driven advancements in thermal insulation properties, foaming techniques, and the use of sustainable precursors like fly ash and slag. By analyzing their contributions, readers can identify foundational studies and cutting-edge research trends.

The correlation between the counts of published works and the prominence of authors is illustrated in Figure 7(a). The density map of connected authors is shown in Figure 7(b). The color gradient indicates author prominence and influence: red and yellow regions represent authors with high publication density and collaboration networks, such as Colombo, Paolo, Zhang, Zuhua, and Rossignol, Sylvie. In contrast, green and blue regions depict authors with lower activity or peripheral involvement. This visualization is significant for identifying leading contributors and collaboration hubs within the field and guiding new researchers in selecting potential mentors, collaborators, or influential studies to enhance their understanding and engagement with the FGP research community.

Figure 7

Science mapping of authors: (a) network visualization and (b) density visualization.

3.5 Science mapping of documents

An article’s citation count is used in academics to determine its importance [24]. In their respective disciplines of study, the most frequently referenced works are frequently regarded as groundbreaking. Only 26 out of 361 articles, which were expected to have at least 100 citations, were kept. The top five articles in the FGP research are shown in Table 4, along with their citations. There were 584 citations found in Zhang et al.’s [7] study, “Geopolymer foam concrete: An emerging material for sustainable construction.” This study has become a cornerstone in FGP research due to its novel approach to developing lightweight, thermally insulating construction materials using industrial by-products such as fly ash and slag. By addressing key challenges in pore structure optimization and demonstrating significant environmental benefits, Zhang’s work laid the foundation for integrating FGPs into sustainable construction practices. Additionally, Zhang et al. [36] and Liu et al. [37] ranked the best three for their articles with 397 and 320 citations, respectively. Only 6 articles had received more than 200 citations as of 2023, though. The evolution of citation trends in FGP research reflects the growing interest and advancements in the field over time. Early studies in the 2000s focused on the potential of geopolymers as sustainable alternatives to traditional cement, but it was not until Zhang et al.’s [7] landmark study in 2014.

The article network map based on citations is shown in Figure 8. A network of publications with a hundred or more citations through 2023 is displayed in Figure 8(a). A graph displaying the citation density of related publications may be found in Figure 8(b). The color gradient indicates the citation density, with red and yellow areas representing articles with a high number of citations, highlighting their significant impact on the field. For instance, the studies of Zhang et al. [7,36] appear as highly cited works, reflecting their foundational contributions to the development of sustainable geopolymer materials. Articles in green and blue regions have lower citation densities, indicating less academic attention but potentially emerging areas of research. This visualization helps researchers identify key landmark studies that shape the field and understand the evolution of research priorities in FGP development, aiding new researchers in locating influential studies and guiding future investigations.

Figure 8

Mapping of documents: (a) network and (b) density.

3.6 Science mapping of countries

Certain countries have submitted a larger quantity of articles to the journal than others, and they intend to maintain this pattern. The purpose of the scientific graph is to provide readers with data that is pertinent to the FGP research. Twenty-five countries satisfied the prerequisite of having a minimum of five articles. The list of nations with at least five publications on FGP research is presented in Table 5. The top three countries were China with 83, Italy with 47, and Australia with 39 publications. Australia (2,767 citations), Italy (2,132 citations), and China (1,987 citations) were found to be the three nations with the most number of citations. Figure 9 displays the scientific illustration as well as the density of the regions associated with citations. The magnitude of a nation’s impact on a subject is determined by the extent of its frame size in Figure 9(a), which is based on the quantity of articles published on the subject. For instance, China’s large node reflects its leading role in FGP research, driven by its extensive industrial by-products and significant funding for sustainable construction technologies [40,41]. Similarly, Italy’s contributions are promoted by its expertise in materials science, particularly in thermal insulation and fireproof materials, aligning with its focus on energy efficiency [42]. Australia, with its strong academic-industry collaborations and commitment to carbon emission reduction, has demonstrated leadership in utilizing locally sourced industrial waste for geopolymer development [43,44].

Figure 9

Mapping of countries: (a) network and (b) density.

The high publication counts and citation density observed in countries like China, Italy, and Australia underline the importance of international collaboration in FGP research. These leading nations act as global hubs of innovation, offering opportunities for partnerships that can enhance knowledge exchange and accelerate advancements. For example, collaborative efforts between countries with established FGP expertise and those with emerging research infrastructure can address region-specific challenges, such as the availability of raw materials or climatic considerations for FGP applications. Furthermore, international collaborations can help standardize geopolymer production techniques and testing protocols, facilitating the widespread adoption of these materials across diverse industries.

The density map in Figure 9(b) reveals high-density regions corresponding to countries with active FGP research activities, providing young researchers insights into collaboration opportunities. By engaging with high-impact nations, researchers can access cutting-edge methods, participate in joint projects, and co-author influential publications. Such partnerships foster interdisciplinary approaches and drive innovation in advanced FGP composites, which are critical for addressing global sustainability challenges. These statistical data and graphical illustrations serve as a valuable resource for identifying leading nations and establishing scientific alliances. Collaborating with subject matter experts and institutions in these countries can significantly enhance researchers’ contributions to the growing body of knowledge in FGP research and promote the global application of sustainable construction materials.

6.1 Thermal insulation: Technical properties and innovations

A substantial body of literature exists due to the significant technical advantages that geopolymer foams possess in comparison to conventional low thermal conductivity materials (e.g., excellent resistance to heat, environmentally friendly synthesis, and fire resistance). A pioneering inquiry was conducted in 2010 by Prud’homme et al. [55]. As a foaming agent, silica fume was employed in the synthesis of geopolymer foams characterized by thermal conductivities ranging from 220 to 240 mW·m⁻¹·K⁻¹ and a density of 534 kg·m⁻³. Additionally, two investigations were conducted in the same year [56,57]. Vaou and Panias [56] documented significantly reduced thermal conductivities in perlite-based geopolymers, reaching as low as 30 mW·m⁻¹·K⁻¹ through the foaming of H₂O₂. In comparison, conventional insulation materials such as fiberglass and mineral wool typically have thermal conductivity values around 40 mW·m⁻¹·K⁻¹ [58]. This indicates that while FGPs offer beneficial thermal insulation properties, they may have higher thermal conductivity than traditional insulators like fiberglass and mineral wool.

Aluminum powder was utilized by Shu-Heng et al. [57] to create highly porous geopolymers based on phosphoric acid. Despite the fact that the samples’ thermal conductivity was not revealed, they were manufactured with a compressive strength of 7 MPa and a porosity of 83%. Since then, there has been a significant surge in the number of studies devoted to porous geopolymers, with a particular emphasis on the development of formulations that provide thermal insulation. In addition to possessing an exceptionally limited ability to transfer heat, thermal insulating materials must also exhibit great resistance to vapor diffusion; failure to do so may permit water to permeate the materials, thereby significantly diminishing their insulating capabilities. Multiple authors have documented significant water uptake values: 26.2–41.1% [59], 31.9–56.3% [60], 30–50% [38], and 42.1–111.7% [61]. This limitation can be surmounted through the fabrication of bi-layered foams, which consist of a denser, bulkier layer that is highly porous and has a thin, dense surface layer that ensures high resistance to vapor diffusion. However, the bi-layered foam approach has complexity in production, requiring precise control over layering and interface bonding, which increases manufacturing demands. Additionally, the higher production costs and potential recyclability issues of multi-material layers may limit its scalability and adoption. The literature has previously showcased this solution [62,63], as evidenced by the images displayed in Figure 10.

Figure 10

Digital pictures depicting bi-layered geopolymers, including (a and b) mortar, (c) paste, and (d) composite that incorporates glass spheres [62].

6.2 Sound absorption: Acoustic characteristics and challenges

In addition, sound-absorbent materials are critical in mitigating the increasing degrees of noise pollution [64]. This is especially pertinent when contemplating construction materials, as sound absorbents improve the acoustic comfort of the interior for occupants and mitigate the health hazards (such as disruption of sleep) [65] linked to exposure to sound pollution. With few exceptions to date, exploration has been conducted into the acoustic properties of geopolymer foams [66,67]. However, these investigations yielded significant insights into the capability of geopolymer foams to function as sound barriers. Furthermore, they examined critical performance factors such as curing temperature [66], porosity and bulk density [36,66], and activating solution [66,68]. An upward trend in the sound absorption coefficient (α) was documented by Luna-Galiano et al. [66] as the open porosity increased from 11.5 to 28.9% within the frequency range of 100–5,000 Hz. One peak appeared in the low-frequency region (approximately 500 Hz), while the other appeared at higher frequencies (approximately 2,500 Hz). Max α was approximately 0.25 or 2,500 Hz. Papa et al. [68] reported comparable results, wherein they also identified two absorption maxima, albeit within distinct frequency ranges: 3,000–5,000 and 500–3,000 Hz. The sound absorption in the frequency ranges of 1,000–1,500 Hz and 4,200–6,500 Hz varied between 0.45 and 0.9, contingent upon the exact variety of metakaolin and activating solution utilized. Stolz et al. documented an alternative pattern of acoustic absorption: they observed an exceptionally high α (approximately 0.85 in magnitude) in the low-frequency range (125–250 Hz), which dropped precipitously to approximately 0.15 in the 500–2,000 Hz range prior to regaining prominence at higher frequencies [67]. In comparison, flexible polyurethane foam can achieve α values of 0.95 at 500 Hz [69]. This comparison indicates that FGPs offer competitive sound absorption performance, particularly in specific frequency ranges, making them a viable alternative to conventional sound-absorbing materials.

Zhang et al. [36] examined the effect of the sample thickness and fraction of fly ash to slag as raw materials on the acoustic performance of geopolymer foams. The substitution of 30% of fly ash using powdered blast furnace slag led to little changes in the sound absorption properties. Nevertheless, the introduction of more closely spaced samples resulted in improved sound absorption at lower frequencies but had no impact at higher frequencies. Extremely high α values (0.7–1.0) were observed in the low-frequency range (40–150 Hz) for these specimens.

The impact of the bulk density of the foams on α was inspected by Hung et al. [70]. The results indicated that sound absorption was greater at lower densities across the frequency range of 100–4,000 Hz. With a very high α ranging from 0.5 (in the low-frequency region) to 0.9 (in the higher frequency region), the lightest specimen (0.4 g·cm⁻³) demonstrated the highest effectiveness of any geopolymer foam soundproofing. The aforementioned inquiries provide evidence for the feasibility of employing geopolymer foams as sound absorbers. However, additional research is necessary to validate prior findings and elucidate the significance of pertinent factors, including the type of pores (whether they are open or closed), their distribution in terms of size, and their tortuosity. Except for Hung et al. [70], in which α was greater than 0.4 across the entire frequency range of 100–4,000 Hz, the aforementioned studies all demonstrated sound absorption behavior that was merely intriguing at particular frequencies but comparatively subpar in other ranges.

The data indicate that FGPs exhibit varying α across different frequency ranges. For instance, studies have demonstrated high α values in the low-frequency range but relatively lower performance at mid-frequencies, with subsequent improvements at higher frequencies. This frequency-dependent behavior suggests that FGPs can be tailored for specific real-world applications. In high-traffic urban areas, where low-frequency noise from vehicles predominates, FGPs’ superior performance in the lower frequency range makes them ideal for sound barriers along roads and highways. Similarly, in industrial settings, where machinery often generates noise across a broader spectrum, FGPs could be adjusted to provide balanced absorption by modifying pore size, density, or material composition. For residential environments, where mid-to high-frequency noise (e.g., voices and appliances) is prevalent, FGPs’ tunability offers potential for acoustic panels or insulation that enhance indoor comfort. Further advancements in material engineering could optimize FGPs for specific frequency ranges. Adjustments such as controlling the porosity, adding fillers, or layering materials with varying densities could significantly enhance their acoustic performance. For example, increasing the proportion of open pores could improve absorption at higher frequencies, while denser compositions might enhance low-frequency damping. This flexibility underscores FGPs’ versatility and points toward their potential development as a customized solution for diverse acoustic challenges.

6.3 Adsorbent: Applications and challenges

Exploring waste-based geopolymers as cost-effective and environmentally sustainable adsorbents could serve as a promising strategy to remediate industrial wastewater contamination and promote cleaner manufacturing processes. This is due to the fact that their synthesis can be conducted at room temperature with relatively low energy input, residues serving as solid precursors, and a minimal amount of activating solution being utilized. These conditions significantly limit the release of greenhouse gases. In addition, the strong binding of pollutants to the geopolymer matrix facilitates their use in alternative applications (e.g., as a filler or aggregate in the manufacturing of new construction materials) after exhaustion is possible and uncomplicated. These are significant advantages over the standard adsorbent activated carbon, whose recovery after use is difficult and has extremely high production costs [71]. The potential of this possibility as a viable substitute for activated carbons has been acknowledged for some time [72,73] due to the capacity of geopolymers to undergo cation exchange with the solution [74]. Recent scholarly articles [75,76,77] cover the subject; nevertheless, none of them made a distinction between bulk-type and pulverized geopolymer adsorbents; this deficiency will be rectified in the following section.

According to Al-Zboon et al. [78], the lead uptake from coal fly ash-based geopolymer powders was 81 mg·g⁻¹. Cheng et al. [79] described an even greater lead uptake of 100 mg·g⁻¹ from metakaolin-based powders. An illustration of the application of the same concept to the extraction of dyes from effluent is the geopolymer block developed by Liu et al. [80], which is both floatable and permeable. However, prior to adsorption, the geopolymers were pulverized and filtered using a 100-mesh screen. Notwithstanding the noteworthy outcomes of methylene blue removal at a rate of 50.7 mg·g⁻¹, the pulverized geopolymers remain unsuitable for direct application in packed beds and are not readily retrievable. Lately, the scientific community has shown a keen interest in the potential utilization of monolithic bodies (such as membranes) as opposed to powders. This approach offers the advantage of safety and simplicity when compared to the utilization of nano or micro-scale powders, as it enables the direct utilization of materials in packed beds without the need for support materials.

In lieu of a post-separation step, Ge et al. [81] employed a cylindrical membrane made of metakaolin-based geopolymer to extract Ni²⁺ from a synthetic effluent. Novais et al. [61] employed cylindrical discs as a means of lead extraction from wastewater. Notwithstanding the relatively low lead uptake exhibited by the monolithic geopolymers (ranging from 0.95 to 6.34 mg·g⁻¹) when compared to pulverized geopolymers [78,79], this study was among the initial to explore the utilization of monolithic adsorbents, namely those in bulk form rather than powder form. A further geopolymer adsorbent of the bulk variety was documented [82]. The maximal adsorption capacity of the zeolites supported on geopolymers was 37.9 mg·g⁻¹. Salehi and Kani reported the existence of a cylindrical mesoporous adsorbent [83] for the purpose of removing dyes. The adsorbent possessed an average pore diameter of 20 nm and a specific surface area (SSA) of 56.6 m²·g⁻¹. Maximum adsorption capacities for malachite green oxalate and basic violet were 46.6 and 46.4 mg·g⁻¹, respectively. Bai et al. [84] utilized monolithic FGPs to extract copper and ammonium, achieving 87% copper elimination and 95% ammonium elimination, corresponding to uptakes of 0.54 and 0.57 mg·g⁻¹, respectively. Tang et al. [85] have documented an additional method of stimulation: the implementation of metakaolin geopolymer spheres characterized by a substantial SSA (approximately 54 m²·g⁻¹) and a total porosity of 60% (refer to Figure 11). The maximal absorption values for Cu²⁺, Pb²⁺, and Ca²⁺ ions bound to the spheres were determined to be 35, 46, and 24 mg·g⁻¹, respectively.

Figure 11

SEM images depicting the surface characteristics of porous geopolymer spheres are as follows: (a) the spheres’ exterior surface and (b) the pore microstructure; (c) the spheres’ exterior surface prior to the addition of K12; and (d) the pore microstructure subsequent to the adsorption process on Cu(ii) [85].

6.4 Catalytic applications

The potential utilization of geopolymers in catalytic processes has been hypothesized for quite some time [86]. Geopolymers are viable catalysts because of the high permeability of their chemical durability, high SSA, and mechanical strength [87]. Nevertheless, the potential of these substances as catalyst supports has been the subject of very few investigations thus far [86,88] and even fewer as catalysts [89,90,91]. However, investigations on the use of geopolymer have been carried out for various catalytic applications, including effluent treatment [92,93,94], syngas production [95], and biodiesel manufacturing [90,96]. The current review is more particular, focused on the utilization of geopolymer foams in their bulk form rather than in particle form (for pertinent literature, see previous studies [87,97]). As far as our understanding, a single study [98] has examined the application of bulk-type geopolymer in the synthesis of biodiesel. An additive manufacturing process was utilized to shape a geopolymer mixture into a lattice structure (refer to Figure 12). Subsequently, this novel material underwent testing to determine the characteristics that make it suitable for the heterogeneous transesterification of soybean oil with methanol. When 3 weight percent of the structured catalyst was utilized to transesterify soybean oil for 4 h, 43.5 weight percent of biodiesel was produced. The results presented are exceedingly encouraging and provide evidence for the practicality of making use of geopolymers of the bulk kind as catalysts in relevant industrial applications.

Figure 12

Photographs of the powdered sodium-based geopolymer (a) and the direct ink writing-fabricated graded lattice structure (b–d) [98].

7.1 Adaptability in the construction sector

In current decades, geopolymers have garnered attention as a possibly more environmentally sustainable alternative to Portland cement due to their lower carbon footprint and ability to incorporate industrial by-products. Their adaptability stems from the customization potential in terms of porosity, adsorption, and leaching properties, which broadens their utility beyond traditional construction materials. These materials are finding innovative applications in renewable energy generation, such as biogas systems, where their thermal stability and resistance to chemical degradation play a crucial role. Moreover, geopolymers are being explored for environmental remediation, particularly in wastewater treatment, where their ability to adsorb contaminants like heavy metals and dyes has shown promise. Their use in energy-efficient construction is particularly noteworthy, as they serve as thermal insulation materials that significantly reduce energy consumption in buildings. The adaptability of geopolymers facilitates progress in sustainable technologies. Additional research and development may enhance their physical and chemical properties for targeted applications. Their versatility guarantees that geopolymers serve not merely as alternatives to conventional materials but as a transformative remedy for urgent worldwide issues in sustainability and energy efficiency. However, interdisciplinary challenges persist in fully realizing their potential. The lack of standardized testing protocols across engineering, materials science, and environmental science disciplines hinders broader adoption. Future studies should focus on developing unified frameworks for evaluating mechanical, thermal, and environmental performance to bridge these gaps and support seamless integration into diverse applications. Exploring advanced computational models and hybrid composites can further accelerate their practical implementation in the construction sector.

7.2 Advancing material properties and processing techniques

Geopolymers have exhibited considerable promise as materials with minimal thermal conductivity. However, additional pertinent characteristics of construction materials, including their capacity to absorb and release moisture and acoustic resistance, have received comparatively less attention thus far and warrant further research. Further research is warranted to ascertain the optimal processing techniques (e.g., replica method and direct frothing) and synthesis conditions necessary to meet the demands of these applications. Further investigation into novel processing methodologies, such as 3D printing, is warranted in order to fabricate monoliths featuring intricate geometries that enable enhanced regulation of pore size distribution and, consequently, improved performance. Notwithstanding the intriguing prospects presented by 3D printing, there exist engineering obstacles pertaining to the temporal alteration of the viscosity of slurries through geopolymerization reactions [99]. These challenges must be surmounted. It is necessary to research various techniques for extending the curing times of slurry in order to guarantee an appropriate printing timeframe. Irrespective of the method of processing, it is critical to utilize precursors that are both readily available and locally sourced. Ideally, untapped waste streams that are unsuitable for Portland cement blends should be utilized. Furthermore, compositions should be meticulously planned to guarantee cost-competitiveness in production while assuming net-zero emissions. Subsequent investigations ought to rely heavily on life cycle analysis to substantiate the ecological advantages linked to the use of geopolymer foams in building settings.

7.3 Advanced adsorption applications

An additional promising application of geopolymer foams is in the realm of adsorption. The majority of research conducted thus far has been devoted to the extraction of heavy metals and cationic dyes from synthetic wastewater. Industrial effluents may also contain anionic species; nevertheless, no study has documented the extraction of these species via geopolymer foams (as opposed to powders), and the application of powdered geopolymers is even less frequent [100]. Further research should be conducted regarding this subject matter. Furthermore, further research should be conducted on emerging sources of contamination, including pharmaceuticals and crude oil. Further research investigating the functionalization of the geopolymer surface and the fabrication of geopolymer composites (e.g., incorporating activated carbons or zeolites) could potentially augment the efficacy of these novel adsorbents. The primary obstacle impeding the broader application of geopolymer adsorbents is the disparity between what has been studied in the lab, frequently simulated using unattainable scenarios (e.g., synthetic wastewaters), and the intricacies of actual effluents [74,101]. It is expected that research concerning industrial effluents will be a popular subject in the near future.

7.4 Potential for pH buffering and catalytic applications

The investigation into the utilization of geopolymer foams as pH buffering materials is limited. However, the limited number of studies conducted thus far indicate their viability for this application with significant added value. To further demonstrate their potential, additional research should be conducted under more stringent conditions and for extended durations so as to more closely resemble their definite industrial application (e.g., continuous anaerobic digestion systems). An investigation into the assessment of the catalytic characteristics exhibited by geopolymer foams is warranted as well. By doping the geopolymer foams with unique reactive species, their catalytic properties could be significantly enhanced.

7.5 Higher production cost

A notable limitation of FGPs in comparison to conventional construction materials such as bricks, concrete blocks, and expanded polystyrene is the comparatively higher production cost under specific conditions. The expense of alkali activators, including sodium silicate and sodium hydroxide, is a major factor in this cost, frequently constituting a substantial share of the overall material expenditure [102]. Moreover, the requirement for specialized apparatus and regulated manufacturing settings, like exact foaming operations or curing environments, significantly increases production expenses [103]. Transportation and storage expenses may escalate if raw materials such as fly ash or slag are not locally accessible, requiring long-distance transportation [7]. Moreover, the absence of economies of scale in numerous places, attributable to restricted adoption and production infrastructure, renders FGPs less cost-competitive compared to extensively manufactured conventional materials [7]. These considerations combined underscore the necessity for advances in production techniques and supply chain efficiency to improve the economic viability of FGPs.

7.6 Limitations in experimental standardization

Despite significant progress in FGP research, a major limitation lies in the lack of standardized testing protocols across studies. Variations in experimental conditions, such as raw material compositions, pore structure measurements, curing processes, and environmental testing, make comparing results and generalizing findings difficult. For instance, thermal conductivity values may vary widely depending on the porosity measurement method or the type of foaming agent used, while differences influence α in frequency ranges and material thicknesses. This lack of uniformity complicates efforts to establish benchmarks for FGP performance in thermal and acoustic applications. Additionally, discrepancies in experimental designs hinder the reproducibility of results, limiting the reliability of studies for industrial-scale applications. Addressing these challenges requires the development of standardized testing protocols. Such protocols should define consistent parameters for evaluating FGP properties, including thermal conductivity, sound absorption coefficients, and durability under controlled environmental conditions. For thermal applications, specify the temperature range, heat flux, and material thickness for thermal conductivity testing. Also, standardize frequency bands, porosity characterization, and sample sizes for acoustic applications for sound absorption tests. By implementing standardized methodologies, researchers can produce more reliable and comparable data, facilitating advancements in material design and enabling broader adoption of FGPs in construction and environmental applications.