Behavior of Fibers in Geopolymer Concrete: A Comprehensive Review

: Over the last decades, cement has been observed to be the most adaptive material for global development in the construction industry. The use of ordinary concrete primarily requires the addition of cement. According to the record, there has been an increase in the direct carbon footprint during cement production. The International Energy Agency, IEA, is working toward net zero emissions by 2050. To achieve this target, there should be a decline in the clinker-to-cement ratio. Also, the deployment of innovative technologies is required in the production of cement. The use of alternative binding materials can be an easy solution. There are several options for a substitute to cement as a binding agent, which are available commercially. Non-crystalline alkali-aluminosilicate geopolymers have gained the attention of researchers over time. Geopolymer concrete uses byproduct waste to reduce direct carbon dioxide emissions during production. Despite being this advantageous, its utilization is still limited as it shows the quasi-brittle behavior. Using different fibers has been started to overcome this weakness. This article emphasizes and reviews various mechanical properties of fiber-reinforced geopolymer concrete, focusing on its development and implementation in a wide range of applications. This study concludes that the use of fiber-reinforced geopolymer concrete should be commercialized after the establishment of proper standards for manufacturing.


Introduction
In recent decades, there has been a rapid growth in urbanization in both less and more developed countries [1].By 2050, approximately more than two-thirds of the world's population is expected to move to urban areas [2].Globally, in 2018, the share of the urban population was around 4.2 billion, whereas 3.4 billion people were reported to live in rural areas.But by 2050, the statistics will change and around 6.7 billion people will live in urban areas and only 3.1 billion people will live in rural areas [2].Urbanization along with the emerging population growth is the major cause of the increasing demand for concrete [3][4][5][6].Concrete existence is mainly due to the cohesive strength of cement, which increases rapidly during setting [7].Globally, no sign of decline was observed in the production of Portland cement (PC), indicating that it has been increasing continuously [8][9][10].A large number of natural resources are used for the production of PC like fossil fuels, limestone, electricity, and natural gas [8].The manufacturing of PC is a very high-temperature and energy-intensive process, which results in an increase in carbon dioxide (CO 2 ) footprint in the atmosphere [11][12][13][14][15][16][17][18][19][20][21][22].The carbon emissions are primary factors contributing to global warming.Between 2015 and 2020, the direct CO 2 intensity of cement manufacturing grew by 1.8 percent per year.An annual reduction of 3% is required by 2030 to meet the final requirement of net zero emissions by 2050 [23].Cement is made up primarily of clinker, which is directly proportional to CO 2 emitted due to limestone decomposition in the clinkermaking process and from fuel combustion [13,[23][24][25][26][27].Therefore, to reduce the effect of carbon footprint due to PC production, particularly process emission, there is a need for alternative (sustainable) binding materials [18,23,[27][28][29][30][31].
The rise in the use of concrete across various applications resulted in an increase in concrete consumption [32][33][34][35][36].This also led to the development of concrete in different perspectives [37][38][39][40][41], which resulted in the invention of geopolymer concrete.For decades, researchers have been showing interest in geopolymer, an inorganic polymer with low density and high-temperature tolerance [42][43][44][45][46].It is a sustainable and environmentally friendly substitute to PC with a low cost of production [42][43][44][47][48][49].Geopolymer concrete technology offers significant promise for the commercialization, standardization, and repurposing of agricultural and industrial byproducts [50][51][52][53].Research has been carried out to address the challenges of environmental contamination caused by the disposal of these wastes in landfills [50].Depending on the use of activators and precursors, an 80% reduction in the footprint of carbon has been observed while using geopolymer [54][55][56][57].Moreover, a comprehensive survey has been conducted and the results demonstrated that alkali-activated binding materials have high strength, low carbon content, good frost resistance, etc. [58,59].Researchers acknowledged the use of geopolymer in retaining walls, boundary blocks, pavements, water tanks, road ramps, and precast beams.Despite all the applications of geopolymer, its application is still considerably limited.This is owing to limited studies on its use in structural elements and due to the requirements of practical design standards [60].
Various studies on geopolymer have identified that it displays quasi-brittle behavior like PC concrete and rocks [61][62][63][64][65]. Structures that exhibit large macroscopic crack growth prior to failure are termed quasi-brittle structures [66,67].To overcome this failure, the use of distinct fibers, whether continuous or short fibers along with alkali-activated binders, is introduced [67][68][69].This article offers a comprehensive and pioneering exploration of the mechanical properties of fiber-reinforced geopolymer concrete (FRGC), emphasizing its development and practical application.While some research works may overlook the importance of incorporating various fibers alongside alkali-activated binders to bolster the structural strength of geopolymer concrete, our study underscores this critical factor.Our examination of the material's mechanical behavior provides valuable insights which are crucial for real-world applications, setting our work apart from studies that primarily focus on binders.Additionally, our research goes a step further by categorizing different types of fibers based on characteristics such as type, aspect ratio, strength, modulus of elasticity, and density.This classification facilitates the prioritization of specific fiber types according to their impact on the mechanical properties, a novel contribution that was previously absent in the existing body of literature on FRGC.
In this study, the research on geopolymer concrete holds remarkable importance for sustainable construction practices.Geopolymer, characterized by low density and hightemperature tolerance, emerges as an eco-friendly alternative to traditional PC, contributing to reduced carbon footprints and waste management.It effectively utilizes agricultural and industrial residues, mitigating environmental contamination from landfill disposal.However, despite its potential, the adoption of geopolymer concrete remains limited, primarily due to the absence of practical design standards for structural elements.This research addresses a critical issue by focusing on FRGC, a vital aspect often overlooked in prior studies.By incorporating various fibers alongside alkali-activated binders, the study enhances the structural strength of geopolymer concrete.Moreover, it classifies fibers based on their characteristics, allowing for the prioritization of specific fiber types in accordance with their influence on the mechanical properties.This novel contribution fills a significant gap in the existing literature and provides valuable insights for real-world applications.This research work extends the potential of geopolymer concrete by improving its structural integrity and providing a comprehensive framework for incorporating different fiber types, making it a crucial step toward wider adoption of this sustainable construction material.
The existing research gap in fiber-reinforced geopolymer composites lies in the insufficient attention to the economic feasibility and scalability of these materials.While extensive studies have focused on their technical properties, there is a need for investigation into the cost effectiveness of large-scale production and implementation in construction projects.Additionally, the environmental impact of fiber-reinforced geopolymer composites, including factors such as raw material sourcing and energy consumption during production, requires further scrutiny.Bridging this research gap will be pivotal in ensuring that geopolymer composites not only exhibit impressive technical performance, but also prove to be economically viable and environmentally sustainable for widespread adoption in the construction industry.
Re-orientation: Monomers of liberated silicate and aluminate combine to form short alumino-silicate oligomers.
Solidification: The 3D geopolymer structure hardens and gains strength.Completion of these three reaction phases is related to the characteristics of various alumino-silicate source materials.Proper dissolution is accomplished by sodium silicate (Na 2 SiO 3 with pH range of 11-13) and sodium hydroxide (NaOH with pH of 14) solutions [90][91][92].The introduction of a resting period between mixing and shaping the material and the commencement of solidification by adding heat are common ways to stimulate the reorientation phase [83].The compressive strength development after geopolymer hardening starts at room temperature for various pozzolanic materials.N-A-S-H gel is a 3D structure, which is found to be the main hydration product of this calcium-free binder [93].The alkaline solution has an impact on both fresh and hardened properties of geopolymer concrete; therefore, the selection should be made carefully [55].Both precursors (alumino-silicate materials) and activators (alkaline solutions) are dependent on the strength development of geopolymeric materials [94].
The objective of this review article on geopolymer concrete is as follows: a.
To develop a better understanding of alumino-silicate material.b.To study the temperature effect and curing time.c.
To gather the knowledge regarding the selection or use of the proper alkaline activator solution.
d.To study the proper water content.

FRGC
When various types of fibers such as composite fillers are added to geopolymer, their composition is considered an FRGC [95][96][97][98].These FRGC composites enhance the structural durability as they can sustain temperatures from as low as room temperature to higher elevated temperatures (for example, 1000 • C) [34,[86][87][88].Consequently, these FRGC composites are an emerging alternative to repair materials in the construction industry [99][100][101].The processing ability of these composites can be affected by the fiber length.As a result, they are classified as short fiber composites (length of less than 1 mm) and long fiber composites (length of more than 2 mm and up to 10 mm) [102].The use of the geopolymer matrix with fibers is found to be admissible as they can sustain elevated temperatures with excellent durability and no release of toxic gas [96,[103][104][105].Therefore, to improve the energy absorption and modulus of rupture (flexural strength), geopolymer composites are reinforced with fibers in various forms of filaments, threads, whiskers, and nanoparticles [106].The selection of fibers to make FRGC composites is based on a sufficient fiber-matrix interaction, material property compatibility with applications, and optimum aspect ratio to control the post-cracking behavior of these composites [106].Accordingly, fibers are classified as metallic fibers (such as steel fiber), mineral fibers (such as asbestos fiber), organic fibers (natural and man-made fiber), natural fibers (such as leaf fiber and silk), natural polymers (such as protein fiber and cellulose), and synthetic fibers (such as polypropylene and nylon fibers) based on the origin of the fibers [107].In regard to the fibers, researchers have proposed that the performance of FRGC composites predominantly depends on the properties of the fibers [106].

Steel Fiber
Steel fibers can be defined as a metal reinforcement with a discrete length and aspect ratio in a range between 20 and 100, in which various cross sections can be randomly dispersed in fresh concrete mixture [108].Steel fiber-reinforced concrete (SFRC) has a wide range of applications in civil engineering [109] such as enhancing the toughness, impact, and abrasion resistance of various composites [108].Owing to the high availability, mechanical strength, and flexibility of steel fibers, they are used in cement composites [106].Modified cold-drawn wires, mill cut, melt extracted, pieces of smooth or deformed colddrawn wires and smooth or deformed cut sheet steel fibers are categorized as types of steel fibers according to ASTM A820-16 [110].The fibers are categorized based on their physical and mechanical properties.From the literature, it is acknowledged that for different types of steel fibers with a distinct aspect ratio, the modulus of elasticity was approximately 2 × 10 5 MPa, and the fiber strength ranges between 800 and 2850 (MPa) with an approximate density of 7.8 kg/m 3 .Table 1 lists the physical and mechanical properties of steel fibers.The microstructural properties (sorptivity, effective porosity, and water absorption) of geopolymer concrete were lower than conventional concrete, which can be further improved with the addition of a lower quantity of steel fibers.Furthermore, it can be concluded that the durability characteristics of geopolymer concrete is better than conventional concrete, which can be further improved with the addition of steel fibers [111].For the ultra-high-performance fiber-reinforced geopolymer concrete (UHPFRGC), 2% (by volume fraction) of steel fibers gives the highest mechanical strength.Whereas the mechanical strength of UHPFRGC reduces when steel fibers are replaced with polypropylene fibers.For concrete with both steel fibers and polypropylene fibers, the flow passing through concrete is reduced [112].Riahi et al. [113] concluded in their research that the interfacial bond between the metakaolin-based polymer composite and steel fibers affects the strength.Figure 1 depicts a scanning electron microscope (SEM) image of an uncoated steel fiber matrix transition zone, revealing the debonding that occurs at the interface.In Figure 2, another SEM image illustrates the same uncoated steel fiber matrix transition zone, but this time, it highlights the presence of a geopolymeric product adhering to the surface of steel fibers.Figure 3 presents a SEM image indicating the interface between a steel-coated fiber matrix and the alumina coating in the transition zone.Meanwhile, Figure 4 showcases another SEM image demonstrating the debonding process and the presence of geopolymer products on the surface of coating.Thus, to increase the interfacial bond strength between the polymer composite and steel fibers, an alumina coating should be done on steel fibers.Compared to the uncoated steel fibers, the alumina-coated steel fibers enhanced the bond strength by 151%.It was also resulted from the study that the peak load deflection and deflection hardening behavior of the alumina-coated steel fibers were higher.be concluded that the durability characteristics of geopolymer concrete is better than conventional concrete, which can be further improved with the addition of steel fibers [111].
For the ultra-high-performance fiber-reinforced geopolymer concrete (UHPFRGC), 2% (by volume fraction) of steel fibers gives the highest mechanical strength.Whereas the mechanical strength of UHPFRGC reduces when steel fibers are replaced with polypropylene fibers.For concrete with both steel fibers and polypropylene fibers, the flow passing through concrete is reduced [112].Riahi et al. [113] concluded in their research that the interfacial bond between the metakaolin-based polymer composite and steel fibers affects the strength.Figure 1 depicts a scanning electron microscope (SEM) image of an uncoated steel fiber matrix transition zone, revealing the debonding that occurs at the interface.In Figure 2, another SEM image illustrates the same uncoated steel fiber matrix transition zone, but this time, it highlights the presence of a geopolymeric product adhering to the surface of steel fibers.Figure 3 presents a SEM image indicating the interface between a steel-coated fiber matrix and the alumina coating in the transition zone.Meanwhile, Figure 4 showcases another SEM image demonstrating the debonding process and the presence of geopolymer products on the surface of coating.Thus, to increase the interfacial bond strength between the polymer composite and steel fibers, an alumina coating should be done on steel fibers.Compared to the uncoated steel fibers, the aluminacoated steel fibers enhanced the bond strength by 151%.It was also resulted from the study that the peak load deflection and deflection hardening behavior of the aluminacoated steel fibers were higher.According to [123], the elevated temperature also affects the compressive strength of steel fiber-reinforced geopolymer concrete (SFRGC) for both sodium and potassium alkali-activator based concretes.The strength up to a temperature of 400 °C was higher compared to concrete cured in ambient temperature and to that of SFRC.However, it tends to decrease in the same pattern for all types of concrete.Alkali-activated based geopolymer concretes (concrete with sodium hydroxide and concrete with potassium hydroxide) at elevated temperatures exhibit a lower elastic modulus compared to that of SFRC.Taking SFRC into consideration, both alkali-activator concretes experience lesser spalling at an ultimate compression load and lesser surface cracking when exposed to elevated temperatures.Also, the damage in the top layer of steel fibers is noticed in the case of SFRC compared to both SFRGC (with sodium hydroxide) and SFRGC (with potassium hydroxide) when exposed to elevated temperatures.
Liu et al. [116] evaluated the influence of steel fibers on the mechanical properties of ground granulated blast-furnace slag (GGBS), silica fume, and class F fly ash-based geopolymer concrete.They compared the effect of straight steel fibers on the compressive and flexural strengths of SFRGC with three different aspect ratios at three various volume fractions.Table 1 provides the details of the fibers used in the research.Three different fiber types are named as SM (straight steel fiber with an aspect ratio of 67 and tensile strength of 2500 MPa), SL (straight steel fiber with an aspect ratio of 108 and tensile strength of 2500 MPa), and SW (straight steel fiber with an aspect ratio of 65 and tensile strength of 2850 MPa).The three volume fractions were 1%, 2%, and 3%.Figures 7 and 8 depict a comparison of 28-day compressive strength and 28-day flexural strength, respectively for various SM, SL, and SW specimens of SFRGC.In Figures 7 and 8, SM1, SM2, and SM3 designate specimens with 1%, 2%, and 3% SM type of steel fibers, respectively.SL1, SL2, and SL3 represent specimens with 1%, 2%, and 3% SL type of steel fibers, respectively.SW1, SW2, According to [123], the elevated temperature also affects the compressive strength of steel fiber-reinforced geopolymer concrete (SFRGC) for both sodium and potassium alkaliactivator based concretes.The strength up to a temperature of 400 • C was higher compared to concrete cured in ambient temperature and to that of SFRC.However, it tends to decrease in the same pattern for all types of concrete.Alkali-activated based geopolymer concretes (concrete with sodium hydroxide and concrete with potassium hydroxide) at elevated temperatures exhibit a lower elastic modulus compared to that of SFRC.Taking SFRC into consideration, both alkali-activator concretes experience lesser spalling at an ultimate compression load and lesser surface cracking when exposed to elevated temperatures.Also, the damage in the top layer of steel fibers is noticed in the case of SFRC compared to both SFRGC (with sodium hydroxide) and SFRGC (with potassium hydroxide) when exposed to elevated temperatures.
Liu et al. [116] evaluated the influence of steel fibers on the mechanical properties of ground granulated blast-furnace slag (GGBS), silica fume, and class F fly ash-based geopolymer concrete.They compared the effect of straight steel fibers on the compressive and flexural strengths of SFRGC with three different aspect ratios at three various volume fractions.Table 1 provides the details of the fibers used in the research.Three different fiber types are named as SM (straight steel fiber with an aspect ratio of 67 and tensile strength of 2500 MPa), SL (straight steel fiber with an aspect ratio of 108 and tensile strength of 2500 MPa), and SW (straight steel fiber with an aspect ratio of 65 and tensile strength of 2850 MPa).The three volume fractions were 1%, 2%, and 3%.Figures 7 and 8 depict a comparison of 28-day compressive strength and 28-day flexural strength, respectively for various SM, SL, and SW specimens of SFRGC.In Figures 7 and 8, SM1, SM2, and SM3 designate specimens with 1%, 2%, and 3% SM type of steel fibers, respectively.SL1, SL2, and SL3 represent specimens with 1%, 2%, and 3% SL type of steel fibers, respectively.SW1, SW2, and SW3 denote specimens with 1%, 2%, and 3% SW type of steel fibers, respectively.The figures point out that for all three different types of fiber specimens (SM, SL, and SW), both the compressive strength and flexural strength increased with an increase in the percentage of fibers from 1% to 3%.Among all the considered specimens, SL3 gave the best result for both compressive strength and flexural strength with values of 170.4 MPa and 33.3 MPa, respectively.The comparison concludes that fibers with a higher aspect ratio would perform well [116].Increase in the fiber content enhances the compressive strength and flexural strength up to the optimum replacement value of 3%.
There is a positive effect on the molarity of sodium hydroxide in the compressive strength increment.

Glass Fiber
Glass fibers are non-crystalline materials with a short-range network structure that comprises an extremely fine fiber of glass [131].Primarily, glass fibers are used to reinforce polymers.Generally, there are three leading glass fiber types including CR (corrosion resistant) glass, E-glass, and HS (high strength) glass [132].When comparing E-glass and HS glass, HS glass has better creep and fatigue resistances.Glass fibers have low electrical as well as thermal conductivity.Kumar et al. [133] concluded that geopolymer concrete (combination of GGBS, fly ash, condensed silica fume, and metakaolin) with steel fibers increases the strength compared to concrete incorporated with glass fibers.But if the strength is not a major criterion, then glass fibers in geopolymer concrete play an important role.They make concrete denser, ductile, and crack free.Sivaraja et al. [134] reported that the compressive strength of geopolymer concrete with glass fibers, when heat cured at 60 • C, was found to be highest at 2% glass fibers by the volume fraction, and the splitting tensile strength was resulted to be highest at 2% fiber incorporation.The use of nano CaCO 3 in geopolymer acts as a nucleation site, which helps in the formation of additional products within the geopolymeric composite and in accelerating the geopolymeric reaction [135].The ability of nanomaterials to fill micro voids helps the geopolymeric composite gain the strength.As a consequence, the research concluded that the incorporation of nano CaCO 3 increased the compressive strength, and the incorporated glass fibers improved the mechanical performance [135].In accordance with Sathanandam et al. [136], the molarity of geopolymer concrete also affects the mechanical performance.The geopolymeric paste with molarity of 16 M and 20 M showed higher compressive strength than geopolymer concrete with 12 M. Additionally, 0.3% glass fibers incorporated with 20 M in geopolymer concrete, when cured thermally, demonstrated higher compressive strength than geopolymer concrete cured in a normal environment.Moreover, the addition of glass fibers to geopolymer concrete influenced the workability; the slump value decreased with the increase in the fiber content up to 1.25% by volume of concrete and the density increased with the increase in the fiber content by volume [137].Geopolymer concrete without glass fibers exhibited both initial and final cracks at the same number of blows.But with glass fibers, the impact of geopolymer composite's strength increased ten times when compared to the control specimens, since fibers increased the modulus of elasticity and stiffness [138].Figure 9 displays a SEM image of geopolymer concrete lacking the glass fiber reinforcement.This image visually elucidates the microstructure and composition of unreinforced concrete.In Figure 10, a SEM image of geopolymer concrete can be observed which has been fortified with 0.3% glass fibers, enabling us to scrutinize alterations in the structure and the distribution of fibers within the geopolymer matrix.
(by volume fraction)

Glass Fiber
Glass fibers are non-crystalline materials with a short-range network structure that comprises an extremely fine fiber of glass [131].Primarily, glass fibers are used to reinforce polymers.Generally, there are three leading glass fiber types including CR (corrosion resistant) glass, E-glass, and HS (high strength) glass [132].When comparing E-glass and HS glass, HS glass has better creep and fatigue resistances.Glass fibers have low electrical as well as thermal conductivity.Kumar et al. [133] concluded that geopolymer concrete (combination of GGBS, fly ash, condensed silica fume, and metakaolin) with steel fibers increases the strength compared to concrete incorporated with glass fibers.But if the strength is not a major criterion, then glass fibers in geopolymer concrete play an important role.They make concrete denser, ductile, and crack free.Sivaraja et al. [134] reported that the compressive strength of geopolymer concrete with glass fibers, when heat cured at 60 °C, was found to be highest at 2% glass fibers by the volume fraction, and the splitting tensile strength was resulted to be highest at 2% fiber incorporation.The use of nano CaCO3 in geopolymer acts as a nucleation site, which helps in the formation of additional products within the geopolymeric composite and in accelerating the geopolymeric reaction [135].The ability of nanomaterials to fill micro voids helps the geopolymeric composite gain the strength.As a consequence, the research concluded that the incorporation of nano CaCO3 increased the compressive strength, and the incorporated glass fibers improved the mechanical performance [135].In accordance with Sathanandam et al. [136], the molarity of geopolymer concrete also affects the mechanical performance.The geopolymeric paste with molarity of 16 M and 20 M showed higher compressive strength than geopolymer concrete with 12 M. Additionally, 0.3% glass fibers incorporated with 20 M in geopolymer concrete, when cured thermally, demonstrated higher compressive strength than geopolymer concrete cured in a normal environment.Moreover, the addition of glass fibers to geopolymer concrete influenced the workability; the slump value decreased with the increase in the fiber content up to 1.25% by volume of concrete and the density increased with the increase in the fiber content by volume [137].Geopolymer concrete without glass fibers exhibited both initial and final cracks at the same number of blows.But with glass fibers, the impact of geopolymer composite's strength increased ten times when compared to the control specimens, since fibers increased the modulus of elasticity and stiffness [138].Figure 9 displays a SEM image of geopolymer concrete lacking the glass fiber reinforcement.This image visually elucidates the microstructure and composition of unreinforced concrete.In Figure 10, a SEM image of geopolymer concrete can be observed which has been fortified with 0.3% glass fibers, enabling us to scrutinize alterations in the structure and the distribution of fibers within the geopolymer matrix.The addition of glass fibers to geopolymer concrete has illustrated varying effects on the compressive strength [136,139].According to Table 3 of this research article, glass fibers are classified on the basis of their aspect ratio, length, diameter, modulus of elasticity, and tensile strength.Glass fibers used in the experimental investigations of Refs.[136,139] had aspect ratios of 35, 62, and 600, modulus of elasticity of 42,000 MPa and 82,000 MPa, respectively, and fibers' tensile strength of 1000 MPa and 2500 MPa, respectively.Zuaiter et al. [139] indicated that the incorporation of glass fibers led to the enhanced compressive strength at different curing ages.The short glass fibers resulted in an increase in the strength up to 24%, while the long fibers initially decreased the strength at 7 days but later increased at 28 days.Hybrid combinations, particularly with more long fibers at 1% volume fraction, showed a substantial 40% increase in the strength.On the other hand, Sathanandam et al. [136] highlighted that the inclusion of 0.3% glass fibers in geopolymer concrete resulted in a notable 16% increase in the compressive strength compared to the conventional geopolymer concrete.However, the addition of fibers led to the reduced strength.From the above comparison, it can be easily concluded that fibers with higher aspect ratios increase the compressive strength of GFRGC at a lower volume fraction with the addition of glass fibers, whereas glass fibers with lower aspect ratios increase the compressive strength of GFRGC at a higher volume fraction with the addition of glass fibers [136,139].Figure 11 depicts what glass fibers look like.Table 4 includes the literature survey on GFRGC.The addition of glass fibers to geopolymer concrete has illustrated varying effects on the compressive strength [136,139].According to Table 3 of this research article, glass fibers are classified on the basis of their aspect ratio, length, diameter, modulus of elasticity, and tensile strength.Glass fibers used in the experimental investigations of Refs.[136,139] had aspect ratios of 35, 62, and 600, modulus of elasticity of 42,000 MPa and 82,000 MPa, respectively, and fibers' tensile strength of 1000 MPa and 2500 MPa, respectively.Zuaiter et al. [139] indicated that the incorporation of glass fibers led to the enhanced compressive strength at different curing ages.The short glass fibers resulted in an increase in the strength up to 24%, while the long fibers initially decreased the strength at 7 days but later increased at 28 days.Hybrid combinations, particularly with more long fibers at 1% volume fraction, showed a substantial 40% increase in the strength.On the other hand, Sathanandam et al. [136] highlighted that the inclusion of 0.3% glass fibers in geopolymer concrete resulted in a notable 16% increase in the compressive strength compared to the conventional geopolymer concrete.However, the addition of fibers led to the reduced strength.From the above comparison, it can be easily concluded that fibers with higher aspect ratios increase the compressive strength of GFRGC at a lower volume fraction with the addition of glass fibers, whereas glass fibers with lower aspect ratios increase the compressive strength of GFRGC at a higher volume fraction with the addition of glass fibers [136,139].Figure 11 depicts what glass fibers look like.Table 4 includes the literature survey on GFRGC.Maximum compressive strength was obtained for the specimen with 0.5% glass fibers (for example, 15% more than conventional).
In comparison to the steel fiber specimen, the glass fiber specimen demonstrated higher mechanical strength increment.

NA Heat curing
The analysis was carried out using glass fibers, and sand was replaced with copper slag.2% was the optimum fiber content.Results for the mechanical strengths (compressive, splitting, and flexural stengths) were maximum for the optimum content.The optimum ratio of sodium silicate and caustic soda was found to be 1.5 for the maximum compressive strength.

Polypropylene Fiber
In recent years, the use of fibers named polypropylene in plain concrete has gained popularity.Further, the use of the fibers in plain concrete improves the crack resistance and reduces the shrinkage [145].To enhance the compressive strength of polypropylene fiberreinforced geopolymer concrete (PPFRGC) in comparison to plain concrete, it is necessary to ensure the optimum amount of fibers to be added.Reed et al. [145] recommended that 0.05% (added by weight) polypropylene fibers was the optimum content, as it indicates the best compressive strength result for all testing days (specimen testing was carried out for 0, 10, 20, 30, 40, 50, and 60 days).Also, due to the random distribution, polypropylene fibers limit the propagation of cracks.Ranjbar et al. [146] discussed the long-term effect of polypropylene fibers on increasing the compressive strength.It was resulted that no significant increase in the compressive strength was witnessed at the early ages, but the addition of polypropylene fibers adversely affected the concrete's compressive strength.Moreover, higher energy absorption up to 3% polypropylene fibers was observed (from 2.5% to 5% higher compared to the geopolymer specimen with no fiber).Rajak and Rai [147] stated that the loss in the compressive strength for PPFRGC specimens was evidenced when oven cured at 80 • C for 24 h.The loss in the compressive strength decreased with increasing the content of polypropylene fibers with a minimum 5% loss at 0.2% fiber inclusion.Aygörmez et al. [148] performed research on PPFRGC with polypropylene fibers of higher strength and higher aspect ratio.The research was conducted to acknowledge the effects of higher temperatures (for example, 300 • C, 600 • C, and 900 • C).The compressive strength decreased considerably when the temperature increased beyond 600 • C. Figure 12 displays a field emission scanning electron microscope (FESEM) image that provides a detailed view of the surface morphology of polypropylene fibers.Figure 13 illustrates a FESEM image that depicts the interaction between the fibers and matrix material within the examined specimen.
Table 5 presents details of various types of polypropylene fibers used in experimental investigations of PPFRGC by several researchers.Comparing the studies done by Ranjbar et al. [146] and Aygörmez et al. [148], the former suggested that the addition of polypropylene fibers to fly ash-based geopolymer concrete had mixed effects.Polypropylene fibers, due to their hydrophobic nature, exhibited weak initial bonding with the binder and eventually led to debonding over time, resulting in reduced flexural strength.However, it improved the energy absorption compared to the fiberless geopolymer paste.In contrast, the incorporation of micro steel fibers enhanced the energy absorption and flexural strength and reduced the shrinkage without noticeably affecting the compressive strength.The study emphasized the importance of considering both fiber-matrix interaction and binder shrinkage for the evaluation of FRGC.Whereas the latter highlighted that the inclusion of polypropylene fibers in geopolymer matrices, along with colemanite waste and silica fume substitution, yielded slight enhancements in the composite's performance.Polypropylene fibers helped increase the residual compressive and flexural strengths, especially under high-temperature conditions (from 600 • C to 900 • C).The use of air-entraining admixture, however, negatively impacted the strength results.Additionally, the freezing-thawing test showed fluctuations in the compressive strength, ultrasonic pulse velocity, and weight changes, which are attributed to the severity of the regime and affect the integrity of the composite materials over service conditions.Table 6 provides the literature survey on PPFRGC.Table 5 presents details of various types of polypropylene fibers used in experimental investigations of PPFRGC by several researchers.Comparing the studies done by Ranjbar et al. [146] and Aygörmez et al. [148], the former suggested that the addition of polypropylene fibers to fly ash-based geopolymer concrete had mixed effects.Polypropylene fibers, due to their hydrophobic nature, exhibited weak initial bonding with the binder and eventually led to debonding over time, resulting in reduced flexural strength.However, it improved the energy absorption compared to the fiberless geopolymer paste.In contrast, the incorporation of micro steel fibers enhanced the energy absorption and flexural strength and reduced the shrinkage without noticeably affecting the compressive strength.The study emphasized the importance of considering both fiber-matrix interaction and binder shrinkage for the evaluation of FRGC.Whereas the latter highlighted that the inclusion of polypropylene fibers in geopolymer matrices, along with colemanite waste and silica fume substitution, yielded slight enhancements in the composite's performance.Polypropylene fibers helped increase the residual compressive and flexural strengths, especially under high-temperature conditions (from 600 °C to 900 °C).The use of air-entraining admixture, however, negatively impacted the strength results.Additionally, the freezing-thawing test showed fluctuations in the compressive strength, ultrasonic pulse velocity, and weight changes, which are attributed to the severity of the regime and affect the integrity of the composite materials over service conditions.Table 6 provides the literature survey on PPFRGC.Thermal conductivity of geopolymer concrete was not affected by the addition of polypropylene fibers.
There was no positive effect of polypropylene fibers on the compressive strength of geopolymer concrete.

Basalt Fiber
Basalt fibers are obtained from a fine-grained solidified volcanic stone, which are commonly recognized as basalt [153,154].Volcanic magma (a very hot fluid beneath the earth's crust that solidifies when exposed to open air) is the source of origin of basalt.Basalt, due to its application as a paving and building stone in its natural form, is known from the Roman age [154].In 1923, the French Paul Dhé received a US patent for his idea to extrude fibers from basalt [155].From the 1960s, just after World War II, basalt fibers were chosen to be the best material for military research by the Soviet Union, Europe, and the United States due to their extensive use in aeronautical and defense applications [144,146].In 1995, the fiber production technology was declassified and commenced civilian research [155].The extrusion of basalt fibers is carried out generally by melting the basalt rock at a temperature of approximately 1400 • C in a fine fiber from 0.009 mm to 0.013 mm in diameter [154,156].The addition of 2% basalt fibers to fly ash and GGBS-based geopolymer composite increases its compressive strength and splitting tensile strength.There was a 10% immediate increase in the strength for the specimen with 0.5% basalt fibers cured for 7 days compared to the reference specimen.However, in the specimens cured for 28 days, the strength increased by 10%, 17%, 25%, and 34% having 0.5%, 1.0%, 1.5%, and 2.0% basalt fibers, respectively [157].Taking the length of basalt fibers into consideration, there were some variations in the strength among different basalt fiber lengths.In addition, 0.1% basalt fibers with the length of 6 mm, when incorporated in geopolymer composite, gave the best result in the compressive strength compared to basalt fibers with the lengths of 3 mm, 12 mm, and 18 mm.Furthermore, the inclusion of basalt fibers demonstrated its effect on the fracture process of fly ash geopolymer concrete.The addition of basalt fibers to fly ash geopolymer concrete leads to linear elastic deformation at the initial loading stage, whereas it results in nonlinear deformation along with microcracks with increasing the load.Also, the inclusion of basalt fibers in fly ash geopolymer concrete affects the fracture toughness and fracture energy [158].Temuujin et al. [159] explained that basalt fiber coating is a prime factor that influences the strength of basalt fiber-reinforced geopolymer concrete (BFRGC).Spooled fibers coated with a carbon layer illustrated the best bonding between the fibers and polymer matrix compared to the chopped fibers.
Details of basalt fibers such as aspect ratio, modulus of elasticity, strength, and density are listed in Table 7 from various literatures.After conducting the literature survey, it was concluded that very few experimental investigations have been done using basalt fibers in geopolymer concrete.From the available literature mentioned in this review article, it was found that the aspect ratio ranges from 600 to 2500, the modulus of elasticity ranges from 75 MPa to 110 MPa, fiber strength ranges from 1450 MPa to 4100 MPa, and the density of basalt fibers ranges from 2630 kg/m 3 to 2660 kg/m 3 .Considering Tables 1, 3, 5 and 7, the strength of polypropylene fibers compared to steel fibers, glass fibers, and basalt fibers was less and ranged between 250 MPa and 750 MPa.Ronad et al. [157] reported a notable 34.74% increase in the compressive strength with the fiber incorporation, pointing out their effectiveness as crack arrestors.Wang et al. [158] corroborates these findings, emphasizing the substantial improvements in the compressive and splitting tensile strengths.Therefore, it can be concluded that the addition of basalt fibers to geopolymer concrete greatly improves its mechanical properties.Table 8 consists of the literature survey on BFRGC.The mechanical and physical properties of the composite were found to be excellent after the incorporation of basalt sand.Also, the use of basalt fibers enhanced the properties of geopolymer concrete.The negative effect of the elevated temperatures beyond 800 • C on BFRGC was observed.

Conclusions
Geopolymers exhibit brittle failures and weak tension.To overcome these failures and weaknesses, numerous studies have concentrated on the inclusion of various fibers in geopolymer to achieve ideal mechanical and thermal qualities for each individual application.This article highlighted numerous issues, recent discoveries, and potential uses for FRGCs.Different types of fibers, including synthetic, organic, inorganic, and natural fibers, are considered for reinforcement in the geopolymer matrix.Additionally, one of the latest approaches for enhancing the mechanical strength of the fibers in geopolymer composites is hybrid fiber reinforcement.Achieving a strong bond between the fibers and geopolymer matrix in concrete involves careful consideration of material selection, mixture design, and environmental conditions.By optimizing these key factors, engineers can improve the performance of geopolymer concrete in various applications.The current review article investigated how a combination of hybrid fibers and other types of fiber reinforcements affect the composite's overall performance.Applications for geopolymer span a broad spectrum, from basic building to cutting-edge foams and thermal insulators.The conclusions for different types of the fibers are provided below:

•
Steel fibers, with their diverse types and properties, play a crucial role in enhancing the performance of concrete composites.SFRC finds wide applications in civil engineering such as improving the toughness, impact resistance, and abrasion resistance.The addition of steel fibers reduces sorptivity and enhances durability, while aluminacoated steel fibers significantly improve the interfacial bond strength and mechanical behavior in polymer composites.Also, elevated temperatures impact SFRGC, with an initial strength improvement up to 400 • C, followed by a decreasing strength.

•
Glass fibers, including CR, E-glass, and HS types, are commonly utilized to reinforce polymers.HS glass fibers exhibit superior creep and fatigue resistance compared to E-glass fibers.Glass fibers improve the density, ductility, and crack resistance of geopolymer concrete.Incorporating nano CaCO 3 enhances the compressive strength, and the higher NaOH molarity leads to improved mechanical performance.The addition of glass fibers affects the workability and density negatively, but remarkably enhances the impact strength due to the increased elasticity and stiffness.• The polypropylene fiber's application in plain concrete has gained popularity for increasing the crack resistance and reducing the shrinkage.Research suggests that an optimal addition of 0.05% polypropylene fibers by weight improves the compressive strength.Meanwhile, the polypropylene fiber's random distribution limits the crack propagation.However, high-temperature exposure, such as 600 • C and 900 • C, significantly reduces the compressive strength in PPFRGC.

•
Basalt fibers, originating from a solidified volcanic stone, have a rich history, known since the Roman times for their natural applications in construction.

Figure 1 .
Figure 1.SEM image of uncoated steel fiber matrix transition zone showing debonding process at interface.Reprinted/adapted with permission from Ref. [113], 2021, Elsevier Ltd.All rights reserved.

Figure 1 .
Figure 1.SEM image of uncoated steel fiber matrix transition zone showing debonding process at interface.Reprinted/adapted with permission from Ref. [113], 2021, Elsevier Ltd.All rights reserved.

Figure 2 .
Figure 2. SEM image of uncoated steel fiber matrix transition zone showing geopolymeric product on surface of steel fibers.Reprinted/adapted with permission from Ref. [113], 2021, Elsevier Ltd.All rights reserved.

Figure 2 . 33 Figure 3 .
Figure 2. SEM image of uncoated steel fiber matrix transition zone showing geopolymeric product on surface of steel fibers.Reprinted/adapted with permission from Ref. [113], 2021, Elsevier Ltd.All rights reserved.Buildings 2024, 14, x FOR PEER REVIEW 7 of 33

Figure 3 .
Figure 3. SEM image of alumina-coated steel fiber matrix transition zone showing steel-coated fiber matrix interface.Reprinted/adapted with permission from Ref. [113], 2021, Elsevier Ltd.All rights reserved.Buildings 2024, 14, x FOR PEER REVIEW 8 of 33

Figure 4 .
Figure 4. SEM image showing debonding process and geopolymeric products on coating surface.Reprinted/adapted with permission from Ref. [113], 2021, Elsevier Ltd.All rights reserved.

Figure 4 .
Figure 4. SEM image showing debonding process and geopolymeric products on coating surface.Reprinted/adapted with permission from Ref. [113], 2021, Elsevier Ltd.All rights reserved.

Figure 5 33 Figure 5 .
Figure 5 displays the coil diameter, nominal length, and pitch length of a spiral steel fiber.Figure 6 illustrates what a steel fiber with hooked ends looks like.Buildings 2024, 14, x FOR PEER REVIEW 9 of 33

Figure 7 .
Figure 7. Effect of straight steel fibers with different aspect ratios on compressive strength of SFRGC [116].

Figure 7 . 33 Figure 8 .
Figure 7. Effect of straight steel fibers with different aspect ratios on compressive strength of SFRGC [116].

Figure 8 .
Figure 8.Effect of straight steel fibers with different aspect ratios on flexural strength of SFRGC [116].

Figure 9 .
Figure 9. SEM image of geopolymer concrete with 0% glass fiber.Reprinted/adapted with permission from Ref. [136], 2017, Chinese Institute of Environmental Engineering, Taiwan.Production and hosting by Elsevier B.V.

Figure 9 .
Figure 9. SEM image of geopolymer concrete with 0% glass fiber.Reprinted/adapted with permission from Ref. [136], 2017, Chinese Institute of Environmental Engineering, Taiwan.Production and hosting by Elsevier B.V.

Buildings 2024 , 33 Figure 11 .
Figure 11.Glass fibers.Reprinted/adapted with permission from Ref. [140], 2019, Elsevier Ltd.All rights reserved.Selection and peer-review under responsibility of scientific committee of Innovative Advancement in Engineering & Technology.

Figure 11 .
Figure 11.Glass fibers.Reprinted/adapted with permission from Ref. [140], 2019, Elsevier Ltd.All rights reserved.Selection and peer-review under responsibility of scientific committee of Innovative Advancement in Engineering & Technology.
by weight when incorporated in concrete improved the compressive strength along with a positive change in the flexural strength.

Table 1 .
Details of different types of steel fibers.

Table 2
consists of the literature survey on SFRGC.
ReferencePrecursor Type Activator Type Molarity Fiber Content Aspect RatioCuring Type EffectEnhanced compressive, flexural, and splitting tensile strength at 28

Table 3 .
Details of different types of glass fibers.

Table 3 .
Details of different types of glass fibers.

Table 5 .
Details of different types of polypropylene fibers.

Table 7 .
Details of different types of basalt fibers.

Table 8 .
Cont.By replacing basalt fibers with RHA, the modulus of elasticity of the specimen was increased with the increase in the density of RHA-based fiber specimen.The optimum results were achieved for 10% basalt fiber replacement with RHA.Poisson's ratio decreased with an increment in the basalt fiber content.Maximum flexural strength was obtained for the specimen having 10% basalt fiber replacement with RHA.
Their industrial use began in 1923 with a US patent and gained prominence post-World War II for military and aerospace applications.By 1995, civilian research commenced, utilizing extrusion techniques at 1400 • C. Incorporating basalt fibers increases the strength of geopolymer composites, with varying effects based on the fibers' length.In addition, they influence the fracture behavior and bonding with the polymer matrix, with carbon-coated spooled fibers showing superior performance.U.S.: conceptualization, methodology, investigation, validation, writingoriginal draft.N.G.: conceptualization, methodology, investigation, validation, writing-original draft.A.B.: conceptualization, methodology, investigation, validation, formal analysis, resources, writing-original draft, writing-review and editing.Y.O.Ö.: conceptualization, methodology, investigation, validation, writing-original draft, writing-review and editing.M.V.: conceptualization, methodology.P.B.: methodology, investigation.E.A.: investigation, validation.S.I.: investigation, formal analysis.M.A.K.: validation, formal analysis.All authors have read and agreed to the published version of the manuscript.
Funding: This research received no external funding.