Mechanical properties and durability of fiber reinforced geopolymer composites: A review on recent progress

Geopolymer concrete has similar mechanical properties to ordinary Portland cement (OPC) concrete, and even better properties in high temperature, high corrosion environment. However, geopolymer still has the disadvantages of large shrinkage and high brittleness, which greatly limit its application. Fiber reinforcement is widely used in various geopolymer systems to overcome the brittleness issue, but still retains the high strength. Over the past 10 years, great progress has been made in the research of fiber reinforced geopolymers in the aspect of toughening efficiency and durability improvement. Many studies have been conducted on fiber reinforced geopolymers, but there are still few systematic summaries on the addition of inorganic fibers and organic fibers to geopolymers. What is more there are many novel studies such as the application of geopolymers to 3D printing and the new concept of fiber factor have not been reviewed. In this paper, inorganic fibers, natural fibers and synthetic fibers in geopolymers are briefly reviewed, and their specific effects on geopolymers' compressive, flexural, tensile strength, fracture toughness, shear strength and durability such as shrinkage, chemical resistance, freeze–thaw resistance are reviewed. The current understanding of bonding mechanism and fiber‐geopolymer interface are also discussed, and related knowledge gaps and future work challenges are correspondingly pointed out.


INTRODUCTION
Geopolymer is considered an alternative cementitious material that alleviates environmental problems, and has the advantages of low CO 2 emission of production and the use of large volume of industrial wastes when compared with Portland cement.The hardened geopolymers contain amorphous and quasi-crystalline three-dimensional network gels by linking of tetrahedrons, such as silicon-oxygen tetrahedron, aluminum-oxygen tetrahedron and phosphorus-oxygen tetrahedron.Considering the special chemical activation process and the resulting three-dimensional network structure, geopolymers usually present the advantages of rapid high strength, excellent corrosion resistance, and high temperature stability, and can be extensively used as suitable binders in building, heavy metal adsorption, traffic repair projects, nuclear waste treatment and other fields.
However, geopolymers with high strength are also highly brittle, and methods overcoming the brittleness of geopolymers can be broadly divided into three categories: (a) adjustment of substrates by chemical or physical means; (b) internal reinforcement; and (c) providing external constraints.Of the three solutions, the latter two are generally more efficient and less costly.One of the ways to improve brittleness is to add fibers to the geopolymer matrix to increase its density and toughness.At present, many scholars have summarized the factors affecting fiber-reinforced geopolymers, such as fiber type, fiber content, geopolymer precursor, fluidity, mechanical properties and durability.Al-Ghazali et al. 1 summarized the physical, mechanical and microstructural properties of geopolymer composites.However, the effect of fiber on the durability of geopolymers was not discussed.Khatib et al. 2 presented the mechanical and durability aspects of fiber reinforced geopolymer and found steel and polypropylene fibers were the most popular geopolymer reinforcements, followed by glass fibers and few other fiber materials.Shahari et al. 3 summarized research works regarding geopolymer which is applied on enhancing fire retardancy of epoxy-based material especially the glass fiber.Rashad et al. 4 summarized the previous studies carried out on the influence of steel fibers on the fresh and the hardened properties of the geopolymers and proved the addition of steel fibers can play a part as propagation of cracks, increase toughness, flexural strength, splitting strength, ductility load carrying capacity, for all geopolymer types.Santana et al. 5 prepared plant fiber-reinforced alkali-activated geopolymers (high and low calcium) and identified the development limitations of this composite.Silva et al. 6 presented a review of recent advances in the production of natural fiber-reinforced geopolymers produced from industrial by-products and waste as promising sustainable construction materials.Shaikh et al. 7 presented state-of-the-art mechanical and durability properties of short fiber reinforced geopolymer composites.In addition, there are some interesting and novel studies that have not been reviewed.Zhang et al. 8 introduced the concept of fiber factor that describes the comprehensive influence of fiber content and fiber length.The concept of fiber factor was used to prove that the toughening effect of polyethylene fiber on geopolymer was significantly better than that of steel fiber, polyvinyl alcohol fiber and basalt fiber on cement-based materials.Su et al. 9 used fly ash and slag as aluminate silicate raw materials to prepare geopolymer, and incorporated different fibers and hollow microspheres into the slurry to improve the performance of the composites.The results show that the improvement effect of fibers on reinforced materials is in turn of polypropylene fiber, alkali-resistant glass fiber and lignin fiber.Whether mixed fiber or single fiber, fiber can prevent the separation of hollow microspheres from geopolymer matrix, inhibit the generation and development of cracks, and finally improve the strength.Liu et al. 10 overcame the brittleness of geopolymer matrix using four kinds of steel fibers.When the fiber content and length increase, the fluidity of the mixture decreases and is not affected by the fiber shape.The increase of fiber content and the decrease of the fiber diameter contribute to the improvement of compressive strength, and the bending performance improves with the increase of fiber volume and length.
At present, many studies have been conducted on fiber reinforced geopolymers, but there are still few systematic summaries on the addition of inorganic fibers and organic fibers to geopolymers.What is more there are many novel studies such as the application of geopolymers to 3D printing and the new concept of fiber factor have not been reviewed.To this end, this review focuses on the current status of research on fiber reinforced geopolymer, and attempts to identify the limitations and advances of fiber-reinforced geopolymer.Finally, the knowledge gaps and remaining challenges for future work are discussed.

ROLE OF FIBERS IN GEOPOLYMER COMPOSITES
In the composite material, the matrix function is to support the fiber and glue the fiber into a whole, as well as maintain the direction and position of the fiber.A good base material can transfer as much external load to the fiber as possible.Steel fibers, glass fibers, basalt fibers, polypropylene fibers, and blended fibers are all fibers currently used in concrete.Given that geopolymers and cement are both brittle materials, fibers play similar roles in geopolymers, mainly the following roles in the polymer such as the skeleton, nanometer filling and crack bridging.The key factors to be considered while selecting fibers for reinforcement modification of cement or geopolymer composites include: (1) compatibility of material properties with application; (2) adequate fiber-matrix interaction to transfer stress; and (3) optimal aspect ratio to ensure effective post-cracking behavior.The addition of fibers affects the fluidity and mechanical properties of viscous carbides, which depends on the surface properties, shape, types and flexibility of the fibers. 11t present, the fibers used in geopolymers can be roughly divided into the following three categories, i.e., inorganic fiber, natural fiber and synthetic fiber.To be specific, inorganic fiber mainly includes steel fiber, glass fiber and basalt fiber; natural fiber mainly consists of plant fiber, animal fiber and mineral fiber; and synthetic fiber mainly refers to polyethylene, polypropylene, polyvinyl alcohol fiber, etc. Table 1 lists the main characteristics of common fibers added to geopolymer.
• Inorganic fibers.Inorganic fiber can be traced back to the application of ancient asbestos. 12These fibers consist of a mixture of alumina and silicon dioxide and are often used in high thermal applications such as refractories due to their high melting point.In addition, these fibers also possess the advantages of low cost, high tensile strength, chemical stability and excellent insulation properties.• Natural fibers.Nature fibers are mainly divided into plant fiber, animal fiber and mineral fiber.Plant fibers mainly include fruit fiber, leaf fiber, stem fiber, seed fiber, and bast fiber; animal fiber mainly refers to the silk fiber in animal hair; And iron asbestos, crocidolite fibers are the main mineral fibers. 14 • Synthetic fibers.Synthetic fibers refer to the fibers made of synthetic polymer materials, including polyester fiber, polyamide fiber, polypropylene fiber, polyvinyl alcohol fiber, polyvinyl chloride fiber, etc.
Considering the different properties of fiber itself, its microstructure is also rather different.The macroscopic and microscopic morphology of some different fibers are listed as Figure 1.
The addition of polypropylene fiber reduces the specific density and water absorption of geopolymer composites, which is mainly due to the increase of microstructure and density of matrix.These effects are directly related to the fiber content instead of the fiber type. 27Although the natural fiber improves the brittleness of concrete, it reduces the pumpability of shotcrete.The workability is the key factor used in the construction of shotcrete, so the content of natural fiber in the concrete matrix needs to grasp properly. 28

BONDING BETWEEN FIBERS AND MATRIX
The properties in fiber reinforced geopolymer depend on the performance of fiber, fiber content, precursor of geopolymer and curing environment.However, the interface between the matrix and the fiber plays a key role in the overall mechanical properties of the composites.If the interface is well bonded and the load can be transferred from the matrix to the fiber with a high carrying capacity, fibers with inert surfaces will also result in weak interface contact. 34Regarding the interface between fiber and matrix, many scholars have studied the interface bonding zone between fiber and geopolymer by establishing mathematical or physical models. 35iber fracture and pull-out from the matrix are the two main failure mechanisms of geopolymer.At the same time, the fiber will agglomerate if the fiber content in the matrix is too high, thereby resulting in the decrease of the matrix strength.Figure 2 depicts the interface bonding between different fibers and geopolymers.
Figure 2A shows that the steel fiber is in good contact with the geopolymer matrix, and there is no interface peeling phenomenon.Considering that steel fibers tend to be hydrophilic, they can significantly improve the energy absorption and bending strength of composites. 34The glass fiber and basalt fiber in Figure 2B,C present obvious interfacial debonding with geopolymer.The addition of glass fiber increases the compressive strength and flexural strength of the matrix by 32.6% and 30.35%, respectively, while the addition of basalt fiber increases the compressive strength and flexural strength of the matrix by 27.3% and 35.36%, respectively. 17Figure 2D describes the interface bonding between polypropylene fiber and geopolymer matrix.Organic fiber also shows the interface peeling phenomenon, and the addition of adding polypropylene fiber increases the compressive strength and flexural strength of the matrix by 27.5% and 16.07%, respectively.Figure 2E,F depicts the interfacial bonding between jute, sisal fiber and geopolymer in natural fibers.The jute with a mass fraction of 1.5% increases the compressive strength and tensile strength of the matrix by 64% and 45%, respectively,  polypropylene fiber 34 ; (E) jute fiber 26 ; (F) sisal fiber. 2617,26,34Reproduced with permission. 17,26,34Elsevier 2023.
while the sisal fiber with a mass fraction of 2.5% increases the compressive strength and tensile strength of the matrix by 76% and 112%, respectively.The high water-absorbing ability of plant fibers can prompt their swelling resulting in size instability.That is the reason of weakening of the adhesive contact at the fiber-matrix interface and formation of cracks upon drying of the composite.Therefore, plant fibers need pretreatment before adding geopolymers.Lazorenko et al. 36 studied the effects of mechanical cleaning of low-grade flax bundles, mermerization of 5% NaOH aqueous solution and ultrasonic (US) treatment in alkaline medium (22 kHz, 500 W) on the mechanical properties and microstructure of polymer composites based on fly ash.According to the results of three-point bending flexural tests of geopolymer composites, the combined treatment by 5 wt% aqueous NaOH solution and high-intensity ultrasound is the most effective way to treat flax tow fibers.
The bonding between steel fiber and geopolymer is found the best, and no interface peeling phenomenon is observed.Considering the high strength and rough surface of steel fiber, the mechanical bonding force with geopolymer is formed, and leads to the optimal mechanical properties of geopolymer.

Compressive strength
Geopolymers are provided with excellent mechanical properties, among which, compressive strength is the most important index.Compressive strength refers to the limit of strength when external forces apply pressure.The compressive strength of the specimen depends on the flow resistance of the mixture, while the flow resistance and slump depend on the type, size and weight of the fiber. 37The proper uniformity and high pressure compaction between the fiber and the cement matrix can improve the compressive strength by the application of fibers.However, for geopolymer, different fibers exercise different effects on the compressive strength. 38he effect of different fiber contents on 28 d compressive strength of geopolymer is listed in Figure 3. Due to the high rigidity of steel fibers, its addition improves the compressive strength of geopolymer.The experimental results of Zada Farhan show that 6 mm steel fibers improve the compressive strength more efficiently than 12 mm steel fibers, 40 and that the alumina coating can improve the interfacial bonding strength between steel fiber and geopolymer, which is 151% higher than that of uncoated steel fiber. 48With the increase of polypropylene fiber content, the decrease in strength is attributed to the formation of interpolymer voids and weak interfacial bonding zone.Murthy's study shows that 1% is the best content for polypropylene fiber to improve the compressive strength of geopolymer, 41 while Bellum's research indicates that 2% is the best content for polypropylene fiber to improve the compressive strength of geopolymer, 16 and Wang's study considers 0.8% polyvinyl alcohol fiber the best dosage to improve the compressive strength of geopolymer.However, Wang and Li's study indicates that a higher polypropylene fiber content represents a higher compressive strength. 2143Xu has found that the optimal dosage of basalt fiber to improve the compressive strength of geopolymer is 0.6%, and that the comprehensive performance of 6 mm basalt fiber is better than that of 3 mm basalt fiber. 44Wang et al. 17 found that the higher the basalt fiber content, the better the compressive strength of geopolymers.Bai et al. 31 believed that 0.4% glass fiber was the best dosage to improve the compressive strength of geopolymer.Glass fiber can eliminate the microcrack of polymer and improve the mechanical properties of geopolymer.Silva et al. 26 found that the higher the content of sisal fiber, the better the compressive strength.Wongsa et al. 24 found that sisal fibers greater than 0.5% reduces the compressive strength of geopolymers, that the jute fiber optimal dosage is 0.5%, and that the cotton fiber optimal dosage is 0.5%. 47More than 0.5% of coconut fiber decreases the compressive strength of the geopolymer. 24s shown in Figure 4, internal tensile stress is generated when the fiber reinforced geopolymer is squeezed by external force.The appropriate amount of fiber can play a bridging role and effectively inhibit the crack propagation.The strength growth percentage of cement-based materials caused by fiber inclusions is usually lower than that of geopolymer, which can be explained by the ceramic-like and polymer-like properties of geopolymers and better fiber matrix interface.Geopolymer has finer pore distribution than cement-based materials.
In short, the strong bond between geopolymer and fiber may be related to the following aspects: (a) different reaction mechanism and reaction products of geopolymer and cement-based materials; (b) strong adhesion of fiber to matrix (N-A-S-H); (c) the formation of an improved interface transition zone (ITZ) in the geopolymer, which is partially attributed to the fact that cement-based materials tend to form low strength silicates at the ITZ, and partially that the fiber matrix interface is affected by higher pH values.

(A) (B)
F I G U R E 4 Effect of fiber content on the compressive strength of geopolymer (A) little; and (B) much. 49Reproduced with permission. 49Elsevier 2023.

Flexural strength
Because geopolymer is a kind of ceramic brittle material, the bending strength is weak.Bending strength refers to the ability of materials to resist bending without breaking, generally measured by three-point bending test.Considering the brittleness caused by the internal structure of geopolymer cross-linked each other, adding fiber can significantly enhance the flexural properties of geopolymer, while considering the crack bridging mechanism of fibers, the bending strength of fiber reinforced geopolymer is significantly higher than that of unreinforced geopolymer.
Besides, it has also been found that flax fibers has advantages in improving the flexural strength of geopolymer.As Figure 5 showed, Alzeer et al. 52 reported new ecologically-friendly composites of inorganic polymers reinforced with 4-10 vol.% unidirectional natural cellulose-based flax fibers to produce materials with excellent mechanical and thermal properties.The greatest improvement in flexural strength (from about 6 MPa to about 70 MPa) was obtained with 10 vol% fiber reinforcement.Lazorenko et al. 53 reported novel fly ash-based geopolymer reinforced with 0.25-1.0wt% short-cut flax tows involving preparation, mechanical properties, and a microstructure.The results show that the bending strength of geopolymers increases by 22% from 4.0 MPa to 4.9 MPa when 1.0 wt% flax tows is added.Lazorenko et al. 36 studied the effects of mechanical cleaning of low-grade flax bundles, mermerization of 5% NaOH aqueous solution and ultrasonic (US) treatment in alkaline medium (22 kHz, 500 W) on the mechanical properties and microstructure of polymer composites based on fly ash.According to the results of three-point bending flexural tests of geopolymer composites, the combined treatment by 5 wt% aqueous NaOH solution and high-intensity ultrasound is the most effective way to treat flax tow fibers.
Adding fiber in geopolymer can improve the flexural strength of the matrix and greatly improve the bending toughness.The reason why the fiber can enhance the flexural strength of the matrix is as follows: (1) the geopolymer has multiple cracks in the process of bending deformation; (2) fiber bridging plays an important role in crack stability and multi-crack induction; and (3) the existence of fiber changes the stress distribution during the specimen deformation process, and distributes the stress evenly in the specimen, thereby improving the flexural strength.

Tensile strength
Tensile strength represents the resistance to maximum uniform plastic deformation of a material.For brittle materials without uniform plastic deformation, it reflects the fracture resistance of materials.The tensile properties of fiber reinforced composites are greatly affected by the matrix type, fiber type, fiber volume, loading rate, interface bonding strength and even the type of the used impact machine.
The tensile failure process of composites is a progressive damage process: considering the existence of defects, some fibers will fracture first in the initial loading process, and local thermoplastic deformation will occur in the matrix and interface near the fiber fracture.Besides, the redistribution of microscopic stress deformation will occur, accompanied by more fiber failure and local plasticity, considerable fiber instability failure and the final failure of composite materials.It can be found that the tensile strength failure of composites depends on various loss evolutions including fiber fracture and inelastic deformation of the matrix and the interface.
The effects of different fiber dosage or length on the 28-day tensile strength of geopolymers are listed in Figure 6.Given the brittleness of the geopolymer, the addition of fiber can greatly improve the tensile properties of the matrix, which is not only attributed to the high tensile strength and elastic modulus of the fibers, but also the fact that stress in the sample can be transferred to the fibers through the interface with the geopolymer matrix.
The tensile strength of the geopolymer increases with the addition of steel fiber because of its greater rigidity.Bellum et al. 16 shows that the best content of polypropylene fiber reinforced geopolymer is 2%, and that the best tensile strength is 7.23 MPa.The research conducted by Farooq indicates that the tensile strength of geopolymer increases with the increase of polyvinyl alcohol fiber content. 55Wang's study presents the maximum splitting tensile strength of 4.84 MPa of the 6 mm basalt fiber reinforced geopolymer. 25The tensile strength of the geopolymer increases with the increase of the glass fiber content.The study of Wongsa shows that sisal fiber reinforced geopolymer possesses the greatest tensile strength, while the tensile strength of coconut fiber reinforced geopolymer increases with the increase of coconut fiber. 24However, Zhang's research shows that the tensile strength increases by 4.6%, 21.3%, 4.3% and 74.3%, respectively by adding 1% single fiber PP, PVA, RPP and steel fiber into the geopolymer. 38Steel fiber is most effective in improving tensile strength because of its high tensile strength and stiffness, but the tensile strength of geopolymer with PVA, PP and RPP increases by 38.2%, 36% and 48.9% by the incorporation of mixed fibers.In this case, it is more effective to mix PVA, PP, RPP and steel fiber to improve the tensile strength of geopolymer than to use single fiber merely. 56onsidering its own characteristics such as high rigidity and high tensile strength, fiber can obviously improve the splitting tensile strength when added to geopolymer.However, the mixed fiber has a better improvement effect, and the mixture of PP and steel fiber presents the best effect in the case of improving the splitting tensile strength.

Fracture toughness
Fracture toughness represents the ability of material to prevent against crack propagation and is a quantitative index to measure the toughness of material.The test methods of fracture toughness include direct tensile method, compact tensile method, wedge splitting method and three-point bending beam method.The fracture toughness of the material is larger in the case of a constant crack size, and fracture energy is defined as the energy required for the unit area of fracture propagation in brittle materials, which reflects the energy change during the crack development. 57iber factors can describe the combined effect of fiber content and length: the fracture toughness increases with the increase of the fiber factor in the case of a fiber factor less than 600, indicating that the fiber matters considerably in restricting the cracks at the initial stage of geopolymer cracking.The fracture toughness of geopolymer decreases when the fiber factor exceeds 600.

RI = V × L∕D
where RI denotes the fiber factor; V, the fiber volume content; and L/D, the fiber length to diameter ratio.
Zhang et al. 8 studied the influence of the ratio of polyethylene fiber to water-binder ratio on the fracture toughness of geopolymers, and found that polyethylene fiber has a good toughening effect on geopolymers.The fracture toughness of the matrix decreases when the fiber content exceeds 1.05%.Due to the high fiber content, the matrix has more pores and interfaces, resulting in more defects.Behzad et al. 58 found that when 2% polyvinyl alcohol fiber is added to fly ash base geopolymer, the fracture toughness can reach 0.436Km.It has also been found that adding 0.5 wt% cotton fiber can increase the fracture toughness of geopolymer by 1.12 MPa⋅m 1/2 .Considering the poor dispersion of cotton fiber in slurry, the fracture toughness decreases with the increase of fiber content.At the same time, the dispersion of cotton fiber in geopolymer has a great influence on the fluidity, adding 0.7 and 1.0 wt% cotton fiber will greatly affect the fluidity of the matrix, which must be compensated by increasing the water content of the mixture, and increasing the porosity and microcracks of the matrix.Alomayri et al. 59 investigated the mechanical and fracture properties of 0.5 wt% cotton reinforced geopolymers, which can be used for siding, roofing, piping and cooling tower panels or shingles.Ghasemzadeh et al. 60 studied the effect of steel fiber and polypropylene fiber in a certain volume on fracture characteristics of abrasive slag and wollast-based ultra-high performance geopolymer concrete.With the increase of fiber content, the fracture energy increases compared with the control group, and the substitution of polypropylene fibers for steel fibers slightly reduces both types of fracture energy.
When the properties of fibers such as the hardness cannot be changed, many scholars improve the fracture toughness of composites by filling admixtures to improve the density of the matrix. 61The fiber plays a bridging role in the matrix and transmits internal stress in the matrix, thereby improving the ability of resisting crack generation and propagation.

Shear strength
Shear strength refers to the ultimate strength of the material when it is cut, and reflects the ability of the material to resist against shear sliding.The influence of different types and volume fractions of fibers on the shear strength of geopolymers is shown in Figure 7.The optimal dosage of steel fiber (SF) to increase the shear strength of geopolymer is 0.5% and the increment is 56%.However, the shear strength of geopolymer decreases when the dosage is increased to 1%.Given that the geopolymer concrete with high blast furnace slag content has poor workability and fiber dispersion may be affected by the high viscosity of sodium silicate solution, the optimal content of steel fiber in geopolymer decreases compared with that of steel fiber in cement of 2.5%.Besides, the mixed fiber is better to improve the shear performance of polymer.In order to obtain good shear strength with low fiber content, it is recommended to add steel fiber and polypropylene fiber as well as steel fiber and polyvinyl alcohol fiber to the geopolymer.

F I G U R E 7
Effects of different fiber types and dosages on shear strength. 62CF, carbon fiber; PF, polypropylene fiber; PVF, polyvinyl alcohol fiber; SF, steel fiber.Reproduced with permission. 62Elsevier 2023.
The high shear resistance of fiber-reinforced polymer is more widely used on walls.According to the study of Harry et al., 13 the tested composite polyvinyl alcohol fiber wrapped macroscopic synthetic fiber reinforced concrete wall samples present significantly higher values of interfacial shear strength compared with the test wall samples filled with ordinary concrete.The maximum shear load and interfacial shear strength increases by 93.5%.Tran et al. 63 studied the shear capacity of fiber reinforced concrete beams under impact load, and found that the addition of fibers significantly improves the impact response of geopolymer beams in terms of maximum and residual displacement and force, and reduces negative cracks and spalling cracks.Lakavath et al. 4 conducted experimental and numerical studies on the shear behavior of large synthetic fiber reinforced concrete beams, and found that a low fiber volume increase of 0.5% does not significantly improve the shear behavior.However, the test and finite element results of the beams with higher volume fractions (1.0% and 1.5%) significantly improves the post-cracking behavior, ductility and ultimate shear resistance of the beams. 64

Drying shrinkage
Composites lose their moisture during air drying, resulting in shrinkage and crack formation.Drying shrinkage is an important durability parameter of concrete specimens, indicating the potential cracks formed in hardened cementitious materials. 65The addition of fiber can offset the stress in the matrix and effectively restrain its drying shrinkage.
The drying shrinkage value of fly ash base geopolymer added with steel fiber is in the range of 264-297 micro-strain, that of fly ash base geopolymer added with polypropylene fiber ranges 394-424 micro-strain, 15 and that of the matrix can be minimized by adding 0.6% polypropylene fiber to the geopolymer. 66Given that basalt fibers promote the development of C-S-H, the addition of this fiber to the geopolymers can reduce the drying shrinkage of the matrix.The combination of CASH and NASH leads to the refinement of the pores, and the fiber acts as aggregation in the matrix, which can well disperse the sample stress. 67As is shown in Figure 8, a longer basalt fiber indicates a better shrinkage rate in the case of constant basalt fiber content, and the shrinkage rate of the sample with 6 mm basalt fiber is lower than that with 3 mm basalt fiber.As is shown in Figure 9, when the fiber content is constant, a longer fiber indicates fewer roots per unit volume.In this case, the binding energy between fiber and slurry increases with the increase of the fiber length. 44Adding 0.4% polypropylene fiber and 0.5% expansion agent MgO to the geopolymer can effectively reduce the shrinkage of the matrix. 68The dry shrinkage rate of fly ash and slag base polymer with 0.3 and 0.6 volume fraction of PVA fiber is 20.53% and 45.69% at Day 7, and 26.92% and 41.18% at Day 28, respectively. 69he drying shrinkage of the matrix can be reduced when fiber is added to the geopolymer.Since the fiber restrains the extension of micro-cracks in the specimen, it bears part of the stress caused by the shrinkage of the matrix, thus reducing the shrinkage strain of the material. 1F I G U R E 8 Effects of basalt fiber content and length on the dry-shrinkage properties of geopolymers. 44Reproduced with permission. 44sevier 2023.

Chemical resistance
There are many types of chemical erosion, mainly including sulfate erosion and chloride ion erosion.Corrosion is caused by the reaction of sulfate ions and chloride ions from the external environment with hydration products, which results in expansion and cracking, and thereby reduces the matrix properties. 70It is of great significance to study the effect of fiber-reinforced geopolymers on chemical erosion, since chloride ion and sulfate erosion and freeze-thaw cycle are easy to occur in marine environment.Figure 10A describes the breakdown mechanism of chemical erosion (SO 4 2− and Cl − attack) and physical processes (freeze-thaw cycles), and Figure 10B,C shows the SEM before and after Cl − and SO 4 2− attack.The matrix eroded by Cl − presents cubic crystals (NaCl after drying), indicating that Cl − invades the internal voids of the geopolymer.However, the geopolymer matrix changes from a uniform and dense one to a porous one after sulfate erosion.Zhu et al. 72 studied the influence of liquid-solid ratio and slag substitution rate on the chloride ion resistance of geopolymer, and found that chloride ion permeability largely depends on the porosity and tortuosity of composites.The porosity of composites decreases with the decrease of the liquid-solid ratio, and the decrease of porosity and increase of tortuosity are beneficial to the decrease of chloride ion permeability.Banana fiber and coconut shell fiber improve the toughness of the matrix, enhance the durability of the composite against acid erosion, and coconut shell fiber presents a better effect. 73The mass loss of bamboo fiber composite is about 18% when exposed to an acidic environment, and it possesses good durability. 74In 10% and 32% hydrochloric acid solutions, the compressive strength of carbon fiber reinforced geopolymer decreases by 66% and 61.3%, and the mass loss decreases by 5.3% and 3.7%, respectively, which can be used for improving the durability of reinforced concrete bridges. 75he degradation of geopolymers by sulfate erosion mainly focuses on the effect of pores and cracks on the matrix.The porosity and pore size of geopolymers increase with the increase of the sulfate concentration, which will thus reduce the compressive strength.However, a mixture of organic fibers (polypropylene and polyethylene) and inorganic minerals (wollastonite) can improve the sulfate resistance of the composite. 29After 15 sulfate cycles, the compressive strength of the polymer with 0.2% polypropylene fiber, 0.3% basalt fiber and 0.4% steel fiber reaches 67.9 MPa. 75Besides, it is also found that steel fiber and propylene fiber are acid resistant, while synthetic fiber performs weakly in this aspect. 76The durability of the geopolymer with a fiber content of 1.2 kg/m 3 remains good after soaking 60 times in 5% sodium sulfate solution. 77he mixture of mineral particles and fibers can effectively improve the resistance of geopolymers to chemical erosion. 78Wang et al. 21proved that geopolymer mortars with a polyvinyl alcohol fiber content of 0.6-0.8vol% and a nano-SiO 2 content of 1.0-2.0wt% present better chemical resistance, as shown in Figure 11.The incorporation of nano-SiO 2 and polyvinyl alcohol fiber has a synergistic effect on the durability of geopolymer mortar.The bridging effect of polyvinyl 29 Reproduced with permission. 21Elsevier 2023.
alcohol fibers limits the crack propagation, and nano-SiO 2 is found conducive to filling the micropores and improving the microstructure. 79The addition of wollastonite, tremolite and basalt fibers to metakaolin base polymers can also improve sulfate and chloride ion erosion.When the proportion of wollastonite, tremolite and basalt fiber samples is 5%, the geopolymers have the highest compressive strength and the strongest erosion resistance. 80If 0.2% polypropylene fiber and 3% nanoparticles are used to prepare geopolymers, the durability can be improved by 67%. 81The addition of basalt fiber to geopolymer can reduce the weight loss after acid erosion and maintain the compressive strength as much as possible, indicating that basalt fiber reinforced geopolymer has good durability. 82The combination of cellulose fiber and fly ash can improve the durability of composite materials under the sulfate dry-wet cycle.In the late cycle, the pore volume increases obviously, the proportion of small hole decreases, and the mesopores increase.In this case, the durability of concrete can be consequently improved by optimizing the ratio to increase the proportion of harmless holes such as micro holes and small holes. 83 I G U R E 12 Mechanism of fiber resistance to SO 4 2− and Cl − . 29Reproduced with permission. 29Elsevier 2023.
In all, fiber-reinforced geopolymer matter considerably in combating against the harsh conditions of the ocean.As Figure 12 showed, The bridging effect of randomly distributed fibers inhibits the initiation, development and penetration of cracks in the matrix, delays the diffusion of SO 4 2− and Cl − into the base, and prevents against physical (freeze-thaw process) and chemical (SO 4 2− and Cl − erosion) attacks. 71,81,84,85When the local geopolymer is subjected to the loading pressure, the fibers with weak bond to the matrix fracture first, forming microcracks, and then producing elastic deformation.The mass loss of geopolymer sulfate erosion leads to the formation of porous structure and sulfate crystal phase, thereby resulting in strength loss. 29

Freezing-thaw resistance
The freeze-thaw cycle is one of the important indexes to evaluate the durability of the geopolymer. 86In geopolymer structures, water mainly exists in three forms, i.e., crystal water, gel water and free water.Crystal water and gel water will not be frozen because of its small pore, while only the free water with capillary pores will freeze at a negative temperature.The resistance against geopolymer can be effectively improved by reducing the porosity and improving the compactness of the structure. 15he pure fly ash base polymer is destroyed after 50 freezing-thawing cycles, but the addition of 0.1% carbon nanotubes and 2% polyvinyl alcohol fiber withstands 175 freezing-thawing cycles. 86Polyamide fiber can also improve the freeze-thaw cycle durability of fly ash base geopolymer. 87Besides, the compressive strength of polypropylene fiber reinforced geopolymer increases during 56 freeze-thaw cycles, but decreases during 300 freeze-thaw cycles. 88With the increase of fly ash and bentonite, the durability of PVA fiber reinforced geopolymer composite decreases.When the ratio of fly ash to cement is 1.8, the water resistence of polyvinyl alcohol fiber reinforced geopolymer composite is the highest. 89fter 150 cycles of wet and dry cycles, the durability behavior of fiber reinforced cement matrix composites in different solutions are as follows: water>MgSO 4 >Na 2 SO 4 (aq). 90As shown in Figure 13, the 0.6%-0.8%polyvinyl alcohol fiber and 1.0%-2.0%nano-SiO 2 can enhance the geopolymer durability to the greatest extent, and the residual compressive strength can reach 57.3-58.8MPa after 25 freeze-thaw cycles. 21The strength of slag fly ash base geopolymer prepared with polypropylene, steel fiber and polyamide fiber does not change significantly after 250 cycles, indicating its excellent durability. 15The strength loss of basalt fiber reinforced metakaolin base geopolymer is small after 90 freeze-thaw cycles, 91 and after 180 freeze-thaw cycles, the compressive strength of the geopolymer with 0.8-1.2%basalt fiber increases while the flexural strength decreases. 45he addition of fibers improves the internal structure and macroscopic mechanical properties of geopolymer. 89First, the fibers are evenly distributed in the matrix to balance the internal stress caused by water freezing.Second, fiber fills the internal pores, and increases the density of the matrix.Therefore, the incorporation of fibers can significantly improve the freeze-thaw cycle resistance of geopolymer. 86,92

Other performances
Additionally, the fiber performs excellently in the electric conductivity and high temperature resistance.Carbon fiber and ceramsite can also be used as conductive admixtures in mortar to effectively reduce the resistivity of mortar, but exercise a freezing-thawing cycles. 21Reproduced with permission. 21Elsevier 2023.
negative impact on its workability and strength development. 93Fiber and hollow glass beads were added into geopolymer to prepare thermal insulation composite.The improvement effects of fiber on the shrinkage resistance of materials are successively as follows: polyvinyl alcohol fiber, glass fiber, lignin fiber. 9The flexural strength and compressive strength of ultrahigh performance concrete (UHPC) are improved by the addition of steel fiber and polyvinyl alcohol fiber, and the flexural strength of UHPC is increased by 70.06% with the increase of length-diameter ratio of steel fiber.Polyvinyl alcohol fiber and Polypropylene fiber absorb more water, which affects the hydration of cement in UHPC, thereby reducing the strength of UHPC. 94In order to enhance the adhesion between fiber and matrix, the surface of fiber is modified with chemical reagents such as graphene and silane to enhance the hydrophilicity and roughness of the fiber surface. 95

CONCLUSIONS AND RECOMMENDATIONS
The appropriate amount of fiber can play a bridging role and effectively inhibit the crack propagation.The existence of fiber changes the stress distribution during the specimen deformation process, and distributes the stress evenly in the specimen, thereby improving the mechanical properties such as compressive strength, flexural strength etc.Since the fiber restrains the extension of micro-cracks in the specimen, it bears part of the stress caused by the shrinkage of the matrix, thus reducing the shrinkage strain of the material.The addition of fibers can improve the internal structure and macroscopic mechanical properties of geopolymer.The mixture of mineral particles and fibers can effectively improve the resistance of geopolymers to chemical erosion.The bridging effect of randomly distributed fibers inhibits the initiation, development and penetration of cracks in the matrix, delays the diffusion of SO 4 2− and Cl − into the base, and prevents against physical (freeze-thaw process) and chemical (SO 4 2− and Cl − erosion) attacks.At present, the research mainly focuses on the influence of fiber type and content on the performance of geopolymer, but the following factors may also affect the geopolymer performance: the selection of geopolymer matrix, interface strength, fiber extraction method, and fiber treatment.Therefore, the following aspects remain to be further explored: 1. Most geopolymers are prepared by alkali activation, but it is actually feasible to carry out acid activation as well.Studies on fiber reinforced acid activated geopolymers should be taken into consideration.2. The present research focuses on the influence of fiber type and content on the geopolymer performance.Indeed, more emphasize can be placed on the fiber treatment, extraction method, matrix-fiber interface characteristics and pore structure of composite materials.3.There are few studies and applications on the influence of unique properties of fibers such as light weight, heat preservation and heat insulation on geopolymer, which requires further exploration.4.More academic attention should be paid to the special application of geopolymer, such as 3D printing, insulation wall materials, etc.
U R E 13 (A) Effect of polyvinyl alcohol fiber content on sulfate resistance of geopolymer (B) Appearance of geopolymer after 13