Study on the Reinforced Properties of Geopolymer Fibers with a Sustainable Development Role

Abstract

Geopolymers are of great significance in reducing the consumption of mineral resources, saving energy, protecting the environment, and realizing sustainable economic and social development. This experiment investigated geopolymer mortar with fly ash and metakaolin as the primary binders, assessing the impact of different fiber types and volume fractions on the mortar’s flexural and compressive strength. The results indicated that optimal mechanical properties could be achieved with a fly ash-to-metakaolin ratio of 35:65. The mechanical performance is the best, with a compressive strength of 54 MPa, a flexural strength of 3.4 MPa, and a split tensile strength of 1.9 MPa at 28 days. Different fibers influenced the splitting tensile strength to varying degrees; with a 1.5% volume fraction of steel fibers, geopolymer mortar exhibited the best reinforcement effect, showing a 70% increase in flexural strength and a 142% increase in tensile strength. Mechanistic analysis revealed that the reinforcement from refined various fibers could refine the structure and further enhance the strength. Of steel geopolymer fibers’ The reinforcing effect of steel fibers is the best among them, and the internal structure is the most compact. The geopolymer mortar hydration products of geopolymer mortar reinforced with PP fibers, PVA fibers, steel fibers, and carbon fibers were amorphous network-structured zeolites (Na₂[Al₂Si₃O₁₀]·2H₂O). The limitations of geopolymers can be effectively addressed through the aforementioned research, which can effectively reduce the use of cement and achieve the goal of sustainable development.

1. Introduction

It is well established that the cement industry accounts for approximately 5–8% of global carbon dioxide emissions and roughly 5–8% of total industry energy consumption worldwide [1]. According to statistics, national cement production in 2022 was approximately 2.46 billion tons. The cement production process consumes significant energy, driving the development of alternative binding materials. One promising substitute is geopolymer, typically produced from silicon- and aluminum-rich inorganic materials like fly ash, metakaolin, slag, and furnace slag via mineral polymerization. In addition to low energy consumption and carbon footprint, geopolymer offers good compressive strength, fire resistance, thermal resistance, corrosion resistance, and durability [2,3,4].

Fly ash and metakaolin are two important solid wastes that play a significant role in environmental protection and nature conservation. Fly ash, an industrial by-product, contains active components that can improve the durability and strength of mortar [5]. By using these materials, the amount of waste can be reduced, thereby reducing pollution and protecting the ecosystem. Metakaolin is a type of clay mineral known for its excellent thickening and binding properties, which can improve the cohesion and strength of mortar. With the rapid development of the coal-based kaolin industry in the province, the accumulation of metakaolin is becoming increasingly important [6].

Although geopolymer materials are excellent in many respects, their low flexural and tensile strength and high sensitivity to micro-cracking pose significant limitations in some applications. To address these issues, fiber materials can be incorporated into geopolymers to produce composites with improved ductility, toughness, and tensile strength. The introduction of fibers helps to control and mitigate the growth of cracks and brittleness, thus improving ductility, toughness, and tensile strength. These improvements can be achieved by controlling or preventing the expansion, initiation, or coalescence of cracks [7]. The use of fibers in mortar began in the early 1960s, and in recent years, the types and forms of fibers used in the construction industry have evolved significantly. Today, a variety of fibers are available for use in cementitious materials, including asbestos, steel, basalt, glass, carbon, polyvinyl alcohol (PVA), polypropylene (PP), polyethylene (PE) and natural fibers [8,9,10]. Fibers can be classified according to their stiffness/strength, i.e., low modulus (such as PP fibers) or high modulus (such as carbon and steel fibers). Due to safety and health considerations, the aim was to replace asbestos fibers, which led to the introduction of PP fibers, a thermoplastic polymer commonly used in textiles, stationery, laboratory equipment, and automotive parts. PVA fibers, also considered a replacement for asbestos, have a modulus of elasticity and tensile strength of around 20–40 GPa and 900–1600 MPa, respectively [11]. PVA fibers have a low specific gravity of about 1.3 and a typical elongation at break of about 7–8% [12]. These fibers have excellent resistance to alkalis and acids. Carbon fibers are unique materials with a wide range of thermophysical properties that are suitable for various applications and enable a wide range of material properties [13]. Currently, carbon fibers are used in the automotive, aerospace, defense, and construction industries due to their high tensile strength and high modulus of elasticity. Steel fibers, which are commonly used to improve the strength and durability of mortar, are made from high-tensile steel wires or chopped wires and typically have a spiral or straight shape [14]. Steel fibers act similarly to a skeleton in mortar by effectively containing the propagation of cracks and increasing tensile strength and impact resistance [15].

The mechanical and organizational properties of geopolymer mortars reinforced with polypropylene (PP) and steel fibers were experimentally investigated by Bellum et al. [16] It was found that the addition of up to 2.0% (v/v) PP fibers improved the flexural properties of geopolymer mortar samples. The compressive strength of the steel fiber-reinforced geopolymer composites reached a maximum of 2.5% by volume, which was 13.26% higher than the control mix. The flexural toughness indices of PP and steel fiber-reinforced composites increased with increasing percentage. However, the steel fiber-reinforced geopolymer specimens showed better flexural strength than the PP fibers. Vatin et al. [17] investigated how different types and amounts of different fiber reinforcements affect the creep strain of fly ash-based polymer composites during compression. The results showed that the highest creep strength was observed in samples with 5% polypropylene fibers, followed by samples with 1% polypropylene fibers, plain fibers, and 1% steel fibers. Ranjbar et al. [18] summarize the development, characterization, and application of fiber-reinforced geopolymers for a wide range of applications. Guo, GL et al. [19] summarize the current research on fiber-reinforced one-component base polymers (FOPGs) in terms of raw material precursors, activators, fibers, physical properties, and curing mechanisms. Ahmed, HQ et al. [20] investigated the effect of carbon fibers on address polymers, and the results showed that the deflection decreased with increasing compressive strength, the first crack load increased, the deflection of geopolymer mortar beams increased slightly, and the bearing capacity values were almost the same; the crack width values of geopolymer mortar beams were also lower than those of ordinary Portland mortar beams.

In this study, geopolymers were prepared using metakaolin and fly ash as starting materials. The flexural and tensile properties of the geopolymer mortar were improved by adding different types and proportions of fibers (PP fibers, PVA fibers, carbon fibers, and steel fibers). In addition, the microstructure and transformation patterns of the cementitious materials were investigated using scanning electron microscopy (SEM) and X-ray diffraction (XRD).

2. Experiments

2.1. Raw Materials

The main raw materials for geological polymer mortar include kaolinite, fly ash, alkaline solution, and aggregate. Their performance parameters are as follows:

2.1.1. Fly Ash

The experiment uses fly ash to prepare geopolymer mortar, which is purchased from Taiyuan Iron and Steel Thermal Power Plant with a classification of level two fly ash. It has a residual content of 21.4% on a 45 μm sieve and an activity index of 83%. The chemical composition is as

Table 1. Percentage composition of fly ash chemical components.

Composition	Al2O3	SiO2	CaO	MgO	TFe	LOI
Percentage/%	31.70	50.77	1.66	0.77	5.06	5.31

:

2.1.2. Metakaolin

The metakaolin with Al₂O₃·2SiO₂ was purchased from Jin Yu Ke Lin Technology Co., Ltd. (Xinzhou, China). It is of grade 90B, with a whiteness of 86%, median diameter of 0.8 μm, oil absorption value of 72, fineness of 500 mesh, and was calcined at a temperature of 850 °C. The chemical composition is as follows: [chemical composition] (

Table 2. Chemical composition of metakaolin as a percentage.

Composition	Al2O3	SiO2	Fe2O3	TiO2	H2O
Percentage/%	40	55	0.5	1.5	0.5

).

2.1.3. Alkaline Solution

The alkaline solution activator used in this experiment is prepared by mixing sodium silicate (Na₂SiO₃) and sodium hydroxide (NaOH). Sodium hydroxide flakes are dissolved in water at a concentration of 10 mol/L. Afterwards, the NaOH solution is mixed with the Na₂SiO₃ solution to prepare the alkaline activator.

2.1.4. Sand

Sand was produced by Xiyuefa International Environmental Protection Co., Ltd. (Taiyuan, China). It has a clay content of 23.8%, a moisture content of 4.5%, an alkali-aggregate reaction value of 5.0, a fineness modulus of 2.86, and falls into the medium sand category for Zone 2.

2.2. Preparation of Mortar

Five different mortar mixtures were prepared by varying the proportions of fly ash and metakaolin. The water–cement ratio (H0-H5) was fixed at 0.45, and the dosage of water-reducing agent was 1.2%. The research results indicate that the proportions of fly ash and metakaolin have a certain influence on the performance of geopolymers in mortar.

The following are the mix proportions and specimen preparation methods for geopolymer mortar.

2.2.1. Selection of Mix Proportions

A total of five mortar mixture proportions were prepared to investigate the influence of different proportions of fly ash and metakaolin on the compressive strength, flexural strength, and split tensile strength of geopolymers in mortar (

Table 3. Presents the mixture proportions of geopolymers in mortar with different ratios of fly ash and metakaolin.

Serial Number	Metakaolin (kg/m3)	Fly Ash (kg/m3)	Water Cement Ratio	Water Reducing Agent Dosage (kg/m3)	Waterglass (kg/m3)	NaOH (kg/m3)	Granule (kg/m3)
H0	0	371.2	0.45	4.45	350	90	504
H1	129.92	241.28	0.45	4.45	327	82	504
H2	185.6	185.6	0.45	4.45	245	62	504
H3	241.28	129.92	0.45	4.45	163	41	504
H4	100	0	0.45	4.45	141	21	504

).

2.2.2. Preparation of Test Specimens

• Measure a certain amount of water glass and flake caustic soda, put them into a barrel, stir well, then cool and set aside for use.
• Measure a certain amount of manufactured sand and coarse and fine aggregates in a shallow iron pan, and then measure different proportions of metakaolin and fly ash in a tin barrel.
• Measure the water quantity and add a measured amount of naphthalene-based water-reducing agent, stir, and dissolve evenly.
• Pour the coarse and fine aggregates, metakaolin, and fly ash into a horizontal mixer for dry mixing for 3 min, then add the water-reducing agent solution and continue stirring for 3 min, followed by adding the water glass solution and stirring for 5 min, then discharge onto a wet stainless-steel plate to test the spreadability. (When adding fibers, it is only necessary to put them into the powder to participate in dry mixing, and other things remain unchanged.)
• Place the mortar into a 100 × 100 × 100 mm cubic mold and 80 mm × 10 mm × 4 mm cubic mold, fully vibrate it, and send it to a curing room (temperature 20 °C, relative humidity above 70%) for curing. Demold after 1 day, and continue curing until the ages of 1, 3, and 28 days.
• A flowchart of the address polymer preparation is shown in Figure 1.

2.3. Characterization

The geopolymer mortar prepared in this experiment was tested for its compressive strength, flexural strength, and splitting tensile strength.

2.3.1. Mechanical Performance Testing

In this experiment, the DYE-3000S (Wuxi Jianyi Experiment Instrument Co., Ltd., Wuxi, China) fully automatic constant stress pressure testing machine was used for compressive strength testing and splitting tensile strength testing. This experiment used the SH64-DKZ-5000 (Wuxi Jianyi Experiment Instrument Co., Ltd., Wuxi, China) electric bending tester for bending strength testing. The machine used for the pilot test is shown in Figure 2.

2.3.2. Microscopic Mechanism Research

The surface morphology of the specimen was observed using a Japanese JSM-7200F (JEOL Ltd., Tokyo, Japan) high-speed analytical thermal field emission scanning electron microscope, and the XRD pattern of the specimen was determined using a Japanese physics D/MAX-2200PC (Rigaku, Tokyo, Japan) X-ray diffractometer.

3. Results and Discussion

3.1. Effect of Fly Ash Metakaolin Ratio on the Mechanical Properties of Geopolymer Mortar

Figure 3, Figure 4 and Figure 5 show the effects of different ratios of fly ash to metakaolin on the compressive strength, flexural strength, and splitting tensile strength of mortar at different ages. The experimental results show that an appropriate ratio of fly ash to metakaolin can improve the strength and durability of mortar. However, too high a ratio leads to lower strength. In particular, the best mechanical properties were observed at a fly ash to metakaolin ratio of 35:65, with a 28-day compressive strength of 54 MPa, a 28-day flexural strength of 3.4 MPa, and a 28-day splitting tensile strength of 1.9 MPa. These phenomena indicate that with a certain type of metakaolin and fly ash, the early strength of the geopolymer mortar can be significantly increased by increasing the metakaolin content. When the metakaolin content exceeds 65%, the setting time becomes too fast and solidifies essentially immediately after mixing with the water glass solution. This is due to the high pozzolanic activity of metakaolin, which quickly enters into a geopolymerization reaction with the water glass solution, causing the geopolymer mortar to harden before it can even be formed.

3.2. The Impact of Fiber Reinforcement on the Performance of Geopolymer Mortar

According to the above research results, when geopolymer mortar was produced under the condition of a ridge soil to fly ash ratio of 65:35, its mechanical properties were found to be optimal. However, the flexural strength of this mortar only reached 3.4 MPa after 28 days, and the splitting tensile strength only reached 1.9 MPa, indicating lower performance and limiting its application in certain areas. To improve the flexural and tensile properties of the geopolymer mortar, fiber reinforcement will be used. Potential fibers include PP fibers, PVA fibers, carbon fibers, and steel fibers. We plan to add 0.5%, 1%, and 1.5% by volume of these fibers, respectively.

Table 4. Physical properties of PP fibers.

Section Shape	Lengths/mm	Calibre/μm	Densities/(g/cm3)	Tensile Strength/MPa	Initial Modulus/GPa
Flattened	12	30	0.91	485	7.1

,

Table 5. Physical properties of PVA fibers.

Section Shape	Lengths/mm	Calibre/μm	Densities/(g/cm3)	Tensile Strength/MPa	Initial Modulus/GPa
Trilobate	15	40	1.30	1500	42.8

,

Table 6. Physical properties of carbon fibers.

Section Shape	Lengths/mm	Calibre/μm	Densities/(g/cm3)	Tensile Strength/MPa	Initial Modulus/GPa
Elliptical	20	7	1.76	3530	230

and

Table 7. Physical properties of steel fibers.

Section Shape	Lengths/mm	Calibre/mm	Densities/(g/cm3)	Tensile Strength/MPa	Initial Modulus/GPa
Orbicular	16	0.2	7.8	6500	450

show the physical properties of PP fiber, PVA fiber, carbon fiber, and steel fiber, respectively.

3.2.1. PP Fiber

Figure 6 shows the effects of different volume fractions of PP fibers on the flexural and tensile strength of geopolymer mortar. The experimental results show that the volume fraction of PP fibers has a significant effect on the flexural and tensile strength of the geopolymer. When the volume fraction of PP fibers increases, the flexural and tensile strengths of the geopolymer first increase and then decrease. This phenomenon can be explained as follows: When the amount of PP fibers is below a certain threshold, its reinforcing effect is limited and cannot significantly improve the flexural and tensile strength of the geopolymer mortar. On the other hand, if the amount of PP fibers exceeds a certain threshold, fiber aggregation may occur in the mortar, resulting in deterioration of the workability of the mortar. PP fibers can prevent the expansion of internal cracks in the mortar by forming a three-dimensional, uniformly distributed network structure, thus improving the toughness and crack resistance of the mortar. When the mortar is subjected to an external load, PP fibers can effectively distribute and bear the load, preventing the formation of cracks and increasing the flexural capacity of the mortar. In addition, the network structure formed by the PP fibers in the mortar can effectively absorb and distribute tensile stresses, thus counteracting the tensile failure of the mortar. In addition, the PP fibers increase the ductility and toughness of the mortar and improve its seismic performance.

In summary, an appropriate amount of PP fibers can significantly improve the flexural and tensile strength of geopolymer mortar, and the amount is a critical factor. When the volume fraction of PP fibers is 1.0%, the reinforcement effect is optimal, with a 58% increase in flexural strength and a 94% increase in tensile strength.

3.2.2. PVA Fiber

Figure 7 shows the effects of different volume fractions of PVA fibers on the flexural and tensile strength of geopolymer mortar. It can be seen that as the volume fraction of PVA fibers increases, the flexural and tensile strength of the geopolymer mortar first increases and then decreases. This is due to the fact that if the PVA fiber content is too low, the reinforcing effect is limited, and the flexural and tensile strength of the geopolymer mortar cannot be significantly improved. Conversely, if the PVA fiber content is too high, the flowability of the geopolymer mortar may decrease, which reduces the workability of the mortar. PVA fibers form a three-dimensional network structure in the mortar, which effectively blocks the expansion of cracks and thus improves the toughness and crack resistance of the mortar. Under external loads, PVA fibers can effectively distribute and bear the load, preventing cracking and increasing the bending capacity of the mortar. In addition, PVA fibers can absorb and distribute tensile stresses in the mortar, preventing tensile failure. In addition, PVA fibers can also improve the ductility and toughness of the mortar, increasing its seismic performance.

In summary, by properly adjusting the volume fraction of PVA fibers, the flexural and tensile strength of geopolymer mortar can be significantly improved, as can its toughness, cracking resistance, seismic performance, and other engineering properties. However, in certain engineering applications, it is necessary to select the appropriate PP fiber content based on the specific requirements and design criteria of the mortar to take full advantage of the reinforcing effect while ensuring that the flowability and workability of the mortar are not affected. With a PVA fiber volume content of 0.5%, the reinforcing effect is optimal, with a 38% increase in flexural strength and a 52% increase in tensile strength.

3.2.3. Carbon Fiber

According to the experimental results, Figure 8 shows the effects of different volume fractions of carbon fibers on the flexural and tensile strength of geopolymer mortar. The study found that as the volume fraction of carbon fibers increased, the flexural and tensile strength of the geopolymer first increased and then decreased. Initially, the appropriate addition of carbon fibers can effectively increase the flexural properties of the mortar and improve its load-bearing capacity. Carbon fibers, which have exceptionally high tensile strength and stiffness, can prevent the propagation of cracks in the mortar under load. At the same time, the incorporation of carbon fibers results in a uniformly distributed three-dimensional reinforcement system, which increases its toughness and crack resistance, thus improving its durability and reliability under bending stress. Secondly, the carbon fibers significantly improve the tensile strength of the mortar. Since mortar is prone to cracking and failure under tensile stress, carbon fibers, with their high strength and high modulus, can effectively absorb and distribute tensile stresses, preventing the propagation of cracks and improving the ductility of the mortar. In addition, the addition of carbon fibers forms a network structure that further increases the tensile strength and durability of the mortar while reducing the width and number of cracks.

Carbon fiber is a high-strength, low-density fiber material whose surface is usually subject to electrostatic forces, making it more likely to be clumped together in concrete. Unlike this, polypropylene fibers have relatively weaker and better electrostatic forces and do not tend to clump together.

In summary, the appropriate addition of carbon fibers can significantly improve the flexural and tensile strength of geopolymer mortar. However, in practical application, it is necessary to select an appropriate amount of carbon fibers according to the specific needs and design requirements and take appropriate processing and design measures to make full use of the reinforcing effect of carbon fibers and ensure the durability and safety of the mortar structure. The reinforcing effect is optimal when the volume fraction of carbon fibers is 1.0%. The flexural strength increases by 41%, and the tensile strength increases by 94%.

3.2.4. Steel Fiber

According to the experimental results, Figure 9 shows the effect of steel fibers on the flexural and tensile strength of geopolymer mortar. With the increase in steel fiber content, the flexural and tensile strength of geopolymers gradually increase. The addition of steel fibers significantly increases the flexural strength of the mortar. The steel fibers are characterized by their high strength and high modulus and can effectively resist stress concentration and crack propagation when the mortar is subjected to stress. The introduction of steel fibers forms a three-dimensional uniformly distributed reinforcement system, which gives the mortar better toughness and crack resistance under bending loads. In addition, the steel fibers can effectively limit the width and number of cracks, improving the durability and reliability of the mortar structure. In addition, the addition of steel fibers significantly increases the tensile strength of the mortar. Mortar tends to crack and break under tensile stress, and due to their high strength and high modulus of elasticity, steel fibers can effectively absorb and distribute tensile stresses, preventing the propagation of cracks. The incorporation of steel fibers creates a fabric-like structure that improves the tensile strength and ductility of the mortar, increases its resistance to cracking, and slightly reduces the width and number of cracks.

In summary, it can be said that an appropriate proportion of steel fibers can significantly improve the flexural and tensile strength of geopolymers. At a volume fraction of 1.5% of steel fibers, the reinforcement effect is best, with an increase in flexural strength of 70 and an increase in tensile strength of 142.

Based on the above results, the effects of PP fibers, PVA fibers, carbon fibers, and steel fibers on flexural and tensile strength show a trend of increasing and then decreasing with increasing content. This is due to the fact that too high a fiber content can lead to an accumulation and entanglement of fibers in the mortar. As a result, excessively dense particle areas are created in the mortar, which can increase the likelihood of internal defects and micro-cracks, thus weakening the overall strength of the mortar. Secondly, too high a dosage of fibers can lead to a reduction in the fluidity of the mortar, making it difficult for the mortar to adequately fill the molds, which affects the compactness of the mortar. This can lead to a reduction in the compressive strength of the mortar. However, the optimal dosage of steel fiber is the highest because it has a better surface roughness and friction coefficient compared to other fibers, which reduces the friction between the fibers and thus the probability of aggregation and interlocking. In addition, the steel fiber has a very high tensile and flexural strength. It can distribute stresses very efficiently and thus prevent further propagation of cracks. It therefore offers the best improvement in tensile and flexural strength in geopolymer mortar.

3.3. Microscopic Mechanism Analysis

3.3.1. XRD Analysis

Figure 10 shows the X-ray diffraction spectra of geopolymer mortar with different proportions of metakaolin and fly ash. As can be seen from Figure 10, this geopolymer contains a large amount of unhydrated quartz SiO₂ and mullite (3Al₂O₃-2SiO₂) crystals as well as the hydration product sodium silicate (Na₂Si₄O₉). Figure 10 shows the X-ray diffraction spectra of metakaolin fly ash-based geopolymer mortar and metakaolin-based geopolymer mortar, respectively, where the significant similarity of the spectra is clearly visible. Further observations of the X-ray diffraction spectrum in Figure 10 show that the peaks corresponding to sodium zeolite (Na₂[Al₂Si₃O₁₀]-2H₂O) are significantly increased in the metakaolin-based geopolymer mortar. This suggests that sodium zeolite (Na₂[Al₂Si₃O₁₀]-2H₂O) is the main product of the geopolymerization reaction. This substance is a network-like amorphous silicon–aluminate gel with a diffraction peak area between 25° and 30°.

Figure 11 shows the X-ray diffraction spectra of various fiber-reinforced geopolymer mortars. It can be seen that the geopolymer mortar contains a large amount of unhydrated quartz (SiO₂) and aluminum hydroxide (Al₂O₃) crystals, as well as hydrated sodium zeolite (Na₂[Al₂Si₃O₁₀]-2H₂O) and sodium sulfate (Na₂SO₄) crystals after reinforcement with various fibers. The X-ray diffraction spectrum of the steel fiber-reinforced geopolymer mortar shows a significant increase in the diffraction peak of sodium zeolite (Na₂[Al₂Si₃O₁₀]-2H₂O) and a significant decrease in quartz (SiO₂), indicating that steel fibers have the best reinforcing effect on geopolymer mortar. Therefore, by observing the X-ray diffraction spectra of various fiber-reinforced geopolymer mortars, it can be found that the main product formed is sodium zeolite (Na₂[Al₂Si₃O₁₀]-2H₂O), which is a network-like amorphous silicon–aluminate gel. This can be clearly observed in the region of the diffraction peaks between 25° and 30°.

3.3.2. SEM Analysis

Figure 12 shows the SEM images of samples with different ratios of fly ash to metakaolin. The images show that as the proportion of metakaolin increases, the microcracks in the geopolymer mortar decrease significantly, the porosity decreases continuously, and the interfacial bonding between the aggregates and the hydration products of the geopolymer mortar improves, resulting in a denser mortar structure and a significant increase in strength. This is due to the relatively higher reactivity of the volcanic ash of metakaolin, which undergoes a rapid hydration reaction. In the early stages of hydration, a large amount of gel-forming hydration products with cementitious activity are formed, which improve the encapsulation of the aggregates by the cementitious materials and strengthen the interfacial bonding between the aggregates and the geopolymer mortar, thereby significantly reducing the internal microcracks in the mortar.

Figure 13 shows SEM images of geopolymer mortar samples with various volumetric inclusions of PP fibers. As can be seen in the images, as the volume of PP fibers increases, the porosity and microcracks in the geopolymer mortar first decrease and then increase. This is because as the volume fraction of PP fibers increases, the number of contact points between the fibers increases, increasing the interaction forces between the fibers, which reduces porosity and microcracks. However, once the volumetric content of PP fibers reaches a certain limit, the PP fibers have a certain surface tension, which causes the fibers to attract each other when they are close to each other, resulting in agglomeration.

Figure 14 shows the SEM images of geopolymer mortar samples with different volumetric inclusions of PVA fibers. As can be seen in the figure, with the continuous increase in volumetric content of PVA fibers, the porosity and microcracks in the geopolymer mortar decrease steadily. This is due to the fact that the PVA fibers are distributed more evenly in the matrix at a low volumetric content, which leads to a more uniform arrangement. At a high volumetric content, however, agglomeration can occur between the PVA fibers, which leads to a lower fiber distribution and an uneven arrangement.

Figure 15 shows the SEM images of geopolymer mortar samples with different volumetric inclusions of carbon fibers. As can be seen in the figure, similar to PP fibers, the porosity and microcracks in the geopolymer mortar first decrease and then increase with the continuous increase of the volumetric content of carbon fibers. This is because carbon fibers can increase the flexural and tensile strength of the geopolymer mortar at an appropriate volumetric content. However, if the volumetric content is too high, the agglomeration between the fibers and the shear effect at the fiber/matrix interface can lead to a deterioration of the mechanical properties.

Figure 16 displays the SEM of geopolymer mortar specimens with different volumetric additions of steel fibers. Observing the images, it is evident that with the reinforcement from various fibers, the geopolymer’s structure becomes more refined and significantly stronger. As seen in Figure 16b, the SEM image of the steel fiber-reinforced geopolymer shows an abundance of hydration products, with the tightest structure, where micro-cracks and pores are almost nonexistent. This indicates that steel fibers significantly enhance the geopolymer’s structure. The incorporation of steel fibers creates a mesh-like structure, which improves the mortar’s tensile strength and ductility, enhances its resistance to cracking, and somewhat reduces the width and number of cracks.

In conclusion, as observed from Figure 13b and Figure 14a, PP fibers and PVA fibers form a three-dimensional, uniformly dispersed mesh structure within the geopolymer, effectively impeding the path of crack propagation and thereby enhancing the toughness and crack resistance of the mortar. Figure 15b showcases that carbon fibers exhibit excellent bridging effects, capable of traversing through the pores and micro-cracks of the geopolymer mortar, resulting in tighter internal connections and effectively improving both the flexural and tensile strength.

4. Conclusions

According to the experimental results, the strength of the geopolymer is related to the ratio of metakaolin to fly ash. As the proportion of metakaolin increases, the strength of the geopolymer exhibits an increasing trend, followed by a decreasing trend. The optimal ratio is 65:35, whereby the 28-day compressive strength reaches 47 MPa, the 28-day flexural strength reaches 3.4 MPa, and the 28-day splitting tensile strength reaches 1.9 MPa.

The flexural and tensile strengths of geopolymer mortar can be significantly improved through fiber reinforcement. Here is the enhancement effect at different fiber dosages: when PP fiber volume content is 1.0%, the enhancement effect is optimal, and the flexural and tensile strengths increase by 58% and 94%, respectively; when PVA fiber volume content is 0.5%, the enhancement effect is the best, with 38% and 52% increases in flexural and tensile strengths, respectively; when carbon fiber volume content is 1.0%, the enhancement is the best, with a 41% and 94% increase in flexural and tensile strengths, respectively; when the steel fiber volume content is 1.5%, the enhancement effect is optimum, with a 70% and 142% increase in flexural and tensile strengths, respectively.

Mechanism analysis shows that different fibers contribute to the densification of geopolymer structures and significantly enhance their strength. Among them, steel fibers present the most prominent effect on internally compact structures and further strengthen the performance of geopolymers. The hydration products of geopolymer reinforced with PP fiber, PVA fiber, carbon fiber, and steel fiber are non-crystalline reticular sodium zeolite (Na₂[Al₂Si₃O₁₀]·2H₂O).