Research on the Mechanical Properties and Microstructure of Fiber Geopolymer Mortar

Abstract

It is known that geopolymer mortar exhibits high compressive strength but relatively low flexural strength, high brittleness, and poor toughness. Engineering practices for cement-based materials have demonstrated that incorporating fibers can effectively prevent the expansion of existing cracks and the formation of new ones in the materials. Adding polypropylene fibers to geopolymer mortar can, on the one hand, improve the crack resistance of the mortar, and on the other hand, enhance the impact resistance of the geopolymer mortar. In this paper, slag, metakaolin, and fly ash are utilized as silico-aluminous raw materials, standard sand is employed as aggregate, and a mixture of water glass and NaOH in a specific proportion is used as the alkali activator to prepare geopolymer mortar. Polypropylene fibers are incorporated to improve its mechanical properties. The effects of fiber length and mixing method on the mechanical properties of geopolymer mortar are studied to determine the optimal fiber length and mixing method. The mechanism of the mechanical properties of fiber-reinforced geopolymer mortar is analyzed by combining SEM. The research results indicate that the geopolymer mortar with 15 mm single-doped fibers exhibits the best flexural strength and toughness. In contrast, the geopolymer mortar with 12 mm single-doped fibers demonstrates the best compressive strength. The geopolymer with 9 mm and 18 mm hybrid-doped fibers has the best mechanical properties and is superior to the geopolymer mortar with single-doped fibers.

1. Introduction

Against the backdrop of the in-depth integration of the concept of sustainable development into the field of civil engineering, geopolymer materials have emerged as a research hotspot for replacing traditional cement-based materials, thanks to their dual advantages of low carbon emissions and environmental protection, as well as the resource utilization of industrial solid waste. Geopolymers are cementitious materials with a three-dimensional network structure formed through alkali activation reactions, using silicon-aluminum industrial solid wastes such as slag, fly ash, and metakaolin as raw materials [1,2,3]. Research indicates that the carbon emissions associated with the production of this material are approximately 80% lower than those of traditional cement. Moreover, it can effectively consume large amounts of solid waste generated by industries such as steel and power [4,5,6], providing a new approach to addressing environmental load and resource shortages.

However, the inherent brittle defect of geopolymer mortar seriously restricts its engineering applications. The flexural strength of ordinary geopolymer mortar is only 1/10 to 1/15 of its compressive strength, which is far lower than the toughness level of cement-based materials [7,8,9]. When subjected to dynamic loads, such as bending and impact, geopolymer mortar is prone to sudden cracking, which can result in structural failure [10,11,12]. Harle et al. measured the 28-day flexural strength of geopolymer mortar without fiber to be only 6.0 MPa, and the flexural-compressive strength ratio was as low as 0.13 [13]. This brittle characteristic makes it difficult to meet the requirements of structures such as pavements and bridges for the crack resistance and durability of materials.

Engineering practices with cement-based materials have demonstrated that incorporating fibers is an effective means to improve brittleness. Polypropylene fiber has become one of the preferred materials for strengthening geopolymer mortar due to its strong corrosion resistance and good compatibility with the matrix [14,15,16]. Liu et al. found that adding 0.3% of 12 mm polypropylene fiber can increase the flexural strength of geopolymer mortar by 20%, but it has no significant effect on the later strength growth [17]. Feng et al. further studied the effect of fiber length and found that 15 mm fibers had the most effective inhibition of early microcrack expansion. The growth rate of the flexural strength of the strengthened geopolymer mortar at 14 days reached 22.6% [18].

There are still controversies in the existing research on optimizing fiber length. Some scholars indicate that 12 mm fibers are more conducive to the improvement of compressive strength [19,20,21]. When the slag content increases, the strengthening effect of long fibers (such as 15 mm) is more significant [22,23]. Additionally, the synergistic effect of hybrid fibers has garnered increasing attention. Sun et al. found that the hybrid of 9 mm and 18 mm fibers can form a spatial network structure, which can increase the crack resistance of geopolymer mortar by more than 30% [24].

From a microscopic mechanism perspective, the interfacial bonding performance between the fiber and the geopolymer matrix is crucial in determining the reinforcement effect. Zhou et al. observed through SEM that the 15 mm fibers were broken rather than pulled out, indicating that the chemical bonding force between them and the matrix was strong [25]. Although the 9 mm fibers had good dispersibility, the porosity of the interfacial transition zone was relatively high [26]. This microscopic structural difference directly affects the material’s macroscopic properties. The toughness index of the geopolymer mortar with a single addition of 15 mm fibers is increased by 21.3% compared to that without fiber addition [27,28]. When mixing short and long fibers, the short fibers inhibit early microcracks, and the long fibers impede the later crack propagation, forming a complementary effect [29,30].

At the engineering application level, the application requirements of geopolymer mortar in fields such as pavement repair and marine engineering are driving in-depth research on fiber reinforcement technology. Chakravarthy et al. pointed out that the fiber geopolymer mortar used for airport pavements must meet the technical requirements of a flexural strength of ≥7.5 MPa and a flexural strength-to-compressive strength ratio of ≥0.18 [31]. The 28-day flexural strength of the geopolymer mortar with a mixture of 9 mm and 18 mm fibers in this paper reached 7.3–10.3 MPa, and the ratio of flexural strength to compressive strength was 0.14–0.21, which was close to or met the relevant engineering standards [32,33].

In conclusion, research on the mechanical properties of Geopolymer Mortar incorporating single and binary solid waste materials, along with their fiber modification, has reached a relatively advanced stage. However, the potential effects of hybrid fibers on Geopolymer Mortar composed of ternary solid waste mixtures remain unclear. Based on the above background, this paper utilizes slag, metakaolin, and fly ash as silico-aluminous raw materials, standard sand as aggregate, and a mixture of water glass and NaOH in a specific proportion as the alkali activator to prepare geopolymer mortar. By adding polypropylene fibers of 9 mm, 12 mm, 15 mm, and 18 mm, as well as their mixed combinations, the influence of fiber parameters on the flexural strength, compressive strength, and F/C ratio of geopolymer mortar is systematically studied. Combined with SEM analysis, the microscopic mechanism of fiber reinforcement is revealed, providing a basis for parameter optimization and theoretical support for the engineering application of fiber-reinforced geopolymer mortar.

2. Experimental Programs

2.1. Materials

In this study, metakaolin, slag, and fly ash, which have relatively high silicon and aluminum contents and intense activity, were selected as silico-aluminous raw materials. The chemical compositions of each raw material were then analyzed. The X-ray fluorescence spectrometer (XRF) was used to determine the composition of the raw materials used in this paper. The chemical compositions are presented in

Table 1. Chemical composition of metakaolin (%).

Chemical Composition	SiO2	Al2O3	Fe2O3	K2O	TiO2	CaO	Na2O	MgO	LoI
content	50.14	41.11	0.76	0.55	0.24	0.17	0.06	0.06	6.91

,

Table 2. Chemical composition of slag (%).

Chemical Composition	CaO	SiO2	Al2O3	MgO	SO3	TiO2	MnO	Fe2O3	K2O	Na2O	SrO	LOI
content	35.99	34.11	15.36	6.58	2.5	2.41	1.07	0.83	0.62	0.4	0.13	-

and

Table 3. Chemical composition of fly ash (%).

Chemical Composition	SiO2	Al2O3	Fe2O3	CaO	TiO2	MgO	SO3	Na2O	P2O5	SrO	LoI
content (%)	62.04	25.5	4.28	3.96	1.33	1.27	0.73	0.46	0.31	0.12	-

. The XRD results of metakaolin (Raw-P) and slag (Raw-S) used in the experiment are shown in Figure 1.

Geopolymer mortar is prepared using standard sand as aggregate and sodium hydroxide and water glass as alkali activators. The leading technical indicators for standard sand are presented in

Table 4. Technical parameter of the sand.

Technical Parameters	Bulk Density (kg/m3)	Apparent Density (kg/m3)	Porosity (%)	Fineness Modulus
Standard sand	1610	2570	37.4	2.4

, while the leading technical indicators for water glass are listed in

Table 5. Technical parameter of the water glass.

Technical Parameters	Na2O Content (%)	SiO2 Content (%)	Modulus	Density (kg/m3)	Brix Degree
Measured value	8.5	26.5	3.1	1480	40

. Granular sodium hydroxide with a purity greater than 96% is used. Calculate based on the modulus and use sodium silicate, NaOH, and water to prepare an alkali-activated solution. Allow the mixture to stand for 24 h, followed by cooling to room temperature.

2.2. Mix Proportion Design

In this paper, the mix proportion of fiber geopolymer mortar is the same as that of the cement mortar specified in the code, that is, the ratio of binder to standard sand is 1:3, and the water-solid ratio is 0.5. Polypropylene fibers are used as the fibers, with lengths of 9 mm, 12 mm, 15 mm, and 18 mm. Combining relevant research and the application of polypropylene fibers in actual projects, the dosage is 0.9 kg/m³. The mix proportion design of the fiber mortar is shown in

Table 6. Design table of geopolymer mortar mix proportion (kg/m³).

Serial Number	Number	Na2SiO3	NaOH	Silicoaluminate Materials			Water	Fiber/g
				Metakaolin	Fly Ash	Slag
1	P100F0S0M1.4N16	221	31.6	269.9	0	0	0	—
2								0.7
3	P60F40S0M1.2N12	156.3	28.9	178.8	119.2	0	40.8	—
4								0.7
5	P60F0S40M1.2N14	153.4	27.9	172.8	0	115.2	39.4	—
6								0.7
7	P60F20S20M1.2N12	156.3	28.9	178.8	59.6	59.6	40.8	—
8								0.7

Note: — represents the control group without fiber incorporation.

. The XRD results of the P₁₀₀F₀S₀M_1.4N₁₆ and P₆₀S₄₀M_1.2N₁₄ groups at the age of 28 days are shown in Figure 2.

2.3. Test Methods

2.3.1. Strength Test of Geopolymer Mortar

The flexural and compressive strengths of geopolymer mortar are determined by referring to the test method for the strength of cement mortar (GB/T 17671–2021 [34]). Pour the prepared geopolymer mortar into a mold measuring 40 mm × 40 mm × 160 mm. Vibrate it on a vibrating table for 30 s to expel the air bubbles. Cover the specimen’s surface with a layer of plastic film to prevent moisture loss during the curing process. Place the specimens in the curing chamber and cure them under conditions of 20 °C ± 1 °C and a relative humidity exceeding 90%. Demold after 24 h and continue curing. Measure its flexural strength and compressive strength after reaching age (the experiment adopted three-point bending), as shown in Figure 3.

2.3.2. Microstructural Characterization

(1)

Molecular structure analysis

The molecular structure of geopolymer paste is analyzed by Fourier transform infrared spectroscopy. The milled sample is mixed with potassium iodide to form small pieces with a thickness of 1 mm and a diameter of 10 mm, and then the test is conducted. The Nicolet 50 Fourier transform infrared spectrometer, produced by Thermo Fisher Scientific (China) Co., Ltd. (Harbin, China), is used. Resolution: 0.09 cm⁻¹; wavenumber accuracy: 0.005 cm⁻¹; full-spectrum linear accuracy: better than 0.07%; spectral range: 7800–350 cm⁻¹; stability: it has a fast-scanning performance of more than 60 spectra per second.

(2)

Microanalysis

The morphological characteristics of the geopolymer paste were characterized using a field-emission scanning electron microscopy (FE-SEM) analysis method to study the reaction mechanism of the geopolymer further. A small sample with a relatively flat surface was taken. After dust removal treatment, it was fixed on the sample stage with conductive glue. A gold conductive layer was sprayed on its surface and then the test was conducted. In this paper, a JSM-7500F variable vacuum scanning electron microscope produced by JEOL Ltd. (Tokyo, Japan) is used.

3. Results and Discussion

3.1. Mechanical Properties of Geopolymer Mortar with Fibers

3.1.1. Research on Flexural Strength Test

The influence of fiber length on the flexural strength of geopolymer mortar was studied. According to the mixed proportion of fiber geopolymer mortar in Table 6, the flexural strength at 3 d, 7 d, 14 d, and 28 d was measured, respectively, and the growth law of flexural strength was analyzed. The test results are shown in

Table 7. Flexural strength of fiber geopolymer mortar.

Serial Number	Number	Fiber Length (mm)	Flexural Strength (MPa)
			3 d	7 d	14 d	28 d
1	P100F0S0M1.4N16	——	1.8	3.6	5.3	6.0
2		9	1.9	3.9	6.0	6.5
3		12	1.9	4.1	6.2	6.8
4		15	2.0	4.2	6.5	7.0
5		18	1.9	4.0	6.3	6.8
6	P60F40S0M1.2N12	——	1.5	2.9	5.2	6.3
7		9	1.6	3.1	5.6	6.8
8		12	1.7	3.3	6.1	7.1
9		15	1.8	3.5	6.3	7.3
10		18	1.6	3.2	6.0	7.1
11	P60F0S40M1.2N14	——	4.5	6.2	7.3	7.7
12		9	4.8	7.0	8.0	8.2
13	P60F0S40M1.2N14	12	5.3	7.4	8.5	8.9
14		15	5.5	7.6	8.8	9.2
15		18	5.1	7.3	8.4	8.8
16	P60F20S20M1.2N12	——	4.3	6.0	7.2	8.0
17		9	4.5	6.8	8.2	8.6
18		12	4.9	7.2	8.7	9.3
19		15	5.2	7.4	8.9	9.7
20		18	5.0	7.3	8.7	9.4

. The influence of fiber length on the flexural strength at 3 d, 7 d, 14 d, and 28 d of four kinds of geopolymer mortars, namely P₁₀₀F₀S₀M_1.4N₁₆, P₆₀F₄₀S₀M_1.2N₁₂, P₆₀F₀S₄₀M_1.2N₁₄, and P₆₀F₂₀S₂₀M_1.2N₁₂, is shown in Figure 4a–d.

Figure 4a,d show that the addition of polypropylene fibers effectively improves the flexural strength of the geopolymer mortar. Moreover, with the increase in fiber length, the growth rate of flexural strength exhibits a changing pattern, first increasing and then decreasing. Among them, when the fiber length is 15 mm, the strengthening effect reaches its best. Specifically, the P₁₀₀F₀S₀M_1.4N₁₆ geopolymer mortar shows the most significant growth effect at 14 days. Its flexural strength at 3 days, 7 days, 14 days, and 28 days is increased by 11.1%, 16.7%, 22.6%, and 16.7%, respectively. The P₆₀F₄₀S₀M_1.2N₁₂ geopolymer mortar shows the most significant growth effect at 28 days. The flexural strengths at 3 days, 7 days, 14 days, and 28 days are increased by 20%, 20.7%, 21.2%, and 21.7%, respectively. The optimal growth effect of the P₆₀F₀S₄₀M_1.2N₁₄ geopolymer mortar appears at 7 days. The flexural strengths at 3 days, 7 days, 14 days, and 28 days are increased by 22.2%, 22.6%, 20.5%, and 19.5%, respectively. The growth effect of P₆₀F₂₀S₂₀M_1.2N₁₂ geopolymer mortar is most prominent at 14 days. The flexural strengths at 3 days, 7 days, 14 days, and 28 days are increased by 20.9%, 23.3%, 23.6%, and 21.3%, respectively.

The reason polypropylene fiber can improve the flexural performance of geopolymer mortar is its ability to resist bending deformation. During the hardening process of geopolymer mortar, an appropriate amount of polypropylene fibers can be evenly distributed throughout the slurry and thoroughly combined with it. When subjected to bending, they can effectively withstand shear stress. Combined with the analysis of the growth and change law of flexural strength, it further shows that 15 mm is the ideal length for the fiber to exert the best reinforcement effect.

3.1.2. Experimental Research on Compressive Strength

The influence of fiber length on the compressive strength of geopolymer mortar is studied. According to the mix ratios of fiber geopolymer mortar in Table 6, the compressive strengths at 3 days, 7 days, 14 days, and 28 days are measured, respectively, and the growth law of compressive strength is analyzed. The test results are shown in

Table 8. Compressive strength of fiber geopolymer mortar.

Serial Number	Number	Fiber Length (mm)	Compressive Strength (MPa)
			3 d	7 d	14 d	28 d
1	P100F0S0M1.4N16	—	18.5	35.8	42.6	45.0
2		9	18.7	36.5	43.8	45.8
3		12	20.3	39.5	47.3	49.5
4		15	19.8	38.8	45.4	47.3
5		18	19.5	38.0	45.2	47.3
6	P60F40S0M1.2N12	—	15.2	26.5	41.7	45.8
7		9	15.8	27.9	45.0	49.5
8		12	16.5	29.5	47.0	51.9
9		15	16.2	29.0	46.5	51.5
10		18	16.0	28.9	46.5	51.1
11	P60F0S40M1.2N14	—	24.2	35.2	40.8	44.6
12		9	25.0	36.8	42.2	45.8
13		12	25.8	38.2	43.5	46.8
14		15	25.5	37.8	42.9	46.5
15		18	25.3	37.6	42.6	46.3
16	P60F20S20M1.2N12	—	22.8	32.8	41.5	46.0
17		9	23.9	34.6	43.8	47.5
18		12	24.4	35.8	45.0	48.6
19		15	24.2	35.5	44.0	48.3
20		18	23.8	35.0	43.7	48.1

. The influence of fiber length on the compressive strengths at 3 days, 7 days, 14 days, and 28 days of four kinds of geopolymer mortars, namely P₁₀₀F₀S₀M_1.4N₁₆, P₆₀F₄₀S₀M_1.2N₁₂, P₆₀F₀S₄₀M_1.2N₁₄, and P₆₀F₂₀S₂₀M_1.2N₁₂, is shown in Figure 5a–d.

Figure 5a–d show that although the addition of polypropylene fibers improves the compressive strength of geopolymer mortar, the improvement effect is not significant. Especially when the fiber length is 9 mm, the growth rate of strength is almost negligible. Overall, as the fiber length increases, the growth rate of compressive strength first increases and then decreases. When the fiber length is 12 mm, the strengthening effect is optimal. Specifically, the P₁₀₀F₀S₀M_1.4N₁₆ geopolymer mortar shows the most significant growth effect at 14 days. Its compressive strengths at 3 days, 7 days, 14 days, and 28 days are increased by 9.7%, 10.3%, 11.0%, and 10.0%, respectively. The geopolymer mortar of P₆₀F₄₀S₀M_1.2N₁₂ showed the most significant growth effect at 28 days. The compressive strengths at 3 days, 7 days, 14 days, and 28 days increased by 8.6%, 11.3%, 12.7%, and 13.3%, respectively. The best growth effect of P₆₀F₀S₄₀M_1.2N₁₄ geopolymer mortar appears at 7 days. The compressive strengths at 3 days, 7 days, 14 days, and 28 days are increased by 6.6%, 8.5%, 6.6%, and 4.9%, respectively. The growth effect of P₆₀F₂₀S₂₀M_1.2N₁₂ geopolymer mortar is most prominent at 7 days. The compressive strengths at 3 days, 7 days, 14 days, and 28 days are increased by 7.0%, 9.1%, 8.4% and 5.7%, respectively.

The addition of polypropylene fibers has a limited effect on improving the compressive performance of geopolymer mortar. Especially when the fiber length is 9 mm, the increase in strength is minimal. This is mainly because too short a fiber length will reduce the bonding area with the paste interface, thus reducing the bonding force with the paste. In addition, when the dosage is fixed, the shorter the fibers are, the greater the number of fibers that can be incorporated. This can easily lead to uneven distribution, thereby increasing the pores within the slurry and making it difficult to achieve the desired strengthening effect. Combined with the analysis of the growth and change in law of compressive strength, it further confirms that 12 mm is the ideal length for the fiber to exert the best reinforcement effect.

3.1.3. Comparative Analysis of Flexural-to-Compressive Ratio

The flexural-compressive ratio of geopolymer mortar refers to the ratio of flexural strength to compressive strength. It is a significant indicator reflecting the toughness of the mortar. The greater the toughness, the stronger the mortar’s impact resistance and the better its fatigue performance. The influence of fiber length on the F/C ratios of four kinds of geopolymer mortars, namely P₁₀₀F₀S₀M_1.4N₁₆, P₆₀F₄₀S₀M_1.2N₁₂, P₆₀F₀S₄₀M_1.2N₁₄, and P₆₀F₂₀S₂₀M_1.2N₁₂, at 3 days, 7 days, 14 days, and 28 days is shown in Figure 6a–d.

It can be analyzed from Figure 4a–d that the addition of polypropylene fibers to geopolymer mortar plays a role in improving its toughness. With the increase in fiber length, the flexural-to-compressive strength ratio exhibits a change rule of first increasing and then decreasing, with the best effect achieved when the fiber length is 15 mm. After fibers are incorporated into P₁₀₀F₀S₀M_1.4N₁₆ and P₆₀F₄₀S₀M_1.2N₁₂, the flexural-to-compressive strength ratio increases slowly with the age of the specimens. It reaches the maximum value at 28 days, but the growth rate is relatively small. This indicates that the toughness improvement of the two types of mortar mainly occurs slowly over the long term, and the overall toughness level is relatively low. The flexural-to-compressive ratios of P₆₀F₀S₄₀M_1.2N₁₄ and P₆₀F₂₀S₂₀M_1.2N₁₂ are significantly higher than those of the former two, and show a trend of “rising first and then falling” with age. The peak value is reached at 14 days or 7 days. This indicates that the toughness of the two types of mortar can be fully exerted in the early stage (7–14 days), which may be related to the optimization of the fiber-matrix interfacial transition zone by the ratio of silico-aluminous raw materials.

In the technical index system for cement concrete pavements, flexural strength is the primary index. Meanwhile, the impact resistance of the pavement is equally indispensable. This is especially true for airport cement concrete pavements, as they endure high-frequency and high-load impacts, thus having even more stringent requirements for impact resistance. Based on the performance characteristics of geopolymer concrete and the results of experimental research, if polypropylene fibers of a single length are used for reinforcement in actual pavement engineering, from the perspective of meeting the core performance requirements of the pavement, it is recommended to give priority to fibers with a length of 15 mm. Fibers of this length can not only effectively improve the flexural strength of geopolymer concrete, but also enhance the toughness of the material by optimizing the adhesion with the matrix, thus better meeting the service requirements of pavements, especially airport pavements.

3.2. Mechanical Properties of Geopolymer Mortar Mixed with Fibers

3.2.1. Research on Flexural Strength Test

The impact of mixing fibers with varying lengths on the strength of geopolymer mortar are investigated. Two lengths of fibers for an equal amount mixing are selected, each accounting for 0.1% of the binder material. According to the mixed proportion of fiber geopolymer mortar in Table 6, the flexural strength at 3 d, 7 d, 14 d, and 28 d are measured, respectively, and the growth law of the flexural strength is analyzed. The test results are shown in

Table 9. The flexural strength of fiber geopolymer mortar.

Serial Number	Number	Mixing Type (mm)	Flexural Strength (MPa)
			3 d	7 d	14 d	28 d
1	P100F0S0M1.4N16	—	1.8	3.6	5.3	6.0
2		9 + 12	1.9	3.9	5.7	6.5
3		9 + 15	2.0	4.1	6.3	6.8
4		9 + 18	2.2	4.3	6.8	7.3
5		12 + 15	2.0	4.0	6.1	6.6
6		12 + 18	2.0	4.1	6.2	6.9
7		15 + 18	1.9	3.8	5.9	6.7
8	P60F40S0M1.2N12	—	1.5	2.9	5.2	6.3
9		9 + 12	1.5	3.0	5.5	6.6
10		9 + 15	1.6	3.2	5.9	7.0
11		9 + 18	1.9	3.7	6.8	7.8
12		12 + 15	1.6	3.2	6.0	7.1
13		12 + 18	1.7	3.5	6.3	7.3
14		15 + 18	1.7	3.4	6.2	7.2
15	P60F0S40M1.2N14	—	4.5	6.2	7.3	7.7
16		9 + 12	4.7	6.7	7.8	8.0
17		9 + 15	5.2	7.4	8.5	8.7
18		9 + 18	5.9	8.3	9.5	9.6
19		12 + 15	5.2	7.2	8.4	8.7
20		12 + 18	5.4	7.6	8.8	9.0
21		15 + 18	5.3	7.5	8.6	9.0
22	P60F20S20M1.2N12	—	4.3	6.0	7.2	8.0
23		9 + 12	4.4	6.3	7.5	8.3
24		9 + 15	5.1	7.2	8.4	9.2
25		9 + 18	5.8	8.3	9.6	10.3
26		12 + 15	4.8	6.8	8.0	8.8
27		12 + 18	5.3	7.5	8.8	9.6
28		15 + 18	5.2	7.4	8.6	9.4

. The influence of fiber mixing on the flexural strength of four types of geopolymer mortars, namely P₁₀₀F₀S₀M_1.4N₁₆, P₆₀F₄₀S₀M_1.2N₁₂, P₆₀F₀S₄₀M_1.2N₁₄, and P₆₀F₂₀S₂₀M_1.2N₁₂ at 3 d, 7 d, 14 d, and 28 d is shown in Figure 7a–d.

From the analysis results in Figure 7a–d, the hybrid mixing of polypropylene fibers of different lengths can effectively improve the flexural strength of geopolymer mortar. Moreover, the greater the difference in fiber length, the more significant the effect on growth and flexural strength. Among them, the mixing combination of 9 mm and 18 mm fibers shows the best performance. In contrast, the strengthening effects of other mixing methods are all inferior to the level of single addition of 15 mm fibers. Specifically, examining the improvement in flexural strength of mortars with different ratios, the geopolymer mortar of P₁₀₀F₀S₀M_1.4N₁₆ exhibits the most pronounced growth effect at 14 days. The flexural strengths at 3 days, 7 days, 14 days, and 28 days are increased by 22.2%, 19.4%, 28.3%, and 21.7%, respectively. The P₆₀F₄₀S₀M_1.2N₁₂ geopolymer mortar also showed the most significant growth at 14 days. The flexural strengths at 3 days, 7 days, 14 days, and 28 days increased by 26.7%, 27.6%, 30.8%, and 23.8%, respectively. The best growth of P₆₀F₀S₄₀M_1.2N₁₄ geopolymer mortar occurred at 7 days. The flexural strengths at 3 days, 7 days, 14 days, and 28 days increased by 31.1%, 33.9%, 30.1%, and 24.7%, respectively. The geopolymer mortar of P₆₀F₂₀S₂₀M_1.2N₁₂ shows the most obvious growth effect at 14 days. The flexural strengths at 3 days, 7 days, 14 days, and 28 days are increased by 27.9%, 30.0%, 29.2% and 26.3%, respectively.

3.2.2. Research on Compressive Strength Test

The impact of mixing fibers with varying lengths on the strength of geopolymer mortar are investigated. Two types of fibers with different lengths for an equal amount of mixing are selected. According to the mixed proportion of fiber geopolymer mortar in Table 6, the compressive strengths at 3 d, 7 d, 14 d, and 28 d are measured, respectively, and the growth law of flexural strength is analyzed. The test results are shown in

Table 10. The compressive strength of fiber geopolymer mortar.

Serial Number	Number	Mixing Type (mm)	Compressive Strength (MPa)
			3 d	7 d	14 d	28 d
1	P100F0S0M1.4N16	—	18.5	35.8	42.6	45.0
2		9 + 12	18.7	36.5	44.5	46.5
3		9 + 15	19.5	38.6	46.8	48.5
4		9 + 18	20.5	39.9	48.5	50.2
5		12 + 15	19.5	38.0	45.5	47.6
6		12 + 18	20.0	38.8	48.0	48.3
7		15 + 18	19.6	38.5	47.3	48.5
8	P60F40S0M1.2N12	—	15.2	26.5	41.7	45.8
9		9 + 12	15.5	27.5	43.8	47.3
10		9 + 15	16.0	28.8	45.9	49.0
11		9 + 18	16.8	29.8	48.0	52.0
12		12 + 15	16.0	28.3	46.5	50.1
13		12 + 18	16.5	29.3	47.1	50.8
14		15 + 18	16.3	29.1	46.3	50.4
15	P60F0S40M1.2N14	—	24.2	35.2	40.8	44.6
16		9 + 12	25.7	37.5	42.4	46.0
17		9 + 15	26.7	39.3	44.5	46.8
18		9 + 18	27.3	40.2	46.1	48.2
19		12 + 15	25.8	38.5	43.0	46.5
20		12 + 18	27.3	39.8	45.3	47.3
21		15 + 18	27.1	39.5	45.0	47.0
22	P60F20S20M1.2N12	—	22.8	32.8	41.5	46.0
23		9 + 12	23.7	35.0	43.8	48.2
24		9 + 15	26.1	37.9	47.0	50.9
25		9 + 18	26.8	39.5	47.6	51.6
26		12 + 15	24.6	35.9	45.0	49.3
27		12 + 18	26.5	38.9	47.3	51.3
28		15 + 18	26.2	38.5	47.0	51.1

. The influence of fiber mixing on the compressive strengths of four types of geopolymer mortars, namely P₁₀₀F₀S₀M_1.4N₁₆, P₆₀F₄₀S₀M_1.2N₁₂, P₆₀F₀S₄₀M_1.2N₁₄, and P₆₀F₂₀S₂₀M_1.2N₁₂, at 3 d, 7 d, 14 d, and 28 d is shown in Figure 8a–d.

It can be observed that the hybrid mixing of polypropylene fibers of different lengths has a positive effect on improving the compressive strength of geopolymer mortar, as shown in Figure 8a–d. Moreover, the greater the length difference between the two kinds of fibers, the more significant the growth effect on the compressive strength. Among them, the hybrid mixing of 9 mm and 18 mm fibers has the best strengthening effect. From the performance of mortars with different ratios, in the geopolymers of P₆₀F₀S₄₀M_1.2N₁₄ and P₆₀F₂₀S₂₀M_1.2N₁₂, except for the two mixing ratios of 9 mm and 12 mm, and 12 mm and 15 mm, the compressive strength growth of the rest of the mixing methods is better than that of single-doping of 12 mm fiber. For the geopolymer mortars of P₁₀₀F₀S₀M_1.4N₁₆ and P₆₀F₄₀S₀M_1.2N₁₂, only when the fibers of 9 mm and 18 mm are mixed, the growth effect of compressive strength is better than that of single doping of 12 mm fibers. Specific to the growth data of each age period, the P₁₀₀F₀S₀M_1.4N₁₆ geopolymer mortar shows the most significant growth effect at 14 days. The compressive strengths at 3 days, 7 days, 14 days, and 28 days are increased by 10.8%, 11.5%, 13.8%, and 11.6%, respectively. The P₆₀F₄₀S₀M_1.2N₁₂ geopolymer mortar also showed the most prominent growth at 14 days. The compressive strengths at 3 days, 7 days, 14 days, and 28 days increased by 10.5%, 12.5%, 15.1%, and 13.5%, respectively. The best growth of P₆₀F₀S₄₀M_1.2N₁₄ geopolymer mortar occurred at 7 days. The compressive strengths at 3 days, 7 days, 14 days, and 28 days increased by 12.8%, 14.2%, 13.0%, and 8.1%, respectively. The geopolymer mortar of P₆₀F₂₀S₂₀M_1.2N₁₂ shows the most obvious growth effect at 14 days. The compressive strengths at 3 days, 7 days, 14 days, and 28 days are increased by 17.5%, 20.4%, 14.7%, and 12.2%, respectively.

3.2.3. Comparative Analysis of Flexural-to-Compressive Ratio

The influence of fiber mixing on the F/C ratios of four kinds of geopolymer mortars, namely P₁₀₀F₀S₀M_1.4N₁₆, P₆₀F₄₀S₀M_1.2N₁₂, P₆₀F₀S₄₀M_1.2N₁₄, and P₆₀F₂₀S₂₀M_1.2N₁₂, at 3 days, 7 days, 14 days, and 28 days is shown in Figure 9a–d. Analysis from the figure shows that the mixed addition of polypropylene fibers of different lengths has a positive effect on improving the toughness of geopolymer mortar. The specific manifestations vary depending on the type of mortar. After adding fibers to the P₁₀₀F₀S₀M_1.4N₁₆ and P₆₀F₄₀S₀M_1.2N₁₂ geopolymer mortars, the F/C ratio shows an upward trend with the increase in age. However, the growth rate gradually slows down, indicating that the improvement in their toughness exhibits the characteristic of long-term, slow improvement. The F/C ratios of geopolymer mortars P₆₀F₀S₄₀M_1.2N₁₄ and P₆₀F₂₀S₂₀M_1.2N₁₂ are significantly higher than those of the first two types of mortars. Moreover, their early F/C ratios are even higher. With age, they gradually decline and tend to stabilize. This phenomenon mainly stems from the fact that the improvement effect of fibers on the early flexural strength of mortar is more prominent. In contrast, the growth rate of the later compressive strength is relatively faster, resulting in a slight decline in the F/C ratio.

Comparing the effects of single doping and mixed doping methods, it can be seen that when only 9 mm and 18 mm fibers are mixed and doped, the toughness enhancement level reaches the same effect as single doping of 15 mm fibers. The improvement amplitudes of the other mixed doping methods are not significant, indicating that the overall improvement effect of mixed doping on toughness is limited.

3.3. Mechanical Properties of Fiber-Reinforced Geopolymer Mortar

By observing the SEM images of geopolymer mortar specimens with single-doped 15 mm polypropylene fibers and hybrid-doped 12 mm and 18 mm polypropylene fibers, the mechanism of fiber-reinforced geopolymer mortar was deeply understood, as shown in Figure 10 and Figure 11.

It can be clearly observed from Figure 7 and Figure 8 that a strong bonding effect exists between the polypropylene fiber and the geopolymer mortar. When the fiber is broken or pulled out, a large amount of geopolymer hydration products will adhere to its surface. This indicates that incorporating polypropylene fiber has a significant effect on improving the mortar’s strength.

For fibers of the same type, since the surface roughness is the same, their bonding performance with geopolymer mortar is mainly determined by the length. Therefore, in terms of enhancing the strength of geopolymer mortar, long fibers perform significantly better than short fibers. However, if the fibers are too long, agglomeration will occur during the mixing process, resulting in more bubbles forming in the slurry and thereby reducing the density of the geopolymer mortar, which leads to a decrease in strength. This also fully explains why the polypropylene fiber with a single length of 15 mm has the best reinforcement effect, as can be clearly seen from Figure 7, when the specimen is damaged, the fiber breaks rather than being pulled out. This phenomenon strongly proves that there is good adhesion between the fiber of this length and the geopolymer mortar.

Figure 8 visually shows that the 9 mm and 18 mm polypropylene fibers overlap with each other in the sand geopolymer mortar, forming a spatial network structure. There are numerous tiny cracks in the geopolymer mortar itself. In the initial stage of loading, the 9 mm-long fibers can effectively inhibit the development of cracks due to their relatively small spacing. As the external load continues to increase, the cracks gradually expand. At this time, all the 9 mm fibers are completely pulled out or broken and are therefore unable to continue restricting the crack expansion. However, the 18 mm fibers begin to take effect at this moment, further preventing the cracks from expanding until the geopolymer mortar is completely damaged and the fibers are completely pulled out or broken. Thus, it can be seen that during the entire process of the geopolymer mortar specimen’s stress, deformation, and failure, fibers of different lengths do not act alone but cooperate to produce a complementary effect. This also suggests that the greater the difference in fiber lengths, the more pronounced the strengthening effect after hybrid mixing.

Specifically, adding short fibers can reduce the bubble content in geopolymer mortar, improve its density, and simultaneously inhibit the expansion of early microcracks. Adding long fibers can enhance the bonding properties of geopolymer mortar and inhibit the development of macroscopic cracks in the later stages. Only by organically combining the two can we achieve complementary advantages and reach the best strengthening effect. Based on the comprehensive fiber reinforcement mechanism and from the perspective of improving the strength performance of geopolymer mortar, it is optimal to mix two types of fibers with lengths of 9 mm and 18 mm.

4. Conclusions

In this study, fly ash, slag, and metakaolin were used as raw materials, standard sand was used as aggregate, and water glass and NaOH were used as alkali activators. Polypropylene fibers were incorporated into the geopolymer mortar to enhance its mechanical properties. Study the influence of fiber length and mixing method on the flexural strength, compressive strength, and flexural-compressive ratio of geopolymer mortar, determine the optimal fiber length and mixing method, and combine SEM to analyze the mechanical property mechanism of fiber-reinforced geopolymer mortar. The main research results are as follows:

(1)

By analyzing the strength variation laws of geopolymer mortars with different fiber lengths and mixing methods and combining this with SEM images to investigate the mechanical property mechanism of fiber-reinforced mortars, both approaches have improved the mechanical properties of geopolymer mortars.

(2)

When 15 mm fibers are incorporated individually into the geopolymer mortar, the bridging effect provided by the longer fibers significantly enhances flexural strength and toughness, achieving optimal performance in these properties. In contrast, 12 mm fibers exhibit superior dispersion homogeneity during mixing, resulting in minimal disruption to the matrix structure and a relatively minor reduction in workability. This favorable distribution contributes to a more effective enhancement of the matrix’s compressive load-bearing capacity, thereby yielding the highest compressive strength.

(3)

When 9 mm and 18 mm fibers are incorporated into the geopolymer mortar, the combined fiber reinforcement effect enhances the mechanical properties, resulting in superior performance compared to geopolymer mortars containing only a single fiber type.

This study presents a preliminary investigation into the performance of fiber-reinforced ternary solid waste polymer mortar. Future work will focus on in-depth analyses of its durability and mesoscopic modeling.