Strain hardening geopolymer composite material based on natural fiber reinforcement and preparation method thereof

Abstract

The invention belongs to the technical field of building materials, and particularly relates to a geopolymer composite material and a preparation method and application thereof. The geopolymer composite material provided by the invention comprises a geopolymer matrix and fibers doped in the geopolymer matrix, the volume mixing amount of the fibers in the geopolymer composite material is 1.5-2%; the fibers comprise modified natural fibers and synthetic fibers, and the volume ratio of the modified natural fibers to the synthetic fibers is (0.5-1): (0.5-1); the preparation method of the modified natural fiber comprises the following steps: soaking natural fiber in a strong alkali solution, and modifying to obtain the modified natural fiber; the natural fibers comprise jute fibers and / or coconut fibers. According to the invention, the modified natural fiber and the synthetic fiber are compounded to form a mixed fiber reinforced system, so that the usage amount of the synthetic fiber is reduced; and by controlling the dosage ratio of the modified natural fibers to the synthetic fibers, the natural fibers play a role in skeleton reinforcement in the cracking stage, and the overall strength of the geopolymer composite material is improved.

Description

Technical Field

[0001] The present invention belongs to the technical field of building materials, and in particular relates to a geopolymer composite material and a preparation method and application thereof. Background Art

[0002] Geopolymers are widely used as cement substitutes due to their wide availability of raw materials, low energy consumption, strong chemical resistance, and excellent high-temperature resistance. However, their brittleness, poor deformation capacity, and tensile properties far inferior to their compressive properties still pose significant limitations in their application in structural engineering.

[0003] To overcome these challenges, researchers have introduced reinforcing fibers into geopolymer materials to develop strain-hardening engineered geopolymer composites (EGCs). EGCs can achieve strain-hardening behavior and multiple crack distribution during tensile stress, significantly improving their ductility, crack control, and durability. Currently, synthetic fibers, such as polyethylene (PE) and polypropylene (PP), are widely used as reinforcing fibers. These fibers exhibit excellent interfacial bonding with the geopolymer matrix, enabling stable fiber bridging and crack control. However, synthetic fibers have significant drawbacks: their production consumes significant amounts of non-renewable resources and energy, resulting in a relatively high price, which limits their application in large-scale engineering projects. Furthermore, synthetic fibers are non-degradable, making them difficult to dispose of after their service life. Therefore, finding low-cost, widely available, environmentally friendly, and reinforcing alternative fibers has become a hot topic and a challenge in current research.

[0004] Natural fibers, a renewable plant-based resource, not only possess high specific strength and modulus but are also biodegradable, inexpensive, and abundant, theoretically offering the potential to replace synthetic fibers. However, previous studies have shown that natural fibers face significant challenges in geopolymer matrices: they struggle to form stable bridging mechanisms with geopolymers, making strain hardening difficult to achieve. Summary of the Invention

[0005] In view of this, the present invention provides a geopolymer composite material, a preparation method and an application thereof. The present invention uses modified natural fibers to replace part of the synthetic fibers, so that the fibers have good dispersion and good bridging performance in the geopolymer matrix material, and the formed geopolymer composite material has good strain hardening ability.

[0006] In order to solve the above technical problems, the present invention provides a geopolymer composite material, comprising a geopolymer matrix and fibers doped in the geopolymer matrix; the volume content of the fibers in the geopolymer composite material is 1.5-2%;

[0007] The fibers include modified natural fibers and synthetic fibers, and the volume ratio of the modified natural fibers to the synthetic fibers is 0.5-1:0.5-1;

[0008] The preparation method of the modified natural fiber comprises the following steps: soaking the natural fiber in a strong alkaline solution to modify the natural fiber to obtain the modified natural fiber; the natural fiber comprises jute fiber and / or coconut shell fiber.

[0009] Preferably, the length of the natural fiber is 12 to 18 mm;

[0010] The aspect ratio of the natural fiber is 130-330.

[0011] Preferably, the strong alkaline solution includes sodium hydroxide solution, sodium carbonate solution or potassium hydroxide solution;

[0012] The mass concentration of the strong alkaline solution is 5 to 8%;

[0013] The soaking time is 3 to 5 hours.

[0014] Preferably, the raw materials for preparing the geopolymer matrix include a gelling material, an activator, a water reducing agent, silica sand and water;

[0015] The mass ratio of the activator to the cementitious material is 0.32-0.36:1, the mass ratio of the water reducer to the cementitious material is 0.008-0.012:1, the mass ratio of the silica sand to the cementitious material is 0.18-0.22:1, and the mass ratio of the water to the cementitious material is 0.18-0.22:1.

[0016] Preferably, the cementitious material comprises fly ash and slag, and the fly ash accounts for 65-75% by mass of the cementitious material;

[0017] The activator includes sodium hydroxide and sodium silicate, and the modulus of the activator is 1.4 to 1.6;

[0018] The water reducer includes a polycarboxylate water reducer.

[0019] Preferably, the synthetic fibers include polyethylene fibers, polyvinyl alcohol fibers or polypropylene fibers.

[0020] The present invention also provides a method for preparing the geopolymer composite material described in the above technical solution, comprising the following steps:

[0021] Mix the raw materials for preparing the geopolymer matrix to obtain a slurry

[0022] Modified natural fibers and synthetic fibers are dispersed in the slurry to obtain the geopolymer composite material.

[0023] Preferably, the method for preparing the slurry comprises the following steps: first mixing a gelling material and silica sand to obtain a first mixture;

[0024] performing a second mixing of the first mixture and the exciter to obtain a second mixture;

[0025] The second mixture, the water reducing agent and water are mixed for the third time to obtain the slurry.

[0026] Preferably, the dispersing comprises sequentially performing low-speed stirring and high-speed stirring;

[0027] The rotation speed of the low-speed stirring is 90-110 rpm, and the time of the low-speed stirring is 0.8-1.2 min;

[0028] The rotation speed of the high-speed stirring is 130-140 rpm, and the time of the high-speed stirring is 2.5-3.5 min.

[0029] The present invention also provides the use of the geopolymer composite material described in the above technical solution or the geopolymer composite material prepared by the preparation method described in the above technical solution in civil engineering structures.

[0030] The present invention provides a geopolymer composite material, comprising a geopolymer matrix and fibers doped in the geopolymer matrix; the volume content of the fibers in the geopolymer composite material is 1.5-2%; the fibers comprise modified natural fibers and synthetic fibers, and the volume ratio of the modified natural fibers to the synthetic fibers is 0.5-1:0.5-1; the preparation method of the modified natural fibers comprises the following steps: soaking the natural fibers in a strong alkaline solution to modify them and obtain the modified natural fibers; the natural fibers comprise jute fibers and / or coconut shell fibers. The present invention modifies the natural fibers to improve the uniform distribution and good interface bonding of the modified natural fibers in the geopolymer matrix, effectively avoiding failure mechanisms such as natural fiber agglomeration and breakage. The present invention combines modified natural fibers and synthetic fibers to form a mixed fiber reinforcement system, reducing the amount of synthetic fibers used and significantly improving the economic efficiency and environmental friendliness of the geopolymer composite material. The present invention controls the dosage ratio of modified natural fibers and synthetic fibers so that the natural fibers play a skeleton reinforcement role in the cracking stage and improve the overall strength of the geopolymer composite material, while the synthetic fibers provide a stable bridging effect in the crack propagation stage, thereby achieving the coordinated development of multi-crack control and strain hardening behavior. BRIEF DESCRIPTION OF THE DRAWINGS

[0031] Figure 1 The following is a schematic diagram of the process for preparing modified natural fibers using jute fiber as an example;

[0032] Figure 21 is a bar chart comparing the fluidity and setting time of the geopolymer composite materials prepared in Examples 1 to 3 and Comparative Examples 1 to 3;

[0033] Figure 3 This is a bar chart comparing the compressive strength of the geopolymer composite materials prepared in Examples 1 to 3 and Comparative Examples 1 to 3 after being molded and cured for different times;

[0034] Figure 4 The compression failure mode and internal fiber stress diagram of the geopolymer composite material after 28 days of molding and curing;

[0035] Figure 5 This is a comparison chart of the tensile properties of the geopolymer composite materials prepared in Examples 1 to 3 and Comparative Examples 1 to 3;

[0036] Figure 6 This is a diagram of the test process of a single fiber pulling test;

[0037] Figure 7 The stress-displacement curves of different fibers during the single fiber pulling test;

[0038] Figure 8 The SEM and BSEM images of the single fiber before and after the single fiber pulling test;

[0039] Figure 9 The results of the interface performance between different fibers and geopolymer matrix are shown;

[0040] Figure 10 3D images and roughness comparison charts of different fibers after pulling tests;

[0041] Figure 11 The three-dimensional pore structure and crack morphology of the geopolymer composite materials prepared in Examples 1 to 3 and Comparative Examples 1 and 3 are shown;

[0042] Figure 12 The bar graph shows the effect of natural fiber replacement amount and type on the porosity of geopolymer composites.

[0043] Figure 13 Graph showing the environmental and economic benefit analysis results of the geopolymer composite materials prepared in Examples 1 to 3 and Comparative Examples 1 to 3;

[0044] Figure 14 This is a comparison chart of the engineering performance, material cost and environmental impact of the geopolymer composite materials prepared in Examples 1 to 3 and Comparative Examples 1 to 3. DETAILED DESCRIPTION

[0045] The present invention provides a geopolymer composite material comprising a geopolymer matrix and fibers doped in the geopolymer matrix; the fibers are incorporated into the geopolymer composite material in an amount of 1.5 to 2% by volume. In the present invention, an excessive amount of fibers (greater than 2%) will hinder the uniform distribution of the fibers within the matrix and reduce the engineering properties of the oligomer composite material.

[0046] In the present invention, the fibers include modified natural fibers and synthetic fibers, and the volume ratio of the modified natural fibers to the synthetic fibers is 0.5-1:0.5-1, and may be specifically 0.5:1 or 1:0.5.

[0047] In the present invention, the synthetic fiber may include polyethylene fiber (PE fiber), polyvinyl alcohol fiber (PVA fiber) or polypropylene fiber (PP fiber); the aspect ratio of the synthetic fiber may be 700 to 1000, and may be specifically 750.

[0048] In the present invention, the method for preparing the modified natural fiber comprises the following steps: soaking the natural fiber in a strong alkaline solution to modify the natural fiber, thereby obtaining the modified natural fiber; the natural fiber comprises jute fiber and / or coconut shell fiber, and can be specifically jute fiber or coconut shell fiber. As a specific embodiment of the present invention, the length of the natural fiber can be 12 to 18 mm, and can be specifically 12 mm, 13 mm, 14 mm, 15 mm, 16 mm, 17 mm, or 18 mm; the aspect ratio of the natural fiber can be 130 to 330, and can also be 200 to 300.

[0049] As a specific embodiment of the present invention, the strong alkaline solution may include a sodium hydroxide solution, a sodium carbonate solution, or a potassium hydroxide solution; the mass concentration of the strong alkaline solution may be 5-8%, specifically 5%, 6%, 7%, or 8%; the present invention has no particular limitation on the amount of the strong alkaline solution used, as long as it can completely immerse the natural fiber. As a specific embodiment of the present invention, the soaking time may be 3-5 hours, specifically 3 hours, 4 hours, or 5 hours; the soaking temperature may be room temperature, which may be 20-35°C, or 25-30°C.

[0050] As a specific embodiment of the present invention, the roughness of the jute fiber after modification is 17.88-27.31 μm, and the roughness of the coconut shell fiber after modification is 26.15-37.83 μm.

[0051] The present invention can remove lignin, wax and impurities (such as pectin, inorganic salts, natural pigments, protein residues, etc.) on the surface of natural fibers through modification, so that more microgrooves and active groups (such as hydroxyl groups) appear on the surface of natural fibers, increase the surface roughness of natural fibers, and enhance the mechanical intercalation and chemical bonding ability of the interface between natural fibers and geopolymer matrix (active groups can form hydrogen bonds with the geopolymer matrix); thereby effectively enhancing the bonding strength with the geopolymer matrix and improving the mechanical properties of the interface weakened zone, thereby facilitating the bridging and expansion control of cracks.

[0052] As a specific embodiment of the present invention, the modification may further include: washing the modified natural fiber until the pH value is neutral and then drying to obtain the modified natural fiber. As a specific embodiment of the present invention, the water used for washing may be deionized water. The present invention has no particular requirements for the number of washings, as long as the pH value is neutral. As a specific embodiment of the present invention, the drying may be air-drying, and the air-drying time may be 22 to 26 hours, specifically 24 hours.

[0053] Figure 1 The figure is a schematic diagram of the process for preparing modified natural fibers using jute fiber as an example. Specifically, the jute fiber is soaked in a sodium hydroxide solution, washed with water until the pH value is neutral, and dried to obtain the modified jute fiber.

[0054] As a specific embodiment of the present invention, the raw materials for preparing the geopolymer matrix may include a cementitious material, an activator, a water reducer, silica sand and water; the cementitious material may include fly ash and slag, and the fly ash may account for 65-75% of the cementitious material by mass, and may be specifically 68%, 70% or 72%; the activator may include sodium hydroxide and sodium silicate, and the modulus of the activator may be 1.4-1.6, and may be specifically 1.4, 1.5 or 1.6; the water reducer A polycarboxylate water reducer may be included; the mass ratio of the activator to the cementitious material may be 0.32-0.36:1, specifically 0.34:1; the mass ratio of the water reducer to the cementitious material may be 0.008-0.012:1, specifically 0.01:1; the mass ratio of the silica sand to the cementitious material may be 0.18-0.22:1, specifically 0.2:1; and the mass ratio of the water to the cementitious material may be 0.18-0.22:1, specifically 0.2:1. In the present invention, the fly ash and slag serve as silicon and aluminum sources.

[0055] As a specific embodiment of the present invention, the porosity of the geopolymer composite material can be 1.89% to 3%, specifically 1.89%, 2.26%, 2.48%, or 2.81%. The geopolymer composite material prepared by using modified natural fibers to replace some synthetic fibers has a lower porosity, which helps improve the cracking strength of EGC. This invention achieves effective synergy between natural fibers and geopolymers, overcoming the bottleneck of poor strain hardening performance of traditional natural fiber-reinforced geopolymers.

[0056] The present invention also provides a method for preparing the geopolymer composite material described in the above technical solution, comprising the following steps:

[0057] Mixing raw materials for preparing a geopolymer matrix to obtain a slurry;

[0058] Modified natural fibers and synthetic fibers are dispersed in the slurry to obtain the geopolymer composite material.

[0059] The present invention mixes raw materials for preparing a geopolymer matrix to obtain a slurry. Specifically, a gelling material, silica sand, an activator, a water reducer, and water are mixed to obtain the slurry. As a specific embodiment of the present invention, the slurry preparation method may include the following steps:

[0060] performing a first mixing of the cementitious material and silica sand to obtain a first mixture;

[0061] performing a second mixing of the first mixture and the exciter to obtain a second mixture;

[0062] The second mixture, the water reducing agent and water are mixed for the third time to obtain the slurry.

[0063] As a specific embodiment of the present invention, the first mixing can be carried out under stirring conditions, the stirring speed can be 90-110 rpm, specifically 95 rpm, 100 rpm or 105 rpm; the stirring time can be 2.5-3.5 min, specifically 3 min.

[0064] As a specific embodiment of the present invention, the second mixing can be carried out under stirring conditions, the stirring speed can be 90-110 rpm, specifically 95 rpm, 100 rpm or 105 rpm; the stirring time can be 1.8-2.2 min, specifically 2 min.

[0065] As a specific embodiment of the present invention, the third mixing can be carried out under stirring conditions, the stirring speed can be 90-110 rpm, specifically 95 rpm, 100 rpm or 105 rpm; the stirring time can be 0.8-1.2 min, specifically 1 min.

[0066] After obtaining the slurry, the present invention disperses the modified natural fiber and the synthetic fiber in the slurry to obtain the geopolymer composite material. As a specific embodiment of the present invention, the dispersion may include low-speed stirring and high-speed stirring performed sequentially; the rotation speed of the low-speed stirring may be 90 to 110 rpm, specifically 95 rpm, 100 rpm or 105 rpm; the time of the low-speed stirring may be 0.8 to 1.2 min, specifically 1 min; the rotation speed of the high-speed stirring may be 130 to 140 rpm, specifically 135 rpm or 138 rpm; the time of the high-speed stirring may be 2.5 to 3.5 min, specifically 3 min. The present invention can evenly disperse the fibers in the slurry through two-step dispersion to prevent the occurrence of agglomeration.

[0067] In a specific embodiment of the present invention, after obtaining the geopolymer composite material, the process may further include: molding the geopolymer composite material; the molding may be performed in a mold. The present invention has no particular requirements for the size and shape of the mold; conventional molds in the art may be used. The molding may specifically include placing the geopolymer composite material in the mold, followed by defoaming, allowing it to rest, and demolding. In a specific embodiment of the present invention, the defoaming may be performed by oscillating on a vibration table; the resting time may be 22 to 26 hours, and more specifically 24 hours.

[0068] The present invention can also place the demoulded product in a sealed environment for curing. The temperature of the curing environment can be 18-22°C, specifically 20°C; the relative humidity of the curing environment can be 90-100%, specifically 95%; the curing time can be 7-30 days, specifically 7 days or 28 days.

[0069] The present invention uses jute fiber and coconut shell fiber as the main reinforcing materials, adopts an alkaline pretreatment process to improve the interfacial bonding performance between the natural fiber and the geopolymer matrix, and prepares a geopolymer composite material with multi-crack distribution characteristics and strain hardening performance through standardized mixing and molding processes.

[0070] The present invention improves the environmental friendliness of geopolymer composite materials through the synergistic reinforcement mechanism of modified natural fibers and synthetic fibers, while achieving ductility and multi-crack control performance comparable to or even better than traditional EGC, thereby effectively solving the technical problems of insufficient strain hardening capacity and unstable bridging performance of existing natural fiber-reinforced geopolymer materials.

[0071] The present invention also provides for the use of the geopolymer composite material described in the above technical solution, or the geopolymer composite material prepared by the preparation method described in the above technical solution, in civil engineering structures. As a specific embodiment of the present invention, the civil engineering structure may include a seismic-resistant component, a protective shell structure, or a lightweight enclosure structure. The geopolymer composite material provided by the present invention is suitable for a wide range of engineering scenarios, particularly in the fields of green building, rural construction, and low-carbon infrastructure, and has excellent prospects for promotion and application.

[0072] The geopolymer composite system provided by the present invention not only exhibits good crack resistance and crack strength in the cracking stage, but also exhibits obvious strain hardening behavior and multi-crack pattern in the crack propagation stage. In particular, alkali treatment improves the fiber-matrix interface bonding strength, enhances the crack bridging stability, and reduces the critical load of fiber debonding and slippage, thereby achieving a more durable and stable tensile deformation capacity in the geopolymer composite material. Compared with natural fiber reinforcement systems that have not been treated with alkali modification, the present invention greatly improves the mechanical homogeneity and ductility of geopolymer composite materials, and significantly improves the long-standing problems of natural fiber reinforced geopolymer composite materials such as large performance fluctuations and unstable crack control.

[0073] In order to further illustrate the present invention, the technical solutions provided by the present invention are described in detail below in conjunction with the embodiments, but they should not be construed as limiting the scope of protection of the present invention.

[0074] Example 1

[0075] Jute fibers with a length of 18 mm and an aspect ratio of 224 were soaked in a sodium hydroxide solution with a mass concentration of 8% at 25° C. for 4 hours, the soaked jute fibers were washed with deionized water until the pH value was neutral, and placed in a fume hood for natural ventilation for 24 hours to obtain modified jute fibers;

[0076] Fly ash and slag are mixed in a mass ratio of 7:3 to obtain a cementitious material; sodium hydroxide and sodium silicate are mixed to form an activator with a modulus of 1.4;

[0077] The cementitious material and silica sand were stirred and mixed at a speed of 100 rpm for 3 minutes, an activator was added and stirred and mixed at a speed of 100 rpm for 2 minutes, and then a polycarboxylate water reducer with a solid content of 40% and water were added and stirred and mixed at a speed of 100 rpm for 1 minute to obtain a slurry; wherein the mass ratio of silica sand to cementitious material is 0.2:1, the mass ratio of the activator to cementitious material is 0.34:1, the mass ratio of the polycarboxylate water reducer to cementitious material is 0.01:1, and the mass ratio of water to cementitious material is 0.2:1.

[0078] Modified jute fiber and polyethylene fiber with an aspect ratio of 750 were added to a slurry and stirred at 100 rpm for 1 minute, and then stirred at 135 rpm for 3 minutes to obtain a geopolymer composite material, recorded as J-0.5%; wherein the volume ratio of the modified jute fiber to the polyethylene fiber was 0.5:1, and the volume fraction of the modified jute fiber and the polyethylene fiber in the formed geopolymer composite material was 1.5%.

[0079] Example 2

[0080] A geopolymer composite material was prepared according to the method of Example 1, except that the volume ratio of the modified jute fiber and the polyethylene fiber was adjusted from 0.5:1 to 1:0.5. The prepared geopolymer composite material was recorded as J-1.0%.

[0081] Example 3

[0082] A geopolymer composite material was prepared according to the method of Example 1, except that jute fiber was replaced with coconut shell fiber. The prepared geopolymer composite material was recorded as C-0.5%.

[0083] Comparative Example 1

[0084] A geopolymer composite material was prepared according to the method of Example 1, except that jute fiber was replaced with ramie fiber. The prepared geopolymer composite material was recorded as R-0.5%.

[0085] Comparative Example 2

[0086] A geopolymer composite material was prepared according to the method of Example 1, except that modified jute fiber was directly added to the slurry without adding polyethylene fiber. The volume fraction of the modified jute fiber in the geopolymer composite material was 1.5%. The prepared geopolymer composite material was recorded as J-1.5%.

[0087] Comparative Example 3

[0088] A geopolymer composite material was prepared according to the method of Example 1, except that polyethylene fiber was directly added to the slurry without adding modified jute fiber. The volume fraction of polyethylene fiber in the geopolymer composite material was 1.5%. The prepared geopolymer was recorded as J-0%.

[0089] The physical and chemical properties of the natural fibers and polyethylene fibers selected in the examples were tested, and the results are listed in Tables 1 and 2.

[0090] 1.Physical and chemical properties test

[0091] Table 1 Physical properties of polyethylene fibers and modified natural fibers

[0092]

[0093]

[0094] Table 2 Chemical composition and properties of modified natural fibers

[0095] Properties / fiber types Modified ramie fiber Modified jute fiber Modified coconut shell fiber Cellulose (%) 83.14 72.20 50.54 Hemicellulose (%) 0.90 18.04 15.16 Lignin (%) 14.52 6.56 31.36

[0096] 2. Flowability test

[0097] The fluidity test was performed on the geopolymer composite materials prepared in Examples 1 to 3 and Comparative Examples 1 to 3, and the diffusion diameter (mm) of the samples was measured after 25 bounces on the jumping table. The results are listed in Table 3. The initial setting time and final setting time of the geopolymer composite materials prepared in Examples 1 to 3 and Comparative Examples 1 to 3 were tested using a structural setting time measuring instrument, and the results are listed in Table 3.

[0098] Table 3 Mobility and setting time of geopolymer composite materials prepared in Examples 1 to 3 and Comparative Examples 1 to 3

[0099]

[0100] According to Table 3, a bar chart comparing the fluidity and setting time of the geopolymer composite materials prepared in Examples 1 to 3 and Comparative Examples 1 to 3 is drawn, as shown in FIG. Figure 2 As shown, (a) is a fluidity bar graph comparison chart, and (b) is a setting time bar graph comparison chart.

[0101] Depend on Figure 2 In (a), it can be seen that the diffusion diameter of EGC shows an increasing trend with the increase of modified jute fiber content, reaching a maximum value of 205 mm at a fiber dosage of 1.5%, which is 18% higher than the mixture without natural fiber (174 mm). This shows that the partial replacement of polyethylene fiber with modified jute fiber significantly improves the fluidity of the EGC mixture. The improvement in fluidity can be attributed to the reduction of yield stress caused by the fiber contact network inside the EGC matrix, and the reduction of fiber aspect ratio can further weaken this stress. The aspect ratio of polyethylene fiber (750) is 3.3 times that of jute fiber, resulting in an increase in the overall yield stress. When the modified natural fiber addition amount is 0.5%, the maximum spreading diameter of C-0.5% is 185.8 mm, which is 4.7% and 2.5% higher than R-0.5% (177.5 mm) and J-0.5% (181.2 mm), respectively. This can be attributed to the lower aspect ratio of coconut shell fiber (137) than that of ramie fiber (335) and jute fiber (224).

[0102] Figure 2(b) shows that the setting time of EGC increases with increasing modified jute fiber content. Compared to EGC without modified natural fibers, the EGC mixture containing 1.5% modified jute fiber increased the initial setting time by 17.9% (37.3 minutes) and the final setting time by 33.6% (54.3 minutes). Regardless of the amount or type of modified natural fiber, the presence of modified natural fibers delays the setting time of EGC. This is because the cellulose in modified natural fibers is composed of cyclic D-glucose units connected by β-1,4-glycosidic bonds. Under alkaline conditions, glucose is degraded to form organic acids, which lower the pH of the alkali-activated solution and delay the dissolution process of the geopolymer reaction. The insoluble salts produced by acid-base neutralization coat the surface of the cementitious material, thereby reducing the reaction rate. When the modified natural fiber content was 0.5%, the EGC mixture containing modified ramie fiber exhibited the longest setting time.

[0103] 3. Compressive strength test

[0104] The geopolymer composite materials prepared in Examples 1 to 3 and Comparative Examples 1 to 3 were respectively poured into a cubic mold with a size of 50×50×50 mm, vibrated on a vibration table for defoaming, and then allowed to stand for 24 hours. The demolded products were placed in a sealed environment with a temperature of 20±2°C and a relative humidity of 95±5% for curing for 7 days or 28 days. The compressive strength of the samples was tested using a universal testing machine. The results are listed in Table 4.

[0105] Table 4 Compressive strength of geopolymer composite materials prepared in Examples 1 to 3 and Comparative Examples 1 to 3

[0106] sample Compressive strength (7d) Compressive strength (28d) J-0％ 62.6 86.7 J-0.5％ 62.2 85.4 J-1.0％ 58.9 79.3 J-1.5％ 55.5 70.5 R-0.5％ 62.0 80.7 C-0.5％ 56.5 76.9

[0107] According to Table 4, a columnar comparison chart of the compressive strength of the geopolymer composite materials prepared in Examples 1 to 3 and Comparative Examples 1 to 3 after different curing times is drawn, as shown in FIG. Figure 3 shown.

[0108] Depend on Figure 3 It can be seen that when the modified natural fiber replacement content is fixed, J-0.5% exhibits the highest compressive strength (85.4 MPa) at 28 days, which is 5.8% and 11.0% higher than R-0.5% (80.7 MPa) and C-0.5% (76.9 MPa), respectively.

[0109] The geopolymer composite material was tested after 28 days of curing, and the compression failure mode of the geopolymer composite material after 28 days and the internal fiber stress diagram were obtained, as shown in the figure below. Figure 4 As shown, Figure 4(a) to (f) are schematic diagrams of geopolymer composites under compression failure mode, and (g) is a schematic diagram of the behavior of fibers inside the geopolymer matrix under pressure.

[0110] Figure 4 Figures (a) to (f) show the differences in fracture modes of geopolymer composites under compression. Due to the bridging effect of the fibers, no obvious cracking was observed on the surface of all EGC samples. The main cracks were mainly concentrated at the interface parallel to the longitudinal axis. After the main cracks were formed, the damage extended from both sides to the center. The compressive strength of all samples at 28 days was between 70 MPa and 85 MPa, meeting the strength requirements of most structural engineering applications. J-0.5% achieved a compressive strength of 85.4 MPa at 28 days, showing better compressive performance than most EGC / ECC samples.

[0111] Depend on Figure 4 It can be seen that the modified jute fiber with a higher elastic modulus enables the EGC sample to effectively transmit and evenly distribute the internal concentrated stress under compression load. Figure 4 As shown in Figure g, this mechanism effectively prevents local premature cracking caused by stress concentration. Therefore, the compressive strength of EGC is positively correlated with the fiber elastic modulus.

[0112] 4. Tensile properties test

[0113] The geopolymer composite materials prepared in Examples 1 to 3 and Comparative Examples 1 to 3 were respectively injected into a dog-bone mold, vibrated on a vibration table for defoaming, and then allowed to stand for 24 hours. The demolded products were placed in a sealed environment with a temperature of 20±2°C and a relative humidity of 95±5% for curing for 7 days or 28 days. The samples were tensile tested using a tensile testing machine, and the results are listed in Table 5.

[0114] Table 5 Tensile properties of geopolymer composite materials prepared in Examples 1 to 3 and Comparative Examples 1 to 3

[0115]

[0116] According to Table 5, a comparison chart of the tensile properties of the geopolymer composite materials prepared in Examples 1 to 3 and Comparative Examples 1 to 3 is drawn, as shown in FIG. Figure 5 shown.

[0117] Combined with Table 5 and Figure 5It can be seen that the tensile strength and strain capacity of geopolymer composites decrease with decreasing polyethylene fiber content, consistent with the changes in compressive properties. Due to the inherent internal defects and structural irregularities of natural fibers, their fiber bridging efficiency decreases significantly. For example, when the modified jute fiber content increases to 1.5%, the 28-day tensile strength decreases by 14.2% to 41.1% compared to the baseline (J-0%). The diameter of modified jute fibers (80.2 μm) is much larger than that of PE fibers (24 μm), resulting in a decrease in fiber bridging efficiency. When the replacement ratio exceeds 1 / 3 by volume, the tensile strain capacity deteriorates sharply (for example, J-1% decreases by 90% compared to J-0.5%). Furthermore, the performance of the modified natural fibers varies significantly. Modified coconut husk fiber (C-0.5%) performs best due to its high elongation, with a 28-day tensile strain of 7.1%, significantly outperforming modified ramie fiber (R-0.5%), 3.3%, and modified jute fiber (J-0.5%), 6.5%. Therefore, the replacement ratio of PE fibers needs to be strictly controlled to ensure that the tensile properties of the EGC mixture meet engineering requirements.

[0118] 5. Test of interface bonding performance

[0119] Single fiber drawing test: Prepare the slurry according to the method of Example 1; place a single polyethylene fiber (PE), modified jute fiber (J), modified coconut shell fiber (C) and modified ramie fiber (R) in a single fiber drawing mold in advance, and pour the prepared slurry into the mold. After demoulding, place the sample in a curing room (temperature: 20±2℃ and humidity: 95±5%) for curing for 28 days. After curing, use a micro universal testing machine for testing. The testing machine is equipped with a high-precision sensor (measuring range: 0-20N, accuracy: 1 / 10,000). The test process is as follows: Figure 6 The sample size is 20×20×3mm and the fiber embedding depth is 3mm. Each type of fiber was tested 12 times to ensure the accuracy of the results. The measured stress-displacement curve is shown in Figure 7 As shown, (a) is the test result of polyethylene fiber, (b) is the test result of jute fiber, (c) is the test result of coconut shell fiber, and (d) is the test result of ramie fiber.

[0120] Figure 7 The results show that in single-fiber pullout tests, the free end of the fiber deforms under the applied tensile load, resulting in an increase in the pullout length. Furthermore, after removing excess fiber from the embedded end, the fiber tip may not be properly aligned with the sample surface, resulting in a slight bulge. As a result, the final pullout length of the fiber slightly exceeds the original embedment depth by 3 mm.

[0121] Figure 7(d) shows that the modified ramie fiber breaks during the drag adhesion stage because its tensile strength is lower than the bond strength with the matrix. In contrast, the curves for the other three fibers show three stages: linear elasticity, slip hardening, and pullout. The interfacial bonding between the modified natural fiber and the matrix relies on chemical and frictional bonding, similar to that of polyvinyl alcohol fibers. In contrast, due to the superhydrophobicity of polyethylene fibers, the bridging stress between the fiber and the matrix is ​​transmitted solely through frictional stress.

[0122] The single fiber before and after the drawing test was examined by scanning electron microscopy to obtain SEM and BSEM images, as shown in Figure 2. Figure 8 As shown; (a) to (c) are the results of polyethylene fiber, (d) to (f) are the results of modified jute fiber, and (g) to (i) are the results of modified coconut shell fiber; (a), (d) and (g) are SEM images of single fibers before drawing, (b), (e) and (h) are SEM images of single fibers after drawing, and (c), (f) and (i) are BSEM images of the cross-section of the sample after drawing. Figure 8 The microscopic morphology of the fibers before and after single-fiber pullout testing is shown. Compared to modified jute and coconut husk fibers, polyethylene fibers retained more matrix residue after pullout, indicating a stronger mechanical engagement with the matrix, thereby enhancing the frictional bond strength at the fiber-matrix interface. This high frictional bond strength can be attributed to the polyethylene fibers' uniform surface and excellent mechanical properties, which ensure more stable frictional resistance. Figure 8 (c) shows that the polyethylene fibers are tightly bonded to the matrix, with no significant interfacial delamination. In contrast, the modified natural fibers exhibit significant interfacial delamination after tensile failure, with delamination widths of 2.4 μm for the modified jute fibers and 4.8 μm for the modified coconut shell fibers. Therefore, the polyethylene fibers maintain better interfacial stability under tensile loading, facilitating more efficient stress transfer. This stable interfacial bonding not only improves the load-bearing capacity of the composite but also increases energy dissipation during crack propagation, a key prerequisite for achieving strain-hardening behavior.

[0123]

[0124] Among them, G d : interface chemical bonding strength; τ0: friction bonding strength; β: sliding hardening coefficient; P a : the maximum force generated when the debonding reaches the entire length of the embedded fiber; P b : The force generated when the load is suddenly dropped but before sliding begins; P max : Peak load; E f : elastic modulus of fiber; d f : fiber diameter; L e : embedded length of the fiber; ΔS′: slip displacement of the fiber.

[0125] The fiber-matrix interface parameters are calculated according to formulas 1 to 3, and the results are as follows: Figure 9 As shown in the figure, (a) is a comparative bar graph of frictional bonding stress of different fibers, (b) is a comparative bar graph of slip hardening coefficient of different fibers, (c) is a comparative bar graph of interfacial bonding strength of different fibers, and (d) is the debonding width of jute fiber and coconut shell fiber from the matrix after tensile failure. The specific results obtained by calculation are listed in Table 6.

[0126] Table 6 Interface parameters between different fibers and geopolymer matrix

[0127]

[0128] Note: In Table 6, “-” indicates not tested. Due to the superhydrophobicity of PE fiber, the bridging stress between it and the matrix is ​​only transmitted through friction stress, so the chemical bonding strength between the fiber and the matrix can be ignored.

[0129] Table 6 shows the fiber-matrix interface parameters obtained through single-fiber pullout tests. Because the modified ramie fibers fractured during debonding, only polyethylene fibers, modified jute fibers, and modified coconut fibers were analyzed. The slip hardening coefficient (β) of the natural fibers ranged from 0.045 to 0.13, which is 2.5 to 7.2 times that of polyethylene fibers (0.018).

[0130] Figure 9 Panel (a) shows that the friction bond strength (τ0) of polyethylene fiber is 2.8 MPa, which is 24.8% and 60.9% higher than that of modified jute fiber (2.13 MPa) and modified coconut shell fiber (1.1 MPa), respectively. This trend is consistent with the change in the tensile strength of the corresponding EGC blends, where the tensile strength of J-0% (7.6 MPa) is 15.2% and 24.6% higher than that of J-0.5% (6.6 MPa) and C-0.5% (6.1 MPa), respectively. Higher friction bond strength helps improve the overall load-bearing capacity of the composite.

[0131] Unlike modified natural fibers, polyethylene fibers lack reactive functional groups (such as hydroxyl and carboxyl groups), resulting in minimal chemical bonding strength. Figure 9 As shown in (c), the chemical bonding strength of the modified jute fiber is 6.896 J / m 2 , is a modified coconut shell fiber (2.081J / m 2) is 3.3 times that of modified jute fiber. This significant difference is attributed to the high crystallinity of cellulose in modified jute fiber (72.7%), and its abundant hydroxyl groups (-OH) on the surface form hydrogen bonds with the geopolymer matrix, thereby enhancing the chemical bonding strength. In comparison, the cellulose content in modified coconut shell fiber (50.5%) is about 70% of that in modified jute fiber. The lignin content in modified coconut shell fiber reaches 31.4%, which is 4.8 times that of modified jute fiber (6.6%). Excessive lignin covering the cellulose surface hinders the direct contact of hydroxyl groups with the geopolymer matrix, thereby reducing the interfacial bonding efficiency. Overall, the interface strengthening mechanism depends not only on the interface parameters (such as friction bonding stress, slip hardening behavior and chemical bonding strength), but also on the multi-scale synergistic effect between fiber morphology, chemical composition and matrix.

[0132] The irregular cross-sectional shape of natural fibers easily induces additional shear and bending stress concentration during the extraction process. To further analyze the interfacial properties of modified natural fibers, a 3D profilometer (VR-5000, KEYENCE) was used to collect images of polyethylene fibers, modified jute fibers, and modified coconut shell fibers. Figure 10 As shown in the figure, 3D imaging clearly reveals significant differences in fiber roughness; (a), (d) and (g) are selected test areas; (b), (e) and (h) are 3D imaging results; (c), (f) and (i) are quantitative roughness test results; the specific roughness test results are listed in Table 7.

[0133] Table 7 Surface roughness of different fibers

[0134]

[0135] Quantitative analysis of captured images using VR-5000 software revealed that the roughness of modified coconut shell fiber (21.6-43.6 μm) was significantly higher than that of modified jute fiber (13.1-31.4 μm) and polyethylene fiber (5.6-17.6 μm). This higher surface roughness facilitates the interlocking of matrix particles with the protrusions on the fiber surface, enhancing the mechanical locking effect.

[0136] The β value of modified coconut fiber (0.094) was significantly higher than that of modified jute fiber (0.05), further reflecting the variations in interfacial shear resistance due to differences in morphology and surface properties among natural fibers. This interfacial mechanism leads to greater fluctuations in interfacial shear resistance during slip, manifested as more pronounced stress fluctuations in the stress-strain curve. The improvement in initial cracking strength achieved by modified natural fibers can be attributed to improved mechanical engagement, while the relatively unstable interfacial shear resistance negatively impacts their ability to maintain effective stress transfer and crack bridging after cracking.

[0137] The progressive interfacial destruction (slip hardening) of the natural fibers in the present invention is more conducive to improving the toughness of the composite material than the brittle debonding of the PE fibers.

[0138] 6. Porosity properties of geopolymer composites

[0139] X-ray computed tomography (XCT) was used to scan the entire cross section of the dog-bone specimen (30×13×80mm gauge length) after the tensile test to improve the accuracy and representativeness of the test data. Voxel Studio Recon and Avizo software were used to reconstruct the specimen in three dimensions and quantitatively analyze the distribution of pores and cracks. The results are shown in Figure 2. Figure 11 The test results show that when the jute fiber content reaches 1.5%, the material exhibits a brittle fracture characteristic, resulting in complete destruction of the specimen structure and inability to perform a complete XCT scan. Figure 11 The three-dimensional pore structure and crack morphology of the geopolymer composite materials prepared in Examples 1 to 3 and Comparative Examples 1 and 3 are shown. Figure 12 This is a bar chart comparing the effects of natural fiber replacement amount and type on the porosity of geopolymer composites. The pore structure and crack morphology of the samples are clearly displayed through three-dimensional reconstruction. For the convenience of comparative analysis, only the pores with a volume greater than 0.1 mm are displayed. 3 The significant pores in the material are effectively revealed, which effectively reveals the influence of fiber content on the material failure mode and internal defects.

[0140] Depend on Figure 11 and Figure 12 It can be seen that the addition of natural fibers can significantly reduce the porosity of EGC. The porosity of its geopolymer composites (1.89% to 2.81%) is 10.9% to 40.0% lower than that of the control group (3.16%) without the addition of modified natural fibers. This is due to the fact that the modified natural fibers improve fluidity and prolong the setting time, thereby improving the compaction effect and promoting the discharge of bubbles. However, when the modified jute fiber content increases from 0.5% to 1.0%, the porosity increases by 9.6%, indicating that the fiber curl morphology may introduce air during the mixing process, offsetting the positive effect of natural fibers. It is worth noting that the modified ramie fiber (R-0.5%) has the lowest porosity (1.89%), highlighting the key influence of the modified natural fiber type and its physical properties on porosity.

[0141] 7. Cost and Environmental Impact Analysis of Geopolymer Composites

[0142] The environmental and economic benefits of partially replacing polyethylene fibers with modified natural fibers to prepare EGC were calculated by combining material cost with life cycle database data and specific material usage of geopolymer composites. In order to evaluate the cost-effectiveness of EGC in engineering applications, the material cost per unit compressive strength, unit tensile strength, and unit tensile capacity of all EGC mixtures was calculated (by dividing the cost by the corresponding strength or strain). The results of the analysis are as follows: Figure 13 and as shown in Table 8; Figure 13 (a) is a cost comparison chart of geopolymer composites with different fiber additions, (b) is a comparison chart of embodied carbon emissions, (c) is a comparison chart of embodied energy, and the comparison charts of engineering performance, material cost and environmental impact are shown in Figure 2. Figure 14 shown.

[0143] Table 8 Cost and environmental impact analysis results of geopolymer composite materials prepared in Examples 1 to 3 and Comparative Examples 1 to 3

[0144]

[0145] In Table 8, M45-ECC is an engineering cement-based composite material.

[0146] From Table 8 and Figure 13 It can be seen that the addition of modified natural fibers significantly improves the cost-effectiveness of EGC. Specifically, the unit compressive strength cost of EGC containing modified natural fibers and synthetic fibers is (2.1~5.3USD / m 3 / MPa) and unit tensile strength cost (32.4~67.3USD / m 3 / MPa) are lower than EGC containing only polyethylene fibers (unit compressive strength cost: 6.2USD / m 3 / MPa, unit tensile strength cost: 70.3USD / m 3 / MPa). As the jute fiber content increased (0.5-1.5%), the unit compressive strength cost and unit tensile strength cost decreased by 23-67% and 12-54%, respectively. J-0.5% was more cost-effective than R-0.5% and C-0.5%. Regardless of the type of modified natural fiber, the unit compressive strength cost of EGC containing only 0.5% modified natural fiber was lower than that of J-0%, while the other mixtures were more expensive. In summary, increasing the modified natural fiber content to 0.5% showed better cost-effectiveness than traditional EGC while maintaining or improving all key engineering properties.

[0147] This invention achieves a breakthrough in environmental and economic benefits by using modified natural fiber (NF) to partially replace PE fiber in geopolymer composites (EGC). Cost-effectiveness is significantly improved: material cost is reduced by 73%, and the unit performance cost advantage is outstanding; unit compressive strength cost (UCS): 2.1-5.3 USD / m 3 / MPa (23-67% lower than PE-EGC), unit tensile strength cost (UTS): 32.4-67.3USD / m 3 / MPa (reduced by 12-54% compared to PE-EGC), among which 0.5% modified jute fiber dosage has the highest comprehensive cost efficiency. Excellent environmental benefits: embodied carbon is reduced by 5% (minimum 305kg CO2·eq / m 3 ), the embodied energy is reduced by 26% (22% lower than the M45-ECC benchmark); the natural fiber production process only requires planting and harvesting (energy consumption 7.58~9.6MJ / m 3 ), while PE fibers rely on high energy consumption melt spinning process (73 ~ 116MJ / m 3 Sustainable development balance: Under the premise of maintaining engineering performance (compressive strength 70-86MPa), achieve "three reductions": material cost reduction > 70%, carbon emission intensity < 320kg CO2·eq / m 3 , production energy consumption <5000MJ / m 3 .

[0148] Although the above embodiment provides a detailed description of the present invention, it is only a part of the embodiments of the present invention, not all of the embodiments. People can also obtain other embodiments based on this embodiment without creativity, and these embodiments all fall within the scope of protection of the present invention.

Claims

1. A geopolymer composite material, characterized in that: The geopolymer composite material comprises a geopolymer matrix and fibers doped in the geopolymer matrix; the volume content of the fibers in the geopolymer composite material is 1.5-2%; The fibers include modified natural fibers and synthetic fibers, and the volume ratio of the modified natural fibers to the synthetic fibers is 0.5-1:0.5-1; The preparation method of the modified natural fiber comprises the following steps: soaking the natural fiber in a strong alkaline solution to modify the natural fiber to obtain the modified natural fiber; the natural fiber comprises jute fiber and / or coconut shell fiber.

2. The geopolymer composite material according to claim 1, characterized in that: The length of the natural fiber is 12 to 18 mm; The aspect ratio of the natural fiber is 130-330.

3. The geopolymer composite material according to claim 1 or 2, characterized in that: The strong alkaline solution includes sodium hydroxide solution, sodium carbonate solution or potassium hydroxide solution; The mass concentration of the strong alkaline solution is 5 to 8%; The soaking time is 3 to 5 hours.

4. The geopolymer composite material according to claim 1, characterized in that: The raw materials for preparing the geopolymer matrix include gelling material, activator, water reducing agent, silica sand and water; The mass ratio of the activator to the cementitious material is 0.32-0.36:1, the mass ratio of the water reducer to the cementitious material is 0.008-0.012:1, the mass ratio of the silica sand to the cementitious material is 0.18-0.22:1, and the mass ratio of the water to the cementitious material is 0.18-0.22:

1.

5. The geopolymer composite material according to claim 4, characterized in that: The cementitious material comprises fly ash and slag, wherein the fly ash accounts for 65-75% of the cementitious material by mass; The activator includes sodium hydroxide and sodium silicate, and the modulus of the activator is 1.4 to 1.6; The water reducer includes a polycarboxylate water reducer.

6. The geopolymer composite material according to claim 1, characterized in that: The synthetic fibers include polyethylene fibers, polyvinyl alcohol fibers or polypropylene fibers.

7. The method for preparing the geopolymer composite material according to any one of claims 1 to 6, characterized in that: The following steps are involved: Mixing raw materials for preparing a geopolymer matrix to obtain a slurry; Modified natural fibers and synthetic fibers are dispersed in the slurry to obtain the geopolymer composite material.

8. The preparation method according to claim 7, characterized in that: The slurry preparation method comprises the following steps: first mixing a gelling material and silica sand to obtain a first mixture; performing a second mixing of the first mixture and the exciter to obtain a second mixture; The second mixture, the water reducing agent and water are mixed for a third time to obtain the slurry.

9. The preparation method according to claim 7, characterized in that: The dispersion includes low-speed stirring and high-speed stirring performed sequentially; The rotation speed of the low-speed stirring is 90-110 rpm, and the time of the low-speed stirring is 0.8-1.2 min; The rotation speed of the high-speed stirring is 130-140 rpm, and the time of the high-speed stirring is 2.5-3.5 min.

10. Use of the geopolymer composite material according to any one of claims 1 to 6 or the geopolymer composite material prepared by the preparation method according to any one of claims 7 to 9 in civil engineering structures.

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