An environmentally friendly cellulose fiber-based composite material for shotcrete and a method for preparing the same

Abstract

This invention relates to the field of concrete reinforcing fiber technology, and discloses an environmentally friendly cellulose fiber-based composite material for shotcrete and its preparation method. The composite material consists of twisted fiber bundles, locally cured rigid rod-shaped non-water-soluble resin, multi-directional raised short fibers attached to its surface, and an overall water-soluble resin layer. When this composite material is mixed into concrete, it initially enhances workability through uniform dispersion, and later, the dense fiber network promotes cement hydration, significantly improving the overall performance and durability of concrete. It is particularly suitable for shotcrete, improving the alkali resistance, durability, ease of mixing, uniform dispersion, and workability of cellulose fibers in concrete.

Description

Technical Field

[0001] This invention relates to the field of concrete reinforcing fiber technology, and in particular to an environmentally friendly cellulose fiber-based composite material for shotcrete and its preparation method. Background Technology

[0002] Concrete reinforcing fibers are short-cut fiber materials that are incorporated into concrete / mortar to improve its mechanical properties and durability. They effectively inhibit micro-cracks in concrete caused by plastic shrinkage, drying shrinkage, and temperature changes, and improve mechanical strength. Cellulose fibers, on the other hand, are environmentally friendly materials processed from natural plants (such as bamboo pulp, cotton, and linen). Due to their high content of hydrophilic hydroxyl groups (-OH), they have strong water absorption properties, absorbing free water from fresh concrete and continuously providing the free water needed for the reaction during cement hydration, thus creating an effect of internal curing.

[0003] However, the water absorption of cellulose fibers can also reduce the fluidity of fresh concrete, making it more difficult to pump and spray, thus affecting construction efficiency. Furthermore, due to the low mechanical properties and initial modulus of cellulose fibers, and the insufficient fluidity of concrete, the fibers are unevenly dispersed, which can easily form voids and reduce the mechanical strength of the concrete after it is formed.

[0004] Therefore, it is necessary to provide an environmentally friendly cellulose fiber-based composite material for shotcrete and its preparation method to solve the above-mentioned technical problems. Summary of the Invention

[0005] This invention overcomes the shortcomings of the prior art and provides an environmentally friendly cellulose fiber-based composite material for shotcrete and its preparation method.

[0006] To achieve the above objectives, the technical solution adopted by the present invention is as follows: a method for preparing an environmentally friendly cellulose fiber-based composite material for shotcrete, comprising the following steps:

[0007] S1. Twist multiple long cellulose fibers to form a continuous fiber bundle;

[0008] S2. Coat a local area of ​​the fiber bundle surface with a non-water-soluble resin and partially cure it so that the surface of the non-water-soluble resin is in a sticky state that is not fully cured.

[0009] S3. Attach short cellulose fibers to the adhesive resin section on the surface of the fiber bundle, so that some short fibers are embedded therein, and then completely cure the resin to form a rigid fiber bundle.

[0010] S4. Coat the surface of the cured rigid fiber bundle with a water-soluble resin layer. After curing, divide the fiber bundle carrier into cellulose fiber-based composite materials with a length of 15-45 mm.

[0011] In a preferred embodiment of the present invention, both the long cellulose fibers and the short cellulose fibers are selected from bamboo pulp fibers, cotton fibers, hemp fibers, wood pulp fibers, or combinations thereof.

[0012] In a preferred embodiment of the present invention, the average diameter of the fiber bundle is 0.5 to 1.5 mm.

[0013] In a preferred embodiment of the present invention, the non-water-soluble resin is selected from bio-based resin, silicone resin, epoxy resin, polyester resin, vinyl ester resin, polyurethane resin or a combination thereof; the coating area of ​​the non-water-soluble resin on the surface of the fiber bundle is a plurality of discretely distributed segments, each segment having a length of 2 to 15 mm, and within a single length of the composite material, there are at least two cured segments of the non-water-soluble resin.

[0014] In a preferred embodiment of the present invention, the water-soluble resin layer is selected from polyvinyl alcohol, polyethylene glycol, carboxymethyl cellulose, starch-based polymer or a combination thereof; the average thickness of the water-soluble resin layer on the surface of the composite material is 50-80 μm.

[0015] In a preferred embodiment of the present invention, in step S2, a non-water-soluble resin is coated onto a localized area of ​​the fiber bundle surface using a coating base plate and a coating top plate.

[0016] The upper surface of the adhesive base plate is provided with several equidistant semi-circular recesses, and the center-to-center distance between adjacent recesses is 10-14 mm.

[0017] The lower surface of the top plate for applying adhesive has a protrusion corresponding to the recess on the bottom plate for applying adhesive, and the upper surface of the top plate for applying adhesive has a glue flow groove, and the bottom of the glue flow groove has a glue flow hole corresponding to the center position of the protrusion.

[0018] In a preferred embodiment of the present invention, the method of locally coating a non-water-soluble resin in step S2 includes the following steps:

[0019] S21. Clamp the multiple fiber bundles prepared in S1 from both ends and transfer them to the surface of the adhesive base plate, so that each fiber bundle corresponds to a row of indentations;

[0020] S22. Fasten the top plate of the adhesive coating to the upper part of the bottom plate of the adhesive coating, and apply pressure to the top plate of the adhesive coating to press the protrusion into the recess of the fiber bundle.

[0021] S23. The non-water-soluble resin is injected into the glue-dispensing groove, and the non-water-soluble resin enters the depression through the glue-dispensing hole to achieve precise coating of the local area of ​​the fiber bundle.

[0022] In a preferred embodiment of the present invention, in step S3, short cellulose fibers are uniformly sprayed onto the viscous surface of the non-water-soluble resin using a spray gun or an electrostatic flocking device.

[0023] In a preferred embodiment of the present invention, in step S3, the fiber bundle slowly passes through a fluidized bed or vibrating trough filled with suspended short cellulose fibers, attaching the short cellulose fibers to the viscous surface of the non-water-soluble resin.

[0024] An environmentally friendly cellulose fiber-based composite material for shotcrete, comprising:

[0025] Fiber bundles formed by twisting long cellulose fibers;

[0026] A non-water-soluble resin is coated on a local area of ​​the fiber bundle, and the non-water-soluble resin, after curing, fixes the local area of ​​the fiber bundle to maintain the twisted shape.

[0027] A large number of short cellulose fibers are attached to the outer surface of the cured area of ​​the water-insoluble resin, and these short cellulose fibers exhibit a multi-directional raised state on the surface of the water-insoluble resin; and

[0028] A water-soluble resin layer covering the entire outer surface of the fiber bundle.

[0029] This invention addresses the shortcomings of the prior art and has the following beneficial effects:

[0030] (1) This invention provides an environmentally friendly cellulose fiber-based composite material for shotcrete. It involves partially coating the surface of a twisted cellulose fiber bundle with a non-water-soluble resin and then partially curing it to form a rigid segment with a viscous surface. Short cellulose fibers are then embedded into this viscous region and completely cured, ultimately covering the entire structure with a water-soluble resin layer. This allows the water-soluble resin layer to effectively isolate moisture during the initial mixing of the composite material into the concrete, inhibiting water absorption by the cellulose fibers and preventing a decrease in concrete fluidity and fiber agglomeration caused by fiber water absorption. This ensures the uniform dispersion and excellent workability of the fresh concrete. It solves the problem in the prior art where the strong hydrophilicity of cellulose fibers easily leads to a sharp drop in concrete fluidity and construction difficulties. It significantly improves the dispersion of fibers in concrete and the pumpability of the mixture, further improving construction efficiency and concrete forming quality.

[0031] (2) In this invention, the non-water-soluble resin is locally cured on the surface of the fiber bundle in discrete segments to form rigid support sections, while retaining some uncured flexible areas. The short fibers are anchored to the resin surface in a multi-directional upturned manner, forming a three-dimensional interlocking network together with the main fibers. After the water-soluble resin dissolves during the concrete hardening stage, the structure releases long and short fibers. The long fibers form a spiral convex surface due to their twisted structure, increasing the friction with the matrix. The short fibers form a large number of secondary anchoring points, significantly enhancing the mechanical interlocking ability and stress transfer efficiency between the fibers and the concrete matrix.

[0032] (3) The composite material of the present invention uses cellulose fiber as the base material. Cellulose fiber has natural hydrophilicity and porous cavity structure. After the water-soluble resin layer dissolves, it begins to absorb and store water. During the cement hydration process, it gradually releases water, promotes the continuous internal hydration reaction, and achieves the effect of internal curing. This mechanism significantly reduces the drying shrinkage rate of concrete and reduces the generation of shrinkage cracks. At the same time, the adhesion of hydration products on the fiber surface enhances the chemical bonding strength between the fiber and the matrix and optimizes the structure of the interface transition zone. Compared with ordinary concrete or fiber concrete wrapped only with non-water-soluble resin, the present invention significantly improves the long-term durability, impermeability and freeze-thaw resistance of concrete while maintaining good construction performance, realizing the functional transformation of cellulose fiber from "construction obstacle" to "performance enhancer". Attached Figure Description

[0033] To more clearly illustrate the technical solutions in the embodiments of the present invention or the prior art, the drawings used in the description of the embodiments or the prior art will be briefly introduced below. Obviously, the drawings described below are only some embodiments recorded in the present invention. For those skilled in the art, other drawings can be obtained based on these drawings without creative effort.

[0034] Figure 1 This is a three-dimensional structural diagram of the glued base plate and glued top plate according to a preferred embodiment of the present invention;

[0035] Figure 2 This is a top view of the glued top plate according to a preferred embodiment of the present invention;

[0036] In the diagram: 1. Glue-coated base plate; 11. Recess; 2. Glue-coated top plate; 21. Protrusion; 22. Glue flow groove; 23. Glue flow hole. Detailed Implementation

[0037] The technical solutions of the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention.

[0038] Many specific details are set forth in the following description in order to provide a full understanding of the invention. However, the invention may also be practiced in other ways different from those described herein. Therefore, the scope of protection of the invention is not limited to the specific embodiments disclosed below.

[0039] This invention provides an environmentally friendly cellulose fiber-based composite material for shotcrete and its preparation method. This cellulose fiber-based composite material aims to overcome the technical problems of water absorption and uneven dispersion of cellulose fibers in the early stages of concrete mixing, while allowing cellulose fibers to exert their advantages in promoting cement hydration and improving long-term durability in the later stages of concrete hardening. The structure of the composite material and its preparation method of this invention will be described in detail below.

[0040] The cellulose fiber-based composite material provided by this invention comprises: a fiber bundle formed by twisting multiple long cellulose fibers, a non-water-soluble resin coated on a local area of ​​the fiber bundle, a large number of short cellulose fibers attached to the outer surface of the cured area of ​​the non-water-soluble resin, and a water-soluble resin layer covering the overall outer surface of the fiber bundle. This cellulose fiber-based composite material exhibits a twisted rod-like shape, possessing sufficient rigidity and anti-agglomeration ability, ensuring its uniform dispersion in freshly mixed concrete.

[0041] Both long and short cellulose fibers are selected from bamboo pulp fiber, cotton fiber, hemp fiber, wood pulp fiber, or any combination thereof. The long cellulose fibers are selected with a length between 2mm and 200mm to ensure effective twisting into fiber bundles, thereby bridging microcracks in concrete and preventing dispersion. The short cellulose fibers are selected with a length between 0.5mm and 5mm, and these short fibers are arranged at a density of at least 50 fibers / cm. 2 The short fibers are uniformly attached to the outer surface of the non-water-soluble resin-cured area. Their outer ends exhibit a multi-directional upward curve on the fiber bundle surface; that is, the short fibers are not laid flat along the fiber bundle axis, but rather extend outwards from the resin surface at a certain angle. This multi-directional upward curve is crucial for constructing a three-dimensional interlocking network. It significantly increases the contact area and friction between the fiber and the matrix, thereby effectively transferring and dispersing stress, and increasing the interfacial contact area between the cellulose fiber-based composite material and concrete.

[0042] The fiber bundle is made from the aforementioned long cellulose fibers through a twisting process, and a water-insoluble resin is precisely coated onto a localized area of ​​the fiber bundle. After curing, this water-insoluble resin fixes the localized area of ​​the fiber bundle to maintain its twisted shape. The water-insoluble resin is selected from bio-based resins, silicone resins, epoxy resins, polyester resins, vinyl ester resins, polyurethane resins, or any combination thereof. These water-insoluble resins exhibit excellent adhesion to cellulose fibers and high stability to water. The coating area of ​​the water-insoluble resin on the fiber bundle surface consists of multiple discrete segments, each segment being 2–15 mm in length, and at least two cured segments of the water-insoluble resin exist within a single fiber length of 15–45 mm in the composite material. In the cured state, the flexural strength of the water-insoluble resin should be no less than 50 MPa to ensure that the localized area of ​​the fiber bundle fixed by it has sufficient rigidity to resist shear forces in the early stages of concrete mixing, preventing large deformation or breakage of the fiber bundle. Meanwhile, the bonding strength of the non-water-soluble resin to the cellulose fiber must be no less than 5 MPa to ensure a strong interfacial bond between the resin layer and the fiber bundle, and to prevent interfacial debonding during stress.

[0043] Furthermore, because the non-water-soluble resin is not continuously coated on the fiber bundle surface, but rather appears as multiple discrete segments, this design, while maintaining the overall shape of the fiber bundle, reserves areas of the fiber bundle that have not been cured by the resin. This allows the long fibers in these areas to stretch freely after the outermost water-soluble resin layer dissolves, fully utilizing their flexibility and physical interlocking with the concrete matrix.

[0044] Furthermore, a water-soluble resin, selected from polyvinyl alcohol, polyethylene glycol, carboxymethyl cellulose, starch-based polymers, or any combination thereof, completely covers the outer surface of the fiber bundle. These resin materials possess controllable solubility in water. The average thickness of the water-soluble resin layer on the surface of the composite material is 50-80 μm, providing a sufficient physical barrier for the composite material in the early stages of concrete mixing. This effectively isolates the cellulose fibers from direct contact with the mixing water, preventing the cellulose fibers from absorbing water prematurely and reducing the fluidity of the concrete from mixing to spraying. After the concrete is sprayed, as the ambient humidity increases and the cement hydration process proceeds, the resin gradually dissolves, releasing the uncured long fibers and one end of the short fibers from the fiber bundle. Only then do these cellulose fibers begin to contact the water and exert their material properties.

[0045] Cellulose fibers have natural hydrophilicity, which can provide moisture for cement hydration, allowing hydration products to adhere to their surface and improving the bond strength between the fiber and the cement matrix.

[0046] The unique cavity structure of cellulose fibers can store moisture, which is slowly released during the hardening process of concrete, promoting further hydration, compensating for concrete shrinkage, and improving the long-term durability of concrete.

[0047] This invention provides a method for preparing an environmentally friendly cellulose fiber-based composite material for shotcrete, comprising the following steps: Step S1, twisting multiple long cellulose fibers to form a continuous fiber bundle; Step S2, coating a local area of ​​the fiber bundle surface with a non-water-soluble resin and partially curing it, so that the surface of the non-water-soluble resin is in a partially cured, viscous state; Step S3, attaching short cellulose fibers to the viscous resin section on the fiber bundle surface, embedding some short fibers therein, and then fully curing the resin to form a rigid fiber bundle; Step S4, coating the cured fiber bundle carrier surface with a water-soluble resin layer, and after curing, dividing the fiber bundle carrier into cellulose fiber-based composite materials with a length of 15-45 mm.

[0048] The following describes in detail each step of the preparation method of the cellulose fiber-based composite material of the present invention.

[0049] In step S1, cellulose fibers with a length between 2mm and 200mm are selected and continuously twisted using a twisting device. The twisting coefficient is set to 2-10 turns / cm to form a continuous fiber bundle. For example, a ring spindle twister or shuttleless twister with a rotation speed of 1000-3000 rpm / min is used, and the twisting coefficient is controlled by adjusting the feeding speed and spindle speed. In one embodiment, for a fiber bundle with a diameter of 1mm, a twisting coefficient of 5 turns / cm produces a fiber bundle with better structural stability.

[0050] It is worth noting that the diameter of the twisted fiber bundle is controlled between 0.5 and 1.5 mm, which ensures the macroscopic operability of the fiber bundle and enables it to form an effective reinforcing unit in concrete.

[0051] In step S2, a non-water-soluble resin is coated on a local area of ​​the surface of the twisted continuous fiber bundle, and the non-water-soluble resin is initially cured to make its surface viscous.

[0052] like Figure 1 and Figure 2 As shown, a non-water-soluble resin is coated on a localized area of ​​the fiber bundle surface using a base plate 1 and a top plate 2.

[0053] The upper surface of the adhesive base plate 1 has several equidistantly distributed semi-circular recesses 11. The depth of each recess 11 is 2-3 times the diameter of the fiber bundle, ensuring that the local area of ​​the fiber bundle can be tightly fitted to the recess 11 after being pressed into it. The center-to-center distance between adjacent recesses 11 is 10-15 mm, ensuring that there are a sufficient number of rigid curing segments within a unit length of the fiber bundle, and that there is sufficient flexible area between the curing segments.

[0054] The lower surface of the top coating plate 2 has a protrusion 21 corresponding to the recess 11 on the bottom coating plate 1, which can accurately press the fiber bundle into the recess 11 when the top coating plate 2 and the bottom coating plate 1 are fastened together. The upper surface of the top coating plate 2 has a glue flow channel 22, which is rectangular in shape and is used to temporarily store and guide non-water-soluble resin. At the bottom of the glue flow channel 22, corresponding to the center of the protrusion 21, there is a glue flow hole 23. The diameter of the glue flow hole 23 is 0.5-1.5 mm. The non-water-soluble resin flows into the recess 11 through the glue flow hole 23, accurately coating a local area of ​​the fiber bundle.

[0055] Furthermore, in step S2, the method of locally coating a non-water-soluble resin includes the following steps:

[0056] Step S21: Clamp and tension the multiple fiber bundles prepared in S1 from both ends using tensioning devices to maintain a straight line and a preset tension, such as 5-20N, during the adhesive coating process. Then, transfer the tensioned fiber bundles parallel and evenly onto the upper surface of the adhesive coating base plate 1, ensuring that each fiber bundle precisely corresponds to a row of recesses 11. The spacing between the fiber bundles can be adjusted according to the design of the adhesive coating base plate 1 to avoid mutual interference.

[0057] Step S22: Attach the top coating plate 2 to the upper part of the bottom coating plate 1. Apply a preset pressure, such as 0.1-1 MPa, to the top coating plate 2 using hydraulic or pneumatic pressure. This pressure causes the protrusions 21 on the lower surface of the top coating plate 2 to precisely press the localized positions of the fiber bundles into the recesses 11 of the bottom coating plate 1, ensuring a tight fit between the top coating plate 2 and the recesses 11. This provides stable positioning for the subsequent precise coating of the non-water-soluble resin.

[0058] Step S23: Use a metering pump to inject non-water-soluble resin into the glue dispensing tank 22. The non-water-soluble resin enters the recess 11 through the glue dispensing hole 23 to achieve precise coating of local areas of the fiber bundle.

[0059] In this step, the non-water-soluble resin is coated on the fiber bundle surface in discrete segments, each segment ranging from 2 to 15 mm in length. Within a single fiber length of 15 to 45 mm in the composite material, there are at least two segments where the non-water-soluble resin has cured. Because the non-water-soluble resin is not continuously coated on the fiber bundle surface but rather appears as discrete segments, this design, while maintaining the overall shape of the fiber bundle, reserves areas of the fiber bundle that have not been cured by the resin. This allows the long fibers in these areas to freely extend after the outermost water-soluble resin layer dissolves, fully utilizing their flexibility and physical interlocking with the concrete matrix.

[0060] Furthermore, after coating, the non-water-soluble resin can be pre-cured by ultraviolet irradiation or gentle heating to ensure that the surface of the non-water-soluble resin has sufficient adhesion to effectively adhere to the subsequent short fibers, while not being fully cured, thus retaining the possibility of short fiber embedding.

[0061] Steps S21 to S23 utilize the mating structure of the coating base plate 1 and the coating top plate 2 to achieve precise coating of non-water-soluble resin onto localized areas of the fiber bundle. This structure, through the design of the semi-circular recess 11 on the base plate and the protrusion 21 and resin flow hole 23 on the top plate, allows the fiber bundle to be locally pressed into the recess 11 and injected with resin, ensuring accurate coating location and controllable resin dosage. This avoids the resin waste and uneven coating problems caused by traditional impregnation or spraying methods, significantly improving coating accuracy and consistency. Furthermore, because the coated area is a discrete segment, it retains some flexibility of the fiber bundle while providing a structural basis for subsequent attachment of short fibers and maintaining rigidity.

[0062] Meanwhile, a tensioning device clamps multiple fiber bundles and maintains a preset tension, ensuring that the fiber bundles remain straight and evenly spaced during coating, preventing them from shifting or twisting. This guarantees that each fiber bundle accurately corresponds to a row of recesses 11, improving coating position consistency and operational stability, significantly increasing production efficiency and product quality, and laying the foundation for subsequent large-scale continuous production.

[0063] In step S3, during the initial curing stage when the non-water-soluble resin is in a viscous state, the continuous fiber bundle coated with resin is continuously passed through the short fiber spraying zone or the short fiber immersion tank, so that the cellulose short fibers adhere to the viscous resin section on the surface of the fiber bundle, and some short fibers are embedded therein. Then the resin is completely cured to form a rigid fiber bundle.

[0064] In one embodiment, short cellulose fibers are attached to a sticky resin section on the surface of a fiber bundle using a spraying method. Specifically, pre-prepared short cellulose fibers are uniformly sprayed onto the sticky surface of the water-insoluble resin using a pneumatic spray gun or an electrostatic flocking device. The pneumatic spray gun uses an air pressure of 0.2-0.5 MPa to spray the short fibers in an atomized manner. The electrostatic flocking device applies a high-voltage electric field (e.g., 30-60 kV) between the short fibers and the fiber bundle, causing the short fibers to be directionally implanted into the resin surface. The spraying or implantation density of the short fibers is controlled at 50-150 fibers / cm². 2 This forms a high-density three-dimensional network.

[0065] In another embodiment, a short fiber immersion method is used, in which the fiber bundle passes through a fluidized bed or vibrating tank filled with suspended short fibers at a speed of 5–20 m / min. In the fluidized bed, the short fibers are kept suspended by airflow, and as the fiber bundle passes through, the short fibers uniformly adhere to and partially embed into the surface of the water-insoluble resin. The vibrating tank, on the other hand, uses mechanical vibration to uniformly distribute the short fibers and bring them into contact with the fiber bundle.

[0066] Furthermore, after the cellulose short fibers are attached, excess short fibers that are not firmly attached are removed by an air knife or negative pressure adsorption device to ensure that the short fibers are firmly attached only to the surface of the non-water-soluble resin and are in a multi-directional raised state, so as to avoid loose short fibers affecting subsequent processes or forming agglomerates in concrete.

[0067] Furthermore, the fiber bundles with attached short fibers are completely cured by using a hot air circulating oven with the temperature set between 60℃ and 120℃ and the heating time between 30 and 120 minutes. This ensures that the non-water-soluble resin is completely cross-linked, achieving its final mechanical properties and chemical stability. The cured segments form a strong rod-like structure and firmly anchor the short fibers.

[0068] In step S4, the fully cured continuous fiber bundle is passed through a water-soluble resin solution impregnation tank or spraying device at a speed of 20–60 m / min. The concentration of the water-soluble resin solution is 5%–20%, such as a 10% polyvinyl alcohol (PVA) aqueous solution with a viscosity of 100–500 cP. After the fiber bundle is impregnated or sprayed, it is immediately dried at a temperature of 40℃–80℃ for 5–30 min, forming a uniform water-soluble resin layer with a thickness of 50–80 μm on the overall outer surface of the fiber bundle, ensuring comprehensive protection of the internal cellulose fibers.

[0069] Furthermore, the continuous fiber bundles coated with a water-soluble resin layer are cut into environmentally friendly cellulose fiber-based composite materials with a length of 15-45 mm for use in shotcrete using an automatic cutting device.

[0070] The water-soluble resin layer has an average thickness of 50-80 μm on the surface of the composite material, providing a sufficient physical barrier for the composite material in the early stage of concrete mixing. This effectively isolates the cellulose fibers from direct contact with the mixing water, preventing the cellulose fibers from absorbing water too early and reducing the fluidity of the concrete from mixing to spraying. After the concrete is sprayed, as the ambient humidity increases and the cement hydration process proceeds, the resin can gradually dissolve and release the uncured long fibers and one end of the short fibers. Only then do these cellulose fibers begin to come into contact with water and exert their material function.

[0071] In one embodiment, in step S2, alumina and magnesium oxide micropowders are uniformly mixed with non-water-soluble resin to form a high-rigidity, high-temperature resistant composite coating. This coating is then precisely applied to localized areas of the cellulose fiber bundles through steps S21-S23, allowing the fiber bundles to maintain local rigidity and enhance fire resistance and interfacial friction characteristics after curing. In step S4, silica fume, fly ash, wood ash, and water-soluble resin are uniformly mixed to form an active slurry. The fully cured continuous fiber bundles are then passed through an impregnation tank or spraying device of this mixed solution at a speed of 20-60 m / min, forming a water-soluble protective layer rich in volcanic ash active substances on the fiber surface. This layer effectively isolates moisture and ensures workability during the initial stage of concrete spraying. After dissolving later, it not only releases fibers but also simultaneously releases active micropowders to participate in cement hydration, significantly enhancing the density and chemical bonding force of the fiber-matrix interface transition zone, achieving a dual synergistic effect of fiber reinforcement and active internal curing.

[0072] In an alternative embodiment, bio-based nanocellulose fibers prepared from agricultural waste (such as buckwheat hulls or empty fruit bunches of oil palm) through modification and dissociation are used as the short fibers attached in step S3. These nanofibers have extremely high specific surface area and abundant surface-active functional groups. When they are precisely attached to the viscous section of the water-insoluble resin through electrostatic flocking or fluidized bed processes, they can form extremely strong physical adsorption and chemical bonds with the water-insoluble resin, thereby greatly enhancing the anchoring strength of the short fibers at the microscale.

[0073] Example 1:

[0074] According to the above-mentioned method for preparing an environmentally friendly cellulose fiber-based composite material for shotcrete, the preparation of the environmentally friendly cellulose fiber-based composite material includes the following steps:

[0075] Step 1: Select wood pulp fibers with an average length of 4.5 mm and an average diameter of 25 μm, and twist the wood pulp fibers using a ring twister with a twisting coefficient of 5 turns / cm to form a fiber bundle with an average diameter of 1.2 mm.

[0076] Step 2: Using bisphenol A type epoxy resin, apply 6mm long segments of resin with a center-to-center spacing of 10mm between adjacent segments to the surface of the fiber bundle through the cooperation of the base plate 1 and the top plate 2. Within a 30mm length, there are 3 curing segments. Initial curing is performed by irradiating with ultraviolet light for 20 seconds to make the surface tacky.

[0077] Step 3: Select wood pulp fibers with an average length of 2mm and an average diameter of 15μm, and attach them using an electrostatic flocking method with a spray density of 100 fibers / cm. 2 Then, bisphenol A type epoxy resin is cured.

[0078] Step 4: Select a 15% concentration, 300 cP polyvinyl alcohol aqueous solution, apply it by impregnation, and after drying, form a water-soluble resin layer with an average thickness of 60 μm. Finally, cut the continuous fiber bundle into 30 mm lengths to obtain an environmentally friendly cellulose fiber-based composite material.

[0079] To evaluate the performance of the composite material, a typical C30 shotcrete mix proportion was selected, and the composite material of this embodiment was added to it.

[0080] Cement: PO 42.5 ordinary Portland cement, 400kg / m³ 3 ;

[0081] Water: 180kg / m 3 ;

[0082] Sand: Fineness modulus 2.6, 800 kg / m³ 3 ;

[0083] Crushed stone: 5-15mm continuous gradation, 900kg / m³ 3 ;

[0084] Water-reducing agent: Polycarboxylate-based high-performance water-reducing agent, dosage 1.0% (by mass of cement).

[0085] Accelerator: Alkali-free accelerator, dosage 7.0% (by cement mass), added during spraying;

[0086] Composite materials: dosage is 0.5% (based on concrete volume, i.e., 5 kg / m³). 3 ).

[0087] Example 2:

[0088] Except for the use of a composite material with an average thickness of 50 μm of water-soluble resin layer, the other mixing parameters are completely consistent with those in Example 1.

[0089] Example 3:

[0090] Except for the use of a composite material with an average thickness of 80 μm for the water-soluble resin layer, the other mixing parameters are completely consistent with those in Example 1.

[0091] Comparative Example 1:

[0092] The same wood pulp fibers as in Example 1 were used, but without any twisting, resin coating, or water-soluble resin encapsulation. The fibers were added directly to the concrete in loose form.

[0093] Except for replacing “composite material” in Example 1 with “untreated loose wood pulp fiber” (with the same dosage, 0.5% by concrete volume), the other mix proportions are completely consistent with those in Example 1.

[0094] Comparative Example 2:

[0095] The same fiber bundles as in Example 1 were used, but bisphenol A type epoxy resin was used to coat the entire outer surface of the fiber bundles, and no short fiber attachment and water-soluble resin coating treatment was performed.

[0096] Except for replacing “composite material” in Example 1 with “20mm fiber bundles coated with non-water-soluble resin” (with the same dosage, 0.5% by concrete volume), the other mix proportions are completely consistent with those in Example 1.

[0097] Comparative Example 3:

[0098] The same shotcrete mix proportions as in Example 1 were used, but no fiber materials were added.

[0099] Comparative Example 4:

[0100] Except for the use of a composite material with an average thickness of 30 μm for the water-soluble resin layer, the other mixing parameters are completely consistent with those in Example 1.

[0101] Based on GB / T 50080-2016 and GB / T 50081-2019, the slump of freshly mixed concrete in Example 1 and Comparative Examples 1-3, as well as the flexural strength, compressive strength, and drying shrinkage after 28 days of standard curing, were tested. The test results are shown in Table 1 below.

[0102] Table 1. Summary of Concrete Performance Tests

[0103]

[0104] As can be seen from Table 1:

[0105] Regarding the performance of fresh concrete, the slump of the concrete used in Example 1 reached 180 mm, similar to Comparative Examples 2 and 3, and significantly higher than the untreated cellulose fiber concrete of Comparative Example 1 (77 mm). This demonstrates that the composite material of the present invention, through its intact water-soluble resin layer acting as the first physical barrier, effectively isolates the cellulose fibers from direct contact with the mixing water, fundamentally suppressing the inherent strong water absorption of cellulose fibers. Therefore, it avoids the imbalance of the mixing water-cement ratio and fiber agglomeration caused by excessive water absorption by the fibers, thereby ensuring the excellent rheological properties, flowability, and pumpability of the fresh concrete, greatly improving construction efficiency, and solving the key problems of uneven dispersion of cellulose fibers and sharp decline in workability in the prior art.

[0106] Regarding the mechanical properties of hardened concrete, the concrete of Example 1 exhibited superior 28-day compressive strength (42.5 MPa) and flexural strength (7.8 MPa) compared to the fiber-bundled concrete of Comparative Example 2, which was treated with non-water-soluble resin, and significantly higher than the ordinary fiberless concrete of Comparative Example 3. This indicates that the present invention, through a unique activation mechanism, allows the inherent advantages of cellulose fibers to be fully utilized in the later stages of concrete hardening, while the cellulose fibers of Comparative Example 2, being coated with resin, could not exert the concrete curing properties inherent in the cellulose fibers themselves. After the concrete was sprayed, the water-soluble resin layer gradually dissolved, allowing the long fibers of the uncured segments of the fiber bundles to freely extend, while the short fibers attached to the outer surface of the cured area of ​​the non-water-soluble resin were also released and curled outwards. At this point, the concrete slurry could fully penetrate the gaps between the long fibers and the tiny voids formed by the short fibers in the uncured area of ​​the fiber bundles, achieving physical interlocking between the fibers and the matrix and maximizing the interfacial contact area. The released long fiber bundles, through their twisted spiral protrusions, significantly increase the coefficient of friction with the cement matrix, improving load transfer efficiency. Meanwhile, the multi-directionally raised short fibers form numerous secondary anchor points within the matrix, working together with the main long fibers to construct a precise, multi-layered, three-dimensional interlocking network structure. This multi-dimensional fiber network effectively disperses and transfers stress, preventing the initiation and propagation of microcracks, thereby significantly improving the tensile and flexural strength of hardened concrete and enhancing its ability to resist external loads and deformation.

[0107] In terms of durability, the 28-day drying shrinkage rate of the concrete in Example 1 was only 0.012%, significantly lower than that of Comparative Example 3 (0.045%) and Comparative Example 2 (0.055%), verifying the internal curing mechanism of cellulose fibers. After the water-soluble resin layer dissolves, the cellulose fibers are exposed and fully contact the concrete matrix, the natural hydrophilicity of the cellulose fibers is activated. The numerous hydroxyl groups on their surface can form hydrogen bonds with cement hydration products, promoting the adhesion and growth of hydration products on the fiber surface, thereby significantly improving the chemical bond strength between the fiber and the cement matrix and optimizing the microstructure of the interfacial transition zone (ITZ). More importantly, the unique porous cavity structure of cellulose fibers can effectively adsorb and store free water from mixing water or the environment after the water-soluble resin layer dissolves, forming an internal micro-reservoir. In the later stages of concrete hardening, when external humidity decreases or the internal hydration rate slows down, the stored moisture can be slowly released, serving as a "water source for internal curing." This continuously promotes secondary or delayed hydration of cement particles, effectively compensating for the autogenous shrinkage and drying shrinkage of concrete, and reducing the risk of shrinkage cracking. This internal curing mechanism not only helps improve the long-term strength development and density of concrete but also significantly improves its durability properties such as impermeability and freeze-thaw resistance, thereby extending the service life of the structure. Therefore, this invention successfully transforms the initial disadvantages of cellulose fibers into later advantages, achieving the efficient utilization of environmentally friendly fiber materials in shotcrete.

[0108] Under environmental conditions of 20±2℃ and 60±5% humidity, spray curing tests were conducted on Examples 1-3 and Comparative Example 4. The time from spraying to curing of the sprayed concrete was recorded. Timing was started from the completion of spraying, and the following time points were recorded: initial setting time (surface not sticky to the touch) and final setting time (needle penetration resistance reaches 3.5MPa), as shown in Table 2 below.

[0109] Table 2. Summary of Spray Curing Tests

[0110]

[0111] Table 2 shows that when the thickness of the water-soluble resin layer is within the range of 50–80 μm, the composite material exhibits the best construction adaptability in shotcrete (initial setting time 16–22 min). When the thickness is less than 50 μm, premature water absorption by the fibers affects construction; when it is greater than 80 μm, the resin dissolution is too slow, delaying fiber activation. This invention achieves a synergistic dual function of "initial protection and later activation" of cellulose fibers in shotcrete by precisely controlling the thickness of the water-soluble resin layer.

[0112] It is worth noting that the greater the thickness of the water-soluble resin layer, the more water and the longer the time required for complete dissolution. Based on the time window for mixing and transporting sprayed concrete, this invention designs a water-soluble resin layer with a thickness of 50-80 μm.

[0113] In summary, this invention, through its ingenious material structure design and phased functional activation mechanism, not only completely solves the technical problems of water absorption and expansion, uneven dispersion, and decreased workability of cellulose fibers in the early stage of concrete mixing due to their strong hydrophilicity, but also significantly improves the mechanical properties and long-term durability of concrete in the later stage of concrete hardening through its unique multi-dimensional interlocking network structure and internal curing mechanism. This provides a new type of composite material with excellent performance and environmental friendliness for the field of shotcrete.

[0114] In a specific application scenario, the dissolution rate of the water-soluble resin layer can be adjusted according to the ambient humidity and temperature of the sprayed concrete environment. For example, in dry, high-temperature environments, polyvinyl alcohol resins with a slower dissolution rate can be selected, and the layer thickness can be appropriately increased to extend the protection time; while in humid, low-temperature environments, carboxymethyl cellulose resins with a faster dissolution rate can be selected, and the layer thickness can be reduced to ensure that the cellulose fibers can be activated in time. Precise control of the dissolution rate can be achieved by adjusting the thickness of the water-soluble resin layer, the molecular weight of the polymer, the type of comonomer, and the concentration of the coating solution.

[0115] The environmentally friendly cellulose fiber-based composite material and its preparation method disclosed in this invention not only provide a high-performance shotcrete reinforcement material, but more importantly, the functional design concept of "initial inert encapsulation and subsequent activation and utilization" reveals a new avenue for the application of other hydrophilic biomass materials in the engineering field. This phased functional activation strategy overcomes the limitations of traditional fiber modification methods that often suffer from unintended consequences, achieving synergistic optimization of material properties and demonstrating enormous potential for engineering applications.

[0116] Based on the preferred embodiments of the present invention described above, those skilled in the art can make various changes and modifications without departing from the inventive concept. The technical scope of this invention is not limited to the contents of the specification, but must be determined according to the scope of the claims.

Claims

1. A method for preparing an environmentally friendly cellulose fiber-based composite material for shotcrete, characterized in that, Includes the following steps: S1. Twist multiple long cellulose fibers to form a continuous fiber bundle; S2. Coat a local area of ​​the fiber bundle surface with a non-water-soluble resin and partially cure it so that the surface of the non-water-soluble resin is in a sticky state that is not fully cured. S3. Attach short cellulose fibers to the adhesive resin section on the surface of the fiber bundle, so that some short fibers are embedded therein, and then completely cure the resin to form a rigid fiber bundle. S4. Coat the surface of the cured rigid fiber bundle with a water-soluble resin layer. After curing, divide the fiber bundle carrier into cellulose fiber-based composite materials with a length of 15-45 mm.

2. The method for preparing an environmentally friendly cellulose fiber-based composite material for shotcrete according to claim 1, characterized in that: Both the long cellulose fibers and the short cellulose fibers are selected from bamboo pulp fibers, cotton fibers, hemp fibers, wood pulp fibers, or combinations thereof.

3. The method for preparing an environmentally friendly cellulose fiber-based composite material for shotcrete according to claim 1, characterized in that: The average diameter of the fiber bundle is 0.5 to 1.5 mm.

4. The method for preparing an environmentally friendly cellulose fiber-based composite material for shotcrete according to claim 1, characterized in that: The non-water-soluble resin is selected from bio-based resin, silicone resin, epoxy resin, polyester resin, vinyl ester resin, polyurethane resin or a combination thereof; the coating area of ​​the non-water-soluble resin on the surface of the fiber bundle is a plurality of discretely distributed segments, each segment having a length of 2 to 15 mm, and within a single length of the composite material, there are at least two cured segments of the non-water-soluble resin.

5. The method for preparing an environmentally friendly cellulose fiber-based composite material for shotcrete according to claim 1, characterized in that: The water-soluble resin layer is selected from polyvinyl alcohol, polyethylene glycol, carboxymethyl cellulose, starch-based polymers, or combinations thereof; the average thickness of the water-soluble resin layer on the surface of the composite material is 50-80 μm.

6. The method for preparing an environmentally friendly cellulose fiber-based composite material for shotcrete according to claim 1, characterized in that: In S2, a non-water-soluble resin is coated onto a localized area of ​​the fiber bundle surface using a coating base plate and a coating top plate. The upper surface of the adhesive base plate is provided with several equidistant semi-circular recesses, and the center-to-center distance between adjacent recesses is 10-14 mm. The lower surface of the top plate for applying adhesive has a protrusion corresponding to the recess on the bottom plate for applying adhesive, and the upper surface of the top plate for applying adhesive has a glue flow groove, and the bottom of the glue flow groove has a glue flow hole corresponding to the center position of the protrusion.

7. The method for preparing an environmentally friendly cellulose fiber-based composite material for shotcrete according to claim 6, characterized in that: In S2, the method of locally coating a non-water-soluble resin includes the following steps: S21. Clamp the multiple fiber bundles prepared in S1 from both ends and transfer them to the surface of the adhesive base plate, so that each fiber bundle corresponds to a row of indentations; S22. Fasten the top plate of the adhesive coating to the upper part of the bottom plate of the adhesive coating, and apply pressure to the top plate of the adhesive coating to press the protrusion into the recess of the fiber bundle. S23. The non-water-soluble resin is injected into the glue-dispensing groove, and the non-water-soluble resin enters the depression through the glue-dispensing hole to achieve precise coating of the local area of ​​the fiber bundle.

8. The method for preparing an environmentally friendly cellulose fiber-based composite material for shotcrete according to claim 1, characterized in that: In step S3, short cellulose fibers are uniformly sprayed onto the viscous surface of the non-water-soluble resin using a spray gun or electrostatic flocking equipment.

9. The method for preparing an environmentally friendly cellulose fiber-based composite material for shotcrete according to claim 1, characterized in that: In step S3, the fiber bundle slowly passes through a fluidized bed or vibrating trough filled with suspended short cellulose fibers, attaching the short cellulose fibers to the viscous surface of the non-water-soluble resin.

10. An environmentally friendly cellulose fiber-based composite material for shotcrete, comprising a method for preparing an environmentally friendly cellulose fiber-based composite material for shotcrete according to any one of claims 1-9, characterized in that, The cellulose fiber-based composite material comprises: Fiber bundles formed by twisting long cellulose fibers; A non-water-soluble resin is coated on a local area of ​​the fiber bundle, and the non-water-soluble resin, after curing, fixes the local area of ​​the fiber bundle to maintain the twisted shape. A large number of short cellulose fibers are attached to the outer surface of the cured area of ​​the water-insoluble resin, and these short cellulose fibers exhibit a multi-directional raised state on the surface of the water-insoluble resin; and A water-soluble resin layer covering the entire outer surface of the fiber bundle.

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