Cellulose cluster fiber reinforced ultra-high performance concrete and preparation method thereof

Abstract

The invention discloses cellulose cluster fiber reinforced ultra-high performance concrete and a preparation method thereof, and relates to the field of building materials, the cellulose cluster fiber reinforced ultra-high performance concrete is prepared by mixing and stirring the following components in parts by weight: 750-800 parts of cement, 200-250 parts of mineral admixture, 1000-1100 parts of fine aggregate, 0.2-1 part of cellulose cluster fiber, 10-20 parts of water reducer and 170-220 parts of water; the cellulose cluster fiber comprises a plurality of cellulose molecules combined through Van der Waals' force and hydrogen bond interaction, the length of the cellulose cluster fiber is 11-17 mm, the diameter of the cellulose cluster fiber is 150-300 [mu] m, and the cellulose content is 85-95 wt%. According to the invention, the cellulose cluster fibers are added in the form of millimeter scale, and the strength of the concrete is improved by optimizing the pore structure, inhibiting self-constriction, inhibiting generation of microcracks and realizing multi-dimensional coordination of crack bridging.

Description

Technical Field

[0001] This invention relates to the field of building materials, and in particular to a cellulose cluster fiber-reinforced ultra-high performance concrete and its preparation method. Background Technology

[0002] Concrete is a general term for engineering composite materials in which aggregates are bound together by cementing materials. Cement is usually used as the cementing material, and sand and stone are used as aggregates. With the in-depth development of modern high-rise and super high-rise buildings and long-span bridges, people have put forward a series of requirements for concrete materials, such as lightweight, high strength, heat insulation, heat insulation and environmental protection, leading to the development of high-performance concrete (HPC) and ultra-high-performance concrete (UHPC).

[0003] Currently, concrete is typically designed based on the theory of dense particle packing, optimizing the proportion of particles of different sizes, controlling the combination of discontinuous pore structure optimization and increased packing density, adjusting the composition of cementitious materials, and adding reinforcing fibers to improve the compressive and flexural strength of concrete. Reinforcing fibers are diverse, classified by material as steel fiber, carbon fiber, glass fiber, etc., and by size as centimeter, millimeter, micrometer, or even nanometer fibers. The material of the fiber determines its weight, strength, and other properties. Different fiber sizes enhance concrete through different mechanisms. Large-sized fibers (millimeter scale or larger) primarily enhance concrete by bridging internal cracks, while micrometer and nanometer fibers primarily enhance it by suppressing microcrack formation and refining pore size distribution. For ultra-high performance concrete with a low water-cement ratio (0.15-0.25), there is a serious problem of autogenous shrinkage; the addition of micrometer and nanometer fibers can further help suppress this autogenous shrinkage.

[0004] However, the addition of micron and nano-sized fibers, due to their high specific surface area and high surface energy, not only easily leads to fiber agglomeration, resulting in the deterioration of the concrete pore structure, but also significantly increases the dynamic yield stress and plastic viscosity, significantly worsening the flow properties of the concrete slurry and increasing the difficulty of construction. Summary of the Invention

[0005] In view of the shortcomings of the above-mentioned related technologies, the present invention provides a cellulose cluster fiber reinforced ultra-high performance concrete and its preparation method. The present invention adds cellulose cluster fibers, formed by multiple cellulose molecules bonded by van der Waals forces and hydrogen bonds, in a millimeter-scale manner. These fibers are discretely distributed in the concrete slurry during its formation. Compared with nanocellulose fibers and microcellulose fibers, this reduces the yield stress and plastic viscosity of the slurry, and improves the flow properties of the concrete slurry. During the mixing, pouring, and vibration of the concrete slurry, as the cellulose cluster fibers rub against other components, some cellulose overcomes the weak van der Waals forces and hydrogen bonds and peels off from the surface of the cellulose cluster fibers. During the curing process of the concrete slurry, these fibers exist uniformly in the form of nanocellulose fibers and microcellulose fibers. The cellulose cluster fibers, nanocellulose fibers, and microcellulose fibers constitute a multi-scale fiber system in the concrete slurry. Through multi-dimensional synergy in optimizing pore structure, suppressing self-shrinkage, suppressing microcrack generation, and crack bridging, the strength of the ultra-high performance concrete is improved.

[0006] In a first aspect, the present invention provides a cellulose cluster fiber-reinforced ultra-high performance concrete using the following technical solution: A cellulose cluster fiber reinforced ultra-high performance concrete is obtained by mixing the following components in parts by weight: 750-800 parts cement, 200-250 parts mineral admixtures, 1000-1100 parts fine aggregate, 0.2-1 parts cellulose cluster fibers, 10-20 parts water-reducing agent, and 170-220 parts water; wherein the cellulose cluster fibers comprise multiple cellulose molecules bonded by van der Waals forces and hydrogen bonds, the length of the cellulose cluster fibers is 11-17 mm, the diameter of the cellulose cluster fibers is 150-300 μm, and the cellulose content is 85 wt%-95 wt%.

[0007] Preferably, the cellulose cluster fiber is prepared by the following steps: taking a cellulose precursor and pretreating it to remove residual air and soluble organic matter to obtain a pretreated precursor; soaking the pretreated precursor in a dissolving solution to obtain a dissolving precursor; treating the dissolving precursor with alkali and then washing it to obtain a hydrated cellulose cluster fiber; drying the hydrated cellulose cluster fiber to a water content of less than 5 wt% to obtain the cellulose cluster fiber.

[0008] Preferably, the pretreatment includes soaking the cellulose precursor in boiling water for 1-3 hours.

[0009] Preferably, the effusive solution is prepared by the following steps: mixing hydrogen peroxide solution and formic acid solution to obtain a mixed solution, adding concentrated sulfuric acid with a mass fraction of 0.4% to 1% of the mixed solution to react, and then diluting to obtain the effusive solution.

[0010] Preferably, the molar ratio of hydrogen peroxide in the hydrogen peroxide solution to formic acid in the formic acid solution is 0.9~1.1:1.0.

[0011] Preferably, the soaking in the dissolving solution includes immersing the pretreated precursor in a dissolving solution at 60-80 °C for 7-9 h, then replacing the dissolving solution, and repeating the treatment 3-5 times.

[0012] Preferably, the cellulose precursor includes one or more of bamboo, wood, and herbaceous plants.

[0013] Preferably, the drying process includes natural air drying for 12 to 48 hours.

[0014] Preferably, the mineral admixture includes one or more of fly ash, blast furnace slag, metakaolin, silica fume, and limestone powder.

[0015] Preferably, the mineral admixture comprises limestone powder and silica fume in a weight ratio of 3 to 5:1.

[0016] Secondly, the method for preparing cellulose cluster fiber-reinforced ultra-high performance concrete provided by the present invention adopts the following technical solution: A method for preparing cellulose cluster fiber reinforced ultra-high performance concrete includes the following steps: mixing a water-reducing agent with water to obtain a mixing solution; mixing and stirring cement, mineral admixtures, fine aggregates and cellulose cluster fibers for 2-4 minutes; then adding the mixing solution and stirring for another 4-6 minutes to obtain a concrete slurry; and taking the concrete slurry for molding and curing to obtain the cellulose cluster fiber reinforced concrete.

[0017] In summary, the present invention has at least one of the following beneficial technical effects: 1. This invention employs cellulose clusters formed by multiple cellulose molecules bonded by van der Waals forces and hydrogen bonds, added in millimeter-scale form. These clusters are discretely distributed within the concrete slurry during its formation. Compared to nanocellulose and microcellulose fibers, this reduces the yield stress and plastic viscosity of the slurry, improving its flowability. During the mixing, pouring, and vibration of the concrete slurry, some cellulose fibers overcome weaker van der Waals forces and hydrogen bonds and peel off from the surface of the cellulose clusters due to friction with other components. These fibers then exist as nanocellulose and microcellulose fibers during the curing process. The cellulose clusters, nanocellulose fibers, and microcellulose fibers constitute a multi-scale fiber system in the concrete slurry. By optimizing the pore structure, suppressing microcrack generation, and promoting multi-dimensional synergy in crack bridging, the strength of ultra-high performance concrete is improved.

[0018] 2. By adding cellulose cluster fibers in millimeter-scale form, this invention helps to improve the uniformity of cellulose cluster fiber dispersion in ultra-high performance concrete, further improving the uniformity of dispersion of nanocellulose fibers and microcellulose fibers formed by exfoliation in ultra-high performance concrete, reducing the agglomeration of nanocellulose fibers and microcellulose fibers, reducing structural defects in ultra-high performance concrete, and improving the strength of ultra-high performance concrete. Attached Figure Description

[0019] Figure 1 This is a schematic diagram illustrating the classification criteria for ultrafine fiber components, fine fiber components, and longer fiber components in nanocellulose fibers and microcellulose fibers. Figure 2 This is a macroscopic morphology diagram of the cellulose cluster fibers obtained during the preparation process of Example 1; Figure 3 This is a scanning electron microscope image of the cellulose cluster fibers obtained during the preparation process of Example 1; Figure 4 This is a comparison chart of the autogenous shrinkage values ​​of the cellulose cluster fiber-reinforced ultra-high performance concrete of Example 1, the ultra-high performance concrete of Comparative Example 1, and the ultra-high performance concrete of Comparative Example 4. Figure 5 This is a comparison chart of the autogenous shrinkage values ​​of the cellulose cluster fiber-reinforced ultra-high performance concrete of Example 2, the ultra-high performance concrete of Comparative Example 2, and the ultra-high performance concrete of Comparative Example 4. Figure 6 This is a comparison chart of the autogenous shrinkage values ​​of the cellulose cluster fiber-reinforced ultra-high performance concrete of Example 3, the ultra-high performance concrete of Comparative Example 3, and the ultra-high performance concrete of Comparative Example 4. Figure 7 This is a distribution diagram of cellulose cluster fibers in cellulose cluster fiber-reinforced ultra-high performance concrete; Figure 8 This is a scanning electron microscope image of the cellulose cluster fiber-reinforced ultra-high performance concrete of Example 1; Figure 9 These are comparative images of the air content scan results of ultra-high performance concrete specimens reinforced with cellulose clusters in Examples 1-3 and ultra-high performance concrete specimens in Comparative Examples 1-4. Detailed Implementation

[0020] The present invention will be further described in detail below with reference to the embodiments. The following embodiments are for illustrative purposes only and should not be considered as limiting the scope of the invention. Unless otherwise specified, specific conditions in the following embodiments were performed under conventional conditions or conditions recommended by the manufacturer. Unless otherwise specified, the methods used are conventional methods known in the art, and the consumables and reagents used are commercially available. Unless otherwise stated, the technical and scientific terms used herein have the same meaning as those familiar to those skilled in the art. Furthermore, any methods or materials similar to or equivalent to those described herein may also be applied to the present invention. The raw materials used in the embodiments and comparative examples are all commercially available.

[0021] Example 1 Example 1 of this invention provides a cellulose cluster fiber-reinforced ultra-high performance concrete, which is prepared by the following steps: Preparation of cellulose cluster fibers: Bamboo tubes were used as cellulose precursors and treated in boiling water for 1 hour to remove residual air and soluble organic matter, resulting in a pretreated precursor. 113.4 g of 30% hydrogen peroxide solution and 52.3 g of 88% formic acid solution were mixed to obtain a mixed solution. 0.8 g of concentrated sulfuric acid was added, and after reaction, an equal weight of water was added to dilute the solution and obtain a dissolving solution. The pretreated precursor was soaked in the dissolving solution heated to 70 °C for 8 hours. The dissolving solution was replaced and the treatment was repeated 4 times to obtain a dissolving precursor. The dissolving precursor was then soaked in 0.5% sodium hydroxide solution for 0.5 hours and washed with water to obtain hydrated cellulose cluster fibers. The hydrated cellulose cluster fibers were air-dried for 24 hours (moisture content below 5 wt%) and then cut to obtain cellulose cluster fibers.

[0022] Preparation of cellulose cluster fiber reinforced ultra-high performance concrete: Take 784.78 g cement, 209.27 g limestone powder, 52.32 g silica fume, 1046.37 g quartz sand, 0.30 g cellulose cluster fiber, 15.70 g polycarboxylate superplasticizer and 200.64 g water; mix and dissolve the polycarboxylate superplasticizer with water to obtain a mixing solution; mix cement, limestone powder, silica fume, quartz sand and cellulose cluster fiber at a speed of 100±5 r / min for 3 min; then add the mixing solution and continue mixing at the same speed for 5 min to obtain concrete slurry; pour the concrete slurry into a mold, vibrate to remove air bubbles, cover the surface with plastic wrap to prevent rapid moisture loss, cure at 20 ℃ for one day, then remove the mold; place the demolded specimen in a standard curing environment (humidity ≥95%RH, temperature 20±2 ℃) for curing to the required age.

[0023] Example 2 Example 2 of the present invention provides a cellulose cluster fiber reinforced ultra-high performance concrete. The difference between Example 2 and Example 1 is that the amount of cellulose cluster fiber used in the preparation step of the cellulose cluster fiber reinforced ultra-high performance concrete in Example 2 is 0.61 g.

[0024] Example 3 Example 3 of the present invention provides a cellulose cluster fiber reinforced ultra-high performance concrete. The difference between Example 3 and Example 1 is that the amount of cellulose cluster fiber used in the preparation step of the cellulose cluster fiber reinforced ultra-high performance concrete in Example 3 is 0.91 g.

[0025] Comparative Example 1 Comparative Example 1 of this invention provides an ultra-high performance concrete. The difference between Comparative Example 1 and Example 1 is that Comparative Example 1 uses 9.63 g of nano-cellulose fiber hydrogel (purchased from Sappi in the Netherlands, with a cellulose content of 2.7 wt%, a water absorption rate of 850.8%, and a specific surface area of ​​26.15 m²) during its preparation. 2 / g. The volume percentage of ultrafine fiber component is 94.0%, fine fiber component is 4.6%, and longer fiber component is 1.4%. The classification criteria for ultrafine fiber, fine fiber, and longer fiber are as follows: Figure 1 (As shown) Replace the cellulose cluster fibers (control the weight of cellulose in the nanocellulose fiber hydrogel to be consistent with the weight of cellulose in the cellulose cluster fibers). Since the nanocellulose fiber hydrogel contains water, the corresponding amount of water used to prepare the mixing solution is reduced by 9.37 g.

[0026] Comparative Example 2 Comparative Example 2 of the present invention provides an ultra-high performance concrete. The difference between Comparative Example 2 and Example 2 is that 19.26 g of nanocellulose fiber hydrogel is used to replace cellulose cluster fibers in the preparation process of Comparative Example 2. Since the nanocellulose fiber hydrogel contains water, the amount of water used to prepare the mixing solution is reduced by 18.74 g.

[0027] Comparative Example 3 Comparative Example 3 of the present invention provides an ultra-high performance concrete. The difference between Comparative Example 3 and Example 3 is that 28.89 g of nanocellulose fiber hydrogel is used to replace cellulose cluster fibers in the preparation process of Comparative Example 3. Since the nanocellulose fiber hydrogel contains water, the amount of water used to prepare the mixing solution is reduced by 28.11 g.

[0028] Comparative Example 4 Comparative Example 4 of this invention provides an ultra-high performance concrete. The difference between Comparative Example 4 and Example 1 is that Comparative Example 4 uses 9.63g of micron-sized cellulose fiber hydrogel (purchased from Sappi in the Netherlands, with a cellulose content of 2.7wt%, a water absorption rate of 762.1%, and a specific surface area of ​​13.94m²) in its preparation process. 2 / g, of which the volume percentage of ultrafine fiber component is 47.90%, the volume percentage of fine fiber component is 23.5%, and the volume percentage of longer fiber component is 28.6%) replaces cellulose cluster fiber (controlling the weight of cellulose in micron cellulose fiber hydrogel to be consistent with the weight of cellulose in cellulose cluster fiber). Since micron cellulose fiber hydrogel contains water, the corresponding water consumption is reduced by 9.37g when preparing the mixing solution.

[0029] Comparative Example 5 Comparative Example 5 of the present invention provides an ultra-high performance concrete. The difference between Comparative Example 5 and Example 2 is that 19.26g of micron cellulose fiber hydrogel is used to replace the cellulose cluster fibers in the preparation process of Comparative Example 5. Since the micron cellulose fiber hydrogel contains water, the amount of water used to prepare the mixing solution is reduced by 18.74g.

[0030] Comparative Example 6 Comparative Example 6 of the present invention provides an ultra-high performance concrete. The difference between Comparative Example 6 and Example 3 is that 28.89g of micron-sized cellulose fiber hydrogel is used to replace the cellulose cluster fibers in the preparation process of Comparative Example 6. Since the micron-sized cellulose fiber hydrogel contains water, the amount of water used to prepare the mixing solution is reduced by 28.11g.

[0031] Comparative Example 7 Comparative Example 7 of the present invention provides an ultra-high performance concrete. The difference between Comparative Example 7 and Example 1 is that no cellulose cluster fibers are added during the preparation process of Comparative Example 7.

[0032] Testing and Inspection (1) The macroscopic morphology of the cellulose cluster fibers obtained in the preparation process of Example 1 is as follows: Figure 2 As shown, the cellulose clusters obtained during the preparation process of Example 1 were observed using scanning electron microscopy, and the resulting scanning electron micrographs are shown below. Figure 3 As shown in (a), Figure 3 Enlarged view of the white box in (a) as shown Figure 3As shown in (b), the length of the cellulose cluster fibers obtained in the preparation process of Example 1 was measured with a ruler; the cellulose content of the cellulose cluster fibers obtained in the preparation process of Example 1 was detected using the Paranoid method; the tensile strength of the cellulose cluster fibers obtained in the preparation process of Example 1 was detected using a universal testing machine; the water absorption rate was measured by placing the cellulose cluster fibers obtained in the preparation process of Example 1 in water and periodically detecting the mass change; and the specific surface area of ​​the cellulose cluster fibers obtained in the preparation process of Example 1 was detected using a high-performance multi-channel fully automatic specific surface area and porosity analyzer. The results are shown in Table 1 below.

[0033] Table 1:

[0034] (2) According to the national standard GB / T 2419~2005 "Method for Determination of Flowability of Cement Mortar", the flowability of concrete slurries obtained in Examples 1~3 and Comparative Examples 1~7 was tested using the jumping table method. The results are shown in Table 1 below. Using an AMETTEK Brookfield RSX SST rotational rheometer, the concrete slurries obtained in Examples 2, Comparative Examples 2, Comparative Examples 5 and Comparative Examples 7 were pre-sheared at a high speed of 100 rad / s for 60 s, and then allowed to stand for 60 s. The formal shearing was started at an angular velocity of 20 rad / s. After shearing for 20 s, the angular velocity was increased by 20 rad / s. This operation was repeated until the angular velocity reached 100 rad / s. The angular velocity was then decreased by 20 rad / s every 20 s until it stopped. The dynamic yield stress was detected, and the results are shown in Table 2 below.

[0035] Table 2:

[0036] (3) The lengths of the cellulose cluster fiber-reinforced ultra-high performance concrete specimens of Examples 1-3 and Comparative Examples 1-7 were measured every other day using a length comparator. The autogenous shrinkage value was obtained by subtracting the lengths of the specimens from the previous two days. The autogenous shrinkage values ​​of the cellulose cluster fiber-reinforced ultra-high performance concrete of Example 1, the ultra-high performance concrete of Comparative Example 1, the ultra-high performance concrete of Comparative Example 4, and the ultra-high performance concrete of Comparative Example 7 are compared as follows: Figure 4 As shown, the autogenous shrinkage values ​​of the cellulose cluster fiber-reinforced ultra-high performance concrete of Example 2, the ultra-high performance concrete of Comparative Example 2, the ultra-high performance concrete of Comparative Example 5, and the ultra-high performance concrete of Comparative Example 4 are compared as follows: Figure 5 As shown; the comparison results of the autogenous shrinkage values ​​of the cellulose cluster fiber-reinforced ultra-high performance concrete of Example 3, the ultra-high performance concrete of Comparative Example 3, the ultra-high performance concrete of Comparative Example 6, and the ultra-high performance concrete of Comparative Example 4 are as follows. Figure 6 As shown.

[0037] (4) To better demonstrate the distribution of cellulose cluster fibers in cellulose cluster fiber-reinforced ultra-high performance concrete, a scale-up experiment was designed. The preparation method of the cellulose cluster fiber-reinforced ultra-high performance concrete in the scale-up experiment was the same as that in Example 1, except that the amount of cellulose cluster fibers added in the scale-up experiment was 7 g. X-CT scanning was used to observe the distribution of cellulose cluster fibers in the cellulose cluster fiber-reinforced ultra-high performance concrete in the scale-up experiment. The results are as follows: Figure 7 As shown, the volume distribution was obtained by analyzing the image, and the results are shown in Table 3 below. The cellulose cluster fiber-reinforced ultra-high performance concrete of Example 1 was observed using scanning electron microscopy, and the resulting scanning electron microscope images are shown below. Figure 8 As shown in (a), Figure 8 Enlarged view of (a) as shown Figure 8 As shown in (b).

[0038] Table 3:

[0039] (5) According to national standards GB / T 31387~2025 and GB / T 50081, the 28-day compressive strength and flexural strength of the cellulose cluster fiber reinforced ultra-high performance concrete of Examples 1~3 and the ultra-high performance concrete of Comparative Examples 1~7 were tested using a universal testing machine. The results are shown in Table 4 below.

[0040] Table 4:

[0041] (6) Microscopic mercury intrusion porosimetry and porosity tests were conducted on the cellulose cluster fiber-reinforced ultra-high performance concrete of Examples 1-3 and the ultra-high performance concrete of Comparative Examples 1-7. The porosity results are shown in Table 4 below. The volume fraction of gel and capillary pores (the pore volume fraction statistics focus on the pore range of 0-1000 nm, and the pores are divided into three categories: gel pores (0-10 nm), medium capillary pores (10-50 nm), and large capillary pores (50-1000 nm)) are shown in Table 4 below. Three cross-sections of the cellulose cluster fiber-reinforced ultra-high performance concrete of Examples 1-3 and the ultra-high performance concrete of Comparative Examples 1-7 were taken from different locations, polished, and the scanning surfaces were blackened with a black marker. Then, the pores were filled with silica powder. Finally, digital image analysis was performed using a high-resolution flatbed scanner to obtain a comparison of the actual scanning results of the gas content of the cellulose cluster fiber-reinforced ultra-high performance concrete of Examples 1-3 and the ultra-high performance concrete of Comparative Examples 1-7, as shown in Table 4 below. Figure 9 As shown; for Figure 9 The gas content obtained after image processing is shown in Table 5 below.

[0042] Table 5:

[0043] Results Analysis The present invention will be described in detail below with reference to the experimental results provided in the test and detection section.

[0044] Reference Figure 2 and Figure 3 , Figure 2 The cellulose clusters obtained during the preparation process of Example 1 are white and straight, indicating that the removal of lignin is significantly effective. Figure 3 Image (a) shows that cellulose clusters are composed of a large number of highly oriented cellulose molecules tightly packed together, with lignin and hemicellulose being difficult to observe. Thin-walled cells are mainly distributed on the outer wall of the cellulose clusters and are rarely distributed between the cellulose molecules. The diameter of the cellulose clusters is approximately 225 μm. Figure 3 As shown in (b), cellulose molecules are tightly packed together, and almost no lignin, hemicellulose, or other substances such as parenchyma cells are visible between the cellulose molecules. Figure 3 The results are consistent with the cellulose content results in Table 1. Cellulose molecules are mainly bonded together by van der Waals forces and hydrogen bonds.

[0045] Referring to Table 2, the flowability and dynamic yield stress of the concrete slurry in Examples 1-3 are closer to the level of the control group in Comparative Example 7. Specifically, the flowability of the concrete slurry in Examples 1-3 is generally higher than that in Comparative Examples 1-7. Furthermore, the decrease in flowability with the increase of cellulose cluster fiber content is much lower than the decrease in flowability caused by the increase of nanocellulose fiber and microcellulose fiber content. Moreover, the dynamic yield stress of the concrete slurry in Example 2 is only half that of the concrete slurry in Comparative Examples 2 and 5. This indicates that using cellulose cluster fiber is beneficial to improving the workability of concrete slurry compared to nanocellulose fiber and microcellulose fiber. If the flowability is significantly reduced and the dynamic yield stress is too high, it is not conducive to the pouring and self-leveling of concrete slurry, and will also cause too many air bubbles and defects in the resulting concrete, thus leading to a decrease in the mechanical properties of the concrete.

[0046] Reference Figures 4-6It can be seen that the rate of change of autogenous shrinkage value of the cellulose cluster fiber-reinforced ultra-high performance concrete in Examples 1-3 is much lower than that of the ultra-high performance concrete in Comparative Example 7, and close to that of the ultra-high performance concrete in Comparative Examples 1-6. This indicates that the addition of cellulose cluster fibers is beneficial to inhibiting the autogenous shrinkage of ultra-high performance concrete, and the inhibition effect is close to that of nanocellulose fibers. Considering that the water absorption rate of cellulose cluster fibers is only one-quarter that of nanocellulose fibers, and that the number of nano- and micro-sized cellulose fibers in the cellulose cluster fiber-reinforced ultra-high performance concrete in Examples 1-3 is much smaller than that in Comparative Examples 1-6, but the effect of alleviating the degree of autogenous shrinkage of ultra-high performance concrete can reach a level close to that of nanocellulose fibers and microcellulose fibers, the analysis is that the multi-scale cellulose fiber system formed in situ after the cellulose cluster fibers are added to the ultra-high performance concrete is conducive to making full use of the water retention advantage of cellulose, achieving a level of autogenous shrinkage mitigation effect close to that of nanocellulose fibers and microcellulose fibers. Figure 7 Table 3 further verifies that the cellulose cluster fiber-reinforced ultra-high performance concrete contains a multi-scale cellulose fiber system consisting of millimeter-scale, micrometer-scale, and invisible-scale (fibers with length and diameter smaller than the resolution parameter (voxel resolution of 28 micrometers), including nanocellulose fibers) cellulose fibers. Fibers of each scale are uniformly distributed and randomly oriented within the ultra-high performance concrete. This is because: when cellulose cluster fibers are mixed with other components to form concrete slurry, larger-sized cellulose cluster fibers are more easily and uniformly dispersed throughout the system; during dispersion, water molecule penetration overcomes the weak hydrogen bonds and van der Waals forces between cellulose molecules, allowing cellulose molecules to easily detach from the cellulose cluster fibers at different sizes under the action of stirring, vibration, and friction, generating nanocellulose fibers and microcellulose fibers in situ near the cellulose cluster fibers, directionally reinforcing the interfacial transition zone, and... Figure 8 The results presented are consistent; and because the concentration of small-sized cellulose fibers is low in certain areas, aggregation is suppressed, which is consistent with... Figure 8 The results shown are consistent, thereby improving the uniformity of the distribution of nanocellulose fibers and microcellulose fibers in the whole, giving full play to the role of small-sized fibers in inhibiting the generation of microcracks in ultra-high performance concrete and refining the pore size distribution of ultra-high performance concrete; as the concrete slurry solidifies, the ambient water and the water stored in the cellulose cluster fibers are gradually consumed, and the cellulose molecules in the cellulose cluster fibers are re-densified under the action of van der Waals forces and hydrogen bonds, the mechanical strength of the cellulose cluster fibers is restored, and they still play the bridging role of large-sized fibers.

[0047] Referring to Table 4, with the same amount of reinforcing fibers, the compressive strength and flexural strength of cellulose cluster fiber-reinforced ultra-high performance concrete are significantly higher than those of ultra-high performance concrete with added nanocellulose; Figure 7 As shown in Table 5, the porosity, gel pore volume fraction, and air content of the cellulose-reinforced ultra-high performance concrete in Examples 1-3 were significantly lower than those in Comparative Examples 1-3. This indicates that the addition of cellulose cluster fibers improved the density and refined the pore structure of the ultra-high performance concrete compared to nanocellulose, thereby increasing its mechanical strength, consistent with the results in Table 4. Furthermore, its compressive strength reached UC1 level in T / CECS 10107-2020 "Technical Requirements for Ultra-High Performance Concrete (UHPC)". The analysis is due to two main reasons: First, the multi-scale fiber system retains most of the large-sized macro-fibers, while the small-sized fibers are located near the large-sized fibers, resulting in uniform dispersion, low local concentration, less interweaving and entanglement between the multi-sized fibers, reduced competition for water, improved workability, and enhanced mechanical properties, consistent with the aforementioned results. Second, the strength enhancement of the cellulose cluster fiber-reinforced ultra-high performance concrete in Examples 1-3 is not only achieved by eliminating defects, but also by rebalancing the types and distribution of defects. This involves synergistic enhancement from multiple aspects, including pore structure optimization, suppression of self-shrinkage, suppression of microcrack generation, and bridging effects, thereby improving the mechanical properties of the ultra-high performance concrete. Regarding the higher the amount of cellulose cluster fiber added in the cellulose cluster fiber-reinforced ultra-high performance concrete of Examples 1-3, the worse the compressive strength of the ultra-high performance concrete is because as the amount of cellulose cluster fiber added increases, the entanglement and aggregation of local cellulose cluster fibers intensifies, which reduces stress transfer efficiency and introduces weak areas in the matrix. However, if the amount of cellulose cluster fiber added is too small, on the one hand, it will lead to reduced friction, a decrease in the content of nanocellulose fibers and microcellulose fibers, and limited optimization of pore structure and microcrack suppression. On the other hand, the bridging effect of large-sized cellulose on macrocracks decreases, which is not conducive to improving the performance of ultra-high performance concrete. Therefore, the amount of cellulose cluster fiber added in the cellulose cluster fiber-reinforced ultra-high performance concrete of Example 1 is the optimal amount.

[0048] In summary, this invention utilizes millimeter-sized cellulose cluster fibers in ultra-high performance concrete. The cellulose cluster fibers are first dry-mixed with other solid components to ensure uniform dispersion. Then, after the mixing solution is added, the cellulose cluster fibers absorb water, further weakening the bonds between cellulose molecules. Simultaneously, during stirring, mechanical collisions and friction cause some cellulose molecules to overcome the weakened van der Waals forces and hydrogen bonds, causing them to peel off from the main body of the cellulose cluster fibers. By controlling the stirring time and speed, the proportion of cellulose fibers peeling off at different sizes can be controlled. If no stirring is performed, or the stirring rate is too low, or the stirring time is too short, the role of the cellulose cluster fibers in ultra-high performance concrete is similar to that of steel fibers, with limited self-shrinkage inhibition. If the stirring speed is too high or the stirring time is too long, it resembles micron-sized or even nano-sized cellulose fibers, with limited bridging effect. Finally, during the curing process of the concrete slurry, the nano-sized and micron-sized cellulose fibers peeled off around the cellulose cluster fibers are less prone to agglomeration and provide directional reinforcement to the interface. Simultaneously, as the moisture decreases, the cellulose cluster fibers become denser again, restoring their mechanical strength and fulfilling their bridging role. Under the combined effect, the workability (increased fluidity and reduced dynamic yield stress) and mechanical properties (increased compressive strength and flexural strength) of cellulose cluster fiber reinforced ultra-high performance concrete are improved.

[0049] The above description is merely a preferred embodiment of the present invention, and should not be construed as limiting the scope of the invention. It should be noted that those skilled in the art can make various improvements and modifications without departing from the principles of the present invention, and these improvements and modifications are also considered to be within the scope of protection of the present invention.

Claims

1. A cellulose cluster fiber-reinforced ultra-high performance concrete, characterized in that: The product is obtained by mixing and stirring the following components in parts by weight: 750-800 parts cement, 200-250 parts mineral admixtures, 1000-1100 parts fine aggregates, 0.2-1 parts cellulose cluster fibers, 10-20 parts water-reducing agent, and 170-220 parts water; wherein the cellulose cluster fibers comprise multiple cellulose molecules bonded by van der Waals forces and hydrogen bonds, the length of the cellulose cluster fibers is 11-17 mm, the diameter of the cellulose cluster fibers is 150-300 μm, and the cellulose content is 85wt%-95wt%.

2. The cellulose cluster fiber-reinforced ultra-high performance concrete according to claim 1, characterized in that: The cellulose clusters are composed of The process includes the following steps: pretreating the cellulose precursor to remove residual air and soluble organic matter, thereby obtaining the pretreated precursor; The pretreated precursor was immersed in the disintegration solution to obtain the disintegration precursor. The disintegration precursor was then treated with alkali and washed to obtain water-containing cellulose cluster fibers. The cellulose cluster fibers were dried to a moisture content of less than 5 wt% to obtain the cellulose cluster fibers.

3. The cellulose cluster fiber-reinforced ultra-high performance concrete according to claim 2, characterized in that: The pretreatment includes soaking the cellulose precursor in boiling water for 1-3 hours.

4. The cellulose cluster fiber-reinforced ultra-high performance concrete according to claim 2, characterized in that: The effusive solution is prepared by the following steps: mixing hydrogen peroxide solution and formic acid solution to obtain a mixed solution, adding concentrated sulfuric acid with a mass fraction of 0.4% to 1% of the mixed solution to react, and then diluting to obtain the effusive solution.

5. The cellulose cluster fiber-reinforced ultra-high performance concrete according to claim 4, characterized in that: The molar ratio of hydrogen peroxide in the hydrogen peroxide solution to formic acid in the formic acid solution is 0.9~1.1:1.

0.

6. The cellulose cluster fiber-reinforced ultra-high performance concrete according to claim 2, characterized in that: The soaking in the annealing solution involves immersing the pretreated precursor in a annealing solution at 60-80 °C for 7-9 h, then replacing the annealing solution, and repeating the treatment 3-5 times.

7. The cellulose cluster fiber-reinforced ultra-high performance concrete according to claim 2, characterized in that: The cellulose precursors include one or more of bamboo, wood, and herbaceous plants.

8. The cellulose cluster fiber-reinforced ultra-high performance concrete according to claim 2, characterized in that: The drying process includes natural air drying for 12 to 48 hours.

9. The cellulose cluster fiber-reinforced ultra-high performance concrete according to claim 1, characterized in that: The mineral admixture includes one or more of the following: fly ash, blast furnace slag, metakaolin, silica fume, and limestone powder.

10. A method for preparing cellulose cluster fiber-reinforced ultra-high performance concrete as described in any one of claims 1 to 9, characterized in that: Includes the following steps: The water-reducing agent is mixed with water to obtain a mixing solution. Cement, mineral admixtures, fine aggregates and cellulose cluster fibers are mixed and stirred for 2-4 minutes. Then, the mixing solution is added and stirring is continued for 4-6 minutes to obtain a concrete slurry. The concrete slurry is then molded and cured to obtain the cellulose cluster fiber-reinforced ultra-high performance concrete.

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