Preparation method and application of bamboo fiber / cement radiation refrigeration composite material

Abstract

The invention discloses a preparation method and application of a bamboo fiber / cement radiation refrigeration composite material, and the preparation method comprises the following steps: cutting moso bamboo chips into preset sizes, and sequentially carrying out acid cooking delignification, alkali liquor soaking modification and air drying treatment to obtain high-purity bamboo fibers; weighing light magnesium oxide, magnesium chloride and deionized water, and mixing and stirring to obtain a cement-based solution; adding polyvinyl alcohol (PVA) and nano titanium dioxide (TiO2) into the cement-based liquid, ultrasonically dispersing and mechanically stirring uniformly to obtain radiation refrigeration cement slurry; the bamboo fibers pretreated in the step S1 are directionally arranged in a mold, the radiation refrigeration cement slurry prepared in the step S2 is poured and placed in a constant-temperature and constant-humidity box to be cured, demolding is conducted after hydration reaction is completed, and the radiation refrigeration cement is obtained. According to the invention, the bamboo fiber reinforced magnesium oxychloride cement is used as a matrix, and the nano refrigeration particles are compounded, so that the balance of the material among efficient radiation refrigeration performance, high mechanical strength and environmental protection is realized, and the passive cooling requirement of a building structure is met.

Description

Technical Field

[0001] This invention relates to the field of building functional materials technology, and in particular to a method for preparing a bamboo fiber / cement radiation cooling composite material and its application. Background Technology

[0002] Cooling plays an indispensable role in many key areas of human production and life. For example, it can reduce air conditioning energy consumption in building energy conservation, alleviate road surface aging due to high temperatures in summer in road engineering, and ensure stable equipment operation in industrial facilities. Passive radiation cooling technology, with its energy-saving and eco-friendly characteristics of requiring no additional electricity consumption and having zero carbon emissions, has become a research hotspot in materials science and engineering in recent decades. The core advantage of this type of radiation cooling material lies in its high solar reflectivity in the solar spectrum (0.3-2.5 micrometers), effectively blocking the absorption of solar radiation heat; at the same time, it has ultra-high emissivity in the infrared region of the atmospheric window (8-13 micrometers), which can efficiently emit the heat energy on the material surface into the near-absolute zero space in the form of thermal radiation, thereby achieving a self-cooling effect without external energy drive, providing a green solution for solving high-temperature problems in various scenarios.

[0003] To date, researchers have conducted extensive research on radiation cooling materials, covering various types including photonic crystal thin films, silica aerogels, ceramic matrix composites, and porous polymer thin films. Although existing radiation cooling materials have demonstrated good cooling efficiency in laboratory environments (some materials can achieve a cooling effect of 5-10°C below ambient temperature), two key bottlenecks remain to be overcome when applying them to large-scale infrastructure applications such as buildings and roads:

[0004] (1) Bottlenecks in preparation process and environmental protection: Traditional radiation cooling materials (such as photonic thin films and ceramic composites) usually require complex preparation processes, such as high-temperature sintering, precision coating, and in-situ synthesis of nanoparticles. Not only are the production processes energy-intensive and expensive, making it difficult to meet the large-scale production requirements for infrastructure construction; at the same time, most synthetic radiation cooling materials rely on non-renewable petrochemical raw materials or toxic and harmful chemical reagents, and their production and disposal processes are prone to causing environmental pollution.

[0005] Therefore, developing new radiation cooling materials with simple preparation processes, low energy consumption, environmental friendliness, and scalability has become a core requirement for promoting the application of this technology in the infrastructure field.

[0006] (2) Performance compatibility bottleneck: Infrastructure such as buildings and roads has strict requirements on the mechanical properties of materials (such as compressive strength, flexural strength, abrasion resistance, and durability). However, existing radiation cooling materials (such as porous polymer films and aerogels) generally suffer from low mechanical strength, easy breakage, and poor compatibility with substrate materials, making them unsuitable for direct application as structural materials or functional coatings in cement-based infrastructure (such as concrete building exteriors and cement pavements). In addition, cement-based materials, as the most widely used building materials globally, have high thermal conductivity and are prone to absorbing solar radiation in summer, leading to a sudden increase in surface temperature. However, suitable radiation cooling solutions have long been lacking.

[0007] Therefore, developing radiation-cooled cement materials that combine excellent radiation cooling efficiency with mechanical properties that meet infrastructure requirements and are highly compatible with cement-based materials has become an urgent need to solve the high-temperature problem of infrastructure.

[0008] In summary, in view of the technical problems existing in the prior art, providing a method for preparing composite cement materials that combine efficient radiative cooling performance and excellent mechanical strength is a technical problem that urgently needs to be solved by those skilled in the art. Summary of the Invention

[0009] In view of this, the present invention provides a method for preparing a bamboo fiber / cement radiation cooling composite material and its application. The method uses bamboo fiber extracted from natural bamboo as the reinforcing phase, magnesium oxychloride cement MOC as the matrix, composite nano-titanium dioxide (TiO2) radiation cooling particles and polyvinyl alcohol (PVA) modifier, and prepares a composite cement material with both high-efficiency radiation cooling performance and excellent mechanical strength through bamboo fiber orientation arrangement, matrix slurry preparation, and mold casting and curing process.

[0010] To solve the above-mentioned technical problems, the present invention adopts the following technical solution:

[0011] A method for preparing a bamboo fiber / cement radiation cooling composite material includes the following steps:

[0012] S1. Pretreatment of bamboo fiber:

[0013] Bamboo strips are cut into preset sizes and then subjected to acid cooking to remove lignin, alkali soaking for modification, and air drying to obtain high-purity bamboo fiber.

[0014] S2. Preparation of magnesium oxychloride cement-based slurry:

[0015] Weigh light magnesium oxide, magnesium chloride and deionized water, mix and stir to prepare cement-based liquid; add polyvinyl alcohol (PVA) and nano titanium dioxide (TiO2) to the cement-based liquid, ultrasonically disperse and mechanically stir evenly to obtain radiation-cooled cement slurry;

[0016] S3. Directional casting and curing:

[0017] The bamboo fibers pretreated in step S1 are oriented in a mold, and the radiation-cooled cement slurry prepared in step S2 is poured in. The slurry is then placed in a constant temperature and humidity chamber for curing. After the hydration reaction is completed, the mold is removed to obtain radiation-cooled cement.

[0018] Preferably, in step S1, the preset size of the bamboo strip cutting is 1mm×20mm×80mm;

[0019] Preferably, in step 1, the specific process of acid cooking to remove lignin is as follows: Prepare an acidic solution, prepare a certain amount of 1.5 wt.% sodium chlorite solution, and then add acetic acid solution to adjust the pH value to 4.6. After mixing evenly, put a certain amount of natural bamboo into the mixed solution and immerse the bamboo pieces in the acidic solution. Boil at 100°C for 6 hours. After cooking, take out the bamboo pieces and wash them repeatedly with deionized water more than 3 times, soaking for 30 minutes each time, until the pH of the washing solution is neutral.

[0020] Preferably, in step S1, the specific process of alkaline soaking modification is as follows: prepare a sodium hydroxide solution with a mass concentration of 1 wt.%, immerse the deligninated bamboo strips in the sodium hydroxide solution, and let them stand at room temperature for 15 hours; after soaking, take out the bamboo strips, wash them with deionized water until there is no alkaline residue, manually peel off hemicellulose and other components, retain multiple long fibers, and then dry them in a forced-air dryer at room temperature for 8 hours, controlling the moisture content of the bamboo fiber to ≤5%.

[0021] Preferably, in step S2, light magnesium oxide, magnesium chloride and deionized water are weighed according to the MgO:MgCl2:H2O molar ratio of 5:1:13, and mixed and stirred to obtain cement-based liquid; 1% polyvinyl alcohol PVA and 10% nano titanium dioxide TiO2 by mass are added to the cement-based liquid.

[0022] Among them, the activity of light magnesium oxide is 65%, and the loss on ignition is ≤6.0%; magnesium chloride is anhydrous magnesium chloride with a purity ≥99%; the particle size of nano titanium dioxide is 20~50nm, the crystal form is anatase, and the purity is ≥99.5%; the degree of polymerization of polyvinyl alcohol is 1700~1800, and the degree of alcoholysis is ≥99%.

[0023] Preferably, in step S2, the mixing and stirring process parameters are as follows: First, a certain amount of PVA is dispersed in deionized water to form a PVA solution with a concentration of 3.7 wt.%. A certain amount of the above solution is taken, and a certain mass of magnesium chloride is dispersed in it. The mixture is stirred at 500 r / min for 5 min until completely dissolved. Then, a corresponding mass of light magnesium oxide is added, and the stirring speed is increased to 700 r / min for 5 min to obtain a cement-based liquid. 10% of MgO mass of nano-TiO2 is added, and the mixture is mechanically stirred again at 800 r / min for 5 min. Then, it is ultrasonically dispersed at 300 W power and 40 kHz frequency for 30 min to ensure that the slurry is uniform and free of particle agglomeration.

[0024] Preferably, in step S3, the cement is placed in a constant temperature and humidity chamber at 25°C and 60% humidity for 24 hours to cure, and after completing the 73-hour hydration reaction, it is demolded to obtain radiation cooling cement.

[0025] Among them, radiation-cooled cement has a solar reflectivity of ≥90%, an infrared emissivity of ≥99%, and a flexural strength of 176±18MPa. In outdoor environments, it can achieve a passive cooling effect of 5-10℃ lower than the ambient temperature.

[0026] Preferably, in step S3, the bamboo fiber is oriented in the mold as follows: it is laid parallel along the length of the mold, and the number of layers is adjusted according to the thickness of the mold to ensure that the volume fraction of bamboo fiber accounts for 3% to 7% of the total mass of the composite material.

[0027] Preferably, in step S3, the curing process is divided into two stages: the first stage is static curing, in which the material is placed in a constant temperature and humidity chamber for 12 hours after pouring to allow the slurry to initially solidify; the second stage is curing, in which the material is kept under constant temperature and humidity conditions for 24 / 48 / 72 hours, during which the surface hardness of the material is tested every 8 hours, and curing is completed when the Shore hardness is ≥80D.

[0028] A method for preparing a bamboo fiber / cement radiation cooling composite material allows for the application of cement in building exterior walls, roof panels, highway pavement surfaces, or traffic signal housings. By adjusting the mold size, components of different shapes can be prepared to meet diverse engineering needs.

[0029] The present invention achieves the following technical effects compared to the prior art:

[0030] (1) The present invention has excellent cooling performance: the prepared radiation cooling cement has a solar reflectivity of 99.9% and an infrared emissivity of 90%. Compared with bamboo, it can achieve a cooling effect of about 5-10℃. In the outdoor 24-hour test, the lowest temperature is 5℃ lower than the ambient temperature, while the existing ceramic cooling materials are usually 5℃ lower than the ambient temperature. Obviously, the present invention is superior to the existing ceramic cooling materials.

[0031] (2) The present invention has high mechanical strength: bending strength 176±18MPa, compressive strength ≥200MPa, while the bending strength of existing cooled transparent bamboo is about 120MPa and the bending strength of mica / TiO2 material is about 150MPa. Obviously, the present invention exceeds most radiation cooling composite materials. The present invention can meet the load-bearing requirements of building structures.

[0032] (3) This invention is green and environmentally friendly: the raw materials are mainly renewable bamboo fiber and low carbon magnesium oxychloride cement, and the carbon emissions throughout the entire life cycle are only 1 / 6 of those of traditional refrigeration plastics and 1 / 10 of those of refrigeration ceramics; bamboo fiber is biodegradable and has no burden on the environment after disposal.

[0033] (4) The process of this invention is simple and low-cost: no precision equipment is required, and conventional stirring and casting processes are used. When produced on a large scale, the cost is about 1 / 3 of that of photonic thin films.

[0034] (5) The invention has a wide range of applications: different shaped components can be prepared by adjusting the mold size, which can be adapted to various scenarios such as building exterior walls, roofs, and roads. Attached Figure Description

[0035] Figure 1 This is a basic morphological diagram of the radiation-cooled cement of the present invention; Figure 2 The diagram shows the radiation cooling performance of the radiation cooling composite material of the present invention. Where 2 on the left represents solar spectral reflectance; 2 on the right represents atmospheric window emissivity; Figure 3 This is a diagram showing the cooling power of the radiation-cooled cement of the present invention; Figure 4 This is a top-view electron microscope image of the radiation-cooled cement of this invention; Among them, 4a is a top view electron microscope image with a size of 10 micrometers, and 4b is a further magnified electron microscope image of 4a with a size of 5 micrometers. Figure 5 This is the Fourier transform infrared spectrum of the present invention; Figure 6 This is the X-ray diffraction pattern of the present invention; Figure 7 This is a stress-strain curve diagram of the bending strength of the present invention; Figure 8 This is a graph of the cone-shaped calorimetric data of the present invention; Where, a is a graph showing the change of heat release rate (HRR) over time; b is a graph showing the change of total heat release (THR) over time; Figure 9 This is a contact angle test diagram of the present invention; Where a represents before hydrophobic treatment; b represents after hydrophobic treatment; Figure 10This is a schematic diagram of the contact angle of the present invention; Among them, a) the contact angle of the untreated radiation cooling composite material is 46°; b) after simple hydrophobic treatment, its contact angle is increased to 101°. Detailed Implementation

[0036] 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.

[0037] This invention discloses a method for preparing a bamboo fiber / cement radiation cooling composite material, comprising the following steps:

[0038] S1. Pretreatment of bamboo fiber:

[0039] Bamboo strips are cut into preset sizes and then subjected to acid cooking to remove lignin, alkali soaking for modification, and air drying to obtain high-purity bamboo fiber.

[0040] S2. Preparation of magnesium oxychloride cement-based slurry:

[0041] Weigh light magnesium oxide, magnesium chloride and deionized water, mix and stir to prepare cement-based liquid; add polyvinyl alcohol (PVA) and nano titanium dioxide (TiO2) to cement-based liquid, ultrasonically disperse and mechanically stir evenly to obtain radiation-cooled cement slurry.

[0042] S3. Directional casting and curing:

[0043] The bamboo fibers pretreated in step S1 are oriented in a mold, and the radiation-cooled cement slurry prepared in step S2 is poured in. The mold is then placed in a constant temperature and humidity chamber for curing. After the hydration reaction is completed, the mold is removed to obtain radiation-cooled cement.

[0044] In step S1, the preset size for cutting bamboo strips is 1mm×20mm×80mm;

[0045] In step 1, the specific process of acid cooking to remove lignin is as follows: Prepare an acidic solution by preparing a certain amount of 1.5 wt.% sodium chlorite solution, then add acetic acid solution to adjust the pH value to 4.6, mix it evenly, put a certain amount of natural bamboo into the mixed solution, immerse the bamboo pieces in the acidic solution, and boil at 100℃ for 6 hours; after cooking, take out the bamboo pieces and wash them repeatedly with deionized water more than 3 times, soaking for 30 minutes each time, until the pH of the washing solution is neutral.

[0046] In step S1, the specific process of alkaline soaking modification is as follows: prepare a sodium hydroxide solution with a mass concentration of 1 wt.%, immerse the deligninated bamboo strips in the sodium hydroxide solution, and let them stand at room temperature for 15 hours; after soaking, take out the bamboo strips, wash them with deionized water until there is no alkaline residue, manually peel off hemicellulose and other components, retain multiple long fibers, and then dry them in a forced-air dryer at room temperature for 8 hours, controlling the moisture content of the bamboo fiber to ≤5%.

[0047] In step S2, light magnesium oxide, magnesium chloride and deionized water are weighed according to the molar ratio of MgO:MgCl2:H2O 5:1:13, and mixed and stirred to obtain cement-based liquid; 1% polyvinyl alcohol PVA and 10% nano titanium dioxide TiO2 are added to the cement-based liquid according to its mass.

[0048] Among them, the activity of light magnesium oxide is 65%, and the loss on ignition is ≤6.0%; magnesium chloride is anhydrous magnesium chloride with a purity ≥99%; the particle size of nano titanium dioxide is 20~50nm, the crystal form is anatase, and the purity is ≥99.5%; the degree of polymerization of polyvinyl alcohol is 1700~1800, and the degree of alcoholysis is ≥99%.

[0049] In step S2, the mixing and stirring process parameters are as follows: First, a certain amount of PVA is dispersed in deionized water to form a PVA solution with a concentration of 3.7 wt.%. A certain amount of the above solution is taken, and a certain mass of magnesium chloride is dispersed in it. The mixture is stirred at 500 r / min for 5 min until completely dissolved. Then, a corresponding mass of light magnesium oxide is added, and the stirring speed is increased to 700 r / min for 5 min to obtain a cement-based liquid. 10% of MgO mass of nano-TiO2 is added, and the mixture is mechanically stirred again at 800 r / min for 5 min. Then, it is ultrasonically dispersed at 300 W power and 40 kHz frequency for 30 min to ensure that the slurry is uniform and free of particle agglomeration.

[0050] In step S3, the cement is placed in a constant temperature and humidity chamber at 25°C and 60% humidity for 24 hours to cure. After completing the 73-hour hydration reaction, it is demolded to obtain radiation cooling cement.

[0051] Among them, radiation-cooled cement has a solar reflectivity of ≥90%, an infrared emissivity of ≥99%, and a flexural strength of 176±18MPa. In outdoor environments, it can achieve a passive cooling effect of 5℃ lower than the ambient temperature.

[0052] In step S3, the bamboo fiber is oriented in the mold as follows: it is laid parallel along the length of the mold, and the number of layers is adjusted according to the thickness of the mold to ensure that the volume fraction of bamboo fiber accounts for 3% to 7% of the total mass of the composite material.

[0053] In step S3, the curing process is divided into two stages: the first stage is static curing, after pouring, the material is placed in a constant temperature and humidity chamber for 12 hours to allow the slurry to initially solidify; the second stage is curing, which continues to be cured under constant temperature and humidity conditions for 24 / 48 / 72 hours, during which the surface hardness of the material is tested every 8 hours, and curing is completed when the Shore hardness is ≥80D.

[0054] The present invention also discloses a method for preparing bamboo fiber / cement radiation cooling composite material. The cement obtained can be used in building exterior walls, roof panels, road surface layers or traffic signal housings. Different shaped components can be prepared by adjusting the mold size to meet diverse engineering needs.

[0055] The basic morphological diagram of radiation-cooled cement is as follows: Figure 1 As shown.

[0056] Example 1: Optical and cooling properties of radiation-cooled cement

[0057] To evaluate the optical properties of radiation-cooled cement, this invention tested the material's solar reflectance and infrared emissivity. In ordinary silicate cement, the molecular structure of cement hydration products (such as calcium hydroxide and hydrated calcium silicate) results in low emissivity within the atmospheric window (8-13 μm). Simultaneously, the dense structure of the cement matrix and the presence of pigment minerals (such as iron oxides) lead to strong absorption of solar radiation (0.3-2.5 μm), resulting in a reflectance typically less than 40%, making it difficult to achieve an effective radiation-cooling effect.

[0058] The selected magnesium oxychloride cement exhibits significantly improved reflectivity in the solar wavelength range (0.3-2.5μm), with a spectral reflectivity of over 90% across the entire solar spectrum.

[0059] This performance improvement can be attributed to two aspects: first, highly reflective fillers (such as titanium dioxide nanoparticles and hollow glass microspheres) were incorporated into the cement matrix, which reduce the absorption of solar radiation through light scattering; second, the cement surface was modified with micro-nano structures to form a rough surface with a light trapping effect, which further enhances the reflection of sunlight.

[0060] However, the radiation-cooled cement prepared in the early stage had a shortcoming in the infrared emission performance. Due to the addition of high reflectivity filler, the molecular vibration characteristics of the cement matrix were changed to a certain extent, resulting in an emissivity of only about 75% in the atmospheric window, which limited the overall cooling efficiency.

[0061] In this invention, we employ a segmented wavelength measurement scheme to obtain the optical properties of radiation-cooled cement: the reflectance in the solar band (0.2-2.5 μm) is measured using a UV-Vis-NIR spectrophotometer (equipped with an integrating sphere attachment) to ensure accurate capture of the material's reflectance characteristics across the entire solar spectrum; the reflectance in the infrared band (2.5-25 μm) is obtained using a Fourier transform infrared spectrometer (FTIR) equipped with a diffuse reflection golden integrating sphere. According to Kirchhoff's law, the emissivity of a material equals its absorptivity; therefore, it can be calculated by subtracting the measured reflectance from 1. The optimized radiation-cooled cement exhibits an emissivity exceeding 90% within the atmospheric window (8-13 μm).

[0062] The formula for calculating solar reflectance Rsolar is as follows:

[0063]

[0064] The formula for calculating the average thermal emissivity εIR is as follows:

[0065]

[0066] In the above formula, λ is the wavelength; R(λ) is the spectral reflectance of the radiation-cooled cement surface at that wavelength; I AM1.5 (λ) represents the AM1.5 standard solar spectral irradiance (simulating solar radiation received by the Earth's surface); ε(λ) = 1 – R(λ) represents the spectral thermal emissivity of the material surface at that wavelength; I BB (λ, T) represents the spectral radiance of a blackbody at temperature T, which is given by Planck's law (I0...). BB (λ,T)=2hc² / [λ 5 (exp (hc / λkT)-1)], where h is Planck's constant, c is the speed of light, and k is Boltzmann's constant) is defined.

[0067] This superior optical performance is mainly attributed to two factors: Firstly, the Ti-O-Mg bonds formed by the titanium dioxide incorporated into the cement matrix and its hydration products, as well as the OH bonds within the cement itself, exhibit strong molecular vibrations in the atmospheric window wavelength range (8-13 μm), significantly enhancing infrared emission capabilities, such as... Figure 2 The left side (solar spectral reflectance) and the right side (atmospheric window emissivity) are shown. On the other hand, the ordered dispersion structure formed by nano-sized titanium dioxide particles and hollow glass microspheres in the cement matrix constructs a scattering system similar to a photonic crystal, which effectively enhances the diffuse reflection efficiency of sunlight without affecting the radiation transmission in the infrared band, and finally achieves the synergistic optimization of high solar reflectance and high infrared emission.

[0068] To reveal the refrigeration performance of radiation-cooled composite materials, considering all heat exchange processes, its net refrigeration power (Pnet) can be determined by the following formula:

[0069]

[0070]

[0071] Among them, 𝑃 𝑟𝑎d The power radiated outward by the composite material for radiation control.

[0072]

[0073] in, This represents the absorbed power due to incident atmospheric thermal radiation.

[0074]

[0075] Among them, P sun The incident solar power absorbed by the composite material for radiation control.

[0076]

[0077] in, This refers to non-radiative heat loss caused by convection and conduction.

[0078] Here, This is the angle integral over a single hemisphere. The emissivity of cooling bamboo can be represented by its absorptivity, according to Kirchhoff's law of radiation. The formula for calculating atmospheric emissivity is: , where t(λ) is the atmospheric transmittance at the zenith. Solar radiation is expressed as I AM1.5 (λ) represents; assuming the radiation-cooled composite material faces the sun, therefore P sun The term has no angular integral, and the zenith emissivity is taken as θ=0. T r T represents the surface temperature of the radiation-cooled composite material. a The ambient air temperature. H = H cond + H conv The combined nonradiative heat transfer coefficient (H) between the radiative composite material and the surrounding air, generated through conduction and convection; according to existing reports, the H value is typically between 2 and 8 W / m². -2 K. Based on this, the net cooling power (P) of the radiation-cooled composite material under different non-radiative heat transfer coefficients H was calculated. net ) and refrigeration temperature difference (T) a -T r T r For the temperature of the radiation-cooled composite material, T a(Ambient temperature).

[0079] Calculation results show that radiation-cooled cement has highly efficient radiation cooling performance, such as... Figure 3 As shown, the cooling power is close to 63W / m. 2 The cooling power of ordinary cement is negative, as we tested, so this invention has a wide range of applications in fields such as construction.

[0080] Example 2: Structure and Mechanical Properties of Radiation-Cooled Cement

[0081] The composite of magnesium oxychloride cement and oriented bamboo fiber exhibits excellent microstructural stability, primarily due to the synergistic effect of strong interfacial bonding, the support of the oriented fiber structure, and the chemical stability of the composite system. Strong interfacial bonding is fundamental. Magnesium oxychloride cement hydrates to produce products mainly composed of 5·1·8 and 3·1·8 phases, forming a dual bond with bamboo fiber. Bamboo fiber contains active groups such as hydroxyl and carboxyl groups, which can coordinate with Mg²⁺ in the hydration products to form Mg-OC chemical bonds. Furthermore, hydrogen bonds can form between hydroxyl groups, preventing interfacial debonding. Simultaneously, the hollow tubular structure and surface pores of bamboo fiber allow cement hydration products to penetrate and harden into them, forming "mechanical anchors." The oriented arrangement also ensures more uniform physical interlocking between the fiber and the matrix, inhibiting fiber slippage and improving the overall structural integrity.

[0082] Oriented bamboo fiber provides crucial structural support. Arranged along the main stress direction, it can "bridge" microcracks generated during the hardening of the cement matrix, bearing and transferring stress, preventing crack propagation, and its high toughness can buffer shrinkage stress, reducing the initiation of microcracks. Furthermore, the oriented fibers form a continuous "fiber skeleton," constituting a composite structure with the rigid cement matrix. This allows for the uniform transfer of external forces and stresses generated by environmental changes, preventing stress concentration from damaging the structure. Its low thermal conductivity also reduces the system's temperature gradient, minimizing thermal expansion and contraction deformation.

[0083] The chemical stability of the composite system ensures long-term stability. Pure magnesium oxychloride cement is prone to efflorescence and decomposition of hydration products, while the porous structure of bamboo fiber can regulate internal humidity and reduce MgCl2 migration. The coordination of its active groups with Mg²⁺ can also stabilize the crystal structure of hydration products. Regarding the problem that the alkaline environment of cement may corrode bamboo fiber, the cement matrix can be wrapped with a fibrous "protective layer" to reduce its contact with water and oxygen.

[0084] The invention adds polyvinyl alcohol as a dispersant to promote the dispersion of nanoparticles in aqueous solution and as a binder to further improve the interfacial bonding between titanium dioxide nanoparticles and cellulose skeleton.

[0085] in, Figure 4 a is a top-view electron microscope image of a 10-micrometer size, and 4b is a further magnification of 4a, showing that the nanoparticles have been successfully dispersed in radiation-cooled cement.

[0086] Alkali-treated bamboo fiber (ATBF) was used as the base sample, and its spectrum reflected the characteristics of its functional groups. The spectrum was observed at 3200-3600 cm⁻¹. -1 Strong absorption indicates the presence of numerous hydroxyl groups; absorption peaks at specific wavenumbers also reveal its original chemical structure. A comparison of the spectra of the composite system of polyvinyl alcohol (PVA), magnesium oxychloride cement (MOC), and ATBF (PVA / MOC / ATBF) with those of ATBF shows that at 1525 cm⁻¹... -1 1442 cm -1 844cm -1 The emergence of new absorption intensity changes indicates that PVA participated in the recombination and interacted with ATBF or MOC, altering the vibrational environment or content of functional groups. A comparison of the spectra of the sample with added titanium dioxide (TiO2) in the PVA / MOC / ATBF system (PVA / MOC / ATBF / TiO2) with that of PVA / MOC / ATBF shows an absorption peak at 3428 cm⁻¹. -1 2921cm -1 The decrease in intensity at the specified location indicates that the addition of TiO2 has affected the chemical structure of the composite system. TiO2 has chemically bonded and physically adsorbed with other components in the system, such as PVA, MOC, and ATBF, altering the vibrational characteristics of functional groups. This reflects the regulatory effect of TiO2 on the structure of the composite system. Figure 5 As shown.

[0087] By comparison, such as Figure 6 As shown, five strong crystallization peaks were found in the final sample at 11.9° (1 0 0), 21.489°, 29.5°, 37.16°, and 37.6°, indicating that the peak values ​​of the doped phase of titanium dioxide were not affected. The peaks shifted from 21.498° (1 0 1) to 21.489°, 29.160° (1 0 4) to 29.5°, 37.072° (-3 0 4) to 37.16°, and 37.571° (-4 0 3) to 37.6°. The two phases were highly concentrated at 11.9° and 21.489°. In PDF#07-0420, the peaks shifted from 21.498° (1 0 1) to 21.489° and 29.160° (1 0 4) to 37.6°. 4) The offset to 29.5° indicates that the five phases may be converted into three phases.

[0088] Due to the oriented arrangement of long bamboo fibers and the bonding between PVA and magnesium oxychloride cement, radiation-cooled cement exhibits excellent mechanical properties, with a flexural strength of 176±18 MPa. This is 1.65 times that of PVA / MOC / ATBF (100.6 MPa) and 9.6 times that of MOC (17.3 MPa), respectively. Its flexural performance surpasses that of most traditional radiation-cooled inorganic materials, such as... Figure 7 As shown.

[0089] Example 3: Flame retardant performance test

[0090] Figure 8 a represents the change in heat release rate (HRR) over time. Natural bamboo has a high heat release rate and the peak appears early. In contrast, the heat release rates of the PVA / MOC / ATBF and PVA / MOC / ATBF / TiO2 systems are significantly lower, with the peak appearing later and the amplitude being smaller. Figure 8 b represents the change in mass loss rate (MLR) over time. The mass loss rate of bamboo rises rapidly and reaches a high value, while the total mass loss rate of both composite systems is slow and ultimately lower. This demonstrates that the addition of magnesium oxychloride, PVA, titanium dioxide, and bamboo fiber effectively improves the flame retardant properties of the material.

[0091] Figure 9 The sample combustion process was recorded using infrared thermal imaging. As time changed, the temperature gradually increased, but the sample did not ignite or break. Based on the combustion performance and flame spread characteristics of the national standard - Chinese standard (GB 8624-2012), it can be classified as Class A.

[0092] Example 4: Hydrophobicity Test

[0093] like Figure 10 As shown in Figure a, the contact angle of the untreated radiation-cooled composite material is 46°. Figure 10 As described in b, after simple hydrophobic treatment, its contact angle is increased to 101°, which is twice that of the original state, achieving weak hydrophobicity. By improving the weak hydrophobicity of the nanoparticles, the hydrophobic performance can be further enhanced.

[0094] The above description is merely a preferred embodiment of the present invention and does not constitute any limitation on the technical scope of the present invention. Therefore, any minor modifications, equivalent changes, and alterations made to the above embodiments based on the technical essence of the present invention shall still fall within the scope of the technical solution of the present invention.

Claims

1. A method for preparing a bamboo fiber / cement radiation cooling composite material, characterized in that, Includes the following steps: S1. Pretreatment of bamboo fiber: Bamboo strips are cut into preset sizes and then subjected to acid cooking to remove lignin, alkali soaking for modification, and air drying to obtain high-purity bamboo fiber. S2. Preparation of magnesium oxychloride cement-based slurry: Weigh light magnesium oxide, magnesium chloride and deionized water, mix and stir to prepare cement-based liquid; add polyvinyl alcohol (PVA) and nano titanium dioxide (TiO2) to the cement-based liquid, ultrasonically disperse and mechanically stir evenly to obtain radiation-cooled cement slurry; S3. Directional casting and curing: The bamboo fibers pretreated in step S1 are oriented in a mold, and the radiation-cooled cement slurry prepared in step S2 is poured in. The slurry is then placed in a constant temperature and humidity chamber for curing. After the hydration reaction is completed, the mold is removed to obtain radiation-cooled cement.

2. The preparation method of a bamboo fiber / cement radiation cooling composite material according to claim 1, characterized in that, In step S1, the preset size for cutting bamboo strips is 1mm × 20mm × 80mm.

3. The preparation method of a bamboo fiber / cement radiation cooling composite material according to claim 1, characterized in that, In step 1, the specific process of acid cooking for lignin removal is as follows: Prepare an acidic solution by preparing a certain amount of 1.5 wt.% sodium chlorite solution, then add acetic acid solution to adjust the pH value to 4.6, mix them evenly, put a certain amount of natural bamboo into the mixed solution, immerse the bamboo pieces in the acidic solution, and boil at 100℃ for 6 hours; after cooking, take out the bamboo pieces and wash them repeatedly with deionized water more than 3 times, soaking for 30 minutes each time, until the pH of the washing solution is neutral.

4. The preparation method of a bamboo fiber / cement radiation cooling composite material according to claim 1, characterized in that, In step S1, the specific process of alkaline soaking modification is as follows: prepare a sodium hydroxide solution with a mass concentration of 1 wt.%, immerse the deligninated bamboo strips in the sodium hydroxide solution, and let them stand at room temperature for 15 hours; after soaking, take out the bamboo strips, wash them with deionized water until there is no alkaline residue, manually peel off hemicellulose and other components, retain multiple long fibers, and then dry them in a forced-air dryer at room temperature for 8 hours, controlling the moisture content of the bamboo fiber to ≤5%.

5. The method for preparing a bamboo fiber / cement radiation cooling composite material according to claim 1, characterized in that, In step S2, light magnesium oxide, magnesium chloride and deionized water are weighed according to the MgO:MgCl2:H2O molar ratio of 5:1:13, and mixed and stirred to obtain cement-based liquid; 1% polyvinyl alcohol PVA and 10% nano titanium dioxide TiO2 are added to the cement-based liquid. Among them, the activity of light magnesium oxide is 65%, and the loss on ignition is ≤6.0%; magnesium chloride is anhydrous magnesium chloride with a purity ≥99%; the particle size of nano titanium dioxide is 20~50nm, the crystal form is anatase, and the purity is ≥99.5%; the degree of polymerization of polyvinyl alcohol is 1700~1800, and the degree of alcoholysis is ≥99%.

6. The method for preparing a bamboo fiber / cement radiation cooling composite material according to claim 1, characterized in that, In step S2, the mixing and stirring process parameters are as follows: First, a certain amount of PVA is dispersed in deionized water. Then, a certain amount of the above solution is taken, and a certain mass of magnesium chloride is dispersed in it. The mixture is stirred at 500 r / min for 5 min until completely dissolved. Next, a corresponding mass of light magnesium oxide is added, and the stirring speed is increased to 700 r / min for 5 min to obtain cement-based liquid. Add 10% MgO by mass of nano-TiO2, and mechanically stir again at 800 r / min for 5 min, then ultrasonically disperse at 300 W power and 40 kHz frequency for 30 min to ensure that the slurry is uniform and free of particle agglomeration.

7. The method for preparing a bamboo fiber / cement radiation cooling composite material according to claim 1, characterized in that, In step S3, the cement is placed in a constant temperature and humidity chamber at 25°C and 60% for 24 hours to cure. After completing the 72-hour hydration reaction, it is demolded to obtain radiation cooling cement. Among them, the solar reflectivity of the radiation-cooled cement in the range of 0.25~2.5µm is ≥90%, the infrared emissivity in the range of 2.5µm~16µm is ≥99%, the flexural strength is 176±18MPa, and it can achieve a passive cooling effect of 5-10℃ lower than the ambient temperature in outdoor environment.

8. The method for preparing a bamboo fiber / cement radiation cooling composite material according to claim 1, characterized in that, In step S3, the bamboo fiber is oriented in the mold as follows: it is laid parallel along the length of the mold, and the number of layers is adjusted according to the thickness of the mold to ensure that the volume fraction of bamboo fiber accounts for 3% to 7% of the total mass of the composite material.

9. The method for preparing a bamboo fiber / cement radiation cooling composite material according to claim 1, characterized in that, In step S3, the curing process is divided into two stages: the first stage is static curing, after which the slurry is placed in a constant temperature and humidity chamber for 12 hours to allow it to initially solidify; the second stage is curing, which involves maintaining constant temperature and humidity conditions for 24 / 48 / 72 hours, during which the surface hardness of the material is checked every 8 hours, and curing is completed when the Shore hardness is ≥80D.

10. The application of cement obtained by the preparation method of the bamboo fiber / cement radiation cooling composite material according to claims 1-9 in building exterior walls, roof panels, highway pavement surfaces, or traffic signal housings, characterized in that... By adjusting the mold size, components of different shapes can be prepared to meet diverse engineering needs.

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