A method for preparing fly ash-based nanocellulose composite materials

Abstract

This invention relates to a method for preparing a fly ash-based nanocellulose composite material. The method uses fly ash as a silicon source, and obtains a fly ash-derived silicate precursor through acid washing to remove impurities, alkali fusion or alkali leaching to extract silicon, and silicate modulus control. A directional porous framework is constructed using nanocellulose and polyvinyl alcohol. The silicate precursor is then impregnated into the pores of the directional porous framework using vacuum-assisted impregnation. After drying and curing, a silicon-containing inorganic reinforcing layer is formed on the pore wall surface, resulting in an organic-inorganic composite porous material. By controlling the silicate modulus and directional pore structure, this invention enables the precursor to possess both appropriate fluidity and deposition reactivity, avoiding excessive deposition on the outer surface or blockage of internal pores. This results in a material with good structural load-bearing capacity, thermal insulation performance, and flame retardant safety.

Description

Technical Field

[0001] This invention relates to the fields of building energy-saving insulation materials, flame-retardant and heat-insulating composite materials, and resource utilization of industrial solid waste. It relates to a method for preparing a fly ash-based nanocellulose composite material, and more particularly to a method for preparing a low-cost heat-insulating and flame-retardant composite material for buildings with a nanocellulose / polyvinyl alcohol oriented porous skeleton reinforced by fly ash-derived silicates or silica. Background Technology

[0002] With increasing demands for building energy conservation and fire safety, the thermal insulation materials used in building envelopes not only need to have good thermal insulation capabilities, but also need to maintain sufficient mechanical stability and flame retardant safety during construction, transportation, and long-term service. Traditional organic insulation materials, such as polystyrene foam and polyurethane foam, have low thermal conductivity, but their fire resistance and combustion safety are insufficient, making them prone to flame spread and smoke release in the event of a fire. While some inorganic insulation materials have good flame retardant properties, they are often denser, more brittle, or have insufficient adaptability to different construction methods.

[0003] Nanocellulose possesses characteristics such as renewable source, high aspect ratio, abundant surface hydroxyl groups, and easy formation of three-dimensional networks. It can be freeze-cast into lightweight porous structures, showing potential application in thermal insulation materials. However, single nanocellulose porous materials typically suffer from insufficient compressive strength, limited thermal stability, and poor inherent flame retardant properties, making it difficult to directly meet the comprehensive requirements of structural stability and fire safety in the construction industry. Polyvinyl alcohol can act as a water-soluble polymeric binder, forming hydrogen bonds and interchain entanglements with nanocellulose, which helps improve the integrity of the porous framework. However, relying solely on the organic phase is still insufficient to achieve high flame retardancy and high heat resistance.

[0004] Silica or silicate inorganic phases can improve the structural rigidity, thermal stability, and heat insulation capabilities of porous materials during combustion. Existing silicon-modified porous materials mostly use commercial silicon sources or nano-silica, resulting in high material costs and hindering the large-scale promotion of building insulation materials. Fly ash is a major industrial solid waste generated by coal-fired power plants, containing abundant silicon and aluminum components. If the active silicon components in fly ash are converted into impregnable silicate precursors and combined with a nano-cellulose / polyvinyl alcohol porous framework, a unified approach can be achieved to utilize industrial solid waste resources and improve the performance of composite materials.

[0005] However, the silicate modulus of fly ash-derived silicate precursors affects the degree of polymerization, fluidity, penetration depth, and deposition behavior of silicate species. A high modulus leads to excessive polymerization of silicate species, potentially resulting in insufficient wetting within the pores; a low modulus results in excessive alkalinity and reactivity of the precursor, potentially leading to localized aggregation, coarsening of the pore structure, or damage to the pore walls. Therefore, achieving low-cost, high-strength, thermally insulating, and flame-retardant composite materials for building applications through silicate modulus control, directional pore structure design, and synergistic vacuum-assisted impregnation is a crucial technical challenge that needs to be addressed. Summary of the Invention

[0006] To address the challenges of existing building insulation materials in balancing low cost, structural load-bearing capacity, thermal insulation performance, and flame retardant safety, this invention provides a method for preparing fly ash-based nanocellulose composite materials. By controlling the silicate modulus and oriented pore structure, this invention enables the precursor to possess both appropriate flowability and deposition reactivity, avoiding excessive deposition on the outer surface or blockage of internal pores. This results in a material with superior structural load-bearing capacity, thermal insulation performance, and flame retardant safety.

[0007] The technical solution of this invention is as follows: This invention provides a method for preparing a fly ash-based nanocellulose composite material, characterized in that the method for preparing the fly ash-based nanocellulose composite material includes the following steps:

[0008] 1) After acid washing to remove impurities, fly ash is contacted with alkaline components for alkaline fusion or alkaline leaching to extract silicon. Solid-liquid separation is performed to obtain fly ash-derived silicate precursors, and the silicate modulus of the fly ash-derived silicate precursors is adjusted.

[0009] 2) Mix the nanocellulose aqueous dispersion with the polyvinyl alcohol aqueous solution to obtain a nanocellulose / polyvinyl alcohol composite slurry;

[0010] 3) The composite slurry was subjected to directional freezing and freeze-drying to obtain a porous framework of nanocellulose / polyvinyl alcohol with directional interconnected channels;

[0011] 4) The porous framework is placed in the fly ash-derived silicate precursor obtained in step 1) for vacuum-assisted impregnation;

[0012] 5) The impregnated porous skeleton is dried and cured to deposit silicate components on the pore wall surface and form a silicon-containing inorganic reinforcing layer, thus obtaining a fly ash-based nanocellulose composite material.

[0013] Further, the alkaline fusion silicate extraction process in step 1) is as follows: the acid-washed and impurity-removed fly ash is washed until neutral, dried, ground, and sieved to obtain pretreated fly ash; the pretreated fly ash is mixed with alkaline components at a mass ratio of 1:0.8-2.5, placed in a muffle furnace, and calcined at 500-750℃ for 0.5-3 hours, cooled, and ground to obtain an alkaline fusion activated product; the alkaline fusion activated product is then added to deionized water for extraction, with a solid-liquid ratio of 1g:5-20mL, an extraction temperature of 60-100℃, and an extraction time of 0.5-4 hours; after extraction, solid-liquid separation is performed to obtain fly ash-derived silicate precursors.

[0014] Further, the alkaline leaching process for silica treatment in step 1) is as follows: the acid-washed and impurity-removed fly ash is washed until neutral, dried, ground, and sieved to obtain pretreated fly ash; the pretreated fly ash is added to a solution of alkaline components with a mass concentration of 5wt% to 20wt%, the solid-liquid ratio is controlled at 1g:5 to 25mL, and the mixture is stirred and leached at 80 to 120℃ for 2 to 8 hours; or it is leached at 100 to 180℃ for 2 to 6 hours under closed hydrothermal conditions; after leaching, the mixture is cooled and the solid and liquid are separated to obtain fly ash-derived silicate precursors containing soluble silicate components.

[0015] Furthermore, in step 1), the fly ash is fly ash from a coal-fired power plant; the alkaline components include one or both of sodium hydroxide and sodium carbonate.

[0016] Furthermore, in step 1), the silicate modulus is 2.52.

[0017] Furthermore, in step 2), the nanocellulose is TEMPO oxidized cellulose nanofiber, and the solid content of the nanocellulose aqueous dispersion is 0.5wt%~2.0wt%; the weight average molecular weight of polyvinyl alcohol is 20000g / mol~50000g / mol.

[0018] Furthermore, the preparation conditions for the polyvinyl alcohol aqueous solution in step 2) are as follows: polyvinyl alcohol is added to deionized water and stirred at 85℃~100℃ for 0.5h~2h until completely dissolved; the volume ratio of nanocellulose aqueous dispersion to polyvinyl alcohol aqueous solution is 0.5:1~2:1.

[0019] Furthermore, in step 3), directional freezing includes: placing the composite slurry in a directional freezing mold, freezing it in a liquid nitrogen bath for 3 min to 10 min, and then maintaining it at -70℃ to -90℃ for 2 h to 6 h; freeze drying is carried out at -45℃ to -65℃ and 0.05MPa to 0.20MPa for 24 h to 72 h.

[0020] Furthermore, step 4) of vacuum-assisted impregnation includes: first, pre-vacuuming at -0.06MPa to -0.09MPa for 5 min to 15 min, then impregnating at -0.08MPa to -0.10MPa for 20 min to 60 min, and then maintaining the impregnation for 1 h to 3 h.

[0021] Furthermore, in step 5), drying and curing are carried out at 40℃~80℃ for 4h~24h, or under freeze-drying conditions for 12h~48h.

[0022] This invention discloses a method for preparing a fly ash-based nanocellulose composite material. Using fly ash as the silicon source, a fly ash-derived silicate precursor is obtained through acid washing to remove impurities, alkali fusion or alkali leaching to extract silicon, and silicate modulus control. A directional porous framework is constructed using nanocellulose and polyvinyl alcohol. The silicate precursor is then impregnated into the pores of the directional porous framework using vacuum-assisted impregnation. After drying and curing, a silicon-containing inorganic reinforcing layer is formed on the pore wall surface, resulting in an organic-inorganic composite porous material. Therefore, compared with the prior art, this invention has the following beneficial effects:

[0023] 1) This invention, by controlling the silicate modulus and directional pore structure, enables the precursor to have both appropriate fluidity and deposition reactivity, avoiding excessive deposition on the outer surface or blockage of internal pores. This results in a material with good structural load-bearing capacity, thermal insulation performance, and flame retardant safety. The optimized material compressive strength can reach 22.6 MPa, the thermal conductivity is about 0.287 W / (m·K), and the vertical flammability rating reaches UL-94 V-0.

[0024] 2) This invention uses fly ash to replace commercial silicon sources, reducing the cost of inorganic modified raw materials and promoting the high-value utilization of industrial solid waste.

[0025] 3) This invention forms anisotropic interconnected channels through directional freezing, providing a pathway for silicate precursors to enter the interior of the material, while improving load transfer under pressure.

[0026] 4) The silicon phase derived from fly ash can form an inorganic shielding layer during combustion or thermal decomposition, which helps to improve the flame retardancy and thermal stability of the material.

[0027] 5) The raw materials of this invention are widely available and low in cost, and can be used for building walls, roofs, sandwich panels and fireproof and heat-insulating filling layers. Attached Figure Description

[0028] Figure 1 This is a schematic diagram of the preparation process of the present invention.

[0029] Figure 2 This is a schematic diagram of the structure of anisotropic interconnected channels formed by directional freezing according to the present invention.

[0030] Figure 3This is a schematic diagram of the vacuum-assisted impregnation and pore wall thickening and enhancement mechanism of the present invention.

[0031] Figure 4 This is a schematic diagram illustrating the relationship between different alkali treatment concentrations and the modulus of fly ash-derived silicates in this invention.

[0032] Figure 5 This is a comparison diagram of the pore structure parameters of samples with different moduli in this invention. Detailed Implementation

[0033] The overall solution of the present invention will be further described in detail below with reference to specific embodiments and accompanying drawings:

[0034] See Figure 1 The preparation process of a specific embodiment of the present invention is as follows:

[0035] 1) After acid washing to remove impurities, fly ash is contacted with alkaline components for alkaline fusion silicon extraction or alkaline leaching silicon extraction. Solid-liquid separation is performed to obtain fly ash-derived silicate precursors, and the silicate modulus of the fly ash-derived silicate precursors is adjusted.

[0036] Among them, fly ash is preferably fly ash from coal-fired power plants. After acid washing to remove impurities, fly ash can remove some metallic impurities and soluble salts. Then, it is treated with sodium hydroxide or sodium carbonate to convert the silicon and aluminum components into soluble silicate species.

[0037] The steps of alkaline fusion silicate extraction are as follows: After acid washing and impurity removal, the fly ash is washed until neutral, dried, ground, and sieved to obtain pretreated fly ash; the pretreated fly ash is mixed with an alkaline component at a mass ratio of 1:0.8-2.5, placed in a muffle furnace, and calcined at 500-750℃ for 0.5-3 hours, cooled, and ground to obtain an alkaline fusion activated product; the alkaline fusion activated product is then added to deionized water for extraction, with a solid-liquid ratio of 1g:5-20mL, an extraction temperature of 60-100℃, and an extraction time of 0.5-4 hours; after extraction, solid-liquid separation is performed to obtain fly ash-derived silicate precursors.

[0038] The steps of alkaline leaching for silica treatment are as follows: after acid washing and impurity removal, the fly ash is washed until neutral, dried, ground, and sieved to obtain pretreated fly ash; the pretreated fly ash is added to a solution of alkaline components with a mass concentration of 5wt% to 20wt%, and the solid-liquid ratio is controlled at 1g:5 to 25mL, and leaching is carried out at 80 to 120℃ for 2 to 8 hours; or leaching is carried out at 100 to 180℃ for 2 to 6 hours under closed hydrothermal conditions; after leaching, the mixture is cooled and the solid and liquid are separated to obtain fly ash-derived silicate precursors containing soluble silicate components.

[0039] The silicate modulus can be controlled by the concentration of alkali treatment and the degree of silicon source dissolution.

[0040] The alkaline components include one or both of sodium hydroxide and sodium carbonate.

[0041] The silicate modulus is 1.14 to 3.18, preferably 1.96 to 2.52, and more preferably 2.52.

[0042] 2) Mix the nanocellulose aqueous dispersion with the polyvinyl alcohol aqueous solution to obtain a nanocellulose / polyvinyl alcohol composite slurry.

[0043] The nanocellulose is TEMPO oxidized cellulose nanofiber, and the solid content of the nanocellulose aqueous dispersion is 0.5wt%~2.0wt%; the weight average molecular weight of polyvinyl alcohol is 20000g / mol~50000g / mol.

[0044] The preparation conditions for polyvinyl alcohol aqueous solution are as follows: add polyvinyl alcohol to deionized water and stir at 85℃~100℃ for 0.5h~2h until completely dissolved; the volume ratio of nanocellulose aqueous dispersion to polyvinyl alcohol aqueous solution is 0.5:1~2:1.

[0045] 3) The composite slurry was subjected to directional freezing and freeze-drying to obtain a nanocellulose / polyvinyl alcohol porous framework with directional interconnected channels.

[0046] Directional freezing includes: placing the composite slurry in a directional freezing mold, freezing it in a liquid nitrogen bath for 3 min to 10 min, and then maintaining it at -70℃ to -90℃ for 2 h to 6 h; freeze drying is carried out at -45℃ to -65℃ and 0.05MPa to 0.20MPa for 24 h to 72 h.

[0047] The directional freezing method of this invention refers to the crystallization and growth of an aqueous phase in a mold with a temperature gradient along a specific direction. Nanocellulose and polyvinyl alcohol are repelled and enriched in the interstitial spaces between the ice crystals, and after freeze-drying, interconnected channels arranged along the freezing direction are formed. This structure provides channels for subsequent impregnation of silicate precursors and also helps to improve the stress transfer efficiency of the material in the compressive direction.

[0048] See Figure 2 During directional freezing, the aqueous phase gradually crystallizes and grows along the temperature gradient, while nanocellulose and polyvinyl alcohol are displaced and enriched in the interstitial regions of the ice crystals, forming a continuous pore wall structure aligned along the freezing direction. After freeze-drying to remove the ice crystal template, directional interconnected channels are formed inside the material. This directional pore structure helps reduce the mass transfer resistance of subsequent fly ash-derived silicate precursors into the framework, and also provides a relatively continuous load transfer path in the compressive direction, thereby improving the structural stability and compressive strength of the composite material.

[0049] 4) The porous skeleton is placed in the fly ash-derived silicate precursor obtained in step 1) for vacuum-assisted impregnation.

[0050] The vacuum-assisted impregnation method of this invention includes two stages: pre-vacuuming and impregnation holding. Pre-vacuuming is used to remove air from the porous framework and reduce the resistance of the liquid precursor entering the pores; impregnation holding is used to ensure that the fly ash-derived silicate precursor fully enters the internal pores and adheres to the pore walls.

[0051] The vacuum-assisted impregnation process includes: first, pre-vacuuming at -0.06MPa to -0.09MPa for 5 to 15 minutes, then impregnating at -0.08MPa to -0.10MPa for 20 to 60 minutes, followed by maintaining the impregnation for 1 to 3 hours.

[0052] 5) The impregnated porous skeleton is dried and cured to allow silicate components to deposit on the pore wall surface and form a silicon-containing inorganic reinforcing layer, thus obtaining a fly ash-based nanocellulose composite material.

[0053] Drying and curing are carried out at 40℃~80℃ for 4h~24h, or under freeze-drying conditions for 12h~48h.

[0054] See Figure 3 Before impregnation, the CNF / PVA porous framework is mainly composed of nanocellulose and polyvinyl alcohol, with thin pore walls and predominantly organic phase. Vacuum pretreatment removes air from the pores, allowing fly ash-derived silicate precursors to enter the porous framework under the combined influence of pressure difference and capillary action. During impregnation and subsequent drying and curing, silicate species undergo adsorption, dehydration condensation, or deposition on the pore wall surface, forming a silicon-containing inorganic reinforcing layer. This reinforcing layer thickens the pore walls and improves their rigidity, while also forming an inorganic shielding structure under high temperature or combustion conditions, thereby improving the material's mechanical strength, thermal stability, and flame retardant properties.

[0055] The following are specific application examples of the present invention.

[0056] Example 1.

[0057] A method for preparing a low-cost fly ash-based nanocellulose thermal insulation and flame-retardant composite material for building applications includes the following steps:

[0058] 1) Preparation of fly ash-derived silicate precursor: After acid washing to remove impurities, fly ash was subjected to alkaline leaching with a 5 wt% sodium hydroxide solution for silica extraction. The fly ash was added to the 5 wt% sodium hydroxide solution, and the solid-liquid ratio was controlled at 1 g:15 mL. The mixture was stirred and leached at 100℃ for 6 h. After leaching, the mixture was cooled and the solid and liquid components were separated to obtain a fly ash-derived silicate precursor containing soluble silicate components. Solid-liquid separation yielded a fly ash-derived silicate precursor with a silicate modulus of 3.18.

[0059] 2) Preparation of CNF / PVA composite slurry: Weigh 8g of polyvinyl alcohol and add it to 50mL of deionized water. Stir continuously at 95℃ for 1h until completely dissolved. Then add the obtained polyvinyl alcohol solution to 50mL of TEMPO oxidized nanocellulose aqueous dispersion with a solid content of 1wt%. Stir and mix and sonicate for 15min to remove air bubbles.

[0060] 3) Preparation of directional porous framework: The composite slurry obtained in step 2) is poured into a directional cryogenic mold, kept in a liquid nitrogen bath for 5 min, then frozen at -80℃ for 4 h, and then freeze-dried at -55℃ and 0.15MPa for 48 h to obtain CNF / PVA directional porous framework.

[0061] 4) Vacuum-assisted impregnation: Place the porous skeleton obtained in step 3) into the silicate precursor obtained in step 1), first pre-evacuate at -0.08MPa for 10 min, then vacuum impregnate at -0.10MPa for 30 min, and then continue to impregnate for 2 h.

[0062] 5) Drying and curing: The impregnated porous skeleton is removed and dried to obtain a fly ash-based nanocellulose thermal insulation and flame retardant composite material, denoted as CNF / PVA / FA-5.

[0063] Example 2.

[0064] Compared with Example 1, the difference is that in step 1), a 10wt% sodium hydroxide solution was used to treat fly ash to obtain a fly ash-derived silicate precursor with a silicate modulus of 2.52. The remaining steps are the same as in Example 1, and the resulting composite material is denoted as CNF / PVA / FA-10.

[0065] In this embodiment, the fly ash-derived silicate precursor possesses both good pore permeability and appropriate deposition reactivity, enabling the formation of a relatively uniform silicon-enriched pore wall reinforcement layer within the CNF / PVA porous framework. The resulting composite material exhibits well-preserved directional pores, high specific surface area and pore volume, and superior compressive and flame-retardant properties.

[0066] Example 3.

[0067] Compared with Example 1, the difference is that in step 1), a 15 wt% sodium hydroxide solution was used to treat fly ash to obtain a fly ash-derived silicate precursor with a silicate modulus of 1.96. The remaining steps are the same as in Example 1, and the resulting composite material is denoted as CNF / PVA / FA-15.

[0068] Example 4.

[0069] Compared with Example 1, the difference is that in step 1), a 20wt% sodium hydroxide solution was used to treat fly ash to obtain a fly ash-derived silicate precursor with a silicate modulus of 1.14. The remaining steps are the same as in Example 1, and the resulting composite material is denoted as CNF / PVA / FA-20.

[0070] Comparative Example 1.

[0071] Compared with Example 1, the difference is that the fly ash-derived silicate precursor is not vacuum impregnated, and CNF / PVA oriented porous framework is prepared only according to steps 2) and 3) in Example 1. The resulting material is referred to as CNF / PVA.

[0072] Comparative Example 2.

[0073] Compared with Example 1, the difference is that polyvinyl alcohol is not added, and only a nanocellulose aqueous dispersion is used for directional freezing and freeze-drying. The resulting material is denoted as CNF.

[0074] Performance testing.

[0075] See Figure 4 As the alkali treatment concentration changes, the degree of dissolution of silicate and alumina components and the composition of silicate species in fly ash change, leading to corresponding changes in the silicate modulus. The figure shows that different moduli correspond to different precursor fluidity, polymerization degree, and pore wall deposition behavior. When the modulus is too high, the silicate species in the precursor have a high degree of polymerization, which may affect their full penetration into the framework; when the modulus is too low, the precursor is highly reactive, easily causing local deposition, pore structure coarsening, or pore blockage. Therefore, obtaining a suitable silicate modulus by controlling the alkali treatment concentration is key to achieving uniform impregnation and effective pore wall reinforcement.

[0076] In this embodiment of the invention, the silicate modulus and sample number of different samples are shown in Table 1 below.

[0077] Table 1. Parameters of fly ash-derived silicate precursors in different embodiments

[0078]

[0079] The pore structure, density, thermal conductivity, compressive properties, and flame retardant properties of the materials prepared in Examples 1-4 and the comparative examples were tested respectively. The test results show that the introduction of fly ash-derived silicon source can transform the pore wall from a single organic framework to an organic-inorganic composite reinforced structure, thereby improving the compressive strength, thermal stability, and flame retardant properties of the material.

[0080] The pore structure and density parameters of the embodiments and comparative examples are shown in Table 2 below.

[0081] Table 2. Pore structure and density parameters of the materials in the examples and comparative examples.

[0082]

[0083] As shown in Table 2, CNF / PVA / FA-10 corresponding to Example 2 has the highest specific surface area and the largest total pore volume, while the average pore diameter remains at a relatively small level. This indicates that the fly ash-derived silicate precursor with a modulus of 2.52 is beneficial for achieving pore wall reinforcement while maintaining pore connectivity. In contrast, when the modulus is too high, silicate species penetration is insufficient, and when the modulus is too low, pore structure coarsening or local aggregation is likely to occur.

[0084] Further testing revealed the key properties of the optimized CNF / PVA / FA-10 sample, as shown in Table 3. Although this sample has a higher thermal conductivity than some ultralight aerogel materials, it exhibits superior structural load-bearing capacity and flame-retardant safety, making it more suitable for construction, handling, installation, and service applications.

[0085] Table 3 Key performance indicators of the optimized embodiments

[0086]

[0087] See Figure 5 Different fly ash-derived silicate moduli affect the pore structure characteristics of composite materials, such as specific surface area, total pore volume, and average pore diameter. When the silicate modulus is within a suitable range, the precursor can fully penetrate the CNF / PVA oriented porous framework and form a relatively uniform silicon-containing reinforcing layer on the pore wall surface, allowing the material to achieve a high specific surface area and pore volume while maintaining pore connectivity. Conversely, excessively high or low moduli can lead to insufficient pore penetration, local aggregation, or pore structure destruction. Figure 5 It is evident that composite materials obtained under suitable modulus conditions have better pore structure retention and interface reinforcement effects, which is conducive to achieving synergistic improvement in thermal insulation, mechanical support and flame retardant properties.

[0088] Mechanism explanation:

[0089] The performance improvement of this invention stems from multi-scale synergistic effects: First, the hydrogen bonding between nanocellulose and polyvinyl alcohol constructs a continuous organic framework, improving the initial structural integrity of the porous material; second, directional freezing forms interconnected channels arranged along the freezing direction, providing pathways for precursor impregnation and improving load transfer under pressure; third, fly ash-derived silicates are deposited on the pore walls during the drying and curing stage, forming a silicon-containing inorganic reinforcement layer, improving the rigidity and thermal stability of the pore walls; fourth, under combustion or high-temperature conditions, the silicon-containing inorganic phase can form a heat-insulating shielding layer, reducing the release of combustible volatiles and inhibiting flame spread.

[0090] Especially when the silicate modulus is around 2.52, the diffusion and aggregation abilities of silicate species are relatively balanced, which can prevent the precursor from mainly remaining on the outer surface and reduce pore blockage caused by excessively rapid aggregation. Therefore, Example 2 achieves a better balance between density, pore structure, strength, thermal insulation, and flame retardancy.

[0091] Industrial applicability:

[0092] This invention uses readily available raw materials such as fly ash, nanocellulose, and polyvinyl alcohol as main components. The preparation process centers on an aqueous system, cryogenic pore formation, and vacuum impregnation, with a clear process route suitable for further scale-up preparation into building insulation boards, roof insulation layers, filling layers for prefabricated building sandwich panels, or irregularly shaped fireproof and heat-insulating components. This material combines the advantages of utilizing industrial solid waste, low-cost preparation, mechanical support, thermal insulation, and flame retardancy, demonstrating promising prospects for application in building engineering.

[0093] The technical contents of this invention and those not specifically described in the above embodiments are the same as those in the prior art.

[0094] The above are merely specific embodiments disclosed in this invention, but the scope of protection disclosed in this invention is not limited thereto. The scope of protection disclosed in this invention should be determined by the scope of the claims.

Claims

1. A method for preparing a fly ash-based nanocellulose composite material, characterized in that: The preparation method of the fly ash-based nanocellulose composite material includes the following steps: 1) After acid washing to remove impurities, fly ash is contacted with alkaline components for alkaline fusion silicon extraction or alkaline leaching silicon extraction. Solid-liquid separation is performed to obtain fly ash-derived silicate precursors, and the silicate modulus of the fly ash-derived silicate precursors is adjusted. 2) Mix the nanocellulose aqueous dispersion with the polyvinyl alcohol aqueous solution to obtain a nanocellulose / polyvinyl alcohol composite slurry; 3) The composite slurry was subjected to directional freezing and freeze-drying to obtain a porous framework of nanocellulose / polyvinyl alcohol with directional interconnected channels; 4) The porous framework is placed in the fly ash-derived silicate precursor obtained in step 1) for vacuum-assisted impregnation; 5) The impregnated porous skeleton is dried and cured to allow silicate components to deposit on the pore wall surface and form a silicon-containing inorganic reinforcing layer, thus obtaining a fly ash-based nanocellulose composite material.

2. The method for preparing fly ash-based nanocellulose composite material according to claim 1, characterized in that: The alkaline fusion silicon extraction process in step 1) is as follows: the acid-washed and impurity-removed fly ash is washed until neutral, dried, ground and sieved to obtain pretreated fly ash. Pretreated fly ash and alkaline components were mixed evenly at a mass ratio of 1:0.8-2.5 and calcined in a muffle furnace at 500-750℃ for 0.5-3 hours. After cooling, the mixture was ground to obtain an alkali-activated product. The alkali-activated product was then added to deionized water for extraction at a solid-liquid ratio of 1g:5-20mL, an extraction temperature of 60-100℃, and an extraction time of 0.5-4 hours. After extraction, solid-liquid separation was performed to obtain fly ash-derived silicate precursors.

3. The method for preparing fly ash-based nanocellulose composite material according to claim 1, characterized in that: The alkaline leaching silicon treatment in step 1) is as follows: the acid-washed and impurity-removed fly ash is washed until neutral, dried, ground and sieved to obtain pretreated fly ash. Pretreated fly ash is added to a solution of alkaline components with a mass concentration of 5wt% to 20wt%, and the solid-liquid ratio is controlled at 1g:5 to 25mL. The mixture is stirred and extracted at 80 to 120℃ for 2 to 8 hours; or it is extracted at 100 to 180℃ for 2 to 6 hours under closed hydrothermal conditions. After extraction, the mixture is cooled and the solid and liquid are separated to obtain fly ash-derived silicate precursors containing soluble silicate components.

4. The method for preparing fly ash-based nanocellulose composite material according to claim 2 or 3, characterized in that: In step 1), the fly ash is fly ash from a coal-fired power plant; the alkaline components include one or both of sodium hydroxide and sodium carbonate.

5. The method for preparing fly ash-based nanocellulose composite material according to claim 4, characterized in that: In step 1), the silicate modulus is 2.

52.

6. The method for preparing fly ash-based nanocellulose composite material according to claim 5, characterized in that: In step 2), the nanocellulose is TEMPO oxidized cellulose nanofiber, and the solid content of the nanocellulose aqueous dispersion is 0.5wt%~2.0wt%; the weight average molecular weight of polyvinyl alcohol is 20000g / mol~50000g / mol.

7. The method for preparing fly ash-based nanocellulose composite material according to claim 6, characterized in that: The preparation conditions for the polyvinyl alcohol aqueous solution in step 2) are as follows: polyvinyl alcohol is added to deionized water and stirred at 85℃~100℃ for 0.5h~2h until completely dissolved; the volume ratio of nanocellulose aqueous dispersion to polyvinyl alcohol aqueous solution is 0.5:1~2:

1.

8. The method for preparing fly ash-based nanocellulose composite material according to claim 7, characterized in that: The directional freezing in step 3) includes: placing the composite slurry in a directional freezing mold, freezing it in a liquid nitrogen bath for 3 min to 10 min, and then maintaining it at -70℃ to -90℃ for 2 h to 6 h; freeze drying is carried out at -45℃ to -65℃ and 0.05MPa to 0.20MPa for 24 h to 72 h.

9. The method for preparing fly ash-based nanocellulose composite material according to claim 8, characterized in that: The vacuum-assisted impregnation in step 4) includes: first, pre-vacuuming at -0.06MPa to -0.09MPa for 5 min to 15 min, then impregnating at -0.08MPa to -0.10MPa for 20 min to 60 min, and then maintaining the impregnation for 1 h to 3 h.

10. The method for preparing fly ash-based nanocellulose composite material according to claim 9, characterized in that: In step 5), drying and curing are performed at 40℃~80℃ for 4h~24h, or under freeze-drying conditions for 12h~48h.

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