Method for in-situ growth of nano-silica based on fly ash

Abstract

The invention discloses a fly ash-based in-situ growth method of nano silicon dioxide, which comprises the following steps: (1) adding fly ash, a silicon source and a dispersing agent into a mixed solution of ethanol and water to obtain a turbid liquid, and adjusting the pH value of the turbid liquid to be greater than 12; fully reacting in a water bath; and (2) heating the solution after the reaction in the step (1), then introducing pure carbon dioxide or gas containing carbon dioxide, stopping introducing gas when the pH value of the solution is reduced to 7-8, standing and aging, and washing, centrifuging and drying or freeze-drying the obtained reaction product to obtain the fly ash-based in-situ growth nano silicon dioxide. By optimizing the fly ash pretreatment step, the Si-O-Si covalent bond interface layer between the nano silicon dioxide particles and the fly ash is constructed, the preparation effect is improved, and the preparation cost is reduced.

Description

Technical Field

[0001] The invention relates to a method for in-situ growth of fly ash-based nano silicon dioxide. Background Art

[0002] Fly ash-based polymers, especially low-calcium types, exhibit low compressive strength and slow strength growth when cured at room temperature. While fly ash-based polymers can achieve satisfactory compressive strength when cured at elevated temperatures, improving their compressive strength at room temperature is crucial for their widespread application, particularly in cast-in-place concrete. In recent years, researchers have attempted to enhance the room-temperature strength of fly ash-based polymers by adding nanosilica. However, the large surface area of ​​commercial nanosilica significantly reduces the workability of the slurry due to its high water absorption and aggregation. In-situ nanomaterial preparation technology can uniformly coat the fly ash surface with a layer of nanosilica to achieve a well-prepared dispersion, thus fundamentally preventing agglomeration. This in-situ growth of fly ash-based nanosilica minimizes the negative impact on the fluidity of the geopolymer slurry. Furthermore, the nanosilica uniformly dispersed on the fly ash surface rapidly reacts to produce polycondensation products, acting as nuclei to accelerate the geopolymerization reaction, significantly improving the room-temperature strength of the fly ash-based polymer.

[0003] Patent 202111209928.7 discloses a method for preparing in situ grown nano-SiO2 on the surface of solid waste. It uses the sol-gel method to prepare in situ grown nano-silica with excellent dispersibility, but the silicon source ethyl orthosilicate and catalyst ammonia water used are expensive. Patent 201711143543.9 discloses a method for preparing nanoparticles grown in situ on the surface of solid waste. The method uses a carbonization method to grow nano-silica particles in situ on the surface of a solid waste matrix. During the use of this method, acid and alkali solution pretreatment will increase costs and require the construction of a separate fly ash pretreatment production line, resulting in a complicated process flow and a long pretreatment time. At the same time, during pretreatment, part of the Si on the surface of the fly ash will be dissolved into the solution. After the fly ash pretreatment is completed, the fly ash needs to be separated from the pretreatment liquid, causing this part of the Si to be lost with the solution, and the pretreatment liquid needs to be additionally treated. Furthermore, washing and drying during the separation of the fly ash will also cause changes in the active chemical bonds (mainly Si-O bond cleavage) on its surface, thereby weakening the pretreatment effect. However, if pretreatment is not performed, nano-silica cannot grow on the fly ash. Physical adsorption is the main method between fly ash and nano-silica. The fly ash-based in situ grown nano-silica obtained by this method will fall off on a large scale during the slurry stirring process. Summary of the Invention

[0004] Purpose of the invention: The purpose of the present invention is to provide a method for in-situ growth of fly ash-based nano-silica, which can not only construct a Si-O-Si covalent bond interface layer between nano-silica particles and fly ash, but also effectively shorten the preparation time, simplify the process, improve efficiency, and significantly reduce production costs.

[0005] Technical solution: The method for in-situ growth of fly ash-based nano-silicon dioxide of the present invention comprises the following steps:

[0006] (1) adding fly ash, silicon source and dispersant to a mixture of ethanol and water to obtain a suspension, adjusting the pH of the suspension to be greater than 12; and fully reacting in a water bath;

[0007] (2) heating the solution after the reaction in step (1), then introducing pure carbon dioxide or a gas containing carbon dioxide, stopping the aeration when the pH of the solution drops to 7-8, and allowing the solution to stand for aging. The obtained reaction product is washed, centrifuged, and dried or freeze-dried to obtain fly ash-based in-situ grown nano-silica, wherein the particle size of the in-situ grown nano-silica is 20-100 nm.

[0008] Wherein, in step (1), the mass ratio of the fly ash, silicon source, and dispersant (the dispersant mainly plays an auxiliary dispersing effect to prevent the nano-silicon dioxide on the surface of the fly ash balls from generating a large number of agglomerates, thereby ensuring that the nanoparticles are evenly distributed on the periphery of the fly ash) is 100:12.5~37.5:5~10. The ratio of fly ash to silicon source can control the growth amount of silicon dioxide nanoparticles on the surface of the fly ash. The dispersant can effectively disperse the nanoparticles. Too little will lead to poor synthesis effect, and too much will lead to a reduction in the adsorption amount of nanoparticles on the fly ash surface; the fly ash is Class F I ash or Class II ash; the silicon source is sodium metasilicate, sodium metasilicate pentahydrate, or sodium metasilicate nonahydrate powder; and the dispersant is polyethylene glycol or hexadecyltrimethylammonium bromide.

[0009] In step (1), the volume ratio of ethanol to water in the mixed solution of ethanol and water is 1:8-12.

[0010] In step (1), during the reaction, the water bath temperature can be room temperature, the stirring rate is 400-800 rpm, and the reaction time is no more than 6 h.

[0011] In step (2), the mixture is heated to 40-50° C. and pure carbon dioxide gas is introduced at a ventilation rate of 500-800 mL / min; and the aging time is 0.5-1 h.

[0012] The fly ash-based in-situ grown nano-silica prepared above is used to prepare fly ash geopolymer slurry, specifically, the fly ash-based in-situ grown nano-silica, fly ash, an alkali activator and water are mixed to obtain the fly ash geopolymer slurry.

[0013] The added mass ratio of fly ash-based in-situ grown nano-silica, fly ash, alkali activator and water is 5-15:88-96:31.3:26.9.

[0014] When fly ash is used to in-situ grow nanosilica on fly ash to modify fly ash geopolymers, the fly ash drives the nanosilica particles to disperse evenly throughout the slurry, effectively preventing nanosilica particle agglomeration and water absorption, thereby reducing the negative impact on the slurry's performance. Simultaneously, the activator within the fly ash geopolymer dissolves the active silica and aluminum components in the fly ash, triggering a polycondensation reaction to produce a gel, which provides strength to the geopolymer. This process accelerates with increasing temperature and is extremely slow at room temperature, resulting in low strength and slow growth of fly ash geopolymers cured at room temperature. At this point, the highly active nanosilica grown on the fly ash surface, under the action of the activator, rapidly dissociates into monomers, serving as nucleation sites to promote the polycondensation reaction. This results in the formation of a larger number of polymeric gel products, which fill the gaps between particles and bond adjacent fly ash particles together, ensuring that the geopolymer maintains sufficient compressive strength even at room temperature.

[0015] Silicon source solutions with a pH greater than 12 corrode the fly ash surface, producing a large number of broken Si-O bonds and increasing the roughness of the fly ash. According to the chemical principle of strong acid to weak acid conversion, carbonic acid is more acidic than silicic acid. Therefore, after the introduced carbon dioxide dissolves in water to become carbonic acid, it undergoes a double decomposition reaction with an alkaline aqueous solution (an alkaline solution formed by sodium metasilicate, sodium metasilicate pentahydrate, or sodium metasilicate nonahydrate powder) to form orthosilicate Si(OH)4 monomers. According to the theory of heterogeneous nucleation, silicate has low solubility and nucleates after reaching a critical nucleation concentration in the solution. The fly ash in the solution serves as the interface for its heterogeneous nucleation. The Si(OH)4 monomers polymerize with the broken SO bonds on the fly ash surface to form Si-O-Si chemical bonds, connecting the two through covalent bonds and then gradually growing into nanoparticles. These silicate nanoparticles easily decompose into hydrated silica, which leaves nanosilica particles on the fly ash surface after heating or freeze-drying to drive out moisture, thereby producing fly ash-based in situ grown nanosilica.

[0016] Beneficial effects: Compared with the prior art, the present invention has the following significant advantages: (1) The method of the present invention can greatly shorten the process time for obtaining fly ash-based in-situ grown nano-silicon dioxide. The existing carbonization method has a pretreatment process, which includes etching reaction, washing, and drying. The entire treatment cycle requires 24 hours, while the present invention uses a precursor solution to treat fly ash, which only takes 6 hours to complete. Then, carbon dioxide is directly introduced to carry out in-situ growth of nano-silicon dioxide reaction; (2) The method of the present invention uses a precursor solution to treat fly ash, which simplifies the experimental process, shortens the reaction time, and significantly reduces costs and energy consumption. On the one hand, this is because the Si dissolved in the solution during the etching of the fly ash in the precursor solution will also be re-fixed to the fly ash. The fly ash surface is cleaned, thereby increasing the growth amount of nano-silica under the premise of adding the same silicon source; conversely, the amount of silicon source added can be reduced, thereby reducing production costs; on the other hand, after the washing and drying steps without pretreatment, the Si-O broken bonds generated on the fly ash surface obtained by the present invention are always in a highly active state, and have a strong ability to bind with the Si(OH)4 monomer dissociated from the precursor solution, making it easier to undergo condensation to produce a strong Si-O-Si chemical bond to connect the fly ash and nano-silica, thereby further improving the reaction efficiency and reducing energy consumption; (3) The highly active nano-silica particles grown on the fly ash surface can serve as crystal nuclei to accelerate the production of geopolymer gel, so that the fly ash geopolymer can also have high compressive strength in a normal temperature curing room. BRIEF DESCRIPTION OF THE DRAWINGS

[0017] Figure 1 This is a SEM image of fly ash-based in-situ grown nano-silicon dioxide obtained in Example 1;

[0018] Figure 2 This is a SEM image of fly ash-based in-situ grown nano-silicon dioxide obtained in Example 4;

[0019] Figure 3 This is a SEM image of fly ash-based in-situ grown nano-silicon dioxide obtained in Example 5;

[0020] Figure 4 The SEM images of the original fly ash used in Examples 1 to 5 and Comparative Examples 1 to 5 are shown;

[0021] Figure 5 XRD patterns of fly ash-based in-situ grown nano-silicon dioxide obtained in Example 1, commercial nano-silicon dioxide, monodisperse nano-silicon dioxide prepared by carbonization, and the original fly ash used in the present invention;

[0022] Figure 6 This is the Si2p-XPS graph of the fly ash-based in-situ grown nano-silica obtained in Example 1;

[0023] Figure 7This is a locally enlarged SEM image of the fly ash-based in-situ grown nano-silica obtained in Comparative Example 5. DETAILED DESCRIPTION

[0024] Example 1

[0025] The method for in-situ growth of fly ash-based nano-silicon dioxide of the present invention comprises the following steps:

[0026] (1) Weigh 100 parts of fly ash, 25 parts of sodium metasilicate, and 5 parts of polyethylene glycol by weight, add them to a solution of ethanol and water in a volume ratio of 1:8 to obtain a mixed solution, adjust the pH of the mixed solution to greater than 12, and transfer the solution to a water bath and stir at 400 rpm for 6 h;

[0027] (2) The mixed solution was heated to 40°C, and carbon dioxide gas was introduced at a ventilation rate of 500 mL / min. The ventilation was stopped when the pH of the solution dropped to 7. The reaction was aged for 1 hour. The reaction product was washed with anhydrous ethanol and deionized water three times respectively, centrifuged at 5000 rpm, and dried at 100°C to obtain fly ash-based in situ grown nano-silica.

[0028] The growth amount of the fly ash-based in-situ grown nano-silica calculated by thermogravimetry is 19.83% of the total mass of the fly ash-based in-situ grown nano-silica.

[0029] The fly ash-based in-situ grown nano-silica prepared above is used to prepare a fly ash geopolymer slurry. Specifically, 5 parts by weight of fly ash-based in-situ grown nano-silica, 96 parts by weight of fly ash, 31.3 parts by weight of an alkali activator (sodium silicate) and 26.9 parts by weight of water are mixed and stirred evenly to obtain a fly ash geopolymer slurry.

[0030] Example 2

[0031] The method for in-situ growth of fly ash-based nano-silicon dioxide of the present invention comprises the following steps:

[0032] (1) Weigh 100 parts of fly ash, 25 parts of sodium metasilicate, and 5 parts of polyethylene glycol by weight, add them to a solution of ethanol and water in a volume ratio of 1:8 to obtain a mixed solution, adjust the pH of the mixed solution to greater than 12, and transfer the solution to a water bath and stir at 400 rpm for 6 h;

[0033] (2) The mixed solution was heated to 40°C, and carbon dioxide gas was introduced at a ventilation rate of 500 mL / min. The ventilation was stopped when the pH of the solution dropped to 7. The reaction was aged for 1 hour. The reaction product was washed with anhydrous ethanol and deionized water three times respectively, centrifuged at 5000 rpm, and dried at 100°C to obtain fly ash-based in situ grown nano-silica.

[0034] The fly ash-based in-situ grown nano-silica prepared above is used to prepare a fly ash geopolymer slurry. Specifically, 10 parts by weight of fly ash-based in-situ grown nano-silica, 92 parts by weight of fly ash, 31.3 parts by weight of an alkali activator (sodium silicate) and 26.9 parts by weight of water are mixed and stirred evenly to obtain a fly ash geopolymer slurry.

[0035] Example 3

[0036] The method for in-situ growth of fly ash-based nano-silicon dioxide of the present invention comprises the following steps:

[0037] (1) Weigh 100 parts of fly ash, 25 parts of sodium metasilicate, and 5 parts of polyethylene glycol by weight, add them to a solution of ethanol and water in a volume ratio of 1:8 to obtain a mixed solution, adjust the pH of the mixed solution to greater than 12, and transfer the solution to a water bath and stir at 400 rpm for 6 h;

[0038] (2) The mixed solution was heated to 40°C, and carbon dioxide gas was introduced at a ventilation rate of 500 mL / min. The ventilation was stopped when the pH of the solution dropped to 7. The reaction was aged for 1 hour. The reaction product was washed with anhydrous ethanol and deionized water three times respectively, centrifuged at 5000 rpm, and dried at 100°C to obtain fly ash-based in situ grown nano-silica.

[0039] The fly ash-based in-situ grown nano-silica prepared above is used to prepare a fly ash geopolymer slurry. Specifically, 15 parts by weight of fly ash-based in-situ grown nano-silica, 88 parts by weight of fly ash, 31.3 parts by weight of an alkali activator (sodium silicate) and 26.9 parts by weight of water are mixed and stirred evenly to obtain a fly ash geopolymer slurry.

[0040] Example 4

[0041] The method for in-situ growth of fly ash-based nano-silicon dioxide of the present invention comprises the following steps:

[0042] (1) Weigh 100 parts of fly ash, 12.5 parts of sodium metasilicate, and 5 parts of polyethylene glycol by weight, add them to a solution of ethanol and water in a volume ratio of 1:8 to obtain a mixed solution, adjust the pH of the mixed solution to greater than 12, and transfer the solution to a water bath and stir at 400 rpm for 6 h;

[0043] (2) The mixed solution was heated to 40°C, and carbon dioxide gas was introduced at a ventilation rate of 500 mL / min. The ventilation was stopped when the pH of the solution dropped to 7. The reaction was aged for 1 hour. The reaction product was washed with anhydrous ethanol and deionized water three times respectively, centrifuged at 5000 rpm, and dried at 100°C to obtain fly ash-based in situ grown nano-silica.

[0044] The growth amount of the fly ash-based in-situ grown nano-silica calculated by thermogravimetry is 9.90% of the total mass of the fly ash-based in-situ grown nano-silica.

[0045] The fly ash-based in-situ grown nano-silica prepared above is used to prepare a fly ash geopolymer slurry. Specifically, 5 parts by weight of fly ash-based in-situ grown nano-silica, 96 parts by weight of fly ash, 31.3 parts by weight of an alkali activator (sodium silicate) and 26.9 parts by weight of water are mixed and stirred evenly to obtain a fly ash geopolymer slurry.

[0046] Example 5

[0047] The method for in-situ growth of fly ash-based nano-silicon dioxide of the present invention comprises the following steps:

[0048] (1) Weigh 100 parts of fly ash, 37.5 parts of sodium metasilicate, and 5 parts of polyethylene glycol by weight, add them to a solution of ethanol and water in a volume ratio of 1:8 to obtain a mixed solution, adjust the pH of the mixed solution to greater than 12, and transfer the solution to a water bath and stir at 400 rpm for 6 h;

[0049] (2) The mixed solution was heated to 40°C, and carbon dioxide gas was introduced at a ventilation rate of 500 mL / min. The ventilation was stopped when the pH of the solution dropped to 7. The reaction was aged for 1 hour. The reaction product was washed with anhydrous ethanol and deionized water three times respectively, centrifuged at 5000 rpm, and dried at 100°C to obtain fly ash-based in situ grown nano-silica.

[0050] The growth amount of the fly ash-based in-situ grown nano-silica calculated by thermogravimetry is 29.74% of the total mass of the fly ash-based in-situ grown nano-silica.

[0051] The fly ash-based in-situ grown nano-silica prepared above is used to prepare a fly ash geopolymer slurry. Specifically, 5 parts by weight of fly ash-based in-situ grown nano-silica, 96 parts by weight of fly ash, 31.3 parts by weight of an alkali activator (sodium silicate) and 26.9 parts by weight of water are mixed and stirred evenly to obtain a fly ash geopolymer slurry.

[0052] Comparative Example 1

[0053] The fly ash geopolymer slurry was prepared by mixing 100 parts by weight of fly ash, 31.3 parts by weight of an alkali activator (sodium silicate) and 26.9 parts by weight of water, and stirring the mixture to obtain the fly ash geopolymer slurry.

[0054] Comparative Example 2

[0055] The fly ash geopolymer slurry was prepared by mixing 100 parts by weight of fly ash and 31.3 parts by weight of an alkali activator (sodium silicate) evenly, adding 1 part by weight of commercial nano-silica into 26.9 parts by weight of water and ultrasonically mixing for 30 minutes, pouring the ultrasonicated commercial nano-silica aqueous dispersion into the fly ash and activator mixture, and stirring evenly to obtain the fly ash geopolymer slurry.

[0056] Comparative Example 3

[0057] The fly ash geopolymer slurry was prepared by uniformly mixing 100 parts by weight of fly ash and 31.3 parts by weight of an alkali activator (sodium silicate), adding 2 parts by weight of commercial nano-silica into 26.9 parts by weight of water and ultrasonically mixing for 30 minutes, pouring the ultrasonicated commercial nano-silica aqueous dispersion into the fly ash and activator mixture, and stirring evenly to obtain the fly ash geopolymer slurry.

[0058] Comparative Example 4

[0059] The fly ash geopolymer slurry was prepared by uniformly mixing 100 parts by weight of fly ash and 31.3 parts by weight of an alkali activator (sodium silicate), adding 3 parts by weight of commercial nano-silicon dioxide into 26.9 parts by weight of water and ultrasonically mixing for 30 minutes, pouring the ultrasonicated commercial nano-silicon dioxide aqueous dispersion into the fly ash and activator mixture, and stirring evenly to obtain the fly ash geopolymer slurry.

[0060] Comparative Example 5

[0061] The only difference between Comparative Example 5 and Example 2 is that in step (1), the pH of the mixed solution is adjusted to 11; specifically:

[0062] (1) Weigh 100 parts of fly ash, 37.5 parts of sodium metasilicate, and 5 parts of polyethylene glycol by weight, add them to a solution of ethanol and water in a volume ratio of 1:8 to obtain a mixed solution, adjust the pH of the mixed solution to 11, and transfer the solution to a water bath and stir at 400 rpm for 6 h;

[0063] (2) The mixed solution was heated to 40°C, and carbon dioxide gas was introduced at a ventilation rate of 500 mL / min. The ventilation was stopped when the pH of the solution dropped to 7. The reaction was aged for 1 hour. The reaction product was washed 3 times with anhydrous ethanol and deionized water respectively, centrifuged at 5000 rpm, and dried at 100°C to obtain fly ash-based in situ grown nano-silica. The product was as shown in the figure. Figure 7 As shown, through Figure 7As can be seen, because the pH of the pretreatment alkali solution is less than 12, the fly ash surface is not effectively etched, resulting in a decrease in the number of active sites on the surface, ultimately leading to a significant decrease in the amount of nano-silica growth. The compressive strength in Table 1 also shows that samples with insufficient nano-silica growth cannot effectively increase the compressive strength.

[0064] The fly ash-based in-situ grown nano-silica prepared above is used to prepare a fly ash geopolymer slurry. Specifically, 10 parts by weight of fly ash-based in-situ grown nano-silica, 92 parts by weight of fly ash, 31.3 parts by weight of an alkali activator (sodium silicate) and 26.9 parts by weight of water are mixed and stirred evenly to obtain a fly ash geopolymer slurry.

[0065] pass Figures 1 to 3 and Figure 4 By comparison, it can be seen that the surface of the fly ash matrix is ​​coated with nano-scale particles, namely nano-silicon dioxide. Figure 5 The XRD patterns of the fly ash-based in-situ grown nano-silica obtained in Example 1, commercial nano-silica, monodisperse nano-silica prepared by carbonization, and the original fly ash used in Examples 1 to 5 and Comparative Examples 1 to 5 are shown. Figure 5 The XRD patterns of the nano-silica prepared by carbonization are essentially identical to those of commercial nano-silica, exhibiting a diffuse peak characteristic of amorphous nano-silica at 15-35°. Compared to virgin fly ash, the XRD pattern of the fly ash-based in-situ grown nano-silica exhibits a significantly enhanced diffuse peak at 15-35°, indicating that the carbonization method successfully grew amorphous nano-silica onto the fly ash surface. Figure 6 The Si-2p-XPS patterns of the fly ash-based in-situ grown nanosilica obtained in Example 1, the monodisperse nanosilica prepared by carbonization, and the undisturbed fly ash used in Examples 1-5 and Comparative Examples 1-5 show a chemical reaction between the in-situ grown nanosilica and the fly ash, resulting in a shift in the binding energy peak. This chemical bonding ensures that the in-situ grown nanosilica is tightly bonded to the fly ash, preventing it from falling out in the slurry and ensuring that it is evenly dispersed throughout the slurry along with the fly ash. This is fundamentally different from the fly ash-based in-situ grown nanosilica reported in existing literature, resulting in a superior reinforcement effect. Specifically, X-ray photoelectron spectroscopy (XPS) demonstrates that the fly ash-based in-situ grown nanosilica prepared in the present invention forms a chemical adsorption of Si-O-Si chemical bonds with the fly ash surface. After incorporation of the geopolymer, the in-situ grown nanosilica does not fall out and remains evenly dispersed on the fly ash particle surface.

[0066] The properties of the fly ash geopolymer slurries obtained in Examples 1 to 5 and Comparative Examples 1 to 5 were tested, and the relevant indicators are shown in Table 1:

[0067] Table 1

[0068]

[0069] In Examples 1-3, the fly ash-based in-situ grown nanosilica prepared under the same preparation conditions was used. The in-situ grown nanosilica content relative to fly ash in the fly ash geopolymer slurries was 1%, 2%, and 3%, respectively. In Examples 4 and 5, the silicon source concentration was varied during the preparation process, resulting in in-situ grown nanosilica content relative to fly ash of 0.5% and 1.5%, respectively. Comparative Example 1 is a fly ash geopolymer prepared without nanosilica. In Comparative Examples 2-4, the commercial nanosilica content relative to fly ash was 1%, 2%, and 3%, respectively. As shown in Table 1, compared to conventional commercial nanosilica, at the same nanosilica content, the fly ash-based in-situ grown nanosilica prepared by the present invention has a less negative impact on the fluidity of the geopolymer slurry. This demonstrates that fly ash-based in-situ grown nanosilica exhibits superior dispersibility in geopolymer slurries. At the same dosage, nanoparticles do not agglomerate, thus preventing them from encapsulating free water in the slurry. Compared to commercial nanosilica, the fly ash-based in-situ grown nanosilica prepared in this invention is more effective in improving the compressive strength of fly ash geopolymers under room temperature curing.

Claims

1. A method for in-situ growth of fly ash-based nano-silicon dioxide, characterized in that: The steps include: (1) adding fly ash, silicon source and dispersant to a mixture of ethanol and water to obtain a suspension, adjusting the pH of the suspension to be greater than 12; and fully reacting in a water bath; (2) heating the solution after the reaction in step (1), then introducing pure carbon dioxide or a gas containing carbon dioxide, stopping the aeration when the pH of the solution drops to 7-8, and allowing the solution to stand for aging, washing, centrifuging, and drying or freeze-drying the obtained reaction product to obtain fly ash-based in-situ grown nano-silica.

2. The method according to claim 1, wherein: In step (1), the mass ratio of the fly ash, silicon source and dispersant is 100:12.5-37.5:5-10.

3. The method according to claim 2, wherein: The silicon source is sodium metasilicate, sodium silicate pentahydrate or sodium metasilicate nonahydrate powder.

4. The method according to claim 2, wherein: The dispersant is polyethylene glycol or hexadecyltrimethylammonium bromide.

5. The method according to claim 1, wherein: In step (1), the volume ratio of ethanol to water in the mixed solution of ethanol and water is 1:8-12.

6. The method according to claim 1, wherein: In step (1), during the reaction, the stirring rate is 400-800 rpm; and the reaction time is no more than 6 h.

7. The method according to claim 1, wherein: In step (2), the mixture is heated to 40-50° C. and pure carbon dioxide gas is introduced at a ventilation rate of 500-800 mL / min.

8. The method according to claim 1, wherein: In step (2), the standing and aging time is 0.5 to 1 hour.

9. The method according to claim 1, wherein: The fly ash-based in-situ grown nano-silica prepared in step (2) is used to prepare fly ash geopolymer slurry, specifically, the fly ash-based in-situ grown nano-silica, fly ash, an alkali activator and water are mixed to obtain the fly ash geopolymer slurry.

10. The method according to claim 9, wherein: The added mass ratio of fly ash-based in-situ grown nano-silica, fly ash, alkali activator and water is 5-15:88-96:31.3:26.9.

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