Preparation of Janus-type double-faced modified superfine fly ash cenospheres and application thereof in ultra-high performance concrete

Abstract

The application belongs to the field of solid waste resource utilization and high-performance building materials, and particularly relates to preparation of Janus type double-face modified superfine fly ash microbeads and application of the superfine fly ash microbeads in ultra-high performance concrete. The modification and preparation method is: original microbead-hydroxylation pretreatment-activation and loading-Pickering emulsion preparation-directional modification-post treatment-Janus microbead, wherein sodium dihydrogen phosphate is used for surface modification to introduce phosphate groups, and nano-SiO2 is anchored on the surface of the microbead by electrostatic and hydrogen bond action; then Pickering emulsion interface shielding technology is used to construct hydrophobic lubricating surface and hydrophilic active enhanced surface on the two hemispherical surfaces of the microbead respectively. In the application in ultra-high performance concrete, the Janus microbead has interface activity and spontaneously enriches in the steel fiber / aggregate-paste interface; the hydrophobic surface faces the reinforcing phase to reduce the interface friction, and the hydrophilic surface faces the cement paste to make the interface transition zone significantly improve in density, thereby improving the mechanical properties and durability of the concrete.

Description

Technical Field

[0001] This invention belongs to the field of solid waste resource utilization and high-performance building materials, specifically relating to the preparation of Janus-type double-sided modified ultrafine fly ash microspheres and their application in ultra-high performance concrete. Background Technology

[0002] Ultra-high performance concrete (UHVPC) has become an ideal material for major projects such as cross-sea bridges and high-rise buildings due to its ultra-high strength, high toughness, and excellent durability. However, its extremely low water-cement ratio and ultra-high binder content result in extremely high viscosity of the freshly mixed mortar, poor workability, difficult construction, and easy fiber clumping, which seriously affects homogeneity. At the same time, the severe early auto-shrinkage caused by the extremely low water-cement ratio can easily lead to cracking, endangering the long-term safety of the structure.

[0003] In existing technologies, the viscosity of slurry is mainly reduced through physical methods such as optimizing the water-cement ratio, developing viscosity-reducing water-reducing agents, and using spherical aggregates and particles. Studies have shown that modifying the surface of fly ash microspheres can, to some extent, adjust the rheological properties of ultra-high performance concrete. However, these methods are usually single, overall modifications, only addressing a single problem (rheological or mechanical properties), and it is difficult to achieve a balance between viscosity reduction and enhanced activity and toughness.

[0004] For example, patent CN114573298A discloses a silane-modified fly ash microsphere and its preparation method. This method reduces slurry viscosity by grafting hydrophobic silane groups onto the microspheres. However, the hydrophobic groups completely cover the active sites on the microsphere surface, leading to a significant decrease in pozzolanic activity and ultimately reducing the later-stage strength and durability of ultra-high performance concrete. Patent CN113860792A discloses a method for preparing alkali-activated ultrafine fly ash microspheres. This method enhances microsphere activity through overall hydroxylation and alkali activation, but it significantly increases the adsorption of water-reducing agents and free water by the microspheres, resulting in a sharp increase in the viscosity of the ultra-high performance concrete slurry and a deterioration in workability.

[0005] Therefore, developing a novel modification technology that can simultaneously address both "surface hydrophobicity and viscosity reduction" and "active site hydrophilicity and water retention" to prepare ultra-high performance concrete with excellent workability, high strength, and high toughness is a key technical problem that urgently needs to be solved in this field. Summary of the Invention

[0006] To address the problem that existing fly ash microsphere single modification technologies cannot simultaneously achieve "viscosity reduction and toughness retention," and the industry challenge of balancing workability and toughness in traditional ultra-high performance concrete, this invention provides a method for preparing Janus-type double-sided modified ultrafine fly ash microspheres and their application in ultra-high performance concrete.

[0007] To achieve the above objectives, the present invention provides the following technical solution: A method for preparing Janus-type double-sided modified ultrafine fly ash microspheres includes the following steps: (1) Hydroxylation pretreatment: The original ultrafine fly ash microspheres are added to a hydrogen peroxide solution with a mass fraction of 25%-35%, and stirred at 60-80℃ for 2-4 hours. After filtration, washing and drying, hydroxylated microspheres with hydroxyl groups on the surface are obtained. (2) Activation loading: The hydroxylated microbeads were dispersed in deionized water, sodium dihydrogen phosphate was added, the pH was adjusted to 4.0-5.0, and the reaction was stirred at 50-60℃ for 1-2 h. Then nano-SiO2 was added and ultrasonically dispersed for 10-30 min. The mixture was filtered, washed and dried to obtain phosphorylated microbeads loaded with nano-SiO2 composite particles. (3) Pickering emulsion preparation: The phosphorylated microbeads loaded with nano-SiO2 composite particles are dispersed in deionized water, n-hexane is added, and high-speed emulsification is carried out at 10000-15000 r / min for 5-10 min to form a stable oil-in-water Pickering emulsion. (4) Directional modification: slowly add a hexane solution of tridecafluorooctyltriethoxysilane to the oil phase of Pickering emulsion and stir the reaction at 30-40℃ for 3-5 h; (5) Post-processing: Demulsification, washing and drying are performed to obtain Janus-type double-sided modified ultrafine fly ash microspheres with hydrophobic hemispheres / hydrophilic-nano-reinforced hemispheres.

[0008] As a preferred embodiment, by mass, the amount of undisturbed ultrafine fly ash microspheres in step (1) is 100 parts, and the amount of hydrogen peroxide solution is 50-80 parts; the amount of sodium dihydrogen phosphate in step (2) is 5-10 parts, and the amount of nano-SiO2 is 2-5 parts; the amount of n-hexane in step (3) is 50-100 parts; and the amount of tridecafluorooctyltriethoxysilane in step (4) is 3-8 parts.

[0009] Preferably, the original ultrafine fly ash microspheres have a D50 of 2-8 μm, a loss on ignition of ≤3%, and a sphericity of ≥90%; the nano-silica has a particle size of 10-20 nm.

[0010] The reaction principle in the above key steps is: Step (2) Phosphorylation activation and preloading of nano-SiO2: Phosphate groups are introduced by surface modification with sodium dihydrogen phosphate, and nano-SiO2 is pre-anchored to the surface of microbeads by electrostatic and hydrogen bonding to form a hydrophilic and activated "rough" surface matrix. Step (3) Pickering emulsion interface masking: The composite particles are mixed with n-hexane and emulsified at high speed to form a stable O / W type emulsion. The microbeads spontaneously arrange themselves in a monolayer at the oil-water interface, with the hydrophilic nano-SiO2 composite surface pointing towards the aqueous phase and the other half of the smooth surface pointing towards the oil phase. Step (4) Directional hydrophobic modification: Fluorine-containing silane is dropped into the oil phase. The silane reacts only with the microspheres exposed in the oil phase at the oil-water interface, grafting low surface energy long chains, while the water phase side does not react because it is protected by water and nano SiO2 layer.

[0011] The application of Janus-type double-sided modified ultrafine fly ash microspheres obtained by the above preparation method in ultra-high performance concrete is as follows: Firstly, it forms a low-viscosity, high-toughness, ultra-high-performance concrete composition as a component. Secondly, it can be used as a component in the preparation of a low-viscosity, high-toughness, ultra-high-performance concrete.

[0012] This relates to a low-viscosity, high-toughness, ultra-high-performance concrete composition, comprising, by weight: Cementitious material: 100 parts; Quartz sand: 100-120 parts; Steel fiber: 2-3 parts; Water-reducing agent: 1.0-3.0 parts; Water: 14-18 parts; The gel material is composed of 50-70 parts of P·Ⅱ 52.5 silicate cement, 15-30 parts of Janus type double-sided modified ultrafine fly ash microspheres, and 10-20 parts of silica fume, totaling 100 parts.

[0013] Preferably, the silica fume has a D50 of 0.1-0.3 μm and a SiO2 content of ≥92%; the quartz sand is continuously graded with a particle size range of 0.15-1.18 mm; the steel fibers are 12-13 mm in length and 0.18-0.22 mm in diameter; and the water-reducing agent is a polycarboxylate-based high-efficiency water-reducing agent with a water reduction rate of ≥40% and a solid content of 40%±2%.

[0014] This invention relates to a method for preparing low-viscosity, high-toughness, ultra-high-performance concrete, comprising the following steps using the aforementioned composition: (1) Add P·Ⅱ 52.5 silicate cement, Janus type double-sided modified ultrafine fly ash microspheres, silica fume and quartz sand to a forced mixer and dry mix for 1-2 minutes until uniform; (2) Dissolve the water-reducing agent in water beforehand, slowly add it to the mixer, and wet mix for 3-5 minutes until the slurry is uniform; (3) Sprinkle steel fibers evenly while stirring, and continue stirring for 2-3 minutes until the fibers are completely dispersed to obtain a low-viscosity, high-toughness, ultra-high-performance concrete mixture.

[0015] Furthermore, after molding, the specimens were cured at room temperature (20±2℃), relative humidity ≥95%, and curing time ≥28 days.

[0016] Beneficial effects: 1. Synergistic effect of viscosity reduction, slump retention, and activity maintenance: The Janus-type double-sided modified ultrafine fly ash microspheres prepared in this invention have a hydrophobic hemisphere / hydrophilic-nano-reinforced hemisphere structure. The hydrophobic hemispheres significantly reduce the friction between microspheres and the unnecessary adsorption of water-reducing agents, providing an excellent "ball lubrication" effect; the hydrophilic-nano-reinforced hemispheres retain and enhance the activity of pozzolanic ash. After being incorporated into ultra-high performance concrete, the spreadability of the paste is improved, and the amount of water-reducing agent required is reduced.

[0017] 2. Interfacial Directional Enrichment and Gradient Toughening: In ultra-high performance concrete applications, Janus-type double-sided modified ultrafine fly ash microspheres exhibit interfacial activity, spontaneously enriching at the steel fiber / aggregate-slurry interface. The hydrophobic surface faces the reinforcing phase to reduce interfacial friction, while the hydrophilic surface faces the cement paste. Through nano-SiO2 and phosphorylated surfaces, directional growth of CSH is promoted, significantly increasing the density of the interfacial transition zone and improving the mechanical and durability properties of the concrete. Attached Figure Description

[0018] The accompanying drawings, which form part of this specification, are used to provide a further understanding of the invention. The illustrative embodiments of the invention and their descriptions are used to explain the invention and do not constitute an undue limitation of the invention. Wherein: Figure 1 This is a process flow diagram for preparing Janus-type double-sided modified ultrafine fly ash microspheres (hereinafter referred to as Janus microspheres) in this invention.

[0019] Figure 2 This is a SEM comparison image of Janus microspheres and unmodified microspheres in this invention (showing the differences in surface morphology).

[0020] Figure 3 This is a comparison chart of the plastic viscosity of Examples 1-3 and Comparative Examples 1-3.

[0021] Figure 4 The diagram shows a comparison of the compressive strength and flexural strength of Examples 1-3 and Comparative Examples 1-3. Detailed Implementation

[0022] The technical solutions in the embodiments of the present invention will be clearly and completely described below. Obviously, the described embodiments are only a part of the embodiments of the present invention, and not all of them. All other embodiments obtained by those skilled in the art based on the embodiments of the present invention are within the scope of protection of the present invention.

[0023] This invention provides a method for preparing Janus-type double-sided modified ultrafine fly ash microspheres, such as... Figure 1 As shown: Uncirculated microspheres (uncirculated ultrafine fly ash microspheres) - hydroxylation pretreatment - activation and loading - Pickering emulsion preparation - directional modification - post-treatment - Janus microspheres (Janus-type double-sided modified ultrafine fly ash microspheres). Specifically, it includes the following steps: (1) Hydroxylation pretreatment: Add the original ultrafine fly ash microspheres to a hydrogen peroxide solution with a mass fraction of 25%-35%, stir and react at 60-80℃ for 2-4h, filter, wash until neutral, and dry to obtain hydroxylated microspheres with hydroxyl groups on the surface. (2) Phosphorylation activation and nano-SiO2 loading: The hydroxylated microbeads obtained in step (1) were dispersed in deionized water, sodium dihydrogen phosphate was added, the pH was adjusted to 4.0-5.0, and the reaction was stirred at 50-60℃ for 1-2 h. Then nano-SiO2 was added and ultrasonically dispersed for 10-30 min, so that nano-SiO2 was adsorbed on the phosphorylation sites on the surface of the microbeads through hydrogen bonding and electrostatic interaction, forming phosphorylated microbeads loaded with nano-SiO2 composite particles; filtered, washed and dried. (3) Pickering emulsion interface masking: The phosphorylated microbeads loaded with nano-SiO2 composite particles obtained in step (2) are redispersed in deionized water, and n-hexane is added. The mixture is emulsified at a high speed of 10,000-15,000 r / min for 5-10 min to form a stable oil-in-water Pickering emulsion. At this time, the composite particles are spontaneously arranged at the oil-water interface, with the hydrophilic side loaded with nano-SiO2 facing the aqueous phase and the other half facing away from the aqueous phase exposed to the oil phase. (4) Directional hydrophobic modification: slowly add a hexane solution of tridecafluorooctyltriethoxysilane to the oil phase of the emulsion obtained in step (3), and stir the reaction at 30-40℃ for 3-5 hours to expose the microbeads to the hemispherical surface of the oil phase to undergo silane hydrolysis and condensation reaction, forming a low surface energy hydrophobic layer. (5) Post-processing: After the reaction is complete, add a demulsifier to demulsify, centrifuge, filter, and wash alternately with anhydrous ethanol and deionized water 3-5 times. Dry under vacuum at 60-80℃ for 12-24h to obtain Janus-type double-sided modified ultrafine fly ash microspheres (hereinafter referred to as Janus microspheres). The structure involved is hydrophobic hemisphere / hydrophilic-nano-reinforced hemisphere Janus microspheres.

[0024] Preferably, the reagent amounts (by mass) in each of the above steps are as follows: in step (1), 100 parts of undisturbed ultrafine fly ash microspheres and 50-80 parts of hydrogen peroxide solution; in step (2), 5-10 parts of sodium dihydrogen phosphate and 2-5 parts of nano-silica; in step (3), 50-100 parts of n-hexane; and in step (4), 3-8 parts of tridecafluorooctyltriethoxysilane.

[0025] Preferably, the original ultrafine fly ash microspheres have a D50 of 2-8 μm, a loss on ignition of ≤3%, and a sphericity of ≥90%; the nano-silica has a particle size of 10-20 nm.

[0026] A low-viscosity, high-toughness, ultra-high-performance concrete composition, comprising Janus-type double-sided modified ultrafine fly ash microspheres prepared by the above method, wherein the components are as follows (parts by weight): Cementitious material: 100 parts, specifically composed of 50-70 parts of P·Ⅱ 52.5 silicate cement, 15-30 parts of Janus type double-sided modified ultrafine fly ash microspheres, and 10-20 parts of silica fume, with a total of 100 parts. Quartz sand: 100-120 parts; Steel fiber: 2-3 parts; Water-reducing agent: 1.0-3.0 parts; Water: 14-18 parts.

[0027] Preferably, the silica fume has a D50 of 0.1-0.3 μm and a SiO2 content of ≥92%; the quartz sand is continuously graded with a particle size range of 0.15-1.18 mm; the steel fibers are 12-13 mm in length and 0.18-0.22 mm in diameter; and the water-reducing agent is a polycarboxylate-based high-efficiency water-reducing agent with a water reduction rate of ≥40% and a solid content of 40%±2%.

[0028] A method for preparing low-viscosity, high-toughness, ultra-high-performance concrete, using the aforementioned low-viscosity, high-toughness, ultra-high-performance concrete composition, includes the following steps: (1) Add cement, fly ash, Janus type double-sided modified ultrafine fly ash microspheres, silica fume and quartz sand to a forced mixer and dry mix for 1-2 minutes until uniform; (2) Dissolve the water-reducing agent in the water of the formula amount in advance, slowly add it to the mixer, and wet mix for 3-5 minutes until the slurry is uniform; (3) Sprinkle steel fibers evenly while stirring, and continue stirring for 2-3 minutes until the fibers are completely dispersed to obtain a low-viscosity, high-toughness, ultra-high-performance concrete mixture.

[0029] After molding, the specimens were cured at room temperature (20±2℃) and relative humidity (≥95%) for ≥28 days.

[0030] The present invention will now be described in detail with reference to embodiments. It should be noted that, unless otherwise specified, the embodiments and features described in the embodiments of the present invention can be combined with each other.

[0031] Example 1 A method for preparing Janus-type double-sided modified ultrafine fly ash microspheres, wherein all components are expressed in parts by mass, includes the following steps: (1) Take 100 parts of original ultrafine fly ash microspheres with D50=5.2μm, loss on ignition 1.9% and sphericity 94%, add 60 parts of 30% hydrogen peroxide solution, stir at 70℃ for 3h, filter, wash with water until neutral, and dry at 105℃ to obtain hydroxylated microspheres.

[0032] (2) Disperse hydroxylated microbeads in 250 parts of deionized water, add 7 parts of sodium dihydrogen phosphate, adjust pH to 4.5 with dilute hydrochloric acid, and react at 55℃ for 1.5h; then add 3 parts of nano-SiO2 with an average particle size of 15nm, and ultrasonically disperse for 20min to load nanoparticles onto the surface of microbeads. Filter, wash and dry to obtain a composite powder of phosphorylated microbeads loaded with nano-SiO2.

[0033] (3) The obtained composite powder was redispersed in 150 parts of deionized water, 80 parts of n-hexane were added, and emulsified at 12000 r / min for 7 min to form a stable O / W type Pickering emulsion; microscopic examination confirmed that the microbeads were arranged in a monolayer at the droplet interface.

[0034] (4) Dissolve 5 parts of tridecafluorooctyltriethoxysilane in 20 parts of n-hexane, slowly add the above emulsion, stir at 35°C for 4 hours to carry out directional hydrophobic modification.

[0035] (5) Then add a small amount of ethanol to break the emulsion, filter, wash with hot ethanol and deionized water 4 times in sequence, and vacuum dry at 70℃ for 18h to obtain Janus type double-sided modified ultrafine fly ash microspheres.

[0036] The Janus-type double-sided modified ultrafine fly ash microspheres obtained by the above method were applied to the preparation of a low-viscosity, high-toughness, ultra-high-performance concrete. The component proportions involved, by mass parts, are as follows: P·Ⅱ 52.5 cement 60 parts, Janus type double-sided modified ultrafine fly ash microspheres 20 parts, silica fume (D50=0.2μm, SiO2 content 94%) 20 parts, the above are cementing materials, totaling 100 parts; continuously graded quartz sand (0.15-1.18mm) 110 parts; steel fiber (13mm, 0.2mm) 2.5 parts; water-reducing agent (water reduction rate 42%, solid content 40%) 1.4 parts; water 16 parts (water-cement ratio 0.16).

[0037] The molding process using the above components is as follows: P·Ⅱ 52.5 cement, Janus-type double-sided modified ultrafine fly ash microspheres, silica fume, and quartz sand are dry-mixed for 1.5 min; water-reducing agent is dissolved in water and added for wet mixing for 4 min; steel fibers are evenly sprinkled in and mixed for 2.5 min. Standard curing (20±2℃, RH≥95%) is applied for 28 days after pouring.

[0038] Example 2 This embodiment utilizes the Janus-type double-sided modified ultrafine fly ash microspheres prepared in Example 1, applied to the preparation of a low-viscosity, high-toughness, ultra-high-performance concrete. The reagents and processes used are the same, the difference from Example 1 is: In the preparation of ultra-high performance concrete, the amount of Janus-type double-sided modified ultrafine fly ash microspheres was changed to 15 parts, cement 65 parts, silica fume 20 parts, water-reducing agent 1.5 parts, and the rest were the same as in Example 1.

[0039] Example 3 This embodiment utilizes the Janus-type double-sided modified ultrafine fly ash microspheres prepared in Example 1, applied to the preparation of a low-viscosity, high-toughness, ultra-high-performance concrete. The reagents and processes used are the same, the difference from Example 1 is: In the preparation of ultra-high performance concrete, the amount of Janus-type double-sided modified ultrafine fly ash microspheres was changed to 30 parts, cement 50 parts, silica fume 20 parts, water-reducing agent 1.2 parts, and the rest was the same as in Example 1.

[0040] Comparative Example 1 (Blank Group) In the preparation of ultra-high performance concrete, the Janus-type double-sided modified ultra-fine fly ash microspheres in Example 1 were replaced with an equal mass of unmodified undisturbed ultrafine fly ash microspheres (undisturbed microspheres). The amount of water-reducing agent was adjusted to 2.0 parts to ensure basic fluidity. The remaining proportions and processes were the same as in Example 1.

[0041] Comparative Example 2 (fully hydrophobic modified microspheres) Following the preparation method of Janus-type double-sided modified ultrafine fly ash microspheres in Example 1, step (2) phosphorylation and nano-SiO2 loading and step (3) high-speed emulsification are omitted. The hydroxylated microspheres obtained in step (1) are directly added to an ethanol-water solution containing 5 parts of tridecafluorooctyltriethoxysilane for overall hydrophobic modification for 4 hours to obtain fully hydrophobic microspheres. In the preparation of ultra-high performance concrete, the dosage of fully hydrophobic microspheres is 20 parts, water-reducing agent is 1.4 parts, and the rest is the same as in Example 1.

[0042] Comparative Example 3 (fully hydrophilic activated microbeads) Perform only steps (1) to (2) of Example 1, without performing steps (3) to (5), to obtain phosphorylated nano-SiO2 fully hydrophilic microspheres. In the preparation of ultra-high performance concrete, the dosage of fully hydrophilic activated microspheres is 20 parts, the dosage of water-reducing agent is 1.8 parts, and the rest is the same as in Example 1.

[0043] The performance of the ultra-high performance concrete prepared in Examples 1-3 and Comparative Examples 1-3 was tested, and the results are shown in Table 1:

[0044] As shown in Table 1, the fresh mix performance, mechanical properties, and durability of Examples 1-3 are significantly superior to those of the comparative examples: with the same or lower water-reducing agent dosage, the spread is more than 100 mm higher; compared to Comparative Example 1, the viscosity is reduced by about 50%, the 28-day compressive strength is increased by 4.5%~11%, the 28-day tensile strength is increased by 22%~38%, the 28-day flexural strength is increased by 21%~29%, and the chloride ion diffusion coefficient is reduced by 33%~43%. The strength of Comparative Example 2 is significantly lower than that of the examples, and the workability of Comparative Example 3 is significantly lower than that of the examples, fully verifying that the Janus double-sided structure of the present invention has the synergistic advantages of "reducing viscosity, maintaining vitality, and increasing toughness".

[0045] like Figure 3 As shown, the plastic viscosity of Examples 1-3 remained stable between 1.8 and 2.4 Pa·s, while the comparative examples exhibited drastic fluctuations, indicating that the viscosity-reducing effect of the examples was controllable and stable; Figure 4 As shown, the flexural strength and compressive strength of the embodiments change synchronously, indicating the synergistic effect of the toughening mechanism and matrix reinforcement; although the viscosity of Comparative Example 2 drops to 2.0 Pa·s, the strength drops to the bottom simultaneously, and although the viscosity is low, the strength collapses, making it unsuitable for engineering; the compressive strength of Comparative Example 3 rebounds to 165 MPa, but the flexural strength is only 23.7 MPa, indicating poor high-strength brittleness workability; all of the above demonstrate that Examples 1 to 3 achieve low viscosity while maintaining high strength.

[0046] The above description is merely a preferred embodiment of the present invention and is not intended to limit the invention. Various modifications and variations can be made to the present invention by those skilled in the art. Any modifications, equivalent substitutions, improvements, etc., made within the spirit and principles of the present invention should be included within the scope of protection of the present invention.

Claims

1. A method for preparing Janus-type double-sided modified ultrafine fly ash microspheres, characterized in that, Includes the following steps: (1) Hydroxylation pretreatment: The original ultrafine fly ash microspheres are added to a hydrogen peroxide solution with a mass fraction of 25%-35%, and stirred at 60-80℃ for 2-4 hours. After filtration, washing and drying, hydroxylated microspheres with hydroxyl groups on the surface are obtained. (2) Activation loading: The hydroxylated microbeads were dispersed in deionized water, sodium dihydrogen phosphate was added, the pH was adjusted to 4.0-5.0, and the reaction was stirred at 50-60℃ for 1-2 h. Then nano-SiO2 was added and ultrasonically dispersed for 10-30 min. The mixture was filtered, washed and dried to obtain phosphorylated microbeads loaded with nano-SiO2 composite particles. (3) Pickering emulsion preparation: The phosphorylated microbeads loaded with nano-SiO2 composite particles are dispersed in deionized water, n-hexane is added, and high-speed emulsification is carried out at 10000-15000 r / min for 5-10 min to form a stable oil-in-water Pickering emulsion. (4) Directional modification: slowly add a hexane solution of tridecafluorooctyltriethoxysilane to the oil phase of Pickering emulsion and stir the reaction at 30-40℃ for 3-5 h; (5) Post-processing: Demulsification, washing and drying are performed to obtain Janus-type double-sided modified ultrafine fly ash microspheres with hydrophobic hemispheres / hydrophilic-nano-reinforced hemispheres.

2. The preparation method according to claim 1, characterized in that, By mass, in step (1), the amount of undisturbed ultrafine fly ash microspheres is 100 parts, and the amount of hydrogen peroxide solution is 50-80 parts; in step (2), the amount of sodium dihydrogen phosphate is 5-10 parts, and the amount of nano SiO2 is 2-5 parts; in step (3), the amount of n-hexane is 50-100 parts; and in step (4), the amount of tridecafluorooctyltriethoxysilane is 3-8 parts.

3. The preparation method according to claim 1 or 2, characterized in that, The original ultrafine fly ash microspheres have a D50 of 2-8 μm, a loss on ignition of ≤3%, and a sphericity of ≥90%; the nano-SiO2 has a particle size of 10-20 nm.

4. A low-viscosity, high-toughness, ultra-high-performance concrete composition, characterized in that, The composition comprises Janus-type double-sided modified ultrafine fly ash microspheres prepared by the preparation method according to any one of claims 1 to 3, wherein the composition comprises, by weight, the following: Cementitious material: 100 parts; Quartz sand: 100-120 parts; Steel fiber: 2-3 parts; Water-reducing agent: 1.0-3.0 parts; Water: 14-18 parts; The gel material is composed of 50-70 parts of P·Ⅱ 52.5 silicate cement, 15-30 parts of Janus type double-sided modified ultrafine fly ash microspheres, and 10-20 parts of silica fume.

5. The composition according to claim 4, characterized in that, The silica fume has a D50 of 0.1-0.3μm and a SiO2 content of ≥92%; the quartz sand is continuously graded with a particle size range of 0.15-1.18mm; the steel fibers are 12-13mm in length and 0.18-0.22mm in diameter; the water-reducing agent is a polycarboxylate-based high-efficiency water-reducing agent with a water reduction rate of ≥40% and a solid content of 40%±2%.

6. A method for preparing low-viscosity, high-toughness, ultra-high-performance concrete, characterized in that, The following steps are performed using the composition according to claim 4 or 5: (1) Add P·Ⅱ 52.5 silicate cement, Janus type double-sided modified ultrafine fly ash microspheres, silica fume and quartz sand to a forced mixer and dry mix for 1-2 minutes until uniform; (2) Dissolve the water-reducing agent in water beforehand, slowly add it to the mixer, and wet mix for 3-5 minutes until the slurry is uniform; (3) Sprinkle steel fibers evenly while stirring, and continue stirring for 2-3 minutes until the fibers are completely dispersed to obtain a low-viscosity, high-toughness, ultra-high-performance concrete mixture.

7. The preparation method according to claim 6, characterized in that, After molding, the specimens were cured at room temperature (20±2℃) and relative humidity (≥95%) for ≥28 days.

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