A nanocrystal seed additive for UHPC and its preparation method

Abstract

The present invention discloses a nano-seed additive for UHPC and its preparation method, which relates to the field of new concrete materials; the additive is composed of nano CO2 bubbles, fibrous magnesium phosphate nanocrystals and bubble stabilizers; the particle size of the nano-bubbles is 50-200 nm, and the volume ratio is 30-60%; the aspect ratio of the nanocrystals is 5-10, and the size is 10-20 nm; different-scale mutual filling is achieved among the nano-bubbles, magnesium phosphate nanocrystals and bubble stabilizers to form a multi-scale and multi-phase suspension aqueous solution. In this additive, the nano-bubbles disperse the magnesium phosphate nanocrystals to avoid the shielding of the surface active sites of the seeds by chemical dispersants; the carbon dioxide rapidly released by the nano-bubbles during the rupture in the cement mixture reacts with calcium hydroxide to generate nano-calcium carbonate crystals as the crystal nuclei for the hydration reaction, promoting the precipitation and growth of hydration products and accelerating the hydration; the fibrous magnesium phosphate nano-seeds effectively enhance the toughness and tensile strength of UHPC.

Description

Technical Field

[0001] The present invention relates to the technical field of functional new materials for concrete, and particularly relates to a nano-seed additive for UHPC and a preparation method thereof. Background Art

[0002] As a new type of high-strength and high-durability building material, UHPC has attracted much attention in the field of civil engineering in recent years. UHPC has ultra-high compressive strength (usually exceeding 150 MPa), excellent toughness and durability, as well as good freeze-thaw resistance and chemical corrosion resistance. These excellent properties enable UHPC to have broad application prospects in fields such as bridges, super high-rise buildings, and ocean engineering, and can effectively improve the safety and service life of structures.

[0003] Although ultra-high performance concrete (UHPC) exhibits excellent performance in terms of strength and durability, there are still some problems in practical applications. Due to its dense microstructure and lack of sufficient crack bridging and energy absorption mechanisms, UHPC is more likely to undergo sudden fracture and lacks plastic deformation ability when subjected to external shocks or overloads. Currently, adding 1-2% by volume of steel fibers is often used to improve its ductility, but this doubles the construction cost of UHPC. In addition, the mix proportion of UHPC contains a large amount of cementitious materials such as cement, silica fume, and mineral powder. The high dosage of cementitious materials not only increases the material cost but also significantly increases its carbon footprint. This is contrary to the global advocacy of green buildings and sustainable development goals, and limits the popularization and application of UHPC in modern building projects pursuing low-carbon environmental protection.

[0004] With the development of nanotechnology, researchers have tried to use nanomaterials in the production of UHPC. Due to the characteristics of nanomaterials such as fine particles, large specific surface area, and high activity, they can play a role in filling micro-pores, promoting hydration reactions, and improving the performance of the interfacial transition zone in UHPC. Chinese Patent CN116283043A provides an ultra-early-strength admixture for UHPC precast components and its preparation method. This patented technology uses reactive ball milling technology to grind industrial solid waste with reactive grinding aids, non-reactive grinding aids, and nanomaterials (such as nano-calcium carbonate, nano-silica, nano-aluminum oxide), changing the chemical properties of the surface of solid waste particles, improving their reactivity, and using coagulants and retarders in combination to increase the early strength of UHPC while reducing its viscosity, improving fluidity, and reducing early autogenous shrinkage. Chinese Patent CN113416046A discloses an ultra-high-performance concrete for orthotropic steel bridge deck pavement. This technology uses modified highly thixotropic nano-calcium carbonate to fill the gaps between cement and admixtures, displace free water in the binder system, reduce the water film thickness, lower the viscosity of the UHPC paste, and at the same time improve the density of the matrix and increase strength. Chinese Patent CN111018394A provides a nanocrystalline nucleus material for UHPC precast components and its preparation and application methods. The nanocrystalline nucleus material (nano-hydrated calcium silicate) for preparing UHPC precast components can effectively improve the fluidity of its mixture when incorporated into ultra-high-performance concrete; significantly improve the early strength of ultra-high-performance concrete, greatly shorten the formwork curing time of UHPC precast components, and improve the formwork turnover rate and construction efficiency. Chinese Patent CN117886529B discloses a mineral admixture for improving the mixing efficiency of ultra-high-performance concrete. This mineral admixture is prepared by compounding fly ash, calcium carbonate powder, and nanoparticles (silica fume and titanium powder), and reasonably controlling the specific composition of fly ash and calcium carbonate powder among them. After its application to UHPC, the slurry discharge time can be shortened by 20% - 88%, greatly improving the mixing efficiency of UHPC, and it can also reduce the viscosity of the UHPC mixture by more than 20% and increase the 28-day compressive strength by 1% - 10%, achieving a comprehensive improvement in the preparation efficiency, construction performance, and mechanical properties of ultra-high-performance concrete.

[0005] From the existing publicly available technologies, many nanomaterials (such as silica fume, nano-aluminum oxide, nano-SiO2, nano-CaCO3, nano-hydrated calcium silicate) have been used to improve the workability of UHPC mixtures and increase the density of hardened UHPC, thereby enhancing the compressive strength of UHPC. There are few reports on the simultaneous enhancement and toughening of UHPC through nanomaterials. In addition, although the nanoseed technology can achieve the growth of induced crystals on its surface in ordinary concrete, in the UHPC system, due to the serious lattice mismatch between the hydration products and the crystalline phase, the existing technologies show an insignificant increase in early strength. Summary of the Invention

[0006] 1. Technical problems to be solved:

[0007] In view of the above technical problems, the present invention provides a nano-seed additive for UHPC and a preparation method thereof. In this additive, small molecules, nano-crystals and nano-bubbles fill each other at different scales, and the gas-liquid-solid three phases cooperate with each other to form a stable multi-scale and multi-phase suspension aqueous solution. At the same time, the nano-seed additive for UHPC uses nano-CO2 bubbles to disperse fibrous magnesium phosphate nano-crystals, avoiding the shielding of the surface active sites of the seeds by chemical dispersants. The nano-bubbles quickly burst in the cement mixture, and the released carbon dioxide reacts with calcium hydroxide to form nano-calcium carbonate crystals. These crystals serve as crystal nuclei for the hydration reaction, significantly promoting the precipitation and growth of hydration products and accelerating the hydration process. The fibrous magnesium phosphate nano-crystals cooperate with nano-calcium carbonate to improve the compactness and toughness of UHPC.

[0008] 2. Technical solutions:

[0009] A nano-seed additive for UHPC, which is composed of nano-CO2 bubbles, fibrous magnesium phosphate nano-crystals and a bubble stabilizer. The magnesium phosphate nano-crystals serve as crystal nuclei for cement materials during the cement hydration reaction. Among them, the particle size of the nano-CO2 bubbles is 50 - 200 nm, and the volume accounts for 30 - 60% of the total volume; the aspect ratio of the fibrous magnesium phosphate nano-crystals is 5 - 10, and the particle size is 10 - 20 nm; the bubble stabilizer is composed of a composite of inorganic salt electrolytes and organic surfactant molecules; different scales of mutual filling are achieved among the nano-CO2 bubbles, fibrous magnesium phosphate nano-crystals and the bubble stabilizer, forming a stable multi-scale and multi-phase suspension aqueous solution.

[0010] A preparation method of a nano-seed additive for UHPC, comprising the following steps:

[0011] Step 1: Preparation of fibrous magnesium phosphate nanocrystal suspension; Weigh 35 - 50 parts by mass of soluble magnesium salt and dissolve it in 500 parts by mass of deionized water to prepare a magnesium salt solution; Subsequently, weigh 16.7 - 32.5 parts by mass of soluble phosphate and dissolve it in 500 parts by mass of deionized water, and add 0.1 - 6 parts by mass of polymer modifier, stir until the solution is completely transparent to prepare a phosphate solution; Then, slowly drip the phosphate solution into the magnesium salt solution through a constant flow pump, and at the same time use 1M ammonia water solution to adjust the pH value of the solution to 8 - 10 in real time. A milky white suspension gradually appears in the solution, indicating the formation of fibrous magnesium phosphate nanocrystals; After the dripping is completed, continue to stir for 30 - 120 minutes; Subsequently, transfer the reaction solution to a centrifuge tube, centrifuge at a speed of 6000 - 8000 revolutions per minute for 10 - 15 minutes to separate the precipitate, and wash it with deionized water 3 - 5 times to remove residual impurities and unreacted substances; Redisperse 51.8 - 88.5 parts by mass of the washed fibrous magnesium phosphate nanocrystals in 500 parts by mass of deionized water, and add 0.1 - 0.3 parts by mass of stabilizer, and use ultrasonic assisted dispersion technology to obtain fibrous magnesium phosphate nanocrystal suspension.

[0012] Step 2: Using a microfluidic device, uniformly disperse 0.1 - 3 parts by mass of nano-carbon dioxide bubbles in the suspension obtained in Step 1; Supply CO2 gas stably to the gas phase inlet of the microfluidic device through a pressure reducing valve and a flow meter. At this time, the gas pressure of the pressure reducing valve is 0.1 - 0.5 MPa, and the flow rate of the flow meter is set to 0.5 - 1 L / min; At the same time, inject the suspension obtained in Step 1 into the liquid phase inlet of the microfluidic device at a flow rate of 10 - 20 mL / min through a liquid injection module; At the nano-nozzle at the outlet of the microfluidic device, the carbon dioxide gas is sheared into nano-bubbles with a diameter of 50 - 200 nm and uniformly dispersed in the suspension.

[0013] Step 3: Enhance the stability of nano-bubbles; Add 0.02 - 0.05 parts by mass of inorganic salt electrolyte and 0.02 - 0.05 parts by mass of surfactant to the suspension generated in Step 2 and gently stir to form a stable protective film on the surface of the bubbles, enhance the interfacial stability of the bubbles, and thus obtain the target nano-seed additive for UHPC.

[0014] Furthermore, the soluble magnesium salt is one or more combinations of magnesium sulfate, magnesium chloride, magnesium carbonate, magnesium nitrate, magnesium acetate, and magnesium citrate.

[0015] Furthermore, the soluble phosphate is one or more combinations of sodium dihydrogen phosphate, potassium dihydrogen phosphate, disodium hydrogen phosphate, dipotassium hydrogen phosphate, trisodium phosphate, tripotassium phosphate, ammonium phosphate, and ammonium hydrogen phosphate.

[0016] Further, the polymer modifier is any proportion combination of polyethylene glycol with a molecular weight of 2000 - 8000 and polyvinyl alcohol with a molecular weight of 500 - 3000.

[0017] Further, the stabilizer is one or a combination of more than one of polyvinylpyrrolidone with a molecular weight of 8000 - 15000, sodium polyacrylate with a molecular weight of 30000 - 50000, and polyacrylamide with a molecular weight of 50000 - 100000.

[0018] Further, the inorganic salt in the inorganic salt electrolyte is one or a combination of more than one of sodium chloride, potassium chloride, sodium nitrate, sodium nitrite, potassium nitrate, and potassium nitrite.

[0019] Further, the surfactant is one or a combination of more than one of sodium dodecyl sulfate, cetyltrimethylammonium bromide, polyethylene glycol octylphenyl ether, coconut oil amide propyl betaine, dodecyl dimethyl alkyl bisquaternary ammonium salt, and cetylpyridinium chloride.

[0020] Further, in step one, the ultrasonic - assisted dispersion technology is to make the solution uniform through ultrasonic waves. At this time, the frequency of the ultrasonic waves is 40 - 60 kHz, and the treatment time is 5 - 10 minutes.

[0021] 3. Beneficial effects:

[0022] (1) The nano - seed additive for UHPC disclosed by this method firstly discloses the use of magnesium phosphate nanocrystals as the crystal nuclei of cement materials. The cement hydration products can epitaxially grow on their surfaces, showing perfect compatibility, which can greatly promote the positive progress of cement hydration. Applying it in UHPC can overcome the problem of slow strength development.

[0023] (2) The nano - seed additive for UHPC disclosed by this method, in which the magnesium phosphate nano - seeds are a kind of fibrous nano - crystals, can significantly improve the toughness of UHPC.

[0024] (3) The nano - seed additive for UHPC disclosed by this method, in which there is excellent bonding performance between the magnesium phosphate nano - seeds with struvite - like structure and the cement hydration products. Therefore, it can maximize the anti - pull - out ability of the nano - fibers and greatly improve the tensile strength of UHPC.

[0025] (4) The nano - seed additive for UHPC disclosed by this method significantly optimizes the dispersion of magnesium phosphate nano - seeds by introducing nano - carbon dioxide bubbles, avoiding the shielding of the active sites on the surface of the seeds by the dispersant, thereby greatly increasing its specific surface area and reaction efficiency.

[0026] (5) For the nanocrystal seed additive for UHPC disclosed by this method, the nanobubbles rapidly burst in the cement mixture, and the released carbon dioxide reacts with calcium hydroxide to form nanocalcite crystals. These crystals serve as crystal nuclei for the hydration reaction, significantly promoting the precipitation and growth of hydration products and accelerating the hydration process.

[0027] (6) For the nanocrystal seed additive for UHPC disclosed by this method, magnesium phosphate nanocrystals cooperate with nanocalcite. Through the micro-nano aggregate effect, they fill the internal pores of the material and induce the crystallization growth of hydration products on the surface, further improving the compactness and compressive strength of UHPC. Specific Embodiments

[0028] The following provides a detailed description of the embodiments of the present invention. Implementations are carried out on the premise of the technical solution of the present invention, and detailed implementation methods and operation processes are given. However, the protection scope of the present invention is not limited to the following embodiments.

[0029] Example 1

[0030] Weigh 40 parts of magnesium sulfate and dissolve it in 500 parts of deionized water to obtain a soluble magnesium salt solution; separately, weigh 25 parts of disodium hydrogen phosphate and dissolve it in 500 parts of deionized water, add 2 parts of a polymer modifier composed of polyethylene glycol with a molecular weight of 4000 and polyvinyl alcohol with a molecular weight of 2000 in a 1:1 ratio, and stir until the solution is transparent. Use a constant flow pump to slowly drip the phosphate solution into the magnesium salt solution within 60 minutes, and at the same time adjust the pH value to 9 with 1M ammonia water. A milky white suspension gradually appears in the solution, indicating the formation of magnesium phosphate nanoparticles; after the dripping is completed, continue to stir for 60 minutes. Transfer the reaction solution to a centrifuge tube, centrifuge at 7000 revolutions per minute for 12 minutes, separate the precipitate and wash it 3 times with deionized water. Redisperse 60 parts of the washed magnesium phosphate nanoparticles in 500 parts of deionized water, add 0.2 parts of polyvinylpyrrolidone with a molecular weight of 10000 as a stabilizer, and use ultrasonic waves (frequency 50 kHz, time 7 minutes) to assist in dispersion to obtain a suspension of magnesium phosphate nanoparticles. Through a microfluidic device, supply carbon dioxide gas at a pressure of 0.3 MPa and a flow rate of 0.8 L / min, and inject the suspension into the device at a flow rate of 15 mL / min; at the nano-nozzle, the carbon dioxide is sheared into nanobubbles of about 100 nm and uniformly disperse 1.5 parts of nano-carbon dioxide bubbles in the suspension. Finally, add 0.03 parts of sodium chloride and 0.04 parts of sodium dodecyl sulfate, gently stir, and form a stable protective film on the surface of the bubbles to enhance the stability of the bubble interface, obtaining a nanocrystal seed additive for UHPC.

[0031] Example 2

[0032] Weigh 35 parts of magnesium nitrate and dissolve it in 500 parts of deionized water; separately, weigh 30 parts of ammonium dihydrogen phosphate and dissolve it in 500 parts of deionized water, add 5 parts of polyvinyl alcohol with a molecular weight of 2500, and stir until the solution becomes transparent. Use a constant flow pump to drip the phosphate solution into the magnesium salt solution within 90 minutes, and at the same time adjust the pH value to 8.5 with 1M ammonia water to form a milky suspension; after the dripping is completed, continue to stir for 90 minutes. Centrifuge the reaction solution at 6000 revolutions per minute for 15 minutes and wash the precipitate 4 times. Redisperse 65 parts of the particles in 500 parts of deionized water, add 0.1 part of sodium polyacrylate with a molecular weight of 40000 as a stabilizer, and perform ultrasonic dispersion (45 kHz, 8 minutes) to obtain a suspension. Through a microfluidic device, supply carbon dioxide gas at a pressure of 0.2 MPa and a flow rate of 0.6 L / min, and inject the suspension at a flow rate of 12 mL / min; generate nanobubbles with a diameter of about 150 nm, and uniformly disperse 1 part of nano-carbon dioxide bubbles in the suspension. Finally, add 0.05 part of potassium nitrate and 0.03 part of cetyltrimethylammonium bromide, and stir to form a protective film to obtain the nano-seed additive for UHPC.

[0033] Example 3

[0034] Weigh 45 parts of magnesium acetate and dissolve it in 500 parts of deionized water; separately, weigh 20 parts of trisodium phosphate and dissolve it in 500 parts of deionized water, add 4 parts of polyethylene glycol with a molecular weight of 6000, and stir until the solution becomes transparent. Drop the phosphate solution into the magnesium salt solution within 45 minutes, and maintain the pH value at 9.5 with 1M ammonia water to form a milky suspension; after the dripping is completed, continue to stir for 30 minutes. Centrifuge the reaction solution at 8000 revolutions per minute for 10 minutes and wash the precipitate 5 times. Redisperse 67 parts of the particles in 500 parts of deionized water, add 0.3 part of polyacrylamide with a molecular weight of 80000 as a stabilizer, and perform ultrasonic dispersion (55 kHz, 5 minutes) to obtain a suspension. Through a microfluidic device, supply carbon dioxide gas at a pressure of 0.5 MPa and a flow rate of 1 L / min, and inject the suspension at a flow rate of 20 mL / min; generate nanobubbles with a diameter of about 50 nm, and uniformly disperse 3 parts of nano-carbon dioxide bubbles in the suspension. Finally, add 0.02 part of sodium nitrite and 0.05 part of polyethylene glycol octyl phenyl ether to form a stable protective film to obtain the nano-seed additive for UHPC.

[0035] Example 4

[0036] Weigh 50 parts of magnesium citrate and dissolve it in 500 parts of deionized water; separately, weigh 16.7 parts of dipotassium hydrogen phosphate and dissolve it in 500 parts of deionized water. Add 6 parts of a polymer modifier composed of polyethylene glycol with a molecular weight of 8000 and polyvinyl alcohol with a molecular weight of 3000 in a ratio of 2:1, and stir until the solution becomes transparent. Drop the phosphate solution into the magnesium salt solution within 75 minutes, adjust the pH value to 10 with 1M ammonia water to form a milky suspension; after the dropping is completed, continue to stir for 120 minutes. Centrifuge the reaction solution at 7500 revolutions per minute for 10 minutes and wash the precipitate 3 times. Redisperse 68.5 parts of the particles in 500 parts of deionized water, add 0.1 part of polyvinylpyrrolidone with a molecular weight of 12000 as a stabilizer, and disperse ultrasonically (60 kHz, 10 minutes) to obtain a suspension. Through a microfluidic device, supply carbon dioxide gas at a pressure of 0.4 MPa and a flow rate of 0.9 L / min, and inject the suspension at a flow rate of 18 mL / min; generate nanobubbles with a diameter of about 75 nm, and uniformly disperse 2 parts of nano-carbon dioxide bubbles in the suspension. Finally, add 0.04 part of potassium chloride and 0.02 part of cocamidopropyl betaine, and stir to form a protective film to obtain a nano-seed additive for UHPC.

[0037] Example 5

[0038] Weigh 37 parts of magnesium carbonate and dissolve it in 500 parts of deionized water; separately, weigh 32.5 parts of ammonium phosphate and dissolve it in 500 parts of deionized water. Add 0.1 part of polyvinyl alcohol with a molecular weight of 3000, and stir until the solution becomes transparent. Add the phosphate solution into the magnesium salt solution within 30 minutes, adjust the pH value to 8 with 1M ammonia water to form a milky suspension; after the dropping is completed, continue to stir for 30 minutes. Centrifuge the reaction solution at 8000 revolutions per minute for 15 minutes and wash the precipitate 5 times. Redisperse 51.8 parts of the particles in 500 parts of deionized water, add 0.3 part of sodium polyacrylate with a molecular weight of 35000 as a stabilizer, and disperse ultrasonically (40 kHz, 5 minutes) to obtain a suspension. Through a microfluidic device, supply carbon dioxide gas at a pressure of 0.1 MPa and a flow rate of 0.5 L / min, and inject the suspension at a flow rate of 10 mL / min; generate nanobubbles with a diameter of about 200 nm, and uniformly disperse 0.1 part of nano-carbon dioxide bubbles in the suspension. Finally, add 0.02 part of potassium nitrite and 0.05 part of cetylpyridinium chloride, and gently stir to form a stable protective film, and finally obtain a nano-seed additive for UHPC.

[0039] Test Example 1: Hydration promotion effect

[0040] The test was carried out using the mix proportion of UHPC mixture, and its mix proportion is as follows:

[0041] Portland cement (P.Ⅰ.52.5): 800 kg / m³

[0042] Silica fume: 200 kg / m³

[0043] Quartz sand (particle size 0.15 - 0.6 mm): 1020 kg / m³

[0044] Quartz powder (particle size less than 0.15 mm): 200 kg / m³

[0045] Water: 160 kg / m³

[0046] High - efficiency water - reducing agent: 24 kg / m³ (about 2% of the total amount of cementitious materials)

[0047] Steel fiber: 156 kg / m³ (length 13 mm, diameter 0.2 mm)

[0048] The hydration promotion effects of different nano - seed additives on UHPC are characterized by setting time, 12 - h compressive strength, and 1 - d compressive strength respectively. Control group 1 uses a nano - hydrated calcium silicate - type seed additive from a certain commercial manufacturer, and control group 2 is a nano - calcium carbonate - type seed additive from a certain commercial manufacturer. The dosage of the nano - seed additive is 1% of the cement mass. Benchmark cement is selected for the experiment and cured under standard curing conditions. The test results are shown in Table 1.

[0049] Table 1. Influence of nano - seed additives on the hydration process of UHPC

[0050]

[0051] As can be seen from Table 1, Examples 1 to 5 have almost no negative effects on the fluidity and workability of the UHPC mixture, while control group 2 significantly reduces the fluidity of the UHPC mixture. Adding 1% nano - seed additive not only effectively shortens the initial setting and final setting times, speeds up the setting speed of the concrete, but also greatly improves the early compressive strength, enabling UHPC to reach a high - strength level in a short time. In contrast, the control group and the blank group have longer setting times and slow early strength development, and even fail to harden within 12 hours. Thus, it can be seen that the nano - seed additive for UHPC described in the present invention has a significant promoting effect on the hydration reaction of UHPC.

[0052] Test Example 2: Mechanical property test

[0053] Prepare the UHPC mixture according to the mix ratio described in Test Example 1, carry out time - forming and curing with reference to the relevant provisions in Section 3.3 of T / CECS 864 - 2021 "Standard Test Methods for Ultra - High Performance Concrete", and conduct compressive strength, flexural strength, and splitting tensile strength tests according to the relevant provisions in Sections 5.1, 5.5, and 5.6. The test results are shown in Table 2.

[0054] Table 2. Influence of nano - seed additives on the mechanical properties of UHPC

[0055]

[0056] According to the results in Table 2, the 28-day compressive strength of the UHPC specimens doped with the nanoseed additives described in Examples 1 to 5 is between 172.2 MPa and 179.2 MPa, with an average of approximately 176.8 MPa, which is significantly higher than that of the blank group; the 28-day flexural strength is between 39.5 MPa and 42.8 MPa, which is about 20% higher than that of the blank group; the 8-day tensile strength has also been increased significantly. In contrast, the 28-day compressive strength, 28-day flexural strength, and 28-day tensile strength of the UHPC specimens doped with the nanoseed additives of Control Group 1 and Control Group 2 do not show obvious improvement compared with the blank group. These data clearly show that the nanoseed additive for UHPC in this solution has excellent effects in enhancing the compressive, flexural, and tensile properties of UHPC.

[0057] Although the present invention has been disclosed above with preferred embodiments, they are not used to limit the present invention. Any person skilled in this art can make various changes or modifications without departing from the spirit and scope of the present invention. Therefore, the protection scope of the present invention should be defined by the protection scope of the claims of this application.

Claims

1. A nanocrystalline seed additive for UHPC, characterized in that: The additive consists of nano CO2 bubbles, fibrous magnesium phosphate nanocrystals and bubble stabilizers. The magnesium phosphate nanocrystals serve as the crystal nuclei of cement materials during cement hydration reaction. The particle size of the nano CO2 bubbles is 50-200nm, and the volume accounts for 30-60% of the total volume. The magnesium phosphate nanocrystals have an aspect ratio of 5-10 and a particle size of 10-20nm. The bubble stabilizer is a composite of inorganic salt electrolytes and organic surfactant molecules. Nano CO2 bubbles, magnesium phosphate nanocrystals and bubble stabilizers are filled with each other at different scales to form a stable multi-scale multi-phase suspended aqueous solution.

2. A method for preparing a nanocrystalline seed additive for UHPC, used to prepare the additive according to claim 1, characterized in that: The following steps are involved: Step 1: Preparation of fibrous magnesium phosphate nanocrystal suspension; weigh 35-50 parts by mass of soluble magnesium salt and dissolve them in 500 parts by mass of deionized water to prepare a magnesium salt solution; then, weigh 16.7-32.5 parts by mass of soluble phosphate and dissolve them in 500 parts by mass of deionized water, and add 0.1-6 parts by mass of polymer modifier, and stir until the solution is completely transparent to prepare a phosphate solution; then, slowly drip the phosphate solution into the magnesium salt solution through a constant flow pump, and use 1M ammonia solution to adjust the pH value of the solution to 8-10 in real time, and the solution gradually becomes a milky white suspension, indicating that fibrous magnesium phosphate nanocrystals are formed; after the addition is completed, continue stirring for 30-120 minutes; Subsequently, the reaction solution was transferred to a centrifuge tube, centrifuged at 6000-8000 rpm for 10-15 minutes to separate the precipitate, and washed with deionized water 3-5 times to remove residual impurities and unreacted products; Re-dispersing 51.8-88.5 parts by weight of washed fibrous magnesium phosphate nanocrystals in 500 parts by weight of deionized water, adding 0.1-0.3 parts by weight of a stabilizer, and using an ultrasonic-assisted dispersion technique to obtain a fibrous magnesium phosphate nanocrystal suspension; Step 2: using a microfluidic device, evenly dispersing 0.1 to 3 parts by mass of nano carbon dioxide bubbles in the suspension obtained in step 1; stably supplying CO2 gas to the gas phase inlet of the microfluidic device through a pressure reducing valve and a flow meter, at which time the gas pressure of the pressure reducing valve is 0.1 to 0.5 MPa, and the flow rate of the flow meter is set to 0.5 to 1 L / min; at the same time, injecting the suspension obtained in step 1 into the liquid phase inlet of the microfluidic device through a liquid injection module at a flow rate of 10 to 20 mL / min; at the outlet nanonozzle of the microfluidic device, the carbon dioxide gas is sheared into nanobubbles with a diameter of 50 to 200 nm, which are evenly dispersed in the suspension; Step 3: Enhance the stability of nanobubbles; add 0.02-0.05 parts by mass of an inorganic salt electrolyte and 0.02-0.05 parts by mass of a surfactant to the suspension generated in step 2 and stir gently to form a stable protective film on the bubble surface, enhance the interface stability of the bubble, and then obtain the target UHPC nanocrystal seed additive.

3. The method for preparing a nanocrystalline seed additive for UHPC according to claim 2, characterized in that: The soluble magnesium salt is one or a combination of more than one of magnesium sulfate, magnesium chloride, magnesium carbonate, magnesium nitrate, magnesium acetate and magnesium citrate.

4. The method for preparing a nanocrystalline seed additive for UHPC according to claim 2, characterized in that: The soluble phosphate is one or a combination of more than one of sodium dihydrogen phosphate, potassium dihydrogen phosphate, disodium hydrogen phosphate, dipotassium hydrogen phosphate, trisodium phosphate, tripotassium phosphate, ammonium phosphate, and ammonium hydrogen phosphate.

5. The method for preparing a nanocrystalline seed additive for UHPC according to claim 2, characterized in that: The polymer modifier is a combination of polyethylene glycol with a molecular weight of 2000-8000 and polyvinyl alcohol with a molecular weight of 500-3000 in any proportion.

6. The method for preparing a nanocrystalline seed additive for UHPC according to claim 2, characterized in that: The stabilizer is one or a combination of polyvinyl pyrrolidone with a molecular weight of 8000-15000, sodium polyacrylate with a molecular weight of 30000-50000, and polyacrylamide with a molecular weight of 50000-100000.

7. The method for preparing a nanocrystalline seed additive for UHPC according to claim 2, characterized in that: The inorganic salt in the inorganic salt electrolyte is one or a combination of more than one of sodium chloride, potassium chloride, sodium nitrate, sodium nitrite, potassium nitrate and potassium nitrite.

8. The method for preparing a nanocrystalline seed additive for UHPC according to claim 2, characterized in that: The surfactant is one or a combination of the following: sodium lauryl sulfate, hexadecyltrimethylammonium bromide, polyethylene glycol octylphenyl ether, cocamidopropyl betaine, dodecyldimethylalkyldiquaternary ammonium salt, and hexadecylpyridinium chloride.

9. The method for preparing a nanocrystalline seed additive for UHPC according to claim 2, characterized in that: In step 1, the ultrasonic assisted dispersion technology is to make the solution uniform through ultrasound, at which time the frequency of the ultrasound is 40-60 kHz and the processing time is 5-10 minutes.

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