Method for controllably preparing high-quality porous geopolymer microspheres based on freezing-suspension curing method

Abstract

The invention discloses a preparation method for controllably preparing high-quality porous geopolymer microspheres based on a freezing-suspension curing method, which comprises the following steps: mixing an alkali-activated active precursor material with an alkali activator, and grinding to obtain geopolymer slurry; wherein the density of the geopolymer slurry is controlled within the range of 0.5-2 g / cm < 3 >, and the viscosity of the geopolymer slurry is controlled within the range of 40-100 mPa.s; dropwise adding the geopolymer slurry to the surface of the liquid nitrogen in a liquid drop form; the liquid drops stably suspend for 5-15 seconds on a gas layer formed on the surface of the liquid nitrogen due to the inverse Leidenfrost effect, are completely frozen and sink into the liquid nitrogen; and freeze-drying the polymer microspheres, and sublimating the solid solvent in the polymer microspheres to obtain the geopolymer microspheres with a porous structure. The porous geopolymer microspheres prepared by the method have the advantages of high sphericity, smooth surface and few cracks.

Description

Technical Field

[0001] The present invention relates to a method for forming porous geopolymer materials, and in particular to a method for controllably preparing high-quality porous geopolymer microspheres based on a freezing-suspension solidification method. Background Art

[0002] Geopolymers, a new type of inorganic gelling material, have attracted considerable attention due to their wide availability of raw materials (often using industrial solid waste), low energy consumption during the preparation process, environmental friendliness, and excellent mechanical properties and durability. Preparing them into porous microspheres can impart them with high surface area, low density, and a specific pore structure, greatly expanding their application range and demonstrating great potential in functional areas such as adsorption, catalysis, filtration, sustained-release carriers, and environmental remediation. Currently, methods for preparing porous geopolymer microspheres primarily include suspension polymerization, template processing, foaming, and freeze-drying. However, these methods often have some inherent defects: (1) The suspension polymerization method usually requires the use of organic solvents or oil phases, which may cause environmental pollution problems, and it is difficult to control the sphericity and size uniformity of the microspheres; (2) Although the template method can prepare an ordered pore structure, it is usually complicated and costly, and the template removal process may introduce impurities or destroy the structure; (3) The chemical foaming method generates gas-induced pores by adding a foaming agent during the polymerization process, but the pore size distribution is often wide and difficult to accurately control, and the selection and removal of the foaming agent may also cause problems; (4) Conventional freeze-drying methods (such as freezing the slurry as a whole and then breaking it) are difficult to obtain a regular spherical morphology.

[0003] In recent years, the freeze-suspension solidification method has been considered an effective way to prepare porous microspheres, especially ceramic and polymer microspheres. This method uses the inverse Leidenfrost effect, that is, when a droplet contacts a cryogenic liquid (such as liquid nitrogen) far below its freezing point, a stable vapor film (gas layer) is instantly generated at the interface. This gas layer can support the droplet to suspend on the surface of the cryogenic liquid for a period of time and quickly freeze in a suspended state. The solidified solvent (usually ice crystals formed by water) is then removed by freeze drying to obtain porous microspheres. However, the successful application of this method to the preparation of high-quality, performance-controllable porous geopolymer microspheres still faces significant challenges. Existing research often fails to fully address the following key issues:

[0004] (1) Insufficient suspension stability: The droplets cannot be suspended or the suspension behavior is unstable, making it difficult to achieve uniform freezing.

[0005] (2) Microsphere morphology defects: The droplets have poor spherical properties and the ability to maintain spherical shape. At the same time, the extremely low temperature of liquid nitrogen can easily cause thermal stress, leading to cracking of the microspheres.

[0006] (3) The pore structure is difficult to control, resulting in a large randomness of the pore structure.

[0007] (4) Interference from the freezing environment: The violent boiling of liquid nitrogen will interfere with the stability of the gas layer, affect the suspension and freezing process of the droplets, and lead to product morphology and structural defects.

[0008] Therefore, how to systematically solve the above problems and achieve stable and reproducible preparation of high-quality porous geopolymer microspheres with regular morphology (high sphericity, low cracks) and adjustable pore structure is a technical problem that needs to be solved urgently in this field. Summary of the Invention

[0009] In order to overcome the above-mentioned shortcomings and deficiencies of the prior art, the present invention aims to provide a method for preparing high-quality porous geopolymer microspheres based on a freeze-suspension solidification method, by controlling the density of the geopolymer slurry to 0.5-2 g / cm 3 The viscosity is controlled within the range of 40 to 100 mPa·s, and the droplets are stably suspended on the gas layer on the surface of liquid nitrogen for 5 to 15 seconds. The prepared porous geopolymer microspheres have the advantages of high sphericity, smooth surface and few cracks.

[0010] The purpose of the present invention is achieved through the following technical solutions:

[0011] The present invention provides a method for controllably preparing high-quality porous geopolymer microspheres based on a freeze-suspension solidification method, comprising the following steps:

[0012] (1) Preparation of geopolymer slurry: mixing an alkali-activated active precursor material with an alkali activator and grinding the mixture to obtain a geopolymer slurry;

[0013] The density of the geopolymer slurry is controlled at 0.5-2 g / cm 3 range, and the viscosity is controlled within the range of 40 to 100 mPa·s;

[0014] (2) Forming frozen microspheres: the geopolymer slurry obtained in step (1) is added to the surface of liquid nitrogen in the form of droplets; the droplets are stably suspended for 5 to 15 seconds on the gas layer formed on the surface of the liquid nitrogen due to the reverse Leidenfrost effect, and undergo a freezing and solidification process in the suspended state to form frozen geopolymer microspheres, which are completely frozen and sink into the liquid nitrogen;

[0015] The time during which the droplets are stably suspended in the gas layer formed by the reverse Leidenfrost effect on the liquid nitrogen surface is controlled by the density and viscosity of the geopolymer slurry in step (1);

[0016] (3) freeze-drying the frozen geopolymer microspheres obtained in step (2), sublimating the solid solvent therein to form a porous structure, and curing the dried geopolymer microspheres to obtain the porous geopolymer microspheres.

[0017] Preferably, the forming of the cryomicrospheres in step (2) is carried out in a device having a double-layer structure; the device having a double-layer structure comprises a box made of a heat-insulating material, an outer container and an inner container placed in the box; the outer container contains liquid nitrogen; the inner container contains liquid nitrogen and is placed in the outer container, and the inner container is partially immersed in the liquid nitrogen in the outer container; the liquid nitrogen surface of the inner container is in a calmer state than the liquid nitrogen surface of the outer container;

[0018] In step (2), the geopolymer slurry is added in the form of droplets onto the surface of the liquid nitrogen in the inner container.

[0019] Preferably, the alkali-activated active precursor material includes at least two of slag, metakaolin, fly ash, coal gangue, red mud, tailings containing active silicon and aluminum components, silica fume, expanded perlite powder, and fly ash floating beads;

[0020] The alkali activator is sodium water glass or potassium water glass with a modulus controlled at 1.2 to 1.5.

[0021] Preferably, the water-to-solid ratio in the geopolymer slurry is controlled to be 0.6-1.

[0022] Preferably, in step (2), the volume of the droplet is 1 to 50 μl, and the particle size of the porous geopolymer microspheres obtained is 1 to 3 mm.

[0023] Preferably, the grinding in step (1) is completed by ball milling, the ball milling rate is 25 to 45 r / min, and the ball milling time is 0.5 to 2 h.

[0024] Preferably, the freeze-drying conditions in step (3) are: vacuum degree of 0.3 to 0.5 mbar, temperature of -40 to -20°C, and time of 24 to 48 hours.

[0025] Preferably, the curing in step (3) is one of standard curing, steam curing or immersion curing; wherein, the temperature of standard curing is 15-25°C, the relative humidity is greater than 90%, and the curing time is 2-28 days; the temperature of steam curing is 60-80°C, and the curing time is 12-48 hours; and the immersion curing is to immerse the porous geopolymer microspheres in water, and the curing time is 2-28 days.

[0026] Preferably, the density of the geopolymer slurry is 1.0 to 1.6 g / cm 3 , viscosity is 45~90mPa·s, and suspension time is 7~15s.

[0027] Preferably, the solid phase material of the geopolymer slurry includes 30-80 parts of slag powder and 20-70 parts of fly ash beads; the amount of sodium water glass added is 3-7 wt% of the solid phase material. More preferably, the solid phase material of the geopolymer slurry includes 78-82 parts of slag powder and 18-22 parts of fly ash beads, and the amount of sodium water glass added is 5.8-6.2 wt% of the solid phase material. More preferably, the solid phase material of the geopolymer slurry includes 58-62 parts of slag powder and 38-42 parts of fly ash beads, and the amount of sodium water glass added is 4.8-5.2 wt% of the solid phase material.

[0028] Preferably, the specific surface area of the fly ash beads is greater than 250m 2 / kg.

[0029] The present invention also provides porous geopolymer microspheres obtained by the method for controllably preparing high-quality porous geopolymer microspheres based on the freezing-suspension solidification method.

[0030] The present invention controls the raw materials and their ratios (including the types and ratios of the alkali-activated active precursor material and the alkali activator) and the water-solid ratio so that the resulting slurry satisfies the following two conditions simultaneously: (1) the density is controlled at 0.5-2 g / cm 3 This density range is the physical basis for ensuring that the slurry droplets can achieve stable suspension on the gas layer on the surface of liquid nitrogen based on the inverse Leidenfrost effect in the subsequent steps. (2) The viscosity is controlled in the range of 40 to 100 mPa·s. This viscosity range is intended to ensure that the slurry droplets have sufficient surface tension to form a regular spherical shape, while having a certain fluidity and being able to resist cracking under the temperature difference shock of rapid freezing, thereby ensuring the integrity of the microsphere structure.

[0031] In the formation of frozen microspheres, the present invention ensures that a geopolymer slurry with a specific density and viscosity is stably suspended for 5 to 15 seconds on the stable vapor layer (nitrogen layer) formed on the surface of liquid nitrogen. It is also necessary to ensure that the gas layer on the surface of the liquid nitrogen is relatively calm to avoid violent boiling that interferes with the stability of the gas layer and the suspension of the droplets, thereby ensuring the uniformity of the freezing process. The duration of the droplet suspension on this gas layer is ensured to fall within the range of 5 to 15 seconds. This suspension time is a critical period that affects the freezing of water in the slurry (ice crystal nucleation and growth). By controlling the ice crystal growth state within this time range, the pore structure formed after the microspheres are dried, especially the pore size, can be effectively controlled.

[0032] The present invention performs a curing treatment on the dried microspheres to promote the full progress of the geopolymerization reaction, improve the mechanical strength and chemical stability of the microspheres, and ultimately obtain the desired porous geopolymer microspheres.

[0033] Compared with the existing technology, the present invention achieves the following significant benefits by precisely setting and coordinating the four key factors of geopolymer slurry: density and viscosity, liquid nitrogen state during the freezing suspension process, and suspension time:

[0034] 1. A stable suspension freezing process was achieved: ensuring that the geopolymer slurry droplets could reliably and stably suspend on the liquid nitrogen surface for a long time, laying the foundation for uniform freezing and structure formation.

[0035] 2. Significantly improved the morphological quality of microspheres: porous geopolymer microspheres with high sphericity, smooth surface and significantly reduced cracks can be stably prepared.

[0036] 3. The pore structure (especially pore size) can be controlled: by controlling the critical suspension time range (5 to 15 seconds) and adjusting the slurry ratio, the ice crystal growth can be effectively regulated, and then the pore size and porosity of the final microspheres can be controlled to meet the functional requirements of different applications.

[0037] 4. Improved stability and repeatability of the preparation process: Precise parameter control makes the preparation results more reliable and predictable.

[0038] 5. The method is simple and environmentally friendly: based on the principle of freeze-solidification, it does not require complex equipment and chemical foaming agents and is environmentally friendly. BRIEF DESCRIPTION OF THE DRAWINGS

[0039] Figure 1 Schematic diagram of the structure of an apparatus for achieving a relatively calm state of liquid nitrogen according to an embodiment of the present invention.

[0040] Figure 2 Scanning electron microscope (SEM) photograph of porous geopolymer microspheres prepared in Example 1 of the present invention (low magnification, 23 times)

[0041] Figure 3 This is a scanning electron microscope (SEM) photograph (high magnification, 2000 times) of the porous geopolymer microspheres prepared in Example 1 of the present invention, showing the internal porous structure thereof. DETAILED DESCRIPTION

[0042] To make the objectives, technical solutions, and advantages of the present invention more apparent, the present invention will be further described in detail below with reference to the accompanying drawings and specific embodiments. It should be understood that the specific embodiments described herein are merely intended to illustrate the present invention and are not intended to limit the scope of protection of the present invention. Any modifications, equivalent substitutions, improvements, etc. made within the spirit and principles of the present invention shall be included within the scope of protection of the present invention.

[0043] Example 1

[0044] (1) Preparation of geopolymer slurry

[0045] According to the ratio shown in Table 1, 80 parts by weight of slag and 20 parts by weight of fly ash beads were weighed. Based on the requirement of a water-solid ratio of 1, the total amount of water required was calculated. Weigh a sodium water glass solution (prepared or selected with a modulus of 1.3) with a mass of Na2O equivalent to 6% of the weight of the slag. Add the slag, sodium water glass solution and water to make up the remaining water into a plastic bottle. Place the plastic bottle on a planetary ball mill and ball mill at a ball milling rate of 25r / min for 2 hours to obtain a uniform geopolymer slurry. The density of the slurry was measured to be 1.58g / cm 3 , viscosity is 45mPa·s.

[0046] (2) Formation of frozen microspheres

[0047] use Figure 1 The device with a double-layer structure shown in the figure comprises a box body 1 made of polystyrene foam material, an outer container 2 and an inner container 3 placed in the box body 1; the outer container 2 can be a crystallizing dish filled with liquid nitrogen; the inner container 3 can be a beaker filled with liquid nitrogen and placed in the outer container 2, and the inner container 3 is partially immersed in the liquid nitrogen in the outer container 2; the liquid nitrogen surface of the inner container 3 is in a calmer state than the liquid nitrogen surface of the outer container; the box body is also provided with an air inlet 4 and an air outlet 5 for allowing air to enter.

[0048] Wait until the surface of the liquid nitrogen in the inner container reaches a relatively calm state. Use an adjustable pipette with a range of 2 to 20 μl to aspirate the slurry prepared in step (1) with a volume of 4 μl. Carefully add this 4 μl droplet of slurry to the surface of the inner layer of liquid nitrogen. Observe that the droplet is stably suspended in the nitrogen layer and is spherical. Record its suspension duration as approximately 7.81 s. After the suspension ends, the frozen microspheres sink. Repeat the operation to prepare the required number of microspheres.

[0049] (3) Obtaining porous geopolymer microspheres

[0050] The collected frozen microspheres were transferred to a freeze dryer. Freeze-drying conditions were set as follows: vacuum 0.3 mbar, temperature -40°C, and duration 24 hours. After drying, the microspheres were steam-cured at 80°C for 12 hours. This yielded porous geopolymer microspheres. SEM observations revealed high sphericity and internal porosity. Table 2 indicated virtually no cracks.

[0051] Example 2

[0052] (1) Preparation of geopolymer slurry

[0053] According to the ratio shown in Table 1, weigh 70 parts by weight of slag and 30 parts by weight of fly ash beads. Based on the requirement of a water-solid ratio of 0.9, calculate the total amount of water required. Weigh a sodium water glass solution (modulus of 1.3) equivalent to 6% of the total weight of Na2O of the slag and beads. Add the slag, fly ash beads, sodium water glass solution and the required water to a plastic bottle. Use a planetary ball mill to perform ball milling mixing at a ball milling rate of 25r / min for 2h. The density of the slurry was measured to be 1.31g / cm 3 , viscosity is 56mPa·s.

[0054] (2) Formation of frozen microspheres

[0055] The same double-layered equipment and operating methods as in Example 1 were used to ensure that the surface of the liquid nitrogen was relatively calm. An adjustable pipette with a range of 2 to 20 μl was used to aspirate the slurry prepared in step (1) with a volume of 10 μl. This volume of slurry was added dropwise to the surface of the inner layer of liquid nitrogen. The droplets were observed to be stably suspended. The duration of the suspension was recorded to be approximately 11.01 s. After the suspension was completed, the frozen microspheres sank. The operation was repeated to prepare the desired number of microspheres.

[0056] (3) Obtaining porous geopolymer microspheres

[0057] The collected frozen microspheres were freeze-dried under the following conditions: vacuum 0.35 mbar, temperature -35°C, and drying time for 28 hours. After drying, the microspheres were steam-cured at 70°C for 36 hours. This yielded porous geopolymer microspheres. SEM observations revealed good sphericity and a porous interior. Table 2 indicated minimal cracking.

[0058] Example 3

[0059] (1) Preparation of geopolymer slurry:

[0060] According to the ratio shown in Table 1, weigh 50 parts by weight of slag and 50 parts by weight of fly ash beads. Calculate the amount of water based on the requirement of a water-to-solid ratio of 0.9. Weigh a sodium water glass solution (modulus 1.3) equivalent to 5% of the total solid phase weight of Na2O. Mix all the raw materials. Use a planetary ball mill to mill and mix at a ball milling rate of 30r / min for 1.5h. The density of the slurry was measured to be 1.17g / cm 3 , viscosity is 60mPa·s.

[0061] (2) Formation of frozen microspheres

[0062] The same double-layered equipment and operating methods as in Example 1 were used to ensure that the surface of the liquid nitrogen was relatively calm. An adjustable pipette with a range of 2 to 20 μl was used to aspirate the slurry prepared in step (1) with a volume of 19 μl. This volume of slurry was added dropwise to the surface of the inner layer of liquid nitrogen. The droplets were observed to be stably suspended. The duration of the suspension was recorded to be approximately 12.04 s. After the suspension was completed, the frozen microspheres sank. The operation was repeated to prepare the desired number of microspheres.

[0063] (3) Obtaining porous geopolymer microspheres

[0064] The collected frozen microspheres were freeze-dried under the following conditions: vacuum of 0.4 mbar, temperature of -30°C, and duration of 36 hours. After drying, the microspheres were steam-cured at 60°C for 48 hours, ultimately yielding porous geopolymer microspheres. Figures 2-3 SEM observations showed good sphericity, porous interior, and relatively few cracks.

[0065] Example 4

[0066] (1) Preparation of geopolymer slurry:

[0067] According to the ratio shown in Table 1, weigh 60 parts by weight of slag and 40 parts by weight of fly ash beads. Calculate the amount of water based on the requirement of a water-solid ratio of 0.8. Weigh a sodium water glass solution (modulus 1.5) equivalent to 5% of the total solid phase weight of Na2O. Mix all the raw materials. Use a planetary ball mill to mill and mix at a ball milling rate of 35r / min for 1.3h. The density of the slurry was measured to be 1.24g / cm 3 , viscosity 68mPa·s.

[0068] (2) Formation of frozen microspheres

[0069] The same double-layered equipment and operating methods as in Example 1 were used to ensure that the surface of the liquid nitrogen was relatively calm. An adjustable pipette with a range of 2 to 20 μl was used to aspirate the slurry prepared in step (1) to a volume of 9.5 μl. This volume of slurry was added dropwise to the surface of the inner layer of liquid nitrogen. The droplets were observed to be stably suspended. The duration of the suspension was recorded to be approximately 10.54 s. After the suspension was completed, the frozen microspheres sank. The operation was repeated to prepare the desired number of microspheres.

[0070] (3) Obtaining porous geopolymer microspheres:

[0071] The collected frozen microspheres were freeze-dried under the following conditions: vacuum 0.45 mbar, temperature -25°C, and drying time for 40 hours. After drying, standard curing was performed: the microspheres were placed in an environment with a temperature of 25°C and a relative humidity of 95% for 14 days. The resulting porous geopolymer microspheres were obtained. SEM observations revealed high sphericity and internal porosity. According to Table 2, cracks were virtually absent.

[0072] Example 5

[0073] (1) Preparation of geopolymer slurry:

[0074] According to the ratio shown in Table 1, weigh 40 parts by weight of slag and 60 parts by weight of fly ash beads. Calculate the amount of water based on the requirement of a water-to-solid ratio of 0.7. Weigh a sodium water glass solution (modulus 1.5) equivalent to 4% of the total solid phase weight of Na2O. Mix all the raw materials. Use a planetary ball mill to mill the mixture at a ball milling rate of 40r / min for 1 hour. The density of the slurry was measured to be 1.12g / cm 3 , viscosity is 75mPa·s.

[0075] (2) Formation of frozen microspheres

[0076] The same double-layered equipment and operating methods as in Example 1 were used to ensure that the surface of the liquid nitrogen was relatively calm. An adjustable pipette with a range of 10 to 100 μl was used to aspirate the slurry prepared in step (1) with a volume of 25 μl. This volume of slurry was added dropwise to the surface of the inner layer of liquid nitrogen. The droplets were observed to be stably suspended. The duration of the suspension was recorded to be approximately 13.35 s. After the suspension was completed, the frozen microspheres sank. The operation was repeated to prepare the desired number of microspheres.

[0077] (3) Obtaining porous geopolymer microspheres:

[0078] The collected frozen microspheres were freeze-dried under the following conditions: vacuum 0.5 mbar, temperature -20°C, and drying time of 48 hours. After drying, standard curing was performed: the microspheres were placed in an environment with a temperature of 15°C and a relative humidity of 97% for 28 days. The resulting porous geopolymer microspheres were obtained. SEM examination revealed good sphericity and a porous interior. Table 2 indicated minimal cracking.

[0079] Example 6

[0080] (1) Preparation of geopolymer slurry

[0081] According to the ratio shown in Table 1, weigh 30 parts by weight of slag and 70 parts by weight of fly ash beads. Calculate the amount of water based on the requirement of a water-to-solid ratio of 0.6. Weigh a sodium water glass solution (modulus 1.5) equivalent to 4% of the total solid phase weight of Na2O. Mix all the raw materials. Use a planetary ball mill to perform ball milling at a ball milling rate of 45r / min for 0.5h. The density of the slurry was measured to be 1.06g / cm 3 , viscosity is 86mPa·s.

[0082] (2) Formation of frozen microspheres

[0083] The same double-layered apparatus and operating method as in Example 1 were used to ensure that the surface of the liquid nitrogen was relatively calm. An adjustable pipette with a range of 10 to 100 μl was used to aspirate the slurry prepared in step (1) to a volume of 35 μl. This volume of slurry was added dropwise to the surface of the inner layer of liquid nitrogen. The droplets were observed to be stably suspended. The duration of the suspension was recorded to be approximately 14.78 s. After the suspension was completed, the frozen microspheres sank. The operation was repeated to prepare the desired number of microspheres.

[0084] (3) Obtaining porous geopolymer microspheres

[0085] The collected frozen microspheres were freeze-dried under the following conditions: vacuum 0.4 mbar, temperature -25°C, and drying time of 48 hours. After drying, the microspheres were water-cured by immersing them in 20°C water for 14 days. This yielded porous geopolymer microspheres. SEM observations revealed good sphericity and a porous interior. Table 2 indicated minimal cracking.

[0086] Example 7

[0087] (1) Preparation of geopolymer slurry:

[0088] According to the ratio shown in Table 1, weigh 60 parts by weight of slag and 40 parts by weight of fly ash beads. Calculate the amount of water based on the requirement of a water-solid ratio of 0.8. Weigh a sodium water glass solution (modulus 1.5) equivalent to 5% of the total solid phase weight of Na2O. Mix all the raw materials. Use a planetary ball mill to mill and mix at a ball milling rate of 35r / min for 1.3h. The density of the slurry was measured to be 1.24g / cm 3 , viscosity 68mPa·s.

[0089] (2) Formation of frozen microspheres

[0090] In contrast to Example 4, a device with a double-layer structure is not used, that is, the outer crystallization dish and its liquid nitrogen are removed, and only the inner beaker and its liquid nitrogen are retained as a direct reaction device. An adjustable pipette with a range of 2 to 20 μl is used to absorb the slurry prepared in step (1), and the absorption volume is 9.5 μl. This volume of slurry droplets are added to the surface of the liquid nitrogen in the beaker. It is observed that the liquid nitrogen in the beaker boils violently and the droplets cannot be stably suspended. The suspension duration is recorded to be approximately 10.38 s. After the suspension is completed, the frozen microspheres sink. Repeat the operation to prepare the required number of microspheres.

[0091] Table 1 Raw material ratio, preparation and curing conditions of porous geopolymer microspheres in each embodiment

[0092]

[0093] Table 2 Properties of the geopolymer slurry and final microsphere characteristics of each example

[0094]

[0095]

[0096] As shown in Table 2, the geopolymer slurries in Examples 1 to 6 have a slurry density of 1.06 to 1.58 g / cm 3 The slurry viscosity was 45-86 mPa·s, the suspension time was 7.81-14.78 s, and the average pore size was 0.35-0.72 μm. Examples 1 and 4 were the best examples, and the resulting microspheres had high sphericity and were almost crack-free. In Example 7, the double-layer structure was not used in the molding process, and the resulting microspheres had high sphericity but obvious cracks.

[0097] The above embodiments are preferred implementation modes of the present invention, but the implementation modes of the present invention are not limited to the embodiments. Any other changes, modifications, substitutions, combinations, and simplifications that do not deviate from the spirit and principles of the present invention should be considered as equivalent replacement methods and are included in the scope of protection of the present invention.

Claims

1. A method for controllably preparing high-quality porous geopolymer microspheres based on a freeze-suspension solidification method, characterized in that: The following steps are involved: (1) Preparation of geopolymer slurry: mixing an alkali-activated active precursor material with an alkali activator and grinding the mixture to obtain a geopolymer slurry; The density of the geopolymer slurry is controlled at 0.5-2 g / cm 3 range, and the viscosity is controlled within the range of 40 to 100 mPa·s; (2) Forming frozen microspheres: the geopolymer slurry obtained in step (1) is added to the surface of liquid nitrogen in the form of droplets; the droplets are stably suspended for 5 to 15 seconds on the gas layer formed on the surface of the liquid nitrogen due to the reverse Leidenfrost effect, and undergo a freezing and solidification process in the suspended state to form frozen geopolymer microspheres, which are completely frozen and sink into the liquid nitrogen; The time during which the droplets are stably suspended in the gas layer formed by the reverse Leidenfrost effect on the liquid nitrogen surface is controlled by the density and viscosity of the geopolymer slurry in step (1); (3) freeze-drying the frozen geopolymer microspheres obtained in step (2), sublimating the solid solvent therein to form a porous structure, and curing the dried geopolymer microspheres to obtain the porous geopolymer microspheres.

2. The method for controllably preparing high-quality porous geopolymer microspheres based on the freeze-suspension solidification method according to claim 1, characterized in that: The formation of the cryomicrospheres in step (2) is performed in a device having a double-layer structure; the double-layer structure comprises a box made of a heat-insulating material, an outer container and an inner container placed in the box; the outer container contains liquid nitrogen; the inner container contains liquid nitrogen and is placed in the outer container, and the inner container is partially immersed in the liquid nitrogen in the outer container; the liquid nitrogen surface of the inner container is in a calmer state than the liquid nitrogen surface of the outer container; In step (2), the geopolymer slurry is added in the form of droplets onto the surface of the liquid nitrogen in the inner container.

3. The method for controllably preparing high-quality porous geopolymer microspheres based on the freeze-suspension solidification method according to claim 1, characterized in that: The alkali-activated active precursor material includes at least two of slag, metakaolin, fly ash, coal gangue, red mud, tailings containing active silicon and aluminum components, silica fume, expanded perlite powder, and fly ash floating beads; The alkali activator is sodium water glass or potassium water glass with a modulus controlled at 1.2 to 1.

5.

4. The method for controllably preparing high-quality porous geopolymer microspheres based on the freeze-suspension solidification method according to claim 3, characterized in that: The water-to-solid ratio in the geopolymer slurry is controlled to be 0.6-1.

5. The method for controllably preparing high-quality porous geopolymer microspheres based on the freeze-suspension solidification method according to claim 1, characterized in that: In step (2), the volume of the droplet is 1 to 50 μl, and the particle size of the porous geopolymer microspheres obtained is 1 to 3 mm.

6. The method for controllably preparing high-quality porous geopolymer microspheres based on the freeze-suspension solidification method according to claim 1, characterized in that: The grinding in step (1) is completed by ball milling, the ball milling rate is 25-45 r / min, and the ball milling time is 0.5-2 h.

7. The method for controllably preparing high-quality porous geopolymer microspheres based on the freeze-suspension solidification method according to claim 1, characterized in that: The freeze-drying conditions in step (3) are: vacuum degree of 0.3 to 0.5 mbar, temperature of -40 to -20°C, and time of 24 to 48 hours.

8. The method for controllably preparing high-quality porous geopolymer microspheres based on the freeze-suspension solidification method according to claim 1, characterized in that: The curing in step (3) is one of standard curing, steam curing or immersion curing; wherein, the temperature of standard curing is 15-25°C, the relative humidity is greater than 90%, and the curing time is 2-28 days; the temperature of steam curing is 60-80°C, and the curing time is 12-48 hours; and the immersion curing is to immerse the porous geopolymer microspheres in water, and the curing time is 2-28 days.

9. The method for controllably preparing high-quality porous geopolymer microspheres based on the freeze-suspension solidification method according to claim 1, characterized in that: The density of the geopolymer slurry is 1.0-1.6 g / cm 3 , viscosity is 45~90mPa·s, and suspension time is 7~15s.

10. A porous geopolymer microsphere, characterized in that: The porous geopolymer microspheres are obtained by the method for controllably preparing high-quality porous geopolymer microspheres based on the freezing-suspension solidification method as described in any one of claims 1 to 9.

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