A layered-pore-structure geopolymer based on multi-stage low-temperature in-situ polymerization combined with ice template method and a preparation method thereof

Abstract

This invention discloses a layered porous geopolymer based on a combination of multi-stage low-temperature in-situ polymerization and ice template method, and its preparation method. The preparation method includes: preparing a geopolymer slurry; unidirectionally freezing the slurry to form a frozen preform containing oriented ice crystal templates; before freeze-drying the frozen preform to remove the ice crystal templates, performing a specific multi-stage low-temperature in-situ polymerization treatment, which includes at least two different low-temperature holding stages with increasing temperatures within a temperature range of -10°C to +5°C; subsequently, freeze-drying and curing the treated preform. This invention solves the problems of low mechanical properties and unstable pore structure in geopolymers prepared by the traditional ice template method by introducing a multi-stage low-temperature in-situ polymerization step targeting the reactivity of the geopolymer before ice crystal template removal, significantly improving the mechanical strength and structural integrity of the product while maintaining a good layered porous structure and pore connectivity.

Description

Technical Field

[0001] This invention relates to the field of geopolymer preparation, and particularly to a layered porous geopolymer and its preparation method based on a combination of multi-stage low-temperature in-situ polymerization and ice template method. Background Technology

[0002] Geopolymers, inorganic polymer materials formed through the condensation reaction of silica-alumina precursors under the action of alkaline activators, are considered green alternatives to traditional cement-based materials due to their wide availability of raw materials (such as industrial solid waste slag and fly ash), low energy consumption and low carbon emissions in their preparation process, and excellent mechanical properties, durability, and fire resistance. They have broad application prospects in construction, environmental remediation, and other fields. By introducing porous structures, geopolymers can be endowed with specific functional properties, such as adsorption, filtration, and thermal insulation, thereby further expanding their application range. Ice-templating (or Freeze-casting) is an advanced porous material preparation technology that replicates an ordered pore network within the material by controlling the directional solidification of a solvent (usually water) to form an ice crystal template, followed by sublimation to remove the template. This technology can construct layered, tubular, or cellular pore structures with high anisotropy and good pore connectivity, providing an effective approach for the customized design of functional porous materials. However, applying the traditional ice template method directly to geopolymer systems presents the following key technical challenges:

[0003] (1) The contradiction between inhibited geopolymer polymerization and insufficient matrix strength under low-temperature conditions: The curing of geopolymers depends on their condensation reaction in an alkaline environment. In the typical low-temperature freezing process of the ice template method (e.g., -20°C or even lower), the condensation reaction kinetics of geopolymers are significantly slowed down, resulting in a porous geopolymer matrix with extremely poor mechanical strength due to its low degree of polymerization after the ice crystal template is formed and sublimated and removed. This low-strength matrix skeleton is prone to severe volume shrinkage, cracking, or even complete structural collapse due to capillary forces, external stresses, and other factors during subsequent processing, drying, or actual use, making it difficult to effectively maintain the fine pore structure of the ice template.

[0004] (2) Limitations of the applicability of the traditional "melting water curing" principle for geopolymers: Some studies (e.g., Adv. Sci. 2021, 8, 2000096; Chem. Eng. J. Adv. 2021, 7, 100114) have utilized the water generated from the melting of ice crystals at temperatures slightly above freezing point in the preparation of ice templates for hydraulic cementitious materials such as cement and gypsum to participate in the hydration reaction of the material in situ, achieving self-hardening without freeze-drying. Although this "melting water curing" approach provides a useful insight into solving the curing problem of ice template materials, it cannot be directly and simply applied to geopolymer systems. The fundamental reason is that the curing mechanism of geopolymers is a condensation reaction under alkali-induced conditions, which differs significantly from the hydration reaction of hydraulic materials in terms of chemical nature, reaction kinetics, optimal pH environment, actual water requirements, and product characteristics. For geopolymers, simply using melted water above the freezing point may have two drawbacks. First, the geopolymer's reactivity remains low at low temperatures, preventing complete polymerization and solidification before the ice crystal template is fully destroyed. Second, excessively rapid heating and melting rates may lead to premature and uneven destruction of the ice crystal template, affecting the final ordered pore structure. Therefore, developing a new method that can both utilize ice templates to construct ordered pore structures and ensure effective polymerization of geopolymers at low temperatures to obtain sufficient matrix strength and pore structure stability has become a pressing technical challenge in this field.

[0005] (3) Shortcomings and unresolved technical issues in existing research on geopolymer ice templates: Existing research (e.g., RSC Adv., 2016, 6, 24635; Microporous and Mesoporous Materials, 2015, 215, 206) mainly focuses on influencing the pore morphology of geopolymer ice templates by controlling the degree of maturation or water content of the initial slurry. Although this reveals the tunability of the pore structure to some extent, it has not yet proposed a systematic and effective solution to the core problem of low mechanical strength of the geopolymer matrix after freeze-drying while maintaining the ordered pore structure imparted by the ice template method. For example, Chinese patent application CN113582567A discloses a method for preparing directional porous slag-based geopolymers, which involves conventional room temperature or steam curing after freeze-drying to obtain a green preform. This post-processing method is a conventional curing method after freeze-drying. Although it can improve the final strength to a certain extent, it fails to make full use of the potential for in-situ strengthening to stabilize the pore structure while the ice crystal template is still inside the blank. Furthermore, the green blank has low strength before curing and is still prone to structural damage during handling or the early stages of curing.

[0006] Therefore, there is an urgent need in this field for a new geopolymer ice template preparation technology that can promote the polymerization and strengthening of the geopolymer matrix in situ and effectively, thereby obtaining porous geopolymers with excellent mechanical properties while ensuring the integrity and stability of their layered pore structure. Summary of the Invention

[0007] This invention aims to solve the core technical problems in existing geopolymer ice template technology, such as poor mechanical properties and difficulty in maintaining pore structure after freeze-drying due to the inhibition of geopolymer polymerization at low temperatures. It provides a method for preparing layered porous geopolymers based on a combination of multi-stage low-temperature in-situ polymerization and ice template method, as well as the geopolymer prepared by this method. The core technical concept of this invention lies in the innovative introduction of a specific, multi-stage low-temperature in-situ polymerization process after the traditional ice template method has formed a frozen preform containing ice crystal templates through unidirectional freezing, and before the final sublimation freeze-drying to remove the ice crystal templates from the frozen preform. This step, through precise control of at least two different low-temperature ranges with an increasing temperature trend (e.g., the first stage from -10°C to 0°C (excluding 0°C), and the second stage from 0°C to +5°C) and their corresponding holding times, allows some of the ice crystals in the frozen preform to melt in a controlled and phased manner to form liquid water. These in-situ generated liquid waters, slowly released at low temperatures, can more uniformly penetrate into the gaps between geopolymer precursor particles and, under the action of the existing alkaline activator, undergo in-situ polycondensation with unreacted or partially reacted silica-alumina reactive raw materials. This in-situ polymerization, designed specifically for the reactivity characteristics of geopolymers, carried out in stages under specific low-temperature conditions, and utilizing controlled melting water from ice crystals, is the key innovation of this invention, distinguishing it from existing technologies. Its purpose and advantages are reflected in:

[0008] (1) Low-temperature controlled polymerization to strengthen the matrix and stabilize the pore structure: Unlike existing technologies where hydraulic materials are rapidly hydrated and self-hardened using melted water above 0°C, this invention employs a specific low-temperature range (partially below freezing point) and staged processing to slow down the polycondensation reaction rate of the geopolymer and the melting rate of ice crystals. This allows liquid water more time to contact the raw material particles and participate in the reaction, thus enabling preliminary solidification and significant strengthening of the geopolymer matrix skeleton while maintaining the integrity of the main structure of the ice crystal template before the ice crystal template is completely removed.

[0009] (2) Synergistic effect and fine control of sequential processing: The multi-stage processing of this invention (e.g., preliminary polymerization at a lower temperature, such as -10°C to 0°C, to stabilize the pore walls, followed by more thorough polymerization at a slightly higher temperature, such as 0°C to +5°C, to deeply strengthen the matrix) achieves finer control over the in-situ polymerization process. This sequential, temperature-increasing processing method can more effectively utilize the melting behavior of ice crystals and the reactivity of geopolymers at different temperatures, thereby achieving a synergistic strengthening effect that is difficult to achieve with a single temperature range processing, which has not been revealed in the prior art.

[0010] (3) Compatibility and optimization with subsequent freeze-drying: The low-temperature in-situ polymerization of the present invention is a key pretreatment step before freeze-drying. The geopolymer matrix strengthened by this step has higher mechanical strength and can better resist the capillary shrinkage stress that may be generated by ice crystal sublimation during subsequent freeze-drying, thereby effectively avoiding the collapse and cracking of the pore structure and ensuring that a layered pore structure geopolymer with good pore connectivity, structural stability and excellent mechanical properties is finally obtained.

[0011] The objective of this invention is achieved through the following technical solution:

[0012] This invention provides a method for preparing a layered porous geopolymer based on a combination of multi-stage low-temperature in-situ polymerization and ice template method, comprising the following steps:

[0013] (1) Preparation of geopolymer slurry;

[0014] (2) Forming a frozen blank containing an ice crystal template: The geopolymer slurry obtained in step (1) is injected into a mold and the geopolymer slurry is unidirectionally frozen for 4-8 hours, so that the water in the slurry forms an ice crystal template that is oriented along the freezing direction, thereby obtaining the frozen blank containing the ice crystal template.

[0015] (3) Perform multi-stage low-temperature in-situ polymerization and subsequent freeze-drying: The frozen preform containing ice crystal templates obtained in step (2) is subjected to a first low-temperature in-situ polymerization stage and a second low-temperature in-situ polymerization stage in sequence:

[0016] First low-temperature in-situ polymerization stage: The frozen preform is placed in the temperature range T1 and maintained for at least 6 hours, so that some of the ice crystals in the frozen preform melt in a controlled manner to form liquid water, and initiates a preliminary in-situ polycondensation reaction at the temperature T1; wherein -10℃≤T1<0℃;

[0017] Second low-temperature in-situ polymerization stage: The preform treated in the first low-temperature in-situ polymerization stage is raised to a temperature range of T2 and held for at least 6 hours, ensuring that T2 is higher than T1. This allows the liquid water formed by the further melting of ice crystals to undergo a more complete in-situ polycondensation reaction with the unreacted or partially reacted silica-alumina active raw materials in the geopolymer slurry under the action of the original composite activator in the geopolymer slurry. After freeze-drying, a pre-cured geopolymer with a layered pore structure is obtained; wherein 0℃≤T2≤5℃;

[0018] (4) Curing the pre-cured geopolymer: Curing the pre-cured geopolymer obtained in step (3) under preset conditions to obtain the layered porous structure geopolymer.

[0019] Preferably, step (3) of placing the frozen preform within the temperature range T1 and maintaining it for at least 6 hours specifically involves:

[0020] The frozen preform is kept in an environment of -10°C to -5°C for at least 6 hours, and then kept in an environment of T1' for at least 6 hours; -5°C ≤ T1' < 0°C; to promote a more stable and extensive initial in-situ polycondensation reaction, while avoiding premature and excessive melting of the ice crystal template.

[0021] Preferably, step (3) involves raising the preform treated in the first low-temperature in-situ polymerization stage to a temperature range of T2 and maintaining it for at least 6 hours. Specifically, the preform treated in the first low-temperature in-situ polymerization stage is raised to 0°C to 3°C and maintained for at least 6 hours, and then placed in an environment of T2' and maintained for at least 6 hours; 3°C < T2' ≤ 5°C; to utilize the more abundant liquid water provided by the more significant melting of ice crystals in this temperature range, and combined with the relatively increased reactivity of the geopolymer at this temperature, to achieve deep strengthening and densification of the matrix skeleton.

[0022] Preferably, the geopolymer slurry is prepared as follows: the silica-alumina active raw material, water (including the water contained in the composite activator), and the composite activator are uniformly mixed to obtain a geopolymer slurry with a viscosity controlled between 50-200 mPa·s; wherein, the mass ratio of water to silica-alumina active raw material is 0.8-1.2; and the mass of the alkali metal oxide contained in the composite activator is 4%-8% of the mass of the silica-alumina active raw material.

[0023] Preferably, the freeze-drying process in step (3) is specifically carried out for 6-48 hours under conditions where the vacuum degree is not higher than 10 Pa and the cold trap temperature is not higher than -30°C.

[0024] Preferably, the silica-alumina active raw material in step (1) is selected from at least one of slag, metakaolin, fly ash, coal gangue or tailings containing active silica-alumina components.

[0025] Preferably, the composite activator in step (1) is prepared from a mixture of water glass and sodium hydroxide or potassium hydroxide, and the final modulus of the solution formed by the mixture, i.e. the molar ratio of silicon oxide (SiO2) to alkali metal oxide (Na2O or K2O), is controlled in the range of 1.2 to 1.8 to optimize the reaction kinetics and product properties of the geopolymer.

[0026] Preferably, step (2) involves injecting the geopolymer slurry obtained in step (1) into a mold and subjecting the geopolymer slurry to unidirectional freezing for 4-8 hours. Specifically, the geopolymer slurry obtained in step (1) is injected into a mold, the bottom of which is provided with a high thermal conductivity metal sheet. The high thermal conductivity metal sheet is in contact with a single low-temperature cold source, and the geopolymer slurry is subject to unidirectional freezing for 4-8 hours at a freezing temperature of -150°C to -25°C.

[0027] The sidewall of the mold is made of a polymer material with a thermal conductivity of not more than 0.5 W / (m·K) and a thickness of 8-15 mm; the thermal conductivity of the high thermal conductivity metal sheet is not less than 100 W / (m·K) and a thickness of 0.5-1.5 mm.

[0028] Furthermore, the high thermal conductivity metal sheet is preferably a copper or aluminum sheet with a thermal conductivity of not less than 100 W / (m·K) and a thickness controlled between 0.5-1.5 mm to ensure efficient heat transfer; the sidewall of the mold is made of a polymer material (such as polytetrafluoroethylene, polypropylene, etc.) with a thermal conductivity of not more than 0.5 W / (m·K) and a sidewall thickness designed to be 8-15 mm to minimize radial heat loss and enhance the axial temperature gradient during unidirectional freezing.

[0029] Preferably, the curing in step (4) is standard curing or steam curing; the conditions for standard curing are a temperature of 20±2℃, a relative humidity of not less than 95%, and a curing time of 3-28 days; the conditions for steam curing are a temperature of 50-80℃, a relative humidity of greater than 95%, and a curing time of 12-48 hours; to ensure that the subsequent hydration or polymerization reaction of the pre-cured polymer can proceed fully, thereby obtaining the final stable mechanical properties and microstructure.

[0030] Preferably, the uniform mixing process of the geopolymer slurry in step (1) includes: first, loading the silica-alumina active raw material and the required total water volume (excluding the water introduced by the subsequent activator) into a mixing device such as a planetary ball mill or a horizontal ball mill, and performing thorough premixing and dispersion at a speed of 180-220 rpm for 60-240 minutes; then adding the pre-prepared composite activator solution, and continuing to mix at the same speed for 15-25 minutes to ensure uniform contact and initial reaction between the activator and the raw material.

[0031] Preferably, the single low-temperature cold source in step (2) is selected from any one of the following: low-temperature cold stage, low-temperature incubator, refrigerator, dry ice, liquid nitrogen, or low-temperature bath prepared by mixing dry ice with organic solvents such as ethanol; to meet the needs of different experimental conditions and freezing rates.

[0032] The present invention also provides a layered porous structure geopolymer, which is prepared by the preparation method of the layered porous structure geopolymer based on multi-stage low-temperature in-situ polymerization and ice template method; the layered porous structure geopolymer has a layered pore structure, a porosity of 45%-65%, a most probable pore size in the range of 10-100 μm, and a compressive strength parallel to the unidirectional freezing direction of not less than 4 MPa.

[0033] The layered porous geopolymer of the present invention, based on the combination of multi-stage low-temperature in-situ polymerization and ice template method, has the following characteristics:

[0034] a) The compressive strength of its pore wall matrix, which has undergone sequential, temperature-increasing low-temperature in-situ polymerization treatment in at least two different temperature ranges as described in step (3) of the preparation method, is increased by at least 200% compared with the corresponding compressive strength of the same component polymer that has not undergone the multi-stage low-temperature in-situ polymerization treatment but has been directly freeze-dried and cured in the same way.

[0035] b) When observed under a scanning electron microscope, the pore walls exhibit a microstructure formed by undergoing at least two different temperature ranges of low-temperature in-situ polymerization treatment in step (3) of the preparation method. This microstructure is more compact than the corresponding microstructure of the same component geopolymer that is directly freeze-dried without the multi-stage low-temperature in-situ polymerization treatment, and the geopolymer gel phase is more continuous and the interparticles are more tightly bound.

[0036] Compared with the prior art, the present invention has the following advantages and beneficial effects:

[0037] (1) Significantly improves mechanical properties and effectively maintains the stability of the pore structure.

[0038] By introducing a core multi-stage low-temperature in-situ polymerization step, this invention effectively utilizes the liquid water generated by the controlled and gradual melting of ice crystals at specific low temperatures, promoting the full polymerization and strengthening of the geopolymer matrix before freeze-drying. Compared to samples that are directly freeze-dried without low-temperature in-situ polymerization treatment, the porous geopolymer prepared using the method of this invention exhibits a compressive strength increase of over 200% along the unidirectional freezing direction. More importantly, compared to simple single-stage low-temperature in-situ polymerization treatment, the multi-stage treatment of this invention (e.g., preliminary polymerization at -10°C to 0°C followed by more complete polymerization at 0°C to +5°C) results in superior mechanical property enhancement and pore structure retention. Specifically, the first low-temperature in-situ polymerization stage (e.g., -10°C to 0°C) helps to initiate a preliminary condensation reaction without excessively damaging the ice crystal template, stabilizing the pore wall structure and laying the foundation for subsequent more complete polymerization; the second low-temperature in-situ polymerization stage (e.g., 0°C to +5°C) utilizes more melted water and slightly higher reactivity to achieve deep strengthening of the matrix. This division of labor and synergy allows the samples prepared by this invention to further improve their compressive strength by approximately 25-70% compared to samples treated with single-stage low-temperature in-situ polymerization. Simultaneously, because the matrix is ​​effectively strengthened before freeze-drying, it can better resist the capillary shrinkage force that may occur during sublimation, thus effectively avoiding the pore structure collapse or severe cracking problems that may result from traditional direct freeze-drying or insufficient in-situ polymerization, resulting in layered pores with good channel connectivity and structural stability.

[0039] (2) The process is highly targeted, achieving a balance between low-temperature curing and pore structure preservation.

[0040] The multi-stage low-temperature in-situ polymerization process proposed in this invention is innovatively designed specifically for the unique reaction characteristics of geopolymers (e.g., their curing mechanism is a condensation reaction rather than a hydration reaction; the condensation reaction rate is usually slow at low temperatures; and the reaction process is highly sensitive to pH value, ion concentration, and the dynamic balance and supply rate of water) and the core requirements of the ice template method (i.e., the matrix must be effectively strengthened before removing the ice crystal template to maintain the fine porous structure imparted by the ice crystal template). This is fundamentally different from the principle and process conditions of traditional hydraulic materials such as cement / gypsum, which simply utilize melted water for rapid hydration and self-hardening above 0°C. For geopolymers, simple rapid heating and the instantaneous appearance of a large amount of liquid water (e.g., placing a frozen preform directly in an environment above the freezing point to rapidly melt it) are often unsuitable for their effective in-situ curing. This is not only because the reactivity of geopolymers is still limited at low temperatures, resulting in insufficient polymerization before the ice crystal template is completely melted and destroyed; more importantly, the condensation reaction of geopolymers is more sensitive to environmental conditions, and a rapid, uncontrolled supply of melted water may lead to the following adverse effects:

[0041] 1) Uneven reaction products or stress concentration: An excessively fast reaction rate may cause the polymer gel phase in local areas to form and shrink rapidly, while the reaction in other areas is delayed, resulting in unevenness of the overall structure and internal stress.

[0042] 2) Dilution of alkaline activator concentration: The instantaneous influx of a large amount of liquid water may excessively dilute the concentration of alkaline activator in the pores, thereby lowering the pH value. This will significantly inhibit or slow down the polycondensation reaction process of the geopolymer and affect the final degree of polymerization.

[0043] 3) Disruption of the initial gel structure: If a partial initial and relatively fragile geopolymer gel network has already formed at a lower temperature, the sudden appearance of a large amount of liquid water may cause these initial gel structures to swell, dissolve, or even be destroyed.

[0044] 4) Premature destruction of ice crystal template: An excessively fast melting rate may cause the ice crystal template to disappear prematurely and unevenly before the matrix gains sufficient strength, thus making it impossible to effectively replicate and maintain the pore structure.

[0045] Therefore, this invention, through a precisely controlled, sequential, temperature-increasing cryogenic holding process, and the resulting gradual, controlled melting of ice crystals and supply of liquid water, can better adapt to the relatively slow and complex polycondensation reaction process of geopolymers. Specifically: In the first cryogenic stage (e.g., between -10°C and 0°C, excluding 0°C), a preliminary and mild polycondensation reaction is initiated by controlling a lower temperature and a limited ice crystal melting rate. The main purpose of this stage is to initially stabilize and network the pore wall structure without excessively damaging the integrity of the ice crystal template, laying a solid foundation for subsequent deep strengthening, and minimizing the overall melting rate of ice crystals, thus avoiding the problem of premature destruction of the ice crystal template that may be caused by rapid and large-scale melting of ice crystals. In the subsequent second cryogenic stage (e.g., between 0°C and +5°C), by appropriately increasing the temperature and utilizing more gradually melting liquid water, combined with the relatively higher reactivity of geopolymers at this temperature, a more complete and deeper polycondensation reaction is carried out, thereby achieving deep strengthening and densification of the matrix framework. This phased, gradient heating strategy fully considers the delicate balance between the low-temperature reaction kinetics of geopolymers and the stability of the ice crystal template, a feature not found in existing technologies that simply utilize melted water or single-stage treatment. It not only demonstrates a profound understanding of the reaction characteristics of geopolymers and targeted process optimization, but more importantly, it successfully achieves an organic unity between the effective solidification of the geopolymer matrix under low-temperature conditions and the precise preservation of the resulting fine layered porous structure.

[0046] (3) Improve the microstructure of the pore wall, enhance the connectivity of the pores and reduce blockage.

[0047] This invention significantly improves the pore wall microstructure and pore characteristics of the prepared geopolymer by carrying out fully controlled multi-stage in-situ polymerization at low temperature.

[0048] Optimization of pore wall microstructure: Compared with traditional direct freeze-drying methods (whose products may have loose pore walls, a large number of unreacted particles, and microcracks due to incomplete reaction) and simple single-stage in-situ hydration methods (whose products may have local structural defects due to uneven moisture distribution or uneven migration of reaction products), the geopolymer prepared in this invention exhibits a more dense and continuous microstructure in its pore walls when observed under a scanning electron microscope. Specifically, the geopolymer gel phase can more uniformly and fully coat and bond the aluminosilicate raw material particles, effectively filling the original gaps between particles, resulting in a tighter bond between particles and a significant reduction in the loose accumulation of unreacted particles and interfacial microcracks. This densification and homogenization of the pore wall microstructure is not only an important reason for the significant improvement in the macroscopic mechanical properties of the material, but also lays the foundation for improving the regularity of the pore surface and enhancing potential filtration or catalytic performance.

[0049] Improved pore connectivity and reduced blockage: Multi-stage low-temperature in-situ polymerization helps solutes and early reaction products to be more uniformly distributed and firmly fixed in the forming pore wall framework during polymerization, effectively reducing pore blockage caused by the migration and precipitation of substances (such as unreacted basic activators or oligomers) during subsequent freeze-drying and sublimation. Therefore, the porous material prepared by the method of this invention has better effective pore connectivity. Mercury intrusion porosimetry pore size analysis results also confirm that its pore volume distribution is more ideal, and the pores in the target pore size range are more regular. This improved pore connectivity and reduced blockage are of great significance for the application of materials in filtration separation, fluid transport, and other fields.

[0050] (4) Energy efficiency, wide applicability and application potential brought about by process optimization

[0051] The multi-stage low-temperature in-situ polymerization method proposed in this invention not only achieves significant improvements in material properties, but also demonstrates many advantages in terms of process.

[0052] Energy saving and improved process efficiency: The core polymerization strengthening process of this method is mainly carried out under low-temperature conditions (e.g., -10℃ to +5℃). Compared with the high-temperature sintering or high-temperature curing required for traditional ceramic materials or some polymer materials, this significantly reduces energy consumption, demonstrating energy-saving characteristics. Simultaneously, by achieving effective in-situ strengthening of the matrix before freeze-drying, the complexity of subsequent processing is simplified. For example, the increased matrix strength may shorten the time required for subsequent curing (if traditional curing methods are used), or reduce waste generated during handling and processing due to insufficient preform strength, thereby improving overall process efficiency and material utilization.

[0053] Excellent process compatibility and wide applicability of raw materials: The multi-stage low-temperature in-situ polymerization step of this invention can be used as a key pretreatment module inserted between freeze molding and freeze drying in the existing ice template process, exhibiting excellent compatibility with existing ice template equipment and processes. More importantly, this method is applicable to a variety of common geopolymer active raw materials (such as slag, metakaolin, fly ash, coal gangue, or tailings containing active silica and aluminum components), opening up new avenues for the high-value utilization of industrial solid waste from different sources and demonstrating its wide applicability.

[0054] Expanding the application potential of high-performance geopolymers: The porous geopolymers prepared by the method of this invention, possessing excellent mechanical properties and a regular layered pore structure, can meet more stringent application requirements. For example, they can be used directly as lightweight, high-strength, structurally functional integrated materials, or exhibit superior performance in fields such as high-efficiency filtration, catalyst support, and energy storage. Therefore, this invention provides a new, reliable, and more economical technical approach for the customized preparation and functional application of high-performance geopolymer materials with specific pore structures. Attached Figure Description

[0055] Figure 1 This is a process flow diagram of an embodiment of the present invention.

[0056] Figure 2 The image shows a scanning electron microscope (SEM) image of the porous geopolymer prepared in Comparative Example 1 of this invention; wherein (b) is a magnified view of (a).

[0057] Figure 3 The image shows a SEM image of the porous geopolymer prepared in Comparative Example 2 of this invention; (b) is a magnified view of (a).

[0058] Figure 4 The image shows a SEM image of the porous geopolymer prepared in Example 1 of this invention; (b) is a magnified view of (a).

[0059] Figure 5 The image shows a computed tomography (CT) three-dimensional reconstruction of the porous geopolymer prepared in Example 1 of the present invention; (b) in the image is a cross-sectional view of (a).

[0060] Figure 6 The image shows a SEM image of the porous geopolymer prepared in Example 2 of this invention; (b) is a magnified view of (a).

[0061] Figure 7 The image shows a SEM image of the porous geopolymer prepared in Example 3 of this invention; (b) is a magnified view of (a).

[0062] Figure 8 The image shows a SEM image of the porous geopolymer prepared in Example 4 of this invention; (b) is a magnified view of (a).

[0063] Figure 9 This is a comparison diagram of the pore size distribution of the porous geopolymers prepared in Comparative Examples 1-2 and Examples 1-4 of the present invention. Detailed Implementation

[0064] To make the objectives, technical solutions, and advantages of this invention clearer, the 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 illustrative of the invention and are not intended to limit the scope of protection of the invention. Any modifications, equivalent substitutions, improvements, etc., made within the spirit and principles of this invention should be included within the scope of protection of this invention.

[0065] Comparative Example 1: Preparation of porous geopolymers by conventional direct freeze-drying method

[0066] This comparative example is intended to illustrate the preparation of porous geopolymers using a conventional direct freeze-drying method, serving as a basis for comparison with the method of the present invention.

[0067] (1) Preparation of geopolymer slurry: Weigh 100 parts (by weight, the same below) of S95 grade slag powder and add 90 parts of water. Add the slag powder and water to the ball mill jar of a planetary ball mill and mix at 196 rpm for 220 minutes. Then, add a pre-prepared composite alkali activator, which is prepared from industrial-grade liquid water glass (containing 8.53% Na2O, 26.98% SiO2, and a modulus of 3.26) and analytical grade sodium hydroxide particles, so that the mass of Na2O in the final activator is 4% of the mass of the slag powder, and the modulus of the prepared solution (the molar ratio of SiO2 to Na2O) is adjusted to 1.5. After adding the composite alkali activator, continue mixing at the above speed for 20 minutes to obtain a geopolymer slurry with a viscosity controlled in the range of 50-200 mPa·s.

[0068] (2) Unidirectional freezing to form an ice crystal template: The geopolymer slurry prepared in step (1) is injected into a polytetrafluoroethylene mold with an inner cavity size of 30mm×30mm×100mm and a side wall thickness of 8mm. A 1mm thick brass sheet (thermal conductivity of approximately 386W / (m·K)) is pre-fixed at the bottom of the mold. The mold containing the slurry is placed in an insulated container with dry ice (as a single low-temperature cold source) at the bottom, ensuring good contact between the brass sheet at the bottom of the mold and the dry ice through a perforated metal partition, and unidirectional freezing is performed for 8 hours. During this process, the temperature of the brass sheet at the contact end with the dry ice (i.e., the cold end temperature) is controlled at approximately -45±2℃, thereby forming an ice crystal template oriented in the slurry along the freezing direction (perpendicular to the direction of the brass sheet), and obtaining a frozen blank containing the ice crystal template.

[0069] (3) Freeze-drying: The frozen preform obtained in step (2) is directly transferred to a freeze dryer without any in-situ polymerization treatment. It is freeze-dried for 48 hours under the conditions that the cold trap temperature is not higher than -30°C and the vacuum degree is not higher than 10Pa. Ice crystals and moisture are removed by sublimation to obtain the raw preform.

[0070] (4) Curing: The green body obtained in step (3) is cured for 7 days under standard conditions of temperature of 20±2℃ and relative humidity of not less than 95% to obtain a porous polymer sample.

[0071] Comparative Example 2: Freeze-drying method after single-stage low-temperature in-situ polymerization

[0072] This comparative example aims to illustrate the effect of a simple single-stage low-temperature in-situ polymerization treatment on the properties of porous geopolymers, serving as a reference for comparison with the multi-stage treatment of this invention.

[0073] (1) Preparation of geopolymer slurry: Same as step (1) in Comparative Example 1.

[0074] (2) Unidirectional freezing to form an ice crystal template: Same as step (2) in Comparative Example 1.

[0075] (3) Single-stage low-temperature in-situ polymerization and subsequent freeze-drying treatment: The frozen green body obtained in step (2) is placed directly in an environment of +5°C for 24 hours to utilize the water generated by the melting of ice crystals for in-situ polymerization reaction; then, the green body that has undergone the above single-stage low-temperature in-situ polymerization treatment is transferred to a freeze dryer and freeze-dried for 24 hours under the conditions that the cold trap temperature is not higher than -30°C and the vacuum degree is not higher than 10Pa. The remaining ice crystals and water are removed by sublimation to obtain the green body.

[0076] (4) Curing: The green body obtained in step (3) is cured for 7 days under standard conditions of temperature 20±2℃ and relative humidity not less than 95% to obtain a porous polymer sample.

[0077] Example 1: The method of the present invention (employing multi-stage low-temperature in-situ polymerization with a shorter time)

[0078] (1) Preparation of geopolymer slurry: Same as step (1) in Comparative Example 1.

[0079] (2) Unidirectional freezing to form an ice crystal template: Same as step (2) in Comparative Example 1.

[0080] (3) Multi-stage low-temperature in-situ polymerization and subsequent freeze-drying composite treatment: The frozen preform containing ice crystal template obtained in step (2) is subjected to the following sequential, two different low-temperature in-situ polymerization treatment stages with increasing temperatures:

[0081] First low-temperature in-situ polymerization stage: The frozen preform is placed in a temperature range of -5°C and maintained for 12 hours;

[0082] Second low-temperature in-situ polymerization stage: Subsequently, the preform treated in the first low-temperature in-situ polymerization stage is heated to a temperature range of +5°C and held for 12 hours.

[0083] Freeze-drying: Then, the preform that has undergone the above-mentioned multi-stage low-temperature in-situ polymerization treatment is freeze-dried for 24 hours under the conditions of cold trap temperature not higher than -30℃ and vacuum degree not higher than 10Pa to obtain pre-cured polymer.

[0084] (4) Curing: The green body obtained in step (3) is cured for 7 days under standard conditions of temperature 20±2℃ and relative humidity not less than 95% to obtain a porous polymer sample.

[0085] The process flow diagram of this embodiment is as follows: Figure 1 As shown.

[0086] Example 2: The method of the present invention (employing multi-stage low-temperature in-situ polymerization over a relatively long period of time)

[0087] (1) Preparation of geopolymer slurry: Same as step (1) in Comparative Example 1.

[0088] (2) Unidirectional freezing to form an ice crystal template: Same as step (2) in Comparative Example 1.

[0089] (3) Multi-stage low-temperature in-situ polymerization and subsequent freeze-drying composite treatment: The frozen preform containing ice crystal template obtained in step (2) is subjected to the following sequential, two different low-temperature in-situ polymerization treatment stages with increasing temperatures:

[0090] First low-temperature in-situ polymerization stage: First, the frozen preform is placed in an environment of -10°C for 12 hours, and then transferred to an environment of -5°C for another 12 hours;

[0091] Second low-temperature in-situ polymerization stage: Subsequently, the preform treated in the first low-temperature in-situ polymerization stage is heated to 0°C and kept in an environment for 12 hours, and then heated to +5°C and kept in an environment for 18 hours.

[0092] Freeze-drying: Then, the preform that has undergone the above-mentioned multi-stage low-temperature in-situ polymerization treatment is freeze-dried for 6 hours under the conditions of cold trap temperature not higher than -30℃ and vacuum degree not higher than 10Pa to obtain pre-cured polymer.

[0093] (4) Curing: The green body obtained in step (3) is cured for 7 days under standard conditions of temperature 20±2℃ and relative humidity not less than 95% to obtain a porous polymer sample.

[0094] Example 3: The method of the present invention (different water-to-solid ratios and freezing temperatures)

[0095] (1) Slurry preparation: Weigh 100 parts of S95 grade slag powder and add 100 parts of water (water-to-solid ratio is 1.0). The remaining components and mixing method are the same as step (1) in Example 1.

[0096] (2) Unidirectional freezing to form an ice crystal template: The slurry prepared in step (1) is injected into the same mold as in Example 1. The bottom brass sheet of the mold containing the slurry is brought into controlled contact with a liquid nitrogen (as a single low-temperature cold source) bath, so that the cold end temperature is controlled at about -150±5℃, and unidirectional freezing is performed for 4 hours to obtain a frozen blank containing an ice crystal template.

[0097] (3) Multi-stage low-temperature in-situ polymerization and subsequent freeze-drying composite treatment: The same multi-stage low-temperature in-situ polymerization procedure and freeze-drying procedure as step (3) in Example 1 were used to obtain the pre-cured polymer.

[0098] (4) Curing: The green body obtained in step (3) is cured for 7 days under standard conditions of temperature 20±2℃ and relative humidity not less than 95% to obtain porous polymer.

[0099] Example 4: The method of the present invention (different active raw materials and maintenance methods)

[0100] (1) Slurry preparation: Weigh 70 parts of fly ash and 30 parts of S95 grade slag powder as composite silica-alumina active raw materials, and add a total of 80 parts of water (water-to-solid ratio of 0.8). Add fly ash, slag and water to a ball mill jar and mix for 220 minutes at 196 rpm on a horizontal ball mill. Then add a composite alkali activator prepared from liquid water glass and analytical grade potassium hydroxide granules, so that the mass of K2O in the activator is 8% of the total mass of the active raw materials, and adjust the solution modulus to 1.8 after preparation. Continue mixing for 20 minutes to obtain geopolymer slurry, and control its viscosity between 50-200 mPa·s.

[0101] (2) Unidirectional freezing to form ice crystal template: Same as step (2) in Example 1, but the cold end temperature is controlled at about -25±2℃ by a low temperature incubator.

[0102] (3) Multi-stage low-temperature in-situ polymerization and subsequent freeze-drying composite treatment: The same multi-stage low-temperature in-situ polymerization procedure and freeze-drying procedure as step (3) in Example 2 were used to obtain the pre-cured polymer.

[0103] (4) Curing: The pre-cured geopolymer obtained in step (3) is placed in a steam curing chamber and cured for 24 hours at a temperature of 60°C and a relative humidity of more than 95% to obtain a porous geopolymer sample.

[0104] Performance testing and results analysis:

[0105] To systematically evaluate the effectiveness of the proposed preparation method based on multi-stage low-temperature in-situ polymerization combined with the ice template method, and to elucidate the performance advantages of the prepared geopolymer, detailed mechanical property tests, pore structure characterization, and microstructure observations were performed on the porous geopolymer samples prepared by Comparative Example 1, Comparative Example 2, and Examples 1 to 4. The main performance data are summarized in Table 1, and typical microstructures, computed tomography (CT) images, and pore size distribution characteristics are shown in Table 1. Figures 2 to 9 .

[0106] Table 1. Comparison of properties of porous geopolymers prepared by different methods

[0107]

[0108] (1) Significant improvement in mechanical properties

[0109] As shown in Table 1, the compressive strength data clearly demonstrate that the multi-stage low-temperature in-situ polymerization treatment proposed in this invention has a significant effect on improving the mechanical properties of porous geopolymers.

[0110] Comparative Example 1, using a traditional direct freeze-drying method without any in-situ polymerization treatment, exhibited the lowest z-axis compressive strength (1.63 MPa) and a porosity of 57% in its final sample (water-to-solid ratio 0.9, frozen at -45°C). This was primarily due to the severe inhibition of geopolymer condensation during the low-temperature freezing process, resulting in a low degree of polymerization, a loose structure, and poor mechanical properties in the matrix framework formed after the ice crystal template was removed by sublimation. Comparative Example 2 introduced a single-stage +5°C, 18-hour low-temperature in-situ polymerization treatment after freezing the preform, utilizing the moisture generated by the melting of ice crystals to promote the polymerization reaction of the geopolymer. The z-axis compressive strength of its sample increased to 4.05 MPa, and the porosity reached 55%, representing an increase of approximately 148% compared to Comparative Example 1. This preliminarily indicates that in-situ polymerization treatment before removing the ice crystal template is effective in improving the mechanical properties of materials, but there is still considerable room for improvement.

[0111] In contrast, the embodiments of the present invention exhibit mechanical properties far exceeding those of the comparative examples described above. Example 1 (water-to-solid ratio 0.9, -45°C freezing, using multi-stage low-temperature in-situ polymerization treatment at -5°C for 12 hours and +5°C for 12 hours) achieved a z-axis compressive strength of 6.50 MPa and a porosity of 58%. Example 2 (water-to-solid ratio 0.9, -45°C freezing, using a more complex multi-stage low-temperature in-situ polymerization treatment at -10°C for 12 hours; -5°C for 12 hours; 0°C for 12 hours; +5°C for 18 hours) further improved its z-axis compressive strength to 6.92 MPa and its porosity to 57%. This means that, compared to Comparative Example 1 (conventional direct freeze-drying), the compressive strengths of Examples 1 and 2 of the present invention are significantly increased by 299% and 325%, respectively. More importantly, compared to Comparative Example 2 (single-stage low-temperature in-situ polymerization), the compressive strengths of Examples 1 and 2 of the present invention are also further increased by approximately 60% and 71%, respectively.

[0112] Furthermore, Example 3 (water-to-solid ratio 1.0, -150℃ cryogenic freezing, using the same multi-stage polymerization process as Example 1) achieved a compressive strength of 9.01 MPa and a porosity of 54%. Example 4 (water-to-solid ratio 0.8, -25℃ freezing, using fly ash / slag composite raw materials, and undergoing the same multi-stage polymerization process and subsequent steam curing as Example 2) also achieved a compressive strength of 4.77 MPa and a porosity of 54%.

[0113] These data fully demonstrate that the proposed "multi-stage, sequential, temperature-increasing" low-temperature in-situ polymerization process, compared to traditional direct freeze-drying methods and simple single-stage in-situ polymerization methods, can more effectively promote the polymerization and strengthening of the geopolymer matrix, thereby significantly improving the mechanical properties of the final product. This is attributed to the fact that the multi-stage treatment can more precisely control the melting rate of ice crystals and the reaction kinetics of geopolymers in different low-temperature ranges, achieving a synergistic optimization effect of maximizing the degree of polymerization and matrix density while maintaining the stability of the ice crystal template.

[0114] (2) Pore structure characteristics and pore connectivity

[0115] like Figure 9 As shown in the pore size distribution comparison curve and the most probable pore size data in Table 1, the method of the present invention can effectively control and optimize the pore structure of materials while improving mechanical properties.

[0116] from Figure 9 The differences in pore size distribution among the samples are clearly visible: Comparative Example 1 (orange curve, marked with a square) has a main peak at 45.25 μm, with a distinct secondary peak at approximately 2.50 μm, indicating a wide pore size distribution with multiple pore sizes and poor pore uniformity. Comparative Example 2 (green curve, marked with a circle) has a main peak at 36.19 μm. Compared to Comparative Example 1, the main pore size is smaller, and the secondary peak at 2.50 μm is relatively weaker, indicating that single-stage in-situ polymerization has a certain improvement effect on the pore structure, but the pore size distribution is still relatively wide. Example 1 (dark blue curve, marked with an upper triangle) has a main peak at 25.86 μm. Compared to Comparative Examples 1 and 2, its main peak is sharper and more symmetrical, indicating a more concentrated and uniform pore size distribution, and the secondary peak at 2.50 μm almost disappears. Example 3 (light blue curve, diamond mark) has a main peak in its pore size distribution at 15.42 μm, which is the smallest probable pore size among all samples. Its main peak is also very sharp, indicating a highly uniform pore size distribution. Example 4 (purple curve, lower triangle mark) has a main peak in its pore size distribution at 45.24 μm, very close to the main peak position of Comparative Example 1. However, it is noteworthy that the intensity of the secondary peak at 2.50 μm in Example 4 is much lower than that in Comparative Example 1, indicating that its macropore uniformity is superior to that of Comparative Example 1.

[0117] Based on the above observations, the following analysis can be drawn:

[0118] 1) Multi-stage low-temperature in-situ polymerization significantly optimizes the uniformity of pore size distribution: Comparison of Comparative Examples 1 and 2 with Examples 1 and 3 of the present invention shows that the multi-stage low-temperature in-situ polymerization treatment of the present invention can significantly make the pore size distribution more concentrated and uniform (manifested as sharper and more symmetrical main peaks, and weakened or disappeared secondary peaks). This improvement in the concentration and uniformity of pore size distribution indicates that the multi-stage low-temperature in-situ polymerization treatment of the present invention can promote the formation of more regular ice crystal templates and more faithfully replicate the structure of the ice crystal templates. Specifically, in the multi-stage low-temperature in-situ polymerization process, the geopolymer matrix is ​​significantly strengthened before the ice crystal template is sublimated and removed. This pre-strengthened matrix skeleton can more effectively resist phenomena such as pore wall deformation, pore merging, or local collapse that may occur during the subsequent freeze-drying sublimation process due to capillary forces or loose structure. Thus, it more accurately replicates the pore morphology defined by the ice crystal template, and finally forms a layered pore network with more consistent pore size and more concentrated distribution.

[0119] 2) Effect of freezing temperature on pore size: Comparing Example 1 (frozen at -45℃, most probable pore size 25.86 μm) and Example 3 (frozen at -150℃, most probable pore size 15.42 μm), under the same multi-stage in-situ polymerization procedure, the lower freezing temperature (-150℃) resulted in a smaller most probable pore size and a similarly concentrated distribution. This is consistent with the theory of ice crystal nucleation and growth: low temperature promotes the nucleation of numerous ice crystals and inhibits their growth, thereby forming a finer ice crystal template, ultimately replicating a porous structure with a smaller pore size. This proves that the method of the present invention can precisely control the pore size by adjusting the freezing temperature.

[0120] 3) Influence of raw materials and curing methods: The most probable pore size (45.24 μm) of Example 4 (fly ash / slag, -25℃ freezing, steam curing) is similar to the main peak position of Comparative Example 1 (pure slag, -45℃ freezing, standard curing, 45.25 μm). However, considering the mechanical property data in Table 1 (4.77 MPa for Example 4, much higher than 1.63 MPa for Comparative Example 1), and its... Figure 9 The absence of the secondary peak at 2.50 μm indicates that even with similar most probable pore sizes, the multi-stage in-situ polymerization process of this invention, along with different raw materials and curing methods, can still significantly improve the strength of the pore walls and the regularity of the pores. This means that pore size is not the only determining factor; the microstructure of the pore walls and the quality of the overall pore network are equally important.

[0121] 4) Pore connectivity: Although the pore size distribution map mainly reflects the pore size and its distribution, it can also be observed in conjunction with SEM (e.g., Figure 4 , Figure 6 , Figure 7 , Figure 8The sample of the present invention shown has a denser and more continuous pore wall and improved mechanical properties. It can be inferred that the sample prepared by the method of the present invention has its effective pore connectivity guaranteed due to the strengthening of the pore wall structure and the improvement of pore regularity, and reduces the pore blockage that may be caused by structural collapse or material migration. Figure 5 The μ-CT 3D reconstruction image of Example 1 further confirms on a macroscopic scale that the method of the present invention can form a layered pore network structure with good orientation and connectivity. This ordered pore structure is the basis for achieving efficient fluid transport and specific functions (such as filtration and catalysis).

[0122] In summary, pore structure analysis shows that the multi-stage low-temperature in-situ polymerization method proposed in this invention can not only significantly improve the mechanical properties of materials, but also effectively control the pore size and distribution uniformity of porous geopolymers, forming a layered pore structure with good connectivity. By combining the control of process parameters such as freezing temperature, precise control of pore structure characteristics can be achieved.

[0123] (3) Microscopic morphology analysis

[0124] The microstructure of the sample cross-section was observed using scanning electron microscopy (SEM), and the results are as follows: Figures 2 to 4 as well as Figures 6 to 8 As shown. Figure 2 The SEM image of the Comparative Example 1 (conventional direct freeze-drying) sample is shown. It can be seen that its pore wall structure is loose and irregular, the geopolymer gel phase is underdeveloped or discontinuous, and a large number of silica-alumina active raw material particles (irregular blocks or flakes visible in the figure) are physically stacked, lacking an effective gel phase to bind them tightly. Figure 3 The image shows a SEM image of the sample from Comparative Example 2 (single-stage low-temperature in-situ polymerization), compared to... Figure 2 In comparison, the density of the pore walls and the bonding between particles are improved. A certain amount of geopolymer gel phase (the relatively smooth connection between particles in the figure) can be seen to be formed and begins to partially coat the raw material particles, but the distribution of the gel phase is still not uniform and complete enough, and some areas still show exposed particles and relatively loose accumulation. In contrast, the samples treated with the multi-stage low-temperature in-situ polymerization of this invention (such as...) Figure 4 Example 1 shown, Figure 6 Example 2 shown, Figure 7 Example 3 shown, and Figure 8 Example 4) shown exhibits a significantly improved microstructure. From Figure 4 (Example 1) It can be seen that the pore walls become relatively denser and smoother, and the particles are well filled and connected by the geopolymer gel phase, forming a relatively continuous and complete skeleton structure with clear boundaries of the layered pores. Figure 6The SEM image of (Example 2) more clearly shows this optimization effect: the pore walls exhibit high density and continuity. The geopolymer gel phase forms an effective network structure that tightly encapsulates and binds the raw material particles together. The interfacial bonding between particles is very tight, and almost no loosely packed particles or obvious interfacial gaps are observed. Figure 7 SEM images of (Example 3, prepared at -150℃) also show a similar dense pore wall structure and good interparticle bonding, although its pore size is smaller (compared to...). Figure 9 (According to aperture analysis), but the pore wall remains solid. Figure 8 The SEM image of (Example 4, fly ash / slag composite raw material) also shows that the composite raw material geopolymer prepared by the method of the present invention also has dense pore walls and good interparticle bonding, indicating that the method of the present invention has good applicability to different siliceous aluminate raw material systems.

[0125] These significant differences in microstructure clearly reveal the unique advantages of the multi-stage low-temperature in-situ polymerization treatment of the present invention in promoting the full and uniform polycondensation of the geopolymer to form a robust and continuous gel network, which is the fundamental reason for the significant improvement in its macroscopic mechanical properties and pore structure stability.

[0126] (4) The influence of different process parameters

[0127] The multi-stage low-temperature in-situ polymerization method of this invention demonstrates good applicability and effectiveness for different initial slurry compositions (such as the water-to-solid ratio and type of active raw material in Examples 3 and 4), freezing conditions (such as -150°C in Example 3), and subsequent curing methods (such as steam curing in Example 4). For example, even with a water-to-solid ratio as high as 1.0 or with ultra-low temperature freezing (-150°C) (Example 3), the multi-stage treatment of this invention yields a compressive strength far exceeding that of the comparative sample, reaching 9.01 MPa. For Example 4, which uses fly ash / slag composite raw material and is frozen at -25°C, combined with subsequent steam curing, its compressive strength reaches 4.77 MPa, further demonstrating the good compatibility and synergistic enhancement effect of the core treatment steps of this invention with different systems and subsequent processes. This indicates that the core technical concept proposed in this invention has universal guiding significance and broad application potential.

[0128] In summary, performance testing and result analysis fully demonstrate that the preparation method proposed in this invention, based on the combination of multi-stage low-temperature in-situ polymerization and the ice template method, effectively utilizes controlled melting of ice crystal water to significantly promote the condensation reaction of geopolymers at low temperatures and strengthen the matrix framework by subjecting the frozen preform to sequential, at least two different, and progressively increasing low-temperature in-situ polymerization treatments before freeze-drying to remove the ice crystal template. Compared with traditional direct freeze-drying methods and simple single-stage low-temperature in-situ polymerization methods, the porous geopolymers prepared by this invention achieve a significant improvement in mechanical properties (especially compressive strength) (e.g., more than 200% higher than direct freeze-drying and more than 25-70% higher than single-stage in-situ polymerization), while effectively maintaining the integrity of the layered pore structure and being able to control the formation of a pore network with a concentrated pore size distribution. Microscopic morphology observation confirms that the method of this invention can significantly improve the compactness and continuity of the pore walls, promote the full formation of the geopolymer gel phase, and the tight bonding between particles. The process design of this invention is reasonable and optimized for the reaction characteristics of geopolymers, providing a new, reliable and efficient technical approach for the preparation and application of high-performance, specific-structure geopolymer materials.

[0129] The above embodiments are preferred embodiments of the present invention, but the embodiments of the present invention are not limited to the embodiments described above. Any changes, modifications, substitutions, combinations, or simplifications made without departing from the spirit and principle of the present invention shall be considered equivalent substitutions and shall be included within the protection scope of the present invention.

Claims

1. A method for preparing a layered porous geopolymer based on a combination of multi-stage low-temperature in-situ polymerization and ice-templating method, characterized in that, Includes the following steps: (1) Preparation of geopolymer slurry; (2) Forming a frozen blank containing an ice crystal template: The geopolymer slurry obtained in step (1) is injected into a mold and the geopolymer slurry is unidirectionally frozen for 4-8 hours, so that the water in the slurry forms an ice crystal template that is oriented along the freezing direction, thereby obtaining the frozen blank containing the ice crystal template. (3) Perform multi-stage low-temperature in-situ polymerization and subsequent freeze-drying: The frozen preform containing ice crystal templates obtained in step (2) is subjected to a first low-temperature in-situ polymerization stage and a second low-temperature in-situ polymerization stage in sequence: First low-temperature in-situ polymerization stage: The frozen preform is placed in the temperature range of T1 and maintained for at least 6 hours, so that some of the ice crystals in the frozen preform are melted in a controlled manner to form liquid water, and a preliminary in-situ polycondensation reaction is initiated at the temperature of T1; Where -10℃≤T1<0℃; Second low-temperature in-situ polymerization stage: The preform treated in the first low-temperature in-situ polymerization stage is raised to a temperature range of T2 and held for at least 6 hours, ensuring that T2 is higher than T1. This allows the liquid water formed by the further melting of ice crystals to undergo a more complete in-situ polycondensation reaction with the unreacted or partially reacted silica-alumina active raw materials in the geopolymer slurry under the action of the original composite activator in the geopolymer slurry. After freeze-drying, a pre-cured geopolymer with a layered pore structure is obtained; wherein 0℃≤T2≤5℃; (4) Curing the pre-cured geopolymer: Curing the pre-cured geopolymer obtained in step (3) under preset conditions to obtain the layered porous structure geopolymer.

2. The method for preparing a layered porous geopolymer based on multi-stage low-temperature in-situ polymerization combined with ice template method according to claim 1, characterized in that, Step (3) involves placing the frozen preform within the temperature range T1 and maintaining it for at least 6 hours, specifically as follows: The frozen preform is kept in an environment of -10°C to -5°C for at least 6 hours, and then placed in T1. ’ Keep in the environment for at least 6 hours; -5℃≤T1 ’ <0℃.

3. The method for preparing layered porous geopolymers based on multi-stage low-temperature in-situ polymerization combined with ice template method according to claim 1 or 2, characterized in that, Step (3) involves raising the preform treated in the first low-temperature in-situ polymerization stage to a temperature range of T2 and maintaining it for at least 6 hours. Specifically, this means raising the preform treated in the first low-temperature in-situ polymerization stage to 0°C to 3°C for at least 6 hours, and then placing it in T2. ’ At least 6 hours in the environment; 3℃ < T2 ’ ≤5℃.

4. The method for preparing layered porous geopolymer based on multi-stage low-temperature in-situ polymerization combined with ice template method according to claim 1, characterized in that, The geopolymer slurry is prepared as follows: the silica-alumina active raw material, water, and the composite activator are uniformly mixed to obtain a geopolymer slurry with a viscosity controlled between 50-200 mPa·s; wherein the mass ratio of water to silica-alumina active raw material is 0.8-1.2; and the mass of alkali metal oxide contained in the composite activator is 4%-8% of the mass of the silica-alumina active raw material.

5. The method for preparing a layered porous geopolymer based on multi-stage low-temperature in-situ polymerization combined with ice template method according to claim 1, characterized in that, The freeze-drying process described in step (3) is specifically carried out for 6-48 hours under conditions where the vacuum degree is not higher than 10 Pa and the cold trap temperature is not higher than -30℃.

6. The method for preparing a layered porous geopolymer based on multi-stage low-temperature in-situ polymerization combined with ice template method according to claim 4, characterized in that, The silica-alumina active raw material in step (1) is selected from at least one of slag, metakaolin, fly ash, coal gangue or tailings containing active silica-alumina components.

7. The method for preparing a layered porous geopolymer based on multi-stage low-temperature in-situ polymerization combined with ice template method according to claim 4, characterized in that, The composite activator in step (1) is prepared by a mixture of water glass and sodium hydroxide or potassium hydroxide, with a modulus of 1.2-1.

8.

8. The method for preparing a layered porous geopolymer based on multi-stage low-temperature in-situ polymerization combined with ice template method according to claim 1, characterized in that, Step (2) involves injecting the geopolymer slurry obtained in step (1) into a mold and subjecting the geopolymer slurry to unidirectional freezing for 4-8 hours. Specifically: The geopolymer slurry obtained in step (1) is injected into a mold. The bottom of the mold is provided with a high thermal conductivity metal sheet. The high thermal conductivity metal sheet is in contact with a single low temperature cold source. The geopolymer slurry is unidirectionally frozen for 4-8 hours at a freezing temperature of -150°C to -25°C. The sidewall of the mold is made of a polymer material with a thermal conductivity of not more than 0.5 W / (m·K) and a thickness of 8-15 mm; the thermal conductivity of the high thermal conductivity metal sheet is not less than 100 W / (m·K) and a thickness of 0.5-1.5 mm.

9. The method for preparing a layered porous geopolymer based on multi-stage low-temperature in-situ polymerization combined with ice template method according to claim 1, characterized in that, The curing described in step (4) is standard curing or steam curing; the conditions for standard curing are a temperature of 20±2℃, a relative humidity of not less than 95%, and a curing time of 3-28 days; the conditions for steam curing are a temperature of 50-80℃, a relative humidity of greater than 95%, and a curing time of 12-48 hours.

10. A layered porous geopolymer based on a combination of multi-stage low-temperature in-situ polymerization and ice-templating method, characterized in that... The polymer with a layered porous structure is prepared by the preparation method of the multi-stage low-temperature in-situ polymerization and ice template method according to any one of claims 1 to 9; the layered porous structure polymer has a layered pore structure, a porosity of 45%-65%, a most probable pore size in the range of 10-100 μm, and a compressive strength parallel to the unidirectional freezing direction of not less than 4 MPa.

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