Geopolymer based on multi-source solid waste and preparation method and application thereof

Abstract

The present application belongs to the technical field of solid waste resource utilization and inorganic non-metallic material, and more particularly relates to a geopolymer based on multi-source solid waste and a preparation method and application thereof. The raw material of the geopolymer based on multi-source solid waste provided by the present application comprises fly ash, red mud, sludge and alkali activator solution; the mass ratio of fly ash, red mud and sludge is 60-70:20-30:5-10; the alkali activator solution is a sodium silicate solution with a modulus of 1.5-2.0, and the concentration is 14-16% in terms of the mass percentage of Na2O; and the water-solid ratio of the geopolymer based on multi-source solid waste is 0.5-0.7. Through specific proportioning and preparation process, the present application realizes the synergistic effect of waste treatment with waste and waste-to-resource of fly ash, red mud and sludge, the alkalinity of red mud solves the pH condition required for fly ash stabilization, the phosphate of sludge solves the deep fixation problem of heavy metals in fly ash, and the fly ash constitutes the structural main body of the final product while being safely treated, and has excellent mechanical properties and heavy metal solidification performance after curing.

Description

Technical Field

[0001] This invention belongs to the field of solid waste resource utilization and inorganic non-metallic materials technology, and more specifically relates to a geopolymer based on multi-source solid waste, its preparation method and application. Background Technology

[0002] Geopolymers, a three-dimensional network gel material formed by the polymerization of silicon-oxygen tetrahedra and aluminum-oxygen tetrahedra, are a promising green building material due to their high early strength, high temperature resistance, corrosion resistance, strong ability to cure heavy metals, and carbon emissions from the production process that are far lower than those of traditional cement.

[0003] Currently, there are two main technical routes for the preparation of geopolymers, but both have significant limitations and are difficult to economically and efficiently process complex and diverse solid wastes.

[0004] One approach is based on pure raw material preparation technology. This route relies on a single-component "refined raw material" (such as metakaolin) as the core cementing component, reacted with a specific activator (such as phosphoric acid). Its advantages lie in the well-defined activity of the raw material, controllable reaction process, and the ability to obtain stable products, such as solidified soil exhibiting excellent resistance to seawater erosion. However, its disadvantages are also significant, including poor economic efficiency: the core raw material, metakaolin, requires calcination of kaolin at 900-1000℃, resulting in enormous energy consumption, coupled with expensive chemical activators, leading to high costs; and weak solid waste resource utilization capacity: this technology essentially consumes high-value processing materials to treat low-value waste, failing to transform the solid waste to be treated (such as fly ash and red mud) into the main body of the geopolymer cementing phase, thus limiting environmental benefits and failing to achieve resource-based upgrading through "waste-to-waste" transformation.

[0005] Another approach is based on preparation technologies using simple solid waste. To reduce costs and achieve solid waste utilization, technologies have been developed to prepare geopolymers using single or binary solid waste systems such as fly ash and slag. However, these technologies exhibit serious shortcomings when treating complex, multi-component solid wastes with vastly different properties, such as fly ash, red mud, and sludge. For example, they suffer from poor raw material compatibility: the introduction of multi-component solid wastes leads to an imbalance in the proportions of key elements in the system (such as Si / Al / Na / Fe), making it difficult to control simple mixing and activation processes. For instance, the high alkalinity and high iron content of red mud can damage the traditional geopolymer network structure; low reaction efficiency: the high water content and organic matter in sludge can hinder the diffusion of activators, encapsulate solid waste particles, and lead to incomplete reactions, resulting in products with low strength and a tendency to crack; and significant interference from other substances: the superposition of concentrations of interfering components such as chloride ions, sulfate ions, and organic matter can compete for reaction sites, delay coagulation, or cause harmful expansion, damaging the integrity of the three-dimensional network structure.

[0006] It is evident that existing geopolymer preparation technologies, due to limitations in process structure and raw material adaptability, cannot economically, efficiently, and stably achieve the synergistic resource utilization of complex, multi-component solid wastes. Current technologies often employ single or two types of solid waste to prepare geopolymers, resulting in a limited source of raw materials and a limited capacity for the synergistic treatment and disposal of complex, multi-component solid wastes. Traditional disposal methods (such as stockpiling and landfilling) for fly ash (hazardous waste), red mud (highly alkaline bulk solid waste), and sludge (easily perishable organic solid waste) consume significant land resources and pose secondary environmental pollution risks such as heavy metal leaching, dust pollution, and alkali leakage. Traditional geopolymers primarily use pure raw materials such as metakaolin, leading to high costs; while using specific solid wastes often results in poor mechanical properties (such as compressive strength), durability, or volumetric stability of the products. For example, geopolymers prepared from phosphate residue exhibit extremely poor performance. This waste residue is rich in phosphates, fluorides, and heavy metals, which exert multiple toxic effects on the geopolymerization process. Phosphate and fluoride ions strongly compete for reaction sites in the dissolution and condensation of aluminosilicates, severely disrupting the formation of the three-dimensional network. This results in a 28-day compressive strength typically below 5 MPa, with almost no subsequent growth. Simultaneously, phosphates and heavy metals are difficult to solidify effectively, posing a serious leaching risk. High concentrations of soluble salts also lead to a drying shrinkage rate exceeding 2%, causing rapid cracking and pulverization. This example demonstrates that special solid wastes containing high concentrations of interfering ions are almost impossible to effectively utilize using traditional processes without deep pretreatment or a specially designed activation system. In particular, the high alkalinity of red mud and the high viscosity and high water content of sludge result in complex processes, high energy consumption, and unstable product performance during their material utilization.

[0007] Therefore, how to achieve the synergistic resource utilization of fly ash, red mud, and sludge—three typical difficult-to-treat solid wastes—to prepare fly ash, red mud, and sludge-based geopolymers has become a problem that urgently needs to be solved by those skilled in the art. Summary of the Invention

[0008] The purpose of this invention is to provide a geopolymer based on multi-source solid waste, its preparation method, and its application, in order to solve the problems existing in the prior art.

[0009] To achieve the above objectives, the present invention provides the following solution: One of the technical solutions of this invention is to provide a geopolymer based on multi-source solid waste, the raw materials of which include fly ash, red mud, sludge and alkaline activator solution; The mass ratio of fly ash, red mud, and sludge is 60-70:20-30:5-10; The alkaline activator solution is a sodium silicate solution with a modulus of 1.5-2.0 and a concentration of 14-16% by mass of Na2O. The amount of the alkaline activator solution used is 50-70 wt% of the sum of the mass of fly ash, red mud, and sludge. The water-to-solid ratio of the geopolymer based on multi-source solid waste is 0.5-0.7.

[0010] Furthermore, the mass ratio of the fly ash, red mud, and sludge is 70:25:5.

[0011] Furthermore, the concentration of the alkaline activator solution is 14-15% by mass of Na2O, preferably 14%.

[0012] The second technical solution of the present invention provides a method for preparing the above-mentioned geopolymer based on multi-source solid waste, the steps of which include: Fly ash, red mud, and sludge are mixed and stirred evenly. Then, an alkaline activator solution is added, and the water-to-solid ratio is controlled at 0.5-0.7 (mass ratio). The mixture is stirred at 130-150 rpm for 1-3 minutes, followed by high-speed stirring for 3-7 minutes to obtain the geopolymer based on multi-source solid waste.

[0013] Furthermore, the stirring speed is 130-150 rpm.

[0014] Furthermore, the fly ash, red mud, and sludge have a mesh size of 100.

[0015] Furthermore, the high-speed stirring speed is 250-350 rpm.

[0016] The third technical solution of the present invention provides an application of the above-mentioned geopolymer based on multi-source solid waste in the preparation of non-fired bricks, mine backfilling or roadbed filler.

[0017] The present invention discloses the following technical effects: This invention achieves a synergistic effect of "treating waste with waste and turning waste into treasure" through specific proportions and preparation processes. The alkalinity of the red mud solves the pH conditions required for the stabilization of fly ash, and the phosphate of the sludge solves the problem of deep fixation of heavy metals in fly ash. Meanwhile, the fly ash, while being safely treated, forms the main structure of the final product and has excellent mechanical properties and heavy metal solidification properties after curing. Attached Figure Description

[0018] The accompanying drawings, which form part of this invention, are used to provide a further understanding of the invention. The illustrative embodiments of the invention and their descriptions are used to explain the invention and do not constitute an undue limitation of the invention. In the drawings: Figure 1 This is a photograph of the actual product after geopolymer curing in Example 1.

[0019] Figure 2The image shows the characterization of the geopolymer in Example 1, where a is a SEM image, b is a SEM-EDS image, c is an X-ray fluorescence spectrum, and the inset in c is the selected component analysis result of the EDS spot in b.

[0020] Figure 3 The images are XRD patterns of Examples 1-3 and Comparative Examples 7-10. Detailed Implementation

[0021] Various exemplary embodiments of the present invention will now be described in detail. This detailed description should not be considered as a limitation of the present invention, but rather as a more detailed description of certain aspects, features, and embodiments of the present invention.

[0022] It should be understood that the terminology used in this invention is merely for describing particular embodiments and is not intended to limit the invention. Furthermore, with respect to numerical ranges in this invention, it should be understood that each intermediate value between the upper and lower limits of the range is also specifically disclosed. Any stated value or intermediate value within a stated range, as well as each smaller range between any other stated value or intermediate value within said range, is also included in this invention. The upper and lower limits of these smaller ranges may be independently included or excluded from the range.

[0023] Unless otherwise stated, all technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the art. While only preferred methods and materials have been described herein, any methods and materials similar or equivalent to those described herein may be used in the implementation or testing of this invention. All references to this specification are incorporated by way of citation to disclose and describe methods and / or materials associated with those references. In the event of any conflict with any incorporated reference, the content of this specification shall prevail.

[0024] Various modifications and variations can be made to the specific embodiments described in this specification without departing from the scope or spirit of the invention, as will be apparent to those skilled in the art. Other embodiments derived from this specification will also be readily apparent to those skilled in the art. This specification and embodiments are merely exemplary.

[0025] The terms “include,” “including,” “have,” “contain,” etc., used in this article are all open-ended terms, meaning that they include but are not limited to.

[0026] Unless otherwise specified, all raw materials and reagents involved in the specific embodiments of this invention are commercially available products.

[0027] The main components of fly ash used in specific embodiments of the present invention are shown in Table 1.

[0028] Table 1 The main components of the red mud used in the specific embodiments of the present invention are shown in Table 2.

[0029] Table 2 The main components of the sludge used in the specific embodiments of the present invention are shown in Table 3.

[0030] Table 3 The alkaline activator solution used in the specific embodiments of the present invention is prepared by the following method: To prepare a sodium silicate alkali activator solution with a modulus (i.e., the molar ratio of SiO2 to Na2O) of 1.5-2 and a concentration (based on the mass percentage of Na2O) of 14%-16%, the following is an example of the preparation process: Weigh 0.2772 mL of industrial-grade liquid sodium silicate (modulus 3.2) into a beaker. While stirring continuously in a cold water bath, slowly add 0.0490 g of solid sodium hydroxide (NaOH) in 2-3 batches. Stir until the mixture is completely dissolved, cool to room temperature, and then add water to bring the concentration to 14%. Stir until a homogeneous and transparent solution is formed. After aging at room temperature for 24 hours, a 14% sodium silicate base activator solution is obtained.

[0031] Unless otherwise specified, room temperature and normal temperature in the specific embodiments of this invention refer to 20-30℃.

[0032] It should be noted that any aspects not described in detail in this invention are conventional practices in the field and are not the focus of this invention.

[0033] A specific embodiment of the present invention provides a method for preparing geopolymers based on multi-source solid waste, the steps of which include: S1, fly ash (FA), red mud (RM) and sludge (SS) are placed in a forced-air drying oven at 105±5℃ and dried for at least 24 hours to remove free moisture. The dried raw materials are then crushed and passed through a 100-mesh (0.15mm aperture) standard sieve. After sieving, the dried powder raw materials are obtained and sealed for storage. S2. Mix the dried fly ash, red mud and sludge in a mass ratio of 60-70:20-30:5-10, and stir at 130-150 rpm for 5-15 minutes until the solid powder is evenly mixed, the color is uniform, and there is no visible layering or color difference, to obtain the dry mixture. S3. Add a sodium silicate alkaline activator solution with a modulus of 1.5-2.0 and a concentration of 14-16% (50-70 wt% of the sum of the mass of fly ash, red mud and sludge) to the dry mix. Control the water-cement ratio to 0.5-0.7. Stir at 130-150 rpm for 1-3 minutes to ensure that all powders are fully wetted and to prevent dust. Then stir at 250-350 rpm for 3-7 minutes to obtain a uniform and viscous slurry, which is the geopolymer and exhibits typical workability like cement mortar.

[0034] In this invention, the main function of red mud is to provide a continuous strong alkaline environment, stimulate the volcanic ash activity of fly ash, and promote the depolymerization and repolymerization of the silicon-aluminum network. At the same time, the iron and aluminum oxides rich in red mud can participate in the formation of new hydration products (such as iron-containing geopolymer phases) and have an adsorption and co-precipitation effect on heavy metal ions. The organic matter in sludge may slightly delay hydration in the early stages, but it can play a toughening role in the later stages. More importantly, the phosphates and other components in sludge can form extremely stable phosphate precipitates (such as lead Pb) with heavy metals in fly ash, which can effectively chemically solidify heavy metals and improve the solidification effect of heavy metals. Fly ash, as the primary source of silicon, aluminum, and calcium in the system, is responsible for forming the three-dimensional network gel [SiO4] of the geopolymer. 4- and [AlO4] 5- It is the core framework of the structure; at the same time, it is also the main object that requires heavy metal solidification treatment in this invention.

[0035] This invention achieves complementarity and synergy in the chemical composition of three types of solid waste by designing specific raw material ratios and pretreatment processes. It effectively controls the proportions of key elements such as Ca / Si / Al / Na / Fe in the system and eliminates or suppresses the adverse effects of interfering components such as chloride salts and organic matter. Thus, while significantly reducing raw material costs, it achieves efficient resource utilization of the three types of solid waste and prepares geopolymer materials with stable performance that meet application requirements. This effectively solves the shortcomings of existing geopolymer preparation technologies, such as high raw material costs, poor adaptability to complex multi-component solid wastes, and product performance degradation due to chemical composition imbalance and the enrichment of interfering substances.

[0036] This invention achieves efficient synergistic and deep resource utilization of multiple recalcitrant solid wastes, breaking through the raw material limitations of traditional technologies. Existing technologies typically use high-cost "refined raw materials" as cementing materials to solidify single wastes, or can only treat 1-2 types of solid wastes with similar properties. This invention, through an original "fly ash-red mud-sludge" ternary ratio system, for the first time simultaneously transforms industrial hazardous waste (fly ash), strongly alkaline bulk solid waste (red mud), and highly viscous organic solid waste (sludge) into the main body of the geopolymer cementitious phase, rather than merely treating the waste. This effect stems from the precise utilization and synergistic design of the chemical and physical properties of the three types of solid waste. Red mud provides a sustainable, highly alkaline environment, replacing most of the expensive external alkaline activators, and its iron and aluminum oxides participate in network formation. Organic matter and phosphates in sludge, under specific ratios, not only do not cause serious hindrance, but their phosphate ions form extremely stable chemical precipitates with heavy metals in fly ash, achieving deep fixation of heavy metals. Fly ash, as the main source of silicon, aluminum, and calcium, effectively forms a three-dimensional geopolymer network framework under the alkaline environment activated by red mud and the regulation of sludge components. Through the synergy of "alkaline activation - framework construction - impurity stabilization," the disadvantages of the three wastes cancel each other out and their advantages complement each other, achieving a transformation from "waste-waste antagonism" to "waste-waste synergy," reaching the highest level of resource utilization: "treating waste with waste and turning waste into materials."

[0037] The preparation method of this invention significantly reduces raw material and energy costs, improving the economic efficiency of the solution. This invention completely eliminates expensive raw materials such as metakaolin requiring high-temperature calcination; the main raw materials are raw solid waste requiring no further processing. Simultaneously, the inherent strong alkalinity of red mud significantly reduces the dependence on and usage of purchased alkaline activators (such as sodium hydroxide and water glass). (Red mud typically contributes 30%-70% of the total required effective alkali, allowing the usage of purchased alkaline activators (in Na2O equivalent) to be drastically reduced from 15%-20% in traditional technologies to 14%-15% or even lower in this invention). Through optimized raw material ratios and activator concentrations, the chemical properties and potential activity of the waste are maximized while ensuring a complete reaction. Compared to existing technologies, the material cost of this invention can be reduced by more than 60%, and it avoids the high-temperature energy consumption in metakaolin production, aligning with the development direction of a green, low-carbon, and circular economy.

[0038] This invention overcomes the problems of low product strength, volume instability, and poor durability caused by the imbalance of Si / Al / Na ratio and the enrichment of chloride salts and organic matter in existing technologies for treating multi-component solid waste. The ternary geopolymer solidified body prepared by this invention achieves a compressive strength of over 10 MPa after 28 days, and the heavy metal leaching concentration is far below the national standard limit, solving the problem of chemical imbalance and performance degradation in complex multi-component systems, resulting in stable and excellent product performance. This is achieved through systematic raw material pretreatment (drying and grinding), precise solid-liquid ratio control, and subsequent standard constant temperature and humidity curing, providing the most favorable microenvironment for the ternary system geopolymerization reaction. Its specific ratio ensures a reasonable proportion of key elements, while thorough stirring and curing guarantee the uniformity and density of the reaction product. Microstructural analysis (SEM / XRD) confirms that a large amount of dense geopolymer gel phase is generated in the product, effectively encapsulating or chemically immobilizing heavy metals within it.

[0039] This invention not only achieves the safe and stable treatment of high-risk heavy metals and chlorides in fly ash, but also simultaneously treats red mud and sludge, completely eliminating the environmental risks associated with the separate disposal of these three substances. The solidified body has passed long-term stability tests, including freeze-thaw resistance and sulfate attack resistance, demonstrating its excellent durability. It can be used as a building material such as non-fired bricks and roadbed fillers, exhibiting high environmental safety, good long-term stability, and significant environmental and economic benefits.

[0040] Based on multiple stabilization mechanisms—physical encapsulation by geopolymers, adsorption of iron and aluminum oxides in red mud, and chemical precipitation of phosphates in sludge—these synergistic effects ensure that pollutants will not be re-released under long-term environmental stress. Therefore, the technical solution of this invention not only saves huge amounts of hazardous waste and safe landfill costs but also creates new economic benefits by producing valuable building materials, realizing the transformation from "environmentally negative assets" to "resource-positive assets."

[0041] Overall, this invention, through an innovative and targeted process design, successfully transforms three types of solid waste with vastly different properties and difficult-to-treat characteristics into high-performance, highly stable geopolymer materials. This technical solution demonstrates significant advantages over existing technologies in terms of resource synergy, economic feasibility, technological effectiveness, and environmental safety, providing a novel and highly promising technological path for the large-scale, high-value utilization of bulk and hazardous solid waste.

[0042] Example 1 The preparation steps of geopolymers based on multi-source solid waste include: S1, fly ash (FA), red mud (RM) and sludge (SS) are placed in a forced-air drying oven at 105±5℃ and dried for at least 24 hours to remove free moisture. The dried raw materials are then crushed and passed through a 100-mesh (0.15mm aperture) standard sieve. After sieving, the dried powder raw materials are obtained and sealed for storage. S2. Mix the dried fly ash, red mud and sludge in a mass ratio of 70:25:5, and stir at a speed of 140±5 rpm for 10±1 min until the solid powder is evenly mixed, uniform in color, and without visible layering or color difference, to obtain the dry mixture. S3. Add a sodium silicate alkaline activator solution with a modulus of 1.5 and a concentration of 14% (50 wt% of the sum of the mass of fly ash, red mud and sludge) to the dry mixture, control the water-cement ratio at 0.5, stir at 140±5 rpm for 2±0.5 min to ensure that all powders are fully wetted and to prevent dust. Then stir at 285±10 rpm for 5±2 min to obtain a uniform and viscous slurry, which is the geopolymer based on multi-source solid waste.

[0043] Example 2 The preparation steps of geopolymers based on multi-source solid waste include: S1, fly ash (FA), red mud (RM) and sludge (SS) are placed in a forced-air drying oven at 105±5℃ and dried for at least 24 hours to remove free moisture. The dried raw materials are then crushed and passed through a 100-mesh (0.15mm aperture) standard sieve. After sieving, the dried powder raw materials are obtained and sealed for storage. S2. Mix the dried fly ash, red mud and sludge in a mass ratio of 70:20:10, and stir at a speed of 140±5 rpm for 10±1 min until the solid powder is evenly mixed, the color is uniform, and there is no visible layering or color difference, to obtain the dry mixture. S3. Add a sodium silicate alkaline activator solution with a modulus of 1.5 and a concentration of 15% (50 wt% of the sum of the mass of fly ash, red mud and sludge) to the dry mixture, control the water-cement ratio to 0.5, stir at a speed of 140±5 rpm for 2±0.5 min to ensure that all powders are fully wetted and to prevent dust. Then stir at a speed of 285±10 rpm for 5±2 min to obtain a uniform and viscous slurry, which is the geopolymer based on multi-source solid waste.

[0044] Example 3 The preparation steps of geopolymers based on multi-source solid waste include: S1, fly ash (FA), red mud (RM) and sludge (SS) are placed in a forced-air drying oven at 105±5℃ and dried for at least 24 hours to remove free moisture. The dried raw materials are then crushed and passed through a 100-mesh (0.15mm aperture) standard sieve. After sieving, the dried powder raw materials are obtained and sealed for storage. S2. Mix the dried fly ash, red mud and sludge in a mass ratio of 60:30:10, and stir at a speed of 140±5 rpm for 10±1 min until the solid powder is evenly mixed, the color is uniform, and there is no visible layering or color difference, to obtain the dry mixture. S3. Add a sodium silicate alkaline activator solution with a modulus of 1.5 and a concentration of 15% (50 wt% of the sum of the mass of fly ash, red mud and sludge) to the dry mixture, control the water-cement ratio to 0.5, stir at a speed of 140±5 rpm for 2±0.5 min to ensure that all powders are fully wetted and to prevent dust. Then stir at a speed of 285±10 rpm for 5±2 min to obtain a uniform and viscous slurry, which is the geopolymer based on multi-source solid waste.

[0045] Comparative Example 1 The preparation steps of geopolymers include: S1, fly ash (FA), and red mud (RM) are placed in a forced-air drying oven at 105±5℃ for at least 24 hours to remove free moisture. The dried raw materials are then crushed and passed through a 100-mesh (0.15mm aperture) standard sieve. After sieving, the dried powder raw materials are obtained and sealed for storage. S2. Mix the dried fly ash and red mud at a mass ratio of 70:30, and stir at a speed of 140±5 rpm for 10±1 min until the solid powder is evenly mixed, the color is uniform, and there is no visible layering or color difference, to obtain the dry mixture. S3. Add a sodium silicate alkaline activator solution with a modulus of 1.5 and a concentration of 15% (50 wt% of the sum of the mass of fly ash and red mud) to the dry mix. Control the water-cement ratio to 0.5. Stir at 140±5 rpm for 2±0.5 min to ensure that all powders are fully wetted and to prevent dust. Then stir at 285±10 rpm for 5±2 min to obtain a uniform and viscous slurry, which is the geopolymer.

[0046] After 28 days of standard curing, the mechanical properties of this comparative example were only 5.646 MPa.

[0047] Comparative Example 2 The preparation steps of geopolymers include: Two raw materials, S1, fly ash (FA) and sludge (SS), are placed in a forced-air drying oven at 105±5℃ and dried for at least 24 hours to remove free moisture. The dried raw materials are then crushed and passed through a standard sieve of 100 mesh (0.15mm aperture). After sieving, the dried powder raw materials are obtained and stored in a sealed container for later use. S2. Mix the dried fly ash and sludge at a mass ratio of 70:30, and stir at a speed of 140±5 rpm for 10±1 min until the solid powder is evenly mixed, the color is uniform, and there is no visible layering or color difference, to obtain the dry mixture. S3. Add a sodium silicate alkaline activator solution with a modulus of 1.5 and a concentration of 15% (50 wt% of the sum of fly ash and sludge mass) to the dry mix. Control the water-cement ratio to 0.5. Stir at 140±5 rpm for 2±0.5 min to ensure that all powders are fully wetted and to prevent dust. Then stir at 285±10 rpm for 5±2 min to obtain a uniform and viscous slurry, which is the geopolymer.

[0048] After 28 days of standard curing, the mechanical properties of this comparative example were only 0.424-0.628 MPa.

[0049] Comparative Example 3 The preparation steps of geopolymers include: Two raw materials, S1, red mud (RM) and sludge (SS), are placed in a forced-air drying oven at 105±5℃ and dried for at least 24 hours to remove free moisture. The dried raw materials are then crushed and passed through a standard sieve of 100 mesh (0.15mm aperture). After sieving, the dried powder raw materials are obtained and sealed for storage. S2. Mix the dried red mud and sludge at a mass ratio of 70:30, and stir at a speed of 140±5 rpm for 10±1 min until the solid powder is evenly mixed, the color is uniform, and there is no visible layering or color difference, to obtain the dry mixture. S3. Add a sodium silicate alkaline activator solution with a modulus of 1.5 and a concentration of 15% (50 wt% of the sum of the masses of red mud and sludge) to the dry mixture, control the water-cement ratio at 0.5, and stir at a speed of 140±5 rpm for 2±0.5 min to ensure that all powders are fully wetted and to prevent dust. Then stir at a speed of 285±10 rpm for 5±2 min to obtain a uniform and viscous slurry, which is the geopolymer.

[0050] After 28 days of standard curing, the mechanical properties of this comparative example are less than 1 MPa.

[0051] Comparative Example 4 The preparation steps of geopolymers include: S1, fly ash (FA), red mud (RM) and sludge (SS) are placed in a forced-air drying oven at 105±5℃ and dried for at least 24 hours to remove free moisture. The dried raw materials are then crushed and passed through a 100-mesh (0.15mm aperture) standard sieve. After sieving, the dried powder raw materials are obtained and sealed for storage. S2. Mix the dried fly ash, red mud and sludge in a mass ratio of 40:30:30, and stir at a speed of 140±5 rpm for 10±1 min until the solid powder is evenly mixed, uniform in color, and without visible layering or color difference, to obtain the dry mixture. S3. Add a sodium silicate alkaline activator solution with a modulus of 1.5 and a concentration of 15% (50 wt% of the sum of the mass of fly ash, red mud and sludge) to the dry mixture, control the water-cement ratio at 0.5, stir at 140±5 rpm for 2±0.5 min to ensure that all powders are fully wetted and to prevent dust. Then stir at 285±10 rpm for 5±2 min to obtain a uniform and viscous slurry, which is the geopolymer.

[0052] After 28 days of standard curing, the mechanical properties of this comparative example are less than 2 MPa.

[0053] Comparative Example 5 The preparation steps of geopolymers include: S1, fly ash (FA), red mud (RM) and sludge (SS) are placed in a forced-air drying oven at 105±5℃ and dried for at least 24 hours to remove free moisture. The dried raw materials are then crushed and passed through a 100-mesh (0.15mm aperture) standard sieve. After sieving, the dried powder raw materials are obtained and sealed for storage. S2. Mix the dried fly ash, red mud and sludge in a mass ratio of 70:20:10, and stir at a speed of 140±5 rpm for 10±1 min until the solid powder is evenly mixed, the color is uniform, and there is no visible layering or color difference, to obtain the dry mixture. S3. Add a sodium silicate alkaline activator solution with a modulus of 1.5 and a concentration of 12% (50 wt% of the sum of the mass of fly ash, red mud and sludge) to the dry mixture, control the water-cement ratio at 0.5, stir at 140±5 rpm for 2±0.5 min to ensure that all powders are fully wetted and to prevent dust. Then stir at 285±10 rpm for 5±2 min to obtain the geopolymer.

[0054] In this comparative example, the activator concentration was insufficient to effectively activate the active substances in the raw materials. After adding the activator, the mixture could not be formed into a non-liquid, viscous paste, and the resulting geopolymer was sandy.

[0055] Comparative Example 6 The preparation steps of geopolymers include: S1, fly ash (FA), red mud (RM) and sludge (SS) are placed in a forced-air drying oven at 105±5℃ and dried for at least 24 hours to remove free moisture. The dried raw materials are then crushed and passed through a 100-mesh (0.15mm aperture) standard sieve. After sieving, the dried powder raw materials are obtained and sealed for storage. S2. Mix the dried fly ash, red mud and sludge in a mass ratio of 70:20:10, and stir at a speed of 140±5 rpm for 10±1 min until the solid powder is evenly mixed, the color is uniform, and there is no visible layering or color difference, to obtain the dry mixture. S3. Add a sodium silicate alkaline activator solution with a modulus of 1.5 and a concentration of 15% (40 wt% of the sum of the mass of fly ash, red mud and sludge) to the dry mixture, control the water-cement ratio to be between 0.4 and 0.5, stir at a speed of 140±5 rpm for 2±0.5 min to ensure that all powders are fully wetted and to prevent dust. Then stir at a speed of 285±10 rpm for 5±2 min to obtain the geopolymer.

[0056] In this comparative example, the water-to-solid ratio was insufficient, and after adding the activator, it could not be mixed into a non-liquid, viscous paste, resulting in a sandy geopolymer.

[0057] Comparative Example 7 The only difference from Example 1 is that the mass ratio of fly ash, red mud and sludge in step S2 is 60:25:15.

[0058] The slurry prepared in this comparative ratio is very viscous, has poor fluidity, and solidifies quickly. When poured into a mold, it cannot be well shaped and has a very irregular shape because it solidifies too fast.

[0059] Comparative Example 8 The only difference from Example 1 is that the mass ratio of fly ash, red mud and sludge in step S2 is 60:20:20.

[0060] The slurry prepared in this comparative example is of good quality, with good fluidity and moderate curing speed. However, the amount of fly ash is low, resulting in a low Ca ratio, which prevents the formation of a large amount of CASH gel with higher strength.

[0061] Comparative Example 9 The only difference from Example 1 is that the mass ratio of fly ash, red mud and sludge in step S2 is 50:30:20.

[0062] The slurry prepared in this comparative example is of good quality, with good fluidity and moderate curing speed. However, the amount of fly ash is low, resulting in a low Ca ratio, which prevents the formation of a large amount of CASH gel with higher strength.

[0063] Comparative Example 10 The only difference from Example 1 is that the mass ratio of fly ash, red mud and sludge in step S2 is 50:25:25.

[0064] The slurry prepared in this comparative example is of good quality, with good fluidity and moderate curing speed. However, the amount of fly ash is low, resulting in a low Ca ratio, which prevents the formation of a large amount of CASH gel with higher strength.

[0065] Test case The geopolymer slurry prepared in the above examples and comparative examples was filled into a prism mold with a size of 40mm×40mm×160mm that was pre-coated with a release agent in two batches. After each filling, the mold was vibrated on a vibrating table for 60 seconds to remove internal air bubbles. Finally, the excess slurry on the surface of the mold was scraped off with a scraper and the surface was smoothed. Cover the mold surface with a plastic film to prevent moisture from evaporating too quickly, let it stand at room temperature for 24±0.5 hours, and then perform standard curing. The standard curing process is as follows: After standing, remove the initially hardened test block from the mold and place it in a constant temperature and humidity curing chamber. Under the conditions of a temperature of 20±1℃ and a relative humidity of ≥95%, continue curing until the specified test age (7 days or 28 days). During the curing period, a certain gap should be maintained between the test blocks, and their surfaces should always be kept moist.

[0066] Figure 1 This is a photograph of the actual product after geopolymer curing in Example 1.

[0067] Microstructure and elemental distribution analysis (SEM-EDS): Scanning electron microscopy-energy dispersive spectroscopy (EDS) was used. The solidified samples were vacuum-plated with gold or carbon-sprayed, and their microstructure was observed under high pressure. Point analysis, line scanning, or area distribution analysis were performed on typical regions.

[0068] Figure 2 The figures show the characterization of the geopolymer in Example 1, where a is a SEM image, b is a SEM-EDS image, and c is an X-ray fluorescence spectrum. The inset in c is the selected component analysis result of the EDS spot analysis in b. As can be seen from the figures, a shows that the prepared geopolymer reacted fully, producing a homogeneous gel material. The spot scan results in b and c show that the proportion of CASH gel in the material is higher than that of NASH gel, the CASH gel strength is higher, and an iron geopolymer phase is also present.

[0069] Mechanical property testing: Unconfined compressive strength testing was used to verify the mechanical properties necessary for the solidified body as an engineering material. A universal testing machine was used, referring to the "Test Method for Strength of Cement Mortar (ISO Method)" (GB / T 17671-2021). Solidified body specimens cured to the specified age were placed in the center of the testing machine's pressure plate. A uniform loading rate of (0.5±0.1) mm / min was applied until the specimen failed, and the maximum load at failure was recorded. The average value of three valid specimens from the same group was taken to confirm whether the mechanical properties of the solidified body met the mechanical requirements for landfill materials or the strength requirements for higher resource utilization applications. The results are shown in Table 4.

[0070] Table 4 Heavy metal leaching test: Heavy metal leaching was conducted strictly according to the "Solid Waste Leaching Toxicity Leaching Method - Acetic Acid Buffer Solution Method" (HJ / T 300-2007). The solidified body, cured to the specified age, was crushed into particles smaller than 9.5 mm. It was mixed with an acetic acid buffer solution at a liquid-to-solid ratio of 10:1 (pH = 2.64 ± 0.05) and shaken at (30 ± 2) r / min for (18 ± 2) h. The leachate was then filtered through a 0.45 μm microporous membrane to obtain the leachate. The concentrations of key heavy metals such as Pb, Cd, Cr, Zn, Cu, and As in the leachate were determined using inductively coupled plasma mass spectrometry (ICP-MS). The results are shown in Table 5.

[0071] Table 5 Results of heavy metal leaching test in Example 1 The measurement results were compared with the limits specified in the "Standard for Pollution Control of Hazardous Waste Landfill" (GB 18598-2019), and it was found that the fly ash red mud sludge-based geopolymer prepared by the present invention met the basic requirements for harmlessness.

[0072] Phase composition and evolution analysis (XRD): X-ray diffractometer was used. The solidified powder sample was ground to a fineness of 200 mesh or higher and scanned within the range of 5-70° (2θ). The obtained diffraction patterns were compared with standard powder diffraction databases to qualitatively and semi-quantitatively analyze the phase composition in the solidified body.

[0073] Figure 3 The figures show the XRD patterns of Examples 1-3 and Comparative Examples 7-10. In the figures, Example #1 is Example 1, Example #2 is Example 2, Example #3 is Example 3, Example #4 is Comparative Example 7, Example #5 is Comparative Example 8, Example #6 is Comparative Example 9, and Example #7 is Comparative Example 10.

[0074] The various embodiments in this specification are described in a progressive manner, with each embodiment focusing on the differences from other embodiments. The same or similar parts between the various embodiments can be referred to each other.

[0075] The above description of the disclosed embodiments enables those skilled in the art to make or use the invention. Various modifications to these embodiments will be readily apparent to those skilled in the art, and the general principles defined herein may be implemented in other embodiments without departing from the spirit or scope of the invention. Therefore, the invention is not to be limited to the embodiments shown herein, but is to be accorded the widest scope consistent with the principles and novel features disclosed herein.

Claims

1. A geopolymer based on multi-source solid waste, characterized in that, Raw materials include: fly ash, red mud, sludge, and alkaline activator solution; The mass ratio of fly ash, red mud, and sludge is 60-70:20-30:5-10; The alkaline activator solution is a sodium silicate solution with a modulus of 1.5-2.0 and a concentration of 14-16% by mass of Na2O. The amount of the alkaline activator solution used is 50-70 wt% of the sum of the mass of fly ash, red mud, and sludge. The water-to-solid ratio of the geopolymer based on multi-source solid waste is 0.5-0.

7.

2. The geopolymer based on multi-source solid waste as described in claim 1, characterized in that, The mass ratio of fly ash, red mud, and sludge is 70:25:

5.

3. The geopolymer based on multi-source solid waste as described in claim 1, characterized in that, The concentration of the alkaline activator solution is 14-15% by mass of Na2O.

4. The method for preparing geopolymers based on multi-source solid waste as described in any one of claims 1-3, characterized in that, step include: Fly ash, red mud, and sludge are mixed and stirred evenly. Then, an alkaline activator solution is added, and the water-to-solid ratio is controlled at 0.5-0.

7. The mixture is stirred at 130-150 rpm for 1-3 minutes, followed by high-speed stirring for 3-7 minutes to obtain the geopolymer based on multi-source solid waste.

5. The preparation method according to claim 4, characterized in that, The stirring speed is 130-150 rpm.

6. The preparation method according to claim 4, characterized in that, The fly ash, red mud, and sludge are all 100 mesh.

7. The preparation method according to claim 4, characterized in that, The high-speed stirring speed is 250-350 rpm.

8. The application of a geopolymer based on multi-source solid waste as described in any one of claims 1-3 in the preparation of non-fired bricks, mine backfill, or roadbed filler.

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