Preparation method of fly ash building heavy metal migration retardation composite base material

Abstract

The invention discloses a method for preparing a heavy metal migration retarding composite base material from fly ash as a building material. The method comprises the following steps: S1, pretreatment and dechlorination of waste incineration fly ash: S1-1, multi-stage carbonation pretreatment and stabilization, and S1-2, ultrasonic-assisted multi-stage countercurrent washing dechlorination; s2, preparing a sulfydryl functionalized compound stabilizer: S2-1, screening zeolite and compounding the zeolite with biochar, and S2-2, carrying out in-situ grafting and chelating functionalization on a sulfydryl silane coupling agent; s3, constructing an activated composite precursor; and S4, medium-temperature water thermocuring and gradient carbonization crystal form regulation and control: S4-1, alkali excitation-hydrothermal synergistic curing molding, and S4-2, gradient pressure carbonization and crystal form regulation and control: the method realizes efficient stabilization and resource utilization of fly ash, and the obtained base material has excellent mechanical properties and long-acting heavy metal retarding ability, and has excellent engineering application prospects.

Description

Technical Field

[0001] This invention relates to the field of fly ash composite substrate preparation technology, specifically to a method for preparing a fly ash building material heavy metal migration-inhibiting composite substrate. Background Technology

[0002] Fly ash from municipal solid waste incineration is a solid waste generated during the incineration process. Rich in various high-concentration heavy metals such as lead, cadmium, zinc, and chromium, it is classified as hazardous waste and must undergo harmless treatment to avoid harming the ecological environment. However, fly ash also contains abundant silicon and calcium, giving it potential cementing activity. Therefore, by using appropriate stabilization technologies to transform fly ash into building material substrates, not only can hazardous waste be safely disposed of, but some building materials can also be effectively replaced, meeting the dual goals of solid waste resource utilization and pollution control. This technological approach falls at the intersection of environmental functional materials and solid waste resource utilization, aiming to inhibit the migration of heavy metals during long-term use through material design, thus ensuring the environmental safety of building materials.

[0003] Currently, existing technologies for the building material utilization of fly ash mainly revolve around solidification and stabilization methods. One mainstream technology involves solidifying the fly ash with cementitious materials such as cement, utilizing gels such as calcium silicate hydrates formed by hydration reactions to encapsulate heavy metals and reduce their leaching toxicity. Another technology employs chemical stabilization agents, such as using chelating agents to form complexes with heavy metal ions, or using adsorbent materials such as biochar and zeolite to fix heavy metals. In addition, high-temperature sintering technology has also been applied, using high temperatures to volatilize or solidify heavy metals in fly ash within the silicate mineral lattice, while simultaneously degrading organic pollutants such as dioxins. In recent years, research has also emerged on introducing nanomaterials such as nano-silica or carbonization processes into fly ash modification to further improve the microstructure of the materials and the heavy metal fixation effect.

[0004] While existing technologies offer some support for the resource utilization of fly ash, significant technical challenges remain. Conventional solidification technologies often struggle to balance material mechanical properties with long-term environmental safety. For example, cementitious solids may experience changes in pore structure due to carbonization, potentially increasing the risk of heavy metal leaching. Conversely, relying solely on adsorbent materials cannot guarantee the long-term stability of heavy metals under complex environmental conditions. High concentrations of chlorides, sulfates, and other impurities in fly ash interfere with the hydration process of the cementitious system, not only slowing down the reaction and reducing material strength but also potentially causing product expansion and cracking, thus compromising the structural integrity of the solidified body. This results in uncertainty regarding the heavy metal retention effect during long-term use. Summary of the Invention

[0005] The problem with existing technologies is that existing fly ash composite substrates are difficult to balance structural stability and long-term heavy metal migration blocking performance under long-term service conditions. In order to address the above technical problems, this invention provides a method for preparing fly ash building material heavy metal migration blocking composite substrates.

[0006] The technical solution of this invention is: a method for preparing a fly ash building material heavy metal migration-inhibiting composite substrate, comprising the following steps: S1. Pretreatment and dechlorination of fly ash from waste incineration: S1-1. The fly ash from municipal solid waste incineration is subjected to multi-stage carbonation pretreatment and stabilization to obtain carbonated fly ash. S1-2. The carbonated fly ash obtained in S1-1 is subjected to three-stage countercurrent washing to obtain dechlorinated fly ash; S2. Preparation of thiol-functionalized composite stabilizers: S2-1, Zeolite screening and biochar composite: Take clinoptilolite, crush it, and pass it through a 200-400 mesh sieve to obtain zeolite particles; pyrolyze rice husks at 600-700℃ under oxygen-limited conditions for 2-3 hours to obtain biochar, and mix it with the zeolite particles at a mass ratio of 1:1-2 to obtain a zeolite-biochar composite carrier. S2-2, In-situ grafting and chelation functionalization of mercaptosilane coupling agents: The zeolite biochar composite carrier described in S2-1 was dispersed in an ethanol-water solution to prepare a suspension with a mass concentration of 8-12 wt%; the volume ratio of ethanol to water in the ethanol-water solution was 95:5. Then, sodium polyacrylate, accounting for 2-4% of the mass of the zeolite biochar composite carrier, was added and ultrasonically dispersed for 20-30 minutes. Under continuous stirring, mercaptopropyltrimethoxysilane, accounting for 3-6% of the mass of the zeolite biochar composite carrier, was added dropwise. The pH of the reaction system was adjusted to 8.5-9.5 with ammonia water, and the reaction was refluxed at 65-75℃ for 6-8 hours. After the reaction was completed, 2,5-dimercapto-1,3,4-thiadiazole, accounting for 5-8% of the mass of the zeolite biochar composite carrier, was added, and the reaction was continued for 2-3 hours. After centrifugation, washing with ethanol, and drying, a mercapto-functionalized composite stabilizer was obtained. S3. Constructing an activated composite precursor: Weigh the dechlorinated fly ash obtained from S1-2, the thiol-functionalized composite stabilizer obtained from S2-2, silica fume, and nano-perovskite catalyst (La0.8Sr0.2MnO3) in a mass ratio of 100:10-15:10-20:0.5-1.5 and place them in a planetary ball mill jar. Use zirconia grinding balls with a ball-to-material ratio of 5-8:1 and perform mechanochemical activation at a speed of 450-550 r / min for 90-120 minutes under argon protection to obtain a composite precursor powder with high reactivity. S4, Medium-temperature hydrothermal curing and gradient carbonization crystal form control: S4-1, Alkali activation-hydrothermal synergistic curing molding: The composite precursor powder obtained from S3, low-calcium sulfoaluminate cement, and nano-metakaolin were mixed uniformly at a mass ratio of 45-55:100:5-10 to obtain a gel-polymer composite dry powder. A water glass solution with a modulus of 1.2-1.5 was prepared as an alkali activator and mixed with deionized water at a volume ratio of 1:2-3 to obtain an alkali-activated mixture. The alkali-activated mixture was mixed with the gel-polymer composite dry powder at a water-cement ratio of 0.3-0.35 and stirred in a cement paste mixer at 115-125 r / min for 90 seconds, and then stirred at 275-295 r / min for 150 seconds to obtain a homogeneous slurry. The slurry was injected into a mold and vibrated at a vibration frequency of 50 Hz and an amplitude of 0.5. The cement is compacted on a vibrating table for 60 seconds, sealed, and then steam-cured for 16-24 hours at 75-85℃ and 95-99.9% relative humidity. After demolding, the initial set cement is obtained. The low-calcium sulfoaluminate cement has a specific surface area of ​​450-550 m². 2 / kg; S4-2, Gradient Pressure Carbonization and Crystal Form Regulation: The initial solidified body obtained by S4-1 was subjected to gradient carbonization. After carbonization, it was cooled to room temperature at a rate of 1-2℃ / min under nitrogen protection at 0.1 MPa. After curing, fly ash building material heavy metal migration inhibition composite substrate was obtained.

[0007] Note: This method not only achieves high-value-added, large-scale resource utilization of hazardous waste fly ash, turning waste into treasure and reducing environmental risks and landfill space, but also has a clear process route, with major equipment being mature industrial devices, demonstrating good technical feasibility and engineering transformation prospects. The prepared composite substrate exhibits excellent performance, providing an innovative and efficient technical path for the safe disposal and resource utilization of municipal solid waste incineration fly ash.

[0008] Further, the multi-stage carbonation pretreatment and stabilization method described in S1-1 is as follows: Municipal solid waste incineration fly ash with a moisture content of 10-20% is spread evenly in a controlled atmosphere treatment chamber, with a layer thickness of 3-8 cm; air with a CO2 concentration of 20-30% v / v and a relative humidity of 75-85% is introduced into the treatment chamber at a gas exchange rate of 0.8-1.2 chamber volume / hour, and the chamber is treated at 25-30℃ for 24-36 hours; then, the CO2 concentration is increased to 50-70%, the gas exchange rate is adjusted to 0.5-0.8 chamber volume / hour, the temperature is maintained at 30-35℃, and the chamber is treated for 12-24 hours to obtain carbonated fly ash. The three-stage countercurrent washing method described in S1-2 is as follows: First stage: using citrate buffer solution with a pH of 4-5, mixed with fly ash at a liquid-to-solid ratio of 3 mL:1 g, and washed under ultrasonic conditions for 15-20 minutes, followed by solid-liquid separation; Second and third stages: using deionized water with liquid-to-solid ratios of 2 mL:1 g and 1 mL:1 g respectively, and washed for 10-15 minutes each under the same ultrasonic conditions; after the final washing is completed, the solid filter cake is collected and dried; the ultrasonic conditions are a frequency of 40 kHz and a power density of 0.4-0.6 W / mL.

[0009] Explanation: By controlling CO2 concentration and temperature in stages, this multi-stage carbonation pretreatment method effectively neutralizes the alkalinity of fly ash, stabilizes the heavy metal form, and creates favorable conditions for subsequent dechlorination. The three-stage countercurrent ultrasonic washing utilizes the complexing effect of citric acid and the cavitation synergistic effect of ultrasound to achieve efficient and deep removal of chloride salts, while retaining the active silicon and calcium components in fly ash to the greatest extent. This provides high-quality raw materials with low chloride and high reactivity for subsequent building material applications, ensuring the long-term durability and environmental safety of the final composite substrate from the source. Through this specific combination of ultrasonic parameters, a moderate cavitation effect and microfluidic action can be generated in the liquid phase, effectively stripping chloride salts coated on the surface of fly ash particles and enhancing liquid-solid mass transfer, thereby significantly improving dechlorination efficiency while avoiding excessive breakage of fly ash particles and meaningless dissolution of active components caused by excessive energy.

[0010] Furthermore, the drying method described in S1-2 is as follows: after collecting the solid filter cake, vacuum dry it at 85-95℃ until the moisture content is less than 1% to obtain dechlorination fly ash, wherein the mass content of soluble chloride ions in the dechlorination fly ash is less than 0.8%.

[0011] Note: Medium-temperature vacuum drying is used, which can efficiently remove surface water and some bound water under low thermal stress, preventing local recrystallization of chloride salts or agglomeration of fly ash during the drying process, ensuring that loose and homogeneous dechlorinated fly ash powder is obtained, laying the foundation for subsequent precise batching and uniform mixing; the soluble chloride ion content is controlled below 0.8%, which fundamentally reduces the adverse effects of chloride ions on the subsequent gelation process.

[0012] Further, the concentration of the citrate buffer solution in S1-2 is 0.05-0.1 mol / L, and the solvent is water; the pressure of the solid-liquid separation is 0.3-0.6 MPa.

[0013] Note: Using a citrate buffer solution within this concentration range provides a suitable acidic environment to promote chloride dissolution, while its complexing effect can stabilize the dissolved heavy metals to a certain extent and reduce their re-adsorption. Combined with a filter press pressure of 0.3-0.6 MPa, it can ensure high dewatering efficiency while avoiding excessive compaction of the filter cake, which would affect the subsequent washing effect, thus achieving a balance between washing efficiency and energy consumption.

[0014] Further, the mixing method described in S2-1 is as follows: the biochar and zeolite particles are introduced into a planetary ball mill and mixed at a speed of 350-450 r / min for 30-45 minutes.

[0015] Note: Mechanical blending via planetary ball milling not only achieves uniform composite of zeolite and biochar on a physical scale, but the resulting mechanochemical effect also helps improve the interfacial bonding between the two, exposing more active sites. This creates more favorable conditions for subsequent silane coupling agent grafting reactions, thereby enhancing the structural integrity and functionality of the composite carrier.

[0016] Furthermore, the drying described in S2-2 is carried out under vacuum, at a temperature of 75-85℃, for 8-12 hours; the centrifugation speed is 8000-10000 r / min, and the ethanol is washed 2-3 times until the washing liquid is neutral.

[0017] Note: Vacuum environment combined with medium temperature drying can effectively remove solvents and promote the completion of the hydrolysis and condensation reaction of silane coupling agents, while avoiding the decomposition of organic functional groups or damage to nanostructures caused by high temperature; the combination of high-speed centrifugation and ethanol washing ensures the complete removal of unreacted silane coupling agents, small molecule byproducts and residual chelating agents, thereby ensuring the purity and high efficiency of the functional groups on the surface of the obtained composite stabilizer.

[0018] Furthermore, the specific surface area of ​​the silica fume described in S3 is 15-25 m². 2 / g; the particle size of the nano-perovskite catalyst is 50-100 nm.

[0019] Note: The use of silica fume with high specific surface area can fully utilize its micro-aggregate filling effect and pozzolanic activity, significantly improving the microstructure and density of the composite slurry; the incorporation of nanoscale perovskite catalysts, with their abundant oxygen vacancies and catalytic activity, may promote free radical reactions during subsequent curing, which can help further degrade residual trace organic pollutants and may optimize the formation process of gel products.

[0020] Furthermore, the water glass solution in S4-1 has a solid content of 30-35% by mass; the nano-kaolin has a specific surface area of ​​800-1200 m². 2 / g, with a particle size of 50-200 nm.

[0021] Note: By controlling the solid content of the water glass solution within this range, sufficient alkalinity and silicon source can be provided to effectively activate the raw materials, while avoiding excessively high solution viscosity that could affect mixing and molding. Nano-meta-kaolin with ultra-high specific surface area and ultra-fine particle size is used. Its extremely high reactivity can rapidly consume the alkali in the system and generate a large amount of gel products, which can not only quickly build strength, but also more effectively encapsulate heavy metal ions and unreacted fly ash particles.

[0022] Furthermore, the gradient carbonization method described in S4-2 is as follows: In the first stage, carbonization is carried out for 8-12 hours under the conditions of CO2 pressure of 0.3-0.5 MPa, temperature of 30-40℃, and relative humidity of 70-80%; in the second stage, the CO2 pressure is increased to 0.8-1.2 MPa, the temperature is maintained at 40-50℃, and the relative humidity is 85-95%, and carbonization is carried out for 12-18 hours.

[0023] Explanation: A gradient carbonization strategy from low pressure and low temperature to high pressure and high temperature is adopted. First, under relatively mild conditions, CO2 can be fully diffused into the interior of the specimen to initiate the carbonization reaction, generating products such as calcium carbonate to initially fill the pores. Subsequently, under more intense conditions, the carbonization reaction is accelerated to a deeper level, which not only further increases the density of the matrix, but also converts more heavy metals into more chemically stable carbonates or dissolves them in the carbonization products, achieving the dual purpose of carbonization strengthening and deep stabilization of heavy metals.

[0024] Furthermore, the curing temperature described in S4-2 is 19-21℃, the relative humidity is 95-99.9%, and the curing period is up to 28 days.

[0025] Note: Long-term curing under standard temperature and humidity conditions provides a stable environment for the subsequent continuous hydration and microstructure adjustment of the gelling system, which is conducive to the further development of matrix strength and the homogenization of internal structure, thereby ensuring the long-term stability and reliability of the final product performance and meeting the requirements of practical engineering applications.

[0026] The beneficial effects of this invention are: The preparation method provided by this invention, through the synergy of multi-stage carbonation pretreatment and ultrasonic-assisted countercurrent deep dechlorination, efficiently removes harmful impurities such as chlorides from fly ash while maximally preserving its active components such as silicon and calcium, laying a pure and highly active raw material foundation for subsequent building material applications. Based on this, a thiol-functionalized composite stabilizer is innovatively introduced. This stabilizer integrates the adsorption properties of zeolite, the stability of biochar, and the strong complexing ability of thiol functional groups through chemical grafting, constructing a three-dimensional, multi-layered heavy metal chemical immobilization network within the substrate. This achieves specific and effective anchoring of heavy metal ions, thus inhibiting heavy metal migration. Furthermore, mechanochemical activation significantly enhances the reactivity and homogeneity of the entire composite system. Finally, by utilizing an alkali-activated hydrothermal synergistic curing and gradient pressure carbonization process, a matrix mainly composed of a dense zeolite-like phase and stable carbonate minerals was directionally induced under mild conditions. This eliminated chloride salt interference at the source, constructed multiple barrier layers at the microscopic level, and formed a high-strength, low-permeability dense structure at the macroscopic level. This fundamentally solved the core technical problem of balancing the long-term structural stability of fly ash substrates with the long-term barrier performance of heavy metals in existing technologies, ensuring the long-term safety and reliability of the material in complex service environments. Detailed Implementation

[0027] To further illustrate the methods and effects of this invention, the technical solution of this invention will be clearly and completely described below in conjunction with experiments.

[0028] Example 1: A method for preparing a fly ash building material heavy metal migration-inhibiting composite substrate, comprising the following steps: S1. Pretreatment and dechlorination of fly ash from waste incineration: S1-1, Multi-stage carbonation pretreatment and stabilization: Fly ash from municipal solid waste incineration with a moisture content of 15% was spread evenly in a controlled atmosphere treatment chamber with a layer thickness of 5 cm. Air with a CO2 concentration of 25% v / v and a relative humidity of 80% was introduced into the treatment chamber at a gas exchange rate of 1 chamber volume / hour, and the chamber was treated at 28°C for 30 hours. Then, the CO2 concentration was increased to 60%, the gas exchange rate was adjusted to 0.6 chamber volume / hour, and the temperature was maintained at 32°C for 20 hours to obtain carbonated fly ash with a pH value of 9. S1-2, Ultrasonic-assisted multi-stage countercurrent water washing and dechlorination: The carbonated fly ash obtained from S1-1 was subjected to a three-stage countercurrent washing process. The first stage involved mixing the fly ash with a citrate buffer solution (pH 4.5) at a liquid-to-solid ratio of 3:1 mL / g and washing under ultrasonic conditions for 17 minutes, followed by solid-liquid separation. The second and third stages involved washing with deionized water at liquid-to-solid ratios of 2:1 and 1:1 mL / g, respectively, under the same ultrasonic conditions for 12 minutes each. After the final washing, the solid filter cake was collected and dried to obtain dechlorinated fly ash. The ultrasonic conditions were: a frequency of 40 kHz and a power density of 0.5 W / mL. The drying method involved vacuum drying the collected solid filter cake at 90°C until the moisture content was below 1%, yielding dechlorinated fly ash. The soluble chloride ion content in the dechlorinated fly ash was measured to be 0.7%. The concentration of the citrate buffer solution was 0.08 mol / L, and the solvent was water. The pressure for the solid-liquid separation was 0.4 MPa. S2. Preparation of thiol-functionalized composite stabilizers: S2-1, Zeolite screening and biochar composite: Clinoptilolite was pulverized and passed through a 300-mesh sieve to obtain zeolite particles; rice husks were pyrolyzed at 650℃ under limited oxygen conditions for 2.5 hours to obtain biochar, which was then mixed with the zeolite particles at a mass ratio of 1:1.5 to obtain a zeolite-biochar composite carrier; the mixing method was as follows: the biochar and zeolite particles were introduced into a planetary ball mill and mixed at a speed of 400 r / min for 40 minutes. S2-2, In-situ grafting and chelation functionalization of mercaptosilane coupling agents: The zeolite biochar composite carrier described in S2-1 was dispersed in an ethanol-water solution to prepare a suspension with a mass concentration of 10 wt%; the volume ratio of ethanol to water in the ethanol-water solution was 95:5; then, sodium polyacrylate, accounting for 3% of the mass of the zeolite biochar composite carrier, was added and ultrasonically dispersed for 25 minutes; under continuous stirring, mercaptopropyltrimethoxysilane, accounting for 5% of the mass of the zeolite biochar composite carrier, was added dropwise, and the pH of the reaction system was adjusted to 9 with ammonia water, and the reaction was refluxed at 70°C for 7 hours; after the reaction was completed, 2,5-dimercapto-1,3,4-thiadiazole, accounting for 7% of the mass of the zeolite biochar composite carrier, was added, and the reaction was continued for 2.5 hours; after centrifugation, washing with ethanol, and drying, a mercapto-functionalized composite stabilizer was obtained; the drying was carried out under vacuum at a drying temperature of 80°C for 10 hours; the centrifugation speed was 9000 r / min, and the washing was performed twice with ethanol until the washing liquid was neutral. S3. Constructing an activated composite precursor: The dechlorinated fly ash obtained from S1-2, the thiol-functionalized composite stabilizer obtained from S2-2, silica fume, and nano-perovskite catalyst (La0.8Sr0.2MnO3) were weighed at a mass ratio of 100:12:15:1 and placed in a planetary ball mill jar. Zirconia grinding balls were used, with a ball-to-material ratio of 7:1. Under argon protection, the mixture was mechanically activated at 500 r / min for 105 minutes to obtain a highly reactive composite precursor powder. The silica fume had a specific surface area of ​​20 m². 2 / g; the particle size of the nano-perovskite catalyst is 70-80 nm; S4, Medium-temperature hydrothermal curing and gradient carbonization crystal form control: S4-1, Alkali activation-hydrothermal synergistic curing molding: The composite precursor powder obtained from S3, low-calcium sulfoaluminate cement, and nano-metakaolin were mixed uniformly at a mass ratio of 50:100:8 to obtain a gel-polymer composite dry powder. A water glass solution with a modulus of 1.3 was prepared as an alkali activator and mixed with deionized water at a volume ratio of 1:2.5 to obtain an alkali-activated mixture. The alkali-activated mixture was mixed with the gel-polymer composite dry powder at a water-cement ratio of 0.32 and stirred in a cement paste mixer at 120 r / min for 90 seconds, then at 285 r / min for 150 seconds to obtain a homogeneous slurry. The slurry was poured into a mold and compacted for 60 seconds on a vibrating table with a vibration frequency of 50 Hz and an amplitude of 0.5 mm. After sealing, it was steam-cured for 20 hours at 80℃ and a relative humidity of 97%. After demolding, a preliminary set was obtained. The specific surface area of ​​the low-calcium sulfoaluminate cement was 500 m². 2 / kg; the solid content of the water glass solution is 32.5% by mass; the specific surface area of ​​the nano-kaolin is 1000 m². 2 / g, with a particle size of 80-100 nm; S4-2, Gradient Pressure Carbonization and Crystal Form Regulation: The initial solidified body obtained from S4-1 was subjected to gradient carbonization. The gradient carbonization method was as follows: In the first stage, carbonization was carried out for 10 hours under the conditions of CO2 pressure of 0.4 MPa, temperature of 35℃, and relative humidity of 75%; In the second stage, the CO2 pressure was increased to 1 MPa, the temperature was maintained at 45℃, and the relative humidity was 90%, and carbonization was carried out for 15 hours. After carbonization, the temperature was reduced to room temperature at a rate of 1.5℃ / min under nitrogen protection of 0.1 MPa. After curing, fly ash building material heavy metal migration inhibition composite substrate was obtained. The curing temperature was 20℃, the relative humidity was 97%, and the curing period was 28 days.

[0029] Example 2: This example is basically the same as Example 1, except that the ultrasonic conditions described in S1-2 are 40 kHz frequency and 0.4 W / mL power density.

[0030] Example 3: This example is basically the same as Example 1, except that the ultrasonic conditions described in S1-2 are a frequency of 40 kHz and a power density of 0.6 W / mL.

[0031] Example 4: This example is basically the same as Example 1, except that the drying method described in S1-2 is as follows: after collecting the solid filter cake, it is vacuum dried at 85°C until the moisture content is less than 1% to obtain dechlorination fly ash. After testing, the mass content of soluble chloride ions in the dechlorination fly ash is 0.4%.

[0032] Example 5: This example is basically the same as Example 1, except that the drying method described in S1-2 is as follows: after collecting the solid filter cake, it is vacuum dried at 95°C until the moisture content is less than 1% to obtain dechlorination fly ash. After testing, the mass content of soluble chloride ions in the dechlorination fly ash is 0.8%.

[0033] Example 6: This example is basically the same as Example 1, except that the concentration of the citrate buffer solution in S1-2 is 0.05 mol / L and the solvent is water; the pressure of the solid-liquid separation is 0.3 MPa.

[0034] Example 7: This example is basically the same as Example 1, except that the concentration of the citrate buffer solution in S1-2 is 0.1 mol / L and the solvent is water; the pressure of the solid-liquid separation is 0.6 MPa.

[0035] Example 8: This example is basically the same as Example 1, except that the mixing method described in S2-1 is as follows: the biochar and zeolite particles are introduced into a planetary ball mill and mixed at a speed of 350 r / min for 30 minutes.

[0036] Example 9: This example is basically the same as Example 1, except that the mixing method described in S2-1 is as follows: the biochar and zeolite particles are introduced into a planetary ball mill and mixed at a speed of 450 r / min for 45 minutes.

[0037] Example 10: This example is basically the same as Example 1, except that the drying in S2-2 is carried out in a vacuum environment, the drying temperature is 75°C, and the drying time is 8 hours; the centrifugation speed is 8000 r / min, and the washing is done twice with ethanol until the washing liquid is neutral.

[0038] Example 11: This example is basically the same as Example 1, except that the drying in S2-2 is carried out in a vacuum environment, the drying temperature is 85℃, and the drying time is 12 hours; the centrifugation speed is 10000 r / min, and the washing is done with ethanol 3 times until the washing liquid is neutral.

[0039] Example 12: This example is basically the same as Example 1, except that the specific surface area of ​​the silica fume in S3 is 15 m². 2 / g; the particle size of the nano-perovskite catalyst is 50-60 nm; the solid content of the water glass solution in S4-1 is 30% by mass; the specific surface area of ​​the nano-metakaolin is 800 m². 2 / g, with a particle size of 50-80 nm.

[0040] Example 13: This example is basically the same as Example 1, except that the specific surface area of ​​the silica fume in S3 is 25 m². 2 / g; the particle size of the nano-perovskite catalyst is 150-100 nm; the solid content of the water glass solution in S4-1 is 35% by mass; the specific surface area of ​​the nano-metakaolin is 1200 m². 2 / g, with a particle size of 150-200 nm.

[0041] Example 14: This example is basically the same as Example 1, except that the gradient carbonization method described in S4-2 is as follows: In the first stage, carbonization is carried out for 8 hours under the conditions of CO2 pressure of 0.3 MPa, temperature of 30°C and relative humidity of 70%; in the second stage, the CO2 pressure is increased to 0.8 MPa, the temperature is maintained at 40°C and the relative humidity is 85%, and carbonization is carried out for 12 hours; the curing temperature is 19°C and the relative humidity is 95%, and curing is carried out for 28 days.

[0042] Example 15: This example is basically the same as Example 1, except that the gradient carbonization method described in S4-2 is as follows: In the first stage, carbonization is carried out for 12 hours under the conditions of CO2 pressure of 0.5 MPa, temperature of 40℃ and relative humidity of 80%; in the second stage, the CO2 pressure is increased to 1.2 MPa, the temperature is maintained at 50℃ and the relative humidity is 95%, and carbonization is carried out for 18 hours; the curing temperature is 21℃ and the relative humidity is 99.8%, and curing is carried out until 28 days of age.

[0043] Example 16: This example is basically the same as Example 1, except that it includes the following steps: S1. Pretreatment and dechlorination of fly ash from waste incineration: S1-1, Multi-stage carbonation pretreatment and stabilization: Fly ash from municipal solid waste incineration with a moisture content of 10% was spread evenly in a controlled atmosphere treatment chamber with a layer thickness of 3 cm. Air with a CO2 concentration of 20% v / v and a relative humidity of 75% was introduced into the treatment chamber at a gas exchange rate of 0.8 chamber volume / hour, and the chamber was treated at 25°C for 24 hours. Then, the CO2 concentration was increased to 50%, the gas exchange rate was adjusted to 0.5 chamber volume / hour, and the temperature was maintained at 30°C for 12 hours to obtain carbonated fly ash. S1-2, Ultrasonic-assisted multi-stage countercurrent water washing and dechlorination: The carbonated fly ash obtained from S1-1 was subjected to a three-stage countercurrent washing process. The first stage involved mixing the fly ash with a citrate buffer solution at pH 4 at a liquid-to-solid ratio of 3:1 mL / g and washing under ultrasonic conditions for 15 minutes, followed by solid-liquid separation. The second and third stages involved washing with deionized water at liquid-to-solid ratios of 2:1 and 1:1 mL / g, respectively, under the same ultrasonic conditions for 10 minutes each. After the final washing, the solid filter cake was collected and dried to obtain dechlorinated fly ash. S2. Preparation of thiol-functionalized composite stabilizers: S2-1, Zeolite screening and biochar composite: Take clinoptilolite, crush it, and pass it through a 200-mesh sieve to obtain zeolite particles; pyrolyze rice husks at 600℃ under limited oxygen conditions for 2 hours to obtain biochar, and mix it with the zeolite particles at a mass ratio of 1:1 to obtain a zeolite-biochar composite carrier. S2-2, In-situ grafting and chelation functionalization of mercaptosilane coupling agents: The zeolite biochar composite carrier described in S2-1 was dispersed in an ethanol-water solution to prepare a suspension with a mass concentration of 8 wt%; the volume ratio of ethanol to water in the ethanol-water solution was 95:5; then, sodium polyacrylate, accounting for 2% of the mass of the zeolite biochar composite carrier, was added and ultrasonically dispersed for 20 minutes; under continuous stirring, mercaptopropyltrimethoxysilane, accounting for 3% of the mass of the zeolite biochar composite carrier, was added dropwise, and the pH of the reaction system was adjusted to 8.5 with ammonia water, and the reaction was refluxed at 65°C for 6 hours; after the reaction was completed, 2,5-dimercapto-1,3,4-thiadiazole, accounting for 5% of the mass of the zeolite biochar composite carrier, was added, and the reaction was continued for 2 hours; after centrifugation, washing with ethanol, and drying, a mercapto-functionalized composite stabilizer was obtained. S3. Constructing an activated composite precursor: The dechlorinated fly ash obtained from S1-2, the thiol-functionalized composite stabilizer obtained from S2-2, silica fume, and nano-perovskite catalyst were weighed in a mass ratio of 100:10:10:0.5 and placed in a planetary ball mill jar. Zirconia grinding balls were used with a ball-to-material ratio of 5:1. Under argon protection, the mixture was mechanically activated at a speed of 450 r / min for 90 minutes to obtain the composite precursor powder. S4, Medium-temperature hydrothermal curing and gradient carbonization crystal form control: S4-1, Alkali activation-hydrothermal synergistic curing molding: The composite precursor powder obtained from S3, low-calcium sulfoaluminate cement, and nano-metakaolin were mixed uniformly at a mass ratio of 45:100:5 to obtain a gel-based composite dry powder. A water glass solution with a modulus of 1.2 was prepared as an alkali activator and mixed with deionized water at a volume ratio of 1:2 to obtain an alkali-activated mixture. The alkali-activated mixture was mixed with the gel-based composite dry powder at a water-cement ratio of 0.3 and stirred in a cement paste mixer at 115 r / min for 90 seconds, and then stirred at 275 r / min for 150 seconds to obtain a uniform slurry. The slurry was injected into a mold and compacted on a vibrating table with a vibration frequency of 50 Hz and an amplitude of 0.5 mm for 60 seconds. After sealing, it was steam-cured at 75℃ and 95% relative humidity for 16 hours. After demolding, the initial set body was obtained. S4-2, Gradient Pressure Carbonization and Crystal Form Regulation: The initial solidified body obtained from S4-1 was subjected to gradient carbonization. After carbonization, it was cooled to room temperature at a rate of 1℃ / min under nitrogen protection at 0.1 MPa. After curing, fly ash building material heavy metal migration inhibition composite substrate was obtained.

[0044] Example 17: This example is basically the same as Example 1, except that it includes the following steps: S1. Pretreatment and dechlorination of fly ash from waste incineration: S1-1, Multi-stage carbonation pretreatment and stabilization: Fly ash from municipal solid waste incineration with a moisture content of 20% was spread evenly in a controlled atmosphere treatment chamber with a layer thickness of 8 cm. Air with a CO2 concentration of 30% v / v and a relative humidity of 85% was introduced into the treatment chamber at a gas exchange rate of 1.2 chamber volume / hour, and the chamber was treated at 30°C for 36 hours. Then, the CO2 concentration was increased to 70%, the gas exchange rate was adjusted to 0.8 chamber volume / hour, and the temperature was maintained at 35°C for 24 hours to obtain carbonated fly ash. S1-2, Ultrasonic-assisted multi-stage countercurrent water washing and dechlorination: The carbonated fly ash obtained from S1-1 was subjected to a three-stage countercurrent washing process. The first stage involved mixing the fly ash with a citrate buffer solution at pH 5 at a liquid-to-solid ratio of 3:1 mL / g and washing under ultrasonic conditions for 20 minutes, followed by solid-liquid separation. The second and third stages involved washing with deionized water at liquid-to-solid ratios of 2:1 and 1:1 mL / g, respectively, under the same ultrasonic conditions for 15 minutes each. After the final washing, the solid filter cake was collected and dried to obtain dechlorinated fly ash. S2. Preparation of thiol-functionalized composite stabilizers: S2-1, Zeolite screening and biochar composite: Take clinoptilolite, crush it, and pass it through a 400-mesh sieve to obtain zeolite particles; pyrolyze rice husks at 700℃ under limited oxygen conditions for 3 hours to obtain biochar, and mix it with the zeolite particles at a mass ratio of 1:2 to obtain a zeolite-biochar composite carrier. S2-2, In-situ grafting and chelation functionalization of mercaptosilane coupling agents: The zeolite biochar composite carrier described in S2-1 was dispersed in an ethanol-water solution to prepare a suspension with a mass concentration of 12 wt%; the volume ratio of ethanol to water in the ethanol-water solution was 95:5; then, sodium polyacrylate, accounting for 4% of the mass of the zeolite biochar composite carrier, was added and ultrasonically dispersed for 30 minutes; under continuous stirring, mercaptopropyltrimethoxysilane, accounting for 6% of the mass of the zeolite biochar composite carrier, was added dropwise, and the pH of the reaction system was adjusted to 9.5 with ammonia water, and the reaction was refluxed at 75°C for 8 hours; after the reaction was completed, 2,5-dimercapto-1,3,4-thiadiazole, accounting for 8% of the mass of the zeolite biochar composite carrier, was added, and the reaction was continued for 3 hours; after centrifugation, washing with ethanol, and drying, a mercapto-functionalized composite stabilizer was obtained. S3. Constructing an activated composite precursor: The dechlorinated fly ash obtained from S1-2, the thiol-functionalized composite stabilizer obtained from S2-2, silica fume, and nano-perovskite catalyst were weighed in a mass ratio of 100:15:20:1.5 and placed in a planetary ball mill jar. Zirconia grinding balls were used with a ball-to-material ratio of 8:1. Under argon protection, the mixture was mechanically activated at a speed of 550 r / min for 120 minutes to obtain the composite precursor powder. S4, Medium-temperature hydrothermal curing and gradient carbonization crystal form control: S4-1, Alkali activation-hydrothermal synergistic curing molding: The composite precursor powder obtained from S3, low-calcium sulfoaluminate cement, and nano-metakaolin were mixed uniformly at a mass ratio of 55:100:10 to obtain a gel-based composite dry powder. A water glass solution with a modulus of 1.5 was prepared as an alkali activator and mixed with deionized water at a volume ratio of 1:3 to obtain an alkali-activated mixture. The alkali-activated mixture was mixed with the gel-based composite dry powder at a water-cement ratio of 0.35 and stirred in a cement paste mixer at 125 r / min for 90 seconds, and then stirred at 295 r / min for 150 seconds to obtain a uniform slurry. The slurry was injected into a mold and compacted on a vibrating table with a vibration frequency of 50 Hz and an amplitude of 0.5 mm for 60 seconds. After sealing, it was steam-cured at 85℃ and a relative humidity of 99.9% for 24 hours. After demolding, the initial set body was obtained. S4-2, Gradient Pressure Carbonization and Crystal Form Regulation: The initial solidified body obtained by S4-1 was subjected to gradient carbonization. After carbonization, it was cooled to room temperature at a rate of 2℃ / min under nitrogen protection at 0.1 MPa. After curing, fly ash building material heavy metal migration inhibition composite substrate was obtained.

[0045] Comparative Example 1: Referring to Example 1, the difference is that in S1, raw fly ash with a moisture content of 15% is used directly without carbonation pretreatment; in step S2, an unmodified zeolite-biochar physical mixture with a zeolite to biochar mass ratio of 1:1.5 is used to replace the thiol-functionalized composite stabilizer.

[0046] Comparative Example 2: Referring to Example 1, the difference is that the gradient carbonization in step S4-2 is cancelled and replaced with a single carbonization condition: CO2 pressure 0.4 MPa, temperature 35°C, relative humidity 75%, for 25 hours.

[0047] Comparative Example 3: Referring to Example 1, the difference is that the ball milling activation is omitted in step S3. Instead, the components, namely dechlorinated fly ash, silica fume, mercapto-functionalized composite stabilizer, silica fume and nano-perovskite catalyst, are simply mixed for 10 minutes without ball milling under argon protection.

[0048] To investigate the performance of the fly ash composite substrates in the above embodiments and control examples, the main materials were determined according to the experimental formula, and samples were obtained for testing. The various properties were characterized according to relevant national standard testing methods: the 28-day compressive strength was determined according to GB / T 17671 "Test Method for Strength of Cement Mortar (ISO Method)"; the Pb and Cd leaching concentrations were prepared according to HJ557-2010 "Leaching Toxicity of Solid Waste - Leaching Method - Horizontal Oscillation Method", and the leachate was determined using inductively coupled plasma mass spectrometry (ICP-MS); the soluble chloride ion content was determined according to GB / T 8077 "Test Method for Homogeneity of Concrete Admixtures". The results are shown in Table 1. The specific investigation is as follows: Table 1 Performance test results of fly ash composite substrate samples from Examples 1-17 and Comparative Examples 1-3

[0049] 1. Investigating the influence of dechlorination pretreatment process parameters on the properties of fly ash composite substrates. A comparison of Examples 1-5 shows that the sample prepared using the parameters of Example 1 exhibits the highest 28-day compressive strength and the lowest lead and cadmium leaching concentrations, demonstrating superior overall performance. In Examples 2 and 3, the ultrasonic power density decreased and increased, respectively, resulting in a decrease in compressive strength and a corresponding increase in heavy metal leaching concentrations. This indicates that altering the ultrasonic washing intensity parameters has a clear impact on the dechlorination effect and the final substrate performance. In Examples 4 and 5, the drying temperatures were lower and higher than those of Example 1, respectively, leading to a gradual increase in chloride ion content in the resulting fly ash. Correspondingly, the compressive strength of the sample in Example 4 was superior to that in Example 5, indicating that the drying temperature must be controlled within a suitable range to achieve efficient dechlorination without damaging the fly ash activity. Compared to Control Example 1, the performance of all examples is significantly better than that of the sample without carbonation pretreatment and water washing dechlorination. This confirms the crucial role of deep dechlorination pretreatment in ensuring the smooth progress of subsequent gelation reactions and improving the structural stability and environmental safety of the substrate.

[0050] 2. Investigate the influence of composite stabilizer preparation process parameters on the properties of fly ash composite substrates. A comparison of Examples 1, 8, 9, 10, and 11 shows that the sample prepared according to the method in Example 1 exhibits the best overall performance. Examples 8 and 9 adjusted the ball milling speed; the stabilizer prepared in Example 9 at a higher speed resulted in a substrate with higher compressive strength and better heavy metal fixation, indicating that appropriate mechanochemical activation energy enhances the carrier composite effect. Examples 10 and 11 changed the drying temperature and time of the stabilizer, as well as the centrifugation conditions. The performance of the sample in Example 11 was slightly better than that in Example 10, indicating that thorough drying and washing are beneficial for obtaining pure, highly active functionalized stabilizers. All examples showed significantly better heavy metal blocking ability than Control Example 1, which used an unmodified physically mixed carrier. This highlights the indispensability of the step of chemical grafting via silane coupling agents to construct multiple fixation sites.

[0051] 3. Investigating the effects of material ratio and nano-additives on the properties of fly ash composite substrates. A comparison of Examples 1, 12, 13, 16, and 17 shows that Example 1 exhibits the best overall performance. Examples 12 and 13 adjusted parameters such as the specific surface area of ​​silica fume, the particle size and dosage of the nanocatalyst and metakaolin. Example 13, by using nanomaterials with a higher specific surface area, achieved superior strength and lower leaching toxicity compared to Example 12, demonstrating that fine-tuning the properties of nanomaterials can optimize the microstructure. Examples 16 and 17 employed extreme ratio parameters at both ends of the technical solution range, resulting in a significant decrease in compressive strength and an increase in chloride ion content and heavy metal leaching concentration. This indicates that the ratio of key components must be maintained within a reasonable range; excessively high or low ratios are detrimental to performance. Compared to Control Example 3, Examples 1, 12, and 13 show a significant advantage in compressive strength, proving that mechanochemical activation is crucial for achieving uniform dispersion and tight bonding of the components.

[0052] 4. Investigate the effects of curing and carbonization process parameters on the properties of fly ash composite substrates. A comparison of Examples 1, 14, and 15 shows that Example 15, by employing higher gradient carbonization pressure, temperature, and humidity parameters, achieved the highest compressive strength and the lowest heavy metal leaching concentration among all samples, demonstrating the best overall performance. Example 14, using milder carbonization conditions, showed slightly lower performance than Examples 1 and 15, indicating that the intensity of the carbonization process directly affects the matrix density and the degree of heavy metal stabilization. Compared to Control Example 2, which underwent only single-stage carbonization, Examples 1, 14, and 15, regardless of the gradient parameters used, all exhibited significantly lower heavy metal leaching concentrations than Control Example 2. This confirms the significant superiority of the gradient carbonization strategy in promoting the depth of the carbonization reaction and achieving long-term heavy metal stabilization. Employing a complete and optimized curing and carbonization process is crucial for enabling fly ash composite substrates to simultaneously achieve excellent mechanical properties and superior environmental safety.

Claims

1. A method for preparing a fly ash building material heavy metal migration-inhibiting composite substrate, characterized in that, Includes the following steps: S1. Pretreatment and dechlorination of fly ash from waste incineration: S1-1. The fly ash from municipal solid waste incineration is subjected to multi-stage carbonation pretreatment and stabilization to obtain carbonated fly ash. S1-2. The carbonated fly ash obtained in S1-1 is subjected to three-stage countercurrent washing to obtain dechlorinated fly ash; S2. Preparation of thiol-functionalized composite stabilizers: S2-1, Zeolite screening and biochar composite: Take clinoptilolite, crush it, and pass it through a 200-400 mesh sieve to obtain zeolite particles; pyrolyze rice husks at 600-700℃ under oxygen-limited conditions for 2-3 hours to obtain biochar, and mix it with the zeolite particles at a mass ratio of 1:1-2 to obtain a zeolite-biochar composite carrier. S2-2, In-situ grafting and chelation functionalization of mercaptosilane coupling agents: The zeolite biochar composite carrier described in S2-1 was dispersed in an ethanol-water solution to prepare a suspension with a mass concentration of 8-12 wt%; the volume ratio of ethanol to water in the ethanol-water solution was 95:

5. Then, sodium polyacrylate, accounting for 2-4% of the mass of the zeolite biochar composite carrier, was added and ultrasonically dispersed for 20-30 minutes. Under continuous stirring, mercaptopropyltrimethoxysilane, accounting for 3-6% of the mass of the zeolite biochar composite carrier, was added dropwise. The pH of the reaction system was adjusted to 8.5-9.5 with ammonia water, and the reaction was refluxed at 65-75℃ for 6-8 hours. After the reaction was completed, 2,5-dimercapto-1,3,4-thiadiazole, accounting for 5-8% of the mass of the zeolite biochar composite carrier, was added, and the reaction was continued for 2-3 hours. After centrifugation, washing with ethanol, and drying, a mercapto-functionalized composite stabilizer was obtained. S3. Constructing an activated composite precursor: Weigh the dechlorinated fly ash obtained from S1-2, the thiol-functionalized composite stabilizer obtained from S2-2, silica fume, and nano-perovskite catalyst in a mass ratio of 100:10-15:10-20:0.5-1.5 and place them in a planetary ball mill jar. Use zirconia grinding balls with a ball-to-material ratio of 5-8:1 and perform mechanochemical activation at a speed of 450-550 r / min for 90-120 minutes under argon protection to obtain composite precursor powder. S4, Medium-temperature hydrothermal curing and gradient carbonization crystal form control: S4-1, Alkali activation-hydrothermal synergistic curing molding: The composite precursor powder obtained from S3, low-calcium sulfoaluminate cement, and nano-metakaolin are mixed evenly at a mass ratio of 45-55:100:5-10 to obtain a gel-based composite dry powder. A water glass solution with a modulus of 1.2-1.5 is prepared as an alkali activator and mixed with deionized water at a volume ratio of 1:2-3 to obtain an alkali-activated mixture. The alkali-activated mixture is mixed with the gel-based composite dry powder at a water-cement ratio of 0.3-0.35 and stirred in a cement paste mixer at 115-125 r / min for 90 seconds, and then stirred at 275-295 r / min for 150 seconds to obtain a uniform slurry. The slurry is injected into a mold and compacted on a vibrating table with a vibration frequency of 50 Hz and an amplitude of 0.5 mm for 60 seconds. After sealing, it is steam-cured at 75-85℃ and a relative humidity of 95-99.9% for 16-24 hours. After demolding, the initial set body is obtained. S4-2, Gradient Pressure Carbonization and Crystal Form Regulation: The initial solidified body obtained by S4-1 was subjected to gradient carbonization. After carbonization, it was cooled to room temperature at a rate of 1-2℃ / min under nitrogen protection at 0.1 MPa. After curing, fly ash building material heavy metal migration inhibition composite substrate was obtained.

2. The method for preparing a fly ash building material heavy metal migration-inhibiting composite substrate according to claim 1, characterized in that, The multi-stage carbonation pretreatment and stabilization method described in S1-1 is as follows: Municipal solid waste incineration fly ash with a moisture content of 10-20% is spread evenly in a controlled atmosphere treatment chamber, with a layer thickness of 3-8 cm; air with a CO2 concentration of 20-30% v / v and a relative humidity of 75-85% is introduced into the treatment chamber at a gas exchange rate of 0.8-1.2 chamber volume / hour, and the chamber is treated at 25-30℃ for 24-36 hours; then, the CO2 concentration is increased to 50-70%, the gas exchange rate is adjusted to 0.5-0.8 chamber volume / hour, the temperature is maintained at 30-35℃, and the chamber is treated for 12-24 hours to obtain carbonated fly ash. The three-stage countercurrent washing method described in S1-2 is as follows: First stage: using citrate buffer with a pH of 4-5, mixed with fly ash at a liquid-to-solid ratio of 3 mL:1 g, and washed under ultrasonic conditions for 15-20 minutes, followed by solid-liquid separation; Second and third stages: using deionized water with liquid-to-solid ratios of 2 mL:1 g and 1 mL:1 g respectively, and washed for 10-15 minutes each under the same ultrasonic conditions; after the final washing is completed, the solid filter cake is collected and dried; the ultrasonic conditions are a frequency of 40 kHz and a power density of 0.4-0.6 W / mL.

3. The method for preparing a fly ash building material heavy metal migration-inhibiting composite substrate according to claim 2, characterized in that, The drying method described in S1-2 is as follows: after collecting the solid filter cake, vacuum dry it at 85-95℃ until the moisture content is less than 1% to obtain dechlorination fly ash, wherein the mass content of soluble chloride ions in the dechlorination fly ash is less than 0.8%.

4. The method for preparing a fly ash building material heavy metal migration-inhibiting composite substrate according to claim 1, characterized in that, The concentration of the citrate buffer solution in S1-2 is 0.05-0.1 mol / L, and the solvent is water; the pressure of the solid-liquid separation is 0.3-0.6 MPa.

5. The method for preparing a fly ash building material heavy metal migration-inhibiting composite substrate according to claim 1, characterized in that, The mixing method described in S2-1 is as follows: the biochar and zeolite particles are introduced into a planetary ball mill and mixed at a speed of 350-450 r / min for 30-45 minutes.

6. The method for preparing a fly ash building material heavy metal migration-inhibiting composite substrate according to claim 1, characterized in that, The drying described in S2-2 is carried out under vacuum, at a temperature of 75-85℃, for 8-12 hours; the centrifugation speed is 8000-10000 r / min, and the ethanol is used for washing 2-3 times until the washing solution is neutral.

7. The method for preparing a fly ash building material heavy metal migration-inhibiting composite substrate according to claim 1, characterized in that, The specific surface area of ​​the silica fume described in S3 is 15-25 m². 2 / g; the particle size of the nano-perovskite catalyst is 50-100nm.

8. The method for preparing a fly ash building material heavy metal migration-inhibiting composite substrate according to claim 1, characterized in that, The water glass solution in S4-1 has a solid content of 30-35% by mass; the nano-kaolin has a specific surface area of ​​800-1200 m². 2 / g, with a particle size of 50-200 nm.

9. The method for preparing a fly ash building material heavy metal migration-inhibiting composite substrate according to claim 1, characterized in that, The gradient carbonization method described in S4-2 is as follows: In the first stage, carbonization is carried out for 8-12 hours under the conditions of CO2 pressure of 0.3-0.5 MPa, temperature of 30-40℃, and relative humidity of 70-80%; in the second stage, the CO2 pressure is increased to 0.8-1.2 MPa, the temperature is maintained at 40-50℃, and the relative humidity is 85-95%, and carbonization is carried out for 12-18 hours.

10. The method for preparing a fly ash building material heavy metal migration-inhibiting composite substrate according to claim 1, characterized in that, The curing temperature described in S4-2 is 19-21℃, the relative humidity is 95-99.9%, and the curing period is up to 28 days.

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