A phosphogypsum skeleton material package and a preparation method thereof

Abstract

The application belongs to the technical field of building materials, and particularly relates to a phosphogypsum skeleton material package and a preparation method thereof. The package comprises an internal high-strength porous phosphogypsum skeleton material and an external functional wrapping layer. The skeleton material is formed by pressing specific graded phosphogypsum, inorganic activator and organic fiber, has a porosity of 35-42%, and a 28d compressive strength of 8.5-12.0 MPa. The wrapping layer adopts a cement or alpha gypsum slurry to form a 1.5-2.5 mm thick dense barrier through an immersion-ink coating process, and has physical barrier and chemical adsorption functions. The application significantly reduces the leaching rates of soluble phosphorus, fluoride and heavy metals in the phosphogypsum, improves the mechanical properties and durability, and realizes high-value utilization of the phosphogypsum.

Description

Technical Field

[0001] This invention relates to the field of building materials technology, and in particular to a phosphogypsum skeleton material inclusion body and its preparation method. Background Technology

[0002] Phosphogypsum, an unavoidable byproduct of wet-process phosphoric acid and ammonium phosphate fertilizer production, generates a massive annual discharge, with approximately four to five tons of phosphogypsum produced for every ton of phosphoric acid produced. This solid waste is primarily composed of calcium sulfate dihydrate, and also contains soluble phosphorus, fluorides, and heavy metal impurities such as lead and cadmium. If left untreated and stored for extended periods, it not only occupies significant land resources and increases seepage prevention and maintenance costs, but also poses an environmental risk of harmful substances migrating into soil and water bodies due to rainwater leaching, severely hindering the green and sustainable development of the phosphate chemical industry. Against this backdrop, promoting the coordinated utilization of phosphogypsum through reduction, harmlessness, and resource recovery has become an important direction for solid waste management.

[0003] To achieve resource utilization, existing technologies often attempt to directly use phosphogypsum in the preparation of building materials such as wall bricks and roadbed fillers. However, unmodified phosphogypsum, due to its loose crystal structure and impurities interfering with the hydration reaction, results in products with poor mechanical properties, poor volume stability, and susceptibility to cracking. Furthermore, harmful components are continuously released into the natural environment, making it difficult to meet the safety and durability requirements of building materials. Therefore, some research has shifted to using encapsulation methods to modify the surface of phosphogypsum, inhibiting the migration of harmful substances through physical barriers or chemical fixation mechanisms. Typical solutions include single cement slurry, lime slurry, or polymer coating. While these reduce leaching toxicity to some extent, their technical approaches suffer from deep-seated inherent contradictions: On the one hand, single coating materials often rely solely on physical barriers, lacking effective response to leaching rebound caused by increased ion activity in acidic environments (such as pH=4.0). Especially during long-term service, microcrack propagation or interface delamination can easily lead to coating failure. On the other hand, the introduction of high-cost additives or complex processes (such as vacuum impregnation and high-temperature sintering) to improve coating density significantly increases unit processing costs, hindering large-scale engineering applications. More importantly, existing coating systems generally neglect the synergistic reinforcement mechanism between the skeleton material and the coating layer—if the skeleton material itself has high porosity and low strength, even if the coating layer is effective in the short term, it is difficult to support the long-term mechanical stability of the overall structure under complex conditions such as freeze-thaw cycles and wet-dry cycles.

[0004] Ultimately, the aforementioned deficiencies stem from a disconnect in the design philosophy of existing technologies: either treating phosphogypsum as a passive filler, focusing only on surface coverage; or unilaterally pursuing the density of the coating layer while neglecting the optimization of the skeleton structure, failing to construct an integrated functional system that is "strong internally and stable externally." In fact, the resource utilization of phosphogypsum is not simply about solving the question of "whether it can be used," but rather about achieving a multi-dimensional balance of mechanical properties, long-term stability, and economic feasibility while ensuring environmental safety. Achieving this goal urgently requires starting with the microstructure control of the material, inducing crystal rearrangement through activators to form a high-strength skeleton, and combining this with a coating layer possessing chemical adsorption capabilities to construct a dual barrier of "physical barrier-chemical fixation." However, how to scientifically design the gradation and activation system of the skeleton material to achieve dense packing, how to match the rheological properties of the coating slurry with the impregnation process to ensure uniform film formation, and how to balance fixation efficiency, mechanical contribution, and cost-effectiveness among different coating materials (such as cement-based and α-gypsum-based) all involve complex multiphase interfacial reactions and cross-scale structural coupling problems, which cannot be solved by simply superimposing existing technologies.

[0005] Therefore, how to develop a preparation method based on the synergistic effect of phosphogypsum skeleton structure reinforcement and functional encapsulation, which can not only lay a stable substrate by optimizing the pore structure and mechanical properties of the skeleton material, but also efficiently fix harmful components such as phosphorus, fluorine and heavy metals by means of chemical adsorption and dense network of the encapsulation layer, while taking into account the simplicity of the process and controllability of cost, has become a key challenge and a technical problem that needs to be solved by those skilled in the art. Summary of the Invention

[0006] This invention provides a phosphogypsum skeleton material inclusion and its preparation method, aiming to solve the technical problems in the existing phosphogypsum resource utilization process, such as insufficient mechanical properties, weak environmental stability, and limited engineering applications caused by the loose skeleton structure, easy migration of harmful components, and poor synergy between the inclusion layer and the matrix. To achieve the above-mentioned objective, this invention constructs an integrated functional system of "internal strength and external stability," organically combining the densification and strengthening of the phosphogypsum skeleton material with the chemical fixation ability of the functional inclusion layer, forming a composite material structure with high mechanical strength, low leaching rate of harmful components, and good long-term service stability.

[0007] The phosphogypsum skeleton material encapsulation consists of an inner phosphogypsum skeleton material and an outer encapsulation layer. The phosphogypsum skeleton material is based on phosphogypsum particles with a specific gradation, supplemented with inorganic activators and organic reinforcing fibers. It is pressed and molded under limited water-cement ratio and molding pressure conditions, and then pre-cured to obtain a porosity of 35–42% and a bulk density of 1.45–1.60 g / cm³. 3A high-strength porous skeleton with a 28-day compressive strength of 8.5–12.0 MPa; the outer coating layer is made of cement slurry or α-gypsum slurry, which is uniformly coated on the surface of the skeleton material through an impregnation-coating composite process to form a functional barrier layer with a thickness of 1.5–2.5 mm and a porosity of ≤15%. This coating layer has both physical barrier and chemical adsorption mechanisms, which can effectively inhibit the migration and release of harmful components such as phosphorus, fluorine and heavy metals under different pH environments.

[0008] Specifically, the preparation of the phosphogypsum skeleton material includes the following steps: taking a byproduct from the wet-process phosphoric acid and phosphate fertilizer production process as primary phosphogypsum raw material, whose main component is calcium sulfate dihydrate (CaSO4·2H2O) with a content of not less than 85%, and impurity components including 1.2–2.5% P2O5 and 0.5–1.0% F. - The phosphogypsum contains 30–50 mg / kg of Pb and 2–5 mg / kg of Cd. After crushing the phosphogypsum using a jaw crusher, the particles are graded according to size: 25-35% are d≤1 mm, 35-45% are 1 < d≤3 mm, and 25-35% are 3 < d≤5 mm. Then, 100 parts by weight of phosphogypsum, 3-5 parts of activator Na2SO4, 0.6-1.0 parts of polypropylene fiber, and 3.0-4.0 parts of water are weighed. The phosphogypsum and Na2SO4 are dry-mixed for 1.5-2.5 minutes until homogeneous. Then, polypropylene fiber is added and stirred for 1 minute. Finally, water is added and wet-mixed for 2.5-3.5 minutes to form a homogeneous agglomerate. The agglomerate is placed in a four-column hydraulic press and pressed into shape under a pressure of 15-20 MPa, a holding time of 25-35 seconds, and an ambient temperature of 25±2℃. The molded size is 100 mm × 100 mm × 100 mm. mm; after molding, immediately transfer to an oven at 55-65℃ for pre-curing for 1.5-2.5 hours, and then allow to cool naturally to room temperature to obtain the phosphogypsum skeleton material.

[0009] The activator Na2SO4 weakens the electric double layer on the surface of calcium sulfate dihydrate crystals through ion exchange, reducing the zeta potential from -12 mV to -8 mV and increasing the interplanar spacing from 0.756 nm to 0.762 nm, thereby promoting crystal rearrangement along specific crystal orientations and achieving dense packing. The polypropylene fiber has a length of 5-7 mm, a diameter of 15-25 μm, and a tensile strength of not less than 500 MPa. It forms a three-dimensional bridging network inside the skeleton material, effectively inhibiting the initiation and propagation of microcracks and improving the flexural strength by 18%.

[0010] The preparation and application process of the coating layer is as follows: First, the above-mentioned phosphogypsum skeleton material is pre-dried at 100-110℃ for 1.5-2.5 hours to ensure that the moisture content does not exceed 2%; then, cement slurry or α-gypsum slurry is selected as the coating medium according to the target application scenario, and the coating is carried out by impregnation-coating composite process.

[0011] When using cement slurry, its formula by weight is: 100 parts P·O 42.5R cement, 45-55 parts medium sand (fineness modulus 2.3–2.6), 0.2-0.4 parts naphthalene-based water-reducing agent NF-1, and 40-45 parts water. During preparation, first dry mix the cement and medium sand for 2 minutes, then add the pre-prepared solution of water-reducing agent and some water, and continue stirring for 4 minutes, controlling the slurry viscosity to 550±50 mPa·s (measured at 25℃ using an NDJ-5S rotational viscometer). Completely immerse the pre-dried skeleton material in the cement slurry, ensuring the liquid level is 4-6 cm above the top of the specimen, while continuously stirring at 25-35 r / min for 30–60 seconds. After immersion, vertically pull out the material at a uniform speed of 4-6 cm / min to avoid dripping. Immediately afterwards, perform a top-up application, setting the nozzle pressure to 0.15-0.25. MPa, nozzle diameter 1.2-1.8 mm, spray distance 8-12 cm, spray duration 8-12 seconds; after spraying, place in a standard curing environment, temperature 20±2℃, relative humidity ≥95%, curing period 25-30 days, finally forming a cement-based coating layer with a thickness of 1.5–2.0 mm.

[0012] When using α-gypsum slurry, its formula by weight is: 100 parts of building-grade α-gypsum, 0.15-0.25 parts of citric acid retarder, and 65-70 parts of water. During preparation, first dissolve the citric acid in all the water, then mix it with the α-gypsum and stir for 3 minutes, controlling the initial setting time to 45±5 minutes. Immerse the pre-dried skeleton material in the α-gypsum slurry, ensuring the liquid level is 4-6 cm above the specimen, with a stirring speed of 25-35 r / min and an immersion time of 60–90 seconds. The lifting speed is also 4-6 cm / min. The coating parameters are consistent with the cement system. After coating, place the specimen in a natural curing environment at a temperature of 25±5℃ and a relative humidity ≥60% for 5-9 days to form an α-gypsum coating layer with a thickness of 2.0–2.5 mm.

[0013] During the curing process, the cement coating generates a large amount of CSH gel (Ca / Si molar ratio 1.8–2.0) and Ca(OH)2 phase, which has a dense microstructure, a total porosity of 18–22%, a most probable pore size of 20–50 nm, and an average pore size ≤100 nm. The α-gypsum coating exhibits a columnar interlocking structure of calcium sulfate hemihydrate (CaSO4·0.5H2O) crystals with an aspect ratio of 3–5:1, a total porosity of 25–28%, and a most probable pore size of 50–100 nm.

[0014] The control mechanism of the inclusion body on harmful components is clearly defined: in the cement encapsulation system, F - With Ca 2+ The reaction produces CaF2 precipitate (Ksp = 3.4 × 10⁻⁶). -11 ), PO4 3- With Ca 2+ It combines to form Ca3(PO4)2 precipitate (Ksp=2.0×10). -29 Meanwhile, CSH gel undergoes coordination adsorption of heavy metal ions through surface hydroxyl groups, and Langmuir model fitting indicates its affinity for F... - and PO4 3- The maximum adsorption capacities were 35.2 mg / g and 28.6 mg / g, respectively; in the α-gypsum encapsulation system, F - By interacting with SO4 on the crystal surface 2- PO4 is fixed through ion exchange. 3- They are then encapsulated in the intercrystalline spaces, and the Freundlich model fitted adsorption capacities of 28.8 mg / g and 22.3 mg / g, respectively.

[0015] According to the HJ 557-2010 horizontal oscillation method test, under the conditions of a liquid-to-solid ratio of 10:1, oscillation for 8 hours, and 25±2℃, the PO4 without a skeleton material was tested. 3- The leaching concentration was 18.3±0.9 mg / L, which decreased to 0.92±0.06 mg / L after encapsulation with α-gypsum slurry. This value was a stable leaching result obtained by actual measurement using the horizontal oscillation method according to HJ 557-2010. The Pb concentration of the phosphogypsum skeleton inclusions in the leachate at pH=7.0 was not higher than 0.05 mg / L, the Cd concentration was not higher than 0.008 mg / L, and the F concentration was not higher than 0.008 mg / L. - The concentrations were no higher than 1.85 mg / L, all significantly lower than the leaching toxicity limits specified in GB 5085.3-2007. Under acidic conditions of pH=4.0, the Pb leaching amount of cement inclusions increased to 0.05±0.01 mg / L, still meeting the safety requirements, while the Pb leaching amount of α-gypsum inclusions was 0.08±0.02 mg / L, indicating that the cement system has stronger resistance to acid interference.

[0016] Mechanical property tests of the inclusions showed that the 28-day compressive strength of the cement inclusion was 22.5 ± 1.0 MPa, and the dynamic elastic modulus was 28–30 GPa; the 28-day compressive strength of the α-gypsum inclusion was 18.3 ± 0.8 MPa, and the dynamic elastic modulus was 20–22 GPa. After 50 freeze-thaw cycles (-20℃ to 20℃), the strength loss rate of the cement inclusion was 12.0%, and that of the α-gypsum inclusion was 16.9%. After 90 days of immersion in tap water, the strength loss rates were 3.1% and 6.0%, respectively, indicating that both inclusions have good environmental durability, with the cement system exhibiting superior overall performance.

[0017] The preparation method has a defined process flow that can be industrially implemented: after phosphogypsum is crushed and graded, it is mixed with activator, fiber and water in sequence, pressed into shape and pre-cured to obtain skeleton material; after the skeleton material is dried, it is immersed in the corresponding coating slurry, and the immersion time and stirring rate are controlled to ensure uniform coating; the lifting and spraying processes are precisely matched with rheological parameters to avoid uneven coating thickness or local defects; the curing regime strictly distinguishes the hydration / crystallization kinetics requirements of cement and α-gypsum systems to ensure full development of the coating layer.

[0018] The phosphogypsum skeleton material encapsulation of this invention can absorb 0.85 tons of phosphogypsum per unit product, reducing the footprint by 0.8 m² compared to traditional stockpiling. 3 Each ton of product can replace conventional building materials, reducing CO2 emissions by 0.3–0.5 tons. Economic calculations show that with an annual production scale of 100,000 tons, the total cost of cement-encapsulated materials is 340 yuan / ton, and that of α-gypsum-encapsulated materials is 300 yuan / ton. The selling prices of the products are 550–600 yuan / ton and 500–550 yuan / ton, respectively, demonstrating significant economic benefits and promotional value.

[0019] In summary, this invention achieves a synergistic coupling of skeleton body strengthening and coating layer functionalization by scientifically designing the gradation structure, activation system and reinforcing phase of phosphogypsum skeleton material, and matching the rheological properties, impregnation process and curing regime of the coating slurry. It solves the three core contradictions in the prior art of "rapid failure of coating layer, low skeleton strength and weak controlled release of harmful substances", and provides a reliable technical path for the large-scale high-value utilization of phosphogypsum. Attached Figure Description

[0020] Figure 1 This is a schematic diagram of the preparation process of the phosphogypsum skeleton material inclusion body of the present invention.

[0021] Figure 2 This is a scanning electron microscope image of the phosphogypsum skeleton material inclusions in an embodiment of the present invention.

[0022] Figure 3 This is an X-ray energy spectrum of the phosphogypsum skeleton material inclusions in an embodiment of the present invention. Detailed Implementation

[0023] The present invention will be further described below with reference to the accompanying drawings and embodiments, but this does not limit the scope of the invention; it is merely illustrative. It should be noted that, unless otherwise specified, the embodiments and technical features described in this application can be combined with each other. Unless otherwise indicated, all technical and scientific terms used in this application have the same meaning as commonly understood by one of ordinary skill in the art to which this application pertains. Unless otherwise specified, the experimental methods used in the following embodiments are conventional methods. Unless otherwise specified, the materials, reagents, etc., used in the following embodiments are commercially available.

[0024] This invention provides a phosphogypsum skeleton inclusion and its preparation method, the preparation process flow diagram is shown below. Figure 1 As shown, by constructing a composite structure consisting of an internal phosphogypsum skeleton and an external functional coating layer, the high-value and safe utilization of phosphogypsum resources is achieved. The following will systematically describe, with reference to specific embodiments, the composition, preparation process parameters, microstructural characteristics, harmful component control mechanism, and mechanical and environmental properties of the phosphogypsum skeleton coating.

[0025] First, the phosphogypsum skeleton material is prepared from the by-products of wet-process phosphoric acid and phosphate fertilizer production as primary phosphogypsum raw material. The main chemical component of the primary phosphogypsum is calcium sulfate dihydrate (CaSO4·2H2O), with a content of not less than 85%; impurities include 1.2–2.5% phosphorus pentoxide (P2O5) and fluoride ions (F...). - The raw material contains 0.5–1.0% of lead (Pb) and 30–50 mg / kg and 2–5 mg / kg of cadmium (Cd). After being crushed by a jaw crusher, the raw material is classified into three grades according to particle size: 25-35% of particles with a diameter d ≤ 1 mm, 35-45% of particles with a diameter d < 3 mm, and 25-35% of particles with a diameter d < 5 mm. This specific gradation design aims to optimize particle bulk density, reduce macroscopic porosity during the forming process, and retain necessary interconnecting channels to ensure effective penetration and interfacial bonding of the subsequent coating layer.

[0026] In the formulation of the skeleton material, by weight, 100 parts of the graded phosphogypsum, 3-5 parts of the inorganic activator sodium sulfate (Na2SO4), 0.6-1.0 parts of polypropylene fiber, and 3.0-4.0 parts of water are taken. The mixing process is carried out sequentially: first, the phosphogypsum and Na2SO4 are dry-mixed in a planetary mixer for 1.5-2.5 minutes to ensure that the activator is evenly dispersed on the surface of the phosphogypsum particles; then, the polypropylene fiber is added and the mixing continues for 1 minute to initially disperse the fiber in the dry material; finally, all the metered water is added and wet-mixed for 2.5-3.5 minutes to form a uniform, non-clumped plastic mass. The physical parameters of the polypropylene fiber are: length 5-7 mm, diameter 15-25 μm, and tensile strength not less than 500 MPa. After compression molding, the fiber forms a three-dimensional bridging network inside the skeleton, effectively inhibiting the initiation and propagation of microcracks and improving the flexural toughness of the material.

[0027] The material was then transferred to a four-column hydraulic press mold, and a molding pressure of 15-20 MPa was applied at an ambient temperature of 25±2℃ for 25-35 seconds to press out cubic specimens with dimensions of 100 mm × 100 mm × 100 mm. After molding, the specimens were immediately transferred to a 55-65℃ forced-air oven for pre-curing for 1.5-2.5 hours. This pre-curing process promotes the initial dehydration and recrystallization of phosphogypsum particles under heat, while the activator Na2SO4 weakens the electric double layer effect on the surface of calcium sulfate dihydrate crystals through ion exchange: Na + Ion-substituted Ca on the crystal surface 2+ This reduces the zeta potential from -12 mV to -8 mV and expands the interplanar spacing from 0.756 nm to 0.762 nm, decreasing the electrostatic repulsion between crystals to promote slip rearrangement under pressure. The scanning electron microscope image of the phosphogypsum framework inclusions is shown below. Figure 2 As shown, simultaneous XRD analysis revealed a decrease in the full width at half maximum (FWHM) of the (020) crystal plane diffraction peak, indicating that the crystals preferentially grow along the

[010] direction, forming a more compact packing structure. This conclusion is corroborated by the regular crystal arrangement and uniform pore distribution observed in the SEM image. The phosphogypsum skeleton material prepared by the above process has a porosity of 35–42%, a bulk density of 1.45–1.60 g / cm³, and a 28-day compressive strength of 8.5–12.0 MPa, meeting the mechanical requirements for load-bearing / semi-load-bearing aggregates. Figure 3 The X-ray energy dispersive spectroscopy (EDS) spectrum of the phosphogypsum skeleton inclusions in this embodiment clearly shows that Ca, S, and O are the main elements, with minor impurities such as Si, Al, and Fe. This confirms that the main mineral phase is phosphogypsum and reveals the characteristics of element exchange / coexistence, providing a microscopic basis for composition optimization and performance control.

[0028] Furthermore, to enhance the long-term stability of the skeleton material under complex service environments, a functional coating layer needs to be applied to its surface. Before coating, the skeleton material needs to be pre-dried: placed in an oven at 100-110℃ for 1.5-2.5 hours to reduce its moisture content to below 2% to avoid residual moisture interfering with the wettability and interfacial reaction of the coating slurry.

[0029] The coating layer is applied using an impregnation-coating composite method, and either a cement-based or α-gypsum-based system is selected depending on the application scenario.

[0030] When using a cement-based coating, the slurry formulation by weight is as follows: 100 parts of P·O 42.5R ordinary Portland cement, 45-55 parts of medium sand (fineness modulus 2.3–2.6), 0.2-0.4 parts of naphthalene-based high-efficiency water-reducing agent NF-1, and 40-45 parts of water. The preparation process is as follows: First, dry mix the cement and medium sand in a forced mixer for 2 minutes. Then, dissolve the water-reducing agent NF-1 in a portion of the water to form a mother liquor, and add it along with the remaining water to the dry mix. Continue mixing for 4 minutes until a uniform slurry is obtained. The prepared cement slurry is measured at 25℃ using an NDJ-5S rotational viscometer, and its apparent viscosity is controlled at 550±50 mPa·s to ensure good fluidity and adhesion during impregnation, while avoiding excessive flow that would result in an excessively thin coating.

[0031] The pre-dried phosphogypsum skeleton material is completely immersed in the cement slurry, with the liquid level 4-6 cm above the top of the specimen. The slurry is continuously stirred at a low speed of 25-35 r / min to maintain homogeneity and promote air bubble removal. The immersion time is strictly controlled within 30-60 seconds to ensure the slurry fully wets the skeleton surface without excessively penetrating the internal pores. After immersion, the specimen is vertically lifted at a uniform speed of 4-6 cm / min. This speed effectively reduces localized thickness unevenness caused by gravity dripping. Immediately after lifting, a spray coating process is initiated: using an air-compressed nozzle, the spray pressure is set to 0.15-0.25 MPa, the nozzle inner diameter is 1.2-1.8 mm, the nozzle is 8-12 cm from the specimen surface, and spraying is continued for 8-12 seconds. This spray coating operation can precisely reinforce weak areas caused by surface tension or geometric shielding effects during immersion, ensuring a uniform coating thickness distribution within the range of 1.5-2.0 mm.

[0032] After coating, the specimens were immediately transferred to a standard curing room under environmental conditions of 20±2℃ and relative humidity ≥95% for 28 days. During this period, the cement hydration reaction proceeded fully, generating a large amount of CSH gel (Ca / Si molar ratio 1.8–2.0) and calcium hydroxide (Ca(OH)2) phase. Microstructural analysis showed that the total porosity of the cement coating was 18–22%, with the most probable pore size concentrated in the 20–50 nm range and the average pore size not exceeding 100 nm, forming a dense nanoscale porous barrier.

[0033] When using an α-gypsum-based coating, the slurry formulation, by weight, is: 100 parts building-grade α-type hemihydrate gypsum, 0.15-0.25 parts citric acid retarder, and 65-70 parts water. During preparation, the citric acid is first completely dissolved in the total measured amount of water, then mixed with the α-gypsum powder and stirred in a high-speed mixer for 3 minutes to obtain a homogeneous slurry. The initial setting time of this slurry is controlled at 45±5 minutes to match the subsequent impregnation and coating operation window.

[0034] The impregnation process parameters are basically the same as those for the cement system: the skeleton material is completely immersed in the α-gypsum slurry, with the liquid level 4-6 cm above the surface, and the stirring speed is 25-35 r / min. However, the impregnation time is extended to 60-90 seconds to compensate for the higher viscosity (approximately 800-1000 mPa·s) and slower wetting kinetics of the α-gypsum slurry. The pulling speed remains at 4-6 cm / min, and the coating parameters (pressure 0.15-0.25 MPa, nozzle diameter 1.2-1.8 mm, distance 8-12 cm, time 8-12 seconds) remain unchanged. After coating, the specimens are placed in a natural curing environment at a temperature of 25±5℃ and a relative humidity ≥60%, and crystallization hardening is completed in 5-9 days. The resulting α-gypsum coating has a thickness of 2.0–2.5 mm. Its microstructure consists of interwoven columnar calcium sulfate hemihydrate (CaSO4·0.5 H2O) crystals with an aspect ratio of 3–5:1, a total porosity of 25–28%, and a most probable pore size of 50–100 nm. Although this is slightly higher than that of the cement system, it still has good physical barrier properties.

[0035] The coating layer not only provides a physical barrier but also synergistically inhibits the migration of harmful components through a chemical fixation mechanism. In the cement coating system, the Ca within the coating layer... 2+ With F in the leachate - The reaction produces calcium fluoride (CaF2) precipitate, with a solubility product Ksp = 3.4 × 10⁻⁶. -11 Meanwhile, PO4 3- With Ca 2+ It combines to form tricalcium phosphate (Ca3(PO4)2) precipitate, Ksp = 2.0 × 10 -29 Furthermore, the CSH gel surface is rich in hydroxyl groups (-OH), which can react with Pb.2+ Cd 2+ Coordination adsorption of heavy metal ions occurs. Fitted by the Langmuir isotherm adsorption model, this coating layer exhibits strong adhesion to F... - and PO4 3- The maximum adsorption capacities were 35.2 mg / g and 28.6 mg / g, respectively.

[0036] In the α-gypsum encapsulation system, F - Mainly through interaction with SO4 on the crystal surface 2- Ion exchange occurs, and the ions are fixed at lattice defects; PO4 3- Due to their large molecular size, they are physically trapped within the micron-sized gaps between the columnar crystals. The Freundlich adsorption model fitting results show that they are effective against F... - and PO4 3- The adsorption capacities were 28.8 mg / g and 22.3 mg / g, respectively. Although the adsorption capacity was slightly lower than that of the cement system, the α-gypsum system still has advantages in certain application scenarios due to the wide availability of raw materials, low cost, and the fact that it does not require high-temperature calcination.

[0037] To verify the environmental safety of the encapsulation body described in this invention, intact encapsulation body specimens were crushed to a particle size ≤9.5mm, and deionized water was added at a liquid-to-solid ratio of 10:1. The mixture was then horizontally shaken at 25±2℃ for 8 hours. The pH of the leachate was adjusted to 4.0 (simulating acid rain environment) and 7.0 (neutral environment) for comparison. The test results are shown in Table 1.

[0038] Table 1: Leaching concentration of harmful components in phosphogypsum skeleton inclusions (mg / L)

[0039]

[0040] Note: The leaching toxicity limits specified in GB 5085.3-2007 are: Pb 5.0 mg / L, Cd 1.0 mg / L, F - 100 mg / L, PO4 3- There is no direct limit, but when calculated as total phosphorus, the Class III standard for surface water (0.2 mg / L) is usually referenced. Here, the measured stable leaching result of 0.92 mg / L in this embodiment of the invention is used as the actual engineering safety threshold. It should be noted that PO4 3- A concentration of 0.92 mg / L translates to approximately 0.3 mg / L of total phosphorus (P), which is close to the Class III standard for surface water. The measured PO4 concentration is used here. 3- The stable leaching value is the engineering safety threshold.

[0041] As shown in Table 1, under neutral conditions, the leaching concentrations of harmful substances in both inclusions were far below the national standard limits. Under acidic conditions, the cement inclusions exhibited stronger resistance to interference, with Pb leaching only slightly increasing to 0.05 mg / L, which remained at an extremely low level. In contrast, the α-gypsum inclusions showed a slight increase in Pb release due to the partial dissolution of gypsum in acid, but the release was still below the limit.

[0042] Regarding mechanical properties, the intact cement-encapsulated specimens were tested according to GB / T 50081-2019 "Standard for Test Methods of Physical and Mechanical Properties of Concrete". After 28 days of standard curing, the compressive strength of the cement-encapsulated specimens reached 22.5±1.0 MPa, and the dynamic modulus of elasticity was 28–30 GPa. After 7 days of natural curing, the compressive strength of the α-gypsum-encapsulated specimens was 18.3±0.8 MPa, and the dynamic modulus of elasticity was 20–22 GPa. To further evaluate durability, freeze-thaw cycles and long-term immersion tests were conducted.

[0043] Freeze-thaw cycle test: The cycle regime was -20℃ for 4 hours and 20℃ for 4 hours, for a total of 50 cycles. The results showed that the strength loss rate of cement inclusions was 12.0% and that of α-gypsum inclusions was 16.9%, indicating that both had good freeze-thaw resistance, with the cement system performing better due to the higher density of its hydration products.

[0044] Long-term immersion test: The specimens were completely immersed in tap water (pH≈7.2) for 90 days. The strength loss rate of cement inclusions was 3.1%, and that of α-gypsum inclusions was 6.0%, indicating that the structures of both inclusions were stable in a neutral aquatic environment, with no obvious dissolution or softening.

[0045] In a preferred embodiment of the present invention, the preparation method has been scaled up to pilot-scale. Based on an annual production capacity of 100,000 tons, each ton of product can absorb 0.8-0.9 tons of phosphogypsum, equivalent to a reduction of 0.7-0.9 m² in storage space per ton of product. 3 Meanwhile, by replacing traditional building materials (such as natural aggregates and cement products), each ton of product can reduce CO2 emissions by 0.3–0.5 tons. Economic calculations show that the comprehensive production cost of cement-encapsulated materials is 340 yuan / ton, with a market price of 550–600 yuan / ton; the cost of α-gypsum-encapsulated materials is 300 yuan / ton, with a market price of 500–550 yuan / ton, demonstrating significant economic benefits and promising industrialization prospects.

[0046] In one specific embodiment, phosphogypsum, a byproduct of the wet-process phosphoric acid and phosphate fertilizer production process of a certain phosphate chemical enterprise, was tested and found to contain 87.3% CaSO4·2H2O, 1.8% P2O5, and 1.8% F. -0.75%, Pb 42 mg / kg, Cd 3.5 mg / kg. After crushing with a jaw crusher, the skeleton material was prepared according to the aforementioned process: particle size distribution: 25-35% particles d≤1 mm, 35-45% particles 1<d≤3 mm, and 25-35% particles 3<d≤5 mm; the ingredients were 100 kg phosphogypsum, 3-5 kg ​​Na2SO4, 0.6-1.0 kg polypropylene fiber, and 3.0-4.0 kg water; pressed at 15-20 MPa, and pre-cured at 55-65℃ for 1.5-2.5 h. The resulting skeleton material had a porosity of 38.5% and a bulk density of 1.52 g / cm³. 3 The 28-day compressive strength was 10.2 MPa. It was then dried at 100-110℃ for 1.5-2.5 h. The water-reducing agent was pre-dissolved in water to form an aqueous solution before being added to the cement slurry, resulting in a slurry with a viscosity of 560 mPa·s. The skeleton material was immersed in this cement slurry for 45 seconds, pulled at 4-6 cm / min, and coated at 0.15-0.25 MPa for 8-12 seconds, followed by standard curing for 28 days. The final product had a coating thickness of 1.8 mm, a 28-day compressive strength of 22.3 MPa, and pH=7.0. The leachate contained 0.04 mg / L Pb, 0.007 mg / L Cd, and F... - 1.78 mg / L, PO4 3- 0.89 mg / L, which fully meets the requirements for safe use.

[0047] In another example, if the activator Na2SO4 is omitted and only phosphogypsum, fiber, and water are used for pressing, the 28-day compressive strength of the skeleton material is only 6.1 MPa, the porosity reaches 48%, and the overall strength after encapsulation is less than 15 MPa. Furthermore, the leachate contains F... - The concentration increased to 3.2 mg / L, indicating that the activator plays an irreplaceable role in the densification of the skeleton. If the coating layer is simply sprayed without an impregnation-coating composite process, the coating layer thickness is uneven (locally <1.0 mm), and pinhole defects exist, leading to an increase in the leaching rate of harmful substances of more than 30%. These comparative examples inversely verify the necessity and synergistic effect of the various technical elements of this invention.

[0048] In summary, this invention successfully constructs an integrated functional structure that is "strong internally and stable externally" by precisely controlling the gradation, activation system, fiber reinforcement, and molding parameters of the phosphogypsum skeleton material, combined with the rheological design of the encapsulated slurry, the impregnation-coating composite application process, and differentiated curing regimes. This structure not only enables the large-scale utilization of phosphogypsum but also fundamentally solves its bottleneck problems such as weak mechanical properties, high environmental risks, and poor engineering applicability, providing a replicable and scalable technical paradigm for the high-value utilization of industrial by-product gypsum.

Claims

1. A method for preparing a phosphogypsum skeleton inclusion body, characterized in that, Includes the following steps: Preparation of phosphogypsum skeleton material: Mix 100 parts of phosphogypsum, 3-5 parts of inorganic activator, 0.6-1.0 parts of organic reinforcing fiber and 3.0-4.0 parts of water by weight, press and mold under 15-20 MPa pressure, and then pre-cur at 55-65℃ for 1.5-2.5 hours to obtain phosphogypsum skeleton material; The phosphogypsum skeleton material is pre-dried to ensure that its moisture content is not higher than 2%. An impregnation-coating composite process is used to uniformly coat the surface of the pre-dried phosphogypsum skeleton material with a coating slurry selected from cement slurry or α-gypsum slurry, forming a coating layer with a thickness of 1.5–2.5 mm. The particle size distribution of the phosphogypsum is as follows: 25-35% of particles with d≤1 mm, 35-45% of particles with 1<d≤3 mm, and 25-35% of particles with 3<d≤5 mm.

2. The method for preparing the phosphogypsum skeleton inclusion body according to claim 1, characterized in that, The inorganic activator is sodium sulfate; the organic reinforcing fiber is polypropylene fiber with a length of 5-7 mm and a diameter of 15-25 μm.

3. The method for preparing the phosphogypsum skeleton inclusion body according to claim 1, characterized in that, When the coating slurry is made of cement slurry, the cement slurry is composed of 100 parts of P·O 42.5R cement, 45-55 parts of medium sand, 0.2-0.4 parts of water-reducing agent and 40-45 parts of water by weight, wherein the water-reducing agent needs to be dissolved in water in advance to form an aqueous solution before being added.

4. The method for preparing the phosphogypsum skeleton inclusion body according to claim 1, characterized in that, When the coating slurry is α-gypsum slurry, the α-gypsum slurry is composed of 100 parts by weight of building grade α-gypsum, 0.15-0.25 parts by weight of retarder and 65-70 parts by weight of water, and its initial setting time is 45±5 minutes.

5. The method for preparing the phosphogypsum skeleton inclusion body according to claim 1, characterized in that, The impregnation-coating composite process specifically includes: fully immersing the pre-dried phosphogypsum skeleton material into the coating slurry, with the liquid level 4-6 cm above the top of the specimen; after immersion for 30-90 seconds under stirring at 25-35 r / min, the material is pulled out at a uniform speed of 4-6 cm / min, and then immediately coated with additional material.

6. The method for preparing the phosphogypsum skeleton inclusion body according to claim 5, characterized in that, The parameters for the spray coating are: nozzle pressure 0.15-0.25 MPa, nozzle diameter 1.2-1.8 mm, spray distance 8-12 cm, and duration 8-12 seconds.

7. A phosphogypsum skeleton material inclusion body, characterized in that, The phosphogypsum skeleton material inclusion body prepared by any one of claims 1 to 6 comprises an inner phosphogypsum skeleton material and an outer coating layer, wherein the porosity of the phosphogypsum skeleton material is 35–42% and the thickness of the coating layer is 1.5–2.5 mm.

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