Environment-friendly roadbed filler with high-doped phosphogypsum and multiple solid wastes and preparation and application thereof

Abstract

The present application relates to a kind of high-doped phosphogypsum environmental protection type roadbed filler of multiple solid wastes and its preparation and application, the environmental protection type roadbed filler includes the following weight percentage of raw material components: phosphogypsum 72~95%, alkali-activated solid waste cementing material 4%~10%, the rest is clay.Compared with prior art, the present application significantly improves the mechanical properties, water stability and environmental safety of phosphogypsum-based filler by the synergistic effect of phosphogypsum, alkali-activated solid waste cementing material and clay.Using high-doped phosphogypsum and multiple solid wastes as roadbed filler can meet the requirements of roadbed engineering and solve the problem of large-scale resource utilization of phosphogypsum.

Description

Technical Field

[0001] This invention belongs to the technical field of industrial solid waste resource utilization and road engineering materials, and relates to an environmentally friendly roadbed filler with high content of phosphogypsum and multiple solid wastes, as well as its preparation and application. Background Technology

[0002] Phosphogypsum is a byproduct of phosphate fertilizer and phosphoric acid production, with a massive global stockpile. Currently, global phosphogypsum stockpiles exceed 6 billion tons, mostly stored in the open air. This not only occupies vast amounts of land but also, due to its content of soluble phosphorus, fluorine, and heavy metals, easily causes serious environmental problems such as water and soil pollution. Furthermore, while global annual phosphogypsum production is approximately 200 million tons, the recycling rate is only about 15%, indicating a huge potential for resource utilization.

[0003] Due to the large volume of fill required in road construction, phosphogypsum is characterized by its high usage and utilization rate in road engineering, making it one of the most effective strategies for large-scale resource utilization. However, phosphogypsum's insufficient mechanical properties (powder texture, low plasticity), high water sensitivity (swelling or cracking upon contact with water, especially at high phosphogypsum concentrations), and the high environmental risks posed by leaching pollutants severely limit its direct application in road engineering. To address these issues, phosphogypsum is typically modified by compounding it with other materials, with lime and fly ash being the most commonly used additives.

[0004] Although existing technologies have proposed various schemes for the resource utilization of phosphogypsum, the following problems still need to be addressed in practical applications: First, the application of phosphogypsum in road materials is mainly as an auxiliary additive, rarely used as a main aggregate, which to some extent limits the efficiency of resource utilization of phosphogypsum in road engineering; Second, the few utilization schemes for high-dosage phosphogypsum mainly rely on traditional high-energy-consuming materials such as lime for stabilization treatment, lacking low-energy-consuming and sustainable additive modification schemes, making it difficult to meet the requirements of sustainable development; Third, when traditional materials such as lime stabilize high-dosage phosphogypsum, insufficient consideration is given to the water stability performance and environmental safety performance of the material in water-rich environments, resulting in limited applicability in complex environments.

[0005] For example, CN202210539775.0 discloses a phosphogypsum roadbed filler cured with aluminosilicate cementitious material and its preparation method, which includes the following ingredients: aluminosilicate cementitious material, phosphogypsum, and water. The aluminosilicate cementitious material comprises slag powder, fly ash, and an activator. The molar ratios of the components in the phosphogypsum roadbed filler are as follows: SO3:Al2O3 7.3–12.4; CaO:Al2O3 6.9–11.5; SiO2:Al2O3 5.0–6.9. However, analysis reveals that this patent uses a traditional chemical alkali activator composed of sodium hydroxide and water glass, which may lead to soil salinization due to residual sodium ions. Furthermore, sodium hydroxide is a high-energy-consuming chemical product and does not meet low-carbon goals. In addition, the resulting roadbed filler is prone to brittle fracture, affecting long-term reliability. Summary of the Invention

[0006] The purpose of this invention is to provide an environmentally friendly roadbed filler with high phosphogypsum content and multiple solid wastes, as well as its preparation and application. By using a specific all-solid waste-based alkali-activated cementitious material to effectively solidify and stabilize the high phosphogypsum content, the water stability and environmental safety of the high phosphogypsum content are significantly improved, and the problem of its substandard leaching performance is solved.

[0007] The objective of this invention can be achieved through the following technical solutions:

[0008] In one aspect, the present invention provides an environmentally friendly roadbed filler with high dosage of phosphogypsum and multiple solid wastes, comprising the following raw material components by weight percentage: 72-95% phosphogypsum, 4%-10% alkali-activated solid waste cementitious material, and the remainder being clay.

[0009] Furthermore, the phosphogypsum is a modified phosphogypsum pretreated with lime, dried at 40-55°C until the moisture content is ≤2%, its CaSO4·2H2O content is ≥90%, and the soluble impurity content is ≤0.5%.

[0010] Furthermore, the alkali-activated solid waste cementitious material is composed of fly ash, mineral powder and carbide slag.

[0011] Furthermore, the mass ratio of fly ash, mineral powder and carbide slag is (1.5-2.5):(1.5-2.5):1, preferably 2:2:1.

[0012] Furthermore, the fly ash is a gray-black powdery solid, with a particle size of less than 0.075 mm accounting for 4.5% by mass, and the content of silicon oxide and aluminum oxide in the fly ash is greater than 80%.

[0013] Furthermore, the mineral powder is S95 grade high-activity mineral powder, with a grayish-white appearance, and its particle size is less than 0.075mm, accounting for 4% by mass.

[0014] Furthermore, the carbide slag is high-calcium carbide slag, with a light gray appearance and a calcium hydroxide content of over 80% by mass.

[0015] Furthermore, the clay is a low liquid limit clay, with a liquid limit ≤50% and a plasticity index ≤30%.

[0016] In a second aspect, the present invention provides a method for preparing an environmentally friendly roadbed filler with high dosage of phosphogypsum and multiple solid wastes. The method involves mixing phosphogypsum, alkali-activated solid waste cementitious material and clay evenly, then adding water (based on the optimal moisture content determined by compaction tests), and compacting (the degree of compaction is determined by the maximum dry density obtained from compaction tests) to obtain the environmentally friendly roadbed filler.

[0017] In a third aspect, the present invention provides an application of an environmentally friendly roadbed filler with high phosphogypsum content and multiple solid wastes in road construction filling.

[0018] In the above scheme, the present invention utilizes the synergistic effect of three components: phosphogypsum, alkali-activated solid waste cementitious material, and clay (see appendix). Figure 1 This method employs efficient solidification and stabilization treatment for high-dosage phosphogypsum, not only meeting the requirements for physical and mechanical properties, road performance indicators, and environmental safety, but also significantly overcoming the problems of poor water stability and insufficient environmental safety associated with high-dosage phosphogypsum. The specific principle is as follows:

[0019] The alkali-activated solid waste gelling material of this invention plays a role in the overall solidification and stabilization of the material. It solidifies high doses of PG to enhance its physical properties and stabilizes toxic substances to meet environmental compliance requirements.

[0020] In this invention, the high-dosage phosphogypsum, as the object to be solidified, can be disposed of in large quantities. Simultaneously, phosphogypsum can serve as a calcium source, providing abundant Ca. 2+ and SO4 2- Ions promote hydration reactions.

[0021] The clay in this invention (1) improves toughness: the addition of clay soil increases peak strain energy and reduces brittle fracture. (2) optimizes pore structure: it forms a finer pore distribution dominated by micropores, improving water permeability resistance and long-term durability. (3) promotes CASH gel formation: clay-derived Al3+ promotes the formation of CASH gel, which has better long-term stability than CSH gel. Therefore, the clay component significantly improves the long-term performance of the sample. The ternary synergistic relationship between phosphogypsum, alkali-activated solid waste cementitious material and clay in this invention is as follows: Figure 1 .

[0022] The alkali-activated solid waste cementitious material of this invention is a compound made from fly ash, mineral powder, and carbide slag. The selection of these solid waste raw materials is based on their complementary characteristics. Mineral powder mainly provides early strength development, fly ash exhibits excellent long-term strength enhancement due to its low reactivity, while carbide slag is added in a controlled proportion to adjust the pH value of the mixture. This alkali-activated system can effectively increase the alkalinity of the cementitious system, significantly promote the activation of fly ash and mineral powder, and reduce the risk of alkali-aggregate reaction caused by excessive calcium hydroxide.

[0023] The comprehensive strength development mechanism in this invention effectively improves the physical and mechanical properties and long-term stability of environmentally friendly roadbed fillers that utilize high-dosage phosphogypsum and various solid wastes in synergistic utilization. The main interaction mechanism between hydration products and mechanical strength properties can be summarized as follows (see appendix). Figure 2 ).

[0024] One of the strength development mechanisms of the subgrade filler in this invention is the skeletal reinforcement effect of ettringite crystals (AFt): needle-like ettringite forms an interwoven network structure, tightly binding phosphogypsum and soil particles together. This rigid skeleton significantly improves load-bearing capacity by redistributing stress in the matrix, directly contributing to higher unconfined compressive strength (UCS). The calcium sulfate provided by phosphogypsum reacts in an alkaline environment, promoting the formation of ettringite, thus providing sufficient early strength. The ettringite content is closely related to the development of early strength.

[0025] Traditional alkaline activating materials such as sodium hydroxide and water glass directly activate the activity of cementitious materials through strong alkalinity. However, this invention uses carbide slag (containing high levels of Ca(OH)2) to slowly release an alkaline environment, synergistically regulating the pH value of the system and avoiding the risk of alkali-aggregate reaction caused by localized excessive alkalinity. Furthermore, carbide slag is an industrial solid waste, enabling the synergistic disposal of multiple solid wastes without the need for additional production, thus aligning with low-carbon goals.

[0026] The second strength development mechanism of the roadbed filler in this invention is the bonding and strengthening effect of C-(A)-SH gel: the amorphous C-(A)-SH gel fills the micropores and mesopores, thus making the structure more compact. It binds loose particles together and reduces stress concentration at the interfaces. The degree of polymerization of the gel determines the stability of long-term strength. Higher gel content can significantly improve unconfined compressive strength. Hydration products (including AFt, CSH, and CASH) enhance the adhesion between particles, thereby significantly improving strength.

[0027] The third strength development mechanism of the roadbed filler material of this invention is ion exchange and bonding optimization: ion exchange between phosphogypsum, soil particles, and added alkali-activated solid waste cementitious materials alters the bonding state and promotes mass redistribution, thereby further improving strength. The Ca in phosphogypsum... 2+Al in soil and fly ash / slag 3+ and Si 4+ The exchange process enhances the cohesive force between particles, thereby increasing the overall strength.

[0028] The fourth strength development mechanism of the roadbed filler in this invention is the carbonization reaction and pore-filling effect: the carbonization reaction driven by atmospheric CO2 enhances the long-term stability of the material by precipitating CaCO3 in the pores. Phosphogypsum provides an ample source of calcium ions, which react with carbon dioxide and water in the air under alkaline conditions to generate CaCO3. Although CaCO3 is a precipitate and not a crystalline structure, and cannot directly contribute to strength, it fills some of the voids in the structure, reducing porosity and thus improving the material's density and long-term strength.

[0029] This invention demonstrates through toxicity leaching and radioactivity tests that the reaction system can effectively solidify harmful components in phosphogypsum. The mechanism effectively immobilizes toxic elements within the material structure through physical or chemical means, minimizing the risk of leaching and ensuring the material's environmental safety. This enhanced environmental safety is primarily attributed to the following mechanisms:

[0030] (1) Physical encapsulation: The dense C-(A)-SH gel fixes the heavy metals through physical adsorption and encapsulation. At the same time, the mutual filling and bonding of the needle-shaped calcium aluminum feldspar crystals and the gel further enhances the curing effect.

[0031] (2) Chemical bonding: Toxic ions are chemically fixed by AFt crystals through ion exchange. For example, Cr 3 + can replace Ca in AFt 2+ And Al 3+ chromate (CrO4) 2- ) and arsenate (AsO4) 3- It can replace sulfate (SO4) 2- This allows for the chemical solidification of harmful ions.

[0032] (3) Precipitation: F - With Ca 2+ The reaction produces sparingly soluble CaF2, F - It is mainly fixed in the material system in the form of CaF2, thereby significantly reducing its leaching risk. Attached Figure Description

[0033] Figure 1 A ternary synergistic effect diagram of phosphogypsum, alkali-activated solid waste cementitious materials, and clay;

[0034] Figure 2 A diagram illustrating the strength growth mechanism of environmentally friendly roadbed filler with high phosphogypsum content and multiple solid wastes.

[0035] Figure 3 This is a method and flowchart for preparing and using environmentally friendly roadbed filler with high phosphogypsum content and multiple solid wastes.

[0036] Figure 4 This is a typical cross-sectional microscopic image magnified 10,000 times from a scanning electron microscope (SEM) image of the environmentally friendly roadbed filler material with high phosphogypsum content and multiple solid wastes according to the present invention. Detailed Implementation

[0037] The present invention will now be described in detail with reference to the accompanying drawings and specific embodiments. These embodiments are based on the technical solution of the present invention and provide detailed implementation methods and specific operating procedures. However, the scope of protection of the present invention is not limited to the following embodiments.

[0038] Unless otherwise defined, all technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the art to which this application belongs. The terminology used herein is for the purpose of describing particular embodiments only and is not intended to be limiting of the application.

[0039] As used herein, the terms "and / or," "or / and," and "and / or" encompass any one of two or more of the related listed items, as well as any and all combinations of the related listed items. These arbitrary and all combinations include any two related listed items, any more related listed items, or a combination of all related listed items. It should be noted that when at least three items are connected using at least two conjunctions selected from "and / or," "or / and," and "and / or," it should be understood that, in this application, the technical solution undoubtedly includes solutions connected by "logical AND," and also undoubtedly includes solutions connected by "logical OR."

[0040] In this application, the technical features described in an open-ended manner include both closed technical solutions consisting of the listed features and open technical solutions that include the listed features.

[0041] In this application, numerical ranges are referred to as continuous unless otherwise specified, and include the minimum and maximum values ​​of the range, as well as every value between the minimum and maximum values. Furthermore, when the range refers to integers, it includes every integer between the minimum and maximum values ​​of the range. Additionally, when multiple ranges are provided to describe a feature or characteristic, the ranges may be merged. In other words, unless otherwise specified, all ranges disclosed herein should be understood to include any and all subranges to which they are incorporated.

[0042] This document only specifically discloses some numerical ranges. However, any lower limit can be combined with any upper limit to form an unspecified range; and any lower limit can be combined with other lower limits to form an unspecified range, just as any upper limit can be combined with any other upper limit to form an unspecified range. Furthermore, each individually disclosed point or single value can itself serve as a lower or upper limit and be combined with any other point or single value or with other lower or upper limits to form an unspecified range.

[0043] Unless otherwise specified, the temperature parameters in this application may be either constant temperature processing or processing within a certain temperature range. The constant temperature processing allows temperature fluctuations within the precision range controlled by the instrument, such as ±5℃, ±4℃, ±3℃, ±2℃, or ±1℃.

[0044] In this document, the term "suitable" as used in phrases such as "suitable combination," "suitable method," and "any suitable method" refers to the ability to implement the technical solution of this application, solve the technical problem of this application, and achieve the expected technical effect of this application.

[0045] In this application, terms such as "further," "even further," and "particularly" are used to describe purposes and indicate differences in content, but should not be construed as limiting the scope of protection of this application.

[0046] In this application, "optionally," "optionally," and "optional" mean that something is optional, that is, it means that it is selected from either "with" or "without." If there are multiple "optional" entries in a technical solution, unless otherwise specified, and there are no contradictions or mutual constraints, each "optional" entry shall be independent.

[0047] In the description of the application, "multiple" means at least two, such as two, three, etc., unless otherwise explicitly specified.

[0048] Unless otherwise specified, all preparations and tests described herein took place at 25°C.

[0049] The terms “comprising,” “including,” “containing,” “having,” “comprising,” or other variations thereof are intended to cover non-closed inclusion, and no distinction is made between these terms. The term “comprising” means that other steps and ingredients may be added without affecting the final result. The compositions and methods / processes of the present invention comprise, consist of, and substantially consist of the essential elements and limitations described herein, as well as any additional or optional ingredients, components, steps, or limitations described herein. No distinction is made between the terms “efficacy,” “performance,” “effect,” and “potency” herein.

[0050] Unless otherwise specified, all embodiments and optional embodiments of this application can be combined to form new technical solutions. Unless otherwise specified, all technical features and optional technical features of this application can be combined to form new technical solutions.

[0051] Unless otherwise specified, all steps of this application may be performed sequentially or randomly, but sequentially is preferred.

[0052] In the following embodiments, the phosphogypsum used is modified phosphogypsum that has been pretreated by lime neutralization and stored in a phosphate mine in Guizhou Province. It is dried at 40-55°C until the moisture content is ≤2%, the CaSO4·2H2O content is ≥90%, and the soluble impurity content is ≤0.5%.

[0053] The fly ash used comes from Lingshou County, Hebei Province, China. It is a gray-black powdery solid with a particle size of less than 0.075 mm accounting for 4.5%. The fly ash contains more than 80% silica and alumina.

[0054] The mineral powder used is sourced from Zhengzhou, Henan, China. It is S95 grade high-activity mineral powder, grayish-white in appearance, and 4% of its particles are smaller than 0.075mm.

[0055] The carbide slag used is from Dingyuan County, Chuzhou City, China. It is a high-calcium carbide slag, with a light gray appearance and a calcium oxide content of over 80%.

[0056] The molar ratios of the chemical components of the above raw materials are as follows:

[0057] Table 1: Chemical composition of raw materials

[0058]

[0059] Note: LOI a This indicates the loss on ignition.

[0060] Unless otherwise specified, all other raw materials or processing techniques are commercially available materials or conventional processing techniques in the field.

[0061] Example 1

[0062] This embodiment provides an environmentally friendly roadbed filler with high dosage of phosphogypsum and multiple solid wastes. This embodiment uses three components: phosphogypsum, alkali-activated solid waste cementitious material, and clay, with the following mass percentage ratio: phosphogypsum 81%, alkali-activated solid waste cementitious material 9%, and clay 10%.

[0063] The alkali-activated solid waste cementitious material used is composed of fly ash, mineral powder and carbide slag, which are compounded in a mass ratio of 2:2:1.

[0064] The clay used in this embodiment is low liquid limit clay, and its physical properties are shown in Table 2.

[0065] Table 2 Basic physical properties of the clay used in the examples.

[0066] <![CDATA[Maximum dry density (g / cm 3 )]]> Optimal moisture content (%) Liquid limit (%) Plastic Limit (%) Plasticity index 1.95 12 45.1 18.2 26.9

[0067] Please refer to the appendix for the preparation process of the roadbed filler in this embodiment. Figure 3 The specific preparation steps are as follows:

[0068] Step 1: Add the solid waste material to the mixing equipment according to the mass ratio and dry mix evenly for 3 minutes to ensure that the components are evenly distributed, to obtain mixture 1 (alkali-activated solid waste cementitious material).

[0069] Step 2: Mix the dried phosphogypsum and clay with mixture 1 thoroughly to obtain mixture 2.

[0070] Step 3: Based on the optimum moisture content determined by the compaction test, add the corresponding amount of water to mixture 2, stir thoroughly to prepare mixture 3 (high-content phosphogypsum roadbed filler).

[0071] In this embodiment, the optimal moisture content of the material mixture system, obtained from compaction tests, is 16.06%, and the maximum dry density is 1.548 g / cm³. 3 Based on this data, moisture content was prepared and compaction was carried out. Three cylindrical specimens with a diameter of 50 × 100 mm were made using the static pressing method. After demolding, both specimens were placed in a standard curing room with a humidity of 95% and a temperature of 20 ± 2℃ for curing. After curing for 7 days and 28 days, unconfined compressive strength tests were conducted to obtain the unconfined compressive strength of the specimens. Simultaneously, the specimens were immersed in water for 24 hours before the unconfined compressive strength test to obtain the water stability coefficient. The test results are shown in Table 3. Based on the stress-strain curve, the peak strain energy of the material was calculated (Table 7) to characterize the material's energy absorption capacity and toughness.

[0072] The high-dosage phosphogypsum roadbed filler prepared in this embodiment was subjected to CBR and CBR expansion tests in accordance with JTG 3430-2020. The test results are shown in Table 4.

[0073] The high-dosage phosphogypsum roadbed filler prepared in this embodiment was subjected to ICP-OES toxicity leaching test and radioactivity test on the sample after 28 days of curing. The test results are shown in Tables 5 and 6.

[0074] Example 2

[0075] This embodiment provides an environmentally friendly roadbed filler with high dosage of phosphogypsum and multiple solid wastes. This embodiment uses three components: phosphogypsum, alkali-activated solid waste cementitious material, and clay, with the following mass percentage ratio: phosphogypsum 85.5%, alkali-activated solid waste cementitious material 4.5%, and clay 10%.

[0076] The alkali-activated solid waste cementitious material used is composed of fly ash, mineral powder and carbide slag, which are compounded in a mass ratio of 2:2:1.

[0077] The clay used in this embodiment is low liquid limit clay, and its physical properties are shown in Table 1.

[0078] Please refer to the appendix for the preparation process of the roadbed filler in this embodiment. Figure 3 The specific preparation steps are as follows:

[0079] Step 1: Add the solid waste material to the mixing equipment according to the mass ratio and dry mix evenly for 3 minutes to ensure that the components are evenly distributed, to obtain mixture 1 (alkali-activated solid waste cementitious material).

[0080] Step 2: Mix the dried phosphogypsum and clay with mixture 1 thoroughly to obtain mixture 2.

[0081] Step 3: Based on the optimum moisture content determined by the compaction test, add the corresponding amount of water to mixture 2, stir thoroughly to prepare mixture 3 (high-content phosphogypsum roadbed filler).

[0082] In this embodiment, the optimal moisture content of the material mixture system, obtained from compaction tests, is 15.833%, and the maximum dry density is 1.538 g / cm³. 3 Based on this data, moisture content was prepared and compaction was carried out. Three cylindrical specimens with a diameter of 50 × 100 mm were made using the static pressing method. After demolding, both specimens were placed in a standard curing room with a humidity of 95% and a temperature of 20 ± 2℃ for curing. After curing for 7 days and 28 days, unconfined compressive strength tests were conducted to obtain the unconfined compressive strength of the specimens. Simultaneously, the specimens were immersed in water for 24 hours before the unconfined compressive strength test to obtain the water stability coefficient. The test results are shown in Table 3. Based on the stress-strain curve, the peak strain energy of the material was calculated (Table 7) to characterize the material's energy absorption capacity and toughness.

[0083] The high-dosage phosphogypsum roadbed filler prepared in this embodiment was subjected to CBR and CBR expansion tests in accordance with JTG 3430-2020. The test results are shown in Table 4.

[0084] The high-dosage phosphogypsum roadbed filler prepared in this embodiment was subjected to ICP-OES toxicity leaching test and radioactivity test on the sample after 28 days of curing. The test results are shown in Tables 5 and 6.

[0085] Table 3. Test results of unconfined compressive strength and water stability of the examples

[0086]

[0087] Table 4. CBR and CBR expansion test results of the examples

[0088] serial number CBR value CBR expansion value Example 1 81.47％ 0.68％ Example 2 77.01％ 0.74％

[0089] Table 5. Radioactivity detection results of the examples

[0090]

[0091] Table 6. Toxicity leaching test results of the examples

[0092]

[0093]

[0094] Remark: a Not detected

[0095] Comparative Example 1:

[0096] This comparative example provides a roadbed filler material for phosphogypsum and multiple solid wastes. This embodiment uses three components: phosphogypsum, alkali-activated solid waste cementitious material, and clay. The mass percentage ratio is as follows: phosphogypsum 90%, alkali-activated solid waste cementitious material 10%, and clay 0%.

[0097] The alkali-activated solid waste cementitious material used is composed of fly ash, mineral powder and carbide slag, which are compounded in a mass ratio of 2:2:1.

[0098] The clay used in this embodiment is low liquid limit clay, and its physical properties are shown in Table 1.

[0099] Please refer to the appendix for the preparation process of the roadbed filler in this embodiment. Figure 3 The specific preparation steps are as follows:

[0100] Step 1: Add the solid waste material to the mixing equipment according to the mass ratio and dry mix evenly for 3 minutes to ensure that the components are evenly distributed, to obtain mixture 1 (alkali-activated solid waste cementitious material).

[0101] Step 2: Mix the dried phosphogypsum thoroughly with mixture 1 to obtain mixture 2.

[0102] Step 3: Based on the optimum moisture content determined by the compaction test, add the corresponding amount of water to mixture 2, stir thoroughly to prepare mixture 3 (high-content phosphogypsum roadbed filler).

[0103] The optimal moisture content of the comparative material mixture system, obtained from compaction tests, was 17.045%, and the maximum dry density was 1.607 g / cm³. 3 Based on this data, the moisture content was prepared and the samples were compacted. Three cylindrical specimens with a diameter of 50 × 100 mm were made using the static pressing method. After demolding, both specimens were placed in a standard curing room with a humidity of 95% and a temperature of 20 ± 2℃ for curing. After curing for 7 days and 28 days, unconfined compressive strength tests were conducted to obtain the unconfined compressive strength of the specimens. Based on the stress-strain curve, the peak strain energy of the material was calculated (Table 7) to characterize the material's energy absorption capacity and toughness.

[0104] Comparative Example 2:

[0105] This comparative example provides a roadbed filler material for phosphogypsum and multiple solid wastes. This embodiment uses three components: phosphogypsum, alkali-activated solid waste cementitious material, and clay, with the following mass percentage ratio: phosphogypsum 45%, alkali-activated solid waste cementitious material 5%, and clay 50%.

[0106] The alkali-activated solid waste cementitious material used is composed of fly ash, mineral powder and carbide slag, which are compounded in a mass ratio of 2:2:1.

[0107] The clay used in this embodiment is low liquid limit clay, and its physical properties are shown in Table 1.

[0108] Please refer to the appendix for the preparation process of the roadbed filler in this embodiment. Figure 3 The specific preparation steps are as follows:

[0109] Step 1: Add the solid waste material to the mixing equipment according to the mass ratio and dry mix evenly for 3 minutes to ensure that the components are evenly distributed, to obtain mixture 1 (alkali-activated solid waste cementitious material).

[0110] Step 2: Mix the dried phosphogypsum and clay with mixture 1 thoroughly to obtain mixture 2.

[0111] Step 3: Based on the optimum moisture content determined by the compaction test, add the corresponding amount of water to mixture 2, stir thoroughly to prepare mixture 3 (high-content phosphogypsum roadbed filler).

[0112] The optimal moisture content of the comparative material mixture system, obtained from compaction tests, was 17.045%, and the maximum dry density was 1.607 g / cm³. 3Based on this data, the moisture content was prepared and the samples were compacted. Three cylindrical specimens with a diameter of 50 × 100 mm were made using the static pressing method. After demolding, both specimens were placed in a standard curing room with a humidity of 95% and a temperature of 20 ± 2℃ for curing. After curing for 7 days and 28 days, unconfined compressive strength tests were conducted to obtain the unconfined compressive strength of the specimens. Based on the stress-strain curve, the peak strain energy of the material was calculated (Table 7) to characterize the material's energy absorption capacity and toughness.

[0113] Table 7. Calculation results of peak strain energy at 7d and 28d for the Examples and Comparative Examples

[0114]

[0115]

[0116] Compared to Example 1, Comparative Example 1 maintained a constant ratio of phosphogypsum to alkali-activated solid waste cementitious material. The difference lay in whether clay was introduced into the material system. By comparing peak strain energies, the 7-day and 28-day peak strain energies of Example 1 were significantly better than those of Comparative Example 1, demonstrating the importance of introducing clay in this invention. Compared to Example 1, Comparative Example 2 also maintained a constant ratio of phosphogypsum to alkali-activated solid waste cementitious material. The difference was that the clay content in Comparative Example 2 was increased to 50%. By comparing peak strain energies, the 7-day and 28-day peak strain energies of Example 1 were significantly better than those of Comparative Example 2, demonstrating the importance of ensuring that the clay content is appropriate and does not exceed the limits defined in this application.

[0117] According to the test results of this invention, the environmentally friendly roadbed filler with high-dosage phosphogypsum and multi-solid waste synergistic utilization proposed in this invention possesses material strength, load-bearing capacity, and environmental safety that meet practical application requirements. This material system achieves large-scale utilization of high-dosage phosphogypsum and synergistic disposal with other solid waste materials. The C-(A)-SH gel, AFt needle-like crystals, and cementitious network structure formed by the hydration of specific alkali-activated solid waste cementitious materials fill and bind the phosphogypsum matrix, significantly improving the material's microstructure and pore distribution, thereby enhancing its mechanical properties and water stability. Figure 4As shown, SEM observation of the microstructure allows for direct analysis of the material's surface structure and particle morphology. Numerous needle-like ettringite (AFt) gel phase products are generated around unreacted phosphogypsum particles, filling pores and densifying the microstructure. Simultaneously, needle-like crystals bind the particles together to form an integral structure, while hydration products aggregate to form dense flocculents, creating a dense surface layer on the gypsum particles. Furthermore, needle-like AFt crystals intertwine and connect with gypsum and soil particles, forming a stable network lattice structure. Ion exchange between phosphogypsum, soil particles, and alkali-activated solid waste cementitious materials improves the material's bonding state, promotes mass redistribution, and enhances overall mechanical properties. In addition, phosphogypsum provides an ample source of calcium ions, reacting with carbon dioxide and water in the air to form CaCO3, filling some voids in the overall structure and further strengthening the material. The high-dosage phosphogypsum roadbed filler, after curing / stabilization treatment, exhibits leaching concentrations of heavy metal ions and radioactive substances below regulatory limits, confirming the material's environmental compliance. This is mainly attributed to the physical encapsulation and chemical bonding of the hydration gel products, as well as the mutual filling and cementation between the needle-like ettringite crystals and the gel, effectively fixing harmful ions in phosphogypsum. Furthermore, ion exchange between toxic ions and crystals also plays a crucial role in the stabilization of heavy metal ions. The clay in the material system enhances the material's toughness, preventing brittle fracture, while simultaneously promoting CASH gel formation and optimizing the pore structure distribution. This is beneficial for improving the long-term service performance of high-phosphogypsum-based roadbed materials in water-rich environments.

[0118] The above description of the embodiments is provided to enable those skilled in the art to understand and use the invention. It will be apparent to those skilled in the art that various modifications can be made to these embodiments, and the general principles described herein can be applied to other embodiments without inventive effort. Therefore, the present invention is not limited to the above embodiments, and any improvements and modifications made by those skilled in the art based on the disclosure of the present invention without departing from the scope of the invention should be within the protection scope of the present invention.

Claims

1. An environmentally friendly roadbed filler with high-content phosphogypsum and multiple solid wastes, characterized in that, The raw material components include the following weight percentages: 72-95% phosphogypsum, 4%-10% alkali-activated solid waste cementitious material, and the remainder is clay; The phosphogypsum is a modified phosphogypsum pretreated with lime, dried at 40-55℃ until the moisture content is ≤2%, its CaSO4·2H2O content is ≥90%, and the soluble impurity content is ≤0.5%. The alkaline activated solid waste cementitious material is composed of fly ash, mineral powder and carbide slag; The mass ratio of fly ash, mineral powder and carbide slag is 1.5-2.5:1.5-2.5:

1.

2. The environmentally friendly roadbed filler with high phosphogypsum content and multiple solid wastes as described in claim 1, characterized in that, The fly ash is a gray-black powdery solid with a particle size of less than 0.075 mm accounting for 4.5% by mass, and the content of silicon oxide and aluminum oxide in the fly ash is greater than 80%.

3. The environmentally friendly roadbed filler with high phosphogypsum content and multiple solid wastes as described in claim 1, characterized in that, The mineral powder is S95 grade high-activity mineral powder, with a grayish-white appearance, and its particle size is less than 0.075mm, accounting for 4% by mass.

4. The environmentally friendly roadbed filler with high-dosage phosphogypsum and multiple solid wastes as described in claim 1, characterized in that, The calcium carbide slag is a high-calcium calcium carbide slag, which is light gray in appearance and has a calcium hydroxide content of more than 80% by mass.

5. The environmentally friendly roadbed filler with high phosphogypsum content and multiple solid wastes as described in claim 1, characterized in that, The clay is a low liquid limit clay, with a liquid limit ≤50% and a plasticity index ≤30%.

6. The preparation method of the environmentally friendly roadbed filler with high phosphogypsum content and multiple solid wastes as described in any one of claims 1 to 5, characterized in that, Phosphogypsum, alkali-activated solid waste cementitious material are mixed evenly with clay, then water is added and the mixture is compacted to obtain an environmentally friendly roadbed filler.

7. The application of the environmentally friendly subgrade filler material with high phosphogypsum content and multiple solid waste as described in any one of claims 1 to 5 in road construction filling.

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