Modified phosphogypsum and its use in road base materials

Abstract

This invention discloses a modified phosphogypsum and its application in roadbed materials, relating to the field of building materials technology. The invention includes preparing a composite modifier comprising an alkaline activating component, a sulfate activating component, a crystal form regulating and gradation optimizing component, and an impurity curing component. The pretreated phosphogypsum fines are mixed with the prepared composite modifier at a mass ratio of 85-92:8-15, followed by a primary dry mixing and a secondary wet mixing process. During wet mixing, water is sprayed in to adjust the moisture content, followed by a curing process to obtain a mixture. The resulting mixture is then spread and compacted to form a roadbed filling layer. This invention, through the synergistic effect of the alkaline activating component, sulfate activating component, crystal form regulating and gradation optimizing component, and impurity curing component in the composite modifier, combined with the curing process, allows the modifier to fully react and cure with harmful impurities in the phosphogypsum, effectively improving the bonding density, water stability, and environmental safety of phosphogypsum-based materials.

Description

Technical Field

[0001] This invention belongs to the field of building materials technology, and in particular relates to a modified phosphogypsum and its application in roadbed materials. Background Technology

[0002] Phosphogypsum is an industrial byproduct of wet-process phosphoric acid production, and its main component is calcium sulfate dihydrate (Calcium sulfate dihydrate). Approximately 4-5 tons of phosphogypsum are produced for every ton of phosphoric acid produced. Roadbed filler refers to engineering materials used for highway roadbed filling. It needs to possess certain strength, water stability, and compaction performance to withstand road surface loads and ensure long-term stable road operation.

[0003] With the rapid development of the phosphorus chemical industry, the stockpile of phosphogypsum continues to grow, but the comprehensive utilization rate is less than 15%. Large-scale open-air stockpiling of phosphogypsum not only occupies land resources, but also poses a risk of environmental pollution as soluble phosphorus, fluorine, and heavy metals it contains may leach into the soil and water bodies through rainwater. Using phosphogypsum in highway subgrade filling is one of the effective ways to dispose of this solid waste, offering advantages such as large usage and low treatment costs.

[0004] However, there are many technical obstacles to using undisturbed phosphogypsum directly as a roadbed filler. Phosphogypsum particles are finely crushed, have a simple gradation, and a low plasticity index, making compaction difficult. Its soluble phosphorus content can delay the hydration reaction of cement and other cementitious materials, affecting strength development. Soluble fluorine and heavy metals pose a risk of environmental leaching. Furthermore, phosphogypsum is prone to softening or swelling under the influence of water, exhibiting poor water stability. These problems severely restrict the large-scale application of phosphogypsum in roadbed engineering.

[0005] To address the aforementioned issues, existing technologies typically employ cement or lime to stabilize phosphogypsum. However, the modification effect of a single cementitious material is limited, making it difficult to simultaneously meet the requirements for strength, water stability, and environmental safety. Some studies have attempted to introduce industrial solid wastes such as fly ash and slag for compound modification, but these often overlook the interference of harmful impurities in phosphogypsum on the cementitious system and fail to fully consider the long-term stability of the material in humid environments. Therefore, the following solutions are proposed to address these problems. Summary of the Invention

[0006] The purpose of this invention is to provide a modified phosphogypsum and its application in roadbed materials. Through the multi-component synergy of alkaline activating components, sulfate activating components, crystal form regulation and gradation optimization components, and impurity curing components in the composite modifier, combined with the curing process, the modifier and harmful impurities in the phosphogypsum can fully react and cure. This can effectively improve the cementitious density, water stability, and environmental safety of phosphogypsum-based materials, and solve the problems of limited modification effect of single cementitious materials, interference of harmful impurities with the cementitious system, and insufficient long-term stability in humid environments in the prior art.

[0007] To solve the above-mentioned technical problems, the present invention is achieved through the following technical solution:

[0008] This invention relates to a method for preparing a phosphogypsum-based highway subgrade filling material, characterized in that the method includes the following steps:

[0009] Step S1: Pre-treat the raw phosphogypsum to remove impurities and adjust the moisture content to obtain pre-treated phosphogypsum fines;

[0010] Step S2: Prepare a composite modifier, wherein the composite modifier includes an alkaline activation component, a sulfate activation component, a crystal form regulation and gradation optimization component, and an impurity curing component;

[0011] Step S3: Mix the pretreated phosphogypsum fines obtained in step S1 with the composite modifier prepared in step S2 at a mass ratio of 85-92:8-15, and perform first-stage dry mixing and second-stage wet mixing in sequence. During wet mixing, water is sprayed in to adjust the moisture content, and then the mixture is subjected to a curing process to obtain the mixture.

[0012] Step S4: Spread and compact the mixture obtained in step S3 to form the roadbed filling layer;

[0013] Step S5: Curing the compacted roadbed filling layer to obtain phosphogypsum-based highway roadbed filling material.

[0014] Further, the composite modifier in step S2 is composed of the following raw materials in parts by weight: 20-35 parts quicklime, 10-20 parts cement clinker, 8-15 parts natural anhydrite, 3-5 parts polyaluminum chloride, 5-8 parts diatomaceous earth, and 10-15 parts carbide slag; the fineness of the composite modifier is all passed through a 180-mesh sieve.

[0015] Further, the pretreatment in step S1 includes the following sub-steps: pre-homogenizing the undisturbed phosphogypsum, controlling the moisture content fluctuation within ±3% by layering and vertical material handling; drying the pre-homogenized phosphogypsum to a moisture content of 8%-12%; sieving through a 5mm aperture vibrating screen, crushing the material on the screen and mixing it with the material under the screen to obtain pre-treated phosphogypsum fines with a particle size of less than 5mm.

[0016] Further, the primary dry mixing in step S3 is carried out in a twin-shaft horizontal mixer with a mixing speed of 60-80 rpm and a dry mixing time of 45-60 seconds; the secondary wet mixing is carried out by spraying water accounting for 8%-12% of the total mass of the material through an atomizing spray device, with the water pressure controlled at 0.3-0.5 MPa, and stirring continues for 120-180 seconds after adding water.

[0017] Furthermore, the curing time in step S3 is 12-24 hours. During curing, the mixture is piled up to a height of no more than 2 meters and covered with plastic film to keep it moist.

[0018] Furthermore, the compaction in step S4 includes initial compaction, intermediate compaction, and final compaction: initial compaction is performed by static compaction with a light roller 1-2 times; intermediate compaction is performed by strong vibration compaction with a heavy vibratory roller 3-5 times, with the wheel tracks overlapping by 1 / 3 of the wheel width; final compaction is performed by finishing compaction with a rubber-tired roller or a double-drum roller 1-2 times.

[0019] Furthermore, the maintenance period described in step S5 is more than 7 days, during which the roadbed surface is kept moist and traffic is closed.

[0020] A phosphogypsum-based roadbed filling material is prepared by any of the above-described preparation methods. The roadbed filling material includes pretreated phosphogypsum fines and a composite modifier. The composite modifier is uniformly distributed between the phosphogypsum particles to form a dense structure with ettringite and CSH gel as the main cementing products.

[0021] The present invention has the following beneficial effects:

[0022] 1. This invention achieves dual modification of phosphogypsum through the synergistic effect of multiple components in the composite modifier, including alkaline activation components, sulfate activation components, crystal form regulation and gradation optimization components, and impurity solidification components. Quicklime and cement clinker provide an alkaline environment to activate the pozzolanic activity of fly ash, mineral powder, and other siliceous alumina materials. Natural anhydrite supplements sulfate and promotes the formation of ettringite. Polyaluminum chloride acts as a crystal form regulator to optimize the crystal morphology of hydration products. Diatomite improves particle gradation and participates in secondary hydration reactions. The components work together to form a cemented network mainly composed of ettringite and CSH gel between phosphogypsum particles, resulting in a dense overall structure and improved mechanical properties.

[0023] 2. This invention incorporates a curing process after mixing, allowing the composite modifier to fully react with soluble phosphorus, fluorine, and other impurities in the phosphogypsum under a humid environment, generating stable calcium phosphate and calcium fluoride precipitates, thus chemically solidifying the harmful components. Simultaneously, the curing process ensures uniform moisture migration within the mixture and complete lime dissolution, preventing volume expansion or weak areas caused by uneven localized reactions during subsequent compaction. This process helps improve the material's water stability, reduces the risk of harmful substance leaching, and ensures the environmental safety of roadbed filling materials.

[0024] 3. This invention uses various industrial solid wastes as raw materials for composite modifiers, realizing the synergistic utilization of solid wastes such as carbide slag, fly ash, and mineral powder, and reducing the consumption of traditional cementitious materials such as cement and lime. In the preparation process, pre-homogenization treatment reduces the impact of raw material fluctuations, screening and crushing ensure reasonable particle size distribution, and dry and wet two-stage mixing ensures uniform distribution of modifiers. The overall process route has strong adaptability to phosphogypsum, large processing capacity, and is conducive to the large-scale consumption of phosphogypsum, which is in line with the development direction of resource recycling and low-carbon environmental protection.

[0025] Of course, any product implementing this invention does not necessarily need to achieve all of the advantages described above at the same time. Attached Figure Description

[0026] To more clearly illustrate the technical solutions of the embodiments of the present invention, the accompanying drawings used in the description of the embodiments will be briefly introduced below. Obviously, the drawings described below are only some embodiments of the present invention. For those skilled in the art, other drawings can be obtained based on these drawings without creative effort.

[0027] Figure 1 This is a schematic diagram illustrating the process of a modified phosphogypsum and its application in roadbed materials according to the present invention. Detailed Implementation

[0028] The technical solutions of the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention.

[0029] Please see Figure 1 As shown, this invention relates to a modified phosphogypsum and its application in roadbed materials, comprising the following steps:

[0030] Step S1: Pretreatment and homogenization of undisturbed phosphogypsum

[0031] Raw material receiving and initial inspection: The raw phosphogypsum, a byproduct of the wet-process phosphoric acid plant, is transported to the raw material storage shed; the initial moisture content of this raw phosphogypsum is usually 20%-30%, and the pH value is 2.5-5.0; during receiving, larger lumps of impurities mixed in (such as rubber lining that has fallen off the equipment, sawdust, etc.) are picked out manually or mechanically.

[0032] Pre-homogenization treatment: Pre-homogenization is carried out by flat-laying and direct extraction; different batches of phosphogypsum are laid in layers in the homogenization silo according to the feeding sequence, using a loader or a special stacker, with the thickness of each layer controlled at 30cm-50cm; after the layering is completed, the material is vertically extracted from the cross section of the pile from top to bottom. This method initially reduces the fluctuation of chemical indicators of the fed phosphogypsum, ensuring the stability of the subsequent modification process; the fluctuation range of the attached water content of the pre-homogenized phosphogypsum is controlled within ±3%;

[0033] Natural air drying or baking: Transfer the pre-homogenized phosphogypsum to the drying yard, spread it into a thin layer with a thickness of 20cm-30cm, and turn it over to dry using natural ventilation and sunlight, or use a rotary dryer to dry it when weather conditions do not permit, reducing the moisture content of the phosphogypsum to 8%-12%; this step is intended to create good flow conditions for the next step of screening and mixing of modifiers.

[0034] Screening and crushing: The dried phosphogypsum is fed into the screening system via a belt conveyor; a vibrating screen with a 5mm aperture is used for screening. The material on the screen (mainly gypsum lumps with a particle size greater than 5mm and a small amount of other impurities) is crushed to less than 5mm in a small crusher. The crushed material and the material under the screen (particle size less than 5mm) are mixed evenly on a mixing belt to obtain pretreated phosphogypsum fines with a particle size of less than 5mm.

[0035] Step S2: Preparation of the composite modifier

[0036] On a dedicated powder compounding production line, prepare and manufacture composite modifiers according to the following mass proportions:

[0037] Raw material weighing:

[0038] Alkaline activating components: Weigh 20-35 parts of quicklime (effective CaO content ≥80%, fineness passing through a 200-mesh sieve); weigh 10-20 parts of cement clinker (mainly tricalcium silicate and dicalcium silicate, fineness passing through a 200-mesh sieve).

[0039] Sulfate activating component: Weigh out natural anhydrite (anhydrous calcium sulfate, Content ≥85%, fineness passing through a 200-mesh sieve) 8-15 parts;

[0040] Crystal form regulation and gradation optimization components: Weigh 3-5 parts of polyaluminum chloride (industrial grade, alumina content ≥28%); weigh diatomaceous earth (mainly amorphous) after drying. (5-8 parts, fineness passing through a 325-mesh sieve);

[0041] Impurity solidification component: Weigh out carbide slag (effective) Content ≥65%, fineness passing through a 100-mesh sieve) 10-15 parts;

[0042] Mixing process: The weighed quicklime, cement clinker, natural anhydrite, polyaluminum chloride, diatomaceous earth and carbide slag are put into a vertical high-power stirring mill or a double cone high-efficiency mixer.

[0043] Grinding and homogenization: Turn on the equipment to mix and grind for no less than 15 minutes to ensure that all components are in full contact and further refined. The final output fineness is controlled so that all components pass through a 180-mesh sieve. After uniform mixing, a grayish-white powdery composite modifier is obtained. The modifier should be sealed in packaging to prevent moisture and failure.

[0044] Step S3: Mixing phosphogypsum and modifier

[0045] Ingredient metering: The pretreated phosphogypsum fines obtained in step S1 are metered using a belt scale; the composite modifier prepared in step S2 is metered using a screw scale; the mass ratio of the two is: 85-92 parts of pretreated phosphogypsum fines and 8-15 parts of composite modifier; this ratio can be adjusted according to the design strength requirements of the subgrade filling section, for example, a lower modifier dosage is used for the lower subgrade filling and a higher modifier dosage is used for the upper subgrade filling;

[0046] Primary mixing: The metered pretreated phosphogypsum fines and composite modifier are simultaneously fed into a twin-shaft horizontal mixer; the mixer speed is set to 60-80 rpm; the materials are continuously tumbled and propelled by the blades in the mixer, and preliminary dry mixing is carried out so that the modifier powder is evenly coated on the surface of the phosphogypsum particles; the dry mixing time is controlled to 45-60 seconds.

[0047] Secondary wet mixing and moisture adjustment: Simultaneously with the discharge of the primary mixing material, water accounting for 8%-12% of the total mass of the material (phosphogypsum + modifier) ​​is evenly sprayed in through an atomizing spray device installed at the rear of the mixer; the spray water pressure is controlled at 0.3-0.5MPa to ensure that the water is evenly distributed in a mist form; after adding water, the material continues to be mixed in the mixer for 120-180 seconds; during this process, the quicklime in the composite modifier rapidly dissipates and releases heat upon contact with water, and occurs simultaneously with the hydration reaction of cement clinker, providing an alkaline and sulfate-rich liquid phase environment; polyaluminum chloride, as an early strength agent and flocculant, promotes the rapid generation and flocculation sedimentation of hydration products; the goal of this stage is to precisely control the moisture content of the mixture within the range of the optimum moisture content ±1%, and to ensure that the mixture has a uniform appearance and color, without clumps or agglomerates;

[0048] Curing process: The mixed material is transferred to the indoor curing area via belt conveyor, piled into heaps no more than 2 meters high, and covered with plastic film to retain moisture; the curing time is 12-24 hours; this process is crucial, on the one hand, it allows the modifier to fully react with the soluble phosphorus and fluorine impurities in phosphogypsum to generate stable calcium phosphate and calcium fluoride precipitates, achieving chemical solidification of the impurities; on the other hand, it allows the internal moisture of the mixture to migrate more evenly and the lime to dissolve completely, laying the foundation for subsequent compaction and strength formation; after the curing is completed, if the surface of the mixture is slightly dry, a small amount of water can be sprayed to keep it moist.

[0049] Step S4: Subgrade filling and compaction

[0050] Construction preparation: The subgrade base to be filled shall be treated to ensure that the base is flat and dense and meets the specified compaction degree and deflection value requirements; piles and lines shall be driven according to the loose paving coefficient determined by the test section to control the paving thickness;

[0051] Paving: Transport the phosphogypsum-based mixture that has undergone curing to the construction site; use a soil stabilizer paver or grader for paving operations; the paving speed should be uniform and continuous to ensure that the surface of the paved layer is flat and free from segregation of coarse and fine particles; if local segregation occurs, it should be manually mixed immediately.

[0052] Crushing:

[0053] Initial compaction: After paving, compaction should be carried out immediately when the mixture is at or slightly above the optimum moisture content (not exceeding +2%). First, use a light roller (such as an 18-20 ton vibratory roller, without vibration) to statically compact the paved layer 1-2 times to initially stabilize it and obtain a smooth surface.

[0054] Secondary compaction: After the initial compaction, use a heavy vibratory roller (such as 22-26 tons) to compact 3-5 times with strong vibration. When compacting, follow the principle of light to heavy, slow to fast, and edge to center, with the wheel tracks overlapping by 1 / 3 of the wheel width. Vibratory compaction is to rearrange the mixture particles, expel air, achieve maximum density, and further stimulate the gelation reaction of the modifier.

[0055] Final compaction: After the secondary compaction is completed, use a rubber-tired roller or a double-drum roller (without vibration) to compact the surface 1-2 times to eliminate wheel tracks and make the surface smooth.

[0056] Compaction degree test: After compaction, the compaction degree is tested by sand cone method or nuclear density meter method; for the subgrade of expressway and first-class highway, the compaction degree is required to be no less than 96% (according to heavy compaction standard); if the compaction degree is insufficient, additional compaction should be carried out until the requirements are met.

[0057] Step S5, Maintenance

[0058] Initial curing: Curing should be carried out immediately after compaction and passing inspection; curing should be carried out by covering with geotextile or by watering with a water truck;

[0059] Watering maintenance: If watering maintenance is used, ensure that the water is sprayed evenly to keep the roadbed surface moist at all times and prevent the surface from drying and cracking; the watering frequency is determined according to the weather conditions, generally 2-4 times a day; during the maintenance period, traffic should be closed and vehicles are strictly prohibited from passing through.

[0060] Curing period: The curing period shall not be less than 7 days. During the curing period, the composite modifier in the phosphogypsum-based mixture will continue to undergo hydration, hardening and pozzolanic reactions, generating cementitious substances mainly composed of hydrated calcium silicate and ettringite. These substances will firmly bind the phosphogypsum particles into a whole and fill the pores between the particles, thereby forming a slab structure with sufficient strength and water stability. The next layer of filling or road structure construction can only be carried out after the 7-day curing period is completed.

[0061] The specific application of this embodiment is as follows:

[0062] Example 1

[0063] This embodiment provides a method for preparing a phosphogypsum-based highway subgrade filling material, which follows the steps described above.

[0064] Step S1: Pretreatment and homogenization of undisturbed phosphogypsum

[0065] Raw material receiving and initial inspection: Raw phosphogypsum, a byproduct of the wet-process phosphoric acid unit of a chemical plant in Hubei Province, was collected with an initial moisture content of 26.5% and a pH value of 3.2; visible impurities were manually removed.

[0066] Pre-homogenization treatment: Different batches of phosphogypsum are layered and piled in the homogenization silo, with each layer being 40cm thick and 2.5m high; after the piling is completed, material is taken vertically from the cross section of the pile to control the moisture content fluctuation within ±2.5%;

[0067] Natural sun-drying: The pre-homogenized phosphogypsum was transferred to the drying yard, spread to a thickness of 25cm, and sun-dried for 3 days. The moisture content was measured to have dropped to 10.5%.

[0068] Screening and crushing: A 5mm aperture vibrating screen is used for screening. The material on the screen is crushed to less than 5mm in a small crusher and mixed evenly with the material under the screen to obtain pretreated phosphogypsum fines.

[0069] Step S2: Preparation of the composite modifier

[0070] Raw material weighing: Prepare raw materials according to the following mass proportions: quicklime (effective CaO content 82%, 5% residue on 200 mesh sieve) 28 parts; cement clinker (PI 42.5 grade cement clinker, 3% residue on 200 mesh sieve) 15 parts; natural anhydrite ( 12 parts of polyaluminum chloride (88% content, 6% residue on 200-mesh sieve); 4 parts of polyaluminum chloride (industrial grade, alumina content 29%); 4 parts of diatomaceous earth (fineness 325 mesh). 6 portions of calcium carbide slag (75% content); 12 samples (68% purity, 8% residue on 100-mesh sieve);

[0071] Mixing process: Put all the above raw materials into a double cone high-efficiency mixer;

[0072] Grinding and homogenization: Turn on the equipment and mix for 20 minutes. The fineness of the output material is tested and all of them pass through a 180-mesh sieve to obtain a grayish-white powdered composite modifier, which is then sealed and packaged for later use.

[0073] Step S3: Mixing phosphogypsum and modifier

[0074] Ingredient metering: 88 parts of pretreated phosphogypsum fines and 12 parts of composite modifier are precisely metered by a belt scale and a screw scale according to the mass ratio.

[0075] Primary mixing: The metered materials are synchronously conveyed to the twin-shaft horizontal mixer, with a set speed of 70 rpm and a dry mixing time of 50 seconds;

[0076] Secondary wet mixing and moisture adjustment: After dry mixing, water accounting for 10% of the total mass of the material is sprayed in through an atomizing spray device at a water pressure of 0.4 MPa; after adding water, continue stirring for 150 seconds. The discharged mixture has a uniform color, can be formed into a ball when squeezed by hand, and can be dispersed when dropped.

[0077] Curing process: Transfer the mixture to the indoor curing area, stack it to a height of 1.8m, cover it with plastic film to keep it moist, and cure it for 18 hours; after curing, check that the moisture content of the mixture is within the range of ±1% of the optimum moisture content;

[0078] Step S4: Subgrade filling and compaction

[0079] Construction preparation: The foundation treatment is qualified, and piles and lines are driven according to a loose laying coefficient of 1.25;

[0080] Paving: Use a stabilized soil paver to evenly spread the mixture after curing, to a thickness of 25cm;

[0081] Crushing:

[0082] Initial compaction: 1 pass of static compaction with a 20t vibratory roller;

[0083] Secondary compaction: 22t vibratory roller with strong vibration for 4 passes, speed 2.5km / h;

[0084] Final compaction: The surface is compacted twice with a rubber-tired roller;

[0085] Compaction degree test: After rolling, the compaction degree was tested using the sand cone method and found to be 97.5%.

[0086] Step S5, Maintenance

[0087] Initial curing: Cover with geotextile immediately after compaction;

[0088] Watering maintenance: Water three times a day to keep the roadbed surface moist;

[0089] Maintenance period: 7 consecutive days of maintenance, during which traffic will be closed.

[0090] Comparative Example 1

[0091] This comparative example aims to verify the synergistic effect of the composite modifier of the present invention, rather than the effect of a single cementitious material;

[0092] The difference between this comparative example and Example 1 lies in steps S2 and S3:

[0093] Step S2: Cancel the preparation of the composite modifier; instead, directly purchase PO 42.5 ordinary Portland cement as the single modifier.

[0094] Step S3: When metering the ingredients, replace the composite modifier with an equal mass of 12 parts of PO 42.5 cement; the pretreated phosphogypsum fines remain at 88 parts; the water mixing, curing, rolling and curing processes are the same as in Example 1.

[0095] Comparative Example 2

[0096] This comparative example aims to verify the crucial role of blanching treatment in material properties, especially water stability and environmental safety;

[0097] The difference between this comparative example and Example 1 lies in step S3:

[0098] In step S3, the material processing step is omitted; that is, after the material completes the secondary wet mixing in the mixer, it is no longer piled up, covered and aged, but directly transported by belt conveyor to the construction site for paving and compaction; all other steps and parameters are consistent with those in Example 1.

[0099] Comparative Example 3

[0100] This comparative example aims to verify the necessity of crystal form regulation and gradation optimization components in composite modifiers;

[0101] The difference between this comparative example and Example 1 lies in step S2:

[0102] Step S2: When preparing the composite modifier, adjust the raw material formula; remove the addition of polyaluminum chloride and diatomaceous earth, and distribute their mass parts to quicklime and cement clinker according to the original proportion; the adjusted formula is: 33 parts quicklime, 20 parts cement clinker, 12 parts natural anhydrite, and 12 parts carbide slag (the total number of parts remains unchanged at 77 parts, and the proportion of phosphogypsum needs to be slightly adjusted accordingly, but to maintain the comparison, the materials are still mixed in step S3 according to the mass ratio of phosphogypsum:modifier = 88:12); the remaining preparation, mixing, rolling, and curing steps are the same as in Example 1.

[0103] Comparative Example 4

[0104] This comparative example aims to verify the rationality and criticality of the raw material ratio range defined in this invention;

[0105] The difference between this comparative example and Example 1 lies in the ingredient measurement in step S3:

[0106] Step S3: Adjust the mass ratio of ingredients; increase the amount of pretreated phosphogypsum fines to 96 parts, while reducing the amount of composite modifier to 4 parts (i.e., phosphogypsum:modifier = 96:4, the modifier dosage is lower than the lower limit of 8-15 parts specified in this invention); all other steps and parameters are consistent with those in Example 1.

[0107] Comparative Example 5

[0108] This comparative example aims to verify the superiority of the alkaline activation system of the present invention, which uses carbide slag, cement clinker and quicklime as alkaline activation agents, compared with traditional strong alkaline chemical activators, especially in terms of environmental safety and long-term stability.

[0109] The difference between this comparative example and Example 1 lies in steps S2 and S3:

[0110] Step S2: The preparation of the composite modifier is cancelled; instead, a traditional alkali activation solution is prepared; a composite alkali activator composed of sodium hydroxide and water glass (modulus 2.0) is prepared, wherein the concentration of NaOH is 4 mol / L and the amount of water glass is 8% of the mass of the mineral powder;

[0111] Step S3: During the batching and metering, the solid material is still 88 parts of pretreated phosphogypsum fines, but the cementing material is replaced by commercially available S95 mineral powder used alone, with a dosage of 12 parts; in the secondary wet mixing stage, ordinary water is not sprayed in, but an alkali activator solution prepared above with the same volume as in Example 1 is sprayed in; the mixed material is not subjected to a curing process (because the strong alkali activation reaction is rapid), and is directly spread and compacted, with the curing process being the same as in Example 1.

[0112] Performance Testing and Comparison

[0113] According to relevant methods, the performance of the roadbed filler prepared in the above embodiments and comparative examples was tested; the test items included: 7-day unconfined compressive strength, 28-day unconfined compressive strength, 28-day water stability coefficient, bearing ratio (CBR), and toxicity leaching after 28 days of curing; the test results are summarized in Table 1;

[0114] Table 1 Comparison of performance test results between the examples and the comparative examples

[0115] serial number 7d UCS(MPa) 28d UCS (MPa) 28-day water stability coefficient CBR(%) Toxicity leaching (mg / L) Example 1 1.25 2.01 0.79 83.5 Total phosphorus 0.3%, fluoride 2.1%, heavy metals not detected. Comparative Example 1 1.02 1.56 0.61 65.2 Total phosphorus 0.9%, fluoride 4.5%, heavy metals detected in trace amounts. Comparative Example 2 1.18 1.75 0.58 71.0 Total phosphorus 0.8%, fluoride 3.8%, heavy metals detected in trace amounts. Comparative Example 3 1.10 1.68 0.68 72.5 Total phosphorus 0.5%, fluoride 3.0%, heavy metals not detected. Comparative Example 4 0.65 0.92 0.45 38.7 Total phosphorus 2.1, fluoride 7.2, heavy metals detected. Comparative Example 5 1.30 (7d) 1.85 (28d) 0.71 78.0 Total phosphorus 0.2%, fluoride 2.5%, sodium ions leached in large quantities.

[0116] Comparative Example 1, which uses an equal amount of cement to replace the composite modifier, has lower 7-day and 28-day strength, water stability coefficient, and CBR value than Example 1. This indicates that the chemical activation and filling effect generated by the compounding of multiple solid waste components in this invention is superior to the hydration effect of single cement, demonstrating the synergistic effect of the composite modifier.

[0117] Comparative Example 2 omitted the curing step. Although the early strength was acceptable, the water stability coefficient dropped to 0.58 at 28 days, and the risk of toxic leaching increased. This proves that the curing treatment allows the modifier to fully react and solidify with the harmful impurities in phosphogypsum and allows moisture to migrate evenly. This is crucial for ensuring the long-term water stability and environmental safety of the material. This step is not optional.

[0118] The modifier in Comparative Example 3 lacked polyaluminum chloride and diatomaceous earth, resulting in a decrease in both its strength and water stability. This indicates that as components for crystal form regulation and gradation optimization, polyaluminum chloride can promote the early formation of ettringite and optimize crystal morphology, while diatomaceous earth can improve gradation, adsorb moisture, and participate in the pozzolanic reaction. Both work together and are indispensable.

[0119] Comparative Example 4 reduced the modifier dosage, resulting in a sharp decline in various mechanical properties and excessive toxicity leaching. This indicates that the modifier dosage range (8-15%) defined in this invention is the critical value for achieving effective curing and stabilization of phosphogypsum. Below this range, the basic requirements for roadbed filling cannot be met.

[0120] Comparative Example 5 used traditional strong alkali activation, which achieved higher early strength, but the 28-day strength growth rate slowed down, and there was a potential environmental risk of large-scale sodium ion leaching (which may lead to soil salinization); while Example 1 used an alkaline activation system based entirely on solid waste, which resulted in stable strength development and no risk of secondary pollution, and was more in line with the requirements of green, low-carbon and sustainable development.

[0121] In the description of this specification, references to terms such as "an embodiment," "example," "specific example," etc., indicate that a specific feature, structure, material, or characteristic described in connection with that embodiment or example is included in at least one embodiment or example of the invention. In this specification, illustrative expressions of the above terms do not necessarily refer to the same embodiment or example. Furthermore, the specific features, structures, materials, or characteristics described may be combined in any suitable manner in one or more embodiments or examples.

[0122] The preferred embodiments of the present invention disclosed above are merely illustrative of the invention. These preferred embodiments do not exhaustively describe all details, nor do they limit the invention to the specific implementations described. Clearly, many modifications and variations can be made based on the content of this specification. This specification selects and specifically describes these embodiments to better explain the principles and practical applications of the invention, thereby enabling those skilled in the art to better understand and utilize the invention. The invention is limited only by the claims and their full scope and equivalents.

Claims

1. A method for preparing a phosphogypsum-based highway subgrade filling material, characterized in that, The method includes the following steps: Step S1: Pre-treat the raw phosphogypsum to remove impurities and adjust the moisture content to obtain pre-treated phosphogypsum fines; Step S2: Prepare a composite modifier, wherein the composite modifier includes an alkaline activation component, a sulfate activation component, a crystal form regulation and gradation optimization component, and an impurity curing component; Step S3: Mix the pretreated phosphogypsum fines obtained in step S1 with the composite modifier prepared in step S2 at a mass ratio of 85-92:8-15, and perform first-stage dry mixing and second-stage wet mixing in sequence. During wet mixing, water is sprayed in to adjust the moisture content, and then the mixture is subjected to a curing process to obtain the mixture. Step S4: Spread and compact the mixture obtained in step S3 to form the roadbed filling layer; Step S5: Curing the compacted roadbed filling layer to obtain phosphogypsum-based highway roadbed filling material.

2. The method for preparing a phosphogypsum-based highway subgrade filling material according to claim 1, characterized in that, The composite modifier in step S2 is composed of the following raw materials in parts by weight: 20-35 parts quicklime, 10-20 parts cement clinker, 8-15 parts natural anhydrite, 3-5 parts polyaluminum chloride, 5-8 parts diatomaceous earth, and 10-15 parts carbide slag; the fineness of the composite modifier is all passed through a 180-mesh sieve.

3. The method for preparing a phosphogypsum-based highway subgrade filling material according to claim 1, characterized in that, The pretreatment in step S1 includes the following sub-steps: pre-homogenizing the undisturbed phosphogypsum, controlling the moisture content fluctuation within ±3% by layering and vertical material handling; drying the pre-homogenized phosphogypsum to a moisture content of 8%-12%; sieving through a 5mm aperture vibrating screen, crushing the material on the screen and mixing it with the material under the screen to obtain pre-treated phosphogypsum fines with a particle size of less than 5mm.

4. The method for preparing a phosphogypsum-based highway subgrade filling material according to claim 1, characterized in that, The first-stage dry mixing in step S3 is carried out in a twin-shaft horizontal mixer with a mixing speed of 60-80 rpm and a dry mixing time of 45-60 seconds; the second-stage wet mixing is carried out by spraying water accounting for 8%-12% of the total mass of the material through an atomizing spray device, with the water pressure controlled at 0.3-0.5 MPa, and stirring continues for 120-180 seconds after adding water.

5. The method for preparing a phosphogypsum-based highway subgrade filling material according to claim 1, characterized in that, The curing process described in step S3 takes 12-24 hours. During curing, the mixture is piled up to a height of no more than 2 meters and covered with plastic film to keep it moist.

6. The method for preparing a phosphogypsum-based highway subgrade filling material according to claim 1, characterized in that, The compaction described in step S4 includes initial compaction, intermediate compaction, and final compaction: initial compaction is carried out by static compaction with a light roller for 1-2 passes; intermediate compaction is carried out by strong vibration compaction with a heavy vibratory roller for 3-5 passes, with the wheel tracks overlapping by 1 / 3 of the wheel width; final compaction is carried out by finishing compaction with a rubber-tired roller or a double-drum roller for 1-2 passes.

7. The method for preparing a phosphogypsum-based highway subgrade filling material according to claim 1, characterized in that, The maintenance period described in step S5 is more than 7 days. During the maintenance period, the roadbed surface should be kept moist and traffic should be closed.

8. A phosphogypsum-based highway subgrade filling material, characterized in that, The roadbed filling material is prepared by any one of the preparation methods described in claims 1-7, and includes pretreated phosphogypsum fines and a composite modifier. The composite modifier is uniformly distributed between the phosphogypsum particles to form a dense structure with ettringite and CSH gel as the main cementing products.

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