A preparation method of a water-stable reinforced cement-soil mixing pile of a large-mixing-amount phosphogypsum

Abstract

This invention discloses a method for preparing water-stabilized and reinforced cement-soil mixing piles with a large dosage of phosphogypsum, belonging to the field of roadbed treatment technology. This invention constructs a fully dry powder modification system composed of phosphogypsum, cement, alkali activator, and crystal nucleation strengthener in a mass ratio of 1.8:8:0.2:0.2, and uses a two-step dry mixing method combined with ball milling to prepare the dry-mixed foundation material. During construction, the dosage of the foundation material is dynamically adjusted according to the moisture content of the soft soil, and piles are formed by combining pre-mixing, re-mixing, and compaction processes, followed by moisture retention curing. This invention utilizes the alkali activator to neutralize the acidity of phosphogypsum and solidify harmful impurities. Nano-calcium carbonate induces gypsum crystals to transform from plate-like to short columnar shapes, forming a dense structure together with the C-S-H gel generated by cement hydration, significantly improving the water stability and durability of the pile. This invention achieves high-dosage resource utilization of phosphogypsum, reduces costs, and achieves a 28-day water stability coefficient of 0.93 for the pile, outperforming traditional cement piles, making it suitable for reinforcing medium-layer soft soil subgrades.

Description

Technical Field

[0001] This invention belongs to the field of roadbed treatment technology in road engineering, and specifically relates to a method for preparing cement-soil mixing piles with high dosage of phosphogypsum and water-stabilized reinforcement. Background Technology

[0002] Cement-soil mixing piles, with their core advantages of convenient construction and controllable cost, have become the mainstream technology for soft soil subgrade reinforcement in transportation engineering, and are widely used in subgrade treatment scenarios for infrastructure such as highways and railways. However, the traditional cement-soil mixing pile process has long relied on pure cement or low-dosage industrial waste residue as cementing materials, which has exposed two major bottlenecks in practical applications: First, pure cement materials are expensive, and the economic benefits are poor when used on a large scale, making it difficult to meet the cost control requirements of engineering construction; second, when trying to add industrial solid waste such as phosphogypsum to reduce costs, the phosphogypsum itself contains harmful impurities such as soluble phosphorus and fluorine, and its crystal morphology is loose and flaky, which leads to a sharp decrease in the water stability of the pile body. When used for a long time in a high-humidity underground environment, it is prone to disintegration and strength decay, and cannot meet the engineering requirements for long-term load-bearing of the subgrade.

[0003] Currently, the improvement technologies in the industry for addressing the above-mentioned problems have obvious limitations: single alkali modification technology can only neutralize the acidic impurities of phosphogypsum, but cannot solve the inherent defect of its loose crystal structure. After the mixing pile is exposed to water, it is still prone to interface peeling and strength loss, and cannot fundamentally solve the contradiction between the application of large amounts of phosphogypsum and the water stability of the pile body.

[0004] With the advancement of policies promoting the resource utilization of industrial solid waste, the demand for large-scale disposal of phosphogypsum is becoming increasingly urgent, while the requirements for the water stability and bearing capacity of materials in the reinforcement of medium-layer soft soil subgrades are continuously increasing. Against this backdrop, there is an urgent need to develop a high-dosage phosphogypsum modification technology adapted to the mixing pile construction process. This technology would address the water stability defects caused by high-dosage phosphogypsum application at the mechanistic level, achieving the triple goals of large-scale solid waste disposal, improved engineering performance, and cost reduction, thus providing an efficient, economical, and environmentally friendly solution for the treatment of medium-layer soft soil subgrades. Summary of the Invention

[0005] To address the aforementioned shortcomings in the prior art, this invention aims to provide a method for preparing cement-soil mixing piles with high phosphogypsum content and water-stabilized reinforcement, thereby solving the problem of poor water stability of the pile body caused by high phosphogypsum content during the preparation process of existing cement-soil mixing piles.

[0006] To solve the above-mentioned technical problems, the present invention provides a method for preparing a water-stabilized and reinforced cement-soil mixing pile with a large amount of phosphogypsum, comprising: Step 1, Material Pretreatment: Dry the phosphogypsum to a moisture content of ≤4% and pass it through a 180-mesh sieve; preheat the 42.5 grade ordinary Portland cement to 50℃; prepare sodium carbonate, calcium hydroxide and nano calcium carbonate; Step 2: Preparation of the all-dry powder modification system: Weigh the pretreated phosphogypsum, cement, alkali activator, and crystal nucleation agent according to the mass ratio of 1.8:8:0.2:0.2. The alkali activator is a mixture of sodium carbonate and calcium hydroxide in a mass ratio of 1:1, and the crystal nucleation agent is nano-calcium carbonate with a particle size of 50-80nm. Step 3, Dry mixing of base materials: Add the phosphogypsum, alkali activator and crystal nucleation agent weighed in Step 2 into a mixer at a mass ratio of 9:2, and dry mix at a high speed of 350 r / min for 25 min; then add the cement weighed in Step 2, and continue to dry mix at a speed of 250 r / min for 20 min to obtain the dry-mixed base material; ball mill the dry-mixed base material to a particle size of 100 nm; Step 4: Soft soil compatibility test: Test the moisture content of the soft soil in the construction area, and select an area with a moisture content of 35%-50% for construction; Step 5, Construction of Mixing Pile: After pre-mixing the soft soil for 2 cycles, lift the pile driver at a uniform speed from the bottom to the top of the pile while simultaneously adding the dry-mixed foundation material obtained in Step 3. When the moisture content of the soft soil is 35%-40%, the amount of dry-mixed foundation material added is 20% of the dry soil mass; when the moisture content of the soft soil is 40%-50%, the amount of dry-mixed foundation material added is 30% of the dry soil mass. After the addition is completed, re-mix the entire pile at a speed of 120 r / min and compact the top 30 cm of the pile. Step 6, Water-stabilized curing: Cover the piles with geotextile after pile formation, and begin moisture-retaining curing 24 hours later.

[0007] The basic principle and beneficial effects of the method for preparing water-stabilized reinforced cement-soil mixing piles with high phosphogypsum content in this invention are as follows: Precise raw material proportioning and pretreatment: By limiting the mass ratio of phosphogypsum, cement, alkali activator (sodium carbonate and calcium hydroxide 1:1), and crystal nucleation strengthener (nano-calcium carbonate) to 1.8:8:0.2:0.2, a foundation was laid for subsequent chemical and physical reactions. This proportion ensured a large dosage of phosphogypsum (accounting for 20% of the total cementitious material), while the synergistic effect of the alkali activator and crystal nucleation strengthener solved the problems of acidity, impurity leaching, and loose crystal structure of phosphogypsum. Drying and sieving the phosphogypsum ensured its activity and uniformity; preheating the cement reduced its hygroscopicity, which is beneficial for uniform mixing with the dry powder.

[0008] The unique "two-step dry mixing method" and ultrafine ball milling: First, phosphogypsum is dry-mixed at high speed with an alkali activator and a crystal nucleation enhancer, allowing the alkaline substances and nanocrystal nuclei to preferentially and uniformly adhere to the surface of the phosphogypsum particles, creating optimal conditions for subsequent interfacial reactions and crystal reconstruction. Cement is then added for medium-speed mixing, avoiding interference from the cement with the preceding reactions. Finally, the particles are ball-milled to an ultrafine particle size of 100 nm, greatly increasing the specific surface area and reactivity of each component, ensuring sufficient contact between the modifier and the phosphogypsum particles, and providing ideal interfacial conditions for subsequent hydration reactions and structure formation in soft soil.

[0009] Precise matching and dynamic control of soft soil moisture content: The moisture content of the soft soil used in construction is limited to the optimal range of 35%-50% through testing. Furthermore, the dosage of dry-mixed foundation material is dynamically adjusted (20% or 30%) according to different moisture contents, solving the compatibility problem between soft soils with varying moisture content and dry powder materials, and ensuring the uniformity and density of the pile body.

[0010] Optimized construction process: Pre-mixing breaks up the agglomeration structure of soft soil, and uniform lifting and synchronous feeding ensure the even distribution of dry powder material in the pile body. Full pile re-mixing and pile top compaction further improve the integrity and density of the pile body, especially strengthening the key stress-bearing pile top area.

[0011] Scientific maintenance system: After pile formation, cover with geotextile in time to prevent moisture evaporation. After 24 hours, start watering maintenance with "moisturizing as the main function" to provide suitable humidity and temperature conditions for cement hydration and cementitious reaction, and avoid strength loss caused by early water loss and cracking or soaking in water.

[0012] Further, in step 1, the drying temperature of the phosphogypsum is 105-110℃, and the drying time is 4-6 hours; the cement preheating uses a constant temperature oven with a temperature fluctuation range of ±2℃. The above technical solution specifies the specific parameters for phosphogypsum drying and cement preheating. A drying temperature of 105-110℃ effectively removes free water from the phosphogypsum without damaging its crystal structure; a drying time of 4-6 hours ensures that the moisture content meets the standard. Using a constant temperature oven for cement preheating and controlling the temperature fluctuation within ±2℃ ensures the stability of the preheating effect, avoids abnormal cement hygroscopicity due to sudden temperature changes, and thus ensures the uniformity of mixing with the dry powder and the stability of the subsequent hydration reaction.

[0013] Furthermore, in step 3, the grinding media used in the ball mill is zirconia balls, with a ball-to-material ratio of 10:1, and an inert gas is introduced for protection during the ball milling process. In the above technical solution, using high-hardness, low-wear zirconia balls as the grinding media, with a ball-to-material ratio of 10:1, can efficiently grind the dry-mixed base material to an ultrafine particle size of 100nm. The introduction of an inert gas (such as nitrogen or argon) during the ball milling process effectively prevents the oxidation or agglomeration of nano-sized particles (especially nano-calcium carbonate) due to their high surface energy, ensuring the dispersibility and activity of the powder, thereby ensuring its full reaction with the soft soil during subsequent construction.

[0014] Furthermore, in step 3, the mixer used for dry mixing is a twin-shaft mixer with impellers made of high-manganese steel and chrome-plated, and the impeller gap is 1-2 mm. In the above technical solution, the twin-shaft mixer can provide stronger shear force and mixing efficiency. The impellers are made of high-manganese steel, which has good wear resistance; the chrome-plated surface further improves corrosion resistance and surface smoothness, reducing material adhesion. Controlling the impeller gap to 1-2 mm generates a strong shearing effect, ensuring that phosphogypsum and trace components such as alkali activators and nucleation strengtheners achieve ≥95% mixing uniformity during high-speed dry mixing, avoiding problems such as local agglomeration or incomplete reaction.

[0015] Furthermore, in step 5, the addition rate of the dry-mixed foundation material is 0.5 ± 0.05 kg / s. The above technical solution specifies the addition rate of the dry-mixed foundation material. A uniform addition rate of 0.5 ± 0.05 kg / s matches the lifting speed of the pile driver, ensuring that the dry powder material is evenly dispersed in the soft soil. This avoids localized accumulation of dry material due to excessively rapid addition or blank mixing areas due to excessively slow addition, thereby ensuring the uniformity of the pile material distribution and the consistency of its performance.

[0016] Furthermore, in step 5, the number of re-mixing cycles for the entire pile is 2.5 cycles, and the compaction time for the top 30cm area of ​​the pile is 60 seconds. In the above technical solution, the 2.5 cycles of re-mixing ensure thorough mixing of the dry-mix foundation material and the soft soil, resulting in a more complete reaction and a more uniform pile body. The additional 60 seconds of compaction for the top 30cm area of ​​the pile is a reinforcement treatment targeting the critical stress area at the pile top, which can significantly improve the density and bearing capacity of the pile top, effectively preventing settlement and damage caused by the upper load.

[0017] Furthermore, in step 5, before adding the dry-mixed base material, the soft soil is pre-stirred twice to break up its agglomerated structure. In the above technical solution, pre-stirring the in-situ soft soil before adding the dry powder material effectively breaks up the agglomerated structure present in the soft soil, making the soil loose and uniform. This creates favorable initial conditions for the subsequent addition and mixing of the dry powder material, thereby improving the mixing efficiency and uniformity of the dry powder material and the soft soil.

[0018] Furthermore, in step 6, the geotextile covering is carried out within 8 hours of pile formation; the moisturizing curing involves watering twice daily at 9:00 AM and 3:00 PM, ensuring the geotextile is moist but not dripping, with a total curing period of no less than 28 days. In the above technical solution, the first 8 hours after pile formation are the critical period for cement hydration reaction. Timely covering with geotextile can effectively prevent excessive evaporation of moisture from the pile surface, avoiding the formation of shrinkage cracks. After 24 hours, "moisturizing-oriented" watering curing is adopted, twice daily, ensuring the geotextile is moist but not dripping. This ensures sufficient moisture inside the pile for hydration reaction while avoiding excessive water soaking that could adversely affect early strength. The 28-day curing period ensures the full development of pile strength.

[0019] Furthermore, in step 6, the geotextile is a short-fiber needle-punched nonwoven geotextile with a unit area mass ≥200g / m² and a permeability coefficient ≥1×10⁻⁶. - ³cm / s. In the above technical solution, the short-fiber needle-punched nonwoven geotextile has good water retention and a certain degree of permeability. A unit area mass ≥200g / m² ensures its strength and water retention effect, and the permeability coefficient ≥1×10³ cm / s. - ³cm / s ensures that water can slowly penetrate, keeping the surface of the pile moist while preventing surface water accumulation due to the non-breathable covering, achieving the ideal maintenance state of "moisturizing" rather than "soaking".

[0020] Further, in step 6, the sodium carbonate has a purity ≥98% and a particle size ≥160 mesh; the calcium hydroxide has a purity ≥92% and a free water content ≤1%; and the nano-calcium carbonate has a purity ≥98%. In the above technical solution, the purity, particle size, and free water content of the alkali activator and crystal nucleation enhancer are strictly limited to ensure the stability of their reactivity and effect. High-purity, fine-particle-size sodium carbonate and calcium hydroxide can quickly and fully dissolve and participate in neutralization and precipitation reactions. Low free water content in calcium hydroxide ensures the concentration of its effective components. High-purity nano-calcium carbonate ensures its activity and effect in inducing crystal transformation and avoids interference from impurities in the reaction.

[0021] Compared with existing methods for preparing cement-soil mixing piles, the present invention has the following specific advantages: 1. Significantly Improved Water Stability and Durability of Pile: This invention utilizes a triple synergistic mechanism of "alkali activation - crystal nucleation induction - gel filling" to induce phosphogypsum crystals to transform from loose, flaky shapes into short columnar shapes, forming a dense microstructure of "crystal framework + gel filling," which significantly enhances the pile's resistance to water erosion. Tests show that the pile of this invention achieves a 28-day water stability coefficient of 0.93, a strength loss rate of only 7.2% after 7 days of water immersion, and a stability coefficient as high as 0.89 after 20 freeze-thaw cycles. Furthermore, the strength attenuation rates after immersion in 5% NaCl and 5% Na2SO4 solutions for 28 days are only 6.0% and 6.9%, respectively, far superior to traditional pure cement piles. This allows the pile to adapt to complex service environments such as rainy, cold, and saline-alkali lands, ensuring long-term stability of the roadbed.

[0022] 2. Achieving large-scale resource utilization of phosphogypsum with outstanding environmental benefits: This invention breaks through the technical bottleneck of existing technologies where the phosphogypsum content does not exceed 10%, increasing the content of phosphogypsum in cementitious materials to 20%, with a single pile capable of absorbing approximately 0.8 tons of phosphogypsum. Through an alkali activator, soluble phosphorus and fluorine are converted into hydroxyapatite and calcium fluoride precipitates, effectively solidifying harmful impurities. The soluble phosphorus leaching amount is only 0.017 mg / L, far below environmental safety standards, achieving large-scale, high-value, and safe disposal of industrial solid waste.

[0023] 3. Significantly reduced engineering material costs and substantial economic benefits: This invention uses phosphogypsum to replace 20% of cement and employs a dry construction process, significantly reducing raw material costs. Material costs have decreased from 140 yuan / meter for traditional pure cement piles to 92 yuan / meter, a reduction of 34%. Simultaneously, the dry construction method eliminates the need for slurry mixing, transportation, and storage, increasing construction efficiency by approximately 20% and shortening the construction period by approximately 15%, resulting in considerable overall economic benefits.

[0024] 4. Optimized construction techniques and improved project quality controllability: The "two-step dry mixing method" proposed in this invention, combined with ultrafine ball milling, ensures uniform adhesion and reactivity of the modified components on the phosphogypsum surface; the dynamic adjustment of the dry powder content (20%-30%) based on the soft soil moisture content (35%-50%) solves the compatibility problem between soft soil with different moisture levels and the curing agent; the combined process of pre-mixing, re-mixing, and pile top compaction ensures the uniformity and density of the pile body. These process optimizations significantly improve the stability and controllability of construction quality and reduce the project quality risks caused by construction fluctuations.

[0025] 5. Excellent comprehensive mechanical properties, meeting high engineering standards: The mixing piles prepared by this invention achieve an unconfined compressive strength of 1567 kPa after 28 days, which is 39.5% higher than that of traditional cement piles; the vertical bearing capacity of a single pile is 185 kPa, meeting the engineering design requirement of ≥180 kPa for intermediate roadbeds. Furthermore, the pile body exhibits no shrinkage in later stages, and the CSH gel continues to form with age, further filling the pores and ensuring stable and reliable long-term bearing capacity.

[0026] In summary, this invention achieves the resource utilization of phosphogypsum with large dosage while comprehensively improving the water stability, durability, mechanical properties, and economy of cement-soil mixing piles. It provides an efficient, economical, and environmentally friendly solution for the treatment of medium-layer soft soil subgrades and has broad prospects for promotion and application. Attached Figure Description

[0027] Figure 1 This is a flowchart of a method for preparing a water-stabilized, reinforced cement-soil mixing pile with a large amount of phosphogypsum according to the present invention.

[0028] Figure 2 A scanning electron microscope schematic diagram of a cross-section of a cement-soil mixing pile prepared by the method of the present invention.

[0029] Figure 3 This is a schematic diagram of a scanning electron microscope image of a pure cement pile.

[0030] Figure 4 X-ray diffraction analysis of a slice of cement-soil mixing pile prepared by the method of the present invention.

[0031] Figure 5 This is an X-ray diffraction analysis diagram of a pure cement pile. Detailed Implementation

[0032] The specific embodiments of the present invention are described below to enable those skilled in the art to understand the present invention. However, it should be understood that the present invention is not limited to the scope of the specific embodiments. For those skilled in the art, various changes are obvious as long as they are within the spirit and scope of the present invention as defined and determined by the appended claims. All inventions utilizing the concept of the present invention are protected.

[0033] Example 1 like Figure 1 As shown in the figure, this embodiment provides a method for preparing cement-soil mixing piles with high dosage of water-stabilized and reinforced phosphogypsum. The specific steps are as follows: Step 1: Material Preparation and Pretreatment: Phosphogypsum: Dihydrate phosphogypsum, a byproduct of a phosphate chemical company, is presented as a grayish-white, loose powder. The phosphogypsum is dried in a 105℃ constant temperature oven for 5 hours. After drying, the moisture content is determined using the Karl Fischer method to be 3.2%, meeting the requirement of ≤4%. The dried phosphogypsum is then fed into a ball mill, milled, and passed through a 180-mesh standard sieve. The residue rate is less than 5%, and it is ready for use.

[0034] Cement: PO 42.5 grade ordinary Portland cement is used, with an initial setting time of not less than 45 min, a final setting time of not more than 600 min, a 3-day compressive strength ≥ 17.0 MPa, and a 28-day compressive strength ≥ 42.5 MPa. The cement should be preheated to 50℃ in a constant temperature oven 24 hours in advance, with temperature fluctuations controlled within ±2℃.

[0035] Alkali activator: Accurately weigh sodium carbonate and calcium hydroxide at a mass ratio of 1:1. The sodium carbonate is industrial grade, with a purity of 98.5% and a particle size of 180 mesh (≥160 mesh); the calcium hydroxide is industrial grade, with a purity of 92.3% and a free water content of 0.8% (≤1%). Mix the two in a V-type mixer at a speed of 30 r / min for 30 min to obtain a homogeneous composite alkali activator.

[0036] Crystal nucleation enhancer: Industrial-grade nano calcium carbonate with a purity of 98.5%, an average particle size of 60nm (within the range of 50-80nm), and a specific surface area ≥30m² / g.

[0037] Step 2: Preparation of the all-dry powder modification system: Weigh 180 kg of pretreated phosphogypsum, 800 kg of cement, 20 kg of alkali activator, and 20 kg of crystal nucleation strengthener according to the mass ratio of 1.8:8:0.2:0.2, for a total of 1020 kg.

[0038] Step 3, Dry Mixing and Blending of Base Materials: First step dry mixing (high-speed premixing): Weigh out 180kg of phosphogypsum, 20kg of alkali activator, and 20kg of crystal nucleation enhancer (totaling 220kg, with a mass ratio of phosphogypsum to alkali activator and crystal nucleation enhancer of 9:2) and add them to a twin-shaft mixer. This mixer is equipped with high-manganese steel impellers, the impeller surface is chrome-plated, and the impeller gap is controlled at 1.5mm (within the range of 1-2mm). Start the mixer and dry mix at a high speed of 350r / min for 25 minutes to ensure that the alkali activator and crystal nucleation enhancer are evenly adhered to the surface of the phosphogypsum crystals, activating their reactivity.

[0039] The second step is dry mixing (medium-speed mixing): Keep the mixer running, add 800 kg of preheated cement (the mass ratio of phosphogypsum to cement is 1.8:8), adjust the mixer speed to 250 r / min, and continue dry mixing for 20 minutes to fully mix all components and form a dry-mixed base material with uniform particle size and no lumps.

[0040] Ultrafine ball milling: The above-mentioned dry-mixed base material was fed into a nanoscale ball mill. The milling media were zirconia balls, with a ball-to-material ratio of 10:1. During the ball milling process, high-purity nitrogen was continuously introduced as an inert gas to prevent oxidation and agglomeration of nanoparticles. The ball milling was continued until the D50 particle size, as measured by a laser particle size analyzer, was 100 nm, thus obtaining a completely dry powder modified system. The specific surface area of ​​the powder after ball milling was significantly increased, and the contact between the components was more sufficient, providing ideal interfacial conditions for subsequent hydration reactions.

[0041] Step 4: Soft Soil Compatibility Testing: In a subgrade treatment section of a highway project, the moisture content of the soft soil in the construction area was tested using a ring cutter method at a grid size of 5m × 5m. A total of 30 points were tested, with the moisture content ranging from 35% to 48%. Based on the test results, an area with a moisture content of 38% was selected as construction zone A, and an area with a moisture content of 45% was selected as construction zone B. For areas with a moisture content below 35% or above 50%, the method of this invention was not used for construction to ensure the quality of the pile body.

[0042] Step 5, Construction of Mixing Pile Formation: Pile Driver Positioning: A φ500 type variable frequency mixing pile driver is used, with a drill rod verticality deviation of ≤1% and a pile position deviation of ≤50mm.

[0043] Pre-mixing: Start the piling machine and lower the drill rod to the designed elevation of the pile bottom. Before lifting, pre-mix the material by 2 rotations to effectively break the natural agglomeration structure and stratification of the soft soil, creating favorable conditions for the uniform mixing of dry powder materials.

[0044] Lifting and Synchronous Material Feeding: The drill rod is lifted uniformly from the pile bottom at a speed of 0.5 m / min, while the matching spiral dry powder feeder is activated to synchronously feed the dry-mixed foundation material prepared in step 3 at a rate of 0.5 ± 0.05 kg / s. Specifically, for construction area A with a moisture content of 38%, the amount of dry-mixed foundation material added is 20% of the dry soil mass; for construction area B with a moisture content of 45%, the amount of dry-mixed foundation material added is 30% of the dry soil mass. The control system ensures precise matching between the lifting speed and the feeding rate to avoid localized accumulation of dry material or blank mixing areas.

[0045] Re-mixing and compaction: After the material is fed in, the drill rod is lowered back to the bottom of the pile and the entire pile is re-mixed at a speed of 120 r / min for 2.5 revolutions to ensure that the dry-mixed foundation material and soft soil are uniformly mixed at the molecular level. After re-mixing, the top 30cm of the pile is additionally compacted for 60 seconds to improve the density and bearing capacity of the upper part of the pile.

[0046] Step 6, Water-Stabilized Curing: Early Covering: Within 8 hours of pile completion, immediately cover the top of the pile with a layer of short-fiber needle-punched nonwoven geotextile. The geotextile has a unit area mass of 220g / m² (≥200g / m²) and a permeability coefficient of 5×10⁻⁶. - ³cm / s (≥1×10 - (³cm / s), which can effectively prevent the moisture on the pile surface from evaporating too quickly, and also has good air permeability.

[0047] Moisturizing and Curing: Formal curing will begin 24 hours after pile completion. The curing method involves watering twice daily, at 9:00 AM and 3:00 PM, ensuring the geotextile is kept moist but not dripping. The total curing period should be no less than 28 days. During the curing period, large-scale water accumulation or heavy machinery compaction around the pile is strictly prohibited.

[0048] Performance test results According to the "Test Procedure for Inorganic Binder Stabilized Materials in Highway Engineering" (JTG E51-2009) and the "Standard for Geotechnical Testing Methods" (GB / T 50123-2019), the performance of the mixing piles prepared in Example 1 was tested after 28 days of curing. The results are as follows: (1) Verification of the neutralization effect of the alkali activator The all-dry powder modified system prepared in Example 1 was used to prepare a slurry at a mass concentration of 10%. The pH value was measured using a pH meter with an accuracy of 0.01, and the result was 9.33, which is completely consistent with the experimental data (pH 9.33) when the alkali activator dosage was 2.0% as stated in the disclosure. This proves that the formulation of the present invention (alkali activator accounting for 2.0% of the dry weight of phosphogypsum) can effectively raise the pH of the phosphogypsum slurry from the initial 4.20 to 9.33, providing an ideal alkaline environment for the subsequent conversion of soluble phosphorus and fluorine into hydroxyapatite and calcium fluoride precipitation.

[0049] (2) The mechanical properties and water stability properties are shown in the table below:

[0050] (3) Microstructure verification: SEM (Scanning Electron Microscopy) observation: such as Figure 2 and Figure 3 As shown, in the pile body slices of this invention, the phosphogypsum crystals are short columnar, and the contact between crystals changes from point contact to surface contact. They are tightly interwoven with the amorphous CSH gel generated by cement hydration, forming a dense "crystal skeleton + gel filling" structure. At the same time, hydroxyapatite and calcium fluoride precipitates adhere to the crystal surface, forming an effective "impurity barrier." In contrast, the phosphogypsum crystals in the control group are loosely flaky, with large gaps, no obvious gel filling, and a loose structure.

[0051] XRD (X-ray diffraction) analysis: such as Figure 4As shown in Figure 5, the present invention group exhibits calcium hydroxyphosphate (Ca) at 2θ = 25.9°. 10 The characteristic diffraction peaks of (PO4)6(OH)2 were observed, and the characteristic diffraction peaks of calcium fluoride (CaF2) appeared at 2θ=28.2°. The diffraction peak intensity of CSH gel at 2θ=29.4° was significantly higher than that of the control group. This confirms the effective synergy of the triple mechanism of "alkali-induced impurity removal-nucleation reconstruction-silicon-based gelation".

[0052] (4) Moisture content suitability verification The 28-day unconfined compressive strength of piles was tested under different soft soil moisture contents using a 20% dry powder foundation material addition method. The results are shown in the table below:

[0053] The results show that the optimal range for soft soil moisture content is 35%-50%, which is completely consistent with the construction moisture content range defined in this invention.

[0054] (5) Water stability test verification Short-term water stability: Using the formulation of this invention, the water stability coefficient was tested at different curing ages, and the results are as follows:

[0055] Long-term water stability (freeze-thaw cycles): Freeze-thaw cycle tests were conducted according to JTG E51-2009, and the results are as follows:

[0056] Resistance to ionic corrosion: Tested after immersion in 5% NaCl solution and 5% Na2SO4 solution for 28 days:

[0057] Example 2 This embodiment is basically the same as Embodiment 1, except that in step 3, the ball milling process was not protected by inert gas. The remaining process parameters are consistent with Embodiment 1. After 24 hours of storage, scanning electron microscopy revealed slight agglomeration of the prepared dry-mixed base material, with uneven particle size distribution. After 28 days of curing, the prepared mixing piles showed the following test results: 28-day unconfined compressive strength of 1480 kPa, 28-day water stability coefficient of 0.91, and 7-day strength loss rate after water immersion of 8.5%. Although these indicators are slightly lower than those of Embodiment 1, they are still significantly better than traditional pure cement piles, demonstrating that the use of inert gas protection is crucial for ensuring the dispersibility and modification effect of nanomaterials.

[0058] Example 3 This embodiment is basically the same as Embodiment 1, except that in step 5, the number of re-mixing cycles for the entire pile is adjusted to 1.5 cycles, while the remaining process parameters are the same as in Embodiment 1. Due to insufficient re-mixing cycles, the uniformity of the dry-mixed foundation material mixed with the soft soil decreased. After 28 days of curing, the test results showed that the 28-day unconfined compressive strength was 1430 kPa, the 28-day water stability coefficient was 0.90, and the strength loss rate after 7 days of water immersion was 8.8%. The results indicate that appropriately increasing the number of re-mixing cycles helps improve the uniformity and overall performance of the pile body, and the 2.5 re-mixing cycles verified in the disclosure document are the optimal process parameters.

[0059] Example 4 This embodiment is basically the same as Embodiment 1, except that in step 5, for construction area B with a moisture content of 45%, the dry-mixed foundation material is still added at 20%, not adjusted to 30%. After 28 days of curing, the test results show that the 28-day unconfined compressive strength is 1250 kPa, the 28-day water stability coefficient is 0.88, and the strength loss rate after 7 days of water immersion is 9.5%. The results indicate that in soft soil with high moisture content, if the amount of solidifying agent is not increased accordingly, the strength and stability of the pile will decrease significantly. This verifies the necessity of dynamically adjusting the dosage according to the moisture content, which is consistent with the conclusion in the instruction manual that the dry material addition should be increased to 30% when the moisture content is 40%-50%.

[0060] Comparative Example 1 (Traditional pure cement pile) This comparative example uses a traditional wet shotcrete process to prepare pure cement-soil mixing piles. Only PO 42.5 grade ordinary Portland cement is used as the cementing material, with a water-cement ratio of 0.5 and a cement content of 20% of the dry soil mass. During construction, cement slurry is injected into the soft soil through a drill rod and mixed to form piles. The curing regime is the same as in Example 1. After 28 days of curing, the test results show: 28-day unconfined compressive strength is 1120 kPa, 28-day water stability coefficient is 0.90, 7-day strength loss rate after water immersion is 10.2%, single pile vertical bearing capacity is 190 kPa, and material cost is 140 yuan / meter. Compared with this invention, although the bearing capacity is basically the same, the water stability is slightly worse, the cost is 34% higher, and it cannot dispose of phosphogypsum solid waste.

[0061] Comparative Example 2 (without using the "two-step dry mixing method" and ultrafine ball milling) This comparative example used the same material ratio as Example 1 (phosphogypsum:cement:alkali activator:nucleation agent = 1.8:8:0.2:0.2), but did not employ the "two-step dry mixing method" and ultrafine ball milling process. Instead, all dry powder materials were simply mixed once (stirring time 10 min) and used directly. The remaining construction and curing processes were the same as in Example 1. After 28 days of curing, the test results showed: the 28-day unconfined compressive strength was 1280 kPa, the 28-day water stability coefficient was 0.85, the strength loss rate after 7 days of water immersion was 12.5%, and the soluble phosphorus leaching amount was 0.35 mg / L. SEM observation showed that the phosphogypsum was still mainly in a loose flake state, with no obvious crystal reconstruction or gel filling. The results indicate that without the "two-step dry mixing method" and ultrafine ball milling treatment, the alkali activator and nucleation agent cannot effectively adhere to the surface of the phosphogypsum to exert their modifying effect, resulting in a significant decrease in the modification effect.

[0062] Comparative Example 3 (different proportions of alkali activator) This comparative example used the same material ratios and preparation process as Example 1, except that the mass ratio of sodium carbonate to calcium hydroxide in the alkali activator was 2:1, while the remaining process parameters were the same as in Example 1. After 28 days of curing, the test results showed that the 28-day unconfined compressive strength was 1350 kPa, the 28-day water stability coefficient was 0.87, and the strength loss rate after 7 days of water immersion was 9.8%. Analysis suggests that the change in the alkali activator ratio resulted in insufficient pH adjustment capability of the system, affecting the neutralization effect of free acids in phosphogypsum and the precipitation efficiency of soluble phosphorus and fluorine.

[0063] Comparative Example 4 (different particle sizes of crystal nucleation reinforcing agents) This comparative example used the same material ratios and preparation process as Example 1, except that ordinary light calcium carbonate (average particle size 5 μm) was used as the nucleation strengthener instead of nano-calcium carbonate. All other process parameters were the same as in Example 1. After 28 days of curing, the test results showed that the 28-day unconfined compressive strength was 1320 kPa, the 28-day water stability coefficient was 0.86, and the strength loss rate after 7 days of water immersion was 10.1%. SEM observation showed that the phosphogypsum crystals were still predominantly platy, with no obvious reconstruction of short columnar crystals. The results indicate that ordinary particle size calcium carbonate cannot effectively induce crystal transformation in gypsum crystals, and the nanoscale is key to the nucleation strengthening effect.

[0064] In summary, the dry powder modification system ratio (phosphogypsum: cement: alkali activator: crystal nucleation agent = 1.8:8:0.2:0.2) defined in this invention can effectively neutralize the free acid in phosphogypsum (pH increases from 4.2 to 9.3) and convert soluble phosphorus and fluorine into hydroxyapatite and calcium fluoride precipitates, thereby solidifying impurities.

[0065] The "two-step dry mixing method" combined with ultrafine ball milling to 100nm ensures that the alkali activator and nano-calcium carbonate are uniformly attached to the phosphogypsum surface, providing ideal interface conditions for subsequent crystal reconstruction and gelation reaction.

[0066] Nano-sized calcium carbonate (50-80nm) reacts with gypsum crystals (Ca²⁺) via surface hydroxyl groups. + Hydrogen bonds were formed, successfully inducing the transformation of platy dihydrate gypsum crystals into short columnar crystals. The crystal contact mode changed from point contact to surface contact, which significantly reduced the water penetration channels (confirmed by SEM observation).

[0067] Based on the dynamic dosage control (20%-30%) of soft soil moisture content (35%-50%), the compatibility problem of mixing soft soil with dry powder with different moisture content was solved, ensuring the quality and stability of the pile body.

[0068] The mixing pile prepared by this invention has a 28-day water stability coefficient of 0.93, a strength loss rate of only 7.2% after 7 days of water immersion, and an unconfined compressive strength of 1567 kPa after 28 days. The material cost is reduced by 34% compared with pure cement piles, achieving the triple goals of large-volume resource utilization of phosphogypsum, improved engineering performance, and reduced cost.

Claims

1. A method for preparing a water-stabilized, high-dosage phosphogypsum-based cement-soil mixing pile, characterized in that, include: Step 1, Material Pretreatment: Dry the phosphogypsum until the moisture content is ≤4% and pass it through a 180-mesh sieve; Preheat the 42.5 grade ordinary Portland cement to 50℃; Prepare sodium carbonate, calcium hydroxide, and nano-calcium carbonate; Step 2: Preparation of the all-dry powder modification system: Weigh the pretreated phosphogypsum, cement, alkali activator, and crystal nucleation agent according to the mass ratio of 1.8:8:0.2:0.

2. The alkali activator is a mixture of sodium carbonate and calcium hydroxide in a mass ratio of 1:1, and the crystal nucleation agent is nano-calcium carbonate with a particle size of 50-80nm. Step 3, Dry mixing of base materials: Add the phosphogypsum, alkali activator and crystal nucleation agent weighed in step 2 into the mixer at a mass ratio of 9:2, and dry mix at a high speed of 350 r / min for 25 min; Add the cement weighed in step 2, and continue dry mixing at 250 r / min for 20 min to obtain the dry-mixed base material; ball mill the dry-mixed base material to a particle size of 100 nm; Step 4: Soft soil compatibility test: Test the moisture content of the soft soil in the construction area, and select an area with a moisture content of 35%-50% for construction; Step 5, Construction of Mixing Pile: After pre-mixing the soft soil for 2 cycles, lift the pile driver at a uniform speed from the bottom to the top of the pile while simultaneously adding the dry-mixed foundation material obtained in Step 3. When the moisture content of the soft soil is 35%-40%, the amount of dry-mixed foundation material added is 20% of the dry soil mass; when the moisture content of the soft soil is 40%-50%, the amount of dry-mixed foundation material added is 30% of the dry soil mass. After the addition is completed, re-mix the entire pile at a speed of 120 r / min and compact the top 30 cm of the pile. Step 6, Water-stabilized curing: Cover the piles with geotextile after pile formation, and begin moisture-retaining curing 24 hours later.

2. The method for preparing water-stabilized reinforced cement-soil mixing piles with high phosphogypsum content according to claim 1, characterized in that, In step 1, the drying temperature of the phosphogypsum is 105-110℃ and the drying time is 4-6 hours; the cement preheating is carried out in a constant temperature oven with a temperature fluctuation range of ±2℃.

3. The method for preparing water-stabilized reinforced cement-soil mixing piles with high phosphogypsum content according to claim 1, characterized in that, In step 3, the ball milling medium is zirconia balls, the ball-to-material ratio is 10:1, and an inert gas is introduced for protection during the ball milling process.

4. The method for preparing water-stabilized reinforced cement-soil mixing piles with high phosphogypsum content according to claim 1, characterized in that, In step 3, the mixer used for dry mixing is a twin-shaft mixer with impellers made of high manganese steel and chrome-plated, and the impeller gap is 1-2mm.

5. The method for preparing water-stabilized reinforced cement-soil mixing piles with high phosphogypsum content according to claim 1, characterized in that, In step 5, the addition rate of the dry-mixed base material is 0.5 ± 0.05 kg / s.

6. The method for preparing water-stabilized reinforced cement-soil mixing piles with high phosphogypsum content according to claim 1, characterized in that, In step 5, the number of rounds of full pile re-mixing is 2.5, and the compaction time for the top 30cm of the pile is 60s.

7. The method for preparing water-stabilized reinforced cement-soil mixing piles with high phosphogypsum content according to claim 1, characterized in that, In step 5, before adding the dry-mixed foundation material, the soft soil is pre-mixed twice to break up the soft soil agglomeration structure.

8. The method for preparing water-stabilized reinforced cement-soil mixing piles with high phosphogypsum content according to claim 1, characterized in that, In step 6, the geotextile covering is carried out within 8 hours of pile formation; the moisturizing maintenance is to sprinkle water twice a day at 9:00 and 15:00, so that the geotextile is moist but not dripping, and the total maintenance period is not less than 28 days.

9. The method for preparing water-stabilized reinforced cement-soil mixing piles with high phosphogypsum content according to claim 8, characterized in that, In step 6, the geotextile is a short fiber needle-punched nonwoven geotextile with a unit area mass of ≥ 200 g / m2and a permeability coefficient of ≥ 1 x 10 - ³ cm / s.

10. The method for preparing water-stabilized reinforced cement-soil mixing piles with high phosphogypsum content according to claim 1, characterized in that, In step 6, the sodium carbonate has a purity ≥ 98% and a particle size ≥ 160 mesh; the calcium hydroxide has a purity ≥ 92% and a free water content ≤ 1%; and the nano-calcium carbonate has a purity ≥ 98%.

Images