Roadbed material of phosphogypsum treated by soil coagulated rock and preparation method of roadbed material

Abstract

The invention discloses a roadbed material of ardealite treated by soil coagulated rock and a preparation method of the roadbed material, and relates to the technical field of roadbed material preparation, the preparation method comprises the following steps: 1) pretreatment of ardealite; (2) mixing roadbed materials; and (3) paving, compacting and maintaining. The geopolymerization reaction of the soil coagulated rock is utilized, the soil coagulated rock and the ardealite are cooperated to generate a'gel-ettringite 'compact structure, harmful impurities are thoroughly solidified, and the roadbed material is endowed with the characteristics of high strength and stability; by introducing amido and phosphonic acid groups into the modified naphthalene water reducer, efficient water reduction is realized, the compactness is improved, heavy metal ions can be strongly chelated, chemical fixation is formed, and phosphogypsum is harmless; the two components cooperate with each other, so that large-scale resource utilization of the phosphogypsum is realized, the long-term mechanical property and environmental safety of roadbed engineering are ensured, and remarkable environmental protection and economic benefits are achieved.

Description

Technical Field

[0001] This invention relates to the field of roadbed material technology, and in particular to roadbed materials for soil-concrete-rock treated phosphogypsum and their preparation methods. Background Technology

[0002] Phosphogypsum is a large-scale industrial solid waste generated during the wet-process phosphoric acid production process, with approximately 4.5-5 tons of phosphogypsum produced as a byproduct for every ton of phosphoric acid produced. Its large-scale stockpiling not only occupies land but also poses a serious environmental pollution risk due to its content of harmful impurities such as soluble phosphorus, fluorine, organic matter, and trace heavy metals.

[0003] Currently, the resource utilization of phosphogypsum is mainly concentrated in the building materials field, such as the preparation of gypsum board and cement retarders. However, the utilization rate is low and it faces key technological bottlenecks. These impurities, especially soluble phosphorus and fluorine, can severely delay or even inhibit the hydration process of traditional silicate cement, resulting in products made from it having low strength, unstable setting, and poor durability. Although pretreatment methods such as water washing and lime neutralization are often used, they often have problems such as high cost, high energy consumption, or incomplete curing. Harmful ions still pose a risk of leaching during long-term use, making it difficult to meet the requirements of high strength, high durability, and environmental safety for roadbed materials.

[0004] Soil-coagulated rock is a chemical agent designed to re-coagulate and gradually lithify soil based on its material source and physical and chemical properties, aiming to achieve certain engineering mechanics goals. It is based on the theories of rock and soil mechanics and the principles of petrology and geology, taking into account the chemical element composition requirements of geological diagenesis and the conditions and laws of geological mineralization. It can be designed for specific engineering mechanics needs by using geochemical coagulation and mineralization technology to design the chemical and mineral composition.

[0005] The process of treating phosphogypsum with soil-concrete composites is expected to develop a synergistic treatment technology that can completely solidify harmful impurities while stimulating the potential cementitious activity of phosphogypsum itself, thereby promoting the harmless, large-scale, and high-value utilization of phosphogypsum resources. Summary of the Invention

[0006] The purpose of this invention is to address the shortcomings of existing technologies by proposing a roadbed material for treating phosphogypsum with soil-concrete composite and its preparation method.

[0007] To achieve the above objectives, the present invention adopts the following technical solution: This invention first proposes a method for preparing roadbed materials using soil-concrete-rock treated phosphogypsum, comprising the following steps: S1. Pretreatment of phosphogypsum: Raw phosphogypsum and steel slag powder were added to a mixing mill at a mass ratio of 10:1, passed through a 45μm square hole sieve, and 15% of the material mass of process water was added. The mixture was stirred in a forced mixer at 30-40 rpm for 10-15 minutes until a uniform wet mixture was formed. The wet mixture was then transferred to an aging chamber and sealed and aged for 3-4 days under ambient temperature ≥25℃ and relative humidity ≥85%. After passing through a 5mm sieve, the pretreated phosphogypsum mixture was obtained. Steel slag powder is rich in active components such as f-CaO (free calcium oxide) and calcium silicate. During the mixing and aging process, f-CaO reacts with water to generate Ca(OH)2, providing an alkaline environment that neutralizes acidic impurities in phosphogypsum, such as soluble phosphorus, forming insoluble calcium phosphate precipitates and reducing harmful ions. Simultaneously, the 7-day activity index of steel slag powder is ≥75%, indicating its potential hydraulic properties. It can undergo a micro-expansion reaction with calcium sulfate in phosphogypsum to form products such as ettringite, enhancing the cementitious properties of the pretreated material.

[0008] S2. Roadbed material mixing: Add 100 parts of pretreated phosphogypsum mixture and 10-20 parts of soil-concrete mixture to a twin-shaft forced mixer and dry mix for 1-2 minutes; then add mixing water containing 4.5-7.5 parts of modified naphthalene-based water-reducing agent, control the total water-cement ratio to 0.30-0.35, and wet mix for 3-5 minutes to obtain the roadbed material mixture. The steel slag powder added during the pretreatment stage provides an initial strongly alkaline environment for the soil-concrete mixture to participate in the geological polymerization reaction.

[0009] The main component of phosphogypsum is calcium sulfate dihydrate (CaSO4·2H2O); the core component of soil-concrete is usually a highly reactive aluminosilicate precursor, which undergoes a geological polymerization reaction under alkaline conditions. In soil-concrete rock, silica and aluminum oxides dissolve to form [SiO4] and [AlO4] tetrahedral units. These tetrahedral units are connected by shared oxygen atoms, forming a three-dimensional network structure (-Si-O-Al-O-Si-). This network structure itself possesses early strength, high strength, and corrosion resistance. The main component of phosphogypsum, calcium sulfate dihydrate (CaSO4·2H2O), partially dissolves in an alkaline environment, providing a large amount of Ca. 2+ and SO4 2- AlO2 will dissolve during the reaction of soil and concrete. - (Aluminate ion), with Ca 2+ and SO4 2- It reacts rapidly to form ettringite.

[0010]

[0011] Traditional geological polymerization reaction matrices are low-calcium or calcium-free, while phosphogypsum introduces a large number of calcium ions. Calcium ions, aluminum ions, and silicate ions react to form hydrated calcium aluminosilicate gel (CASH gel), which is the main product of ordinary silicate cement hydration and has high strength.

[0012] Simultaneously, sodium / potassium ions, aluminum ions, and silicate ions condense to form a geopolymer gel, namely sodium / potassium polyaluminosilicate gel (NASH gel), which is a three-dimensional network structure with excellent durability and chemical stability.

[0013] Ca 2+ It acts as both a "bridge" and a "crosslinking agent" here. It can not only form its own CASH gel, but also interact with [AlO4] in the NASH gel network. 5- Tetrahedral equidistant sites bind the two gel networks together tightly to form a C-(N)-ASH composite gel system. The mechanical properties and microstructure of this composite gel are generally superior to those of a single gel system. It can also more thoroughly neutralize the acidic impurities remaining in phosphogypsum and convert them into stable compounds, fixing harmful substances such as soluble phosphorus, fluorine, organic matter, and heavy metal ions.

[0014] As the gel system gradually densifies and pores are filled, the strength of the structure increases, eventually forming a hard, dense, and well-integrated rock-like solid. The resulting needle-like ettringite crystals intertwine and coexist with the gel network of the argillaceous conglomerate, marking the end of the geological polymerization reaction.

[0015] This composite structure, which combines a geopolymer gel network with crystal reinforcement, is far denser and stronger than a simple phosphogypsum system or a simple geopolymer structure.

[0016] S3. Paving, compaction and curing: The roadbed material mixture should be spread and compacted within 6 hours after mixing. The initial compaction should be carried out by static compaction and weak vibration compaction once with a double-drum vibratory roller, followed by 3-5 compaction passes with a pneumatic tire roller, and 1-2 final compaction passes with static compaction of a double-drum roller, ensuring a compaction degree of ≥96%. After compaction, the mixture should be covered with geotextile and watered for curing for ≥7 days, so that the various mechanical performance indicators of cement-stabilized crushed stone can be used for road base.

[0017] Preferably, in step S1, the original phosphogypsum has a moisture content ≤25% and a calcium sulfate dihydrate content ≥85%; the steel slag powder has a specific surface area ≥450 m². 2 / kg, f-CaO content <5%, 7d activity index ≥75%.

[0018] The pH value of the pretreated phosphogypsum mixture is 6-9, the water-soluble phosphorus leaching concentration is ≤0.12mg / L, and the water-soluble fluorine leaching concentration is ≤0.3mg / L.

[0019] Preferably, in S2, the total content of active SiO2 and Al2O3 in the soil-concrete rock is >75%, and the specific surface area is ≥400m². 2 For a weight of / kg, the loss on ignition is ≤5%; The preparation process of modified naphthalene-based water-reducing agents includes the following steps: Industrial naphthalene was added to a reactor, heated to 130°C to melt, and 98% concentrated sulfuric acid was added. The mixture was then kept at 167±2°C for 3 hours for sulfonation.

[0020] The sulfonation reaction is essentially an aromatic electrophilic substitution reaction of the naphthalene ring. It introduces a sulfonic acid group onto the naphthalene ring, providing a water-soluble basis for the subsequent product. At the same time, the strong electron-withdrawing effect of the sulfonic acid group activates other active sites on the naphthalene ring, creating conditions for subsequent polycondensation reactions.

[0021] At high sulfonation temperatures, some naphthalenesulfonic acid undergoes a side reaction to generate sulfone impurities. Sulfones have no hydrophilic groups and do not participate in subsequent reactions, which reduces the dispersion performance of the water-reducing agent. Cooling the temperature to 120°C and holding it for 1 hour causes a hydrolysis reaction that breaks the sulfone bonds, allowing the sulfone impurities to be converted back into naphthalenesulfonic acid. This process regenerates byproducts, improves raw material utilization, and purifies the reaction system.

[0022] Cool to 120℃ and hydrolyze for 1 hour; add 37% formaldehyde aqueous solution at 85℃, condense at 105-115℃ for 6 hours, add 30% NaOH aqueous solution, and neutralize pH to 7-9.

[0023] The nucleophilic addition-elimination reaction of naphthalenesulfonic acid with formaldehyde involves the addition of the unsulfonated active site on the naphthalene ring of naphthalenesulfonic acid to the hydroxymethyl group (-CH2OH) formed by formaldehyde under acidic / neutral conditions, forming hydroxymethylated naphthalenesulfonic acid. The hydroxymethyl group then undergoes dehydration condensation with the active hydrogen of another molecule of naphthalenesulfonic acid to form a -CH2- bridging bond, ultimately polymerizing into naphthalenesulfonic acid-formaldehyde condensate. Randomly neutralize excess concentrated sulfuric acid to terminate the sulfonation side reaction, making the system neutral to weakly alkaline, thus avoiding excessive polymerization of formaldehyde under acidic conditions and the generation of paraformaldehyde impurities.

[0024] Add p-aminobenzenesulfonic acid, stir to dissolve, and keep the reaction at 90-95℃ for 5 hours to obtain an aminated naphthalene-based water-reducing agent liquid intermediate.

[0025] The -NH2 group of p-aminobenzenesulfonic acid undergoes a dehydration condensation reaction with the hydroxymethyl group at the end of the naphthalenesulfonic acid formaldehyde condensate molecular chain to form a -CH2-NH- covalent bond, thus achieving graft modification of p-aminobenzenesulfonic acid. The amino group is a strongly hydrophilic group and can react with cement hydration products, such as Ca... 2+ Calcium silicate hydrate (CSH) undergoes chemical adsorption on its surface, enhancing the bonding force between the water-reducing agent and cement particles; each molecule of p-aminobenzenesulfonic acid provides an additional -SO3H, further increasing the negative charge density of the molecular chain, strengthening the electrostatic repulsion effect, and preventing aggregation; the grafted benzene ring structure causes the molecular chain to form moderate branching, enhancing the steric hindrance effect.

[0026] Add phosphorous acid to the liquid intermediate of the naphthalene-based water-reducing agent, stir until completely dissolved, adjust the pH to 5.0-6.5 with dilute hydrochloric acid, add formaldehyde aqueous solution again, and keep the reaction at 75-80℃ for 4-5 hours.

[0027] Schiff bases are formed under weakly acidic conditions. The aromatic amino group on the naphthalene-based intermediate undergoes a condensation reaction with formaldehyde, losing one molecule of water to form a Schiff base. The phosphorus atom in the phosphate molecule, with its lone pair of electrons, is an excellent nucleophilic center. It nucleophilically attacks the activated, electron-deficient methylene carbon in the Schiff base. Proton transfer and product formation occur through a four-membered ring transition state, where the phosphate bond breaks, and a hydrogen atom transfers to a nitrogen atom, ultimately forming a stable β-aminophosphonic acid structure.

[0028] After the reaction is complete, the pH of the product is adjusted to 7-8 with a 30% NaOH aqueous solution; water is added to adjust the solid content to 40%, and the product is filtered and packaged to obtain a modified naphthalene-based water-reducing agent liquid product; or it is spray-dried to obtain a brown powder, which is a modified naphthalene-based water-reducing agent powder product.

[0029] Preferably, in the preparation process of the modified naphthalene-based water-reducing agent, the mass ratio of industrial naphthalene, concentrated sulfuric acid, formaldehyde solution, p-aminobenzenesulfonic acid, phosphorous acid, and added formaldehyde solution is 300:320:350-370:20-30:60-70:35-40.

[0030] Preferably, in step S3, the watering frequency is 3-4 times per day to keep the geotextile surface moist.

[0031] The present invention also proposes a roadbed material of soil-concrete-rock treated phosphogypsum prepared by the aforementioned preparation method, which has a seven-day unconfined compressive strength ≥4.0MPa and a splitting strength ≥0.45MPa, meeting the technical requirements of road base course.

[0032] Compared with the prior art, the beneficial effects of the present invention are: 1. Soluble phosphorus and fluorine impurities in raw phosphogypsum severely inhibit and delay cement hydration, resulting in low strength and poor durability of the finished product. The continuously strong alkaline environment (pH > 13) provided by the soil-concrete mixture during its preparation process completely neutralizes these acidic impurities and transforms them into stable, insoluble minerals such as calcium phosphate and fluorapatite, which are permanently fixed within the three-dimensional network structure. This eliminates the negative effects of phosphogypsum on the cementing system, removing the biggest obstacle to the preparation of high-performance roadbed materials. The soil-concrete mixture generates an amorphous three-dimensional network CASH gel (hydrated calcium aluminosilicate) with a [-Si-O-Al-O-] framework through polymerization; simultaneously, the Al dissolved from the soil-concrete mixture... 3+ / [AlO4] 5- With the Ca provided by phosphogypsum 2+ and SO4 2- It reacts rapidly in an alkaline environment to produce a large number of needle-like / rod-like ettringite (AFt) crystals;

[0033] The hard ettringite crystals, acting as "microfibers," intertwine and permeate with the flexible CASH gel, forming a microstructure resembling a "crystal-reinforced composite material." This structure significantly enhances the matrix's density, compressive strength, and resistance to shrinkage cracking, ensuring the long-term stability and high load-bearing capacity of the roadbed.

[0034] 2. The water-reducing agent in this invention undergoes precise chemical modification, exceeding the limitations of traditional water-reducing agents that merely disperse particles through electrostatic repulsion. The densely packed sulfonic acid groups (-SO3-) on the molecular backbone give the surface of the soil / concrete particles a strong negative charge, generating a powerful electrostatic repulsion force that prevents particle aggregation. The amination reaction grafting of p-aminobenzenesulfonic acid introduces benzene ring branches, producing a significant steric hindrance effect. The combined effect of these two processes enables perfect particle dispersion with minimal water usage, releasing trapped water and significantly reducing porosity, directly improving the strength, impermeability, and durability of concrete. The amino group (-NH2) introduced by the amination reaction and the phosphonic acid group (-PO3H2) introduced by the phosphonation reaction are strong coordinating groups. They can react with the [AlO4] produced during soil / concrete hydration. 5- Unit and Ca produced by cement hydration 2+ A strong complexation reaction occurs, acting like an "anchor" to firmly fix the water-reducing agent molecules to the particle surface. This strong adsorption ensures the persistence of the dispersion effect, maintains workability stability during long-distance transportation and paving, and avoids excessive dissolution of the water-reducing agent in pore water, which could lead to system bleeding or segregation. This is crucial for the uniformity of roadbed materials. Water-reducing agents, through their dispersing effect, provide a large specific surface area for concrete particles, accelerating the reaction rate with water. Simultaneously, the directional arrangement of their molecules on the particle surface provides favorable sites for the nucleation and growth of ettringite and CSH gel, thereby optimizing the formation process of hydration products. This contributes to the rapid development of early concrete strength, improves the early strength of pavement base courses, and shortens the curing cycle.

[0035] 3. The concrete and modified water-reducing agent do not work in isolation; they produce a profound synergistic effect: Water-reducing agents create conditions for the reaction of soil-concrete mixtures. The extremely low water-cement ratio means a higher particle concentration and a shorter ion migration distance, which creates an extremely favorable environment for the polymerization reaction of soil-concrete mixtures and the secondary reaction with phosphogypsum (to generate ettringite), resulting in close packing and rapid mass transfer.

[0036] The soil-concrete system provides adsorption sites for water-reducing agents. The amorphous gel generated by the soil-concrete reaction has more unsaturated coordination sites than traditional cement hydration products. This provides more "anchoring points" for amino and phosphonic acid groups in modified water-reducing agents, allowing the effectiveness of water-reducing agents to be fully realized.

[0037] 4. In addition to soluble phosphorus, fluorine, and organic matter, raw phosphogypsum usually also contains various heavy metal ions (such as Cd). 2 + Pb 2+ Cr 3+ Cu 2+ Zn 2+ These ions (etc.) originate from phosphate rock itself. If these ions are leached by rainwater during the use of roadbeds, they will cause serious pollution to the soil and groundwater. Traditional naphthalene-based water-reducing agents have a weak adsorption capacity for heavy metal ions. However, after the two-step chemical modification of "amineation" and "phosphonation" described in this invention, functional groups specifically targeting heavy metals are introduced into its molecular structure, thereby giving it a strong adsorption and fixation capacity.

[0038] The nitrogen atom in the aromatic amino group (-NH2) introduced during the amination step possesses a lone pair of electrons, which can act as an electron donor and acceptor, along with heavy metal ions (such as Cd). 2+ Pb 2+ Phosphonic acid groups form stable coordination bonds, generating complexes. Phosphonic acid groups are well-known strong complexing groups, with a much higher affinity for divalent and trivalent heavy metal ions than carboxyl groups (-COOH) or even sulfonic acid groups (-SO3H). Under alkaline conditions, phosphonic acid groups are partially or completely deprotonated, forming -PO3H⁻ or -PO3 2-It has extremely high electronegativity; it can form very stable, cyclic complexes with heavy metal ions through chelation, similar to the effect of EDTA. These chelates have extremely low solubility, effectively "locking" heavy metal ions onto the water-reducing agent molecular chain. The water-reducing agent molecules are thus locked within the concrete, preventing the heavy metal ions from leaching out.

[0039] In summary, this invention utilizes the geological polymerization reaction of soil-concrete composites to synergistically generate a dense "gel-ettringite" structure with phosphogypsum, thoroughly solidifying harmful impurities and endowing the roadbed material with high strength and stability. The modified naphthalene-based water-reducing agent, by introducing amine and phosphonic acid groups, not only achieves efficient water reduction and increased density but also strongly chelates heavy metal ions, forming chemical fixation. The synergistic effect of these two components enables large-scale, harmless, and resource-efficient utilization of phosphogypsum while ensuring the long-term mechanical properties and environmental safety of the roadbed engineering, resulting in significant environmental and economic benefits. Attached Figure Description

[0040] Figure 1 The 1H NMR spectrum of the modified naphthalene-based water-reducing agent produced by this invention. Detailed Implementation

[0041] The technical solutions of the present invention will be clearly and completely described below with reference to the accompanying drawings of the embodiments of the present invention. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments.

[0042] Preparation Example 1: Industrial naphthalene was added to a reactor and heated to 130°C to melt it. 98% concentrated sulfuric acid was added, and the mixture was kept at 167°C for 3 hours for sulfonation. The temperature was then lowered to 120°C, and the mixture was hydrolyzed for 1 hour. A 37% formaldehyde aqueous solution was added at 85°C, and the mixture was condensed at 110°C for 6 hours. A 30% NaOH aqueous solution was added to neutralize the pH to 8. P-aminobenzenesulfonic acid was added, stirred to dissolve, and the mixture was kept at 90-95°C for 5 hours to obtain an amination naphthalene-based water-reducing agent liquid intermediate. Phosphorous acid was added to the liquid intermediate of the naphthalene-based water-reducing agent and stirred until completely dissolved. The pH was adjusted to 6 with dilute hydrochloric acid, and formaldehyde aqueous solution was added again. The reaction was kept at 80°C for 4-5 hours. After the reaction was completed, the pH of the product was adjusted to 8 with a 30% NaOH aqueous solution. Water was added to adjust the solid content to 40%, and the product was filtered and packaged to obtain a modified naphthalene-based water-reducing agent liquid product.

[0043] In the preparation process of the modified naphthalene-based water-reducing agent, the mass ratio of industrial naphthalene, concentrated sulfuric acid, formaldehyde solution, p-aminobenzenesulfonic acid, phosphorous acid, and added formaldehyde solution is 300:320:370:20:70:35.

[0044] A portion of the modified naphthalene-based water-reducing agent was dissolved in dimethyl sulfoxide and then subjected to 1H NMR spectroscopy. The results are as follows. Figure 1 As shown; The multiple peaks in the aromatic hydrogen region (δ6.0-8.5ppm) correspond to the aromatic hydrogens of the naphthalene ring and the benzene ring. At δ8.54, 8.53, 8.49, and 8.48ppm, the aromatic hydrogens on the naphthalene ring are affected by the strong electron-withdrawing effect of the sulfonic acid group (-SO3H), and their chemical shifts are significantly high-field shifted, proving that the naphthalene ring has been successfully sulfonated. At δ8.13, 8.10, and 8.09ppm, the aromatic hydrogens at other sites on the naphthalene ring exhibit characteristic peak shapes due to the change in electron cloud distribution after sulfonation. At δ7.83-6.88ppm, the overlapping peaks of the aromatic hydrogens of the naphthalene ring and the p-aminobenzenesulfonic acid benzene ring correspond to the aromatic hydrogens that have not been substituted or have different substitution environments, proving that the aromatic ring skeleton structure of the "naphthalene ring-benzene ring" is complete.

[0045] The peaks at δ8.03 and 8.00 ppm correspond to the hydrogen atoms of the aromatic amino group (-NH-). Since the amino hydrogen atoms of the aromatic amine are conjugated with the aromatic ring, the chemical shift is around 8.0, which proves that the amination reaction successfully introduced the -NH- functional group.

[0046] δ4.77 and 4.76 ppm correspond to hydrogen in the -CH2-NH-methylene group; δ4.32 and 4.32 ppm correspond to hydrogen in the -CH2-PO3H2-methylene group. The peaks in these two regions directly prove the success of the amination and phosphonate modification steps, which introduce characteristic structural units of amino and phosphonic acid groups.

[0047] Preparation Example 2: The preparation method is the same as in Preparation Example 1, but the mass ratio of industrial naphthalene, concentrated sulfuric acid, formaldehyde solution, p-aminobenzenesulfonic acid, phosphorous acid, and added formaldehyde solution is 300:320:360:25:65:37.5.

[0048] Preparation Example 3: The preparation method is the same as in Preparation Example 1, but the mass ratio of industrial naphthalene, concentrated sulfuric acid, formaldehyde solution, p-aminobenzenesulfonic acid, phosphorous acid, and added formaldehyde solution is 300:320:350:30:60:40.

[0049] Example 1: A method for preparing subgrade materials for soil-concrete-treated phosphogypsum includes the following steps: S1. Pretreatment of phosphogypsum: Raw phosphogypsum and steel slag powder were added to a mixing mill at a mass ratio of 10:1, passed through a 45μm square hole sieve, and 15% of the material mass of process water was added. The mixture was stirred in a forced mixer at 30-40 rpm for 10-15 minutes until a uniform wet mixture was formed. The wet mixture was then transferred to an aging chamber and sealed and aged for 3-4 days under ambient temperature ≥25℃ and relative humidity ≥85%. After passing through a 5mm sieve, the pretreated phosphogypsum mixture was obtained. S2. Roadbed material mixing: 100 kg of pretreated phosphogypsum mixture and 10 kg of soil-concrete mixture were put into a twin-shaft forced mixer and dry-mixed for 2 min; then, mixing water containing 4.5 kg of modified naphthalene-based water-reducing agent obtained in Preparation Example 3 was added, and the total water-cement ratio was controlled at 0.30-0.35. The mixture was wet-mixed for 3 min to obtain concrete mixture. S3. Paving, compaction and curing: The concrete mixture should be spread and compacted within 6 hours after mixing. The initial compaction should be carried out by static compaction and weak vibration compaction once with a double-drum vibratory roller, followed by 4 compaction compaction passes with a pneumatic tire roller, and 2 final compaction passes with static compaction of a double-drum roller to ensure a compaction degree of ≥96%. After compaction, the mixture should be covered with geotextile and watered for 7 days.

[0050] Example 2: A method for preparing subgrade materials for soil-concrete-treated phosphogypsum includes the following steps: S1. Pretreatment of phosphogypsum: Raw phosphogypsum and steel slag powder were added to a mixing mill at a mass ratio of 10:1, passed through a 45μm square hole sieve, and 15% of the material mass of process water was added. The mixture was stirred in a forced mixer at 30-40 rpm for 10-15 minutes until a uniform wet mixture was formed. The wet mixture was then transferred to an aging chamber and sealed and aged for 3-4 days under ambient temperature ≥25℃ and relative humidity ≥85%. After passing through a 5mm sieve, the pretreated phosphogypsum mixture was obtained. S2. Roadbed material mixing: 100 kg of pretreated phosphogypsum mixture and 15 kg of soil-concrete mixture were put into a twin-shaft forced mixer and dry-mixed for 2 min; then, mixing water containing 6 kg of modified naphthalene-based water-reducing agent obtained in Preparation Example 3 was added, and the total water-cement ratio was controlled at 0.30-0.35. The mixture was wet-mixed for 4 min to obtain concrete mixture. S3. Paving, compaction and curing: The concrete mixture should be spread and compacted within 6 hours after mixing. The initial compaction should be carried out by static compaction and weak vibration compaction once with a double-drum vibratory roller, followed by 4 compaction compaction passes with a pneumatic tire roller, and 2 final compaction passes with static compaction of a double-drum roller to ensure a compaction degree of ≥96%. After compaction, the mixture should be covered with geotextile and watered for 7 days.

[0051] Example 3: A method for preparing subgrade materials for soil-concrete-treated phosphogypsum includes the following steps: S1. Pretreatment of phosphogypsum: Raw phosphogypsum and steel slag powder were added to a mixing mill at a mass ratio of 10:1, passed through a 45μm square hole sieve, and 15% of the material mass of process water was added. The mixture was stirred in a forced mixer at 30-40 rpm for 10-15 minutes until a uniform wet mixture was formed. The wet mixture was then transferred to an aging chamber and sealed and aged for 3-4 days under ambient temperature ≥25℃ and relative humidity ≥85%. After passing through a 5mm sieve, the pretreated phosphogypsum mixture was obtained. S2. Roadbed material mixing: 100 kg of pretreated phosphogypsum mixture and 20 kg of soil-concrete mixture were put into a twin-shaft forced mixer and dry-mixed for 1 min; then, mixing water containing 7.5 kg of modified naphthalene-based water-reducing agent obtained in Preparation Example 1 was added, and the total water-cement ratio was controlled at 0.30-0.35. The mixture was wet-mixed for 5 min to obtain concrete mixture. S3. Paving, compaction and curing: The concrete mixture was spread and compacted within 6 hours after mixing. The initial compaction was carried out by static compaction and weak vibration compaction with a double-drum vibratory roller, followed by 4 compaction compaction with a pneumatic tire roller, and 2 final compaction compaction with a double-drum vibratory roller to ensure a compaction degree of ≥96%. After compaction, the mixture was covered with geotextile and watered for 7 days.

[0052] Based on this, the following design was also created: Comparative Example 1: Same formulation and experimental method as Preparation Example 2, but without the addition of phosphorous acid; Comparative Example 2: The formulation and experimental method are the same as in Example 2, but the amount of p-aminobenzenesulfonic acid is increased so that the mass ratio of industrial naphthalene, concentrated sulfuric acid, formaldehyde solution, p-aminobenzenesulfonic acid, phosphorous acid, and added formaldehyde solution is 300:320:350:30:120:40. Comparative Example 3: The formulation and experimental method are the same as those in Example 2, but in S1, steel slag is not added, and phosphogypsum is treated by first neutralizing with alkali and then washing with acid. Comparative Example 4: The formulation and experimental method are the same as those in Example 2, but in S2, the soil concrete is replaced with an equal mass of silicate cement.

[0053] For each embodiment and comparative example, according to standards such as "Identification Standard for Hazardous Waste - Leaching Toxicity Identification", "Determination of Soil pH Value - Glass Electrode Method", "Test Procedure for Inorganic Binder Stabilized Materials in Highway Engineering", "Test Procedure for Cement and Cement Concrete in Highway Engineering", and "Test Procedure for Cement and Cement Concrete in Highway Engineering", the leaching concentrations of water-soluble phosphorus, water-soluble fluorine, and heavy metals of the present invention; pH value of the solidified body; seven-day unconfined compressive strength; elastic modulus; and slump of the mixture, the corresponding results are shown in Table 1. Table 1. Test data of various properties of subgrade materials

[0054] Data Analysis: Leaching toxicity data is the primary indicator for evaluating the environmental safety of phosphogypsum resource utilization. Data shows that all examples (leached phosphorus ≤ 0.18 mg / L, fluorine ≤ 4.18 mg / L, heavy metals far below the limits) performed excellently, while the comparative examples showed varying degrees of risk of exceeding the limits.

[0055] In the examples, the f-CaO of the steel slag powder reacts with water to generate Ca(OH)2, providing the necessary alkaline environment (pH ~ 12.5) for pretreatment. During this stage, most of the soluble phosphorus (e.g., H3PO4) and fluorine (e.g., F2PO4) are dissolved in water. - The leaching is neutralized, producing insoluble or sparingly soluble precipitates such as calcium phosphate and CaF2, achieving the first stage of fixation. Comparative Example 3 (without steel slag pretreatment) provides a contrasting example: its leaching concentration is the highest across the board, indicating that the lack of pretreatment results in a large amount of residual acidic impurities, severely interfering with subsequent hydration / polymerization reactions and directly leading to environmental risks.

[0056] Entering the S2 stage, the persistently strongly alkaline environment generated after the soil-concrete reaction is crucial for complete solidification. Under this environment, any residual substances, and even intermediate products formed during pretreatment, will be further transformed. The strongly alkaline environment promotes the reaction of soluble phosphorus and fluoride ions with Ca in the system. 2+ [AlO4] 5+ Through these reactions, thermodynamically more stable minerals such as fluorapatite (Ca5(PO4)3F) with extremely low solubility are eventually formed and permanently fixed in the three-dimensional network structure of the soil-concrete rock.

[0057] While the leaching concentration of Comparative Example 4 (using silicate cement) was partially better than that of Comparative Example 3, it was significantly worse than that of the Example. This is because cement hydration products (such as CSH gel) primarily immobilize ions through physical adsorption and surface bonding, which has limited capacity. In contrast, the amorphous CASH gel generated from soil-concrete composites possesses a large number of unsaturated coordination sites in its aluminosilicate three-dimensional network, enabling it to immobilize Cd through ion exchange and the formation of inner-layer complexes. 2+ Pb 2+ Heavy metal ions are directly "woven" into its mesh, achieving molecular-level chemical fixation, which is far more stable than physical adsorption.

[0058] Under strong alkaline conditions, the soil-concrete rock dissolves and condenses, forming a flexible, amorphous CASH gel, which constitutes the continuous phase of the matrix. Simultaneously, phosphogypsum provides Ca... 2+ and SO4 2- AlO2 dissolved from soil and concrete -The reaction proceeds rapidly in an alkaline environment, generating a large number of needle-like ettringite (AFt) crystals. The high strength and high elastic modulus of the examples are a result of the intertwined symbiosis and complementary advantages of these two products. The flexible gel fills the voids and encapsulates the crystals, giving the material toughness and continuity; while the hard ettringite crystals, acting as "microfibers," run through the gel, playing a reinforcing and toughening role similar to the steel bars in reinforced concrete, greatly improving compressive and splitting strength. Comparative Examples 3 and 4, lacking this effective synergistic reaction (Insufficient pretreatment in Comparative Example 3, and no polymerization reaction of soil-aggregate in Comparative Example 4), could not form this dense structure and therefore had the lowest strength.

[0059] The slump (78-86 mm) of the mixtures in the examples was significantly higher than that of all comparative examples, demonstrating the superior dispersing ability of the modified water-reducing agent. Its molecular chain contains densely packed sulfonic acid groups (-SO3-). - The water-reducing agent provides electrostatic repulsion, while the benzene ring branches introduced through amination create steric hindrance. The combined effect of these two factors effectively prevents particle agglomeration and releases the trapped free water, thus achieving good workability at an extremely low water-cement ratio. A low water-cement ratio means denser particle packing and shorter ion migration paths, creating excellent conditions for the polymerization of soil-concrete and the formation of ettringite. Simultaneously, the directional adsorption of water-reducing agent molecules on the particle surface provides abundant nucleation sites for hydration products, guiding the more uniform and orderly growth of ettringite and CSH gel, further optimizing the quality of the "gel-crystal" composite structure. In contrast, the water-reducing agents in Comparative Examples 1 and 2, due to incomplete modification, exhibited decreased dispersion and adsorption capabilities, leading to increased porosity and simultaneous deterioration of strength and workability.

[0060] This invention introduces an aromatic amino group (-NH2) onto the water-reducing agent molecule through the amination reaction of p-aminobenzenesulfonic acid; and further introduces a phosphonic acid group (-PO3H2) through the phosphonation reaction of phosphorous acid with formaldehyde. These two groups are recognized as strong metal chelating functional groups. In the highly alkaline environment of concrete, the phosphonic acid group is partially deprotonated, exhibiting extremely high electronegativity. It synergistically with the amino group, and can bind to heavy metal ions (such as Pb). 2+ Cd 2+ This forms stable five- or six-membered ring chelates. These chelates have extremely high stability constants and extremely low solubility, effectively "chemically locking" heavy metal ions onto the water-reducing agent molecular chain. The heavy metal leaching concentration in Comparative Example 1 (without phosphorous acid or phosphonic acid groups) increased dramatically, even far exceeding that of Comparative Example 2 (with only increased amino groups), eloquently demonstrating that phosphonic acid groups play a dominant role in heavy metal fixation. Furthermore, the water-reducing agent molecules themselves are firmly adsorbed or coated within the hydration products of the soil and concrete, ensuring that the chelated heavy metal ions are difficult to leach. This endows the water-reducing agent with the function of actively capturing and fixing heavy metals, which is one of the most prominent innovations of this invention.

[0061] The above description is only a preferred embodiment of the present invention, but the scope of protection of the present invention is not limited thereto. Any equivalent substitutions or modifications made by those skilled in the art within the scope of the technology disclosed in the present invention, based on the technical solution and inventive concept of the present invention, should be covered within the scope of protection of the present invention.

Claims

1. A method for preparing subgrade materials for soil-concrete-rock treatment of phosphogypsum, characterized in that, Includes the following steps: S1. Pretreatment of phosphogypsum: Raw phosphogypsum and steel slag powder were added to a mixing mill at a mass ratio of 10:1, passed through a 45μm square hole sieve, and 15% of the material mass of process water was added. The mixture was stirred in a forced mixer at 30-40 rpm for 10-15 minutes until a uniform wet mixture was formed. The wet mixture was then transferred to an aging chamber and sealed and aged for 3-4 days under ambient temperature ≥25℃ and relative humidity ≥85%. After passing through a 5mm sieve, the pretreated phosphogypsum mixture was obtained. S2. Roadbed material mixing: Add 100 parts of pretreated phosphogypsum mixture and 10-20 parts of soil-concrete mixture to a twin-shaft forced mixer and dry mix for 1-2 minutes; then add mixing water containing 4.5-7.5 parts of modified naphthalene-based water-reducing agent, control the total water-cement ratio to 0.30-0.35, and wet mix for 3-5 minutes to obtain the roadbed material mixture. S3. Paving, compaction and curing: The roadbed material mixture should be spread and compacted within 6 hours after mixing. The initial compaction should be carried out by static compaction and weak vibration compaction once with a double-drum vibratory roller, followed by 3-5 compaction passes with a pneumatic tire roller, and 1-2 final compaction passes with static compaction of a double-drum roller, ensuring a compaction degree of ≥96%. After compaction, the mixture should be covered with geotextile and watered for curing for ≥7 days, so that the various mechanical performance indicators of cement-stabilized crushed stone can be used for road base.

2. The method for preparing roadbed material of soil-concrete-rock treated phosphogypsum according to claim 1, characterized in that, In S1, the original phosphogypsum has a moisture content ≤25% and a calcium sulfate dihydrate content ≥85%; the steel slag powder has a specific surface area ≥450m². 2 / kg, f-CaO content <5%, 7d activity index ≥75%; The pH value of the pretreated phosphogypsum mixture is 6-9, the water-soluble phosphorus leaching concentration is ≤0.12mg / L, and the water-soluble fluorine leaching concentration is ≤0.3mg / L.

3. The method for preparing roadbed material of soil-concrete-rock treated phosphogypsum according to claim 1, characterized in that, In S2, the total content of active SiO2 and Al2O3 in the soil-concrete rock is >75%, and the specific surface area is ≥400 m². 2 For a weight of / kg, the loss on ignition is ≤5%; The preparation process of modified naphthalene-based water-reducing agents includes the following steps: Industrial naphthalene was added to a reactor and heated to 130℃ to melt it. 98% concentrated sulfuric acid was added, and the mixture was kept at 167±2℃ for 3 hours for sulfonation. The temperature was then lowered to 120℃, and the mixture was hydrolyzed for 1 hour. A 37% formaldehyde aqueous solution was added at 85℃, and the mixture was condensed at 105-115℃ for 6 hours. A 30% NaOH aqueous solution was added to neutralize the pH to 7-9. P-aminobenzenesulfonic acid was added, stirred to dissolve, and the mixture was kept at 90-95℃ for 5 hours to obtain an amination naphthalene-based water-reducing agent liquid intermediate. Phosphorous acid is added to the liquid intermediate of the amination naphthalene-based water-reducing agent and stirred until completely dissolved. The pH is adjusted to 5.0-6.5 with dilute hydrochloric acid, and formaldehyde aqueous solution is added again. The reaction is carried out at 75-80℃ for 4-5 hours. After the reaction is completed, the pH of the product is adjusted to 7-8 with 30% NaOH aqueous solution. Water is added to adjust the solid content to 40%, and the product is filtered and packaged to obtain the modified naphthalene-based water-reducing agent liquid product; or it is spray-dried to obtain a brown powder, which is the modified naphthalene-based water-reducing agent powder product.

4. The method for preparing roadbed material of soil-concrete-rock treated phosphogypsum according to claim 1, characterized in that, In the preparation process of the modified naphthalene-based water-reducing agent, the mass ratio of industrial naphthalene, concentrated sulfuric acid, formaldehyde solution, p-aminobenzenesulfonic acid, phosphorous acid, and added formaldehyde solution is 300:320:350-370:20-30:60-70:35-40.

5. The method for preparing roadbed material of soil-concrete-treated phosphogypsum according to claim 1, characterized in that, In S3, the watering frequency is 3-4 times a day to keep the geotextile surface moist.

6. A roadbed material for soil-concrete-treated phosphogypsum prepared by the preparation method according to any one of claims 1-5, characterized in that, The roadbed material mixture obtained by S2 has a seven-day unconfined compressive strength ≥4.0MPa and a splitting tensile strength ≥0.45MPa, which meets the technical requirements of the road base course.

Images