A low-carbon type solidifying agent for phosphogypsum composite construction slag road base and a preparation method thereof

Abstract

The application discloses a kind of low-carbon type solidifying agent for phosphogypsum composite construction slag road base and preparation method thereof.The solidifying agent includes the following components by weight parts: acid-resistant composite gel 5~15 parts, acid-resistant bacillus liquid 10~25 parts, mineralized substrate solution 60~85 parts.The application utilizes the synergistic mechanism of acid-resistant bacillus and composite gel, without pre-adjusting alkali, by inducing calcium carbonate to deposit on the gel reticular skeleton, simultaneously realizing the interface bridging of phosphogypsum fine particles and continuous gradation construction slag aggregate, and constructing the multi-level nested hybrid structure of "skeleton-filling".The application realizes the high proportion of phosphogypsum and construction slag double solid waste collaborative utilization (consumption rate reaches 80%~95%), significantly improves the compressive strength (28d reaches 4.0 MPa) and water stability of road base, and the harmful impurities leaching rate is extremely low.Compared with traditional technology, the process is simplified, the carbon reduction efficiency reaches 89%, and has high engineering application value and ecological benefits.

Description

Technical Field

[0001] This invention relates to the field of road engineering and solid waste resource utilization technology, specifically to a low-carbon curing agent for phosphogypsum composite construction waste road base and its preparation method. Background Technology

[0002] With the rapid development of my country's phosphate chemical industry, phosphogypsum (PG), as an industrial by-product, suffers from large stockpiles and low utilization rates, severely restricting the sustainable development of phosphate chemical enterprises. The resource utilization of this by-product has become a challenge in the environmental protection field. Phosphogypsum contains soluble phosphorus and fluorine, among other acidic impurities, making it highly acidic. In road engineering applications, this highly acidic environment severely inhibits the hydration reaction of traditional inorganic cementing materials (such as cement and lime), resulting in low early strength and poor water resistance of the solidified body. Furthermore, harmful impurities are easily leached out with rainfall, causing secondary environmental pollution. Therefore, direct utilization is extremely difficult. Simultaneously, with the acceleration of urbanization, the output of construction waste (including waste concrete and broken bricks) has increased dramatically. Currently, construction waste is mainly disposed of through landfill, which not only occupies a large amount of land resources but also has a low resource utilization rate, especially lacking high-value-added technologies for the co-conversion of construction waste with industrial solid waste (such as phosphogypsum).

[0003] Currently, the main solution for curing phosphogypsum relies on pretreatment with large amounts of quicklime or cement. However, this "adjust alkali before curing" approach has many drawbacks: First, the acid-base neutralization reaction is violent, resulting in high dust levels at the construction site, complex procedures, and high energy consumption; second, residual phosphorus and fluorine impurities in the phosphogypsum system after alkali adjustment can interfere with the cement hydration process, leading to low early strength, easy cracking, and poor long-term stability of the cured body.

[0004] In recent years, microbial induced calcium carbonate deposition (MICP) technology has provided a new approach for solid waste treatment. However, studies have found that the urease-producing bacteria used in conventional MIP are mostly alkaliphilic bacteria, which have extremely low activity in the acidic environment of phosphogypsum, making it difficult to directly trigger the mineralization reaction. Furthermore, the calcium carbonate crystals generated by MIP alone are isolated, lacking a continuous cementing network, and the minerals are unevenly distributed among the phosphogypsum crystals, resulting in slow early strength development and susceptibility to debonding and cracking under complex road conditions. Moreover, the solidified phosphogypsum body lacks coarse aggregate support, has a low modulus of deformation, and is highly susceptible to fatigue cracking and softening / debonding under heavy loads or wet-dry cycles.

[0005] Therefore, developing a solidification technology that can directly adapt to the native acidic environment of phosphogypsum, eliminate the need for pre-adjusting pH, and synergistically absorb construction waste aggregates to achieve the triple goals of "high strength, low carbon emissions, and impurity passivation" has become a key issue that urgently needs to be addressed in the fields of solid waste resource utilization and green transportation construction. Summary of the Invention

[0006] To address the shortcomings of existing technologies, the technical problem this invention aims to solve is to provide a low-carbon curing agent and its preparation method for phosphogypsum composite construction waste road base courses. This addresses the technical deficiencies of existing phosphogypsum road base course resource utilization processes, such as poor activity of traditional curing agents in strongly acidic environments, the need for pre-alkali adjustment leading to complex processes and high carbon emissions, and the uneven distribution, high brittleness, low early strength, and high risk of leaching of soluble impurities like phosphorus / fluorine from traditional microbial curing products. This invention aims to achieve high-proportion, in-situ, and efficient curing of phosphogypsum and construction waste without the need for pre-alkali adjustment, simultaneously improving the mechanical strength, water stability, crack resistance, and environmental safety of the road base course.

[0007] To solve the above-mentioned technical problems, the present invention adopts the following technical solution: a low-carbon curing agent for phosphogypsum composite construction waste road base, characterized in that it comprises the following components by weight: 5-15 parts of acid-resistant composite gel, 10-25 parts of acid-resistant Bacillus liquid, and 60-85 parts of mineralized substrate solution; the acid-resistant composite gel is a composite product with a three-dimensional network structure constructed by oligomeric carboxylated chitosan and organically intercalated sodium montmorillonite through a weakly acidic crosslinking agent; the optimal growth pH of the acid-resistant Bacillus liquid is 3-5, and it can synthesize and secrete extracellular urease; the solute in the mineralized substrate solution includes urea.

[0008] Thus, a biochemical solidification system based on "bacterial-gel synergy" was constructed. The acid-resistant composite gel serves as a scaffold and protective medium, providing an acid-resistant barrier for bacteria and guiding mineral growth. Extracellular urease secreted by acid-resistant Bacillus drives the mineralization reaction, while the mineralization substrate solution provides the necessary reaction substrate. Through the synergy of these three components, in-situ solidification of phosphogypsum was directly induced without the addition of an alkaline regulator. Specifically, acid-resistant Bacillus maintains a stable capacity for urease synthesis and secretion in an acidic phosphogypsum substrate. Its metabolic process directly drives the core MIP (micro-microparticle polyphosphate) reaction, while simultaneously coupling in-situ stabilization of phosphorus and fluorine. This is the core theoretical basis for its modification and a key characteristic distinguishing it from other acid-resistant bacteria. Acid-resistant Bacillus can continuously synthesize and secrete extracellular urease. The urease-catalyzed urea hydrolysis reaction can proceed efficiently at pH 3-5, as shown in the following reaction formula: The CO3 produced by the reaction 2− With free Ca in phosphogypsum base 2+In an acidic environment, it rapidly combines to form calcite-type CaCO3 (this crystal form is more stable and has stronger cementing properties under acidic conditions). The generated CaCO3 preferentially deposits on the surface and in the pores of phosphogypsum particles and soil particles, achieving particle encapsulation, pore filling, and aggregate cementation, thereby improving the density of the mixture and the interparticle adhesion at a microscopic level. The metabolic processes and extracellular secretions of acid-resistant Bacillus can achieve in-situ stabilization of soluble phosphorus and phosphorus in phosphogypsum through a dual action of chemical precipitation and complexation adsorption, thus mitigating the environmental leaching risk of acidic phosphogypsum base layers. This is achieved by acid-resistant strains hydrolyzing urea to produce CO3 in an acidic phosphogypsum environment. 2- and with Ca in the system 2+ By combining and utilizing the skeletal support of CMCG gel, a multi-level curing system was constructed, achieving efficient bonding of phosphogypsum and construction waste without the need for pre-adjustment of pH, significantly improving the early and later strength of the solidified body.

[0009] Preferably, the mass ratio of the oligomeric carboxylated chitosan to the organically intercalated sodium montmorillonite is 4:1 to 5:1. This ensures that the organic polymeric carboxylated chitosan can be fully intercalated into the interlayer structure of montmorillonite, forming a stable intercalated composite system. This guarantees the structural stability of the gel and the uniform distribution of nucleation sites, avoiding gel embrittlement due to excessive montmorillonite or insufficient mechanical strength due to excessive chitosan.

[0010] Preferably, the oligomeric carboxylated chitosan has a molecular weight of 1000-3000 Da and a carboxyl substitution degree of ≥85%; the organically intercalated sodium montmorillonite is hexadecyltrimethylammonium bromide-modified sodium montmorillonite with an interlayer spacing of 3.5-4.0 nm. Thus, the oligomeric and highly substituted carboxylated chitosan ensures its high solubility and chelating activity under acidic conditions; the larger interlayer spacing of the montmorillonite provides nanoscale space for the enrichment of bacterial metabolites and calcium ions, possessing microporous adsorption and crystal nucleation induction capabilities, and significantly enhancing the curing agent's chemical capture ability for phosphorus and fluorine impurities, while simultaneously increasing the effective surface area of ​​crystal deposition.

[0011] Preferably, the weakly acidic crosslinking agent is trisodium citrate, disodium tartrate, sodium malate, sodium gluconate, or disodium adipic acid; the amount of the weakly acidic crosslinking agent is 0.5% to 1.0% of the mass of the acid-resistant composite gel. In this way, by using an organic weak acid salt as a crosslinking agent, the carboxyl groups form a moderate ionic crosslinking or hydrogen bond network with the chitosan molecular chain, avoiding the inhibition of acid-resistant Bacillus activity by strong acid or strong base crosslinking agents, thus endowing the gel with good biocompatibility and flexibility, and solving the problem of easy cracking of the cured body under wet-dry cycling.

[0012] Preferably, the concentration of the acid-resistant Bacillus spp. solution is 10. 7 ~109 CFU / mL, preferably 5×10⁻⁶ 7 ~5×10 8 CFU / mL, the optimal value is 1×10⁻⁶. 8 ~3×10 8 CFU / mL. Within this concentration range, the rate of urease hydrolysis of urea produced by acid-resistant Bacillus reached a dynamic equilibrium with the precipitation rate of calcium ions in phosphogypsum and construction waste aggregate. This facilitates the uniform and slow growth of calcium carbonate crystals at the nucleation sites of the CMCG composite gel, avoiding crystal porosity caused by excessively rapid mineralization. The acid-resistant Bacillus is at least one of Bacillus megaterium, Bacillus alkalophilus, Bacillus subtilis, Bacillus lysinophilus, or Bacillus pasteurellium.

[0013] Preferably, the concentration of urea in the mineralized substrate solution is 0.8~2.0 mol / L, more preferably 1.0~1.5 mol / L, and most preferably 1.2~1.4 mol / L. This provides sufficient substrate for the urease secreted by acid-resistant Bacillus, ensuring a sufficient concentration of CO3 is generated in the acidic phosphogypsum substrate. 2- This induces the formation of sufficient calcium carbonate crystals to fill the intergranular spaces. Simultaneously, it effectively avoids a surge in local osmotic pressure caused by excessively high substrate concentration, prevents the high-salt environment from inhibiting the bioactivity of acid-resistant Bacillus, and ensures the continuity and stability of the mineralization reaction.

[0014] Another object of the present invention is to provide a method for preparing the above-mentioned low-carbon curing agent for phosphogypsum composite construction waste road base, comprising the following steps:

[0015] S1: Dissolve oligocarboxylated chitosan in deionized water, adjust the pH to 4.0~4.5, then add organic intercalated sodium montmorillonite for dispersion treatment, then add a weak acidic crosslinking agent, and crosslink at 30~35℃. After crosslinking, it is a light milky white viscous substance, which is the acid-resistant composite gel.

[0016] S2: Inoculate the activated acid-fast Bacillus into liquid culture medium (LB medium), and then incubate at 30~35℃ for 24~48h to obtain acid-fast Bacillus liquid.

[0017] S3: Mix the acid-resistant composite gel obtained in step S1, the acid-resistant Bacillus liquid obtained in step S2, and the mineralized substrate solution, or package the three separately for on-site preparation, to obtain the low-carbon curing agent.

[0018] Preferably, the dispersion treatment in step S1 is ultrasonic dispersion, with an ultrasonic power of 250-350W and an ultrasonic time of 30-45min; the crosslinking time is 30-50min; and the solid content of the acid-resistant composite gel is 4.5%-6.5%. The ultrasonic power, within the range of 250–350W, allows for sufficient intercalation and exfoliation of montmorillonite without damaging the chitosan structure due to excessive power, making it suitable for both laboratory and industrial applications. The ultrasonic time, within the range of 30–45 minutes, ensures uniform dispersion without wasting energy, aligning with the "low-carbon" positioning. The cross-linking time, within the range of 30–50 minutes, ensures sufficient formation of the three-dimensional gel network without excessive reaction leading to embrittlement. The solid content of the composite gel, within the range of 4.5%–6.5%, ensures sufficient effective components to form a complete three-dimensional network framework, achieving stable coating of microorganisms and in-situ induced deposition of calcium carbonate. It also maintains suitable viscosity, resulting in good fluidity, easy dispersion, and convenient on-site mixing. It exhibits optimal synergy with acid-resistant Bacillus and mineralized substrates, balancing curing effect, construction performance, and low-carbon economy.

[0019] Preferably, the inoculum size of the acid-resistant Bacillus in step S2 is 8-10%. This inoculum size range is crucial for ensuring that the acid-resistant Bacillus megaterium maintains high enzyme production activity under specific acidic culture conditions. A size below this range will result in insufficient early-stage reinforcement of the system, while a size above this range will lead to wasted energy consumption and decreased bacterial culture stability.

[0020] Another objective of this invention is to provide a phosphogypsum composite road base course made from construction waste, comprising the following raw materials by mass fraction: 50%~70% phosphogypsum, 5%~10% low-carbon curing agent, and the remainder being construction waste aggregate; the construction waste aggregate has a particle size ≤20mm, and is composed of the following particle size components: 50%~60% by mass of particles with a diameter of 10mm~20mm, 30%~35% by mass of particles with a diameter of 5mm~10mm, and 5%~15% by mass of particles with a diameter <5mm; the initial moisture content of the phosphogypsum is 8%~12%. In this invention, the coarse aggregate (10~20mm) forms the skeleton, playing a major role in load-bearing and support; the medium aggregate (5~10mm) fills the large gaps between the coarse aggregate; and the fine aggregate and powder (<5mm) further fill the smaller pores.

[0021] Another object of the present invention is to provide a construction method for the above-mentioned phosphogypsum composite construction waste road base, comprising the following steps:

[0022] (1) In an environment where pH adjustment is not required, the low-carbon curing agent, phosphogypsum and construction waste aggregate are mixed in proportion and stirred evenly to obtain a mixture.

[0023] (2) The mixture obtained in step (1) is spread in layers with a single layer thickness of 15~25cm. After spreading, it is compacted by a road roller. The compaction procedure is as follows: first lightly vibrate and compact 2~3 times, then heavily vibrate and compact 3~4 times until the compaction degree is ≥96%. The speed of light vibrating compaction is 2~3km / h, and the speed of heavy vibrating compaction is 1.5~2.5km / h. If the moisture content of the mixture is lower than the preset optimum moisture content during the compaction process, an appropriate amount of deionized water is sprayed onto the surface of the mixture. The amount of deionized water sprayed does not exceed 2% of the total mass of the mixture.

[0024] (3) Under normal temperature conditions, the mixture is naturally cured for 7-14 days. The low-carbon curing agent induces the deposition of mineral products in situ inside the mixture, thus obtaining the cured phosphogypsum composite building slag road base.

[0025] In this way, the modification process can be seamlessly integrated with traditional road base construction techniques (layered paving, compaction, and curing). The bacterial solution can be incorporated into the phosphogypsum composite construction waste mixture through spraying or pre-mixing, without the need for additional specialized equipment. Because pre-alkali adjustment is unnecessary, phosphogypsum can be utilized in situ at a high proportion of 50%–70% in the composite road base, simultaneously facilitating the disposal of 20%–40% of construction waste solid waste. This achieves dual large-scale disposal of both phosphorus chemical solid waste and construction solid waste, maximizing the value of solid waste disposal. It replaces traditional inorganic cementitious materials such as cement and lime, reducing CO2 emissions during cementitious material production. Furthermore, the modification process is a biochemical reaction, generating no toxic or harmful substances, meeting the requirements of green road construction. The elimination of the quicklime alkali adjustment process significantly reduces carbon emissions and resolves the environmental safety risks of phosphorus and fluorine leaching from the road base.

[0026] Compared with the prior art, the present invention has the following beneficial effects:

[0027] 1. This invention addresses the characteristics of a dual-solid-waste composite system of phosphogypsum and construction waste road base. It constructs a synergistic solidification mechanism adapted to acidic composite solid-waste systems through deep coupling of acid-resistant composite gel (CMCG) and microbial mineralization reactions. On one hand, the CMCG composite gel, with its porous three-dimensional network structure, provides a microenvironmental protection and buffer barrier for acid-resistant Bacillus, significantly mitigating the inhibitory effects of the inherently strong acidity, high fluoride, high phosphorus, and high salt environment of phosphogypsum on bacterial growth and urease secretion. This ensures that the bacteria can stably and continuously catalyze the hydrolysis of urea to produce carbonate ions in the acidic microenvironment of the composite system, providing sufficient mineralization precursors for the cementation of composite aggregates. On the other hand, the oligocarboxylated chitosan in the gel and the organically intercalated sodium montmorillonite surface are rich in high-density electronegative active sites, which can serve as efficient heterogeneous nucleation centers. This induces the CaCO3 crystals generated by the mineralization of acid-resistant Bacillus to be deposited in situ, uniformly, and directionally on the three-dimensional network framework. Simultaneously, it achieves interfacial bridging between phosphogypsum fine particles and construction waste aggregate, ultimately forming a continuous and stable organic-inorganic composite hybrid cement structure. This solves the technical problems of scattered distribution of single microbial mineralization products, easy crystal agglomeration, weak interfacial bonding with phosphogypsum-construction waste composite particles, and easy redissolution of mineralization products from the microscopic level.

[0028] 2. The curing agent of this invention has stable components and strong construction compatibility. It can be directly mixed with phosphogypsum-construction waste composite mixture through conventional methods such as pre-mixing and spraying, achieving acidic in-situ curing without the need for pre-adjusting alkali. It seamlessly connects with traditional road base construction processes (layered paving, compaction, and curing), without requiring additional specialized construction equipment, and is compatible with the standardized construction process of existing road projects. In terms of structural reinforcement, CMCG gel acts as a crystal nucleus template and tough matrix, driving the directional growth of calcium carbonate mineralization products along the network skeleton, and preferentially filling the gaps between phosphogypsum fine particles and the interface gaps between phosphogypsum and construction waste aggregate, forming a multi-level nested hybrid structure of "phosphogypsum-construction waste composite skeleton-gel tough matrix-calcium carbonate rigid core". The flexible bonding and anchoring effect of the gel greatly improves the interfacial bonding strength between composite aggregates, effectively dissipating external load stress, inhibiting the initiation and propagation of microcracks, and significantly improving the technical problems of high brittleness, poor water stability, and easy softening and degumming in humid environments of the composite matrix. In terms of environmental safety, oligomeric carboxylated chitosan and montmorillonite can first capture soluble phosphorus and fluorine released from phosphogypsum and trace harmful ions carried by construction waste in the composite system through chelation and adsorption. Then, the dense calcium carbonate crystals generated in situ encapsulate, seal, and stabilize these ions, achieving a triple synergistic control effect of adsorption-chelation-mineralization co-solidification. This significantly reduces the risk of leaching of harmful impurities under complex conditions such as long-term rainfall and groundwater erosion, ensuring the long-term environmental safety and service durability of the phosphogypsum composite construction waste road base. The solidified phosphogypsum composite construction waste road base prepared by this invention can achieve an unconfined compressive strength of 4.0 MPa after 28 days, while the carbon emission per unit mass of solidification is only 0.09 kg CO2 / kg, which is about 89% lower than that of traditional cement solidification technology. This achieves a significant reduction in engineering energy consumption and carbon emissions while significantly improving the mechanical strength and water stability of the base, providing a new path for the low-cost synergistic resource utilization of industrial and construction solid waste.

[0029] 3. This invention addresses the dual-solid-waste system characteristics of phosphogypsum composite construction waste road base layers. Its core mechanism is the synergistic solidification of microbially induced calcium carbonate precipitation and biopolymer composite gel. This can completely replace traditional high-carbon-emission cementitious materials such as cement and lime, truly achieving low-carbon, green, and efficient synergistic solidification of the dual-solid-waste system. In terms of resource utilization, since there is no need for pretreatment of acidic phosphogypsum with an added alkaline regulator, phosphogypsum can be utilized in situ at a high proportion of 50%–70% in the composite road base layer, simultaneously synergistically disposing of 20%–40% of construction waste solid waste. This achieves large-scale dual disposal of phosphate chemical solid waste and construction solid waste, providing a new technological path for the large-scale, high-value, and low-cost synergistic resource utilization of industrial and construction solid waste. Regarding green and low-carbon aspects, all raw materials used are biodegradable or natural mineral materials. The preparation process is carried out at room temperature and pressure, with low energy consumption and no toxic byproducts. The carbon emissions throughout the entire life cycle are significantly lower than those of traditional solidification systems. This invention, while satisfying the mechanical properties, water stability, and environmental safety of road base courses, also has multiple advantages such as low-carbon emission reduction, efficient utilization of solid waste, and green construction, providing a creative solution for the resource-based and low-carbon utilization of acidic industrial solid waste in the field of transportation engineering. Detailed Implementation

[0030] The present invention will be further described in detail below with reference to embodiments, but the scope of protection of the present invention is not limited thereto. Unless otherwise specified, the experimental methods in the following embodiments are conventional methods; and unless otherwise specified, the materials used are commercially available.

[0031] The acid-resistant Bacillus used in this invention is *Bacillus megaterium*, purchased from the China General Microbiological Culture Collection Center (CGMCC), strain accession number CGMCC 1.6721. The phosphogypsum used was obtained from a phosphogypsum stockpile of a domestic phosphate chemical company, with a pH value of 3.2–4.8, and the content (mass fraction) of harmful impurities was as follows: soluble phosphorus (PO42-) 3- 0.35%~0.62%, fluoride ions (F - 0.18%~0.32% soluble calcium (Ca 2+ The aggregate contains 8.2%~10.5% calcium sulfate dihydrate (CaSO4・2H2O), with the remaining main component being ≥85% calcium sulfate dihydrate, and a particle size of 0.1~5mm, meeting the requirements for phosphogypsum road base materials. The construction waste used was taken from a building demolition and foundation pit excavation project in a domestic city. After pretreatment such as screening, crushing, and impurity removal to remove impurities such as steel bars, wood chips, plastics, and large bricks and stones, recycled aggregate with a particle size ≤20mm and continuous gradation was obtained, meeting the technical requirements for recycled aggregate for road base.

[0032] I. A method for preparing a low-carbon curing agent for phosphogypsum composite construction waste road base and its application.

[0033] Example 1: This example includes the following steps:

[0034] (1) Take oligocarboxylated chitosan (O-CCS) with a molecular weight of 1000~3000 Da and a carboxyl substitution degree of ≥85%, then add O-CCS to deionized water and stir until the mass concentration is 2%~3%. Adjust the pH to 4.0~4.5 (consistent with the acidic environment of phosphogypsum) to form a transparent acidic solution, i.e. oligocarboxylated chitosan solution.

[0035] (2) Sodium montmorillonite was modified by intercalation with hexadecyltrimethylammonium bromide, and the interlayer spacing was increased from 1.2 nm to 3.5~4.0 nm to obtain organic intercalated sodium montmorillonite.

[0036] (3) Add organically intercalated sodium montmorillonite to the oligocarboxylated chitosan solution, making the mass ratio of oligocarboxylated chitosan to organically intercalated sodium montmorillonite 4:1. Disperse ultrasonically for 30 min (power 300W) to completely exfoliate the layered structure of montmorillonite and form an intercalated dispersion. Then add trisodium citrate and stir at 30~35℃ for 60 min. Utilize the ester bond formed by citrate and the hydroxyl groups of chitosan to construct a three-dimensional network composite gel, finally obtaining an acid-resistant composite reinforcing agent with a solid content of 5%~6%, which is a light milky white viscous substance.

[0037] (4) The activated acid-fast Bacillus megaterium (CGMCC 1.6721) was inoculated into LB liquid medium (pH 4.0–4.5) at an inoculation rate of 10%, and then incubated at 30–32°C for 36–42 h to obtain a bacterial concentration of 1 × 10⁻⁶. 8 ~3×10 8 Acid-resistant Bacillus solution at CFU / mL.

[0038] (5) Dissolve urea in deionized water to prepare a urea solution with a concentration of 1.2 mol / L, which is the mineralization substrate solution.

[0039] (6) The components are prepared by weight: 10 parts of acid-resistant composite gel, 20 parts of acid-resistant Bacillus liquid, and 70 parts of mineralized substrate solution to obtain the curing agent.

[0040] (7) Select waste concrete fragments and broken bricks and tiles, crush and screen them to remove wood, plastic and metal impurities, and control the particle size to 5mm~20mm. By adjusting the screening ratio, a continuous gradation is formed, in which the mass fraction of particles with a particle size of 10mm~20mm accounts for 50%~60%, the mass fraction of particles with a particle size of 5mm~10mm accounts for 30%~35%, and the mass fraction of particles with a particle size <5mm accounts for 5%~15%. Use native acidic phosphogypsum, without the need to add an alkaline neutralizing agent in advance, and let the phosphogypsum air dry to a moisture content of 8%~12% for later use.

[0041] (8) Weigh the low-carbon curing agent prepared in step (6), the pretreated phosphogypsum and the construction waste aggregate in a mass ratio of 1:3:2. First, put the phosphogypsum and the construction waste aggregate into a forced mixer and dry mix for 2 minutes to ensure that the fine material fully coats the coarse aggregate. Then, spray the low-carbon curing agent evenly into the mixer in an atomized form and continue to wet mix for 3 minutes to obtain a uniformly mixed material.

[0042] (9) Transport the mixed material to the test section and pave it in layers using a paver. The thickness of each layer is 20cm. After paving, compact it using a roller. The compaction procedure is as follows: first, lightly vibrate and compact 2-3 times at a speed of 2-3 km / h, then heavily vibrate and compact 3-4 times at a speed of 1.5-2.5 km / h, until the compaction degree is ≥96%. If the moisture content of the mixture is lower than the preset optimum moisture content (optimum moisture content is 10%-13%) during the compaction process, spray an appropriate amount of deionized water onto the surface of the mixture. The amount of deionized water sprayed shall not exceed 2% of the total mass of the mixture.

[0043] (10) After compaction, allow for natural curing at room temperature (20~30℃). During this period, the acid-resistant Bacillus megaterium in the curing agent maintains its metabolic activity in the acidic phosphogypsum microenvironment, catalyzing the hydrolysis of the substrate urea to produce CO3. 2- This process induces the directional deposition of calcite-type calcium carbonate crystals on the three-dimensional network framework of the composite gel. The mineralization products preferentially form a "chemical bridge" at the interface between the fine phosphogypsum particles and the slag aggregate, simultaneously achieving physical filling and chemical bonding.

[0044] Comparative Example 1: Ordinary silicate cement was used as the curing agent, and the amount of curing agent was 10% of the total mass of phosphogypsum-construction waste composite aggregate. Other steps were the same as in Example 1.

[0045] Comparative Example 2: No acid-resistant composite gel was added; other steps were the same as in Example 1.

[0046] Comparative Example 3: No acid-resistant Bacillus and mineralized substrate solution were added; other steps were the same as in Example 1.

[0047] Comparative Example 4: Traditional Bacillus pasteurellii was used instead of acid-resistant Bacillus pasteurellii. Before use, the phosphogypsum was pre-adjusted to pH 8.5-9.5 with quicklime. Other steps were the same as in Example 1.

[0048] II. Performance Verification

[0049] The road base specimens prepared in Example 1 and Comparative Examples 1-4 were subjected to 28-day unconfined compressive strength tests according to JTG 3441-2024 "Test Procedure for Inorganic Binder Stabilized Materials in Highway Engineering". The solidified samples after 28 days of fracture were then subjected to leaching toxicity tests using HJ 557-2010 "Leaching Toxicity of Solid Waste - Horizontal Oscillation Method". Furthermore, carbon emissions from the preparation and construction of the curing agent (excluding phosphogypsum and construction waste) were calculated according to GB / T 51346-2019 "Standard for Calculation of Carbon Emissions from Buildings". The results are shown in Table 1.

[0050] Table 1

[0051]

[0052] As shown in Table 1, the 28-day unconfined compressive strength of Example 1 of this invention reached 4.0 MPa, an increase of 53.8% compared to Comparative Example 2 (single-component bacteria) and 135.3% compared to Comparative Example 3 (single-component gel). This is because although single-microbial solidification (Comparative Example 2) has a mineralization effect, the crystals are randomly distributed in acidic phosphogypsum, resulting in weak bonding force; single-gel solidification (Comparative Example 3) only plays a physical filling role and lacks rigid support. This result strongly proves that the interaction between acid-resistant Bacillus and CMCG composite gel is not a simple physical accumulation, but rather a significant synergistic effect. This invention, through the synergistic mineralization effect of acid-resistant Bacillus and acid-resistant composite gel, can generate uniform and stable calcium carbonate cement products in situ in the acidic phosphogypsum system and form a multi-level nested hybrid structure, significantly improving the mechanical strength of the solidified body. Compared with Comparative Example 1 (cement solidification), the strength of Example 1 is not only 5.3% higher, but also completely solves the problem of poor long-term stability of cement in acidic phosphogypsum due to hindered hydration. Compared to Comparative Example 4 (traditional MIP requiring alkali adjustment), Example 1 showed a 17.6% increase in strength without the need for alkaline additives such as lime and fly ash. This indicates that the present invention utilizes the acid-resistant properties of Bacillus megaterium for "in-situ mineralization," where the generated CaCO3 preferentially deposits on the surface and pores of phosphogypsum particles and soil particles, achieving particle encapsulation, pore filling, and aggregate bonding. This microscopically improves the density of the mixture and the interparticle bonding force, resulting in a denser structure.

[0053] Furthermore, the leaching concentrations of phosphorus and fluorine in Example 1 were far below the national standard limits, the lowest among all groups. This indicates that the present invention, through a triple synergistic control effect of adsorption-chelation-mineralization co-solidification, first utilizes the active sites of the composite gel to capture soluble harmful ions such as phosphorus and fluorine, and then permanently encapsulates and seals them with dense calcium carbonate crystals generated in situ, thereby fundamentally solving the risk of impurity leaching from phosphogypsum during long-term service.

[0054] Furthermore, the carbon emission per unit mass of the road base specimen prepared in Example 1 was only 0.09 kg CO2 / kg, which is approximately 89% lower than that of Comparative Example 1 (0.85 kg CO2 / kg) which uses traditional cement curing; and approximately 76% lower than that of Comparative Example 4 (0.38 kg CO2 / kg) which requires pre-adjusted alkali. This fully demonstrates that the present invention can completely replace high-carbon cementitious materials such as cement and lime, and eliminates the need for pre-adjusted alkali, significantly simplifying the process, reducing energy consumption, and truly achieving low-carbon, green, and efficient curing of phosphogypsum road bases.

[0055] In summary, this invention overcomes the technical bottlenecks of traditional curing technologies, such as the need for pre-adjustment of alkali, poor activity in acidic environments, high carbon emissions, and insufficient water stability, through the deep coupling and synergistic effect of acid-resistant Bacillus and acid-resistant composite gel. It achieves synergistic and efficient curing of phosphogypsum and construction waste, two solid wastes, without the need for pre-adjustment of alkali, while taking into account excellent mechanical properties, environmental safety, and low carbon benefits. It has outstanding substantive features and significant progress, and its engineering application value and ecological and environmental protection benefits are significant.

[0056] The above description is only a preferred embodiment of the present invention and is not intended to limit the present invention. Any modifications, equivalent substitutions, and improvements made within the spirit and principles of the present invention should be included within the protection scope of the present invention.

Claims

1. A low-carbon curing agent for phosphogypsum composite construction waste road base, characterized in that, The product comprises the following components by weight: 5-15 parts of acid-resistant composite gel, 10-25 parts of acid-resistant Bacillus liquid, and 60-85 parts of mineralized substrate solution; the acid-resistant composite gel is a composite product with a three-dimensional network structure constructed from oligocarboxylated chitosan and organically intercalated sodium montmorillonite through a weakly acidic crosslinking agent; the optimal growth pH of the acid-resistant Bacillus liquid is 3-5, and the Bacillus can synthesize and secrete extracellular urease; the solute in the mineralized substrate solution includes urea.

2. The low-carbon curing agent for phosphogypsum composite construction waste road base layer according to claim 1, characterized in that, The mass ratio of the oligocarboxylated chitosan to the organically intercalated sodium montmorillonite is 4:1 to 5:

1.

3. The low-carbon curing agent for phosphogypsum composite construction waste road base layer according to claim 1, characterized in that, The oligomeric carboxylated chitosan has a molecular weight of 1000~3000 Da and a carboxyl substitution degree of ≥85%; the organic intercalated sodium montmorillonite is hexadecyltrimethylammonium bromide modified sodium montmorillonite with an interlayer spacing of 3.5~4.0 nm.

4. The low-carbon curing agent for phosphogypsum composite construction waste road base layer according to claim 1, characterized in that, The weakly acidic crosslinking agent is trisodium citrate, disodium tartrate, sodium malate, sodium gluconate, or disodium adipic acid; the amount of the weakly acidic crosslinking agent is 0.5% to 1.0% of the mass of the acid-resistant composite gel.

5. The low-carbon curing agent for phosphogypsum composite construction waste road base layer according to claim 1, characterized in that, The concentration of the acid-resistant Bacillus spp. solution is 10. 7 ~10 9 CFU / mL, wherein the acid-resistant Bacillus is at least one of Bacillus megaterium, Bacillus alkalophilus, Bacillus subtilis, Bacillus lysine, and Bacillus pasteurellium.

6. The low-carbon curing agent for phosphogypsum composite construction waste road base layer according to claim 1, characterized in that, The concentration of urea in the mineralized substrate solution is 0.8~2.0 mol / L.

7. A method for preparing a low-carbon curing agent for phosphogypsum composite construction waste road base course as described in any one of claims 1 to 6, characterized in that, Includes the following steps: S1: Dissolve oligocarboxylated chitosan in deionized water, adjust the pH to 4.0~4.5, then add organic intercalated sodium montmorillonite for dispersion treatment, then add a weak acidic crosslinking agent, and crosslink at 30~35℃. After crosslinking, it is a light milky white viscous substance, which is the acid-resistant composite gel. S2: Inoculate the activated acid-fast Bacillus into liquid culture medium, and then incubate at 30-35℃ for 24-48 hours to obtain acid-fast Bacillus liquid; S3: Mix the acid-resistant composite gel obtained in step S1, the acid-resistant Bacillus liquid obtained in step S2, and the mineralized substrate solution, or package the three separately for on-site preparation, to obtain the low-carbon curing agent.

8. The method for preparing the low-carbon curing agent for phosphogypsum composite construction waste road base layer according to claim 7, characterized in that, The dispersion treatment in step S1 is ultrasonic dispersion, with an ultrasonic power of 250-350W and an ultrasonic time of 30-45min; the crosslinking time is 30-50min; the solid content of the acid-resistant composite gel is 4.5%-6.5%; and the inoculation amount of the acid-resistant Bacillus in step S2 is 8%-10%.

9. A phosphogypsum composite construction waste road base, characterized in that, It includes the following raw materials by mass fraction: 50% to 70% phosphogypsum, 5% to 10% low-carbon curing agent as described in any one of claims 1 to 6, and the balance being construction waste aggregate; The construction waste aggregate has a particle size ≤20mm, and the construction waste aggregate is composed of the following particle size components: 50%~60% by mass of particles with a particle size of 10mm~20mm, 30%~35% by mass of particles with a particle size of 5mm~10mm, and 5%~15% by mass of particles with a particle size <5mm; the initial moisture content of the phosphogypsum is 8%~12%.

10. A construction method for a phosphogypsum composite construction waste road base as described in claim 9, characterized in that, Includes the following steps: (1) In an environment where pH adjustment is not required, the low-carbon curing agent, phosphogypsum and construction waste aggregate are mixed in proportion and stirred evenly to obtain a mixture; (2) The mixture obtained in step (1) is laid in layers with a single layer thickness of 15-25cm. After laying, it is compacted using a road roller. The compaction procedure is as follows: first lightly vibrate and compact 2-3 times, then heavily vibrate and compact 3-4 times until the compaction degree is ≥96%. The speed of light vibrating compaction is 2-3km / h, and the speed of heavy vibrating compaction is 1.5-2.5km / h. If the moisture content of the mixture is lower than the preset optimum moisture content during the compaction process, an appropriate amount of deionized water is sprayed onto the surface of the mixture. The amount of deionized water sprayed does not exceed 2% of the total mass of the mixture. (3) Under normal temperature conditions, the mixture is naturally cured for 7-14 days. The low-carbon curing agent induces the deposition of mineral products in situ inside the mixture, thus obtaining the cured phosphogypsum composite building slag road base.

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