Modified semi-hydrated phosphogypsum in-situ solidified soil as well as preparation method and application thereof

Abstract

The invention discloses a preparation method of in-situ solidified soil of modified semi-hydrated phosphogypsum. The preparation method comprises the following steps: mixing pretreated phosphogypsum with a composite crystal modifier, homogenizing and retting, and calcining to obtain the modified semi-hydrated phosphogypsum; wherein the mass ratio of the alpha-type semi-hydrated phosphogypsum to the beta-type semi-hydrated phosphogypsum is controlled to be 1: (0.5-2) in the calcining process; and (2) mixing the modified semi-hydrated ardealite with a composite curing agent, uniformly mixing the obtained mixture with the pretreated in-situ soil, adding water to obtain a soil body, and performing layered compaction and maintenance on the soil body to obtain the in-situ solidified soil of the modified semi-hydrated ardealite. According to the method provided by the invention, the pretreatment process of the phosphogypsum is optimized, and the synergistic effect of the composite crystal modifier and the composite curing agent is combined, so that the soil can be quickly cured, and the cured soil is excellent in mechanical property and high in CO2 storage efficiency and heavy metal adsorption rate.

Description

Technical Field

[0001] The invention belongs to the technical field of building materials, and particularly relates to an in-situ solidified soil of modified hemihydrate phosphogypsum, a preparation method thereof and an application thereof. Background Art

[0002] In response to the severe challenges posed by climate warming and industrial solid waste pollution to the earth's environment and social development, researchers at home and abroad have proposed effective and reasonable solutions. Among them, CO2 capture and storage technology, industrial solid waste carbonization technology and soil carbonization solidification technology are key technologies for effectively solving the above problems.

[0003] Phosphogypsum is a representative and common industrial solid waste, and its large-scale accumulation can cause environmental pollution. Currently, the resource utilization of phosphogypsum is a research hotspot. Using phosphogypsum to solidify soil can not only solve the problem of phosphogypsum accumulation, but also improve the mechanical properties of the soil, which has important economic and environmental significance. The main component of phosphogypsum is dihydrate phosphogypsum, which contains a small amount of hemihydrate phosphogypsum and impurities such as P and F. After various pretreatments, hemihydrate phosphogypsum can replace construction phosphogypsum under certain conditions. Hemihydrate phosphogypsum also has very short initial and final setting times. Currently, there are more studies on the direct use of phosphogypsum (dihydrate phosphogypsum) to solidify soil, while there are relatively few studies on hemihydrate phosphogypsum. Most of the studies are on fluidized soil solidification, and there are few reports on in-situ land solidification.

[0004] Currently, CO2 capture and storage (CCS) is one of the mainstream means of CO2 control. Its essence is to capture CO2 produced in the industrial and energy conversion processes, and to fix and store it through physical and chemical processes to isolate it from the atmosphere for a long time. The mainstream view is that CO2 mineral storage is the most promising storage technology, and the CO2 mineral storage process has the advantages of high storage efficiency, thermodynamic stability, permanent stability of carbonization products, low risk of CO2 leakage, and no need for additional energy. In mineral storage technology, carbonized minerals mainly come from two sources: natural sources and industrial sources. Since some industrial solid wastes contain a certain amount of alkaline oxides such as calcium oxide (CaO) and magnesium oxide (MgO), they have high reactivity and can therefore also serve as ideal raw materials for carbon dioxide storage. Summary of the Invention

[0005] The technical problem to be solved by the present invention is to provide an in-situ solidified soil of modified hemihydrate phosphogypsum and its preparation method and application in response to the deficiencies of the existing technology. This method optimizes the pretreatment process of phosphogypsum and combines the synergistic effect of a composite crystallization agent and a composite curing agent to achieve rapid solidification of the soil. The solidified soil has excellent mechanical properties, high CO2 storage efficiency and heavy metal adsorption rate.

[0006] The technical solution adopted by the present invention to solve the above-mentioned problems is: A method for preparing in-situ solidified soil of modified hemihydrate phosphogypsum comprises the following steps: (1) pre-treating the phosphogypsum, wherein the pre-treatment includes countercurrent washing, neutralization, drying and aging; (2) The pretreated phosphogypsum is mixed with a composite crystal-changing agent, homogenized and fermented, and then calcined to obtain modified hemihydrate phosphogypsum; wherein during the calcination process, the contents of α-type hemihydrate phosphogypsum and β-type hemihydrate phosphogypsum in the modified hemihydrate phosphogypsum are monitored in real time by in-situ XRD, and the mass ratio of the α-type hemihydrate phosphogypsum to the β-type hemihydrate phosphogypsum is controlled to be 1:(0.5-2); the composite crystal-changing agent is a mixture of organic matter containing carboxylate groups, sulfate, and nanofiller; (3) mixing the modified hemihydrate phosphogypsum obtained in step (2) with a composite curing agent to obtain a mixture; (4) The mixture obtained in step (3) is mixed evenly with the pretreated in-situ soil, water is added to obtain a soil body, and the soil body is compacted layer by layer and covered with a film for curing to obtain the in-situ solidified soil of the modified hemihydrate phosphogypsum.

[0007] In the above scheme, the organic matter containing carboxylate groups is one or a mixture of sodium benzoate, sodium citrate, and sodium dodecylbenzenesulfonate, and the addition amount is 0.1%-0.5% of the mass of the pretreated phosphogypsum; the sulfate is one or a mixture of aluminum sulfate, iron sulfate, and magnesium sulfate, and the addition amount is 0.1%-0.5% of the mass of the pretreated phosphogypsum; the nanofiller is nano-silica and / or nano-calcium oxide, and the addition amount is 0.1%-0.5% of the mass of the pretreated phosphogypsum.

[0008] In the above scheme, the pretreated in-situ soil is prepared by adding fungi and inorganic nitrogen to the in-situ soil and stirring the mixture evenly.

[0009] In the above scheme, the fungus is one or a mixture of Bacillus subtilis, Trichoderma, Pseudomonas, and actinomycetes, and the inorganic nitrogen is ammonium sulfate and / or ammonium bicarbonate; wherein the fungus dosage is 0.1%-1% of the in-situ soil mass, and the inorganic nitrogen dosage is 0.1%-2% of the in-situ soil mass.

[0010] In the above solution, the mass ratio of the pretreated in-situ soil, the modified hemihydrate phosphogypsum, the composite curing agent, and the water is (4.5-10):1:(0.5-1):(0.5-5).

[0011] In the above scheme, the pretreatment of the pretreated phosphogypsum in step (1) includes countercurrent washing, neutralization, drying and aging, specifically: a multi-stage countercurrent washing process is adopted; calcium hydroxide is used for neutralization; the moisture content of the dried phosphogypsum is less than 10%; the aging time is 1-3 months; the soluble P2O5 content in the washed phosphogypsum is ≤0.3%, and the pH value of the neutralized phosphogypsum is 8-8.5.

[0012] In the above scheme, the temperature of the homogenization and retting in step (2) is 60-80°C and the time is 4-6 hours; the temperature of the calcination is 610-630°C and the time is 2-3 hours.

[0013] In the above scheme, the composite curing agent is a mixture of one or more of wood activated carbon, an early strength agent, low-carbon cement clinker, and zeolite powder; the early strength agent is a nano-composite early strength agent, which is composited with nano-silicon dioxide and nano-hydrated calcium silicate as raw materials; the mass ratio of nano-silicon dioxide to nano-hydrated calcium silicate in the nano-composite early strength agent is 1:3; the preparation steps of the nano-hydrated calcium silicate include: dissolving calcium salt and silicon salt in water, adding alkali solution to adjust the pH value of the resulting mixture to 12-14, bathing the mixture in water at 60-80°C for 7-14 days, and filtering and washing to obtain the nano-hydrated calcium silicate.

[0014] In the above scheme, the film covering and curing time is 7-14 days.

[0015] In the above scheme, step (4) specifically comprises: using a mixing device to crush and mix the in-situ soil evenly, evenly sprinkling the mixture of the modified hemihydrate phosphogypsum and the curing agent, mixing evenly again, adding water and adjusting the water consumption according to the project needs to carry out layered compaction; wherein the compaction process needs to be controlled within 5-20 minutes.

[0016] Another aspect of the present invention provides modified hemihydrate phosphogypsum in-situ solidified soil prepared by the above-mentioned method for preparing modified hemihydrate phosphogypsum in-situ solidified soil.

[0017] Another aspect of the present invention provides the use of the modified hemihydrate phosphogypsum in-situ solidified soil in soft soil foundation treatment projects and riverbank reinforcement projects.

[0018] In this invention, multiple pretreatment steps are used to remove impurities from the phosphogypsum, ensuring it meets environmental and building material standards. A composite crystallization agent is then used to convert the primary component, dihydrate phosphogypsum, into hemihydrate phosphogypsum, while also controlling the mass ratio of α- to β-type phosphogypsum. α-type hemihydrate phosphogypsum exhibits complete crystallization, a low water requirement, and higher strength, while β-type hemihydrate phosphogypsum exhibits very fine crystals, a significantly larger specific surface area, a high water requirement, and a shorter setting time. The mass ratio of α-type hemihydrate to β-type hemihydrate phosphogypsum in this invention is 1:0.5-2. Within this mass ratio range, the water requirement of the solidified soil can be reduced to a certain extent, allowing more water in the solidified soil to participate in the curing reaction with the curing agent. This not only achieves rapid hardening of the solidified soil, improves its strength, and reduces the construction period, but also allows the early curing reaction to absorb more CO2 from the soil.

[0019] In the present invention, nanoparticles (nano-silicon dioxide or nano-calcium oxide) are added to the composite crystal-transforming agent to act as heterogeneous crystal seeds, which can shorten the production cycle, improve production efficiency, and save energy. In addition, both nanoparticles have physical adsorption capacity and can adsorb PO4 in the process of generating hemihydrate gypsum. 3- , reducing the P2O5 content. On the other hand, both nanoparticles have nanofilling effect, which can improve the strength of hemihydrate phosphogypsum.

[0020] In the present invention, the composite curing agent can be low-carbon cement clinker. On the one hand, the raw material of low-carbon cement clinker is magnesium-rich cement, which has gelling properties and can consolidate the soil and provide strength when added to the soil. On the other hand, its carbon emissions during the production process are less than those of conventional cement. Its main component, magnesium-rich mineral, has a strong carbonization potential, which can react with carbon dioxide. The cementing properties of its carbonization product, magnesium carbonate, are also stronger than those of calcium carbonate, the carbonization product of ordinary cement.

[0021] In the present invention, one of the components of the composite curing agent, the early strength agent (nano-composite early strength agent), is a nano-sized particle with high activity, which can promote the reaction of low-carbon cement clinker and increase the hydration rate. In addition, this substance can promote cement-solidified heavy metal ions in the soil; the other two components of the composite curing agent, zeolite powder and wood activated carbon, have a regular three-dimensional pore structure and a large specific surface area. They can capture specific substances through physical adsorption and chemical ion exchange, and can also further adsorb and solidify total metal ions or other harmful substances in the soil.

[0022] Compared with the existing technology, the beneficial effects of the present invention are: (1) The present invention provides a method for preparing in-situ solidified soil of modified hemihydrate phosphogypsum. The method can reduce the impurity content in phosphogypsum by optimizing the pretreatment process of phosphogypsum, and combines the synergistic effect of a composite crystallization agent and a composite curing agent to achieve rapid solidification of soil and solid waste utilization of phosphogypsum. The solidified soil has excellent mechanical properties, high CO2 storage efficiency and heavy metal adsorption rate.

[0023] (2) The present invention provides a method for preparing in-situ solidified soil of modified hemihydrate phosphogypsum, which realizes solid waste resource utilization by pre-treating phosphogypsum to make its soluble P2O5 content ≤0.3% (meeting the building materials standard (GB / T 23456-2018)); by optimizing the type and dosage of the composite crystallization agent, the dihydrate phosphogypsum is converted into hemihydrate phosphogypsum, and the ratio between α-type and β-type in the hemihydrate phosphogypsum can be accurately regulated, thereby regulating the setting time and solidification strength of the solidified soil, so that it can meet the construction requirements of fast hardening and early strength of the solidified soil; the early strength agent, zeolite and other materials in the composite solidifying agent can make the soil have adsorption properties and can absorb harmful substances in the soil; the low-carbon cement clinker in the composite solidifying agent can achieve the storage of carbon dioxide.

[0024] (3) The present invention provides a method for preparing in-situ solidified soil of modified hemihydrate phosphogypsum. By adding nitrogen sources and CO2-releasing fungi to the soil, efficient and safe removal of soil organic matter can be achieved, while CO2 is released at the same time. The combination of the composite solidifying agent and the soil body can activate the soil carbon cycle and achieve CO2 sequestration and engineering performance improvement through the three-step synergistic mechanism of "release-adsorption-solidification".

[0025] (4) The present invention provides a method for preparing in-situ solidified soil of modified hemihydrate phosphogypsum. The raw materials are readily available, the construction is simple, the cost is low, and the carbon dioxide can be stored inside and outside the soil at the same time. DETAILED DESCRIPTION

[0026] The technical solution of the present invention will be fully and clearly described below in conjunction with specific embodiments, but the described embodiments are only some embodiments of the present invention. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without making creative work are within the scope of protection of the present invention.

[0027] The present invention provides a method for preparing in-situ solidified soil of modified hemihydrate phosphogypsum, comprising the following steps: (1) Pretreatment of phosphogypsum, including countercurrent washing, neutralization, drying and aging.

[0028] (2) The pretreated phosphogypsum is mixed with a composite crystal-changing agent, homogenized and fermented at 60-80°C for 4-6 hours, and then calcined at 610-630°C for 2-3 hours to obtain modified hemihydrate phosphogypsum; wherein during the calcination process, the contents of α-type hemihydrate phosphogypsum and β-type hemihydrate phosphogypsum in the modified hemihydrate phosphogypsum are monitored in real time by in-situ XRD, and the mass ratio of α-type hemihydrate phosphogypsum to the β-type hemihydrate phosphogypsum is controlled to be 1:(0.5-2); the composite crystal-changing agent is an organic matter containing a carboxylate group, sulfuric acid The mixture of salt and nano filler, the organic matter containing carboxylate group is one or a mixture of sodium benzoate, sodium citrate and sodium dodecylbenzene sulfonate, and the addition amount is 0.1%-0.5% of the mass of the pretreated phosphogypsum; the sulfate is one or a mixture of aluminum sulfate, iron sulfate and magnesium sulfate, and the addition amount is 0.1%-0.5% of the mass of the pretreated phosphogypsum; the nano filler is nano silicon dioxide and / or nano calcium oxide, and the addition amount is 0.1%-0.5% of the mass of the pretreated phosphogypsum.

[0029] (3) The modified hemihydrate phosphogypsum obtained in step (2) is mixed with a composite curing agent to obtain a mixture; wherein the composite curing agent is a mixture of one or more of wood activated carbon, early strength agent, low carbon cement clinker, and zeolite powder.

[0030] (4) The mixture obtained in step (3) is uniformly mixed with the pretreated in-situ soil, and water is added to obtain a soil body. The soil body is compacted in layers and covered with a film for 7-14 days to obtain the in-situ solidified soil of the modified hemihydrate phosphogypsum; wherein the compaction process needs to be controlled within 5-20 minutes; the pretreated in-situ soil is obtained by adding fungi and inorganic nitrogen to the in-situ soil and stirring evenly, wherein the fungi are a mixture of Bacillus subtilis, Trichoderma, Pseudomonas, and actinomycetes in a mass ratio of 1:1:1:1, and the inorganic nitrogen is ammonium bicarbonate; wherein the amount of fungi is 1% of the mass of the in-situ soil, and the amount of inorganic nitrogen is 2% of the mass of the in-situ soil.

[0031] In a specific embodiment, the phosphogypsum raw material pretreatment steps include three-stage countercurrent washing (soluble P2O5 content in the washed phosphogypsum ≤ 0.3%), neutralization (pH 8-8.5), drying (moisture content <10%), and aging (1 month). The low-carbon cement clinker used is a mixture of magnesium oxychloride cement clinker and magnesium phosphate cement clinker, with a mass ratio of 1:1. The early strength accelerator used is a nanocomposite early strength accelerator composed of nano-silica and nano-hydrated calcium silicate, with a mass ratio of nano-hydrated calcium silicate to nano-silica of 3:1. The specific preparation method of the nano-hydrated calcium silicate is as follows: sodium silicate solution and anhydrous calcium chloride solution are used as raw materials, the water-to-solid ratio of the reaction solution is controlled to 10, and the calcium-silicon molar ratio is 1.2. Sodium hydroxide is added to adjust the pH of the resulting mixture to 14, and the mixture is incubated in a water bath at 60°C for 7 days, followed by filtration and washing to obtain the nano-hydrated calcium silicate. The zeolite powder has a particle size of less than 45 microns. The in-situ soil used was clay with a moisture content of 1.58%.

[0032] Example 1 A method for preparing in-situ solidified soil of modified hemihydrate phosphogypsum comprises the following steps: 1) Raw material processing: Adjust the types and proportions of raw materials of the composite crystal-turning agent: the mass ratio of sodium benzoate, aluminum sulfate and nano-calcium oxide in the composite crystal-turning agent = 2:1:1.

[0033] The pretreated phosphogypsum was mixed with a composite crystal-changing agent (calculated as 0.5% by mass of the pretreated phosphogypsum), homogenized and fermented at 60°C for 4 hours, and then calcined at 620°C for 2.5 hours. During the calcination process, the contents of α-type hemihydrate phosphogypsum and β-type hemihydrate phosphogypsum in the modified hemihydrate phosphogypsum were monitored in real time by in situ XRD, and the mass ratio of α-type hemihydrate phosphogypsum to the β-type hemihydrate phosphogypsum was controlled to be 2:1 to obtain modified hemihydrate phosphogypsum.

[0034] Composite curing agent: The mass ratio of wood activated carbon, low carbon cement clinker, early strength agent and zeolite powder in the composite curing agent is 1:2:1:1.

[0035] 2) Preparation steps: Take 100 parts of modified hemihydrate phosphogypsum and 100 parts of composite curing agent, mix the mixture with 800 parts of pretreated in-situ soil, add 143.3 parts of water, compact the mixture layer by layer (controlled within 20 minutes), and cover and cure for 7 days to obtain in-situ cured soil of modified hemihydrate phosphogypsum.

[0036] Performance testing: The 7-day unconfined compressive strength, setting time, heavy metal adsorption rate and CO2 storage efficiency of the in-situ solidified soil of the modified hemihydrate phosphogypsum prepared in this embodiment were tested.

[0037] Example 2 A method for preparing in-situ solidified soil of modified hemihydrate phosphogypsum comprises the following steps: 1) Raw material processing: Adjust the types and proportions of raw materials of the composite crystal-turning agent: the mass ratio of sodium benzoate, aluminum sulfate and nano-calcium oxide in the composite crystal-turning agent = 2:2:1.

[0038] The pretreated phosphogypsum was mixed with a composite crystal-changing agent (calculated as 0.5% by mass of the pretreated phosphogypsum), homogenized and fermented at 60°C for 4 hours, and then calcined at 620°C for 2.5 hours. During the calcination process, the contents of α-type hemihydrate phosphogypsum and β-type hemihydrate phosphogypsum in the modified hemihydrate phosphogypsum were monitored in real time by in situ XRD, and the mass ratio of α-type hemihydrate phosphogypsum to the β-type hemihydrate phosphogypsum was controlled to be 1:1, thereby obtaining modified hemihydrate phosphogypsum.

[0039] Composite curing agent: The mass ratio of wood activated carbon, low carbon cement clinker, early strength agent and zeolite powder in the composite curing agent is 1:2:1:1.

[0040] 2) Preparation steps: Take 100 parts of modified hemihydrate phosphogypsum and 100 parts of composite curing agent, mix the mixture with 800 parts of pretreated in-situ soil, add 143.3 parts of water, compact the mixture layer by layer (controlled within 20 minutes), and cover and cure for 7 days to obtain in-situ cured soil of modified hemihydrate phosphogypsum.

[0041] Performance testing: The 7-day unconfined compressive strength, setting time, heavy metal adsorption rate and CO2 storage efficiency of the in-situ solidified soil of the modified hemihydrate phosphogypsum prepared in this embodiment were tested.

[0042] Example 3 A method for preparing in-situ solidified soil of modified hemihydrate phosphogypsum comprises the following steps: 1) Raw material processing: Adjust the types and proportions of raw materials of the composite crystal-turning agent: the mass ratio of sodium benzoate, aluminum sulfate and nano-calcium oxide in the composite crystal-turning agent = 1:2:1.

[0043] The pretreated phosphogypsum was mixed with a composite crystal-changing agent (calculated as 0.5% by mass of the pretreated phosphogypsum), homogenized and fermented at 60°C for 4 hours, and then calcined at 620°C for 2.5 hours. During the calcination process, the contents of α-type hemihydrate phosphogypsum and β-type hemihydrate phosphogypsum in the modified hemihydrate phosphogypsum were monitored in real time by in-situ XRD, and the mass ratio of α-type hemihydrate phosphogypsum to the β-type hemihydrate phosphogypsum was controlled to be 1:2 to obtain modified hemihydrate phosphogypsum.

[0044] Composite curing agent: The mass ratio of wood activated carbon, low carbon cement clinker, early strength agent and zeolite powder in the composite curing agent is 1:2:1:1.

[0045] 2) Preparation steps: Take 100 parts of modified hemihydrate phosphogypsum and 100 parts of composite curing agent, mix the mixture with 800 parts of pretreated in-situ soil, add 143.3 parts of water, compact the mixture layer by layer (controlled within 20 minutes), and cover and cure for 7 days to obtain in-situ cured soil of modified hemihydrate phosphogypsum.

[0046] Performance testing: The 7-day unconfined compressive strength, setting time, heavy metal adsorption rate and CO2 storage efficiency of the in-situ solidified soil of the modified hemihydrate phosphogypsum prepared in this embodiment were tested.

[0047] Example 4 A method for preparing in-situ solidified soil of modified hemihydrate phosphogypsum comprises the following steps: 1) Raw material processing: Adjust the types and proportions of raw materials of the composite crystal-turning agent: the mass ratio of sodium benzoate, aluminum sulfate and nano-calcium oxide in the composite crystal-turning agent = 2:1:1.

[0048] The pretreated phosphogypsum was mixed with a composite crystal-changing agent (calculated as 0.5% by mass of the pretreated phosphogypsum), homogenized and fermented at 60°C for 4 hours, and then calcined at 620°C for 2.5 hours. During the calcination process, the contents of α-type hemihydrate phosphogypsum and β-type hemihydrate phosphogypsum in the modified hemihydrate phosphogypsum were monitored in real time by in situ XRD, and the mass ratio of α-type hemihydrate phosphogypsum to the β-type hemihydrate phosphogypsum was controlled to be 2:1 to obtain modified hemihydrate phosphogypsum.

[0049] Composite curing agent: The mass ratio of wood activated carbon, low carbon cement clinker, early strength agent and zeolite powder in the composite curing agent is 2:1:1:1.

[0050] 2) Preparation steps: Take 100 parts of modified hemihydrate phosphogypsum and 100 parts of composite curing agent, mix the mixture with 800 parts of pretreated in-situ soil, add 143.3 parts of water, compact the mixture layer by layer (controlled within 20 minutes), and cover and cure for 7 days to obtain in-situ cured soil of modified hemihydrate phosphogypsum.

[0051] Performance testing: The 7-day unconfined compressive strength, setting time, heavy metal adsorption rate and CO2 storage efficiency of the in-situ solidified soil of the modified hemihydrate phosphogypsum prepared in this embodiment were tested.

[0052] Example 5 A method for preparing in-situ solidified soil of modified hemihydrate phosphogypsum comprises the following steps: 1) Raw material processing: Adjust the types and proportions of raw materials of the composite crystal-turning agent: the mass ratio of sodium benzoate, aluminum sulfate and nano-calcium oxide in the composite crystal-turning agent = 2:1:1.

[0053] The pretreated phosphogypsum was mixed with a composite crystal-changing agent (calculated as 0.5% by mass of the pretreated phosphogypsum), homogenized and fermented at 60°C for 4 hours, and then calcined at 620°C for 2.5 hours. During the calcination process, the contents of α-type hemihydrate phosphogypsum and β-type hemihydrate phosphogypsum in the modified hemihydrate phosphogypsum were monitored in real time by in situ XRD, and the mass ratio of α-type hemihydrate phosphogypsum to the β-type hemihydrate phosphogypsum was controlled to be 2:1 to obtain modified hemihydrate phosphogypsum.

[0054] Composite curing agent: The mass ratio of wood activated carbon, low carbon cement clinker, early strength agent and zeolite powder in the composite curing agent is 1:1:2:1.

[0055] 2) Preparation steps: Take 100 parts of modified hemihydrate phosphogypsum and 100 parts of composite curing agent, mix the mixture with 800 parts of pretreated in-situ soil, add 143.3 parts of water, compact the mixture layer by layer (controlled within 20 minutes), and cover and cure for 7 days to obtain in-situ cured soil of modified hemihydrate phosphogypsum.

[0056] Performance testing: The 7-day unconfined compressive strength, setting time, heavy metal adsorption rate and CO2 storage efficiency of the in-situ solidified soil of the modified hemihydrate phosphogypsum prepared in this embodiment were tested.

[0057] Example 6 A method for preparing in-situ solidified soil of modified hemihydrate phosphogypsum comprises the following steps: 1) Raw material processing: Adjust the types and proportions of raw materials of the composite crystal-turning agent: the mass ratio of sodium benzoate, aluminum sulfate and nano-calcium oxide in the composite crystal-turning agent = 2:1:1.

[0058] The pretreated phosphogypsum was mixed with a composite crystal-changing agent (calculated as 0.5% by mass of the pretreated phosphogypsum), homogenized and fermented at 60°C for 4 hours, and then calcined at 620°C for 2.5 hours. During the calcination process, the contents of α-type hemihydrate phosphogypsum and β-type hemihydrate phosphogypsum in the modified hemihydrate phosphogypsum were monitored in real time by in situ XRD, and the mass ratio of α-type hemihydrate phosphogypsum to the β-type hemihydrate phosphogypsum was controlled to be 2:1 to obtain modified hemihydrate phosphogypsum.

[0059] Composite curing agent: The mass ratio of wood activated carbon, low carbon cement clinker, early strength agent and zeolite powder in the composite curing agent is 1:1:1:2.

[0060] 2) Preparation steps: 100 parts of modified hemihydrate phosphogypsum and 100 parts of composite curing agent were mixed, the mixture was evenly mixed with 800 parts of pretreated in-situ soil, 143.3 parts of water was added, the mixture was compacted layer by layer (controlled within 20 minutes) and covered with a film and cured for 7 days to obtain in-situ cured soil of modified hemihydrate phosphogypsum.

[0061] Performance testing: The 7-day unconfined compressive strength, setting time, heavy metal adsorption rate and CO2 storage efficiency of the in-situ solidified soil of the modified hemihydrate phosphogypsum prepared in this embodiment were tested.

[0062] Comparative Example 1 A method for preparing solidified soil comprises the following steps: 1) Raw material processing: Adjust the types and proportions of raw materials of the composite crystal-turning agent: the mass ratio of sodium benzoate, aluminum sulfate and nano-calcium oxide in the composite crystal-turning agent = 2:1:1.

[0063] The pretreated phosphogypsum was mixed with a composite crystal-changing agent (calculated as 0.5% by mass of the pretreated phosphogypsum), homogenized and fermented at 60°C for 4 hours, and then calcined at 620°C for 2.5 hours. During the calcination process, the contents of α-type hemihydrate phosphogypsum and β-type hemihydrate phosphogypsum in the modified hemihydrate phosphogypsum were monitored in real time by in situ XRD, and the mass ratio of α-type hemihydrate phosphogypsum to the β-type hemihydrate phosphogypsum was controlled to be 2:1 to obtain modified hemihydrate phosphogypsum.

[0064] Composite curing agent: The mass ratio of wood activated carbon, low carbon cement clinker, early strength agent and zeolite powder in the composite curing agent is 1:2:1:1.

[0065] 2) Preparation steps: Take 100 parts of modified hemihydrate phosphogypsum and 30 parts of composite curing agent, mix the mixture with 800 parts of pretreated in-situ soil, add 143.3 parts of water, compact the mixture layer by layer (controlled within 20 minutes), and cover and cure for 7-14 days to obtain solidified soil.

[0066] Performance testing: The 7d unconfined compressive strength, setting time, heavy metal adsorption rate and CO2 storage efficiency of the stabilized soil prepared in this comparative example were tested.

[0067] Comparative Example 2 A method for preparing solidified soil comprises the following steps: 1) Raw material processing: Adjust the types and proportions of raw materials of the composite crystal-turning agent: the mass ratio of sodium benzoate, aluminum sulfate and nano-calcium oxide in the composite crystal-turning agent = 2:1:1.

[0068] The pretreated phosphogypsum was mixed with a composite crystal-changing agent (calculated as 0.5% by mass of the pretreated phosphogypsum), homogenized and fermented at 60°C for 4 hours, and then calcined at 620°C for 2.5 hours. During the calcination process, the contents of α-type hemihydrate phosphogypsum and β-type hemihydrate phosphogypsum in the modified hemihydrate phosphogypsum were monitored in real time by in situ XRD, and the mass ratio of α-type hemihydrate phosphogypsum to the β-type hemihydrate phosphogypsum was controlled to be 2:1 to obtain modified hemihydrate phosphogypsum.

[0069] Composite curing agent: The mass ratio of wood activated carbon, low carbon cement clinker, early strength agent and zeolite powder in the composite curing agent is 1:2:1:1.

[0070] 2) Preparation steps: Take 50 parts of modified hemihydrate phosphogypsum and 100 parts of composite curing agent, mix the mixture with 800 parts of pretreated in-situ soil, add 143.3 parts of water, compact in layers (controlled within 20 minutes) and cover with film for 7-14 days to obtain solidified soil.

[0071] Performance testing: The 7d unconfined compressive strength, setting time, heavy metal adsorption rate and CO2 storage efficiency of the stabilized soil prepared in this comparative example were tested.

[0072] Comparative Example 3 A method for preparing solidified soil comprises the following steps: 1) Raw material processing: Composite curing agent: The mass ratio of wood activated carbon, low carbon cement clinker, early strength agent and zeolite powder in the composite curing agent is 1:2:1:1.

[0073] 2) Preparation steps: Take 100 parts of pretreated phosphogypsum and 100 parts of composite curing agent and mix them, mix the mixture evenly with 800 parts of pretreated in-situ soil, add 143.3 parts of water, compact the mixture layer by layer (controlled within 20 minutes) and cover and cure for 7-14 days to obtain solidified soil.

[0074] Performance testing: The 7d unconfined compressive strength, setting time, heavy metal adsorption rate and CO2 storage efficiency of the stabilized soil prepared in this comparative example were tested.

[0075] Comparative Example 4 A method for preparing solidified soil comprises the following steps: 1) Raw material processing: Adjust the types and proportions of raw materials of the composite crystal-turning agent: the mass ratio of sodium benzoate, aluminum sulfate and nano-calcium oxide in the composite crystal-turning agent = 2:1:1.

[0076] The pretreated phosphogypsum was mixed with a composite crystal-changing agent (calculated as 0.3% by mass of the pretreated phosphogypsum), homogenized and fermented at 60°C for 4 hours, and then calcined at 620°C for 2.5 hours. During the calcination process, the contents of α-type hemihydrate phosphogypsum and β-type hemihydrate phosphogypsum in the modified hemihydrate phosphogypsum were monitored in real time by in-situ XRD, and the mass ratio of α-type hemihydrate phosphogypsum to the β-type hemihydrate phosphogypsum was controlled to be 0.3:1 to obtain modified hemihydrate phosphogypsum.

[0077] Composite curing agent: The mass ratio of wood activated carbon, low carbon cement clinker, early strength agent and zeolite powder in the composite curing agent is 1:2:1:1.

[0078] 2) Preparation steps: Take 100 parts of modified hemihydrate phosphogypsum and 100 parts of composite curing agent, mix the mixture with 800 parts of pretreated in-situ soil, add 143.3 parts of water, compact in layers (controlled within 20 minutes) and cover with film for 7-14 days to obtain solidified soil.

[0079] Performance testing: The 7d unconfined compressive strength, setting time, heavy metal adsorption rate and CO2 storage efficiency of the stabilized soil prepared in this comparative example were tested.

[0080] Comparative Example 5 A method for preparing solidified soil comprises the following steps: 1) Raw material processing: Adjust the types and proportions of raw materials of the composite crystal-turning agent: the mass ratio of sodium benzoate, aluminum sulfate and nano-calcium oxide in the composite crystal-turning agent = 2:1:1.

[0081] The pretreated phosphogypsum was mixed with a composite crystal-changing agent (calculated as 0.8% by mass of the pretreated phosphogypsum), homogenized and fermented at 60°C for 4 hours, and then calcined at 620°C for 2.5 hours. During the calcination process, the contents of α-type hemihydrate phosphogypsum and β-type hemihydrate phosphogypsum in the modified hemihydrate phosphogypsum were monitored in real time by in-situ XRD, and the mass ratio of α-type hemihydrate phosphogypsum to the β-type hemihydrate phosphogypsum was controlled to be 2.5:1 to obtain modified hemihydrate phosphogypsum.

[0082] Composite curing agent: The mass ratio of wood activated carbon, low carbon cement clinker, early strength agent and zeolite powder in the composite curing agent is 1:2:1:1.

[0083] 2) Preparation steps: Take 100 parts of modified hemihydrate phosphogypsum and 100 parts of composite curing agent, mix the mixture with 800 parts of pretreated in-situ soil, add 143.3 parts of water, compact in layers (controlled within 20 minutes) and cover with film for 7-14 days to obtain solidified soil.

[0084] Performance testing: The 7d unconfined compressive strength, setting time, heavy metal adsorption rate and CO2 storage efficiency of the stabilized soil prepared in this comparative example were tested.

[0085] The in-situ solidified soil samples of the modified hemihydrate phosphogypsum prepared in the example and the solidified soil samples prepared in the comparative example were subjected to relevant performance tests, wherein the carbon fixation rate of the soil was tested with reference to the thermogravimetric method, and the leachable ion concentration of the soil was tested with reference to the determination method of leachable heavy metals. The relevant test results are shown in Table 1.

[0086] Table 1 Test results of each sample

[0087] As shown in Table 1, Examples 1-6 controlled the mass ratio of the two hemihydrate gypsums by adjusting the type and dosage of the composite crystallizer, and controlled the strength, carbon fixation rate, and leached ion concentration of the solidified soil by adjusting the mass ratio of the components of the composite curing agent. Examples 1-6 show that adjusting the mass ratio of α-type to β-type hemihydrate phosphogypsum has a significant effect on the setting time of the solidified soil, while adjusting the components and mass of the composite curing agent significantly affects the carbon fixation rate and leached heavy metal ion concentration of the solidified soil. Example 1 has the best overall performance.

[0088] In Comparative Examples 1-5, reducing the amount of composite curing agent (Comparative Example 1) or hemihydrate phosphogypsum (Comparative Examples 2 and 3) reduced the external calcium source, shortening the setting time and strength of the cured soil, and particularly significantly reducing carbon sequestration. By adjusting the ratio of the two hemihydrate phosphogypsums (Comparative Examples 4 and 5) using the composite curing agent, excessive α-type addition improved early strength but also reduced the curing rate. Excessive β-type addition resulted in excessive water demand, which consumed water for the curing agent reaction, reducing overall strength, carbon sequestration, and heavy metal ion curing efficiency.

[0089] In summary, the present invention provides a method for preparing in-situ solidified soil of modified hemihydrate phosphogypsum. This method optimizes the pretreatment process of phosphogypsum and combines the synergistic effect of a composite crystallization agent, a composite bacterial agent, and a composite solidifying agent to achieve rapid soil solidification. The solidified soil has excellent mechanical properties, high CO2 storage efficiency, and high heavy metal adsorption rate.

[0090] 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 person skilled in the art who, within the technical scope disclosed by the present invention, makes equivalent substitutions or changes based on the technical solutions and improved concepts of the present invention shall be covered by the scope of protection of the present invention.

Claims

1. A method for preparing in-situ solidified soil of modified hemihydrate phosphogypsum, characterized in that: The following steps are involved: (1) Pretreated phosphogypsum is mixed with a composite crystal-changing agent, homogenized and fermented, and then calcined to obtain modified hemihydrate phosphogypsum; wherein during the calcination process, the contents of α-type hemihydrate phosphogypsum and β-type hemihydrate phosphogypsum in the modified hemihydrate phosphogypsum are monitored in real time by in-situ XRD, and the mass ratio of the α-type hemihydrate phosphogypsum to the β-type hemihydrate phosphogypsum is controlled to be 1:(0.5-2); wherein the composite crystal-changing agent is a mixture of organic matter containing carboxylate groups, sulfate, and nanofiller; (2) The modified hemihydrate phosphogypsum obtained in step (1) is mixed with a composite curing agent to obtain a mixture, and then mixed evenly with the pretreated in-situ soil, and water is added to obtain a soil body. The soil body is compacted layer by layer and cured to obtain the in-situ cured soil of the modified hemihydrate phosphogypsum.

2. The method for preparing in-situ solidified soil of modified hemihydrate phosphogypsum according to claim 1, characterized in that: In the above scheme, the organic matter containing carboxylate groups is one or a mixture of sodium benzoate, sodium citrate, and sodium dodecylbenzenesulfonate, and the addition amount is 0.1%-0.5% of the mass of the pretreated phosphogypsum; the sulfate is one or a mixture of aluminum sulfate, iron sulfate, and magnesium sulfate, and the addition amount is 0.1%-0.5% of the mass of the pretreated phosphogypsum; the nanofiller is nano-silica and / or nano-calcium oxide, and the addition amount is 0.1%-0.5% of the mass of the pretreated phosphogypsum.

3. The method for preparing in-situ solidified soil of modified hemihydrate phosphogypsum according to claim 1, characterized in that: The mass ratio of the pretreated in-situ soil, the modified hemihydrate phosphogypsum, the composite curing agent, and the water is (4.5-10):1:(0.5-1):(0.5-5).

4. The method for preparing in-situ solidified soil of modified hemihydrate phosphogypsum according to claim 1, characterized in that: The pretreatment of the pretreated phosphogypsum in step (1) includes countercurrent washing, neutralization, drying and aging, specifically: adopting a multi-stage countercurrent washing process; adopting calcium hydroxide for neutralization; the moisture content of the dried phosphogypsum is less than 10%; the aging time is 1-3 months; the soluble P2O5 content in the washed phosphogypsum is ≤0.3%, and the pH value of the neutralized phosphogypsum is 8-8.

5.

5. The method for preparing in-situ solidified soil of modified hemihydrate phosphogypsum according to claim 1, characterized in that: The temperature of the homogenization and retting in step (1) is 60-80°C, and the time is 4-6 hours; the temperature of the calcination is 610-630°C, and the time is 2-3 hours.

6. The method for preparing in-situ solidified soil of modified hemihydrate phosphogypsum according to claim 1, characterized in that: The composite curing agent is one or a mixture of wood activated carbon, an early strength agent, low carbon cement clinker, and zeolite powder; wherein the early strength agent is a nano-composite early strength agent, which is composited with nano-silicon dioxide and nano-hydrated calcium silicate as raw materials.

7. The method for preparing in-situ solidified soil of modified hemihydrate phosphogypsum according to claim 1, characterized in that: The pretreated in-situ soil is prepared by adding fungi and inorganic nitrogen to the in-situ soil and stirring the mixture evenly; the fungi are one or a mixture of Bacillus subtilis, Trichoderma, Pseudomonas, and actinomycetes, and the inorganic nitrogen is ammonium sulfate and / or ammonium bicarbonate; the amount of the fungi is 0.1%-1% of the mass of the in-situ soil, and the amount of the inorganic nitrogen is 0.1%-2% of the mass of the in-situ soil.

8. The method for preparing in-situ solidified soil of modified hemihydrate phosphogypsum according to claim 1, characterized in that: The in-situ soil is crushed and evenly mixed using a mixing device, and the mixture of the modified hemihydrate phosphogypsum and the curing agent is evenly sprinkled in, and evenly mixed again. After adding water, the soil is compacted in layers and covered with a film for curing; wherein the compaction process needs to be controlled within 5-20 minutes; and wherein the film curing time is 7-14 days.

9. In-situ solidified soil of modified hemihydrate phosphogypsum prepared according to the method for preparing in-situ solidified soil of modified hemihydrate phosphogypsum according to any one of claims 1 to 8.

10. Use of the in-situ solidified soil of the modified hemihydrate phosphogypsum as claimed in claim 9 in soft soil foundation treatment projects and riverbank reinforcement projects.

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