A solid waste harmless soft soil curing agent and application thereof in reinforcing soft soil

Abstract

The application discloses a kind of full solid waste harmless soft soil solidifying agent and application in reinforcing soft soil, is by phosphogypsum, red mud, carbide slag, mineral powder is prepared according to the following mass percentage: phosphogypsum 40-60%; red mud 20-40%; carbide slag 5-15%; mineral powder 10-20%, and in carbide slag particle size is 5-10 μm, mineral powder particle size is 10-15 μm, red mud particle size is 15-20 μm, phosphogypsum particle size is 20-40 μm form optimal level allocation.The soft soil solidifying agent prepared by the application with solid waste as component can replace cement and be applied to soft soil treatment, has the outstanding advantages of environmental protection, harmless, free calcination, etc., and has more obvious price advantage compared with cement, positively promotes the resource utilization of phosphogypsum, red mud, carbide slag and the like, protects environment while reducing soft soil solidification cost, has extensive and far-reaching significance.

Description

Technical Field

[0001] This invention belongs to the field of building materials technology, and specifically relates to a soft soil solidification agent that is completely harmless due to solid waste and its application in reinforcing soft soil. Background Technology

[0002] Soft soil refers to fine-grained soil deposited in coastal areas, lakes, valleys, and riverbanks, characterized by high natural water content, large void ratio, high compressibility, and low shear strength. Soft soil has a natural void ratio greater than or equal to 1.0. Its characteristics include low strength, high compressibility, high disturbance, poor permeability, complex layered distribution, and significant differences in physical and mechanical properties between layers. These characteristics often cause considerable problems in road engineering, thus requiring treatment to prevent foundation settlement and collapse.

[0003] The common practice for reinforcing soft soil is the cement mixing method, which uses cement as a solidifying agent, accounting for 10% to 20% of the original soil mass. The cement is mixed evenly in the soft soil by spraying slurry or atomized powder into the soft soil through mixing machinery, and then solidified in situ deep in the foundation into cement-solidified soil with high bearing capacity.

[0004] However, the dust generated during cement production not only pollutes the workplace but also becomes a significant source of environmental pollution when released through large chimneys. Furthermore, the production process generates CO2, which contradicts the current national low-carbon and environmental protection strategy. When used as a soft soil stabilization material, silicate cement cannot effectively fill the pores in the stabilized soil, limiting the improvement of the stabilized soil's strength. The stabilization effect is also limited by soil type, showing unsatisfactory stabilization results for clay, cracked soil, organic soil, and saline soil with high plasticity indices. In addition, the initial and final setting times of silicate cement cannot be adjusted, especially at low temperatures where its setting time is excessively long, hindering project progress; later drying shrinkage is significant, leading to cracking. Therefore, there is an urgent need to develop more suitable soft soil stabilizing agents.

[0005] Phosphogypsum refers to calcium sulfate hydrate, a byproduct of fertilizer production from phosphate rock. Its main component is gypsum (CaSO4·2H2O), and it also contains harmful substances such as phosphorus and fluorine. Gypsum is a widely used material in the construction industry, but phosphogypsum is typically not used. While the utilization rate of phosphogypsum has improved, the existing stockpiles nationwide remain substantial and continue to grow at a net rate of over 50 million tons per year. Therefore, how to utilize phosphogypsum on a large scale has become a very important issue.

[0006] Red mud is a polluting waste residue discharged during the extraction of alumina in the aluminum industry. On average, 1.0-2.0 tons of red mud are generated for every ton of alumina produced. It is typically reddish-brown due to its high iron oxide content. As the world's fourth-largest alumina producer, China discharges millions of tons of red mud annually. With the increasing stockpiles of red mud and its growing environmental pollution, maximizing its resource utilization is now imperative.

[0007] Calcium carbide slag is a waste residue mainly composed of calcium hydroxide, produced after the hydrolysis of calcium carbide to obtain acetylene gas. Its slurry is a grayish-brown, turbid liquid. After settling, it separates into three parts: a clear liquid, a solid sediment layer, and an intermediate colloidal transition layer. Calcium carbide slag is highly alkaline; if discharged without treatment, it will clog sewers, accumulate in riverbeds, and harm fisheries.

[0008] Currently, due to the continuous large-scale stockpiling of phosphogypsum, red mud, and carbide slag, and the need to minimize cement utilization, there is an urgent need to develop a new solid waste formulation to utilize these large-scale stockpiles of environmentally harmful solid wastes in soft soil stabilization, thus demonstrating both commercial value and environmental significance. Summary of the Invention

[0009] The purpose of this invention is to address the aforementioned deficiencies in the existing technology by providing a method for reinforcing soft soil foundations using solid waste as raw material. This method solves the problems of existing soft soil foundation treatments using large amounts of cement, which cannot meet social needs, and the difficulty in treating waste from industrial production.

[0010] To achieve the above objectives, the present invention adopts the following technical solution:

[0011] A soft soil solidification agent for the complete solid waste harmless treatment comprises the following components by mass percentage: 40-60% phosphogypsum, 20-40% red mud, 5-15% carbide slag, and 10-20% mineral powder.

[0012] Preferably, the composition comprises the following components by weight percentage: 40% phosphogypsum, 30% red mud, 10% carbide slag, and 20% mineral powder.

[0013] Preferably, the particle size of the carbide slag is 5-10 μm.

[0014] Preferably, the mineral powder particle size is 10-15 μm. The mineral powder is S95 abundant heavy mineral powder.

[0015] Preferably, the red mud particle size is 15-20 μm. The red mud of the present invention is dried and then ground. The main components of the red mud are alumina and metal oxides such as iron oxide. Red mud is a solid waste generated during the alumina production process. The red mud is ground in a mortar to obtain an average particle size of 15-20 μm.

[0016] Preferably, the phosphogypsum particle size is 20-40 μm. The phosphogypsum used in this invention is naturally dried, and a portion of the phosphogypsum is passed through a 0.15 mm sieve and then through an 80-micron sieve using a cement fineness negative pressure sieve analyzer with a correction factor of 1.2%. The final percentage of residue after sieving is 31.2%, and its main component is calcium sulfate dihydrate. The phosphogypsum powder is then ground in a mortar to obtain an average particle size of 20-40 μm.

[0017] This invention also provides the application of the above-mentioned soft soil solidification agent, which is a complete solid waste harmless treatment agent, in the reinforcement of soft soil, including the following steps:

[0018] The carbide slag is left to stand, and the supernatant and the lower slurry are separated and then air-dried. The red mud and phosphogypsum are directly air-dried, then crushed and sieved to obtain carbide slag, red mud and phosphogypsum powder, which are then stored in a moisture-proof container. The phosphogypsum, red mud, carbide slag, mineral powder and soft soil and water are mixed, stirred and vibrated to form a mold, and then demolded after curing at room temperature for 48 hours.

[0019] Preferably, water accounts for 30-40% of the total mass of the soft soil stabilizer and the soft soil.

[0020] Preferably, the carbide slag is a carbide slag slurry, which requires pretreatment before use. Specifically, the carbide slag slurry is poured into a container and allowed to stand for 5-7 days. After separation, the supernatant is filtered and the lower layer of slurry is collected. The carbide slag is extremely alkaline, with a pH value greater than 12 as measured by a pH meter. The carbide slag (prepared from the carbide slag slurry after pretreatment) is ground in a mortar and pestle to obtain an average particle size of 5-10 μm.

[0021] Preferably, the mass ratio of the soft soil stabilizer to the soft soil is (5-30)%:(70-95)%.

[0022] Compared with the prior art, the present invention has the following advantages:

[0023] (1) This invention is the first to discover that phosphogypsum-red mud-carbide slag synergistically activates mineral powder, resulting in excellent mechanical properties. The reasons are as follows: Carbide slag is extremely alkaline, with a pH value above 12 as measured by a pH meter, and contains over 90% CaO, serving as activation source 1; red mud has a pH value of 10.29-11.83, with an Al2O3 content greater than 27% and an Fe2O3 content greater than 26%, serving as activation source 2; phosphogypsum has a pH value of 2-5 and contains over 70% CaSO4·2H2O, serving as activation source 3. Among the activation sources, carbide slag provides abundant calcium and OH... -Synergistic activation of slag through a pozzolanic reaction can generate hydrated calcium silicate (CSH) and hydrated calcium aluminate (CAH) gels. Aluminum provided by red mud and sulfate provided by phosphogypsum can jointly activate slag to generate crystalline hydration products such as hydrated calcium sulfoaluminate (3CaO·Al₂O₃·3CaSO₄·32H₂O). The system undergoes an alkali-activated reaction and a hydration reaction similar to that of sulfoaluminate cement. The generated hydrated calcium sulfoaluminate not only increases the phase volume by approximately 120%, but also absorbs a large amount of free water. This volume expansion fills the pores in soft soil, making the solidified soil more compact and reducing porosity. The interwoven needle-like structures of ettringite and the formed CSH create a unique spatial network structure, refining the pores and improving its load-bearing capacity.

[0024] (2) The average particle size of the carbide slag used in this invention is 5-10 μm, the average particle size of the mineral powder is 10-15 μm, the average particle size of the red mud is 15-20 μm, and the average particle size of the phosphogypsum is 20-40 μm. This can form a good particle physical gradation, so that the pozzolanic reaction of carbide slag-mineral slag, the alkali-activated reaction of red mud-mineral slag, and the sulfoaluminate cement hydration reaction of red mud-phosphogypsum-carbide slag can achieve the optimal hydration effect, and ultimately the mechanical properties and workability of the curing agent can be optimized. This invention not only considers the synergistic activation of chemicals, but also the good physical gradation, thus forming a full-material-scale, full-solid-waste cementitious material gradation design method that considers physical-chemical interactions.

[0025] (3) Traditionally used alkaline activating materials are sodium hydroxide or water glass, while the curing agent used in this invention is a synergistic activation of carbide slag and red mud. The high calcium content of carbide slag is used to improve the strength of the solidified soil. At the same time, carbide slag can solidify harmful substances in phosphogypsum, such as phosphorus and fluorine, and also solidify heavy metals in red mud, achieving a win-win effect. Furthermore, the curing agent of this invention can be appropriately adjusted in proportion and dosage according to the properties and composition of the soil at the construction site. Moreover, this curing agent does not use any other additives and does not require specified curing or molding conditions, making the curing agent more applicable and effective for road use, with a broader application prospect.

[0026] (4) The raw materials used in this invention are all solid waste, which completely replaces cement. The cost is low, the raw materials are widely available, and large-scale accumulation will cause serious environmental pollution. Therefore, it has good economic and environmental benefits. According to calculations, the soft soil solidifying agent prepared by this invention using solid waste as raw material saves at least 30%-60% of the cost compared with the soft soil solidifying agent using cement as the main material. It has a significant price advantage and is in line with the current national low-carbon emission environmental protection strategy. It has profound significance for the diversified utilization of solid waste. Detailed Implementation

[0027] The following disclosure provides many different embodiments or examples for implementing different structures of the invention. To simplify the disclosure, specific examples of components and arrangements are described below. Of course, these are merely examples and are not intended to limit the invention. Furthermore, reference numerals and / or letters may be repeated in different examples; such repetition is for simplification and clarity and does not in itself indicate a relationship between the various embodiments and / or arrangements discussed.

[0028] According to the "Industry Standard for Urban Construction of the People's Republic of China" (CJ / T 526-2018) for soft soil solidification agents, the unconfined compressive strength of solidified soil at 7 days and 28 days should meet the requirements of Table 1.

[0029] Table 1 Specification for Unconfined Compressive Strength of Stabilized Soil

[0030]

[0031] The specific mixing and curing process used in this invention is as follows: a paste mixer is used for mixing. First, the curing agent and water are added to the mixing pot and slowly mixed for 120 seconds, then paused for 15 seconds. Next, the soft soil to be cured is added and quickly mixed for 120 seconds. Afterward, the mixture is poured into a cylindrical mold with a diameter of 50 mm and a height of 100 mm, and vibrated on a vibrating table for 10 seconds. Excess material on the surface is then removed. Considering that constant temperature and humidity curing conditions are not possible in actual engineering projects, all specimens in this experiment were naturally cured indoors.

[0032] Example 1

[0033] In this embodiment, the components and their mass percentages are as follows: 6% phosphogypsum, 4.5% red mud, 3% mineral powder, and 1.5% carbide slag (equivalent to 40% phosphogypsum, 30% red mud, 20% mineral powder, and 10% carbide slag in the soft soil stabilizer).

[0034] In this embodiment, the above components and their proportions are mixed with 85% of the soft soil to be solidified, and a curing agent and 30% of the total mass of the soft soil are added. The mixture is stirred, vibrated for 10 seconds to form the shape, and then demolded after curing at room temperature for 48 hours.

[0035] Its 7-day unconfined compressive strength was 0.76 MPa, and its 28-day unconfined compressive strength was 1.39 MPa.

[0036] Example 2

[0037] In this embodiment, the components and their mass percentages are as follows: phosphogypsum 8%, red mud 6%, mineral powder 4%, and carbide slag 2%.

[0038] In this embodiment, the above components and their proportions are mixed with 80% of the soft soil to be solidified, and a curing agent and 30% of the total mass of the soft soil are added. The mixture is stirred, vibrated for 10 seconds to form the shape, and then cured at room temperature for 48 hours before demolding.

[0039] Its 7-day unconfined compressive strength was 1.21 MPa, and its 28-day unconfined compressive strength was 1.78 MPa.

[0040] Example 3

[0041] In this embodiment, the components and their mass percentages are as follows: phosphogypsum 4%, red mud 3%, mineral powder 2%, and carbide slag 1%.

[0042] In this embodiment, the above components and their proportions are mixed with 90% of the soft soil to be solidified, and a curing agent and 35% of the total mass of the soft soil are added. The mixture is stirred, vibrated for 10 seconds to form the shape, and then cured at room temperature for 48 hours before demolding.

[0043] Its 7-day unconfined compressive strength was tested to be 0.46 MPa, and its 28-day unconfined compressive strength was tested to be 1.04 MPa.

[0044] Example 4

[0045] In this embodiment, the components and their mass percentages are as follows: phosphogypsum 6%, red mud 4.5%, mineral powder 3%, and carbide slag 1.5%.

[0046] In this embodiment, the above components and their proportions are mixed with 85% of the soft soil to be solidified, and a curing agent and 35% of the total mass of the soft soil are added. The mixture is stirred, vibrated for 10 seconds to form the shape, and then demolded after curing at room temperature for 48 hours.

[0047] Its 7-day unconfined compressive strength was tested to be 0.75 MPa, and its 28-day unconfined compressive strength was tested to be 1.27 MPa.

[0048] Example 5

[0049] In this embodiment, the components and their mass percentages are as follows: phosphogypsum 8%, red mud 6%, mineral powder 4%, and carbide slag 2%.

[0050] In this embodiment, the above components and their proportions are mixed with 80% of the soft soil to be solidified, and a curing agent and 35% of the total mass of water of the soft soil are added. The mixture is stirred, vibrated for 10 seconds to form the shape, and then demolded after curing at room temperature for 48 hours.

[0051] Its 7-day unconfined compressive strength was tested to be 1.32 MPa, and its 28-day unconfined compressive strength was tested to be 2.08 MPa.

[0052] Example 6

[0053] In this embodiment, the components and their mass percentages are as follows: phosphogypsum 10%, red mud 7.5%, mineral powder 5%, and carbide slag 2.5%.

[0054] In this embodiment, the above components and their proportions are mixed with 75% soft soil to be cured, and a curing agent and 35% water (by weight of the total soft soil) are added. The mixture is stirred, vibrated for 10 seconds to form the shape, and then cured at room temperature for 48 hours before demolding.

[0055] Its 7-day unconfined compressive strength was 1.79 MPa, and its 28-day unconfined compressive strength was 2.64 MPa.

[0056] Example 7

[0057] In this embodiment, the components and their mass percentages are as follows: phosphogypsum 12%, red mud 9%, mineral powder 6%, and carbide slag 3%.

[0058] In this embodiment, the above components and their proportions are mixed with 70% of the soft soil to be solidified, and a curing agent and 35% of the total mass of the soft soil are added. The mixture is stirred, vibrated for 10 seconds to form the shape, and then demolded after curing at room temperature for 48 hours.

[0059] Its 7-day unconfined compressive strength was tested to be 2.44 MPa, and its 28-day unconfined compressive strength was tested to be 3.24 MPa.

[0060] Example 8

[0061] In this embodiment, the components and their mass percentages are as follows: phosphogypsum 4%, red mud 3%, mineral powder 2%, and carbide slag 1%.

[0062] In this embodiment, the above components and their proportions are mixed with 90% of the soft soil to be solidified, and a curing agent and 40% of the total mass of the soft soil are added. The mixture is stirred, vibrated for 10 seconds to form the shape, and then demolded after curing at room temperature for 48 hours.

[0063] Its 7-day unconfined compressive strength was 0.48 MPa, and its 28-day unconfined compressive strength was 1.09 MPa.

[0064] Example 9

[0065] In this embodiment, the components and their mass percentages are as follows: phosphogypsum 6%, red mud 4.5%, mineral powder 3%, and carbide slag 1.5%.

[0066] In this embodiment, the above components and their proportions are mixed with 85% of the soft soil to be solidified, and a curing agent and 35% of the total mass of the soft soil are added. The mixture is stirred, vibrated for 10 seconds to form the shape, and then demolded after curing at room temperature for 48 hours.

[0067] Its 7-day unconfined compressive strength was tested to be 0.62 MPa, and its 28-day unconfined compressive strength was tested to be 1.12 MPa.

[0068] Example 10

[0069] In this embodiment, the components and their mass percentages are as follows: phosphogypsum 8%, red mud 6%, mineral powder 4%, and carbide slag 2%.

[0070] In this embodiment, the above components and their proportions are mixed with 80% of the soft soil to be solidified, and a curing agent and 40% of the total mass of the soft soil water are added. The mixture is stirred, vibrated for 10 seconds to form the shape, and then demolded after curing at room temperature for 48 hours.

[0071] Its 7-day unconfined compressive strength was 0.73 MPa, and its 28-day unconfined compressive strength was 1.19 MPa.

[0072] Example 11

[0073] In this embodiment, the components and their mass percentages are as follows: 10% phosphogypsum, 6% red mud, 2% mineral powder, and 2% carbide slag (equivalent to 50% phosphogypsum, 30% red mud, 10% mineral powder, and 10% carbide slag in the soft soil stabilizer).

[0074] In this embodiment, the above components and their proportions are mixed with 80% of the soft soil to be solidified, and a curing agent and 30% of the total mass of the soft soil are added. The mixture is stirred, vibrated for 10 seconds to form the shape, and then cured at room temperature for 48 hours before demolding.

[0075] Example 12

[0076] In this embodiment, the components and their mass percentages are as follows: phosphogypsum 12%, red mud 4%, mineral powder 2%, and carbide slag 2% (equivalent to phosphogypsum accounting for 60%, red mud accounting for 20%, mineral powder accounting for 10%, and carbide slag accounting for 10% in the soft soil stabilizer).

[0077] In this embodiment, the above components and their proportions are mixed with 80% of the soft soil to be solidified, and a curing agent and 30% of the total mass of the soft soil are added. The mixture is stirred, vibrated for 10 seconds to form the shape, and then cured at room temperature for 48 hours before demolding.

[0078] Comparative Example 1

[0079] The components and their mass percentages in this comparative example are: phosphogypsum 8%, red mud 6%, mineral powder 4%, and carbide slag 2%, and the average particle size of the four materials is 15-20 μm.

[0080] This comparative example involves mixing the above components and their proportions with 80% of the soft soil to be cured, adding a curing agent and 35% of the total mass of water of the soft soil, stirring, vibrating for 10 seconds to form the mixture, curing at room temperature for 48 hours, and then demolding.

[0081] Its 7-day unconfined compressive strength was 0.55 MPa, and its 28-day unconfined compressive strength was 0.84 MPa.

[0082] Comparative Example 2

[0083] The components and their mass percentages in this comparative example are: phosphogypsum 8%, red mud 6%, mineral powder 4%, and carbide slag 2%, and the average particle size of the four materials is less than 15μm.

[0084] This comparative example involves mixing the above components and their proportions with 80% of the soft soil to be cured, adding a curing agent and 35% of the total mass of water of the soft soil, stirring, vibrating for 10 seconds to form the mixture, curing at room temperature for 48 hours, and then demolding.

[0085] Its 7-day unconfined compressive strength was 0.71 MPa, and its 28-day unconfined compressive strength was 0.93 MPa.

[0086] Table 2 Performance tests of solidified soft soil in all experimental groups

[0087]

[0088] Table 3. Indices of water-soluble phosphorus pentoxide and water-soluble fluorine in phosphogypsum

[0089]

[0090] As shown in Table 2, with the gradual increase in the amount of the soft soil solidifying agent prepared by this invention using solid waste as the main raw material, the 7-day and 28-day unconfined compressive strength of the solidified soft soil also gradually increased. Compared with the solidifying agent without gradation optimization at the same dosage (Comparative Examples 1-2), the 7-day and 28-day strengths increased by 140% and 148%, respectively. The 28-day samples of solidified soft soil from each embodiment were tested for the concentrations of water-soluble phosphorus pentoxide and water-soluble fluorine. Tables 2 and 3 show that the concentrations of water-soluble phosphorus pentoxide and water-soluble fluorine in the phosphogypsum in the embodiments of this invention meet the first-level indicators in the national standard. Therefore, the solidifying agent achieves the harmless treatment of phosphogypsum. Thus, the effect of particle physical gradation optimization is significant, and the 28-day strength of the solidifying agent without gradation optimization does not meet the specification requirements. As a solidifying agent for reinforcing soft soil, all performance indicators meet or exceed the relevant standards and actual engineering requirements, and it simplifies the production process of soft soil solidification materials, making it widely applicable.

[0091] Other embodiments of the invention will readily occur to those skilled in the art upon consideration of the specification and practice of the invention. This invention is intended to cover any variations, uses, or adaptations that follow the general principles of the invention and include common knowledge or customary techniques in the art. The specification and examples are to be considered exemplary only, and the true scope and spirit of the invention are indicated by the claims.

[0092] It should be understood that the present invention is not limited to the content already described above, and various modifications and changes can be made without departing from its scope. The scope of the present invention is limited only by the appended claims.

Claims

1. A soft soil stabilizing agent for the complete solid waste harmless treatment, characterized in that, The composition by mass percentage is as follows: 40-60% phosphogypsum, 20-40% red mud, 5-15% carbide slag, and 10-20% mineral powder; the particle size of the carbide slag is 5-10 μm; the particle size of the mineral powder is 10-15 μm; the particle size of the red mud is 15-20 μm; and the particle size of the phosphogypsum is 20-40 μm.

2. The soft soil stabilizing agent for the complete solid waste harmless treatment according to claim 1, characterized in that, It is composed of the following components by weight percentage: 40% phosphogypsum, 30% red mud, 10% carbide slag and 20% mineral powder.

3. The application of the soft soil stabilizing agent for the complete solid waste harmlessness treatment according to any one of claims 1 to 2 in the reinforcement of soft soil, characterized in that, Includes the following steps: The phosphogypsum, red mud, carbide slag, mineral powder, soft soil, and water are mixed, stirred, vibrated, and molded. After curing at room temperature for 48 hours, the mold is removed.

4. The application according to claim 3, characterized in that, The water accounts for 30-40% of the total mass of the soft soil stabilizer and the soft soil.

5. The application according to claim 3, characterized in that, The carbide slag is a carbide slag slurry, which needs to be pretreated before use. Specifically, the carbide slag slurry is poured into a container and left to stand for 5-7 days. After separation, the supernatant is filtered and the lower layer of slurry is taken out.

6. The application according to claim 3, characterized in that, The mass ratio of the soft soil solidifying agent to the soft soil is (5-30)%:(70-95)%.

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