Sandy soil improvement liquid combining microorganisms and waste concrete, preparation method and improvement method

Abstract

The invention discloses a sandy soil improvement liquid combining microorganisms and waste concrete, a preparation method and an improvement method. The preparation method of the sandy soil improvement liquid comprises the following steps: preparing an RCF leaching solution, preparing a bacterial solution, preparing a cementing solution and preparing the sandy soil improvement liquid. The RCF is adopted as a main raw material, the ammonium chloride solution is used for improving the calcium solution, a traditional calcium source is replaced, the cost is reduced, and waste recycling is achieved. According to the method, the advantage of calcium enrichment in the waste concrete is fully utilized, and dependence on traditional calcium sources such as calcium chloride or calcium nitrate is reduced. The pH value is naturally increased through microorganism urea hydrolysis, functional groups on the surfaces of microorganisms provide nucleation sites for calcium ions, calcium carbonate precipitation is promoted, and waste recycling is achieved. The process is non-toxic and harmless, corrosive chemical reagents are not needed, and compared with a traditional reinforcing method, the method has the advantages of small environmental influence, no harmful waste or secondary pollution, simplicity and convenience in operation, low energy consumption and remarkable improvement of economical efficiency and environmental friendliness.

Description

Technical Field

[0001] The invention belongs to the field of geotechnical engineering and biological building materials, and specifically relates to a sand improvement liquid combining microorganisms and waste concrete, a preparation method and an improvement method. Background Art

[0002] Microbially induced calcium carbonate precipitation (MICP) is a naturally occurring biomineralization process that has shown broad application potential in geotechnical engineering. Urease catalyzes the hydrolysis of urea to produce CO3 2- Under the action of calcium source, CaCO3 crystals gradually grow with bacteria as nucleation points, fill loose gaps, and form an effective bonding structure between sand particles. In recent years, MICP technology has been increasingly used in many fields such as cultural relics protection, soil stabilization, anti-liquefaction, permeability control, heavy metal solidification and concrete repair. However, with the deepening of research, microbial induced calcium carbonate precipitation technology faces challenges in actual engineering applications, such as high technical cost, low resource utilization efficiency and heavy environmental burden.

[0003] Traditionally, the calcium sources used in MICP research are mainly reagent-grade calcium such as calcium chloride, calcium nitrate, and calcium acetate, which are considered to be costly and the manufacturing process is burdensome to the environment. In order to reduce costs and achieve efficient utilization of waste, a large number of researchers have explored methods to extract effective materials from waste to replace traditional chemicals. Among them, the recycling and reuse of calcium-based waste with abundant sources shows great application potential in the MICP process. For example, researchers have proposed a method for extracting calcium from eggshells using vinegar or hydrochloric acid, but there are still many problems. For example, the supply of raw materials such as eggshells is limited and unstable, and the calcium extraction process is time-consuming, which poses a huge challenge to large-scale application. In addition, industrial reagent hydrochloric acid is not only costly, but may also have adverse effects on the environment. These factors further limit the large-scale promotion and application of this method.

[0004] With the rapid advancement of urbanization and industrialization, the demolition and replacement of old buildings and infrastructure has accelerated significantly. This trend has brought about a large amount of construction and demolition waste, the main component of which is waste concrete. How to achieve sustainable management of these wastes has become a major challenge that the construction industry needs to solve urgently. During the recycling process, waste concrete is usually processed into recycled concrete aggregate (RCA) and recycled concrete fines (RCF), of which RCF accounts for about 20-30% of the total mass of waste concrete. Compared with RCA, RCF faces more restrictions due to its unique physical and chemical properties. Its particle size is small (<150μm), its structure is porous, and its water absorption rate is significantly higher. In addition, RCF contains a large amount of residual cement paste, which has a rough surface, low activity and strong water absorption capacity. These characteristics lead to poor processability of RCF and easy large shrinkage deformation, which has become the main obstacle to its widespread application in concrete production. Nevertheless, some studies have shown that using RCF powder as an inert filler can partially replace stone powder or cement raw materials in concrete products, thereby achieving resource reuse. However, due to the above performance limitations, RCF is usually not used directly to produce new concrete, but is landfilled or used in less efficient applications. It is worth noting that RCF shows good adaptability in carbonation reaction, which is closely related to the cement hydration products contained in it, such as calcium hydroxide, calcium silicate hydrated gel and unhydrated clinker. These characteristics provide new possibilities for further research and high-value utilization of RCF.

[0005] Therefore, the prior art still needs to be improved and developed. Summary of the invention

[0006] In view of the above-mentioned deficiencies in the prior art, the present invention provides a sand improvement liquid, a preparation method and an improvement method combining microorganisms and waste concrete, aiming to solve the problems of high cost, low resource utilization efficiency and heavy environmental burden of the current microbial induced calcium carbonate precipitation technology.

[0007] The method of the present invention combines microbial mineralization with waste concrete resource utilization, uses RCF as the main raw material, uses ammonium chloride solution to prepare RCF leaching solution, and obtains a solution with a high calcium content to replace traditional chemical calcium sources (such as reagent-grade calcium chloride, calcium nitrate and calcium acetate). Then, microorganisms are combined to obtain sand and soil improvement liquid, and the obtained sand and soil improvement liquid is used to treat sand and soil materials, which can promote the precipitation of calcium carbonate, thereby achieving the solidification and enhancement of sand and soil materials.

[0008] The present invention is achieved through the following technical solutions:

[0009] Solution 1)

[0010] The method for preparing a sand-soil improvement liquid combining microorganisms and waste concrete comprises the following steps:

[0011] Step 1, mixing the recycled concrete fine material and the ammonium chloride solution and filtering to prepare the RCF leachate;

[0012] Step 2: culturing urease-producing bacteria to obtain a bacterial solution;

[0013] Step 3: adding urea to the RCF leachate prepared in step 1, and mixing to form a cementing solution;

[0014] Step 4: Adjust the pH value of the cementing solution to neutral, and then mix the bacterial solution and the cementing solution to obtain the sand improvement solution.

[0015] Option 2)

[0016] The sand and soil improvement liquid is obtained by the method for preparing the sand and soil improvement liquid combining microorganisms and waste concrete.

[0017] Option 3)

[0018] The sand improvement method comprises the following steps: at room temperature, soaking, grouting or spraying the sand to be improved with the sand improvement liquid for more than one time, and letting it stand for 24-48 hours each time.

[0019] Compared with the prior art, the present invention has the following beneficial effects:

[0020] The present invention uses recycled concrete fines as the main raw material, uses ammonium chloride solution to leach a solution with a high calcium content, replaces the traditional chemical calcium source, significantly reduces production costs, and realizes efficient recycling of waste. The method gives full play to the advantages of waste concrete fines being rich in calcium, while reducing the dependence on traditional calcium sources (such as calcium chloride or calcium nitrate). In this way, the calcium resources in the waste can be effectively converted into calcium carbonate precipitation, achieving the goal of "waste into resources". By utilizing the natural metabolic process of urea hydrolysis by microorganisms, urea is hydrolyzed by bacteria to generate ammonia and carbon dioxide, resulting in an increase in local pH, while the microorganisms provide nucleation sites for the nucleation of calcium ions through their cell surface functional groups (such as amino, carboxyl, etc.) to further promote the precipitation of calcium. This process does not require the use of toxic or corrosive chemical reagents, and compared with traditional chemical reinforcement methods (such as the use of lime, cement, or polymers), it has little impact on the environment and does not produce harmful waste or secondary pollution. The method of the present invention is simple to operate, has low energy consumption, and has no secondary pollution, which significantly improves the economy and environmental protection of the present invention. BRIEF DESCRIPTION OF THE DRAWINGS

[0021] Figure 1 Schematic diagram of the sand improvement method according to the embodiment of the present invention;

[0022] Figure 21 is a schematic flow chart of a method for preparing a sand-improving liquid according to an embodiment of the present invention;

[0023] Figure 3 is the calcium ion leaching concentration and leaching efficiency under different solid-liquid ratio conditions in the embodiment of the present invention;

[0024] Figure 4 is the relationship between calcium carbonate content and unconfined compressive strength in biocemented sand treated with different calcium sources;

[0025] Figure 5 is the relationship between the hydraulic conductivity of samples treated with different calcium sources and the number of treatments in the embodiment of the present invention;

[0026] Figure 6 is a cost analysis of different calcium sources in the embodiments of the present invention;

[0027] Figure 7 It is a life cycle assessment analysis of the costs of different calcium sources in the embodiments of the present invention. DETAILED DESCRIPTION

[0028] The present invention provides a sand improvement liquid, a preparation method and an improvement method combining microorganisms and waste concrete. In order to make the purpose, technical scheme and effect of the present invention clearer and more specific, the present invention is further described in detail below. It should be understood that the specific embodiments described herein are only used to explain the present invention and are not used to limit the present invention.

[0029] Recovering calcium ions from RCF through the leaching-carbonization process is a value-added utilization method with broad application prospects. This process can not only efficiently extract calcium resources from RCF, but can also be further used to produce high-value-added products. Ammonium chloride plays an important role in the high-value utilization of calcium-based waste, especially in the preparation of high-purity calcium carbonate. In the leaching-carbonization process, ammonium chloride, as an auxiliary agent, can convert the insoluble calcium in calcium-based waste into soluble calcium chloride. Subsequently, calcium carbonate is generated by introducing carbon dioxide into the calcium solution. As an important industrial raw material, calcium carbonate is widely used in construction, papermaking, plastics, coatings and other fields. At present, this process usually generates calcium carbonate by introducing carbon dioxide, but the cost of obtaining and processing carbon dioxide is high, especially in large-scale industrial applications. In addition, the emission of carbon dioxide puts pressure on the environment and fails to fully achieve low-carbon production. Moreover, due to the complexity of the soil and factors such as unsuitable pH environment, solubility and diffusivity, directly introducing carbon dioxide into the soil will not directly react with calcium ions to generate calcium carbonate, and it will not effectively promote the precipitation of calcium carbonate, and thus fail to produce the expected cementation effect.

[0030] The method for preparing the sand-soil improvement liquid combining microorganisms and waste concrete of the present invention comprises the following steps:

[0031] Step 1, mixing the recycled concrete fine material and the ammonium chloride solution and filtering to prepare the RCF leachate;

[0032] Step 2: culturing urease-producing bacteria to obtain a bacterial solution;

[0033] Step 3: adding urea to the RCF leachate prepared in step 1, and mixing to form a cementing solution;

[0034] Step 4: Adjust the pH value of the cementing solution to neutral, and then mix the bacterial solution and the cementing solution to obtain the sand improvement solution.

[0035] Step 1 is the leaching of recycled concrete fines; step 3 is the preparation of cementing solution; and step 4 is the preparation of sand improvement solution. In this embodiment, microbial mineralization and waste concrete resource utilization are combined at the same time, RCF is used as the main raw material, and a high calcium content solution is leached with ammonium chloride solution to replace the traditional chemical calcium source, and then calcium carbonate precipitation is promoted by microbial action, which can be used to achieve the solidification and enhancement of sand materials.

[0036] In one embodiment, the stirring and mixing temperature in step 1 is 25° C.-85° C., the solid-liquid ratio of the recycled concrete fine material and the ammonium chloride solution is 0.2-1 g / mL; and the concentration of ammonium chloride in the ammonium chloride solution is 1-4 mol / L.

[0037] The reaction rate of ammonium chloride solution with soluble components in recycled concrete fines increases with increasing temperature. 25°C is room temperature, ensuring low energy consumption and easy operation; 85°C is close to the operating limit temperature of the solution, which can significantly accelerate the dissolution reaction. If the reaction rate is too low, the dissolution time will be prolonged, affecting the leaching efficiency. In actual operation, a longer soaking time is required to achieve the same dissolution effect, thereby increasing cost and time investment; too high a temperature may cause ammonium chloride volatilization loss and reduce the actual effective concentration of the solution. Higher heat resistance requirements are imposed on the equipment, increasing the complexity and cost of operation.

[0038] The solid-liquid ratio is in the range of 0.2-1g / mL, which can not only ensure the full contact between the solid phase material and the solution during the leaching process, but also avoid excessive waste of solution or uneven stirring caused by excessive solids. The appropriate solid-liquid ratio ensures that the concentration of the ammonium chloride solution will not decrease rapidly during the reaction, thereby maintaining continuous leaching capacity. If the solid-liquid ratio is too low, the amount of solution used will increase significantly due to insufficient effective contact of the solid phase, and the treatment efficiency will be reduced. Too high a solid-liquid ratio may lead to saturation of the solution or incomplete local reaction, thereby reducing the overall leaching efficiency. In addition, too thick a slurry will also affect mixing uniformity and mass transfer efficiency.

[0039] Ammonium chloride concentrations in the range of 1-4 mol / L can provide sufficient ammonium chloride ion concentration to react and dissolve the target components in recycled concrete. Low-concentration ammonium chloride solutions may not be able to fully dissolve the target components, resulting in low leaching efficiency. High-concentration solutions do not significantly improve dissolution efficiency, but will increase the cost of chemical use.

[0040] In one embodiment, the leaching process in step 1 is carried out in a conical flask equipped with an ammonia gas collection device. To ensure temperature stability during the leaching process, the conical flask is placed in a temperature-controlled water bath. After filtration, the filter residue is removed, and the obtained leachate is the RCF leachate.

[0041] In one embodiment, the urease-producing bacteria of step 2 are selected from bacteria with Chinese name Sporosarcina pasteurii, scientific name Sporosarcina pasteurii and deposit number ATCC 11859; step 2 specifically includes: placing the urease-producing bacteria in a culture medium containing 10.00g / L ammonium sulfate, 15.73g / L Tris buffer and 20.00g / L yeast extract for 24-48 hours of shaking culture until the bacteria reach the required activity, and then storing the cultured bacterial solution for standby use; the shaking culture temperature is 28-32°C. The present invention uses Sporosarcina pasteurii to show significant advantages in microbial induced calcium carbonate precipitation (MICP), mainly due to its efficient urease activity, which can quickly decompose urea and increase the pH value of the solution, thereby promoting the precipitation of calcium carbonate. The calcium carbonate precipitate generated by the bacteria has high purity and stability (mainly calcite), and is particularly suitable for engineering applications such as foundation reinforcement and crack self-repair. At the same time, Sporosarcina pasteurii has strong adaptability, can grow in a wide range of pH, temperature and salinity, is non-pathogenic, and is friendly to the environment and human health. In addition, its persistent biological activity reduces the frequency of bacterial supplementation and improves engineering operability. With its wide application in foundation improvement, concrete crack repair, heavy metal pollution control and carbon dioxide capture, Sporosarcina pasteurii is considered to be one of the most promising strains in MICP technology.

[0042] In one embodiment, the initial urease activity of the urease-producing bacterial solution obtained in step 2 is 4-10 U / mL.

[0043] In one embodiment, before use, the stored bacterial solution is activated for a certain period of time to reactivate its enzyme activity.

[0044] In one embodiment, the concentrations of calcium ions and urea in the cementing solution of step three are consistent, and the concentration of calcium ions before mixing is 0.5-2 mol / L. Too low a calcium ion concentration will cause a significant decrease in the precipitation efficiency of calcium carbonate, which cannot meet the engineering requirements. Insufficient precipitation may lead to unsatisfactory cementing effect, so a longer reaction time is required to achieve the target precipitation amount, thereby increasing the construction period and cost. Excessive calcium ion concentration may inhibit microorganisms (such as Pasteurella sporeosarcina), reduce urease activity, and affect the precipitation efficiency of calcium carbonate.

[0045] In one embodiment, the concentration of calcium ions before mixing is obtained based on the analysis results of the RCF leachate, and then urea consistent with the calcium ion concentration is added.

[0046] In one embodiment, the volume ratio of the bacterial solution to the bonding solution in step four is 1-2:1.

[0047] In order to adapt to permeable soil and target precipitation density, the present invention is preferably 1:1. In the MICP process, the volume ratio of the bacterial solution to the cementing solution is set to 1:1, which is an optimized choice. This ratio achieves a good balance between ensuring microbial activity and calcium carbonate precipitation efficiency. The volume ratio of 1:1 can ensure that the concentration of bacteria and the concentration of calcium ions are both in an appropriate range, maximizing the efficiency of microbial induced precipitation. This ratio can maintain a suitable pH and chemical environment in the reaction medium, which is conducive to the generation of high-quality calcium carbonate precipitation. Too low bacterial concentration will lead to limited urease activity, resulting in a decrease in the urea hydrolysis rate, thereby reducing the precipitation efficiency of calcium carbonate. Too much bacterial solution may exceed the effective concentration required for the reaction, resulting in a waste of bacterial solution and increased costs. When the calcium ion concentration in the cementing solution is insufficient, even if the number of bacteria is sufficient, the amount of calcium carbonate precipitation may not meet the engineering requirements.

[0048] In one embodiment, before mixing the bacterial solution and the cementing solution in step 4, an acid is added dropwise to the bacterial solution until the pH of the bacterial solution is adjusted to 4.5-6.5.

[0049] Specifically, the acid may be hydrochloric acid.

[0050] The present invention also provides an implementation method of a sand improvement method, wherein the sand to be improved is treated with the sand improvement liquid (treatment methods include infiltration, grouting or spraying) more than once at room temperature, and each time the sand is left to stand for a certain period of time.

[0051] In one embodiment, the sand to be improved is dry sand, which is first compacted in layers to a target relative density of 60%-90%, and then treated by grouting, followed by retention for a certain period of time.

[0052] To facilitate later testing, Figure 1 As shown, the example in this embodiment uses a vertically arranged PVC pipe of appropriate size as a mold, first compacts the sand to be improved to the target relative density, then specifically selects a grouting method for treatment, and then retains it for a certain period of time to form a biocemented sand sample to be tested.

[0053] Adopt layered compaction method to press dry sand to target relative density, wherein initial relative density is set to specific value, and porosity is kept within predetermined range. Subsequently, the prepared improved liquid is evenly grouted (the improved method adopted in the present embodiment is grouting, under other conditions, infiltration and spraying may also be adopted) into the top surface of the sample, so that it penetrates into the gap between sand grains under gravity. The sample is left to stand at room temperature until the microorganism induces the calcium carbonate precipitation process to be fully carried out. This process is repeated until the predetermined number of treatments (generally more than 2 times) is reached. Finally, dry and remove the mold to obtain a biocemented sand sample.

[0054] Example 1

[0055] The sand improvement method combining microorganisms and waste concrete of this embodiment includes the following steps:

[0056] Step 1, mixing the recycled concrete fine material and the ammonium chloride solution and filtering to prepare the RCF leachate;

[0057] Step 2: culturing urease-producing bacteria to obtain a bacterial solution;

[0058] Step 3: adding urea to the RCF leachate prepared in step 1, and mixing to form a cementing solution;

[0059] Step 4: adjusting the pH value of the cementing solution to neutral, and then mixing the bacterial solution and the cementing solution to obtain the sand improvement solution;

[0060] Step 5: Then compact the sand and soil in layers to the target relative density of 84%, and then use the obtained sand and soil improvement liquid to grout the sand and soil to be improved 12 times. According to the specific requirements of the project for permeability coefficient and strength, the corresponding number of treatments can be adjusted to ensure the expected effect. Each time, let it stand for 36 hours.

[0061] The first four steps are the preparation method of sand improvement liquid, and the preparation method flow is as follows: Figure 2 shown.

[0062] Step 1: The mixing temperature is 85°C, the solid-liquid ratio of the recycled concrete fines and the ammonium chloride solution is 0.2g / mL; the concentration of ammonium chloride in the ammonium chloride solution is 2mol / L. The process of this step is carried out in a conical flask equipped with an ammonia gas collection device, and the temperature is kept stable by a constant temperature magnetic stirrer through a temperature-controlled water bath. The whole process is carried out at a relatively high temperature and stirring is maintained for a certain period of time to ensure that the leaching reaction is fully carried out.

[0063] The urease-producing bacteria in step 2 are selected from bacteria with Chinese name Sporosarcina pasteurii, scientific name Sporosarcina pasteurii and deposit number ATCC 11859;

[0064] The step 2 specifically includes: placing the bacteria in a culture medium containing 10.00 g / L ammonium sulfate, 15.73 g / L Tris buffer and 20.00 g / L yeast extract and shaking culture for 24 hours until the bacteria reach the required activity, and then storing the cultured bacterial solution for later use; storing it under low temperature conditions to enter a dormant state to maintain its activity. The shaking culture temperature is 30°C.

[0065] The initial urease activity of the urease-producing bacterial solution obtained in step 2 is 8 U / mL.

[0066] The concentrations of calcium ions and urea in the cementing solution of step three are consistent, and the concentration of calcium ions before mixing is 0.8 mol / L.

[0067] The volume ratio of the bacterial solution to the cementing solution in step 4 is 1:1.

[0068] Before mixing the bacterial solution prepared in step 2 and the cementing solution prepared in step 3 in step 4, hydrochloric acid is added dropwise to the bacterial solution until the pH of the bacterial solution is adjusted to 5.0.

[0069] Example 2

[0070] The difference from Example 1 is that the solid-to-liquid ratio of the recycled concrete fine material and the ammonium chloride solution in step 1 is 0.5 g / mL.

[0071] Example 3

[0072] The difference from Example 1 is that the solid-to-liquid ratio of the recycled concrete fine material and the ammonium chloride solution in step 1 is 1 g / mL.

[0073] Comparative Example 1

[0074] The difference from Example 1 is that the solid-liquid ratio of the raw concrete fine material and the hydrochloric acid in step 1 is 1 g / mL, wherein the concentration of the hydrochloric acid is 2 mol / L. In this comparative example, hydrochloric acid (HCl) is used as the leaching solvent for RCF. Specifically, a certain mass of RCF is mixed with a hydrochloric acid solution of an appropriate concentration, stirred under a set solid-liquid ratio condition, and kept at room temperature for a certain time to complete the leaching process.

[0075] Comparative Example 2

[0076] The difference from Example 1 is that chemical calcium (calcium chloride) with the same molar concentration as the recycled concrete fine material in Example 1 is used as the calcium source of MICP.

[0077] In order to evaluate the permeability of the biocemented sand samples after different treatment cycles, deionized water was injected through the bottom of the water supply pipe connected to the bottom of the PVC pipe, while the top of the sample was kept overflowing to keep the liquid level constant; the riser was pre-filled with water, and the permeability of the sample was calculated by measuring the time required for the water head to change.

[0078] The UC test was carried out on the cylindrical biocementation samples modified with the modifying fluid using a VJ-Tech Tri-Scan 50 triaxial testing machine.

[0079] The method for determining the calcium carbonate content (CCC) in a biocement sample comprises the following steps: a certain amount of dried biocement sample is mixed with a hydrochloric acid solution of equimolar concentration, and the reaction is sufficient to dissolve the calcium carbonate in the sample. Subsequently, the solid residue is filtered through filter paper and washed with distilled water, and the filtration time is controlled within an appropriate range. Finally, the solid residue on the filter paper is dried and weighed, and the calcium carbonate content is calculated based on the difference between the initial dry mass of the sample and the residual dry mass after the reaction.

[0080] The environmental impact of different calcium sources on microbial induced calcium carbonate deposition is analyzed based on life cycle assessment (LCA). The method constructs a full life cycle model of the MICP process, systematically analyzes the energy consumption and carbon emission characteristics of different calcium sources during the preparation and application process, and quantifies their environmental impact. The present invention focuses on evaluating the impact of calcium source selection on the sustainability of MICP, provides a scientific basis for optimizing process design and reducing environmental load, and is applicable to the fields of green building materials technology and environmental engineering.

[0081] Take Example 1 and Comparative Example 1 for calcium ion leaching concentration and leaching efficiency under different solid-liquid ratios. Figure 3 ;Depend on Figure 3It can be seen that as the amount of RCF increases, the calcium ion concentration in the leachate increases, but the leaching efficiency decreases. A higher RCF dosage may change the pH value of the solution, thereby reducing the solubility of calcium ions and affecting the extraction efficiency. Under the same solid-liquid ratio, the leaching concentration and efficiency of ammonium chloride are slightly higher than those of hydrochloric acid. This provides a theoretical basis for selecting an appropriate leaching agent, especially in terms of improving leaching efficiency and optimizing leaching conditions, ammonium chloride may be a more suitable choice.

[0082] Ammonium chloride has shown significant advantages in the leaching process of RCF. Compared with hydrochloric acid, ammonium chloride can achieve higher calcium ion leaching efficiency at the same solid-liquid ratio. On the whole, ammonium chloride has shown broad application prospects in the RCF leaching-carbonization process due to its high efficiency. Ammonium chloride leachate has shown significant advantages in the MICP process. It can not only effectively increase the calcium carbonate content and significantly reduce the permeability of the sample, but also ensure high unconfined compressive strength, proving its feasibility and environmental friendliness as a substitute for traditional calcium sources. As a substitute for traditional calcium sources, ammonium chloride leachate is comparable in performance while reducing the impact on the environment, providing a more economical and environmentally friendly option for MICP technology.

[0083] The relationship between the calcium carbonate content and the unconfined compressive strength in the biocemented sand provided in Example 1, Comparative Example 1 and Comparative Example 2 is shown in FIG. Figure 4 Different treatment times result in different calcium carbonate contents. According to the specific requirements of the project for permeability coefficient and strength, the corresponding number of treatments can be carried out to ensure the expected effect. The relationship between the hydraulic conductivity of MICP samples treated with different calcium sources and the number of treatments is shown in Figure 5 .like Figure 4 As shown in Figure 2, the biocemented sand samples treated with different calcium sources showed that the calcium carbonate content of all treated samples increased. The unconfined compressive strength of the samples treated with different MICP methods varied greatly, and the calcium carbonate content also changed. Figure 5 As shown in the figure, as the number of MICP treatments increases, the permeability of the samples decreases significantly. The permeability of samples treated with different calcium sources generally decreases, while the unconfined compressive strength changes little. This shows that calcium source substitutes such as ammonium chloride have good application prospects in the MICP process and can effectively improve permeability and maintain high strength.

[0084] According to the analysis of Example 1, Comparative Example 1 and Comparative Example 2, based on the different calcium source costs and life cycle assessment results required to produce 1000 kg of calcium carbonate (see Figure 6 and Figure 7), ammonium chloride leachate performs well in terms of energy demand and carbon emissions, thanks to its efficient production process based on the Hou alkali process. Cost analysis shows that the cost of ammonium chloride leachate is only about 1 / 3 of that of calcium chloride, and is much lower than that of hydrochloric acid leachate, with its cost being less than 1 / 9 of that of hydrochloric acid leachate. As a substitute for calcium chloride, ammonium chloride leachate is not only more economical, but also can significantly reduce carbon emissions and energy consumption, thereby improving the sustainability and practical application value of MICP technology.

[0085] The present invention shows significant environmental and economic advantages in life cycle analysis. Compared with other calcium sources, ammonium chloride leachate performs well in terms of energy demand and carbon emissions, thanks to its efficient production process based on Hou's alkali process. At the same time, cost analysis shows that the cost of ammonium chloride leachate is only about 1 / 3 of that of calcium chloride, and is much lower than that of hydrochloric acid leachate, less than 1 / 9 of its cost. As a substitute for calcium chloride, ammonium chloride leachate is not only more economical, but also can significantly reduce carbon emissions and energy consumption, further enhancing the sustainability and practical application value of MICP technology.

[0086] It can be seen from the results of the above examples that the present invention uses waste concrete fine powder as the main raw material, fully utilizes its high calcium content, and uses ammonium chloride as the leaching solvent, effectively reducing the high cost problem in the traditional calcium source preparation process. The method has a simple process, low energy consumption, and no secondary pollution, which fully embodies the concept of resource recycling and sustainable development. At the same time, the present invention significantly improves the utilization efficiency of waste resources, has significant environmental benefits and social value, and provides an efficient and economical technical solution for green development and ecological protection.

[0087] In summary, the present invention has significant environmental friendliness by combining microbial urease-producing bacteria to promote carbonate precipitation. The present invention is carried out at normal temperature and pressure, with low energy consumption, low carbon emissions, and no toxic by-products. Through microbial metabolic activities, the precipitation of calcium carbonate is promoted, and the generated calcium carbonate is a natural and harmless mineral, and it avoids secondary pollution or waste accumulation that may be produced in traditional methods, so the impact on the environment is minimal. In addition, the present invention can effectively recycle calcium resources in waste and realize the recycling of resources. Unlike traditional chemical reinforcement methods (such as lime, cement or polymers), the present invention does not need to introduce toxic chemical additives or solvents, reducing the potential harm to the ecosystem. Therefore, the present invention is not only a sustainable reinforcement technology, but also has significant environmental advantages.

[0088] Although the above embodiment describes the present invention in detail, it is only a part of the embodiments of the present invention, not all of the embodiments. People can also obtain other embodiments based on this embodiment without creativity, and these embodiments all fall within the protection scope of the present invention.

Claims

1. A method for preparing a sand-soil improvement liquid combining microorganisms with waste concrete, characterized in that: The steps include: Step 1, mixing the recycled concrete fine material and the ammonium chloride solution and filtering to prepare the RCF leachate; Step 2: culturing urease-producing bacteria to obtain a bacterial solution; Step 3: adding urea to the RCF leachate prepared in step 1, and mixing to form a cementing solution; Step 4: Adjust the pH value of the cementing solution to neutral, and then mix the bacterial solution and the cementing solution to obtain the sand improvement solution.

2. The method for preparing the sand and soil improvement liquid combining microorganisms and waste concrete according to claim 1, characterized in that: Step 1: The mixing temperature is 25°C-85°C, the solid-liquid ratio of the recycled concrete fine material and the ammonium chloride solution is 0.2-1g / mL; and the concentration of ammonium chloride in the ammonium chloride solution is 1-4mol / L.

3. The method for preparing the sand and soil improvement liquid combining microorganisms and waste concrete according to claim 1, characterized in that: The urease-producing bacteria in step 2 are selected from bacteria with Chinese name Sporosarcina pasteurii, scientific name Sporosarcina pasteurii and deposit number ATCC 11859; The step 2 specifically comprises: placing the urease-producing bacteria in a culture medium containing 10.00 g / L ammonium sulfate, 15.73 g / L Tris buffer and 20.00 g / L yeast extract and shaking culture for 24-48 hours until the bacteria reach the required activity, and then storing the cultured bacterial solution for later use; the shaking culture temperature is 28-32°C.

4. The method for preparing the sand improvement liquid combining microorganisms and waste concrete according to claim 1 or 3, characterized in that: The initial urease activity of the bacterial solution obtained in step 2 is 4-10 U / mL.

5. The method for preparing the sand improvement liquid combining microorganisms and waste concrete according to claim 1, characterized in that: The concentrations of calcium ions and urea in the cementing solution of step three are consistent, and the concentration of calcium ions in the RCF leachate before mixing in step three is 0.5-2 mol / L.

6. The method for preparing the sand and soil improvement liquid combining microorganisms and waste concrete according to claim 1, characterized in that: The volume ratio of the bacterial solution to the cementing solution in step 4 is 1-2:

1.

7. The method for preparing the sand and soil improvement liquid combining microorganisms and waste concrete according to claim 1, characterized in that: Before mixing the bacterial solution and the cementing solution in step 4, an acid is added dropwise to the bacterial solution until the pH of the bacterial solution is adjusted to 4.5-6.

5.

8. Sand and soil improvement liquid, characterized by: Obtained by the preparation method described in any one of claims 1 to 7.

9. A sand improvement method, characterized in that: The method comprises the following steps: at room temperature, soaking, grouting or spraying the sand to be improved with the sand improvement liquid of claim 8 more than once, and leaving it to stand for 24-48 hours each time.

10. The sand improvement method according to claim 9, characterized in that: The sand to be improved is dry sand. Before infiltration, grouting or spraying, the sand is compacted in layers to a target relative density of 60%-90%.

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