A method for reinforcing coral sand based on MICP-coconut fiber combination

Abstract

The present application relates to the field of building technology, and more particularly to a method for reinforcing coral sand based on MICP-coconut shell fiber, which is suitable for reinforcing the foundation of coral sand in tropical marine island reef engineering and the like. The present application aims to overcome the shortcomings of the existing coral sand reinforcement technology and provide a method for ecologically and low-carbon reinforcing of coral sand based on MICP-coconut shell fiber. By constructing a synergistic system of "fiber skeleton-microbial mineralization-sand body cementation", the ecological, low-carbon and efficient reinforcement of the foundation of coral sand is achieved, and the strength, toughness, uniformity and ecological compatibility of the foundation are simultaneously improved.

Description

Technical Field

[0001] This invention relates to the field of building technology, and in particular to a method for reinforcing coral sand based on MICP-coconut shell fiber, which is applicable to coral sand foundation reinforcement scenarios such as tropical marine island and reef engineering. Background Technology

[0002] Coral sand, as a core dam-building material in tropical marine island and reef engineering, suffers from numerous bottlenecks in traditional foundation reinforcement techniques due to its porous structure (porosity reaching 40%~60%), easily broken particles (Mohs hardness only 3~4), and poor cementation. Physical compaction methods consume large amounts of energy and struggle to address deep loosening issues. Chemical grouting methods, which commonly use cement and epoxy resins, are prone to causing marine environmental pollution and have poor compatibility with the calcium carbonate component of coral sand, leading to interface detachment due to salt spray erosion during long-term service. While microbial induced calcium carbonate precipitation (MICP) technology has become a research hotspot due to its environmental friendliness and the fact that the cementing products are homologous to coral sand, simple MIP reinforcement has significant drawbacks: the generated calcium carbonate crystals are rigid and brittle, resulting in limited improvement in the shear strength of the reinforced coral sand (usually only 2-3 MPa), and it is prone to sudden brittle failure under load without a clear plastic deformation stage; at the same time, the high porosity and irregularity of coral sand can easily cause uneven penetration of MIP solution and cementing solution within the sand body, forming a reinforcement blind zone of "surface enrichment and deep deficiency", which is difficult to meet the synergistic requirements of "strength-toughness-uniformity" for the foundation of island and reef engineering.

[0003] Natural coconut shell fiber, as an agricultural waste, possesses high tensile strength (200-300 MPa dry tensile strength), excellent toughness (15%-25% elongation at break), and biodegradability. Combining it with MIP (Microbial Mineralization-Sand Body Cementation) technology can construct a reinforcement system that combines rigidity and flexibility. However, current technologies have not yet achieved deep synergy between the two. They either focus only on the bonding effect of MIP while neglecting the reinforcing effect of the fiber, or the fiber pretreatment process is too rough, resulting in poor dispersibility. This fails to fully utilize the three-dimensional synergistic effect of "fiber skeleton - microbial mineralization - sand body cementation." There is an urgent need to propose an improved solution that takes into account ecological environmental protection, reinforcement efficiency, and engineering applicability.

[0004] The method for improving the strength of microbial sand fixation based on modified plant fibers, patent application number CN120290192A, although it improves the sand fixation strength by combining alkali-treated modified natural fibers with MIP technology, has strict limitations on the type of sand. It is only applicable to medium-grade sand with a particle size of 0.5-1.0 mm. It does not take into account the compatibility with other types of sand such as fine sand, coarse sand, and coral sand. It cannot reproduce the key engineering properties such as particle size distribution and pore distribution differences of non-medium-grade sand. Furthermore, it does not optimize the compatibility of fiber content and mineralization reaction parameters for different types of sand, making it difficult to meet the engineering needs of diversified sand reinforcement in foundation treatment. Meanwhile, this method has significant limitations and potential problems: First, its limited compatibility with a single type of sand results in a narrow range of engineering applications. When faced with scenarios where fine sand easily agglomerates and clogs pores, or where coarse sand particles have large gaps that make effective bonding difficult, it cannot achieve uniform solidification, leading to large dispersion in the strength of the solidified body. Second, it relies on the particle size of specific medium-grade sand. If the particle size of the sand deviates from the specified range in actual engineering, it will lead to insufficient interfacial friction between the modified fiber and the sand particles, failure of mechanical connection, and consequently, a decrease in the overall structural stability of the solidified body, making it prone to cracking, collapse, and other hidden dangers. Third, it does not conduct durability testing and mineralization efficiency analysis for non-medium-grade sand environments. Even if it is applied to other types of sand, poor particle compatibility will lead to uneven calcium carbonate deposition, insufficient impermeability and erosion resistance of the solidified body, and it is susceptible to strength decay due to environmental factors during long-term service. It is difficult to meet the engineering requirements of "broad-spectrum compatibility - stable durability - long-term reliability" for foundation reinforcement in complex geological environments. The microbial synergistic fiber-stabilized soil, patent application number CN119591353A, although employing modified PE fibers and MIP for synergistic stabilization to improve mechanical and impermeability properties, suffers from several limitations. The fibers are synthetic materials, resulting in higher costs than natural fibers and greater difficulty in biodegradation. Furthermore, it lacks a design for utilizing agricultural waste resources. Its core limitation lies in its focus solely on modifying the PE fibers without pre-treating the sand interface. This leads to the unactivated hydroxyl groups on the sand surface, resulting in only physical contact between the sand and the calcium carbonate generated by the modified fibers and MIP, lacking chemical bonding and resulting in weak interfacial adhesion. Consequently, the stabilization system exhibits poor overall performance. The uniformity and stability are insufficient; it does not adapt to the unique characteristics of coral sand, which is highly porous and easily broken, and it has not conducted durability tests under marine salt spray environment. It only focuses on general sand and soil, which is difficult to meet the comprehensive requirements of "ecological low carbon - engineering adaptability - long-term erosion resistance" for island and reef coral sand foundations. Long-term service is prone to "fiber-sand body-calcium carbonate" interface delamination. However, the dual modification scheme of "pretreated coconut shell fiber + pretreated coral sand" can activate the functional groups of the dual interface through KH550 hydrolysate to build a chemically bonded three-dimensional network system, and simultaneously solve the problems of ecological and environmental protection, interface bonding and engineering adaptability.

[0005] Therefore, given that coral sand in real island and reef foundations generally exhibits a mixed distribution of multiple particle sizes, existing patents, due to their limited particle size compatibility, are prone to problems such as uneven solidification and large strength dispersion when faced with the high porosity and fragile nature of coral sand. How to design an ecological and low-carbon reinforcement method for coral sand by combining MICP is an urgent problem to be solved. Summary of the Invention

[0006] The purpose of this invention is to overcome the shortcomings of existing coral sand reinforcement technologies and provide an ecological and low-carbon coral sand reinforcement method based on MICP-coconut shell fiber. By constructing a synergistic system of "fiber skeleton - microbial mineralization - sand body cementation", the method achieves ecological, low-carbon, and efficient reinforcement of coral sand foundations, while simultaneously improving the foundation strength, toughness, uniformity, and ecological compatibility.

[0007] Specifically, this invention provides a method for reinforcing coral sand based on MICP-coconut shell fiber, which includes the following steps:

[0008] S1. Soak coconut shell fiber in KH550 hydrolysate at a volume ratio of 0.2-0.8% for 2-10 hours, and then wash and dry it. Spread out full-size coral sand, spray it with KH550 hydrolysate at a volume ratio of 1% to 2% of the sand mass, stir and repeat 1-5 times, let it stand and then dry it.

[0009] S2. Mix the coconut shell fiber and coral sand to form a mixture, and place the mixture into a mold in 2-5 layers and hammer it.

[0010] S3. Activate the freeze-dried powder of Bacillus pasteurellii and carry out large-scale culture to form a bacterial solution;

[0011] S4. Use a peristaltic pump to perform 2-5 rounds of grouting on the mixture in the mold. In each round of grouting, inject bacterial solution once, let it stand, and then inject cementing solution 3-5 times. The cementing solution is a mixture of urea solution and calcium chloride solution.

[0012] In step S1, the KH550 hydrolysate consists of 100-150 mmol / L KH550 silane coupling agent, 8-15 mmol / L glacial acetic acid, and 93-98 mol / L deionized water; and / or, the full-size coral sand.

[0013] In step S1, the length of the coconut shell fiber is 10~15mm. The coconut shell fiber is made by soaking fresh coconut shell filaments in KH550 hydrolysate after removing sediment, cutting, drying, and rinsing the fiber with deionized water until the pH of the rinsing solution is stable at 6.5~7.5. Then the fiber is placed in an 80℃ oven to dry for 8-24 hours.

[0014] In step S1, spread the full-size coral sand evenly in the tray, with a thickness of 1-2cm. Hold the sprayer 20-30cm away from the sand surface and spray the hydrolysate in a "Z" shaped path at a uniform speed. Spray the hydrolysate in 2-3 times to complete the total amount, with an interval of 0.2-5 minutes between each spray. Stir immediately after each spray. After stirring, let it stand for 0.2-5 hours and then place it in a 60-100℃ oven for 4-8 hours.

[0015] In step S2, the mold preparation method is as follows: a thin iron sheet is cut into a rectangle, and the rectangular thin iron sheet is formed into a cylindrical mold body through a rolling process. Engineering-grade sealant is used to bond and seal the joint of the thin iron sheet. A cylindrical silicone sealing plug with a diameter matching the inner diameter of the mold is selected. A through hole is machined at the midpoint of the axis of the silicone sealing plug using a drilling tool with a hole diameter of 2-10mm. It is ensured that the axis of the through hole is parallel to the axis of the silicone sealing plug and that the two ends of the through hole are flush with the end face of the sealing plug. A capillary quartz tube is installed and fixed in the through hole of the silicone sealing plug, so that one end of the capillary quartz tube is flush with one end face of the silicone sealing plug, and the other end protrudes 20-30mm from the other end face of the silicone sealing plug, forming a special grouting channel. The pre-made silicone sealing plug with capillary quartz tube is first assembled at one end opening of the mold body. The gap between the sealing plug and the inner wall of the mold is sealed and fixed using engineering-grade sealant.

[0016] In step S2, the amount of coconut shell fiber in the mixture is 0.1-2 wt%.

[0017] In step S3, the culture medium used for the culture includes: ammonium chloride 5-15 g / L, manganese sulfate monohydrate 8-12 mg / L, nickel chloride hexahydrate 20-30 mg / L, and yeast extract 15-25 g / L. The Bacillus pasteurellii freeze-dried bacterial powder was purchased from the Guangdong Provincial Microbial Culture Collection Center, with the number GDMCC-1.803.

[0018] In step S4, the grouting rate is 1 ml / min to 2 ml / min; the cementing solution is prepared by mixing urea solution and calcium chloride solution at equal concentrations, and the concentration of the cementing solution is 0.5 mol / L to 1.5 mol / L.

[0019] In step S4, the cementing solution is prepared by mixing urea solution and calcium chloride solution at equal concentrations, with a concentration of 1 mol / L; and / or, in each round of grouting, a bacterial solution is injected once, allowed to stand for 1-5 hours, and then the cementing solution is injected 3-5 times, with an interval of 6-10 hours between each injection.

[0020] In step S4, after each batch of grouting work is completed, the sample curing stage begins, and the molding mold for loading the sample is flipped. During adjacent curing cycles, the placement of the mold is reversed.

[0021] The core working principle of this invention is as follows: Silane coupling agents pretreat coconut shell fibers and coral sand respectively. The silanol groups generated by hydrolysis form covalent bonds with the hydroxyl groups on the fiber surface and the sand particle surface, respectively. This activates the active sites at both interfaces, providing a more stable growth site for Bacillus pasteurellii, and optimizes the spreading and penetration of the bacterial solution and the cementing solution. Combined with the porous structure of the fibers, it achieves "adsorption-slow release" and avoids uneven mineralization. Coconut shell fibers provide a stable growth site for Bacillus pasteurellii, and their porous structure achieves "adsorption-slow release" of the bacterial solution and the cementing solution, avoiding uneven penetration. Bacillus pasteurellii metabolizes and produces urease, which catalyzes the decomposition of urea to generate carbonate ions, which react with calcium ions in the cementing solution to form calcium carbonate crystals. The silane coupling agent acts as a two-way bridging agent, and its exposed functional groups bond with the calcium carbonate crystals, anchoring the calcium carbonate on the surface of the modified fibers and modified sand particles, constructing a dense three-dimensional network structure of "coral sand particles—silane coupling agent—calcium carbonate—silane coupling agent—coconut shell fiber" (e.g., Figure 1 As shown, it not only improves strength by filling pores with calcium carbonate, but also improves toughness by using coconut shell fiber as a reinforcing agent, and solves the problems of brittleness, interface peeling and insufficient uniformity of simple MIP reinforcement by relying on coupling agent.

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

[0023] (1) Outstanding interface bonding strengthening effect: The silanols generated by the hydrolysis of silane coupling agent form Si-OC covalent bonds with the hydroxyl groups on the surface of coconut shell fiber and Si-O-Si covalent bonds with the hydroxyl groups on the surface of coral sand. Compared with the existing technology where the fiber and sand body only have physical contact, the interface bonding strength is increased by 40%~60%. The calcium carbonate crystals are anchored by the exposed amino and silanols of the coupling agent, avoiding the problem of "fiber-sand-calcium carbonate" interface peeling in the traditional system. After long-term immersion test in the marine environment, the interface integrity retention rate is more than 95%, which is much higher than the less than 60% of the unmodified system. It is suitable for the reinforcement of island and reef foundations and coastal defense projects with strict requirements for interface stability.

[0024] (2) Significantly optimized mineralization uniformity: The porous structure of coconut shell fiber enables the "adsorption-slow release" of bacterial solution and cementing solution. Combined with the adjustment of the wettability of coral sand surface by silane coupling agent (the contact angle is reduced from 72° to below 35°), the penetration depth of bacterial solution is increased to 2 to 3 times that of traditional MIP technology, and the mineralization filling rate of the pores inside the coral sand is increased from below 60% to above 85%. It completely solves the defects of "surface hardening and deep loosening" of existing MIP technology. The coefficient of variation of the overall compressive strength of the solidified body is controlled within 10%, which is far better than the 25% to 30% of the traditional system. It is suitable for the special geological conditions of high porosity and easy permeability of coral sand, and is especially suitable for the reinforcement of thick coral sand foundations.

[0025] (3) Balancing practicality and economy: The silane coupling agent modification process is simple (soaking / spraying + drying), requiring no complex equipment; coconut shell fiber, as an agricultural waste, is widely available and its price is only 1 / 5 to 1 / 3 of that of synthetic PE fiber, significantly reducing the total project cost; with a special molding die, the grouting efficiency is increased to 1.5 times that of the traditional process, and the sample molding accuracy error is controlled within ±1mm. The silane coupling agent does not contain toxic substances such as heavy metals and halogenated compounds, has extremely low VOC emissions, leaves no strong acid or alkali residues during the modification process, and the rinsing wastewater is easy to treat, with no secondary pollution. Its dosage is only 1% to 2% of the material mass, with low environmental impact, and it can eventually be naturally degraded into inorganic silicon compounds, without damaging the ecology and having minimal impact on marine life, which is in line with the concept of green engineering.

[0026] (4) Mold reliability and convenience: The joint of the mold body is sealed with engineering-grade sealant, and the gap between the sealing plug and the inner wall of the mold is sealed. The double sealing structure can effectively prevent the leakage of bacterial liquid / cementing liquid during the grouting process, ensuring the closed environment required for the microbial mineralization reaction, while avoiding material waste. The mold body is formed by rolling process, and the sealing plug and the inner wall of the mold are detachable and have no irreversible fixed structure. When disassembling the sample, only the sealant needs to be peeled off to separate the mold and the sample, which is simple and efficient. Moreover, the thin iron sheet and silicone material have strong corrosion resistance (suitable for marine environment sample preparation) and can be reused after cleaning, reducing the test cost. Excellent non-destructive disassembly to ensure sample integrity: The mold body is made of thin iron sheet material with moderate rigidity and smooth surface. The sealing plug is made of flexible silicone material, which can avoid mechanical damage to the coral sand-coconut shell fiber composite sample during the disassembly process, and completely preserve the internal calcium carbonate mineralization distribution state of the sample, providing a real and effective sample for subsequent microstructure observation and mechanical property testing. Attached Figure Description

[0027] Figure 1 A schematic diagram of the reaction process for strengthening coral sand using microbial-induced calcium carbonate precipitation (MICP) technology;

[0028] Figure 2 Process flow diagram of the MICP-coconut shell fiber-reinforced coral sand method in Embodiment 1 of the present invention;

[0029] Figure 3 This is a scanning electron microscope image of coral sand particles in Comparative Example 1;

[0030] Figure 4 This is a scanning electron microscope image of coral sand particles reinforced with MICP, as shown in Comparative Example 2.

[0031] Figure 5 The image shows a scanning electron microscope (SEM) image of the synergistic modification of coconut shell fiber and coral sand with MICP-0.5% pretreated fiber in Example 1.

[0032] Figure 6 A comparison curve of stress-strain characteristics of coral sand under different treatment methods;

[0033] Figure 7 This is a photograph of the mold used in Embodiment 1 of the present invention. Detailed Implementation

[0034] The present invention will now be described in detail with reference to the accompanying drawings and specific embodiments. These embodiments are based on the technical solution of the present invention and provide detailed implementation methods and specific operating procedures. However, the scope of protection of the present invention is not limited to the following embodiments.

[0035] Example 1

[0036] This embodiment addresses the engineering defects of coral sand on islands and reefs, namely "high porosity, low strength, and fragility." Combining the reinforcing properties of natural coconut shell fiber with the bonding advantages of microbial induced calcium carbonate precipitation (MICP) technology, it proposes an eco-friendly improvement scheme. This scheme is completely free of chemical pollution, has low energy consumption, and is suitable for foundation reinforcement scenarios in island and reef engineering. Figure 1 As shown, the specific steps include:

[0037] Step 1: Select a 0.1mm thick sheet of iron and cut it into rectangular blanks measuring 130mm in length and 80mm in width. Using a rolling process, shape the rectangular sheet into a cylindrical mold body with an inner diameter of 40mm and a height of 80mm. Apply engineering-grade sealant to the joints of the sheet. Select a cylindrical silicone sealing plug (40mm in diameter) with a diameter matching the mold's inner diameter. Using a 5mm bore drill, machine a through-hole at the midpoint of the silicone sealing plug's axis, ensuring the through-hole's axis is parallel to the silicone sealing plug's axis and that both ends of the through-hole are flush with the sealing plug's end face. Select a capillary quartz tube with an inner diameter of 3mm, an outer diameter of 4mm, and a length of 50mm. Insert and fix it into the through-hole of the silicone sealing plug, ensuring one end of the capillary quartz tube is flush with one end face of the silicone sealing plug (collinear axes), and the other end protrudes 20-30mm beyond the other end face of the silicone sealing plug, forming a dedicated grouting channel. The prefabricated silicone sealing plug with capillary quartz tube is first assembled onto the opening at one end of the mold body. Engineering-grade sealant is then used to seal and fix the gap between the sealing plug and the inner wall of the mold, facilitating subsequent sample loading (mold photo as shown). Figure 7 (as described).

[0038] Step 2: Preparation of modified coconut shell fiber-modified coral sand mixture. The coconut shell fiber used in this embodiment is derived from waste coconut shells, following the principles of "resource utilization and ecological low carbon". First, fresh waste coconut shells are taken and soaked in clean water for 48 hours (to soften the woody structure of the coconut shells and facilitate fiber separation). Then, the coconut shells are repeatedly pounded with a wooden pounding tool to separate the coconut shell fibers from the woody core. The pounded mixture is then passed through a 20-mesh sieve to remove residual woody lumps and impurities, retaining coarse fibers with a length of about 5-20 mm. The coarse fibers are then rinsed three times with deionized water (each rinse lasting 5 minutes) to remove dust, coconut shell powder, and other impurities adhering to the surface. The fibers are then combed into bundles using a guillotine and cut to a target length of 10-15 mm (this length ensures the interlocking effect of the fibers with sand particles while avoiding excessive length that could lead to agglomeration). Next, the cut coconut shell fibers are placed in an 80°C constant temperature oven and dried for 12 hours (to thoroughly remove internal moisture from the fibers and prevent subsequent samples from becoming moldy). After drying, the fibers are removed, cooled to room temperature, sealed with plastic wrap, and stored in a dry environment to prevent the fibers from absorbing moisture and affecting dispersibility. Measure 980 mL of deionized water into a beaker, weigh 20 g of KH550 and slowly add it dropwise, turn on the magnetic stirrer (300~400 r / min) and stir for 10 min; add glacial acetic acid dropwise, monitor the pH in real time, adjust to 4.0±0.2, and continue stirring for 30~40 min until the solution is clear and transparent; let stand for 10 min to remove air bubbles, and use immediately (store at room temperature for no more than 24 h). After being cut and screened to a uniform length, the processed coconut shell fibers were immersed in KH550 hydrolysate and statically soaked at room temperature (20~25℃) for 5 hours. During this period, the container was gently shaken once every hour to ensure that the fibers were completely submerged and in full contact with the hydrolysate. After soaking, the fibers were rinsed with deionized water. Each time the fibers were rinsed, they were gently stirred with a glass rod to ensure that the residual hydrolysate on the surface was completely removed. Each rinse lasted 5 minutes and was repeated 3 times. Finally, the pH value of the rinsing solution was tested and found to be stable at 6.5~7.5. The fibers were then laid flat on a clean tray (with a thickness of ≤1cm to avoid stacking) and dried in an 80℃ oven for 12 hours. During the drying process, the fibers were turned over once every 4 hours to ensure that the fibers were dried evenly inside and out. After drying, the fibers were removed and allowed to cool naturally to room temperature (about 30 minutes). They were then immediately wrapped tightly in double-layer plastic wrap and stored for subsequent experiments. This embodiment uses full-size coral sand throughout the experiment, without any deliberate sieving or particle size modification. This fully preserves the original particle size distribution, pore size distribution, and physical properties of coral sand in its natural marine reef environment, accurately simulating its natural state and ensuring a high degree of consistency between the experimental results and actual engineering scenarios. Full-size coral sand is evenly spread in a tray (1-2 cm thick). KH550 hydrolysate (pre-prepared and used immediately) is precisely measured at a ratio of 1%-2% of the dry weight of the coral sand. A handheld pneumatic sprayer is used to spray the solution along a zigzag path (laterally and at a uniform speed) onto the sand surface, ensuring even coverage without localized accumulation.After spraying, immediately stir the sand using a clean glass rod employing a combination of "stirring and circular motions." Afterward, place the tray at room temperature for 1 hour to allow the hydrolysate to fully penetrate the sand particles. Then, place the tray in an 80℃ oven for 6 hours to dry. During drying, gently stir the sand particles with a glass rod every 2 hours to prevent localized agglomeration. Take the dried modified coral sand and mix it with modified coconut shell fiber (0.5% of the dry weight of the coral sand). Observe the mixing state during stirring to ensure that the fiber is not agglomerated and is uniformly dispersed in the sand, thus obtaining a uniformly mixed composite filler. Using a cylindrical rigid mold with a diameter of 38mm and a height of 78mm, fill the mold with the mixture in three layers. After each layer is filled, strike it vertically 15 times with a 1kg hammer (to ensure uniform density of each layer and avoid density differences between layers), finally producing sample I.

[0039] Step 3: Activation and expansion culture of Bacillus pasteurellii. Accurately weigh the following reagents according to the proportions: 10g ammonium chloride (provides nitrogen source to support bacterial metabolism), 10mg manganese sulfate monohydrate (trace element to activate urease activity), 24mg nickel chloride hexahydrate (coenzyme for enzymatic reactions), and 20g yeast extract (provides protein, vitamins, and other complex nutrients). For inoculation, add the above components to 900mL of deionized water and stir with a magnetic stirrer at 500 rpm for 30 minutes until completely dissolved. Then, slowly adjust the pH of the culture medium to 8.5 with 1mol / L sodium hydroxide solution, and finally bring the volume to 1000mL with deionized water. Dispense the culture medium into 100mL portions of 500mL Erlenmeyer flasks, sealing each flask with a breathable sealing film (allowing gas exchange and preventing anaerobic environments from inhibiting bacterial growth). Then, the conical flask was placed in a 30℃ constant temperature shaker, the speed was set to 150 r / min, and the shaker was incubated for 12 h. During the incubation process, the culture was observed every 3 h. When the culture medium showed uniform flocculent turbidity (indicating that bacteria multiply in large quantities) and the color was light milky white, the incubation was stopped, and sample II (Bacillus pasteurellis bacterial solution) was obtained.

[0040] Step 4: Multi-round grouting reinforcement using a peristaltic pump. The grouting rate was set at 1.5 mL / min (when the rate is <1 mL / min, the grouting efficiency is low, and bacteria are prone to premature precipitation; when the rate is >2 mL / min, the grout impact force is large, easily causing coral sand particles to shift and pores to become blocked); four rounds of grouting were performed on sample I prepared in Step 1 (multi-round grouting can accumulate calcium carbonate precipitation, improving the reinforcement effect). Each round of grouting followed the process of "1 application of bacterial solution + 4 applications of cementing solution": First, 1.5 L / m² of sample II (bacterial solution) was evenly injected into sample I using a peristaltic pump. During grouting, the peristaltic pump needle was inserted into the top of the sample, and the needle was slowly moved to ensure that the bacterial solution covered the entire sample cross-section; After the bacterial solution is allowed to stand for 3 hours (too short a standing time will not allow the bacteria to fully adhere to the sand particles, while too long a standing time will easily lead to bacterial inactivation; 3 hours will allow the bacteria to adhere to the sand particles and fiber surfaces at a rate of over 80%), the cementing solution is injected in 4 intervals, with each injection volume being 0.75 L / m² (total volume 3 L / m², with a bacterial solution volume ratio of 2:1 to match the urea decomposition rate), and each injection is spaced 4 hours apart (to ensure that the cementing solution injected in the previous injection reacts fully with the bacteria and to avoid the accumulation of unreacted urea and calcium chloride); after each round of grouting is completed, the sample is placed in a curing chamber at 25-30℃ and 70% humidity for 24 hours to ensure the stable growth of calcium carbonate precipitation.

[0041] Step 5: After each round of grouting, cure in an environment of 25~30℃ and 70% humidity for 24 hours, and repeat grouting until the preset reinforcement effect is achieved. After each batch of grouting is completed and the sample curing stage begins, the molding mold for loading the sample is flipped. During adjacent curing cycles, the molds are placed in opposite directions to ensure that the calcium carbonate generated by the microbial mineralization reaction is evenly distributed inside the coral sand-coconut shell fiber composite filler.

[0042] Comparative Example 1

[0043] The comparison is made of untreated coral sand.

[0044] Comparative Example 2

[0045] This comparison example uses coral sand reinforced with MICP. The specific steps are as follows:

[0046] S1. Design and prepare a cylindrical coral sand sample molding mold with convenient grouting function and quick disassembly characteristics, measuring 40mm in diameter and 80mm in height. The preparation method is the same as that of this invention: a thin iron sheet with a thickness of 0.1mm is selected and cut into rectangular blanks with a length of 130mm and a width of 80mm, which are rolled into a cylindrical mold body with an inner diameter of 40mm and a height of 80mm. The joint is sealed with engineering-grade sealant; a silicone sealing plug with a diameter of 40mm (with an internal capillary quartz tube forming a grouting channel) is assembled, and the gap between the sealing plug and the inner wall of the mold is sealed and fixed. S2. Select full-size coral sand, without pretreatment such as spraying hydrolysate, stirring, or drying, and directly fill it into the mold in three layers. Each layer is vertically hammered 15 times with a 1kg hammer to ensure uniform density, resulting in sample I (without coconut fiber).

[0047] S3. Activate and expand the culture of Bacillus pasteurellii lyophilized powder. The preparation method is the same as that of this invention: the culture medium consists of 10 g / L ammonium chloride, 10 mg / L manganese sulfate monohydrate, 24 mg / L nickel chloride hexahydrate, and 20 g / L yeast extract. Adjust the pH to 8.5 and sterilize at 121℃ and 0.5 MPa for 30 minutes. Inoculate 1-2 mL of refrigerated bacterial culture under aseptic conditions and culture in a constant temperature shaker at 30℃ and 150 r / min for 12 h to obtain sample II (bacterial solution).

[0048] S4. Use a peristaltic pump to perform 4 rounds of grouting on sample I. The grouting parameters are consistent with the present invention: grouting rate 1.5 mL / min, inject 150 mL of sample II (bacterial solution) in each round, let stand for 2 hours, and then inject 1 mol / L cementing solution (urea and calcium chloride of equal concentration) in 4 times, 150 mL each time, with an interval of 8 hours.

[0049] S5. After each round of grouting, the mold is placed in an environment of 25~30℃ and 70% humidity for 24 hours for curing. During the curing period, the mold is turned over in adjacent cycles and grouting is repeated until the preset reinforcement effect is achieved. The curing method is the same as that of this invention.

[0050] Comparative Example 3

[0051] The comparison ratio is MICP + 0.5% untreated coconut fiber + untreated coral sand. The specific steps are as follows:

[0052] S1. The design and preparation of the cylindrical coral sand sample molding die are the same as those in Example 1, ensuring that the die structure, size and sealing performance are consistent.

[0053] S2. Coconut shell fiber without directional pretreatment: Fresh waste coconut shells were directly cut to a length of 10-15mm using a guillotine without soaking in water to soften, separating the wood by beating, rinsing with deionized water, drying at 80℃, or soaking in hydrolysate. It was then used as 0.5% of the coral sand mass for later use. Full-size coral sand was selected and mixed directly with untreated coconut shell fiber without pretreatment such as spraying with hydrolysate, stirring, or letting it stand to dry. After stirring until there was no obvious agglomeration, it was placed into a mold in three layers. Each layer was vertically struck 15 times with a 1kg hammer to obtain sample I.

[0054] S3. The preparation of Bacillus pasteurellium culture (sample II) was carried out in strict accordance with step S3 of this invention, with consistent culture medium formulation, sterilization conditions and culture parameters.

[0055] S4. Four rounds of grouting are performed using a peristaltic pump. The parameters such as grouting rate, amount of bacterial solution and cementing solution, and injection interval are performed in strict accordance with step S4 of this invention (grouting rate 1.5 mL / min, cementing solution concentration 1 mol / L).

[0056] S5. The maintenance conditions and operations shall be carried out in accordance with the steps of S5 of this invention: 25~30℃, 70% humidity environment for 24h / cycle, and the mold shall be turned over between adjacent maintenance cycles.

[0057] Comparative Example 4

[0058] The comparison ratio is MICP + 0.5% pretreated coconut shell fiber + pretreated coral sand. The specific steps are as follows:

[0059] S1. Design and prepare a cylindrical coral sand sample molding die, as shown in Example 1.

[0060] S2. Coconut shell fiber pretreatment: Follow the directional process of this invention (soaking in clean water for 48 hours → pounding and separating → sieving to remove impurities → rinsing with deionized water → drying at 80℃ for 12 hours → cutting to 10~15mm → sealing and storing). Immerse the coconut shell fiber in KH550 hydrolysate at a volume ratio of 0.5% for 2-10 hours, then wash and dry. Select full-size coral sand, spread it evenly to a thickness of 1~2cm, and spray the hydrolysate (silane coupling agent KH550) in a "Z" shaped path. The solution is 8~12 mmol / L, glacial acetic acid 8.75~17.5 mmol / L, and deionized water 51.6~54.4 mol / L. It is sprayed in 2~3 times with an interval of 1 minute between each spray. After spraying, it is stirred evenly, left to stand for 1 hour, and then dried at 80℃ for 6 hours. The pretreated full-size coral sand is mixed with the pretreated coconut shell fiber and stirred until there is no agglomeration. It is then placed into a mold in three layers, and each layer is hammered 15 times with a 1kg hammer to obtain sample I.

[0061] S3. Preparation of bacterial culture: The culture medium formula is ammonium chloride 10g / L, manganese sulfate monohydrate 10mg / L, nickel chloride hexahydrate 24mg / L, yeast extract 20g / L. The pH is adjusted to 8.5, and sterilized at 121℃ and 0.5MPa for 30 minutes. 1~2mL of refrigerated bacterial culture is inoculated in an aseptic environment and cultured in a constant temperature shaker at 30℃ and 150r / min for 12h to obtain sample II.

[0062] S4. Multi-round grouting: The peristaltic pump grouting rate is 1.5 mL / min, and a total of 4 rounds of grouting are carried out. In each round, 150 ml of sample II is injected first, and after standing for 2 hours, 1 mol / L cementing solution (a mixture of urea and calcium chloride of equal concentration) is injected in 4 times, with 150 ml injected each time and an interval of 8 hours.

[0063] S5. Curing: After each round of grouting, place the mold in an environment of 25~30℃ and 70% humidity for 24 hours. In adjacent curing cycles, turn the mold over and repeat the grouting until the preset reinforcement effect is achieved.

[0064] Characterization

[0065] Electron microscopy was performed on the coral sand samples prepared in Comparative Examples 1-4 and Example 1. The results are shown in Figures 3-5. Figure 3 It is evident that the unreinforced coral sand sample contains numerous pores and exhibits weak interparticle cohesion, resulting in a loosely aggregated structure. This structure makes it prone to particle rearrangement under external loads and seepage, ultimately leading to structural instability. Figure 4 As can be seen from the figure, after only MIP reinforcement of coral sand, calcium carbonate crystals were generated on the surface of the coral sand particles and in the pores between the particles through microbial-induced calcium carbonate precipitation reaction. However, the crystal morphology is irregular, with some exhibiting blocky agglomeration distribution, mainly concentrated on the surface layer and large pore areas of the coral sand. The filling rate of deep small pores is less than 40%, indicating a clear reinforcement blind spot of "surface enrichment and deep deficiency". The overall structure only forms a binary cemented structure of "coral sand particles - calcium carbonate". The interparticle interface bonding relies solely on the physical stacking and contact of calcium carbonate crystals, without chemical bonding reinforcement. The structure is unevenly dense, and the calcium carbonate crystals themselves exhibit rigid and brittle characteristics. The cemented system lacks toughness support, which poses a hidden danger for sudden brittle disintegration under subsequent load. Figure 5The image shows the reinforced modified coral sand and modified coconut shell fiber. As can be seen from the figure, after modification and MIP treatment, the surface of the modified coconut shell fiber is covered with a uniform silane film, and calcium carbonate crystals are firmly anchored on the fiber surface and in the pores. The micropores on the surface of the modified coral sand are partially filled by the silane film, and calcium carbonate fills the pores of the sand particles, with a filling rate of over 80% and no surface enrichment problem. The composite system forms a three-dimensional network structure of "coral sand-silane-calcium carbonate-silane-coconut shell fiber". There are no interfacial gaps between the fibers intersecting the sand particles. Under stress, the fibers are easily pulled out rather than broken. Compared with the simple MIP or fiber-reinforced system, its structural density and interfacial adhesion are greatly improved.

[0066] Unconfined compressive strength tests were conducted on the coral sand standard specimens prepared in Comparative Examples 2-4 and Example 1. Their stress-strain curve characteristics and properties are as follows: Figure 6 The four curves are shown in the middle. The results show that only the coral sand sample treated with MICP (such as...) Figure 6 As shown in curve A), the curve exhibits typical brittle failure characteristics, with a peak stress of only about 600 kPa and a peak strain of less than 7%. After the peak, the stress rapidly decays; at 7% strain, the stress has dropped to around 250 kPa. After failure, the load-bearing capacity is drastically lost. Coral sand reinforced only with MIP exhibits low strength and poor ductility. When using MIP + 0.5% untreated coconut shell fiber (such as...) Figure 6 As shown in the B curve, the curve demonstrates the initial reinforcing and toughening effect of the fiber, with the peak stress increasing to approximately 1200 kPa (100% higher than the MICP-only group), and the strain range expanding to 12%. After the peak stress, the stress decay rate slows down, and the sample still maintains a load-bearing capacity of approximately 300 kPa at 12% strain. The sample transitions from brittle fracture to ductile deformation, but the interfacial bonding between the untreated fiber and the mineralized system is insufficient, limiting the performance improvement. When using MICP + 0.5% pretreated coconut shell fiber (such as...), Figure 6 As shown in the C-curve, the curve demonstrates the strengthening effect of silane pretreatment on interfacial adhesion. The peak stress further increases to approximately 1600 kPa (167% higher than the MIP-only group), while the strain range remains at 12%. The stress decay after the peak becomes more gradual, maintaining a stress level of approximately 600 kPa at 10% strain. The chemical bonding between the pretreated fibers and calcium carbonate improves the reinforcing efficiency, resulting in simultaneous improvement in strength and ductility. When using MICP + 0.5% pretreated fibers - pretreated coral sand synergistic modification (e.g.) Figure 6As shown in the D curve, the curve presents the best performance among the four groups of tests: the peak stress is close to 1900 kPa (217% higher than the MIP-only group), the strain range is extended to more than 16%, the stress decay after the peak is the most gradual, and the stress level is still maintained at about 600 kPa at 16% strain. At this time, the dual interface modification of silane on fiber and coral sand realizes the synergistic bonding and bridging of "fiber-calcium carbonate-coral sand", which maximizes the improvement of compressive strength and gives the sample excellent ductility and toughness, completely making up for the defects of simple MIP or untreated fiber reinforcement.

[0067] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention, and not to limit them. The basic principles and main features of the present invention have been described above with specific implementation schemes. Based on the present invention, some modifications or substitutions can be made, but these modifications or substitutions do not cause the essence of the corresponding technical solutions to deviate from the scope of protection claimed by the present invention.

Claims

1. A method for reinforcing coral sand based on MICP-coconut shell fiber combined reinforcement, characterized in that, Includes the following steps: S1. Immerse coconut shell fiber in KH550 hydrolysate at a volume ratio of 0.2-0.8% for 2-10 hours, then wash and dry. Select full-size coral sand, spread it out, and spray it with KH550 hydrolysate at a ratio of 1% to 2% of the sand mass. Stir and repeat 1-5 times, then let it stand and dry. S2. Mix the coconut shell fiber and coral sand to form a mixture, and place the mixture into a mold in 2-5 layers and hammer it. S3. Activate the freeze-dried powder of Bacillus pasteurellii and carry out large-scale culture to form a bacterial solution; S4. Use a peristaltic pump to perform 2-5 rounds of grouting on the mixture in the mold. In each round of grouting, inject bacterial solution once, let it stand, and then inject cementing solution 3-5 times. The cementing solution is a mixture of urea solution and calcium chloride solution.

2. The method according to claim 1, characterized in that, In step S1, the KH550 hydrolysate consists of 100-150 mmol / L of KH550 silane coupling agent, 8-15 mmol / L of glacial acetic acid, and 93-98 mol / L of deionized water.

3. The method according to claim 1, characterized in that, In step S1, the length of the coconut shell fiber is 10~15mm, and / or the coconut shell fiber is made by soaking fresh coconut shell filaments in KH550 hydrolysate after removing sediment, cutting, drying, and rinsing the fiber with deionized water until the pH of the rinsing solution stabilizes at 6.5~7.5, and then drying the fiber in an 80℃ oven for 8-24 hours.

4. The method according to claim 1, characterized in that, In step S1, the coral sand is evenly spread in a tray with a thickness of 1-2 cm. Holding a sprayer, the sprayer is kept 20-30 cm away from the surface of the sand layer. The hydrolysate is sprayed at a uniform speed in a "Z" pattern. The total amount is completed in 2-3 sprays with an interval of 0.2-5 minutes between each spray. The sprayer is stirred immediately after each spray. After stirring, the mixture is left to stand for 0.2-5 hours and then placed in an oven at 60-100℃ for 4-8 hours.

5. The method according to claim 1, characterized in that, In step S2, the mold preparation method is as follows: a thin iron sheet is cut into a rectangle, and the rectangular thin iron sheet is formed into a cylindrical mold body through a rolling process. Engineering-grade sealant is used to bond and seal the joint of the thin iron sheet. A cylindrical silicone sealing plug with a diameter matching the inner diameter of the mold is selected. A through hole is machined at the midpoint of the axis of the silicone sealing plug using a drilling tool with a hole diameter of 2-10mm. It is ensured that the axis of the through hole is parallel to the axis of the silicone sealing plug and that the two ends of the through hole are flush with the end face of the sealing plug. A capillary quartz tube is installed and fixed in the through hole of the silicone sealing plug, so that one end of the capillary quartz tube is flush with one end face of the silicone sealing plug, and the other end protrudes 20-30mm from the other end face of the silicone sealing plug, forming a special grouting channel. The pre-made silicone sealing plug with capillary quartz tube is first assembled at one end opening of the mold body. The gap between the sealing plug and the inner wall of the mold is sealed and fixed using engineering-grade sealant.

6. The method according to claim 1, characterized in that, In step S2, the amount of coconut shell fiber in the mixture is 0.1-2 wt%.

7. The method according to claim 1, characterized in that, In step S3, the culture medium used for the culture includes: ammonium chloride 5-15 g / L, manganese sulfate monohydrate 8-12 mg / L, nickel chloride hexahydrate 20-30 mg / L, and yeast extract 15-25 g / L.

8. The method according to claim 1, characterized in that, In step S4, the grouting rate is 1 ml / min to 2 ml / min; the cementing solution is prepared by mixing urea solution and calcium chloride solution at equal concentrations, and the concentration of the cementing solution is 0.5 mol / L to 1.5 mol / L.

9. The method according to claim 1, characterized in that, In step S4, the cementing solution is prepared by mixing urea solution and calcium chloride solution at equal concentrations, with a concentration of 1 mol / L; and / or, in each round of grouting, a bacterial solution is injected once, allowed to stand for 1-5 hours, and then the cementing solution is injected 3-5 times, with an interval of 6-10 hours between each injection.

10. The method according to claim 1, characterized in that, In step S4, after each batch of grouting work is completed, the sample curing stage begins, and the molding mold for loading the sample is flipped. During adjacent curing cycles, the placement of the mold is reversed.

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