A method for reinforcing sandy soil by utilizing cement hydration and biocement bonding

Abstract

The present invention belongs to the technical field of novel bio-building materials, specifically a method for utilizing cement hydration to coordinate bio-cement bonding and reinforcement of sandy soil. The method comprises inoculating urease bacteria into a culture medium and cultivating the bacteria solution; preparing a bonding solution that matches the urease bacteria; uniformly mixing silicate cement and sand, and filling the mixture into a mold to obtain a sand column blank; and alternately spraying the bacteria solution and bonding solution from the top of the sand column blank to carry out a microbial mineralization reaction, thereby obtaining a molded sand column. The present invention utilizes silicate cement hydration to coordinate bio-cement to fully fill the gaps between sand particles, forming a high-strength composite bonding system. This method significantly improves the compressive strength of sandy soil, and is particularly capable of improving the insufficient strength of sandy soil bonded by a single bio-grouting method. It provides a strong guarantee for the safety and stability of fields such as foundation reinforcement and structural defect repair, and has significant academic innovation and engineering application value.

Description

Technical Field

[0001] The invention belongs to the technical field of novel biological building materials, and particularly relates to a method for reinforcing sandy soil by utilizing cement hydration in coordination with biocement. Background Art

[0002] The large-scale production and use of traditional cement faces challenges such as high energy consumption and high carbon emissions, exacerbating global warming and severely damaging the ecosystem. Green and environmentally friendly alternatives are urgently needed. Biocement, produced using microbial-induced calcium carbonate deposition (MICP) technology, is an environmentally friendly bio-building material that is gaining widespread attention as an alternative to industrial cement. Currently, a large number of research projects are underway to promote the application of biocement as a replacement for traditional cement in engineering projects, such as anti-seepage construction in solid waste landfills, embankment reinforcement, soil improvement, and desertification control.

[0003] However, although single biocement has significant environmental and biocompatibility advantages, it generally faces the problem of low mechanical properties, which is closely related to the yield and crystal structure of its mineralized products. The bonding strength of biocement comes from the stable and continuous formation of calcium carbonate, which is highly dependent on the effective adsorption of bacteria as nucleation sites on the surface of loose sand particles, but the lack of nutrients in the sand environment makes it difficult for bacteria to remain active and effectively adsorb on the sand surface. Patent application number 202311351995.1 provides a method and experimental equipment for the reinforcement and repair of Pb-contaminated sludge using nano-SiO2 synergistic MICP technology, and uses nano-SiO2 synergistic MICP technology to reinforce and repair heavy metal-contaminated soil. Patent application number 201811572106.3 provides a method for the combined modification of sand by microbial solidification and fiber reinforcement. Patent application number 202310243215.5 provides a grouting method for synergistically reinforcing large-scale weak interlayer-matrix composites based on MICP-FR. This method utilizes MICP technology in conjunction with fiber reinforcement to strengthen the soil. However, the production costs of most nanomaterials and fiber materials are relatively high. Patent application number 202210361510.6 provides a method for improving biocement-bonded sand using circulating fluidized bed fly ash. This method utilizes fly ash to improve the biocementation process and enhance the mechanical properties of the sand column, but its compressive strength remains limited.

[0004] To address the challenges of low biocement strength, high calcium source costs, and expensive additives, the application of Portland cement in MICP technology holds promise for addressing these shortcomings. However, the Portland cement mix ratio and the microbial and cementing fluid spraying process directly influence the microbial nucleation process and the amount of nano-calcium carbonate produced, which in turn affects the mechanical properties of the resulting sand. Therefore, it is necessary to develop a method that utilizes cement hydration in conjunction with biocement to reinforce sand and address these issues. Summary of the Invention

[0005] The purpose of the present invention is to provide a method for reinforcing sand soil by utilizing the hydration of cement and the coordinated use of biocement. The method utilizes the hydration of silicate cement and the coordinated use of biocement to fully fill the gaps between sand particles, improve the mechanical properties of sand soil after bonding, reduce its permeability, and solve the problems of low mechanical properties of sand soil treated by a single biological grouting method and high calcium source cost.

[0006] To achieve the above object, the present invention provides a method for reinforcing sandy soil by utilizing cement hydration in conjunction with biocement, comprising the following steps:

[0007] Step 1: Prepare a bacterial solution containing urease bacteria;

[0008] Step 2: preparing a cementing fluid containing calcium chloride and urea;

[0009] Step 3: Mix the Portland cement and sand evenly and fill them into a mold to obtain a sand column blank;

[0010] Step 4: spraying the bacterial solution and the cementing solution from the top of the sand column blank in sequence to carry out microbial mineralization reaction to obtain a formed sand column.

[0011] On the one hand, combined with the characteristics of high strength, high calcium content and fine particles of silicate cement, it can serve as a calcium source for MICP and provide nucleation sites for biomineralization, thereby improving the bonding strength of biocement; on the other hand, MICP technology enables the continuous generation of nano-calcium carbonate during the hydration process of silicate cement, further refining the micropores in the concrete and changing the internal structure of the calcium silicate hydrate (CSH) gel, thereby significantly reducing the permeability of the concrete and improving the mechanical properties and durability of the concrete.

[0012] Furthermore, the mass ratio of Portland cement to sand is (3-10):100, preferably (5-8):100. The Portland cement and sand are evenly mixed, filled into a cylindrical mold, and vibrated and compacted to form a sand column blank. The sand column blank has a porosity of 15%-55%, preferably 18%-50%. The sand column blank does not require prior water curing, resulting in a more uniform pore structure, which facilitates the subsequent flow of bacterial solution and cementing liquid, thereby improving the cementing and curing effect.

[0013] Furthermore, the mass fractions of the components in the Portland cement are: SiO2≥28%, Al2O3≥10%, CaO≥44%, Fe2O3≥4%, MgO3≥5%, SO3≥5%, and others≤4%;

[0014] And / or, the sand is medium-grade sand with a particle size of ≤1 mm and an apparent density of 2.65 g / cm 3 After cleaning, place it in an oven at 105°C for drying before use.

[0015] Furthermore, the urease-producing bacteria is Sporosarcina pasteurii, and the optical density value OD of the bacterial solution obtained by culture is 600 is 0.8-0.9, and urease activity is ≥15mM / min.

[0016] Furthermore, the composition formula of the culture medium is: 20 g / L yeast extract, 10 g / L ammonium sulfate, 1 mL / L mixed solution, and the pH value of the culture medium is adjusted to 8.5 with 1 M NaOH; the solvent is water;

[0017] The mixed solution contains 10 g / L of manganese sulfate monohydrate and 24 g / L of nickel chloride hexahydrate, and the solvent is water.

[0018] Furthermore, the binder fluid comprises 50-60 g / L of calcium chloride and 25-35 g / L of urea, preferably 55.5 g / L of calcium chloride and 30 g / L of urea.

[0019] Furthermore, the spraying of the bacterial liquid and the binder liquid includes 2-10 cycles of spraying, and each cycle of spraying includes: using a syringe to inject the bacterial liquid into the top of the sand column blank, and flowing through the sand column from top to bottom; then using a syringe to inject the binder liquid from the top of the sand column blank 3-4 times at preset intervals, and flowing through the sand column blank from top to bottom.

[0020] Furthermore, the amount of the bacterial solution injected each time is 1.0-1.2 times the initial pore volume of the sand column blank, and the amount of the cementing solution injected each time is 1.0-1.2 times the initial pore volume of the sand column blank; the initial pore volume is the difference between the accumulated volume of the sand column blank in the mold after compaction and the volume of the sand used.

[0021] Furthermore, the spraying of the bacterial solution and the cementing solution is performed for 5-8 cycles, more preferably 8 cycles, with one cycle of spraying performed every two days; the cementing solution is injected 4 times in each cycle of spraying;

[0022] The preset interval time is 10-11 hours; in each cyclic spraying, the time interval between the start of the bacterial liquid injection and the first injection of the cementing liquid is 2-3 hours.

[0023] The present invention also provides a biocement reinforced sand, which is prepared by any of the above methods for utilizing cement hydration in conjunction with biocement cementation to reinforce sand.

[0024] In general, the above technical solutions conceived by the present invention have the following technical advantages compared with the existing technology:

[0025] 1. The present invention provides a method for bonding sandy soil using cement hydration in conjunction with biocement. The hydration of Portland cement and biocement fully fill the gaps between sand particles, forming a high-strength composite bonding system. Portland cement, with its high calcium content and small particles, provides a calcium source for the MICP technique, generating CaCO3 crystals while also providing more nucleation sites for crystallization. Furthermore, MICP technology continuously supplies nano-calcium carbonate to the Portland cement, promoting the formation of a denser CSH. The bonding strength provided by the bio-CaCO3 and the interfacial gelling strength provided by the CSH significantly improve the compressive strength of the biocement-bonded sandy soil.

[0026] 2. The present invention optimizes the dosage based on the hydration of silicate cement. An appropriate amount of silicate cement generates more calcite and cooperates with the generation of CSH by providing an additional calcium source. The surface of hydrated calcium silicate is positively charged, providing a large number of nucleation sites for bacteria with negative charges on the surface, promoting the attachment and colonization of bacteria on the surface of hydrated calcium silicate. At the same time, bacteria secrete extracellular polymers in a living environment with gradually scarce nutrients, further enhancing the accumulation of negative charges on the bacterial surface and enhancing the bacterial surface's affinity for Ca. 2+ adsorption, especially Ca released by cement dissolution 2+ , ultimately significantly enhancing the mechanical properties of biocement.

[0027] 3. This method significantly improves the compressive strength of sand, especially the insufficient strength of sand treated by single bio-grouting method, providing strong guarantee for the safety and stability in fields such as foundation reinforcement and structural defect repair, and has significant academic innovation and engineering application value. BRIEF DESCRIPTION OF THE DRAWINGS

[0028] Figure 1 The diagrams are of the failure modes of the sand columns formed in Example 1, Example 3 and Comparative Example 1;

[0029] Figure 2 The XRD patterns of the sand columns formed in Example 1, Example 3 and Comparative Example 1 are shown;

[0030] Figure 3 TG spectra of the sand column samples of Examples 1 to 3 and Comparative Examples 1 to 2;

[0031] Figure 4 Partial SEM images of the sand column samples formed in Examples 1 to 3 and Comparative Example 1. DETAILED DESCRIPTION

[0032] In order to make the objectives, technical solutions and advantages of the present invention more clearly understood, the present invention is further described in detail below with reference to the following embodiments. It should be understood that the specific embodiments described herein are merely for the purpose of explaining the present invention and are not intended to limit the present invention. In addition, the technical features involved in the various embodiments of the present invention described below may be combined with each other as long as they do not conflict with each other.

[0033] Example 1

[0034] A method for reinforcing sandy soil by utilizing cement hydration in conjunction with biocement, comprising the following steps:

[0035] (1) Liquid culture medium was prepared according to the following formula: 20 g / L yeast extract, 10 g / L ammonium sulfate, 1 mL / L mixed solution (mixed solution was prepared as follows: 1 g manganese sulfate monohydrate, 2.4 g nickel chloride hexahydrate, 100 mL pure water), and the pH was adjusted to 8.5 by titration with 1 M NaOH.

[0036] (2) Prepare NH4-YE culture medium, inoculate pasteurian spore Sarcina into the culture medium, and obtain the optical density value OD of the culture medium. 600 is 0.8-0.9, and urease activity is ≥15mM / min.

[0037] (3) Composition of the cementing fluid: calcium chloride 55.5 g / L, urea 30 g / L, and solvent is water.

[0038] (4) A PVC pipe with an inner diameter of 40 mm, a wall thickness of 1.25 mm, and a height of 110 mm was selected as the mold. The plastic sheet was cut and lubricated evenly on both sides. The sheet was tightly fitted to the inner wall of the PVC pipe to facilitate subsequent demolding.

[0039] (5) Portland cement and medium-grade sand were mixed in a ratio of 5.3 g:100 g, filled into the cylindrical mold of step (4), and vibrated and compacted to form a sand column blank. After testing, the height of the sand column blank was 5.4 cm and its initial pore volume was 30 cm 3 (The height of the sand column blank is about 5.4cm, and the cross section of the mold is 12.56cm 2 , the volume of the container is calculated to be 12.56×5.4=67.82cm 3 ; The apparent density of sand is 2.65g / cm 3 , the sand used is 100g, then the total volume is 100÷2.65=37.74cm 3 The difference between the two is the initial pore volume of about 30cm 3 ).

[0040] (6) Use a small syringe to alternately inject the pasteurized spores of the Sarcina pastoris solution from step (2) and the cementing solution from step (3) into the top of the sand column. Gravity allows the sand column to penetrate and cause calcium carbonate deposition. Use a tray to collect the discharged bacterial solution and cementing solution. After injecting 30 ml of urease solution, inject 30 ml of urease cementing solution again at regular intervals. Repeat this process 4 times and it is considered one round. The entire grouting process needs to be repeated 8 times for 16 days to obtain a molded sand column. The specific cycle program is shown in Table 1.

[0041] Table 1 Circulation perfusion program of Sporosarcina pasteurii bacterial solution and cementing solution

[0042] Number of days content 1 9:00 first round of bacterial injection, 11:30 and 22:00 injection of cementing liquid 2 Fill the cementing liquid at 9:30 and 20:30 3 9:00 second round of bacterial injection, 11:30 and 22:00 injection of cementing liquid 4 Fill the cementing liquid at 9:30 and 20:30 5 The third round of bacterial injection at 9:00, and the cementing fluid at 11:30 and 22:00 6 Fill the cementing liquid at 9:30 and 20:30 7 The fourth round of bacterial injection at 9:00, and the cementing fluid at 11:30 and 22:00 8 Fill the cementing liquid at 9:30 and 20:30 9 The fifth round of bacterial injection at 9:00, and the cementing fluid at 11:30 and 22:00 10 Fill the cementing liquid at 9:30 and 20:30 11 9:00 sixth round of bacterial injection, 11:30 and 22:00 injection of cementing solution 12 Fill the cementing liquid at 9:30 and 20:30 13 The seventh round of bacterial injection at 9:00, and the cementing fluid injection at 11:30 and 22:00 14 Fill the cementing liquid at 9:30 and 20:30 15 9:00 eighth round of bacterial injection, 11:30 and 22:00 injection of cementing liquid 16 Fill the cementing liquid at 9:30 and 20:30

[0043] Example 2

[0044] The method is the same as that of Example 1, except that the perfusion process of the Sporosarcina pasteurianus liquid and the cementing liquid is repeated 4 times.

[0045] Example 3

[0046] The method is the same as that of Example 1, except that Portland cement and sand are mixed in a ratio of 15.9 g:100 g, filled into a cylindrical mold, and vibrated and compacted to form a sand column blank. The grouting process is repeated 4 times.

[0047] Comparative Example 1

[0048] The method is the same as that of Example 1, except that 100 g of sand is filled into a cylindrical mold, vibrated and compacted to form a sand column blank. The grouting process is repeated 8 times.

[0049] Comparative Example 2

[0050] The method is the same as that of Example 1, except that 100 g of sand is filled into a cylindrical mold and vibrated and compacted to form a sand column blank. The grouting process is repeated 4 times.

[0051] The formed sand columns of Examples 1 to 3 and Comparative Examples 1 to 2 were subjected to performance tests on calcium carbonate content and compressive strength. The test results are shown in Table 2.

[0052] Table 2 Calcium carbonate content and compressive strength of the sand columns formed in Examples 1 to 3 and Comparative Examples 1 to 2.

[0053] Sample Calcium carbonate content Compressive strength Example 1 68.86％ 7.92MPa Example 2 86.36％ 2.27MPa Example 3 14.16％ 0.87MPa Comparative Example 1 65.34％ 5.14MPa Comparative Example 2 52.27％ 1.42MPa

[0054] As shown in Table 2, the compressive strength of the sand columns formed in Examples 1 and 2 is significantly higher than that of Comparative Examples 1 and 2, indicating a significant synergistic effect between the interfacial gel strength provided by calcium silicate hydrate and the cementing strength imparted by biogenic calcium carbonate, which together improve the mechanical properties of the biocement. However, the confined compressive strength of the sand columns formed in Example 3 is lower than that of Comparative Example 2, indicating that the introduction of excessive Portland cement results in an excessively alkaline mineralization environment, which reduces the activity of Sporosarcina pasteurianus, restricts the biomineralization process, and reduces its cementation level. This shows that the present invention significantly improves the strength of sand by regulating the amount of Portland cement used and the cyclic injection process of the Sporosarcina pasteurianus bacterial solution and cementing solution.

[0055] Depend on Figure 1 It can be seen that during the compressive performance test, the sand column samples without adding silicate cement (Comparative Example 1) and adding 10% silicate cement as raw materials (Example 1) showed top failure; the sand column sample with 30% silicate cement (Example 3) as raw materials showed middle fault failure.

[0056] Depend on Figure 2 It can be seen that the biocement prepared without adding silicate cement (Comparative Example 1) and adding 10% silicate cement as raw material (Example 1) has a strong calcite diffraction peak in the XRD spectrum of the molded sample, and its main component is calcite-type CaCO3, followed by vaterite-type CaCO3; the biocement prepared by adding 30% silicate cement as raw material (Example 3) has only calcite-type CaCO3, and the diffraction peak is weak.

[0057] From Table 2 and Figure 3 (The test conditions are: heating from room temperature to 950°C at a heating rate of 10°C / min in an N2 atmosphere) It can be seen that the calcium carbonate content of the sand columns formed in Example 1 and Example 2 is greater than that in Comparative Example 1 and Comparative Example 2, respectively, indicating that in the process of preparing biocement using Portland cement as raw material, in addition to calcium chloride, Portland cement also provides a calcium source for biomineralization, generating more CaCO3; the calcium carbonate content of the sand column formed in Example 3 is significantly lower than that in Comparative Example 2 and Example 2, indicating that excessive addition of Portland cement will inhibit the generation of CaCO3 in biomineralization.

[0058] Depend on Figure 4It can be seen that the molded sand column of Example 1 exhibits the typical SEM morphological characteristics of calcite-type CaCO3 crystals, with a rhombohedral crystal structure and a smooth surface, and significant formation of calcium silicate hydrate (CSH) gel can be observed. The close arrangement of calcite crystals effectively fills the gaps between unhydrated silicate cement and sand particles, promoting close bonding between the two, thereby optimizing the overall pore structure and improving the mechanical properties of the material. CSH was also observed in the molded sand column of Example 3, but due to excessive silicate cement, the sand column was clogged prematurely, the number of grouting cycles was reduced, and bacterial activity was reduced, thereby limiting the production of biocement. This change causes the CSH gel in the pore structure to be unevenly distributed, mainly manifested in that most of the gel is only locally attached to the surface of the calcite crystals and fails to fully penetrate into the entire pore network, thereby affecting the overall density and mechanical properties of the material. The molded sand column of Comparative Example 1 did not add silicate cement, and only rhombohedral calcite crystals were formed. Under these conditions, the connection between the calcite crystals appears to be relatively loose, which is not conducive to the stability of the overall structure and the improvement of mechanical properties.

[0059] In summary, the present invention reveals the synergistic effect of silicate cement and the biomineralization process and its influence on the mechanical properties of the material by comparing the preparation of biocement with different silicate cement dosages. An appropriate amount of silicate cement significantly enhances the mechanical properties of biocement by providing an additional calcium source, generating more calcite and coordinating the production of CSH. However, too much silicate cement leads to excessive alkalinity, inhibits the activity of Bacillus pasteurianus, limits the biocementation process, and thus reduces the mechanical properties of the material and the density of the pore structure. The present invention has both biological calcium carbonate crystals and silicate cement hydration products, and the cementing properties are significantly improved, thereby significantly improving the compressive strength of sand, which has a positive effect on improving the safety and stability of engineering structures and is suitable for engineering promotion.

[0060] It will be easily understood by those skilled in the art that the above description is merely a preferred embodiment of the present invention and is not intended to limit the present invention. Any modifications, equivalent substitutions, and improvements made within the spirit and principles of the present invention should be included in the scope of protection of the present invention.

Claims

1. A method for reinforcing sandy soil by utilizing cement hydration in conjunction with biocement, characterized in that: The following steps are involved: Step 1: Prepare a bacterial solution containing urease bacteria; Step 2: preparing a cementing fluid containing calcium chloride and urea; the cementing fluid includes 50-60 g / L of calcium chloride and 25-35 g / L of urea; Step 3: Mix the silicate cement and sand evenly and fill them into a mold to obtain a sand column blank; the mass ratio of the silicate cement and sand is (3-10):100; Step 4: spraying the bacterial solution and the cementing solution from the top of the sand column blank in sequence to carry out microbial mineralization reaction to obtain a formed sand column.

2. The method for reinforcing sandy soil by utilizing cement hydration in conjunction with biocement according to claim 1, characterized in that: The mass ratio of the silicate cement to sand is (5-8):

100.

3. The method for reinforcing sandy soil by utilizing cement hydration in conjunction with biocement according to claim 1, characterized in that: The mass fractions of the components in the Portland cement are: SiO2≥28%, Al2O3≥10%, CaO≥44%, Fe2O3≥4%, MgO3≥5%, SO3≥5%, and others≤4%; And / or, the sand is medium-grade sand with a particle size of ≤1 mm.

4. The method for reinforcing sandy soil by utilizing cement hydration in conjunction with biocement according to claim 1, characterized in that: The urease bacteria is Sporosarcina pasteurii, and the optical density value OD of the bacterial solution is 600 is 0.8-0.9, and urease activity is ≥15 mM / min.

5. The method for reinforcing sandy soil by utilizing cement hydration in conjunction with biocement according to claim 1, characterized in that: The bacterial liquid is obtained by inoculating and culturing the urease bacteria in a culture medium; the composition formula of the culture medium is: 20 g / L yeast extract, 10 g / L ammonium sulfate, and 1 mL / L mixed solution; and the pH value of the culture medium is adjusted to 8.5 with NaOH; The mixed solution contains 10 g / L manganese sulfate monohydrate and 24 g / L nickel chloride hexahydrate, and the solvent is water.

6. The method for reinforcing sandy soil by utilizing cement hydration in conjunction with biocement according to any one of claims 1 to 5, characterized in that: The spraying of the bacterial solution and the cementing liquid includes 2-10 cycles of spraying, and each cycle of spraying includes: using a syringe to inject the bacterial solution into the top of the sand column blank, and flowing through the sand column from top to bottom; then using a syringe to inject the cementing liquid from the top of the sand column blank 3-4 times at preset intervals, and flowing through the sand column blank from top to bottom.

7. The method for reinforcing sandy soil by utilizing cement hydration in conjunction with biocement according to claim 6, characterized in that: In each spraying cycle, the amount of the bacterial solution injected each time is 1.0-1.2 times the initial pore volume of the sand column blank, and the amount of the cementing solution injected each time is 1.0-1.2 times the initial pore volume of the sand column blank.

8. The method for reinforcing sandy soil by utilizing cement hydration in conjunction with biocement according to claim 6, characterized in that: The bacterial solution and the cementing liquid are sprayed for 5-8 cycles, with one cycle of spraying every two days; the cementing liquid is injected 4 times in each cycle of spraying; The preset interval time is 10-11 hours; in each cyclic spraying, the time interval between the start of the bacterial liquid injection and the first injection of the cementing liquid is 2-3 hours.

9. A biocement reinforced sandy soil, characterized in that: The invention is prepared by the method for reinforcing sandy soil by utilizing cement hydration in coordination with biocement as described in any one of claims 1 to 8.

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