Method for consolidating sandy soil through cooperation of cement hydration effect and biological cement bonding

Abstract

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 and biological cement bonding, which comprises the following steps: inoculating urease bacteria into a culture medium for culturing to obtain a bacteria solution; preparing a cementing liquid matched with the urease bacteria; portland cement and sand are evenly stirred and filled into a mold, and a sand column blank is obtained; and spraying the bacterial liquid and the cementing liquid from the top of the sand column blank in turn to carry out a microbial mineralization reaction so as to obtain the molded sand column. According to the invention, the hydration effect of Portland cement is cooperated with biological cement to fully fill gaps of sand grains, so that a high-strength composite cementing system is formed. According to the method, the compressive strength of the sandy soil is remarkably improved, particularly, the defect that the strength of the sandy soil subjected to cementation treatment through a single biological grouting method is insufficient can be overcome, powerful guarantee is provided for safety and stability in the fields of foundation reinforcement, structural defect repair and the like, and remarkable academic innovation and engineering application value are achieved.

Description

Technical Field

[0001] The invention belongs to the technical field of novel biological building materials, and in particular relates to a method for utilizing cement hydration to coordinate biological cement bonding and reinforcement of sandy soil. Background Art

[0002] The mass production and use of traditional cement faces the challenges of high energy consumption and high carbon emissions, exacerbating climate warming and seriously damaging the ecology, and there is an urgent need for green and environmentally friendly alternatives. Biocement produced based on microbial induced calcium carbonate deposition (MICP) technology is an environmentally friendly biological building material, which has gradually attracted widespread attention as an alternative to industrial cement. At present, there have been a lot of research on the engineering promotion and application of biocement to replace traditional cement, such as anti-seepage construction of solid waste landfills, dam reinforcement, soil improvement, and land desertification control.

[0003] However, although a 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 nutrient-poor 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 combined microbial solidification-fiber reinforcement modification of sand. Patent application number 202310243215.5 provides a grouting method based on MICP-FR synergistic reinforcement of large-scale weak interlayer-parent rock complexes, and uses MICP technology to synergize fiber reinforcement to reinforce soil. However, the production cost of most nano-raw materials and fiber raw materials is relatively high. Patent application number 202210361510.6 provides a method for improving biocement-bonded sand using circulating fluidized bed fly ash, using fly ash to improve the biocementation process and improve the mechanical properties of sand columns, but its compressive strength is still limited.

[0004] In order to solve the problems of low strength of biocement, high cost of "calcium source" and expensive additives, the application of silicate cement in MICP technology is expected to make up for the shortcomings of biocement. However, the proportion of silicate cement and the spraying process of microorganisms and cementing liquid directly affect the microbial nucleation process and the amount of nano-calcium carbonate generated, which in turn affects the mechanical properties of the resulting sand. Therefore, it is necessary to develop a method for using cement hydration to coordinate biocement cementation to reinforce sand to solve the above problems. Summary of the invention

[0005] The purpose of the present invention is to provide a method for strengthening sand soil by utilizing cement hydration in coordination with biocement, utilizing the hydration of silicate cement in coordination with 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 strengthening sandy soil by utilizing cement hydration in coordination with biocement, comprising the following steps:

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

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

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

[0010] Step 4: sequentially spraying the bacterial solution and the cementing solution from the top of the sand column blank 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 be used 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 the silicate cement to the sand is (3-10):100, preferably (5-8):100. The silicate cement and the sand are mixed evenly, filled into a cylindrical mold, and then vibrated and compacted to form a sand column blank. The porosity of the sand column blank is 15%-55%, preferably 18%-50%. The sand column blank does not need to be cured by adding water first, and the pore structure is more uniform, which is conducive to the circulation of the subsequent bacterial liquid and the cementing liquid, thereby improving the cementing and curing effect.

[0013] Furthermore, the mass fraction of each component in the silicate cement is: 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 bacteria is Pasteurella sporosarcina, and the optical density value OD of the bacterial solution obtained by culture is 600 It is 0.8-0.9, and the 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 1M 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 solution comprises 50-60 g / L calcium chloride and 25-35 g / L urea. Preferably, the calcium chloride is 55.5 g / L and the urea is 30 g / L.

[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 at preset intervals 3-4 times, 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 liquid and the cementing liquid is performed for 5-8 cycles, more preferably 8 cycles, and one cycle of spraying is performed every two days; the injection number of the cementing liquid in each cycle of spraying is 4 times;

[0022] The preset interval time is 10-11 hours; in each cycle of 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 sandy soil, which is prepared by any of the above methods for utilizing cement hydration in coordination with biocement to cement and reinforce sandy soil.

[0024] In general, the above technical solution conceived by the present invention has the following technical advantages compared with the prior art:

[0025] 1. The method of utilizing cement hydration to coordinate biocement to bond sandy soil provided by the present invention utilizes the hydration of silicate cement to coordinate biocement to fully fill the gaps between sand particles and form a high-strength composite bonding system. Silicate cement has the characteristics of high calcium content and small particles, which provides a calcium source for MICP technology to provide more crystallization nucleation sites while generating CaCO3 crystals. In addition, MICP technology continuously provides nano-calcium carbonate to silicate cement, promoting the generation of denser CSH. The bonding strength provided by biological CaCO3 and the interfacial gelling strength provided by CSH significantly improve the compressive strength of biocement bonded sandy soil.

[0026] 2. The present invention optimizes the dosage of silicate cement based on its hydration. 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 calcium silicate hydrate is positively charged, providing a large number of nucleation sites for bacteria with negative charges on the surface, thereby promoting the attachment and colonization of bacteria on the surface of calcium silicate hydrate. 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 biological grouting method, providing strong guarantee for the safety and stability in the fields of foundation reinforcement and structural defect repair, and has significant academic innovation and engineering application value. BRIEF DESCRIPTION OF THE DRAWINGS

[0028] Figure 1 The diagram is a diagram of the failure forms of the formed sand columns of Example 1, Example 3 and Comparative Example 1;

[0029] Figure 2 The XRD patterns of the formed sand columns of 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 molded sand column samples of Examples 1 to 3 and Comparative Example 1. DETAILED DESCRIPTION

[0032] In order to make the purpose, technical scheme and advantages of the present invention clearer, the present invention is further described in detail below in conjunction with the embodiments. It should be understood that the specific embodiments described herein are only used to explain 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 can 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 coordination 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 medium, inoculate pasteurian spore Sarcina in the culture medium, and obtain the optical density value OD of the bacterial solution obtained by culture. 600 It is 0.8-0.9, and the urease activity is ≥15mM / min.

[0037] (3) Composition of the binder 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 lubricating oil was evenly applied on both sides thereof so that it fits tightly to the inner wall of the PVC pipe for subsequent demoulding.

[0039] (5) Mix silicate cement and medium-grade sand in a ratio of 5.3 g:100 g, fill them into the cylindrical mold of step (4), vibrate and compact them to make sand column blanks. After testing, the height of the sand column blanks is 5.4 cm and the initial pore volume is 30 cm 3 (Sand column blank height is about 5.4cm, mold cross section 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 inject the pasteurian spore Sarcina bacteria solution of step (2) and the cementing solution of step (3) into the top of the sand column in turn, and the calcium carbonate deposition reaction will occur by gravity penetrating the sand column. Use a tray to collect the discharged bacteria solution and cementing solution. After infusing 30 ml of urease bacteria solution, infuse 30 ml of urease bacteria cementing solution again at regular intervals. This process is repeated 4 times and is considered as one round. The entire grouting process needs to be repeated 8 rounds and continued 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 Bacillus pasteurianus 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 solution 2 9:30 and 20:30 fill the cementing liquid 3 9:00: Second round of bacterial injection, 11:30 and 22:00: injection of cementing liquid 4 9:30 and 20:30 fill the cementing liquid 5 9:00: The third round of bacterial injection, 11:30 and 22:00: injection of cementing liquid 6 9:30 and 20:30 fill the cementing liquid 7 9:00 the fourth round of bacterial injection, 11:30 and 22:00 injection of cementing liquid 8 9:30 and 20:30 fill the cementing liquid 9 9:00: Fifth round of bacterial injection, 11:30 and 22:00: injection of cementing liquid 10 9:30 and 20:30 fill the cementing liquid 11 9:00: Sixth round of bacterial injection, 11:30 and 22:00: injection of cementing liquid 12 9:30 and 20:30 fill the cementing liquid 13 9:00 seventh round of bacterial injection, 11:30 and 22:00 injection of cementing liquid 14 9:30 and 20:30 fill the cementing liquid 15 9:00 eighth round of bacterial injection, 11:30 and 22:00 injection of cementing liquid 16 9:30 and 20:30 fill the cementing liquid

[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 in Example 1, except that the silicate cement and sand are mixed in a ratio of 15.9 g:100 g, filled into a cylindrical mold, vibrated and compacted to form a sand column blank. The grouting process is repeated for 4 rounds.

[0047] Comparative Example 1

[0048] The method is the same as in 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 for 8 rounds.

[0049] Comparative Example 2

[0050] The method is the same as in 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 4 times.

[0051] The formed sand columns of Examples 1 to 3 and Comparative Examples 1 to 2 were tested for 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] From the data in Table 2, it can be seen that the compressive strength of the sand columns formed in Examples 1 to 2 is significantly higher than that in Comparative Examples 1 to 2, indicating that there is a significant synergistic effect between the interfacial gel strength provided by calcium silicate hydrate and the cementing strength provided by biological calcium carbonate, thereby jointly improving the mechanical properties of biocement. The lateral confined compressive strength of the sand column formed in Example 3 is lower than that in Comparative Example 2, indicating that the introduction of too much silicate cement leads to excessive alkalinity in the mineralization environment, resulting in low activity of Bacillus pasteurianus, limiting the biological mineralization process and reducing its cementation level. It can be seen that the present invention significantly improves the strength of sand by regulating the amount of silicate cement and the cyclic perfusion process of Bacillus 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 adding 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 a 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 and generates 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 formation of CaCO3 in biomineralization.

[0058] Depend on Figure 4It can be seen that the molded sand column of Example 1 shows the typical SEM morphological characteristics of calcite-type CaCO3 crystals, with a rhombohedral crystal structure and a smooth surface, and significant generation of hydrated calcium silicate (CSH) gel can be observed. The close arrangement of calcite crystals effectively fills the gaps between unhydrated silicate cement and sand particles, promotes the close combination 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 blocked prematurely, the number of grouting cycles was reduced, and the bacterial activity was reduced, thereby limiting the generation of biocement. This change makes the CSH gel in the pore structure 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 does not add silicate cement, and only rhombohedral calcite crystals are formed. Under this condition, 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 biomineralization process and its influence on the mechanical properties of materials 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 generation of CSH. However, too much silicate cement leads to excessive alkalinity, inhibits the activity of Bacillus pasteurianus, limits the biocementation process, and further 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 only 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 protection scope of the present invention.

Claims

1. A method for strengthening sandy soil by utilizing cement hydration and biocement bonding, characterized in that: The following steps are involved: Step 1: Prepare a bacterial solution containing urease bacteria; Step 2: preparing a cementing solution containing calcium chloride and urea; Step 3: Mix the silicate cement and sand evenly, and fill them into the mold to obtain a sand column blank; Step 4: sequentially spraying the bacterial solution and the cementing solution from the top of the sand column blank to carry out microbial mineralization reaction to obtain a formed sand column.

2. The method for strengthening sandy soil by utilizing cement hydration and biocement bonding according to claim 1, characterized in that: The mass ratio of the silicate cement to the sand is (3-10):100, preferably (5-8):

100.

3. The method for strengthening sandy soil by utilizing cement hydration and biocement bonding according to claim 1, characterized in that: The mass fractions of the components in the silicate 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 strengthening sandy soil by utilizing cement hydration and biocement bonding according to claim 1, characterized in that: The urease bacteria is Pasteurella sporosarcina, and the optical density value OD of the bacterial solution is 600 It is 0.8-0.9, and the urease activity is ≥15mM / min.

5. The method for strengthening sandy soil by utilizing cement hydration and biocement bonding according to claim 1, characterized in that: The method for obtaining the bacterial liquid is as follows: 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, 1 mL / L mixed solution, and adjusting the pH value of the culture medium to 8.5 with NaOH; The mixed solution contains 10 g / L of manganese sulfate monohydrate and 24 g / L of nickel chloride hexahydrate, and the solvent is water.

6. The method for strengthening sandy soil by utilizing cement hydration and biocement bonding according to claim 1, characterized in that: The binder solution includes 50-60 g / L of calcium chloride and 25-35 g / L of urea.

7. The method for reinforcing sandy soil by utilizing cement hydration in coordination with biocement according to any one of claims 1 to 6, characterized in that: The spraying of the bacterial liquid and the cementing 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 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.

8. The method for strengthening sandy soil by utilizing cement hydration in coordination with biocement according to claim 7, characterized in that: In each cycle of spraying, 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.

9. The method for strengthening sandy soil by utilizing cement hydration in coordination with biocement according to claim 7, characterized in that: The spraying of the bacterial liquid and the cementing liquid is circulated for 5-8 times, and the circulated spraying is performed once every two days; the injection times of the cementing liquid in each circulated spraying are 4 times; The preset interval time is 10-11 hours; in each cycle of spraying, the time interval between the start of the bacterial liquid injection and the first injection of the cementing liquid is 2-3 hours.

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

Images