Method for increasing the compressive strength of geopolymer based on urease-induced calcium carbonate precipitation

Abstract

The application provides a method for increasing the compressive strength of geopolymer based on urease-induced calcium carbonate precipitation, which comprises the following steps: S1, preparing a urease solution; S2, preparing a cementing solution; S3, preparing fly ash geopolymer; and S4, curing the fly ash geopolymer at room temperature. The method for increasing the compressive strength of geopolymer based on urease-induced calcium carbonate precipitation improves the compressive strength of fly ash geopolymer through the urease-induced calcium carbonate precipitation technology, and does not need complicated pretreatment steps, and is simple to operate and more efficient and convenient compared with traditional enhancement methods.

Description

TECHNICAL FIELD

[0001] The present application relates to the field of civil engineering or building materials, and particularly relates to a method for increasing the compressive performance of geopolymer based on urease-induced calcium carbonate precipitation. BACKGROUND

[0002] Geopolymer (GP) is a new type of green gel material, which was developed and named by French scientist Joseph Davidovits in 1978. It refers to an inorganic cementitious material with a three-dimensional network structure based on -Si-O-Al-O- structural units, which is generated by natural minerals or solid waste under the action of acid or alkali activator. The material has a wide range of sources and is inexpensive, such as kaolin, fly ash, blast furnace slag, rice husk ash, zeolite, clay, etc. Among them, the material with fly ash as the main raw material, which is generated by alkali activator to induce polymerization reaction and form a three-dimensional cross-linked structure, has the characteristics of wide raw material sources, low price and environmental friendliness. Therefore, fly ash-based geopolymer material has become an important green building material, and it has shown excellent potential in structural materials, waste solidification and durable buildings, etc. However, the compressive performance of geopolymer is still affected by the characteristics of its raw materials and preparation process, and improving its compressive strength is still an important research direction.

[0003] The existing methods for improving the compressive performance of geopolymer mainly include adjusting the preparation conditions, mechanical force chemical treatment, composite silicon aluminum calcium material, fiber reinforcement, organic modification and microbial induced calcium carbonate precipitation (MICP) and the like. The method for adjusting the preparation conditions includes, for example, optimizing the concentration of the activator, increasing the curing temperature and the like, the mechanical force chemical treatment method includes, for example, reducing the particle size, increasing the specific surface area and the reaction activity by ball milling and the like, the method for composite silicon aluminum calcium material includes, for example, supplementing active elements to form a dense three-dimensional network structure, adding calcium oxide or silica ash, the method for fiber reinforcement includes, for example, adding natural or synthetic fibers to inhibit crack propagation, such as polyvinyl alcohol fibers and steel fibers can improve the bending strength and toughness, the method for organic modification includes, for example, adding polyethylene glycol, epoxy resin and the like to promote the reaction rate and form a dense network structure and the like. Although these traditional methods can improve the compressive performance to a certain extent, they have many problems and shortcomings. On the one hand, they generally have the problems of low efficiency, high cost and complex operation, and it is often difficult to realize efficient improvement of the compressive performance while ensuring low cost. In addition, microbial induced calcium carbonate precipitation (MICP) as a new technology has attracted attention, but it also has its own shortcomings. MICP needs to cultivate specific microorganisms, and the cultivation environment needs to be precisely controlled, including temperature, pH value, nutrient concentration and the like, which makes the cultivation process complex and prone to failure. Moreover, the whole reaction cycle of MICP is long, and a large amount of time is needed from microbial cultivation to the completion of calcium carbonate precipitation induced by urease, resulting in low efficiency, in addition, the steps are complicated, involving microbial cultivation, urease induction, calcium carbonate precipitation control and the like, and any problem in any link may affect the final effect. Therefore, it is necessary to further develop a new green and efficient technology for changing the compressive performance of geopolymer. SUMMARY

[0004] The purpose of the present application is to at least partially overcome the defects of the prior art, and to provide a method for increasing the compressive performance of geopolymer based on urease induced calcium carbonate precipitation.

[0005] The purpose of the present application is also to provide a method for increasing the compressive performance of geopolymer based on urease induced calcium carbonate precipitation, which is simple to operate and has low cost.

[0006] The purpose of the present application is also to provide a method for increasing the compressive performance of geopolymer based on urease induced calcium carbonate precipitation, which has high environmental friendliness and resource utilization efficiency.

[0007] In order to achieve the above-mentioned purpose or one of the purposes, the technical solution of the present application is as follows:

[0008] A method for increasing the compressive performance of geopolymer based on urease induced calcium carbonate precipitation, the method comprising:

[0009] Step S1: preparing a urease solution;

[0010] Step S2: preparing a cementing solution;

[0011] Step S3: preparing a fly ash-based polymer;

[0012] Step S4: curing the fly ash-based polymer at room temperature.

[0013] According to a preferred embodiment of the present application, the step S1 comprises:

[0014] A predetermined weight of soybeans is weighed and put into a pulverizer for pulverization, and the pulverized soybean powder is sieved to obtain soybean powder with uniform particle size;

[0015] The soybean powder and deionized water are mixed in a predetermined mass-volume ratio, and a magnetic stirrer is used for stirring, and after stirring, the soybean powder is allowed to settle for 2-3 hours, and the urease is released to the upper solution;

[0016] The supernatant after standing is poured into a centrifuge tube, and centrifuged at 3000 r / min and 4℃ for 15 minutes, and the suspended impurities are removed by centrifugation to obtain the supernatant, which is the urease extract.

[0017] According to a preferred embodiment of the present application, the urease extract is stored in a 4℃ environment to avoid the risk of enzyme inactivation caused by temperature fluctuations.

[0018] According to a preferred embodiment of the present application, the step S2 comprises:

[0019] A certain amount of urea is weighed and dissolved in deionized water, and stirred until completely dissolved;

[0020] Calcium chloride is also weighed and dissolved in deionized water to form a 1 mol / L calcium chloride solution;

[0021] The urea solution and the calcium chloride solution are mixed in a volume ratio of 1:1 to form the cementing solution.

[0022] According to a preferred embodiment of the present application, the step S3 comprises:

[0023] Fly ash and water glass are weighed, wherein the modulus of the water glass is 1.4, and the ratio of the water glass to the fly ash is 0.7, and the water glass and the fly ash are pre-mixed in a stirrer, and stirred to uniformly mix the water glass and the fly ash;

[0024] The prepared cementing solution and urease solution are added in sequence, and stirring is continued to obtain a uniform slurry;

[0025] The slurry is poured into a mold, and the bottom of the mold is tapped to remove air bubbles.

[0026] According to a preferred embodiment of the present application, in step S3, a plurality of groups of slurry samples are prepared, each group of slurry samples comprising three samples, and the plurality of groups of slurry samples having different urease concentrations.

[0027] According to a preferred embodiment of the present application, the step S4 comprises:

[0028] The fly ash geopolymer is subjected to a preliminary heat curing process in a 40℃ oven for 24h, and then demolded to obtain the fly ash geopolymer, which is then packaged in a sealed bag to ensure isolation from the external environment, and then placed in a normal temperature environment for further curing treatment.

[0029] According to a preferred embodiment of the present application, different curing times are set for different fly ash geopolymer samples, including at least 7 days, 14 days and 28 days, so as to evaluate and detect the mechanical properties of the fly ash geopolymer at different time periods.

[0030] According to a preferred embodiment of the present application, the method further comprises:

[0031] Step S5: Perform compression performance test of the fly ash geopolymer.

[0032] According to a preferred embodiment of the present application, the compression performance test of the fly ash geopolymer is performed by using an electro-hydraulic servo universal testing machine, and before the test, the cured samples are taken out of the sealed bag, and the samples are placed one by one into the testing machine, and the pressure is slowly applied according to the preset loading speed, and the failure stress and maximum bearing capacity of the sample are recorded.

[0033] According to a preferred embodiment of the present application, the method further comprises:

[0034] Step S6: Perform characterization test of the fly ash geopolymer.

[0035] According to a preferred embodiment of the present application, the cured samples are characterized by using Fourier transform infrared spectroscopy and scanning electron microscopy, including: first, perform Fourier transform infrared spectroscopy test, grind the cured samples into powder and measure the spectrum to confirm the bonding between the calcium carbonate generated by the urease induced reaction and the geopolymer matrix; then, use scanning electron microscopy to observe the microstructure and pore filling of the sample to confirm the distribution effect of the calcium carbonate precipitation.

[0036] The present application has the following beneficial effects compared with the prior art:

[0037] (1) The urease-induced calcium carbonate precipitation technology enhances the compressive performance of fly ash-based geopolymer without the need for complex pretreatment steps, making it simple to operate and more efficient and convenient than traditional reinforcement methods. This technology is widely applicable and can significantly enhance the mechanical strength of materials, making it suitable for various environmental conditions.

[0038] (2) The urease-induced calcium carbonate precipitation technology deposits calcium carbonate in geopolymer, not only enhancing the compressive performance of the material, but also realizing the reuse of waste resources (such as fly ash), with low energy consumption, low carbon dioxide emissions, and no secondary waste, meeting the green and environmental protection requirements.

[0039] (3) In the process flow, the invention optimizes the reaction conditions of urease-induced calcium carbonate precipitation by reasonably matching urease, urea, and calcium source, forming a stable composite structure between the generated calcium carbonate precipitate and the geopolymer matrix, effectively filling the material pores while enhancing the compressive performance.

[0040] (4) Compared to traditional reinforcement techniques that require high-temperature treatment, the invention uses a normal temperature curing process, not only reducing the damage to the geopolymer matrix structure, but also reducing energy consumption, making it stable in various environmental conditions.

[0041] In summary, this invention exhibits excellent advantages in environmental protection, efficiency, durability, and cost control, providing an innovative solution for sustainable building materials and waste resource reuse. BRIEF DESCRIPTION OF DRAWINGS

[0042] Figure 1 A flowchart of the method for increasing the compressive performance of geopolymer based on urease-induced calcium carbonate precipitation according to an embodiment of the invention;

[0043] Figure 2 The compressive strength results of fly ash-based geopolymer under different curing times are shown;

[0044] Figure 3 The Fourier infrared spectrum of fly ash-based geopolymer under different urease concentrations is shown;

[0045] Figure 4 The scanning electron microscope image of fly ash-based geopolymer;

[0046] Figure 5 Images of some fly ash-based geopolymer prepared according to an embodiment of the invention are shown. DETAILED DESCRIPTION

[0047] Exemplary embodiments of the present application are described in detail below with reference to the attached drawing figures, wherein the same or like reference numerals and characters designate the same or like components. Additionally, in the detailed description of embodiments, practices and procedures can be set forth in sequence(s) for purposes of explanation rather than stringent pretenses in order to provide a thorough understanding of embodiments of the present disclosure. It is apparent, however, that one or more embodiments can be practiced without some or all of these specific details. In other instances, well known structures and devices are shown in diagram form in order to avoid unnecessarily obscuring the drawings.

[0048] The present application relates to a method for improving the compressive performance of geopolymer by urease-induced calcium carbonate precipitation. The method introduces urease-induced calcium carbonate precipitation generated by cementing fluid into fly ash geopolymer, optimizes the pore structure of the material, and significantly improves the compressive performance and durability. The technology is particularly suitable for geopolymer materials that require improved strength and durability, and is an innovative method for improving the compressive performance of geopolymer that is environmentally friendly and low cost.

[0049] Specifically, the present application aims to develop an efficient, green, and sustainable technology for improving the compressive performance of fly ash geopolymer. In view of the problems of the microbial-induced calcium carbonate precipitation (MICP) technology, such as harsh microbial culture conditions, long growth and reproduction cycle, complicated process steps, and the need for precise control of inoculation, culture, and reaction parameters, the present application uses urease-induced calcium carbonate precipitation technology to induce the generation of calcium carbonate crystals in an alkaline environment through the catalytic action of urease, without the need for microbial culture. The calcium carbonate crystals are then precipitated in the substrate, improving the strength of the material. Geopolymer is an amorphous aluminosilicate inorganic polymer formed by the complexation of alkali-activated Si and Al materials. Its three-dimensional network structure mainly relies on the alternating linkage of SiO4 and AlO4 tetrahedral chains. Through the combination of urease, calcium chloride, and urea solution, the urease-induced calcium carbonate precipitation process can form a dense calcium carbonate precipitate in the geopolymer substrate, filling the internal pores of the material and improving its compressive performance. In the implementation of the present application, the reaction conditions of urease-induced calcium carbonate precipitation are also optimized to make the generated calcium carbonate precipitate tightly combined with the geopolymer structure, significantly improving the mechanical properties of fly ash geopolymer. Compared with traditional methods for improving compressive performance, the present application is simple to operate, has lower material costs, and uses biological urease to generate calcium carbonate, which is environmentally friendly and has high resource utilization efficiency, making it have good application prospects.

[0050] Urease-induced calcium carbonate precipitation is a process in which urease directly catalyzes the hydrolysis of urea to generate ammonium ions (NH4 + ) and carbonate ions (CO3 2- ). In the presence of soluble calcium ions (Ca 2+ ) in the environment, calcium carbonate precipitates. Urease-induced calcium carbonate precipitation is a precipitation technology based on biochemical reactions. The chemical reactions involved in this process are shown in (1.1)-(1.6):

[0051]

[0052]

[0053] Urease-induced calcium carbonate precipitation does not require microbial participation throughout the reaction process, does not involve biological safety risks and complex processes such as microbial culture, activation, and metabolic rate, and is therefore suitable for substrates with fine particle sizes. Second, urease has a very small size, enabling the filling of small-sized pores. Third, the remaining urease can be naturally degraded, and the entire process is environmentally friendly. Urease is the key to urease-induced calcium carbonate precipitation technology. In previous studies, commercial purified urease was used, which is expensive and cannot be widely used in real-world engineering. The inventors found that urease for urease-induced calcium carbonate precipitation can be extracted from plants and bacteria. Research has found that legumes (such as soybeans, beans) and melon seeds (watermelon seeds) are rich in a large amount of urease, and plant urease is convenient and economical. Urease-induced calcium carbonate precipitation technology has the characteristics of mild reaction conditions and environmentally friendly operation processes. The generated calcium carbonate precipitate can be deposited in the pores of the geopolymer, playing a filling and strengthening role, thereby improving the compressive performance of the material.

[0054] In the present application, fly ash is used as the matrix material of geopolymer, and a three-dimensional network structure containing Si and Al tetrahedral network is formed through the activation of alkali activator. Due to the large number of pores in fly ash-based geopolymer, calcium carbonate precipitates can be generated and fixed in the matrix, enhancing the compactness and strength of the structure. Compared to traditional strengthening methods, urease-induced calcium carbonate precipitation technology uses calcium carbonate generated by biological catalytic reaction to directly fill the material interior, which can better combine with the matrix material, form a stable composite structure between calcium carbonate precipitates and the geopolymer matrix, and improve the compressive performance of the material. In addition, by adjusting the ratio of urease, urea, and calcium source, the amount and distribution of the precipitate can also be optimized, thereby further enhancing the mechanical properties of the material. The specific technical methods of the present application are as follows:

[0055] (1) Experimental preparation for urease-induced calcium carbonate precipitation

[0056] Urease with high efficiency and specific catalytic effect is an important part of urease-induced calcium carbonate precipitation technology. In this study, soybean, which is cheap, is selected as the extraction source of urease. After grinding and sieving, the supernatant is obtained by centrifugation to obtain the urease extract. To ensure the activity of urease, it is stored at 4 DEG C. The cementing liquid is mainly composed of urea and calcium source (such as calcium chloride, calcium nitrate, calcium acetate, etc.), and the specific embodiment of the application selects calcium chloride. The concentration and ratio of the cementing liquid are one of the factors affecting the urease-induced calcium carbonate precipitation. The two-phase injection method is selected, that is, the cementing liquid and the urease solution are injected into the sample in turn, so that the urease-induced calcium carbonate precipitation fully reacts inside the geopolymer.

[0057] (2) Preparation process of fly ash-based geopolymer

[0058] The selection of raw materials plays a decisive role in the preparation process and the final performance of geopolymer materials. In this experimental study, fly ash industrial waste is used as raw material, as well as water glass, sodium hydroxide and other chemical reagents. These raw materials play different roles in the synthesis process of geopolymer: fly ash as a source of silicon and aluminum, provides silicate and aluminate components required for the formation of geopolymer; water glass as an alkaline activator, can promote the hydrolysis and polymerization reaction of the active components of silicate and aluminate; sodium hydroxide is used to adjust the modulus of water glass. The modulus of the activator refers to the molar ratio of SiO2 to equivalent alkaline oxide Na2O in the activator. The ratio of water glass to fly ash is the liquid-solid ratio (L / S), which affects the mechanical properties of geopolymer. After a large number of experiments and optimization, the modulus of water glass is selected as 1.4, and the liquid-solid ratio is selected as 0.7, and the geopolymer has good mechanical properties. According to the preset experimental proportion, the corresponding materials are weighed using an electronic balance and prepared in turn. The urease solution, the cementing liquid, and the alkaline activator are mixed with the fly ash raw material and then transferred to a stirrer for stirring. After 10 minutes, the mixture is poured into a mold to obtain a fly ash-based geopolymer sample based on urease-induced calcium carbonate precipitation and a blank control group.

[0059] (3) Ambient temperature curing of fly ash-based geopolymer

[0060] Considering that high-temperature curing can destroy the particle structure of geopolymer, leading to rapid dehydration and excessive shrinkage of geopolymer, the geopolymer is sealed and stored in a 40 DEG C oven for 24 hours of preliminary heat curing. Then the geopolymer sample is demolded and immediately packaged in a sealed bag to ensure that it is isolated from the external environment. Then it is placed in a normal temperature environment for further curing treatment. The curing time is 7 days, 14 days and 28 days, so that the mechanical properties can be comprehensively evaluated and detected at different time periods.

[0061] (4) Compression resistance test and characterization of geopolymer

[0062] The mechanical properties of the fly ash-based polymer samples maintained at room temperature for 7, 14 and 28 days are tested by using an electro-hydraulic servo universal testing machine to collect data. By testing the compressive strength at different curing periods, the effect of urease-induced calcium carbonate precipitation technology on the improvement of the compressive performance of the fly ash-based polymer is evaluated. The fly ash-based polymer samples are characterized by using Fourier transform infrared spectroscopy and scanning electron microscopy to observe the internal structure characteristics of the samples and the degree of urease-induced calcium carbonate precipitation reaction.

[0063] Taking the urease-induced calcium carbonate precipitation to change the compressive performance of the fly ash-based polymer as an example, more specific experimental steps are described:

[0064] S1: Preparation of urease solution

[0065] An electronic balance is used to weigh 50g of soybeans and put them into a pulverizer to be crushed into powder, ensuring the uniformity of the powder to improve the efficiency of urease extraction. Then, the crushed soybean powder is sieved through a 100-mesh steel sieve to obtain soybean powder with uniform particle size. According to the experimental requirements, a certain amount of soybean powder and deionized water are weighed and mixed in a mass-volume ratio of 1:10, and a magnetic stirrer is used to stir for 30 minutes to ensure that the urease in the soybean powder is fully dissolved and uniformly distributed in the solution. After stirring, let it stand for 2-3 hours to allow the soybean powder to settle and the urease to be fully released into the upper solution. Carefully pour the supernatant into a centrifuge tube after standing, centrifuge at 3000r / min and 4℃ for 15 minutes to remove suspended impurities by centrifugation, and obtain the supernatant as the urease extract. To maintain the catalytic activity of urease, the extract should be stored in a 4℃ environment to avoid the risk of enzyme inactivation caused by temperature fluctuations. The urease concentration selected in the examples of the present invention is 10g / L and 1g / L.

[0066] S2: Preparation of cementing liquid

[0067] The preparation of the cementing liquid includes two parts: urea and calcium chloride, and their concentration and ratio are important factors affecting the effect of urease-induced calcium carbonate precipitation reaction. Through experiments, we found that setting the concentration of urea and calcium chloride to 1mol / L can generate sufficient calcium carbonate precipitation without inhibiting the catalytic activity of urease. First, weigh a certain amount of urea and dissolve it in deionized water, stirring until completely dissolved to ensure the uniformity of the solution; then, weigh the same amount of calcium chloride and dissolve it in deionized water to form a 1mol / L calcium chloride solution. Then mix the urea solution and calcium chloride solution in a volume ratio of 1:1 to form the cementing liquid. Slowly add and gently stir during the mixing process to ensure that the resulting mixed liquid is uniform and transparent, so that it can fully react in subsequent use.

[0068] S3: Preparation of fly ash-based polymer

[0069] The preparation of fly ash geopolymer has high requirements for the selection and proportioning of raw materials. The fly ash used in the application provides a silicon-aluminum source for the geopolymer, and the water glass (sodium silicate) acts as an alkali activator to promote the hydrolysis and polymerization reaction of the active components of the silicate and aluminate in the fly ash. In the preparation process, an electronic balance is used to weigh the fly ash and the water glass, wherein the modulus of the water glass is 1.4, and the liquid-solid ratio is 0.7, so as to ensure that the material has good mechanical properties after forming. The weighed water glass and fly ash are mixed in a stirrer in advance, and low-speed stirring is performed for 5 minutes to uniformly mix the water glass and fly ash. The prepared cementing liquid 2.5 ml and urease solution 2.5 ml are added to each sample in turn. The whole stirring process needs to last for 10 minutes to obtain a uniform slurry. Then the slurry is poured into a mold with a size of 40*40*40mm 3 , and the bottom of the mold is tapped to remove bubbles and improve the compactness of the sample. In this experiment, the urease concentration is set to 1g / L and 10g / L in the experimental groups, and a blank control group is set, and each group has 3 samples, a total of 9 groups of samples.

[0070] S4: Geopolymer normal temperature curing

[0071] In the initial curing stage of the formed fly ash geopolymer sample, in order to avoid high temperature damage to the structure of the geopolymer, a 40℃ oven is selected for 24 hours of heat curing process. This step can ensure the stability and hardness of the material structure in the initial stage. After heat curing, the sample is carefully removed from the mold, and the sample is sealed in a sealed bag to avoid the influence of external humidity and temperature fluctuations, and to maintain the stability of the reaction during the curing process. The sealed sample is placed at room temperature for long-term curing, and the compressive performance test is carried out after 7 days, 14 days and 28 days of curing cycle.

[0072] S5: Geopolymer compressive performance test

[0073] The compressive performance test of the geopolymer sample is carried out by using an electro-hydraulic servo universal testing machine. Before testing, the cured sample is taken out of the sealed bag, and the sample is placed in the testing machine one by one, and the pressure is slowly applied according to the loading speed suitable for the sample size, and the failure stress and maximum bearing capacity of the sample are recorded. The compressive strength test of each group of samples cured for 7 days, 14 days and 28 days is carried out, and the compressive strength data of each group of samples are recorded, and the influence of urease concentration, curing cycle and other parameters on the compressive performance of the geopolymer is compared and analyzed, so as to determine the actual effect of the urease-induced calcium carbonate precipitation technology on improving the compressive performance of the fly ash geopolymer.

[0074] S6: Geopolymer characterization test

[0075] To further analyze the influence of urease-induced calcium carbonate precipitation on the microstructure of fly ash geopolymer matrix, the present application uses Fourier transform infrared spectroscopy (FTIR) and scanning electron microscopy (SEM) to characterize the samples after curing. First, the FTIR test is performed, and the spectrum is measured after the sample is ground into powder to confirm the calcium carbonate generated by the urease-induced reaction and its bonding with the geopolymer matrix. The SEM is used to observe the micro-morphology and pore filling of the sample to confirm the distribution effect of calcium carbonate precipitation.

[0076] The reagent urea is from China National Pharmaceutical Group Chemical Reagent Co., Ltd.; the reagent calcium chloride is from Hunan Bikeman Holding Co., Ltd., China; the alkaline activator is industrial water glass and sodium hydroxide, respectively from Henan Plun Castings Material Co., Ltd., China and Shanghai Aladdin Biochem Technology Co., Ltd.; the stirring machine model is NJ-160B from Shaoxing Shangyu Binxin Instrument Factory; the electronic balance is from Shanghai Yaoneng Electronic Technology Co., Ltd.; the electro-hydraulic servo universal testing machine is from MTS, USA; the Fourier transform infrared spectrometer model is Bruker VERTEX 70V from Billerica, USA; the scanning electron microscope model is FEI Nova Nano SEM450 from Hillsboro, USA.

[0077] The present application obtains the compressive strength results of fly ash geopolymer under different curing times as shown in Figure 2 The present application significantly improves the compressive performance of fly ash geopolymer by using urease-induced calcium carbonate precipitation technology, showing excellent engineering application prospects. The experimental data show that with the increase of curing period, the degree of urease-induced calcium carbonate precipitation reaction gradually increases, and the compressive strength of the experimental group with urease is significantly higher than that of the blank control group at 14 days and 28 days. Among them, the C group with urease concentration of 1 g / L reaches 93.468 MPa at 28 days, while the blank control group is only 78.306 MPa, indicating that appropriate concentration of urease can effectively promote the formation of calcium carbonate precipitation, significantly improve the compactness and compressive strength of the material. At the same time, it is found that with the increase of curing time, the precipitation of calcium carbonate is further stabilized and strengthened, and the mechanical properties of the material are gradually improved.

[0078] The Fourier infrared spectrum shows the chemical structure inside the fly ash geopolymer after curing for 28 days. A is the blank control group, B is urease 10 g / L, and C is urease 1 g / L. There are obvious bands near 3500 cm -1 near 3500 cm -1The peak at 1450cm -1 This region is usually related to C=O (carbonyl) stretching vibration, N-H bending vibration, etc. A-28 has a relatively weak absorption peak in this region. The absorption peaks of B-28 and C-28 in this region are stronger, because the added urease and urea participate in the reaction to generate compounds containing carbonyl or amino groups. -1 The wave number region appears a landmark peak, which is the typical vibration signal of Si-O-T bond formed by silicon oxygen and another element T (which may be silicon or aluminum) in the three-dimensional network structure of geopolymer, and it is a significant identifier of silicate glass phase. 650-400cm -1 This region is usually related to Si-O-Si (silicon-oxygen-silicon) and Si-O-Al (silicon-oxygen-aluminum) stretching vibration. Because the fly ash geopolymer contains aluminosilicate structure, and sodium silicate is used as an activator, there will be obvious absorption peaks in this region.

[0079] The scanning electron microscope image shows the microstructure of the fly ash geopolymer sample B group maintained for 28 days. The surface morphology of the material is shown in the figure, and the urease-induced calcium carbonate crystals and fly ash raw material particles can be seen. The energy dispersive spectroscopy (EDS) spectrum on the left shows the distribution of different elements at point A, and obvious calcium, oxygen and carbon element peaks can be seen, which can prove that the crystals here are calcium carbonate. The spherical fly ash particles are tightly combined with the gel, and a transition layer is formed between the raw material and the matrix phase, indicating that polymerization reaction has occurred between them, forming [-Al-O-Si-] chemical bonds. The cracks in the figure are mainly produced after the sample is subjected to compressive strength test. Through these analyses, the microstructure and element composition of the fly ash geopolymer can be understood in depth.

[0080] The invention has the beneficial effects compared with the prior art, including:

[0081] (1) The compressive performance of fly ash geopolymer is improved by the urease-induced calcium carbonate precipitation technology, without the need for complicated pretreatment steps, and the operation is simple and convenient. Compared with traditional reinforcement methods, this technology is more efficient and convenient. The technology is widely applicable and can significantly enhance the mechanical strength of the material, and is suitable for various environmental conditions.

[0082] (2) The calcium carbonate deposited in the geopolymer by the urease-induced calcium carbonate precipitation technology not only enhances the compressive performance of the material, but also realizes the recycling of waste resources (such as fly ash), has low energy consumption, low carbon dioxide emission, and does not produce secondary waste, which meets the green environmental protection requirements.

[0083] (3) The invention optimizes the reaction conditions of urease-induced calcium carbonate precipitation by reasonably matching urease, urea, and calcium sources in the process flow, forming a stable composite structure between the generated calcium carbonate precipitate and the geopolymer matrix, improving the compressive strength while effectively filling the material pores.

[0084] (4) Compared with traditional reinforcement techniques that require high-temperature treatment, the invention uses a normal-temperature curing process, which not only reduces the damage to the geopolymer matrix structure but also reduces energy consumption, making it stable to implement in various environmental conditions.

[0085] In summary, the invention exhibits excellent advantages in environmental protection, high efficiency, durability, and cost control, providing an innovative solution for sustainable building materials and waste resource recycling.

[0086] It should be noted that the invention uses fly ash geopolymer to test and study the compressive performance, but the types of geopolymer are extensive, including kaolin, blast furnace slag, rice husk ash, zeolite, clay, etc. The method of urease-induced calcium carbonate precipitation to improve the compressive performance of geopolymer is applicable to all geopolymer.

[0087] Although embodiments of the invention have been shown and described, it is to be understood that the embodiments are not limiting, and that changes can be made to these embodiments without departing from the spirit and scope of the invention. The scope of the invention is defined by the appended claims and their equivalents.

Claims

1. A method for increasing the compressive strength of geopolymer by urease-induced precipitation of calcium carbonate, by inducing calcium carbonate to precipitate in geopolymer by urease in an alkaline environment, characterized by, The method comprises: Step S1: preparing urease solution; Step S2: preparing cementing solution; Step S3: preparing fly ash geopolymer; Step S4: curing fly ash geopolymer at room temperature; Step S5: testing the compressive performance of fly ash geopolymer; Step S6: testing the characterization of fly ash geopolymer; The step S1 comprises: A predetermined weight of soybeans is weighed and put into a pulverizer for crushing. The crushed soybean powder is sieved to obtain soybean powder with uniform particle size. The soybean powder and deionized water are mixed in a predetermined mass-volume ratio, and a magnetic stirrer is used for stirring. After stirring, the soybean powder is allowed to settle for 2-3 hours, and the urease is released into the upper solution. The supernatant after standing is poured into a centrifuge tube, and centrifugation is carried out at 3000 r / min and 4℃ for 15 minutes. The suspended impurities are removed by centrifugation, and the supernatant is obtained as the urease extract. The step S2 comprises: A certain amount of urea is weighed and dissolved in deionized water, and stirred until completely dissolved. Calcium chloride is also weighed and dissolved in deionized water to form a 1 mol / L calcium chloride solution. The urea solution and the calcium chloride solution are mixed in a volume ratio of 1:1 to form the cementing solution. The step S3 comprises: Fly ash and water glass are weighed, wherein the modulus of the water glass is 1.4, and the ratio of the water glass to the fly ash is 0.

7. The water glass and the fly ash are pre-mixed in a stirrer to uniformly mix the water glass and the fly ash. The prepared cementing solution and urease solution are added in sequence, and continuous stirring is carried out to obtain a uniform slurry. The slurry is poured into a mold, and the bottom of the mold is tapped to remove air bubbles. The method uses a two-phase injection method, i.e., the cementing solution and the urease solution are injected into the fly ash geopolymer in sequence, so that the urease-induced calcium carbonate precipitation fully reacts inside the geopolymer; The compressive performance of the fly ash geopolymer is tested by using an electro-hydraulic servo universal testing machine. Before testing, the cured samples are taken out of the sealed bag, and the samples are placed one by one into the testing machine. The pressure is slowly applied according to the preset loading speed, and the failure stress and maximum bearing capacity of the sample are recorded. The Fourier transform infrared spectroscopy and scanning electron microscopy are used to characterize the cured samples, including: first, the Fourier transform infrared spectroscopy test is carried out, and the cured samples are ground into powder to measure the spectrum to confirm the calcium carbonate generated by the urease-induced reaction and the bonding between the calcium carbonate and the geopolymer matrix; then, the scanning electron microscope is used to observe the microstructure and pore filling of the sample to confirm the distribution effect of the calcium carbonate precipitation.

2. The method for increasing the compressive performance of geopolymer based on urease-induced calcium carbonate precipitation according to claim 1, wherein: The urease extract is stored in a 4℃ environment to avoid the risk of enzyme inactivation caused by temperature fluctuations.

3. The method for increasing the compressive performance of geopolymer based on urease-induced calcium carbonate precipitation according to claim 2, wherein: In step S3, multiple groups of slurry samples are prepared, each group of slurry samples containing three samples, and the urease concentrations of the multiple groups of slurry samples are different.

4. The method for increasing the compressive performance of geopolymer based on urease-induced precipitation of calcium carbonate according to claim 3, characterized in that, The step S4 comprises: The fly ash-based polymer is sealed and stored in a 40 DEG C oven for a preliminary heat curing process of 24 hours, and then demolded to obtain the fly ash-based polymer, and the demolded fly ash-based polymer is packaged into a sealed bag to ensure isolation from the external environment, and then placed in a normal temperature environment for further curing treatment.

5. The method for increasing the compressive performance of geopolymer based on urease-induced precipitation of calcium carbonate according to claim 4, characterized in that, Different curing times are set for different fly ash-based polymer samples, at least including 7 days, 14 days and 28 days, so as to evaluate and detect the mechanical properties of the fly ash-based polymer at different time periods.

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