A sheet-like nano c-s-h crystal nucleus, a preparation method, a sprayed concrete and a use method

Abstract

This application belongs to the field of tunnel engineering construction technology, and particularly relates to a sheet-like nano C-S-H crystal nucleus, its preparation method, sprayed concrete, and its application method. Addressing the industry pain points of existing high-temperature tunnel shotcrete, such as abnormal hydration, insufficient early strength, high rebound rate, poor durability, and easy agglomeration and low nucleation activity of existing C-S-H crystal nuclei, this application uses calcium nitrate and sodium silicate as raw materials. Through a combination of processes including low-speed dropwise controlled precipitation, in-situ modification with TPEG-type polycarboxylate dispersant, and low-temperature post-treatment, loose, porous, plate-like C-S-H crystal nuclei with a diameter of 50-100 nm are obtained. These nuclei are then incorporated into shotcrete at 1.0%-1.5% of the total mass of cementitious materials. Combined with an online mixing of accelerators and a high-temperature aggregate pre-cooling construction process, the shotcrete strength can be increased by more than 30% in 8 hours to 3 days, the construction rebound rate can be reduced to below 8%, and the 28-day impermeability grade can reach P15. This method is suitable for tunnel conditions with high ground temperatures above 35℃ and high corrosion. The process is simple, cost-controllable, and suitable for large-scale engineering applications.

Description

Technical Field

[0001] This application relates to the field of tunnel engineering construction technology, and in particular to a sheet-like nano-CSH crystal nucleus, its preparation method, sprayed concrete, and its application method. Background Technology

[0002] In tunnel construction, shotcrete, as the core material for initial support, directly determines the safety during construction and the stability of long-term service due to its mechanical properties, bonding properties, and durability. As tunnel construction extends deeper, the number of tunnels with ambient temperatures exceeding 35℃ is increasing. Under these hot conditions, the cement hydration reaction rate is significantly accelerated, leading to a substantial reduction in the initial setting time of concrete. False setting is prone to occur during shotcreting, reducing the workability and sprayability of the concrete, and causing poor bonding between the shotcrete layer and the surrounding rock, as well as between the shotcrete layer and the surrounding rock, resulting in potential problems such as hollow areas and detachment. Simultaneously, high temperatures accelerate the evaporation of internal moisture in the concrete, leading to insufficient cement hydration and slow early strength development, making it difficult to meet the strength requirements of initial tunnel support. Furthermore, high ground temperatures are often accompanied by increased groundwater activity. Corrosive ions such as sulfate and chloride ions in the groundwater migrate and diffuse faster under high temperatures, accelerating the erosion and damage of cement paste, further deteriorating the mechanical properties and durability of shotcrete, and seriously affecting the long-term safety of the tunnel.

[0003] Calcium silicate hydrate (CSH) is the core component of silicate cement hydration products, accounting for 70% to 80% of the total hydration products, and is a key cementitious phase for the strength formation of cement-based materials. Artificially synthesized nano-CSH materials, due to their structural similarity to the CSH products generated during cement hydration, can serve as a nucleation matrix for the cement hydration reaction. This significantly reduces the nucleation barrier of hydration products, accelerates the hydration process of C3S and C2S in cement, and refines the internal pore structure of concrete, improving its strength and durability. Therefore, it has become a novel functional material for improving the performance of shotcrete in high-temperature tunnels.

[0004] While existing technologies include research on the application of nanomaterials in concrete, there is a lack of specifically optimized nano-CSH nucleus preparation processes for the unique conditions of high-temperature tunnels. For example, the sol-gel method and hydrothermal synthesis method suffer from long reaction cycles, high preparation costs, demanding equipment requirements, and difficulties in large-scale production. The single-mineral hydration method exhibits drawbacks such as low product purity, poor dispersibility, and easy agglomeration of nanoparticles, resulting in limited reinforcing effects in shotcrete and failing to effectively address the technical challenges of shotcrete in high-temperature environments. Furthermore, existing technologies do not synergistically optimize the nano-CSH nucleus preparation process with the mix design and construction techniques of shotcrete, leading to a mismatch between nucleus dosage and concrete composition, further impacting the reinforcing effect and construction adaptability.

[0005] Therefore, developing a simple, cost-controllable, well-dispersible, and adaptable sheet-like nano-CSH crystal nucleus for high geothermal conditions, as well as corresponding sheet-like nano-CSH crystal nucleus-reinforced shotcrete, has significant engineering implications and application value. Summary of the Invention

[0006] One of the purposes of this application is to provide a method for preparing sheet-like nano-CSH crystal nuclei to avoid the shortcomings of the prior art. This method has the advantages of low preparation cost, mild and controllable process conditions, suitability for industrial mass production, and the resulting crystal nuclei have the advantages of uniform crystal form, excellent dispersibility and high hydration activity.

[0007] The second objective of this application is to provide a sheet-like nano-CSH crystal nucleus.

[0008] The third objective of this application is to provide a shotcrete.

[0009] The fourth objective of this application is to provide a method for using sprayed concrete.

[0010] One of the objectives of this application is to achieve the following technical solution: A method for preparing sheet-like nano-CSH crystal nuclei is provided, comprising the following steps: S1. Dissolve calcium nitrate and sodium silicate separately in deionized water and stir until completely dissolved to obtain calcium source solution and silicon source solution; S2. Place the calcium source solution in a constant temperature stirring device, and add the silicon source solution dropwise to the calcium source solution while stirring. After the addition is completed, continue the reaction while stirring to obtain the initial reaction solution; the calcium-silicon molar ratio of the calcium source solution to the silicon source solution is 1:(1~1.2). S3. Under stirring, add a polycarboxylic acid dispersant obtained by copolymerizing acrylic acid and polyether macromonomer TPEG-5000 to the initial reaction solution to obtain a modified reaction solution. S4. Centrifuge the modified reaction solution to obtain a solid product; S5. Repeat the washing and centrifugation process on the solid product until the washing solution is neutral. S6. The washed solid product is vacuum dried at a temperature below 80°C, followed by dry grinding and sieving to obtain sheet-like nano CSH crystal nuclei.

[0011] In some embodiments, in step S1, the concentration of the calcium source solution is 0.5~1.0 mol / L, and the concentration of the silicon source solution is 0.5~1.0 mol / L; In step S2, the stirring rate is 300~500 r / min, the reaction temperature is 25~30℃; the silicon source solution drop rate is 1~2 mL / min, the drop time is 30~60 min, and the stirring reaction continues for 60~90 min after the drop is completed; In step S3, the polycarboxylic acid dispersant has a number average molecular weight of 10,000 and a polydispersity index of 1.3; the amount of polycarboxylic acid dispersant added is 3% to 5% of the total mass of calcium nitrate and sodium silicate; after adding the polycarboxylic acid dispersant, stirring is continued at a rate of 300 to 500 r / min for 60 to 90 min.

[0012] In some embodiments, in step S4, the centrifugation speed is 3000~5000 r / min and the centrifugation time is 10~20 min; In step S6, the dry grinding specifically involves grinding for 20-30 minutes using a planetary ball mill at a grinding speed of 200-300 r / min, using zirconia balls as the grinding medium, and a ball-to-material ratio of 5:1. The obtained sheet-like nano-CSH crystal nuclei have a diameter of 50~100nm and possess a porous structure.

[0013] The method for preparing sheet-like nano-CSH crystal nuclei provided in this application has the following beneficial effects: This application selects calcium nitrate, which has high solubility and a slow reaction rate, as the calcium source and sodium silicate as the silicon source. This avoids the irregular agglomeration of grains caused by local ion concentration surges during the reaction, reducing byproduct formation from the reaction source. The entire reaction process is free of violent exothermic reactions, ensuring high operational safety and providing a uniform reaction basis for the directional growth of lamellar grains. Secondly, the silicon source solution is added dropwise to the calcium source solution at a low speed, combined with constant temperature and uniform stirring throughout the reaction, allowing for precise control of Ca2+ concentration. 2+ With SiO3 2- The contact reaction rate guides the preferential growth of CSH grains along the two-dimensional direction, initially forming uniformly sized lamellar grains. Immediately after the precipitation reaction, a polycarboxylate dispersant with a specific structure is added. Its molecular chains can be rapidly adsorbed onto the surface of the newly formed CSH grains. On the one hand, this inhibits grain agglomeration through steric hindrance, reducing the difficulty of subsequent dispersion; on the other hand, it further guides the grains to maintain their lamellar growth trend, avoiding the formation of spherical or irregular agglomerates. Simultaneously, the modified crystal nuclei exhibit excellent compatibility with polycarboxylate additives, ensuring no compatibility issues when subsequently applied to cement-based materials. Finally, a post-processing technique involving medium-low speed centrifugation, room temperature washing, low-temperature vacuum drying, and low-energy ball milling avoids external forces from damaging the formed lamellar porous structure, ultimately yielding target crystal nuclei with uniform morphology, good dispersibility, and high specific surface area.

[0014] The preparation method provided in this application is a reaction at room temperature and pressure throughout the entire process, without the need for special equipment such as high-temperature calcination or high-pressure hydrothermal synthesis. It can solve the problems of high cost, high equipment requirements and long reaction cycle of sol-gel method and hydrothermal synthesis method, and can significantly reduce production energy consumption and waste emissions, significantly reduce the negative impact on the environment, and meet the requirements of low-carbon production. At the same time, it also solves the defects of low purity and poor dispersibility of single mineral hydration method products.

[0015] The second objective of this application is achieved through the following technical solution: A sheet-like nano-CSH crystal nucleus is provided, which is prepared by the above-described method for preparing sheet-like nano-CSH crystal nuclei.

[0016] The sheet-like nano-CSH crystal nucleus provided in this application has the following beneficial effects: The sheet-like nano-CSH crystal nuclei provided in this application have an irregular, loose, porous sheet-like morphology. The sheet diameter can be stably distributed in the range of 50~100nm, with good size uniformity, no hard agglomeration between particles, and excellent dispersibility. Its porous structure can provide a large number of hydration-inducing nucleation sites, with outstanding nucleation activity. After being incorporated into cement-based materials, it can significantly shorten the hydration induction period and accelerate the cement hydration reaction process. Moreover, after the surface is modified with polycarboxylate dispersant, it has good compatibility with admixtures such as polycarboxylate superplasticizers and alkali-free accelerators. It can produce a synergistic effect with accelerators: it can further shorten the setting time of shotcrete and reduce the dosage of accelerators, and can also significantly improve the early strength of concrete from 1d to 3d without deteriorating the later mechanical properties.

[0017] The third objective of this application is achieved through the following technical solution: A shotcrete is provided, comprising cement, fly ash, sand, crushed stone, water, polycarboxylate superplasticizer, liquid alkali-free quick-setting agent, and flake-shaped nano-CSH crystal nuclei; the amount of liquid alkali-free quick-setting agent added is 5.5%~6.5% of the total mass of cement and fly ash, and the amount of flake-shaped nano-CSH crystal nuclei added is 1.0%~1.5% of the total mass of cement and fly ash; The sheet-like nano-CSH crystal nuclei are prepared by the method for preparing sheet-like nano-CSH crystal nuclei according to any one of claims 1 to 3, or are the sheet-like nano-CSH crystal nuclei according to claim 4.

[0018] In some embodiments, the amounts of each component by weight are: 400-450 parts cement, 60-80 parts fly ash, 800-854 parts sand, 750-789 parts crushed stone, 190-200 parts water, and 5.0-5.3 parts polycarboxylate superplasticizer.

[0019] In some embodiments, the fineness modulus of the sand is 2.6 to 3.0, and the particle size of the crushed stone is in the range of 5 to 10 mm; The water reduction rate of the polycarboxylate superplasticizer is ≥25%; The initial setting time of the liquid alkali-free quick-setting agent is <5 min, and the final setting time is <12 min.

[0020] The shotcrete provided in this application has the following beneficial effects: (1) By leveraging the hydration-induced nucleation effect of sheet-like nano-CSH crystal nuclei and the synergistic effect of fly ash pozzolanic activity, the industry pain points of abnormal hydration process and insufficient early strength growth in shotcrete under high geothermal conditions can be addressed. Compared with spherical CSH crystal nuclei, sheet-like nano-CSH crystal nuclei can improve the compressive strength at 8h to 3d age, enabling it to quickly meet the strength requirements of initial tunnel support. At the same time, sheet-like nano-CSH crystal nuclei can effectively refine the internal pore structure of concrete, reducing the rebound rate of shotcrete and thus reducing material loss during construction.

[0021] (2) The content of flaky nano CSH crystal nuclei is controlled in the range of 1.0% to 1.5% of the total mass of cement and fly ash. This not only gives full play to the early strength enhancement effect, but also avoids the problems of excessive hydration rate and deterioration of workability caused by excessive dosage. The concrete mixing and spraying construction process is fully compatible with the existing conventional tunnel wet spraying process. No additional modifications are required to the existing construction equipment. The construction is convenient and easy to promote. For extreme high ground temperature environments above 35℃, water bath temperature control mixing and aggregate pre-cooling and other treatment measures can be adopted to further improve the adaptability of working conditions.

[0022] (3) The sheet-like nano CSH crystal nuclei raw materials used in this application are readily available, the preparation process is simple, and the mass production cost is low. Combined with the advantages of reduced construction rebound rate and reduced post-construction maintenance costs, the overall life-cycle economic efficiency is better than that of traditional shotcrete, which can meet the application needs of large-scale tunnel engineering.

[0023] The fourth objective of this application is achieved through the following technical solution: A method for applying shotcrete is provided, including the following steps: (1) Add cement, fly ash, sand and crushed stone into a mixer according to the proportion, and mix for 3-5 minutes until the mixture is uniform to obtain powder; (2) Dissolve the polycarboxylate superplasticizer in water to obtain a superplasticizer aqueous solution. Then add the superplasticizer aqueous solution to the powder and continue stirring for 5-8 minutes. Finally, add the flaky nano CSH crystal nuclei and continue stirring for 3-5 minutes to obtain a concrete mixture. Send the concrete mixture into the spraying machine. (3) Add liquid alkali-free quick-setting agent to the front end of the spraying machine through a metering pump. After mixing with the concrete mixture online, spray it to the designed thickness in several batches.

[0024] In some embodiments, the spraying pressure of the sprayer is 0.3~0.5MPa, the spraying distance is 1.5~2.0m, the spraying angle is 75°~85°, the single spray thickness is 8~10cm, and the spraying interval is 1~2h; The slump of the concrete mixture is 120~140mm, and the spread is 350~400mm.

[0025] In some embodiments, the applicable ambient temperature for the sprayed concrete is 5℃~35℃; if the ambient temperature is >35℃, the water-reducing agent aqueous solution, powder and flake-shaped nano CSH crystal nuclei are mixed in a water bath at a temperature of 25℃~30℃ using a double-layer mixing tank.

[0026] The method for using shotcrete provided in this application has the following beneficial effects: When applying this method for shotcreting, the liquid alkali-free accelerator is precisely metered and mixed online with the concrete mix using a metering pump. This allows for controllable accelerator dosage and uniform mixing, avoiding problems such as excessively rapid setting of the mix and blockage of delivery pipelines caused by the accelerator prematurely participating in the hydration reaction. Online mixing ensures construction continuity and stably leverages the synergistic early strength effect of the accelerator and CSH crystal nuclei, keeping the shotcrete rebound rate consistently below 8% and ensuring uniform early strength growth of the support structure. For extreme high ground temperatures exceeding 35℃, a 25-30℃ water bath can be used for temperature-controlled mixing during the concrete mix preparation stage. This effectively counteracts the accelerating effect of high-temperature environment on cement hydration, ensuring the mix has sufficient workable time of 1-2 hours. This allows for adaptation to the needs of high-temperature tunnel construction without additional mix proportion adjustments, demonstrating high process flexibility. Attached Figure Description

[0027] The above and other objects, features and advantages of this application will become more apparent from the more detailed description of exemplary embodiments thereof in conjunction with the accompanying drawings, wherein the same reference numerals generally represent the same components in the exemplary embodiments thereof.

[0028] Figure 1 This is a SEM image of the sheet-like nano-CSH crystal nuclei of Example 1 of this application; Figure 2 This is another SEM image of the sheet-like nano-CSH crystal nuclei of Embodiment 1 of this application. Detailed Implementation

[0029] Preferred embodiments of the present application will now be described in more detail with reference to the accompanying drawings. While preferred embodiments of the present application are shown in the drawings, it should be understood that the present application may be implemented in various forms and should not be limited to the embodiments set forth herein. Rather, these embodiments are provided to make the present application more thorough and complete, and to fully convey the scope of the present application to those skilled in the art.

[0030] The first aspect of this application discloses a method for preparing sheet-like nano-CSH crystal nuclei, comprising the following steps: S1. Raw material preparation: Dissolve calcium nitrate and sodium silicate separately in deionized water and stir until completely dissolved to obtain calcium source solution and silicon source solution; S2. Controlled precipitation reaction: Place the calcium source solution in a constant temperature stirring device, and add the silicon source solution dropwise to the calcium source solution while stirring. After the addition is completed, continue the reaction while stirring to obtain the initial reaction solution. S3. In-situ dispersion modification: Under stirring, a polycarboxylic acid dispersant obtained by copolymerizing acrylic acid and polyether macromonomer TPEG-5000 is added to the initial reaction solution to obtain the modified reaction solution; S4. Centrifugal separation: The modified reaction solution is centrifuged to collect the solid product; S5. Washing and purification: Repeatedly wash and centrifuge the solid product until the washing liquid is neutral. S6. Mild post-treatment: The washed solid product is vacuum dried at a temperature not exceeding 80°C, followed by dry grinding and sieving to obtain sheet-like nano CSH crystal nuclei.

[0031] Specifically, in step S1, analytical grade calcium nitrate (Ca(NO3)2·4H2O) and analytical grade sodium silicate (Na2SiO3·9H2O) with a purity ≥99% are used as raw materials, dissolved separately in deionized water, and magnetically stirred until completely dissolved to prepare calcium source solution and silicon source solution with a concentration of 0.5~1.0 mol / L. The calcium-silicon molar ratio of the calcium source to the silicon source is controlled at 1:1~1.2, and a slight excess of silicon source can ensure that Ca 2+ Complete reaction improves the purity of the product from nano-CSH crystal nuclei. Calcium nitrate is chosen as the calcium source due to its high solubility and Ca2+ content. 2+ The release rate is stable, and the raw material cost is low with a mature industrial supply. Sodium silicate is chosen as the silicon source, and its SiO3... 2- The precipitation rate is gradual and can be combined with Ca. 2+ The uniform reaction generates CSH, thereby regulating the reaction rate and avoiding the irregular agglomeration of crystals caused by sudden increases in local ion concentration. Calcium nitrate and sodium silicate are both commonly used industrial raw materials with controllable costs.

[0032] In step S2, the calcium source solution is placed in a constant-temperature stirring device, the stirring speed is adjusted to 300-500 r / min, and the reaction temperature is controlled at 25-30℃. Under stirring, the silicon source solution is slowly added dropwise to the calcium source solution at a rate of 1-2 mL / min, and the total adding time is controlled at 30-60 min. After the addition is completed, the stirring speed is kept constant, and the reaction continues for 60-90 min to allow the Ca to mature. 2+ With SiO3 2- The reaction proceeds fully to form nano-CSH precipitate, yielding the initial reaction solution. In this step, the ambient temperature and pressure reaction conditions avoid the need for the high-temperature, high-pressure specialized equipment required for traditional hydrothermal synthesis methods, significantly reducing equipment investment and energy consumption. By using a low-speed dropwise addition of the silicon source and uniform stirring throughout the process, the Ca2+ content can be controlled. 2+ With SiO3 2- The contact reaction rate guides the preferential growth of CSH grains along the two-dimensional direction, initially forming uniformly sized lamellar grains.

[0033] In step S3, a polycarboxylic acid dispersant is added to the initial reaction solution. This polycarboxylic acid dispersant is synthesized from acrylic acid and the polyether macromonomer TPEG-5000 via a free radical copolymerization reaction. Specifically, the synthesis process involves preparing materials at a molar ratio of acrylic acid to TPEG-5000 of 3:1 to 4:1, while simultaneously preparing a 5% to 8% (w / w) ammonium persulfate aqueous solution as a thermal initiator. The total mass ratio of the three components is TPEG-5000:acrylic acid:ammonium persulfate = 100:15 to 20:2 to 3. Then... TPEG-5000 is dissolved in deionized water, heated to 65-75℃ and stirred continuously. Acrylic monomer solution and initiator aqueous solution are added dropwise simultaneously, with the addition time controlled at 2.5-3h. After the addition is completed, the reaction is kept at the temperature for 1.5-2h to complete the free radical copolymerization. After the reaction solution is cooled to below 40℃, 30% sodium hydroxide solution is added to adjust the pH to 6-7, thus obtaining a polycarboxylic acid dispersant product with a number average molecular weight of 10000 and a polydispersity index (PDI) of 1.3. The amount of polycarboxylic acid dispersant added is 3%~5% of the total mass of calcium nitrate and sodium silicate. After addition, stirring is continued for 60~90 minutes to allow the dispersant molecular chains to be fully adsorbed on the surface of the newly generated nano-CSH crystals. On the one hand, it inhibits crystal agglomeration through steric hindrance effect and improves the dispersion stability of crystal nuclei. On the other hand, it can further guide the crystals to maintain the lamellar growth trend and avoid the formation of spherical or irregular agglomerates. At the same time, the modified crystal nuclei have excellent compatibility with polycarboxylic acid admixtures and there are no compatibility problems when applied to cement-based materials, resulting in a modified reaction solution.

[0034] In step S4, the modified reaction solution is placed in a centrifuge, and the centrifugation speed is adjusted to 3000~5000 r / min for 10~20 min to fully separate the nano CSH solid product from the reaction solution and collect the bottom solid product. The centrifugation parameters in this step have been optimized to ensure thorough separation of the solid product and avoid hard agglomeration of nanoparticles caused by excessive centrifugation speed.

[0035] In step S5, the collected solid product is washed repeatedly with deionized water 3 to 5 times. After each wash, it is centrifuged at 2000 r / min for 5 min to remove residual nitrate, sodium ions and unreacted raw materials from the solid product until the pH of the washing solution is 6.8 to 7.2 and is neutral. Washing and purification can improve the purity of nano CSH crystal nuclei and avoid impurities from having an adverse effect on the subsequent concrete performance.

[0036] In step S6, the washed solid product is placed in a vacuum drying oven at 60~80℃ and dried for 12~24h to remove free water from the product and avoid excessive drying temperature that could cause nanoparticle aggregation and damage to the porous structure. After drying, the product is taken out and dry-milled for 20~30min using a planetary ball mill at a speed of 200~300r / min. The grinding medium is zirconia balls with a ball-to-material ratio of 5:1. After milling, the product is passed through a 200-mesh sieve to obtain nano-CSH crystal nuclei with an irregular sheet-like shape and a loose porous structure with a diameter of 50~100nm. The nuclei are then sealed and stored for later use.

[0037] Through the above technical solution, sheet-like nano-CSH crystal nuclei with uniform size, good dispersibility, no hard agglomerates, and loose porous structure that can provide a large number of hydration nucleation sites can be obtained. These crystal nuclei can fully contact water in cement paste, quickly release nucleation sites to accelerate cement hydration, and at the same time fill the internal pores of concrete to improve the density and mechanical properties of concrete.

[0038] The second aspect of this application discloses a shotcrete adapted to high geothermal conditions, the raw materials of which include cement, fly ash, sand, crushed stone, water, polycarboxylate superplasticizer, liquid alkali-free quick-setting agent, and flake-like nano-CSH crystal nuclei prepared by the above preparation method; wherein the amount of liquid alkali-free quick-setting agent added is 5.5% to 6.5% of the total mass of cement and fly ash, and the amount of flake-like nano-CSH crystal nuclei added is 1.0% to 1.5% of the total mass of cement and fly ash.

[0039] The preparation and construction methods of this shotcrete are as follows: Crystal nucleus preparation: The target crystal nucleus is prepared according to the above-mentioned method for preparing sheet-like nano CSH crystal nuclei, sealed and stored for later use, and prevented from agglomeration due to moisture.

[0040] Concrete mixing: According to the mix proportion, put cement, fly ash, sand, and crushed stone into a forced mixer and dry mix for 3-5 minutes until uniformly mixed, ensuring that the cementitious materials and aggregates are fully dispersed and free of lumps; dissolve polycarboxylate superplasticizer in all the mixing water and stir until completely dissolved to prepare a superplasticizer aqueous solution; slowly add the superplasticizer aqueous solution to the dry-mixed powder and wet mix for 5-8 minutes to fully wet and uniformly mix the aggregates and cementitious materials; finally, add the pre-made flake-shaped nano CSH crystal nuclei and continue stirring for 3-5 minutes to obtain a uniform, lump-free concrete mixture with good workability. Control the slump of the mixture to be 120-140 mm and the spread to be 350-400 mm to meet the requirements of wet spraying construction.

[0041] The mixing sequence has been optimized: first, dry mix the aggregates and cementitious materials, then add the water-reducing agent aqueous solution for wet mixing, and finally add the nano CSH crystal nuclei. This can prevent the nano crystal nuclei from agglomerating in high-concentration aggregates, ensuring that they are evenly dispersed in the concrete mixture and fully exert their nucleation and strengthening effect.

[0042] Spraying construction: The liquid alkali-free quick-setting agent is mixed online with the concrete mixture at the front end of the spraying machine by means of external admixture through a metering pump. After uniform mixing, the wet spraying process is applied immediately. The spraying pressure is controlled at 0.3~0.5MPa, the spraying distance is 1.5~2.0m, the spraying angle is 75°~85°, the thickness of a single spray is 8~10cm, and the layers are sprayed to the design thickness, with an interval of 1~2 hours between adjacent spraying layers.

[0043] The liquid alkali-free quick-setting agent adopts an online mixing method, which can avoid premature initial setting of concrete caused by pre-mixing and ensure construction continuity. For high ground temperature conditions (such as ambient temperature > 35℃), the aggregate can be pre-cooled to 25~30℃ before mixing, which slows down the cement hydration rate, avoids the workability of the mixture deteriorating too quickly, and further improves the adaptability of construction. In specific implementation, the aggregate can be mixed in a double-layer mixing tank at a water bath temperature of 25℃~30℃.

[0044] The present application will be further described below with reference to the accompanying drawings and embodiments, but this is not intended to limit the scope of the application.

[0045] It should be noted that, unless otherwise specified, the embodiments and features described in this application can be combined with each other. All components described in this application are commercially available. The components described in the specific implementation or embodiments are mass ratios or mass parts.

[0046] Example 1 1. Preparation of sheet-like nano-CSH crystal nuclei 1.1 Raw material preparation Analytical grade calcium nitrate (Ca(NO3)2·4H2O) and analytical grade sodium silicate (Na2SiO3·9H2O) were dissolved in deionized water and stirred until completely dissolved to prepare a calcium source solution with a concentration of 0.8 mol / L and a silicon source solution with a concentration of 0.8 mol / L. The calcium-silicon molar ratio of the calcium source to the silicon source was 1:1.1. 1.2 Precipitation reaction The calcium source solution was placed in a constant temperature stirring device, the stirring speed was adjusted to 400 r / min, the reaction temperature was controlled at 28℃, and the silicon source solution was slowly added dropwise to the calcium source solution at a rate of 1.5 mL / min for a total dropping time of 45 min. After the dropping was completed, the stirring was maintained and the reaction continued for 75 min to obtain the initial reaction solution. 1.3 Dispersion Modification Add polycarboxylic acid dispersant (polymerized from acrylic acid and polyether macromonomer TPEG-5000, with a number average molecular weight of 10,000 and a polydispersity index PDI of 1.3) to the initial reaction solution. The amount added is 4% of the total mass of calcium nitrate and sodium silicate. After adding, continue stirring for 75 minutes to obtain the modified reaction solution. 1.4 Centrifugal Separation The modified reaction solution was placed in a centrifuge, the centrifugation rate was adjusted to 4000 r / min, the centrifugation time was 15 min, and the solid product was collected. 1.5 Washing and Purification The solid product was washed four times with deionized water. After each wash, it was centrifuged at 3000 r / min for 5 min until the pH of the washing solution was 7.0 and neutral. 1.6 Drying and Grinding The washed solid product was dried in a vacuum drying oven at 70℃ for 18 hours. After being removed, it was dry-ground for 25 minutes using a planetary ball mill at a speed of 250 r / min. The grinding media was zirconia balls with a ball-to-material ratio of 5:1. After grinding, it was passed through a 200-mesh sieve to obtain irregularly shaped, loosely porous nano-CSH crystal nuclei, which were then sealed and stored for later use.

[0047] Testing revealed that the nano-sized CSH crystal nuclei prepared in this embodiment had a diameter of 50-90 nm and exhibited no hard agglomeration. The SEM images of these sheet-like nanocrystal nuclei are shown below. Figure 1 and Figure 2 As shown.

[0048] 2. Preparation and Construction of Nano-CSH Nucleus-Reinforced Shotcrete 2.1 Raw material ratio By weight, 450 parts of P·O 42.5 grade cement, 75 parts of grade II fly ash, 854 parts of sand with a fineness modulus of 2.8, 789 parts of crushed stone with a particle size of 5~10mm, 194 parts of water, 5.3 parts of polycarboxylate superplasticizer with a water reduction rate of 28%, 29.1 parts of liquid alkali-free quick-setting agent, and 7.8 parts of the above-prepared flaky nano CSH crystal nuclei were weighed out respectively. 2.2 Mixing process Cement, fly ash, sand, and crushed stone are put into a forced mixer and dry-mixed for 4 minutes until uniformly mixed; polycarboxylate superplasticizer is dissolved in 194 parts of water and stirred until completely dissolved to obtain a superplasticizer aqueous solution; the superplasticizer aqueous solution is added to the dry-mixed powder and wet-mixed for 6 minutes; finally, flaky nano CSH crystal nuclei are added and stirred for another 4 minutes to obtain the concrete mixture. Its slump was tested to be 130mm and its spread to be 380mm, which meets the requirements of the wet spraying process. 2.3 Spraying Application Liquid alkali-free quick-setting agent is delivered to the front end of the spraying machine via a metering pump and mixed online with the concrete mixture. The wet spraying process is adopted, and the spraying pressure is controlled at 0.4MPa, the spraying distance at 1.8m, the spraying angle at 80°, the single spraying thickness at 9cm, and the layers are sprayed to the designed thickness, with an interval of 1.5h between adjacent layers. The liquid alkali-free quick-setting agent is a commercially available calcium aluminate-based alkali-free quick-setting agent. The measured initial setting time is 4.2min and the final setting time is 10.8min, which meets the performance requirements of initial setting <5min and final setting <12min.

[0049] Example 1 simulates high ground temperature conditions, with the construction environment temperature selected as 40℃.

[0050] 3. Performance Testing According to GB50086-2015 "Technical Specification for Rock and Soil Anchors and Shotcrete Support Engineering", the performance of the shotcrete in Example 1 was tested, and the results are as follows: 8-hour compressive strength 10.1 MPa, 1-day compressive strength 17.5 MPa, 3-day compressive strength 25.3 MPa, 28-day compressive strength 37.3 MPa; 1-day bond strength 1.12 MPa, 3-day bond strength 1.62 MPa, 28-day bond strength 1.95 MPa; construction rebound rate 7.2%; 28-day seepage pressure 1.5 MPa, seepage height 22 mm; chloride ion diffusion coefficient 4.1 × 10⁻⁶. - With a power output of 12 m² / s and an electrical flux of 1120C, all performance characteristics meet the requirements for use in high-temperature tunnel projects.

[0051] Example 2 1. Preparation of sheet-like nano-CSH crystal nuclei 1.1 Raw material preparation Analytical grade calcium nitrate and analytical grade sodium silicate were dissolved in deionized water and stirred until completely dissolved to prepare a calcium source solution with a concentration of 1.0 mol / L and a silicon source solution with a concentration of 1.0 mol / L. The calcium-silicon molar ratio of the calcium source to the silicon source was 1:1.2. 1.2 Precipitation reaction The calcium source solution was placed in a constant temperature stirring device, the stirring speed was adjusted to 500 r / min, the reaction temperature was controlled at 30℃, and the silicon source solution was slowly added dropwise to the calcium source solution at a rate of 2 mL / min for a total dropping time of 60 min. After the dropping was completed, the stirring was maintained and the reaction was continued for 90 min to obtain the initial reaction solution. 1.3 Dispersion Modification Add polycarboxylic acid dispersant (same as in Example 1) to the initial reaction solution. The amount added is 5% of the total mass of calcium nitrate and sodium silicate. After adding, continue stirring for 90 minutes to obtain the modified reaction solution. 1.4 Centrifugal Separation The modified reaction solution was placed in a centrifuge, the centrifugation speed was adjusted to 5000 r / min, the centrifugation time was 20 min, and the solid product was collected. 1.5 Washing and Purification The solid product was washed five times with deionized water, and centrifuged after each wash until the pH of the washing solution was neutral. 1.6 Drying and Grinding The washed solid product was dried in a vacuum drying oven at 80℃ for 24 hours. After being removed, it was dry-ground for 30 minutes using a planetary ball mill at a speed of 300 r / min. The grinding media was zirconia balls with a ball-to-material ratio of 5:1. After grinding, it was passed through a 200-mesh sieve to obtain irregularly shaped, loosely porous nano-CSH crystal nuclei, which were then sealed and stored for later use.

[0052] Testing revealed that the crystal nuclei obtained in this embodiment had a sheet diameter of 70-100 nm and no hard agglomeration.

[0053] 2. Preparation and Construction of Nano-CSH Nucleus-Reinforced Shotcrete 2.1 Raw material ratio Weigh out the following components by weight: 450 parts of P·O 42.5 grade cement, 70 parts of Grade II fly ash, 854 parts of sand with a fineness modulus of 2.8, 789 parts of crushed stone with a particle size of 5~10mm, 198 parts of water, 5.3 parts of polycarboxylate superplasticizer with a water reduction rate of 28%, and 28.8 parts of liquid alkali-free quick-setting agent. 2.2 Mixing and Construction The mixing process is the same as in Example 1. The slump of the resulting concrete mixture is 135 mm and the spread is 390 mm. The spraying parameters are the same as in Example 1. The ambient temperature for this example is 45℃ (extreme high ground temperature conditions). The aggregate is pre-cooled to 28℃ before being put into use.

[0054] 3. Performance Testing According to GB 50086 standard, the test results are as follows: 8-hour compressive strength 13.4 MPa, 1-day compressive strength 20.2 MPa, 3-day compressive strength 29.4 MPa, 28-day compressive strength 38.1 MPa; 1-day bond strength 1.21 MPa, construction rebound rate 6.8%; 28-day impermeability pressure 1.6 MPa, chloride ion diffusion coefficient 3.8 × 10⁻⁶. - With a speed of ¹²m² / s, it boasts excellent overall performance and is suitable for tunnel working conditions characterized by extreme high ground temperatures and high corrosion.

[0055] Comparative Example 1 Comparative Example 1 uses conventional shotcrete with the same mix proportions and construction process as Example 1, but without the addition of flaky nano-CSH crystal nuclei. Tested under the same high ground temperature environment of 40℃, the results are as follows: 8-hour compressive strength 6.8 MPa, 1-day compressive strength 11.1 MPa, 3-day compressive strength 19.2 MPa, construction rebound rate 15.3%, 28-day impermeability pressure 1.0 MPa, and chloride ion diffusion coefficient 6.8 × 10⁻⁶. -12 m 2 / s, all performance characteristics are significantly lower than those of the shotcrete in Example 1 of this application, which fully demonstrates that the sheet-like nano CSH crystal nuclei of this application have the effect of improving the comprehensive performance of shotcrete in high geothermal tunnels.

[0056] Comparative Example 2 The only difference from Example 1 is that Comparative Example 2 uses spherical nano-CSH crystal nuclei, prepared by the sol-gel method, with a particle size of 60~90nm. The test results are shown in Table 1.

[0057] Comparative Example 3 The only difference from Example 1 is that Comparative Example 3 uses spherical nano-CSH crystal nuclei, prepared by single mineral hydration method, with a particle size of 200~500nm. The test results are shown in Table 1.

[0058] Table 1: Test results of Example 1, Comparative Example 2 and Comparative Example 3

[0059] The test results of Examples 1-2 and Comparative Examples 1-3 show that the sheet-like nano-CSH crystal nuclei of this application have significantly better performance than existing spherical and agglomerated crystal nuclei. Regarding early strength, the high specific surface area of ​​the sheet-like porous structure provides far more hydration nucleation sites than the spherical structure. The loose porous nature ensures sufficient contact between the crystal nuclei and the cement paste, resulting in a prominent nucleation acceleration effect. In contrast, the effective nucleation sites of the agglomerated crystal nuclei are internally encapsulated, leading to a very weak hydration acceleration effect. The spherical crystal nuclei prepared by the sol-gel method in Comparative Example 2 exhibit better dispersibility than those obtained by the single-mineral hydration method in Comparative Example 3. Although the early strength performance of Comparative Example 2 is better than that of Comparative Example 3, it is still lower than that of the sheet-like porous crystal nuclei prepared in this application. This indicates that dispersibility is the core factor affecting the early strength performance of crystal nuclei, and the in-situ modification process of this application can achieve a dispersibility effect far superior to traditional preparation methods. Regarding interfacial bond strength, the interfacial bond strength of the embodiments in this application is significantly better than that of the two control groups. Among them, the bond strength of Comparative Example 3 is the worst, indicating that the overlapping and interlocking characteristics of the sheet-like structure can form a dense cementitious layer at the interface between concrete and surrounding rock and between the sprayed layers, thereby improving the mechanical interlocking force of the interface. In contrast, the interfacial overlapping effect of the spherical structure is poor, and the agglomerated structure is prone to forming weak areas at the interface, ultimately leading to a significant decrease in bonding strength. Regarding construction rebound rate, the sheet-like porous crystal nuclei can optimize the cohesiveness of cement paste, improve the paste's ability to encapsulate aggregates, reduce material loss during spraying, and significantly reduce construction rebound loss.

[0060] In summary, this application achieves stable and controllable preparation of lamellar porous CSH crystal nuclei through process optimization including reaction system selection, controllable precipitation regulation, in-situ dispersion modification, and mild post-treatment to preserve structure. Firstly, this application selects calcium nitrate, which has high solubility and a slow reaction rate, as the calcium source and sodium silicate as the silicon source. This avoids the irregular agglomeration of crystal grains caused by localized surges in ion concentration during the reaction, reducing byproduct formation from the reaction source. The entire reaction process is free of severe exothermic reactions, ensuring high operational safety and providing a uniform reaction basis for the directional growth of lamellar crystal grains. Secondly, this application employs a reaction mode of low-speed dropwise addition of silicon source solution to calcium source solution and constant temperature and uniform stirring throughout the process, allowing for precise control of Ca2+ content. 2+ With SiO3 2- The contact reaction rate can guide the preferential growth of CSH grains along the two-dimensional direction, thereby forming uniformly sized lamellar grains. Furthermore, this application immediately adds a polycarboxylic acid dispersant with a specific structure after the precipitation reaction is completed. Its molecular chains can be rapidly adsorbed onto the surface of the newly formed CSH grains. On the one hand, this inhibits grain agglomeration through steric hindrance, reducing the dispersion difficulty for subsequent applications; on the other hand, it further guides the grains to maintain their lamellar growth trend, avoiding the formation of spherical or irregular agglomerates. Simultaneously, the modified crystal nuclei exhibit excellent compatibility with polycarboxylic acid additives, and there are no compatibility issues when subsequently applied to cement-based materials.

[0061] The various embodiments of this application have been described above. These descriptions are exemplary and not exhaustive, nor are they limited to the disclosed embodiments. Many modifications and variations will be apparent to those skilled in the art without departing from the scope and spirit of the described embodiments. The terminology used herein is chosen to best explain the principles, practical application, or improvement of the technology in the market, or to enable other those skilled in the art to understand the embodiments disclosed herein.

Claims

1. A method for preparing sheet-like nano-CSH crystal nuclei, characterized in that, Includes the following steps: S1. Dissolve calcium nitrate and sodium silicate separately in deionized water and stir until completely dissolved to obtain calcium source solution and silicon source solution; S2. Place the calcium source solution in a constant temperature stirring device, and add the silicon source solution dropwise to the calcium source solution while stirring. After the addition is completed, continue the reaction while stirring to obtain the initial reaction solution; the calcium-silicon molar ratio of the calcium source solution to the silicon source solution is 1:(1~1.2). S3. Under stirring, add a polycarboxylic acid dispersant obtained by copolymerizing acrylic acid and polyether macromonomer TPEG-5000 to the initial reaction solution to obtain a modified reaction solution; S4. Centrifuge the modified reaction solution to obtain a solid product; S5. Repeat the washing and centrifugation process on the solid product until the washing solution is neutral. S6. The washed solid product is vacuum dried at a temperature below 80°C, followed by dry grinding and sieving to obtain sheet-like nano CSH crystal nuclei.

2. The method for preparing sheet-like nano-CSH crystal nuclei according to claim 1, characterized in that, In step S1, the concentration of the calcium source solution is 0.5~1.0 mol / L, and the concentration of the silicon source solution is 0.5~1.0 mol / L; In step S2, the stirring rate is 300~500 r / min, the reaction temperature is 25~30℃; the silicon source solution drop rate is 1~2 mL / min, the drop time is 30~60 min, and the stirring reaction continues for 60~90 min after the drop is completed; In step S3, the polycarboxylic acid dispersant has a number average molecular weight of 10,000 and a polydispersity index of 1.3; the amount of polycarboxylic acid dispersant added is 3% to 5% of the total mass of calcium nitrate and sodium silicate; after adding the polycarboxylic acid dispersant, stirring is continued at a rate of 300 to 500 r / min for 60 to 90 min.

3. The method for preparing sheet-like nano-CSH crystal nuclei according to claim 1, characterized in that, In step S4, the centrifugation speed is 3000~5000 r / min, and the centrifugation time is 10~20 min; In step S6, the dry grinding specifically involves grinding for 20-30 minutes using a planetary ball mill at a grinding speed of 200-300 r / min, using zirconia balls as the grinding medium, and a ball-to-material ratio of 5:

1. The obtained sheet-like nano-CSH crystal nuclei have a diameter of 50~100nm and possess a porous structure.

4. A sheet-like nano-CSH crystal nucleus, characterized in that, It was prepared using the method for preparing sheet-like nano-CSH crystal nuclei as described in any one of claims 1 to 3.

5. A type of shotcrete, characterized in that, The mixture includes cement, fly ash, sand, crushed stone, water, polycarboxylate superplasticizer, liquid alkali-free quick-setting agent, and flake-shaped nano-CSH crystal nuclei; the amount of liquid alkali-free quick-setting agent added is 5.5%~6.5% of the total mass of cement and fly ash, and the amount of flake-shaped nano-CSH crystal nuclei added is 1.0%~1.5% of the total mass of cement and fly ash; The sheet-like nano-CSH crystal nuclei are prepared by the method for preparing sheet-like nano-CSH crystal nuclei according to any one of claims 1 to 3, or are the sheet-like nano-CSH crystal nuclei according to claim 4.

6. The shotcrete according to claim 5, characterized in that, The amounts of each component, by weight, are as follows: 400-450 parts cement, 60-80 parts fly ash, 800-854 parts sand, 750-789 parts crushed stone, 190-200 parts water, and 5.0-5.3 parts polycarboxylate superplasticizer.

7. The shotcrete according to claim 5, characterized in that, The fineness modulus of the sand is 2.6 to 3.0, and the particle size of the crushed stone is 5 to 10 mm. The water reduction rate of the polycarboxylate superplasticizer is ≥25%; The initial setting time of the liquid alkali-free quick-setting agent is <5 min, and the final setting time is <12 min.

8. A method of using shotcrete according to any one of claims 5 to 7, characterized in that, The method includes the following steps: (1) Add cement, fly ash, sand and gravel into a mixer according to the proportion, and mix for 3-5 minutes until the mixture is uniform to obtain powder; (2) Dissolve the polycarboxylate superplasticizer in water to obtain a superplasticizer aqueous solution. Then add the superplasticizer aqueous solution to the powder and continue stirring for 5-8 minutes. Finally, add the flaky nano CSH crystal nuclei and continue stirring for 3-5 minutes to obtain a concrete mixture. Send the concrete mixture into the spraying machine. (3) Add liquid alkali-free quick-setting agent to the front end of the spraying machine through a metering pump. After mixing with the concrete mixture online, spray it to the designed thickness in several batches.

9. The method of using shotcrete according to claim 8, characterized in that, The spraying machine has a spraying pressure of 0.3~0.5MPa, a spraying distance of 1.5~2.0m, a spraying angle of 75°~85°, a single spraying thickness of 8~10cm, and a spraying interval of 1~2h. The slump of the concrete mixture is 120~140mm, and the spread is 350~400mm.

10. The method of using shotcrete according to claim 9, characterized in that, The applicable ambient temperature for the sprayed concrete is 5℃~35℃; if the ambient temperature is >35℃, the water-reducing agent aqueous solution, powder and flaky nano CSH crystal nuclei are mixed in a water bath at a temperature of 25℃~30℃ using a double-layer mixing tank.

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