A concrete internal curing agent for high ground temperature underground caverns and a method for preparing the same

Abstract

The application discloses a kind of concrete internal curing agent for high-temperature underground chamber and its preparation method, belong to concrete material technical field.The internal curing agent is composed of sealing film liquid and superabsorbent polymer, sealing film liquid is compounded by epoxy modified polyurethane emulsion, nano silicon dioxide, carboxyl-containing acrylate copolymer, temperature-sensitive polyether polyol and coupling agent in proportion, form the core-shell structure with sealing film liquid as shell, SAP as core, with good high-temperature resistance and alkali resistance.The internal curing agent does not affect the working performance of concrete, in the high-temperature environment of 30~90 DEG C, when the hydration process of cement causes the pH value of pore solution in concrete to rise, the temperature of concrete rises and internal water shortage occurs, sealing film gradually ruptures under the synergistic effect of high temperature and high alkali, releases the water stored by SAP, provides the required free water for the continuous hydration of cement in concrete, effectively improves the strength and durability of concrete under high-temperature conditions.

Description

Technical Field

[0001] This invention belongs to the field of concrete materials, specifically relating to an internal curing agent for concrete used in high-temperature underground tunnels and its preparation method. Background Technology

[0002] With the rapid development of my country's transportation infrastructure, tunnel engineering has become a key means to overcome complex terrain, shorten spatial distances, and improve road network efficiency. The number of deep-buried long tunnels being built in geologically complex areas such as southwest China for railways, highways, water conservancy, and mining is increasing, and the accompanying problem of high ground temperature is becoming more and more prominent.

[0003] High-temperature tunnels mainly form in deeply buried underground projects or areas with active geothermal activity. Their formation is controlled by both geological and geographical conditions: on the one hand, as the burial depth increases, the ground temperature continues to rise according to the geothermal gradient, causing a large amount of heat energy to accumulate in the deep rock mass; on the other hand, if the project passes through fault zones, igneous rock intrusion areas, or strata with frequent groundwater and thermal activity, the heat flow value in the rock mass will increase significantly, further aggravating heat accumulation.

[0004] Based on the distribution and migration characteristics of groundwater, high ground temperature in tunnels mainly manifests in two types: dry-heat and humid-heat. Humid-heat type high ground temperature is commonly found in water-rich fracture zones or hot water runoff areas, where the interaction between high-temperature rock and groundwater creates a high-humidity and high-temperature environment, resulting in high humidity inside the tunnel. Dry-heat type high ground temperature often occurs in relatively intact geological structures with low permeability, where heat is mainly transferred through rock conduction and radiation, resulting in a high-temperature and low-humidity environment, which easily causes dehydration of the surrounding rock and concrete. The sustained high temperature in dry-heat type tunnel sections rapidly accelerates the evaporation of moisture inside the concrete, leading to significant water loss in the early stages. This process not only significantly reduces the available water for later hydration reactions, causing premature termination of the hydration reaction, but also causes significant drying shrinkage due to excessive early moisture loss, thereby impairing the volume stability and long-term durability of the concrete.

[0005] Currently, there are many concrete improvement measures applicable to high-temperature tunnel sections, most of which are limited to adjusting the components within the concrete, establishing heat insulation structures, and setting up various structures. For example, the high-temperature resistant support structures disclosed in patents with patent numbers 202020216760.7, 201710408796.8, 201510092058.8, and 202111490592.6 can reduce or insulate the tunnel section temperature in the short term, but cannot achieve long-term effective heat insulation. Over time, the high temperature in the tunnel section will still cause the free water inside the concrete to evaporate rapidly, reducing the water required for subsequent hydration reactions, thereby triggering a series of concrete performance degradation problems. Patents with numbers 201510520397.1, 202410005085.6, and 202510138961.7 disclose heat-insulating and heat-resistant concrete materials, using low-thermal-conductivity materials such as expanded clay and vitrified microspheres to prepare heat-insulating concrete, aiming to reduce the thermal conductivity of the materials and delay heat transfer. However, the heat insulation effect is limited in duration, and the temperature in the tunnel section rises again within a few days, causing the internal moisture of the concrete to continue to evaporate, affecting the performance of the concrete. Patents with numbers 202010033964.1, 202510597447.X, and 202510872590.5 disclose methods to alleviate heat damage, such as adjusting the mix proportion, using heat-resistant cement, and adding large amounts of mineral admixtures. However, these methods fail to solve the problem of rapid loss of free water from concrete due to high ground temperatures. Patent 202111238447.9 proposes using superabsorbent resin for internal curing of concrete. However, resins rapidly absorb mixing water, impairing the workability of concrete; and under high-temperature conditions, their internal moisture is quickly released along with the early water loss of the concrete, leading to a significant reduction in the effectiveness of internal curing. Furthermore, the mentioned film-coating curing method presents challenges in practical engineering, including implementation difficulties, high costs, and delays. Therefore, there is an urgent need to develop a new, highly efficient internal curing material that does not affect the workability of concrete and can continuously retain moisture in the later stages of curing in high-temperature tunnel sections, preventing heat damage. Summary of the Invention

[0006] The purpose of this invention is to overcome the shortcomings of the prior art and provide a concrete internal curing agent for high-temperature underground tunnels and its preparation method.

[0007] The objective of this invention is achieved through the following technical solution:

[0008] The first aspect of the present invention is to provide a concrete internal curing agent for high-temperature underground tunnels, comprising, by weight parts: 65-80 parts of sealing film liquid, 20-35 parts of superabsorbent resin particles, forming a core-shell structure with the sealing film liquid as the shell and the superabsorbent resin particles as the core; the sealing film liquid is prepared from the following raw materials in weight parts: 5-8 parts of nano-silica, 6-9 parts of deionized water, 30-40 parts of modified polyurethane emulsion, 13-17 parts of carboxyl-containing acrylate copolymer, 7-11 parts of thermosensitive polyether polyol, and 2-3 parts of coupling agent, wherein the modified polyurethane emulsion refers to a polyurethane emulsion modified by introducing epoxy groups.

[0009] The preparation of the sealing film liquid includes the following steps:

[0010] S1-1: The nano-silica is mixed with deionized water and then ultrasonically dispersed at a power of 400~600W and a frequency of 20~25kHz for 15~25min to obtain a nano-silica aqueous dispersion.

[0011] S1-2: Add the modified polyurethane emulsion to the stirred tank, turn on the disperser, set the initial speed to 500~600 r / min, and stir for 5~10 min to make the modified polyurethane emulsion evenly distributed in the tank;

[0012] S1-3: Inject the nano-silica aqueous dispersion into the stirred tank, maintain the speed of the disperser at 500~600 r / min, and stir for 15~20 min to fully integrate the nano-silica aqueous dispersion with the modified polyurethane emulsion; then add the carboxyl-containing acrylate copolymer and the thermosensitive polyether polyol in sequence, increase the speed of the disperser to 800~1200 r / min, and continue to disperse for 30~45 min to ensure that there are no obvious particles or stratification in the mixture;

[0013] S1-4: Reduce the speed of the disperser to 600-700 r / min, slowly add the coupling agent dropwise at a rate of 0.5-1 part / min, and continue stirring for 20-30 min after the addition is complete to allow the coupling agent to fully crosslink with the components in the mixture. Then add deionized water to adjust the viscosity of the mixture to 200-300 mPa•s, stir for 10-15 min, then turn off the disperser and let it stand for 15-20 min to allow the air bubbles in the mixture to be expelled, thus obtaining the sealing film liquid.

[0014] The internal curing agent of this invention is formed by encapsulating superabsorbent resin particles with a sealing film liquid. Using superabsorbent resin as the primary water-retention technology, a core-shell structure is formed with the sealing film liquid as the shell and SAP as the core, exhibiting both excellent high-temperature resistance and alkali resistance. A temperature-responsive mechanism is also formed within the sealing film layer, enabling it to autonomously respond to temperature changes in the concrete laying area and release superabsorbent resin particles to cope with complex temperature variations. Specifically, the sealing film liquid of this application uses a modified polyurethane emulsion as its base component, employing epoxy modification. The presence of epoxy groups enhances the mechanical strength and alkali resistance of the sealing film liquid, ensuring the film layer remains intact in a weakly alkaline environment with a pH of 7-12. Simultaneously, it can crosslink with the amino groups in the coupling agent, strengthening the cohesion and wear resistance of the film layer.

[0015] Secondly, the use of nano-silica allows for uniform dispersion within the polyurethane substrate, forming an "organic-inorganic interpenetrating network." This enhances the tensile strength and elastic modulus of the sealing film, effectively preventing wear and tear during transportation and mixing. Furthermore, it forms covalent silicon-oxygen bonds with the silanol groups generated by the hydrolysis of the coupling agent and hydrogen bonds with the hydroxyl groups of the polyurethane, improving component compatibility, preventing nanoparticle aggregation, thereby reducing porosity and strengthening water-blocking effects. In addition, the selected nano-silica can fill tiny defects (pore size ≤100nm) within the film, increasing density and preventing the migration of impurity ions, protecting the SAP from corrosion. This application also found that even when the sealing film ruptures under high-temperature and high-alkali conditions, the nano-silica can undergo a secondary pozzolanic reaction in an alkaline water environment, continuously enhancing the later-stage strength of the concrete.

[0016] The use of carboxyl-containing acrylate copolymers to synergistically regulate the functions of each component, with ester bonds acting as alkali-responsive sites, can be slowly hydrolyzed and broken in the high-alkali environment of cement (pH=12.5~13.5), promoting molecular chain degradation and disintegrating the cross-linked structure of the membrane.

[0017] In the high-temperature cave environment (temperature ≥60℃), the molecular chain segments of the temperature-sensitive polyether polyol will soften, leading to a decrease in the rigidity and an increase in the brittleness of the film. This, combined with the hydrolysis of ester bonds in the alkaline copolymer, results in the synergistic slow rupture of the film through physical softening and chemical degradation.

[0018] Preferably, the modified polyurethane molecular chains in the modified polyurethane emulsion simultaneously contain polyurethane urethane bonds, urea bonds, and epoxy groups, with hydroxyl and carboxyl groups at the molecular chain ends. According to experimental verification in this application, this modified polyurethane emulsion can rapidly fuse on the surface of superabsorbent polymer (SAP) particles to form a dense, non-porous, continuous film, effectively blocking water penetration. Furthermore, the urethane bonds impart flexibility to the film to accommodate the water absorption and swelling of SAP, preventing cracking.

[0019] Specifically, when used in conjunction with a coupling agent, the amino groups in the coupling agent react with the hydroxyl groups in the epoxy-modified polyurethane emulsion and the carboxyl groups in the carboxyl-containing acrylate copolymer. Simultaneously, the silanol groups generated from the hydrolysis of ethoxysilane form Si-O-Si covalent bonds with the silanol groups on the surface of nano-silica, acting as an "organic-inorganic bridging" agent. This effectively improves the compatibility between polyurethane and nano-silica, ensuring uniform dispersion of nanoparticles in the membrane solution and preventing membrane defects caused by agglomeration. At the same time, the chemical bonding strengthens the interfacial adhesion between the sealing membrane and the SAP surface, preventing membrane detachment during application processes such as stirring and spraying.

[0020] Preferably, the modified polyurethane emulsion has a solid content of 45-55 wt%, a particle size of 100-300 nm, a viscosity of 200-500 mPa•s, a minimum film-forming temperature of ≤5℃, and a pH value of 6.5-8.0. A modified polyurethane emulsion with uniform particle size and a low film-forming temperature is preferred, resulting in a good morphology of the sealing film liquid and enabling rapid film formation at low temperatures.

[0021] Preferably, the d50 value of the nano-silica is 20~50nm, and the particles are spherical or quasi-spherical.

[0022] Preferably, the carboxyl-containing acrylate copolymer is copolymerized from methyl acrylate, butyl acrylate, and hydroxyethyl methacrylate in a mass ratio of 3:5:2, wherein the ester bond density is 8-10 per 100 repeating units, and the weight-average molecular weight is 50,000-80,000. The butyl acrylate segment imparts flexibility and synergistically toughens with polyurethane; the methyl acrylate segment enhances the rigidity of the film; and the hydroxyl groups of hydroxyethyl methacrylate participate in crosslinking, strengthening the adhesion between the film and SAP.

[0023] Preferably, the thermosensitive polyether polyol is a block copolymer composed of polypropylene oxide and polyethylene oxide, wherein the polyethylene oxide segment accounts for 20-50 wt% and has a weight-average molecular weight of 2500-3500. The ether bonds in the PEO segment impart a certain degree of water resistance to the membrane, preventing premature failure due to swelling caused by moisture in the SAP. Its hydroxyl groups participate in the crosslinking reaction, enhancing the membrane's cohesion and structural stability at room temperature. The "heat-alkali dual-response" system constructed together with the carboxyl-containing acrylate copolymer results in a low membrane rupture rate under either a single heat or alkali environment, while the rupture rate increases under combined heat and alkali conditions, achieving specific control of environmental response.

[0024] Preferably, the coupling agent is γ-aminopropyltriethoxysilane.

[0025] Preferably, the superabsorbent polymer (SAP) particles have a particle size of 0.1~0.3 mm, a saturated water absorption rate of 300~500 g / g, and a particle size of 0.3~1.5 mm after saturation. Limiting the particle size after saturation maximizes its water retention capacity and ensures optimal synergy with the components in the sealing film liquid. In high-temperature environments (30~90℃), when the cement hydration process causes an increase in the pH value of the pore solution in the concrete, an increase in concrete temperature, and internal water shortage, the sealing film gradually ruptures under the synergistic effect of high temperature and high alkali, releasing the water stored in the SAP. This provides the necessary free water for the continuous hydration of cement in the concrete. Simultaneously, the released water, the nano-silica on the SAP surface, and the calcium hydroxide (a cement hydration product) undergo a secondary pozzolanic reaction, further optimizing the internal microstructure of the concrete. This not only does not affect the workability of the concrete but also further optimizes its strength and other properties, making it more suitable for high-temperature environments.

[0026] A second aspect of the present invention is to provide a method for preparing the above-mentioned concrete internal curing agent for high-temperature underground tunnels, comprising the following steps:

[0027] S1 Preparation of Sealing Film Solution:

[0028] S1-1: The nano-silica is mixed with deionized water and then ultrasonically dispersed at a power of 400~600W and a frequency of 20~25kHz for 15~25min to obtain a uniformly dispersed nano-silica aqueous dispersion.

[0029] S1-2: Add the modified polyurethane emulsion to the stirred tank, turn on the disperser, set the initial speed to 500~600 r / min, and stir for 5~10 min to make the modified polyurethane emulsion evenly distributed in the tank.

[0030] S1-3: Inject the nano-silica aqueous dispersion into the stirred tank, maintain the speed of the disperser at 500~600 r / min, and stir for 15~20 min to fully integrate the nano-silica aqueous dispersion with the modified polyurethane emulsion; then add the carboxyl-containing acrylate copolymer and the thermosensitive polyether polyol in sequence, increase the speed of the disperser to 800~1200 r / min, and continue to disperse for 30~45 min. During this period, the system status can be monitored through a visual observation window to ensure that there are no obvious particles or stratification in the mixture.

[0031] S1-4: Reduce the speed of the disperser to 600-700 r / min, slowly add the coupling agent dropwise at a rate of 0.5-1 part / min, and continue stirring for 20-30 min after the addition is complete to allow the coupling agent to fully crosslink with the components in the mixture. Then add deionized water and adjust the viscosity of the mixture to 200-300 mPa•s according to the viscosity of the system. Stir for 10-15 min, then turn off the disperser and let it stand for 15-20 min to allow the air bubbles in the mixture to be expelled, resulting in a milky white, semi-transparent sealing film liquid without precipitation or lumps.

[0032] S2 resin pretreatment: The superabsorbent resin particles are soaked in deionized water for 24-36 hours until they are saturated with water. Then, they are vacuum dried at 30-40°C until there is no free water on the particle surface to obtain saturated superabsorbent resin particles. The saturated superabsorbent resin particles are placed in a fluidized bed, and the bed temperature is adjusted to 40-50°C, the hot air temperature is 50-60°C, the air velocity is controlled at 0.8-1.0 m / s, and the bed is run stably for 10-15 minutes.

[0033] S3 Sealing Film Liquid Atomization Spraying: The sealing film liquid is atomized into tiny droplets of 5~10μm and continuously sprayed onto the surface of the saturated water-absorbing resin particles. After a uniform film layer is formed on the surface of the saturated water-absorbing resin particles without any exposed areas or lumps, the spraying is stopped. The fluidized state and hot air circulation are maintained for 10~15 minutes to allow the water in the sealing film liquid to evaporate, thus obtaining the concrete internal curing agent for high geothermal underground chambers.

[0034] Preferably, in the S3 sealing film liquid atomization spraying, a high-pressure atomizing nozzle with an orifice diameter of 0.2 mm is installed on the fluidized bed for atomization spraying. The atomization pressure during atomization spraying is set to 0.3~0.5 MPa, the spraying rate is 5~8 mL / min, and the spraying time is 30~40 min. During the spraying process, the spraying status of the resin particles can be observed through a viewing window. Spraying is stopped when there is no obvious agglomeration in the resin particles.

[0035] The beneficial effects of this invention are:

[0036] (1) This invention designs an internal curing agent for concrete in high-temperature underground chambers, composed of a sealing film liquid and superabsorbent polymer (SAP), forming a core-shell structure with the sealing film liquid as the shell and SAP as the core, possessing both good high-temperature resistance and alkali resistance. Without affecting the workability of the concrete, in a high-temperature environment of 30–90℃, when the cement hydration process causes an increase in the pH value of the pore solution in the concrete, an increase in the concrete temperature, and an internal water shortage, the sealing film gradually ruptures under the synergistic effect of high temperature and high alkali, rapidly releasing the water stored in the SAP, providing the necessary free water for the continuous hydration of cement in the concrete. Simultaneously, the released water, the nano-silica on the SAP surface, and the calcium hydroxide, a cement hydration product, undergo a secondary pozzolanic reaction, further effectively optimizing the internal microstructure of the concrete and continuously enhancing its later-stage strength. This internal curing agent effectively improves the strength and durability of concrete under high-temperature conditions, has a simple preparation process, and is more suitable for underground engineering projects such as high-temperature underground chambers.

[0037] (2) This invention uses epoxy-modified polyurethane emulsion as the base of the sealing film liquid. Its nano-scale uniform particle size and low film-forming temperature characteristics form a continuous, dense, flexible matrix film layer on the surface of SAP particles, enabling the system to possess excellent water-blocking performance and good flexible load-bearing capacity at room temperature. Nano-silica, as an inorganic reinforcing phase, forms covalent and hydrogen bonds with the coupling agent and polyurethane molecular chains through surface silanol groups, constructing an "organic-inorganic interpenetrating network," significantly improving the mechanical strength, thermal stability, and alkali resistance of the film layer, effectively preventing damage caused by friction or impact during transportation and construction. The coupling agent acts as a "molecular bridge," simultaneously bridging the organic and inorganic phases, strengthening the interfacial bonding, making the film layer structure uniform and stable, and achieving high-strength adhesion between the core material and the sealing film.

[0038] (3) The sealing membrane layer designed in this invention also contains a uniformly distributed carboxyl acrylate copolymer and a thermosensitive polyether polyol, which together constitute a "heat-alkali dual-response" controlled-release unit. In a neutral environment at room temperature, the two maintain chemical stability and physical rigidity, ensuring the integrity of the sealing membrane structure. When entering a high-temperature and high-alkali environment, the heat-sensitive polyether polyol undergoes a transition from a glassy state to a highly elastic state, causing the membrane layer to soften physically and become more brittle; at the same time, the ester bonds in the alkali-hydrolyzed copolymer molecular chain slowly hydrolyze and break under high-temperature and high-alkali conditions, triggering polymer degradation. The synergistic effect of the two leads to the destruction of the membrane layer's cross-linked structure, achieving a reliable switch from "sealing and water retention" to "rupture and water release," and this response mechanism has high environmental specificity, avoiding premature release of moisture. Detailed Implementation

[0039] The technical solution of the present invention will be clearly and completely described below with reference to the embodiments. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention.

[0040] This invention provides a technical solution for preparing a concrete internal curing agent for high-temperature underground tunnels, comprising the following steps:

[0041] S1 Preparation of Sealing Film Solution:

[0042] S1-1: The nano-silica is mixed with deionized water and then ultrasonically dispersed at a power of 400~600W and a frequency of 20~25kHz for 15~25min to obtain a uniformly dispersed nano-silica aqueous dispersion.

[0043] S1-2: Add the modified polyurethane emulsion to the stirred tank, turn on the disperser, set the initial speed to 500~600 r / min, and stir for 5~10 min to make the modified polyurethane emulsion evenly distributed in the tank.

[0044] S1-3: Inject the nano-silica aqueous dispersion into the stirred tank, maintain the speed of the disperser at 500~600 r / min, and stir for 15~20 min to fully integrate the nano-silica aqueous dispersion with the modified polyurethane emulsion; then add the carboxyl-containing acrylate copolymer and the thermosensitive polyether polyol in sequence, increase the speed of the disperser to 800~1200 r / min, and continue to disperse for 30~45 min. During this period, the system status can be monitored through a visual observation window to ensure that there are no obvious particles or stratification in the mixture.

[0045] S1-4: Reduce the speed of the disperser to 600-700 r / min, slowly add the coupling agent dropwise at a rate of 0.5-1 part / min, and continue stirring for 20-30 min after the addition is complete to allow the coupling agent to fully crosslink with the components in the mixture. Then add deionized water and adjust the viscosity of the mixture to 200-300 mPa•s according to the viscosity of the system. Stir for 10-15 min, then turn off the disperser and let it stand for 15-20 min to allow the air bubbles in the mixture to be expelled, resulting in a milky white, semi-transparent sealing film liquid without precipitation or lumps.

[0046] S2 resin pretreatment: The superabsorbent resin particles are soaked in deionized water for 24-36 hours until they are saturated with water. Then, they are vacuum dried at 30-40°C until there is no free water on the particle surface to obtain saturated superabsorbent resin particles. The saturated superabsorbent resin particles are placed in a fluidized bed, and the bed temperature is adjusted to 40-50°C, the hot air temperature is 50-60°C, the air velocity is controlled at 0.8-1.0 m / s, and the bed is run stably for 10-15 minutes.

[0047] S3 Sealing Film Liquid Atomization Spraying: The sealing film liquid is atomized into tiny droplets of 5~10μm and continuously sprayed onto the surface of the saturated water-absorbing resin particles. After a uniform film layer is formed on the surface of the saturated water-absorbing resin particles without any exposed areas or lumps, the spraying is stopped. The fluidized state and hot air circulation are maintained for 10~15 minutes to allow the water in the sealing film liquid to evaporate, thus obtaining the concrete internal curing agent for high geothermal underground chambers.

[0048] In both the examples and comparative examples, the concrete mix proportions are: 1m³ 3 Shotcrete consists of 460 kg of cement, 862 kg of sand, 862 kg of gravel, 175 kg of water, 36.8 kg of quick-setting agent, and 4.6 kg of water-reducing agent. Example 1

[0049] The preparation process of the internal maintenance agent in Example 1 is basically the same as that in the specific implementation method, with only the raw material formula ratio being adjusted. The specific implementation is as follows:

[0050] A nano-silica aqueous dispersion was prepared by mixing 6 parts of nano-silica with 6 parts of deionized water. The nano-silica aqueous dispersion was then added to 35 parts of epoxy-modified polyurethane emulsion, followed by 17 parts of carboxyl-containing acrylate copolymer and 10 parts of thermosensitive polyether polyol. The mixture was stirred continuously at 1000 rpm for 30-45 minutes to form a homogeneous liquid. Then, 2 parts of coupling agent were slowly added dropwise while stirring to prepare a sealing film solution. The sealing film solution was then loaded into a high-pressure atomizing device and evenly sprayed onto the surface of 24 parts of superabsorbent resin particles that had already absorbed water. The solution was then dried at 50-60°C to prepare an internal curing agent.

[0051] In 1m 3 Add 40 kg of the prepared internal curing agent to the mix proportion of the shotcrete. Add cement, sand, gravel, internal curing agent, water-reducing agent, and water to the mixer and mix for 120 seconds. Then add the accelerator and mix for 15 seconds. Pour the concrete into a compression mold, pressurize and vibrate to compact it, and then transfer it to a curing room for 8 hours. After the mold is removed and the concrete is weighed for the first time, it is placed in an 80-degree Celsius oven for curing. Weigh it again at a certain age.

[0052] In 1m 3Add 10 kg of the prepared internal curing agent to the mix proportion of the shotcrete. Add cement, sand, gravel, internal curing agent, water-reducing agent, and water to a mixer and mix for 120 seconds. Test the slump and then add an accelerator and mix for 15 seconds. Pour the concrete into a compressive strength test mold, compact it under pressure, and then place it directly in an 80℃ oven for curing. Test the compressive strength at a certain age. Example 2

[0053] The preparation process of the internal maintenance agent in Example 2 is basically the same as that in the specific implementation method, with only the raw material formula ratio being adjusted. The specific implementation is as follows:

[0054] A nano-silica aqueous dispersion was prepared by mixing 5 parts of nano-silica with 6 parts of deionized water. This dispersion was then added to 32 parts of epoxy-modified polyurethane emulsion, followed by 14 parts of carboxyl-containing acrylate copolymer and 7 parts of thermosensitive polyether polyol. The mixture was stirred continuously at 1000 rpm for 30-45 minutes to form a homogeneous liquid. Then, 2 parts of coupling agent were slowly added dropwise while stirring to prepare a sealing film solution. This sealing film solution was then loaded into a high-pressure atomizing device and evenly sprayed onto the surface of 34 parts of superabsorbent resin that had already absorbed water. The solution was then dried at 50-60°C to prepare an inner curing agent.

[0055] In 1m 3 Add 40 kg of the prepared internal curing agent to the mix proportion of the shotcrete. Add cement, sand, gravel, internal curing agent, water-reducing agent, and water to the mixer and mix for 120 seconds. Then add the accelerator and mix for 15 seconds. Pour the concrete into a compression mold, pressurize and vibrate to compact it, and then transfer it to a curing room for 8 hours. After the mold is removed and the concrete is weighed for the first time, it is placed in an 80-degree Celsius oven for curing. Weigh it again at a certain age.

[0056] In 1m 3 Add 10 kg of the prepared internal curing agent to the mix proportion of the shotcrete. Add cement, sand, gravel, internal curing agent, water-reducing agent, and water to a mixer and mix for 120 seconds. Test the slump and then add an accelerator and mix for 15 seconds. Pour the concrete into a compressive strength test mold, compact it under pressure, and then place it directly in an 80℃ oven for curing. Test the compressive strength at a certain age. Example 3

[0057] The preparation process of the internal maintenance agent in Example 3 is basically the same as that in the specific implementation method, with only the raw material formula ratio being adjusted. The specific implementation is as follows:

[0058] A nano-silica aqueous dispersion was prepared by mixing 7 parts of nano-silica with 6 parts of deionized water. This dispersion was then added to 37 parts of epoxy-modified polyurethane emulsion, followed by 17 parts of carboxyl-containing acrylate copolymer and 10 parts of thermosensitive polyether polyol. The mixture was stirred continuously at 1000 rpm for 30-45 minutes to form a homogeneous liquid. Then, 3 parts of coupling agent were slowly added dropwise while stirring to prepare a sealing film solution. This sealing film solution was then loaded into a high-pressure atomizing device and evenly sprayed onto the surface of 20 parts of superabsorbent resin that had already absorbed water. The solution was then dried at 50-60°C to prepare an inner curing agent.

[0059] In 1m 3 Add 40 kg of the prepared internal curing agent to the mix proportion of the shotcrete. Add cement, sand, gravel, internal curing agent, water-reducing agent, and water to the mixer and mix for 120 seconds. Then add the accelerator and mix for 15 seconds. Pour the concrete into a compression mold, pressurize and vibrate to compact it, and then transfer it to a curing room for 8 hours. After the mold is removed and the concrete is weighed for the first time, it is placed in an 80-degree Celsius oven for curing. Weigh it again at a certain age.

[0060] In 1m 3 Add 10 kg of the prepared internal curing agent to the mix proportion of the shotcrete. Add cement, sand, gravel, internal curing agent, water-reducing agent, and water to a mixer and mix for 120 seconds. Test the slump and then add an accelerator and mix for 15 seconds. Pour the concrete into a compressive strength test mold, compact it under pressure, and then place it directly in an 80℃ oven for curing. Test the compressive strength at a certain age. Comparative Example 1

[0061] Comparative Example 1 served as a blank control group, and the specific implementation details are as follows.

[0062] First, add cement, sand, gravel water-reducing agent and water to the mixer and mix for 120 seconds. After testing the slump, shovel the concrete into the mixing pot, add the quick-setting agent and mix for 15 seconds.

[0063] The concrete was poured into a compression mold, compacted by pressure and vibration, and then placed in a curing room for 8 hours before being demolded. After the first weighing, it was placed in an oven at 80 degrees Celsius for curing, and weighed again at a certain age.

[0064] The concrete is poured into a compressive strength test mold, compacted by pressure vibration, and then placed directly into an 80℃ oven for curing. The compressive strength is then tested at a certain age. Comparative Example 2

[0065] Comparative Example 2 served as a blank control group, and the specific implementation details are as follows.

[0066] In 1m 3Add 40 kg of water-saturated SAP to the mix proportion of the shotcrete. Add cement, sand, gravel, water-saturated SAP, and water to the mixer and mix for 120 seconds. Then add the accelerator and mix for 15 seconds. Pour the concrete into compression molds, compact it under pressure and vibration, and then transfer it to a curing room for 8 hours. After demolding and the first weighing, place it in an 80-degree Celsius oven for curing. Weigh it again at a certain age.

[0067] In 1m 3 Add 10 kg of water-saturated SAP to the shotcrete mix proportion. Add cement, sand, gravel, water-saturated SAP, water-reducing agent, and water to a mixer and mix for 120 seconds. Test the slump, then add an accelerator and mix for 15 seconds. Pour the concrete into a compressive strength test mold, compact it under pressure, and then place it directly in an 80℃ oven for curing. Test the compressive strength at a certain age.

[0068] The test results of workability, weight and mechanical properties of the shotcrete in Examples 1-3 and Comparative Examples 1-2 are shown in Tables 1 and 2 below:

[0069] Table 1. Concrete weight and weight loss in Examples 1-3 and Comparative Examples 1-2

[0070]

[0071] Table 2. Concrete compressive strength of Examples 1-3 and Comparative Examples 1-2

[0072]

[0073] Table 2 shows that the mechanical properties of shotcrete are significantly accelerated in the early stages of hydration under high ground temperatures of 80℃, reaching the design strength of C30 at 3 days. However, the data from Comparative Examples 1 and 2 show that the strength peaks at 7 days due to the high temperature environment, and then declines with prolonged curing time. This strength reduction phenomenon stems from the premature depletion of free water inside the concrete, interruption of subsequent hydration reactions, and the deterioration of the pore structure and crystal form of hydration products caused by high temperature. In stark contrast, the compressive strength of concrete shows a continuous increase after using the internal curing agent of this invention (Examples 1, 2, and 3). This is due to the dual mechanism of action of the internal curing agent: firstly, the sealing film on its surface can slowly rupture in a high-temperature and high-alkali environment, continuously releasing internal moisture to provide the water source required for long-term hydration of unhydrated cement particles; secondly, the free water released after the sealing film ruptures undergoes a secondary pozzolanic reaction with the simultaneously released nano-silica in the alkaline pore solution environment of the concrete, further optimizing the microstructure and thus enhancing the later strength of the concrete.

[0074] The above description is merely a preferred embodiment of the present invention. It should be understood that the present invention is not limited to the forms disclosed herein and should not be construed as excluding other embodiments. It can be used in various other combinations, modifications, and environments, and can be altered within the scope of the concept described herein through the above teachings or related technologies or knowledge. Modifications and variations made by those skilled in the art that do not depart from the spirit and scope of the present invention should be within the protection scope of the appended claims.

Claims

1. A concrete internal curing agent for high-temperature underground tunnels, characterized in that: It comprises, by weight parts: 65-80 parts of sealing film liquid and 20-35 parts of superabsorbent resin particles, forming a core-shell structure with the sealing film liquid as the shell and the superabsorbent resin particles as the core; The sealing film liquid is prepared from the following raw materials in parts by weight: 5-8 parts nano silica, 6-9 parts deionized water, 30-40 parts modified polyurethane emulsion, 13-17 parts carboxyl-containing acrylate copolymer, 7-11 parts thermosensitive polyether polyol, and 2-3 parts coupling agent. The modified polyurethane emulsion refers to a polyurethane emulsion modified by introducing epoxy groups. The preparation of the sealing film liquid includes the following steps: S1-1: The nano-silica is mixed with deionized water and then ultrasonically dispersed at a power of 400~600W and a frequency of 20~25kHz for 15~25min to obtain a nano-silica aqueous dispersion. S1-2: Add the modified polyurethane emulsion to the stirred tank, turn on the disperser, set the initial speed to 500~600 r / min, and stir for 5~10 min to make the modified polyurethane emulsion evenly distributed in the tank; S1-3: Inject the nano-silica aqueous dispersion into the stirred tank, maintain the speed of the disperser at 500~600 r / min, and stir for 15~20 min to fully integrate the nano-silica aqueous dispersion with the modified polyurethane emulsion; then add the carboxyl-containing acrylate copolymer and the thermosensitive polyether polyol in sequence, increase the speed of the disperser to 800~1200 r / min, and continue to disperse for 30~45 min to ensure that there are no obvious particles or stratification in the mixture; S1-4: Reduce the speed of the disperser to 600-700 r / min, slowly add the coupling agent dropwise at a rate of 0.5-1 part / min, and continue stirring for 20-30 min after the addition is complete to allow the coupling agent to fully crosslink with the components in the mixture. Then add deionized water to adjust the viscosity of the mixture to 200-300 mPa•s, stir for 10-15 min, then turn off the disperser and let it stand for 15-20 min to allow the air bubbles in the mixture to be expelled, thus obtaining the sealing film liquid.

2. The concrete internal curing agent for high-temperature underground tunnels according to claim 1, characterized in that: The modified polyurethane emulsion contains polyurethane urethane bonds, urea bonds, and epoxy groups in its molecular chain, and the molecular chain ends contain hydroxyl and carboxyl groups.

3. The concrete internal curing agent for high-temperature underground tunnels according to claim 2, characterized in that: The modified polyurethane emulsion has a solid content of 45-55 wt%, a particle size of 100-300 nm, a viscosity of 200-500 mPa•s, a minimum film-forming temperature of ≤5℃, and a pH value of 6.5-8.

0.

4. The concrete internal curing agent for high-temperature underground tunnels according to claim 1, characterized in that: The d50 value of the nano-silica is 20~50nm.

5. The concrete internal curing agent for high-temperature underground tunnels according to claim 1, characterized in that: The carboxyl-containing acrylate copolymer is copolymerized from methyl acrylate, butyl acrylate, and hydroxyethyl methacrylate in a mass ratio of 3:5:2, wherein the ester bond density is 8-10 per 100 repeating units, and the weight-average molecular weight is 50,000-80,000.

6. The concrete internal curing agent for high-temperature underground tunnels according to claim 1, characterized in that: The thermosensitive polyether polyol is a block copolymer composed of polypropylene oxide and polyethylene oxide, wherein the polyethylene oxide segment accounts for 20-50 wt% and the weight average molecular weight is 2500-3500.

7. The concrete internal curing agent for high-temperature underground tunnels according to claim 1, characterized in that: The coupling agent is γ-aminopropyltriethoxysilane.

8. The concrete internal curing agent for high-temperature underground tunnels according to claim 1, characterized in that: The superabsorbent resin particles have a particle size of 0.1~0.3mm, a saturated water absorption rate of 300~500g / g, and a particle size of 0.3~1.5mm after saturation.

9. A method for preparing a concrete internal curing agent for high-temperature underground tunnels as described in any one of claims 1-8, characterized in that: Includes the following steps: S1: Preparation of sealing film solution; S2 resin pretreatment: The superabsorbent resin particles are soaked in deionized water for 24-36 hours until they are saturated with water. Then, they are vacuum dried at 30-40°C until there is no free water on the particle surface to obtain saturated superabsorbent resin particles. The saturated superabsorbent resin particles are placed in a fluidized bed, and the bed temperature is adjusted to 40-50°C, the hot air temperature is 50-60°C, the air velocity is controlled at 0.8-1.0 m / s, and the bed is run stably for 10-15 minutes. S3 Sealing Film Liquid Atomization Spraying: The sealing film liquid is atomized into tiny droplets of 5~10μm and continuously sprayed onto the surface of the saturated water-absorbing resin particles. After a uniform film layer is formed on the surface of the saturated water-absorbing resin particles without any exposed areas or lumps, the spraying is stopped. The fluidized state and hot air circulation are maintained for 10~15 minutes to allow the water in the sealing film liquid to evaporate, thus obtaining the concrete internal curing agent for high geothermal underground chambers.

10. A method for preparing a concrete internal curing agent for high-temperature underground tunnels according to claim 9, characterized in that: In the S3 sealing film liquid atomization spraying, a high-pressure atomizing nozzle with an orifice diameter of 0.2 mm is installed on the fluidized bed for atomization spraying. The atomization pressure during atomization spraying is set to 0.3~0.5 MPa, the spraying rate is 5~8 mL / min, and the spraying time is 30~40 min.

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