High-temperature triggered directional self-repairing cement-based composite material and preparation method thereof

Abstract

The invention provides a high-temperature triggered directional self-repairing cement-based composite material and a preparation method thereof. The preparation method comprises the following steps: doping surface conductive modified thermal response type core-shell calcium carbonate whiskers into a cement system, wherein the thermal response type core-shell calcium carbonate whiskers are composed of calcium carbonate whisker cores and PNIPAM / nano-silica sol composite shells; in the sprayed concrete forming process, a vertical electric field system is arranged in a spraying track path or a lining support surface, acts on sprayed concrete and induces the PNIPAM / nano silica sol core-shell whiskers to be directionally arranged along a heat flux path, and a structure-enhanced concrete product A is formed; and finally, the product A is placed in a low-humidity and high-temperature environment with the temperature being 103-107 DEG C and the relative humidity being smaller than or equal to 10%, high-wind-speed drying pretreatment is conducted for 18-24 hours, then the same temperature and humidity conditions are maintained for wind-free maintenance till the age is 28 days, forming and performance maturation of the high-temperature self-repairing material are completed, and a final composite product B is obtained. The active crack resistance and high-temperature self-repairing performance of the material can be achieved.

Description

Technical Field

[0001] This application relates to the field of building materials for extreme environments, and in particular to a high-temperature triggered directional self-healing cement-based composite material and its preparation method. Background Technology

[0002] The shotcrete material used in current high-temperature geothermal projects (such as geothermal wells) is prone to structural damage due to microcracks induced by thermal stress when exposed to long-term high temperature and constant low humidity environments.

[0003] To improve the performance of shotcrete under these conditions, admixtures such as minerals, rare earth elements, fibers, and accelerators are commonly added. While these methods can improve the material's thermal stability to some extent, they cannot achieve active crack repair. Rare earth-doped alumina reinforcement is a relatively common approach. Rare earth elements possess unique electronic structures and chemical properties; doping them into alumina can improve its crystal structure and properties, thereby enhancing the thermal stability of the shotcrete material. Through rare earth-doped alumina reinforcement, the coefficient of thermal expansion at high temperatures is adjusted to some extent, reducing thermal stress caused by thermal expansion mismatch and thus improving the material's crack resistance to a certain degree. However, rare earth elements are expensive and still cannot achieve active crack repair.

[0004] Besides rare-earth-doped alumina reinforcement, traditional calcium carbonate whisker reinforcement is also commonly used to improve the performance of shotcrete materials. Calcium carbonate whiskers have advantages such as high strength and high modulus; adding them to concrete can improve the mechanical properties and crack resistance of the material to a certain extent. However, in practical applications, traditional calcium carbonate whisker reinforcement methods have limited effectiveness under high-temperature conditions due to the disordered distribution of whiskers, poor interface stability, and lack of self-healing ability.

[0005] Therefore, there is an urgent need to develop a new type of cement-based reinforcing material that can not only improve crack resistance but also has temperature-triggered self-healing ability and is suitable for spraying construction and high geothermal and low humidity environments. Summary of the Invention

[0006] The purpose of this application is to provide a high-temperature triggered directional self-healing cement-based composite material and its preparation method, so as to solve or alleviate the problems existing in the prior art.

[0007] To achieve the above objectives, this application provides the following technical solution: A method for preparing a high-temperature triggered directional self-healing cement-based composite material includes the following steps: Step 1: Mix the surface-conductive modified thermally responsive core-shell calcium carbonate whiskers with the cement system at a mass ratio of (1-3):100, and add water and water-reducing agent according to the requirements of the shotcrete process to obtain mixture A; the thermally responsive core-shell calcium carbonate whiskers are composed of a calcium carbonate whisker core and a PNIPAM / nano silica sol composite shell. Step 2: During the shotcrete forming process, a vertical electric field system is set up in the spray trajectory path or lining support surface. Before the initial setting of the concrete, a vertical DC electric field of 1-3kV / cm (the direction of the electric field is parallel to the direction of heat flow during the service of the project) is applied for 8-12 minutes to induce PNIPAM / nano silica sol core-shell whiskers to oriented along the heat flux path to form product A. Step 3: Place product A in a low-humidity, high-temperature environment of 103℃-107℃ and relative humidity ≤10%, and first perform a high-speed drying pretreatment for 18-24 hours with a wind speed of 1.5-2.0m / s to enhance the stability of the interface structure and shorten the curing start-up period. Then maintain the same temperature and humidity conditions and cure without wind until the age of 28 days to complete the molding and performance maturation of the high-temperature self-healing material, and obtain the final composite material product B.

[0008] Furthermore, in step one, basalt fiber is also added to mixture A. The amount of basalt fiber is 0.3%-0.8% of the mass of the cement system. The length of the basalt fiber is 50-200μm and the diameter is 0.5-2μm.

[0009] Furthermore, the cement system is pure silicate cement, and the silicate cement is P·O52.5 ordinary silicate cement or high-temperature resistant silicate cement.

[0010] Furthermore, the thickness of the PNIPAM / nano-silica sol composite shell of the thermally responsive core-shell calcium carbonate whiskers is 200-500 nm, and the whisker length is 20-30 μm.

[0011] Furthermore, the thermally responsive core-shell calcium carbonate whisker has a three-segment variable diameter structure: the length of the core of the calcium carbonate whisker is 20-30 μm, and the axial length ratio of the root, middle and tip is (3-2):(2-1):1; the root diameter of the core of the calcium carbonate whisker is 1.0±0.2 μm, the middle diameter is 0.5±0.1 μm, and the tip diameter is 0.2±0.05 μm.

[0012] Furthermore, the three-segment variable diameter structure of the calcium carbonate whisker core is obtained by controlling the whisker growth conditions.

[0013] Furthermore, in step one, the thermally responsive core-shell calcium carbonate whiskers are prepared using the following process: the calcium carbonate whisker cores are dispersed in the following premixed liquid under 365nm ultraviolet light irradiation, so that their surfaces are coated to form a thermally responsive shell layer; The premixed liquid has the following composition by mass: 12% PNIPAM prepolymer, 68% anhydrous ethanol, and 20% nano-silica sol; wherein the nano-silica sol contains 10 wt% silica and has a particle size of 20-30 nm.

[0014] Furthermore, in step one, the surface conductive modification of the thermally responsive core-shell calcium carbonate whiskers involves first performing cerium ion surface modification to improve the core-matrix bonding performance of the calcium carbonate whiskers, and then coating the surface with a polyaniline conductive layer through in-situ oxidative polymerization. The cerium ion loading is 0.5%-1.2% of the mass of the thermally responsive core-shell calcium carbonate whiskers, the thickness of the polyaniline conductive layer is 80-120 nm, and the volume resistivity is ≤10 Ω·cm.

[0015] Furthermore, in step one, the cerium ion surface modification involves immersing the thermally responsive core-shell calcium carbonate whiskers in a 0.05-0.15 mol / L cerium nitrate-ethanol solution for 30-90 minutes, followed by filtration and drying at 60°C for 1-3 hours. The in-situ oxidative polymerization step of the polyaniline conductive layer involves dispersing the thermally responsive core-shell calcium carbonate whiskers in an ethanol-water solution, then adding aniline monomer to a concentration of 0.05-0.2 mol / L, followed by the addition of an oxidant. The mixture is then stirred for 1-2 hours to allow the aniline monomer to polymerize in situ on the whisker surface, forming a polyaniline conductive layer. After washing with alcohol, the mixture is dried at 50-60°C to obtain thermally responsive core-shell calcium carbonate whiskers coated with a polyaniline conductive layer.

[0016] The present invention also proposes a high-temperature triggered directional self-healing cement-based composite material, which is prepared by the above-mentioned preparation method of the high-temperature triggered directional self-healing cement-based composite material.

[0017] This invention also proposes a high-temperature triggered directional self-healing cement-based composite material, which is prepared by the aforementioned method for preparing high-temperature triggered directional self-healing cement-based composite materials and is suitable for concrete spraying construction in constant low humidity (≤10%RH) and high geothermal environment (temperature ≥65℃).

[0018] Compared with the closest prior art, the technical solution of this application has the following beneficial effects: This application constructs a core-shell structure of PNIPAM-coated nano-silicon whiskers, which releases nano-silica sol upon shell contraction at temperatures exceeding 80°C to achieve self-healing of microcracks. Simultaneously, cerium ion interface modification enhances the core-matrix bonding performance of calcium carbonate whiskers. The introduction of electric field vertical orientation technology promotes the orderly arrangement of whiskers along the heat flow direction, significantly improving crack resistance in the vertical heat flow direction. Supplemented by high-temperature and low-humidity pretreatment and curing processes, the material's structural stability and durability are significantly improved, thereby achieving the material's active crack resistance and high-temperature self-healing performance. Detailed Implementation

[0019] The technical solutions in the embodiments of the present invention will be clearly and completely described below. Obviously, the described embodiments are only a part of the embodiments of the present invention, and not all of the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art are within the scope of protection of the present invention.

[0020] Unless otherwise defined, all technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the art to which this disclosure belongs. The terminology used herein is for the purpose of describing embodiments of this disclosure only and is not intended to limit this disclosure.

[0021] A method for preparing a high-temperature triggered directional self-healing cement-based composite material includes the following steps: Step 1: Mix the thermally responsive core-shell calcium carbonate whiskers with the cement system (referred to as cementitious material) at a mass ratio of (1-3):100, and add water and water-reducing agent according to the requirements of the shotcrete process to obtain mixture A; the thermally responsive core-shell calcium carbonate whiskers are composed of a calcium carbonate whisker core and a PNIPAM / nano silica sol composite shell; in this application, the PNIPAM (poly(N-isopropylacrylamide)) is a thermally responsive polymer material, and its number-average molecular weight is preferably 5,000-30,000 Da, to ensure that while forming a dense coating, it also has sensitive phase change release behavior under high temperature conditions; Step 2: During the shotcrete forming process, a vertical electric field system is set up in the spray trajectory path or lining support surface. Before the initial setting of the concrete, a vertical DC electric field of 1-3kV / cm is applied (the direction of the electric field is parallel to the direction of heat flow during the service of the project). The electric field is applied for 8-12 minutes to induce PNIPAM / nano silica sol core-shell whiskers to oriented along the heat flux path, forming structurally reinforced concrete product A. Step 3: Place product A in a low-humidity, high-temperature environment of 103℃-107℃ and relative humidity ≤10%, and first perform a high-speed drying pretreatment for 18-24 hours. The air speed inside the chamber is set to 1.5-2.0m / s to enhance the stability of the interface structure and shorten the curing start-up period. Then maintain the same temperature and humidity conditions and cure without wind until the age of 28 days to complete the molding and performance maturation of the high-temperature self-healing material, and obtain the final composite material product B.

[0022] Step 2: In actual construction, the vertical electric field system adopts a detachable electrode array system, which is deployed on the front end or back plate of the sprayed lining forming area. When the concrete is sprayed at high speed, the slurry can be seen to receive vertical electric field induction before initial setting through the section where the detachable electrode array system is deployed, so as to achieve the collimation of the whiskers.

[0023] Furthermore, the cement system is pure silicate cement, and the silicate cement is P·O52.5 ordinary silicate cement or high-temperature resistant silicate cement.

[0024] Furthermore, the thickness of the PNIPAM / nano-silica sol composite shell of the thermally responsive core-shell calcium carbonate whisker is 200-500 nm, and the whisker length is 20-30 μm. The thermally responsive core-shell calcium carbonate whisker has a three-segment variable diameter structure: the axial length ratio of the three segments is approximately 3:2:1; the total whisker length is 20-30 μm, with a root diameter of 1.0±0.2 μm, a middle diameter of 0.5±0.1 μm, and a tip diameter of 0.2±0.05 μm. The three-segment variable diameter structure of the calcium carbonate whisker core is obtained by controlling the whisker growth conditions. The control methods for whisker growth conditions include oversaturation, temperature, pH, additives (PNIPAM, nano-silica sol), and stirring rate. By using suitable whisker growth conditions, the whisker size required by this invention can be achieved. Preferably, the three-section variable diameter whiskers are obtained by staged growth, and the synthesis process includes three stages of temperature-supersaturation control: first, the root thick growth is induced under low temperature and high pH conditions, then the raw materials are slowly released to regulate the elongation of the middle section, and finally, the tip slender structure is formed under the conditions of rising temperature and high supersaturation rapid growth, so as to realize the axial variable diameter development of the crystal.

[0025] Optionally, in step one, basalt fiber is also added to mixture A. The amount of basalt fiber is 0.3%-0.8% of the mass of the cement system. The length of the basalt fiber is 50-200μm and the diameter is 0.5-2μm.

[0026] Preferably, in step one, the surface conductive modification of the thermally responsive core-shell calcium carbonate whiskers involves first performing cerium ion surface modification to improve the core-matrix bonding performance of the calcium carbonate whiskers, followed by in-situ oxidative polymerization to coat the surface with a polyaniline conductive layer, where cerium ions (Ce) are present. 3+The loading amount is 0.5%-1.2% of the mass of the thermally responsive core-shell calcium carbonate whiskers, the thickness of the polyaniline conductive layer is 80-120 nm, and the volume resistivity is ≤10 Ω·cm. Preferably, the core-shell structure whiskers are first surface-modified with cerium ions to improve the core-matrix bonding performance of the calcium carbonate whiskers, and then the surface is coated with a polyaniline conductive layer through oxidative polymerization. This significantly improves the whiskers' response to an applied electric field and achieves effective directional alignment. This composite modification promotes a synergistic improvement in self-healing ability and mechanical properties.

[0027] The cerium ion surface modification steps are as follows: thermally responsive core-shell calcium carbonate whiskers are immersed in a 0.05-0.15 mol / L cerium nitrate-ethanol solution for 30-90 minutes, preferably 60 minutes, followed by filtration and constant temperature drying and curing at 60°C for 1-3 hours, preferably 2 hours, to form a uniform and stable cerium ion modified layer.

[0028] The method for preparing the polyaniline conductive layer includes the following steps: Thermally responsive core-shell calcium carbonate whiskers are dispersed in a 50-80% (v / v) ethanol or ethanol-water mixture. Aniline monomer is added to the solution to a concentration of 0.05-0.2 mol / L. An oxidant (preferably a 0.05-0.15 mol / L cerium nitrate ethanol solution) is added dropwise at 0-5°C or room temperature, and the mixture is stirred for 1-2 hours to allow the aniline monomer to polymerize in situ on the whisker surface to form a polyaniline conductive layer. After the reaction, the whiskers are washed multiple times with ethanol and dried at 60°C for 1-3 hours to obtain thermally responsive core-shell whiskers coated with polyaniline.

[0029] This invention also proposes a high-temperature triggered directional self-healing cement-based composite material, which is prepared using the aforementioned method for preparing high-temperature triggered directional self-healing cement-based composite materials. The invention will now be described in detail with reference to the embodiments and comparative examples. Specifically, test blocks were prepared using various composite materials and tested. The raw materials used in the embodiments and comparative examples were as follows: the cementitious material was P·O52.5 silicate cement; basalt nanofibers with a content of 0.5%, a length of 100 μm, and a diameter of approximately 1 μm were used; the polycarboxylate superplasticizer was Jiangsu Subote F-108; unless otherwise specified, all raw materials were commercially available products. The testing methods referred to the national standard GB / T 17671-2021 "Test Method for Strength of Cement Mortar," etc. In the embodiments and comparative examples, shotcrete was sprayed into molds to prepare test blocks to facilitate the testing of various indicators.

[0030] Example 1 A method for preparing a high-temperature triggered directional self-healing cement-based composite material includes the following steps: Step 1: First, the thermally responsive core-shell calcium carbonate whiskers are electrically modified by coating their surface with a polyaniline conductive layer of 80-120 nm thickness. Then, they are mixed with cementitious materials (referred to as cementitious materials) at a mass ratio of 2:100, and water, polycarboxylate superplasticizer, and basalt fiber are added. The water-cement ratio is 0.45. The polycarboxylate superplasticizer is Jiangsu Subote F-108, and the dosage is 1.0% of the cementitious materials. All materials are mixed in a horizontal planetary mixer for 5 minutes to obtain mixture A. The thermally responsive core-shell calcium carbonate whiskers consist of a calcium carbonate whisker core and a PNIPAM / nano-silica sol composite shell, and are modified with cerium ions and coated with a polyaniline conductive layer with a thickness of 80-120 nm. The volume resistivity after coating is controlled to ≤10Ω·cm, which is beneficial for electric field stress transmission and orientation. The basalt nanofibers are 0.5% of the cementitious material, with a length of 100μm and a diameter of about 1μm, for synergistic reinforcement. In this embodiment, the molecular weight of PNIPAM (poly-N-isopropylacrylamide) is 5,000-30,000 Da; Step 2: Pour the mixture A obtained in Step 1 into a 40×40×160mm mold and immediately use a special vertical electric field device to orient the whiskers; cast the mixture into shape, and apply a 2kV / cm vertical DC electric field to it before initial setting. The direction of the electric field is parallel to the direction of heat flow transfer, and the action time is 8min to achieve the vertical orientation of the whiskers and obtain the specimen. Step 3: Place product A in a low-humidity, high-temperature environment of 105℃ and 10% relative humidity. First, perform a 24-hour high-speed drying pretreatment to enhance the stability of the interface structure and shorten the curing start-up period. Then, maintain the same temperature and humidity conditions and cure without wind until the age of 28 days to complete the molding and performance maturation of the high-temperature self-healing material, obtaining the final composite material product B. The first 24 hours are the high-speed drying pretreatment stage, with a drying temperature of 105℃, relative humidity of 10%, and an air velocity of 1.5m / s inside the chamber. A hot air convection constant temperature drying chamber is used with the blower on to maintain a hot air flow drying environment, ensuring rapid release of interface moisture and thermal induction of some microcracks to trigger PNIPAM response activation. After that, it enters the long-term curing stage (the blower in the drying chamber is turned off, and it is in constant temperature static curing mode), maintaining the same temperature and humidity atmosphere, and curing until the age of 28 days.

[0031] In this embodiment 1, the thermally responsive core-shell calcium carbonate whisker has a three-segment variable diameter structure: the length of the calcium carbonate whisker core is 20-30 μm, and the axial length ratio of the root, middle and tip is approximately 3:2:1. The root diameter of the calcium carbonate whisker core is 1.0 μm, the middle diameter is 0.5 μm, and the tip diameter is 0.2 μm.

[0032] The preparation process of the thermally responsive core-shell calcium carbonate whiskers is as follows: the calcium carbonate whisker core is dispersed in the following premixed liquid under 365nm ultraviolet light irradiation, so that its surface is coated to form a thermally responsive shell layer. The premixed liquid has the following composition by mass: 12% PNIPAM prepolymer, 68% anhydrous ethanol, and 20% nano-silica sol; wherein the nano-silica sol contains 10 wt% silica and has a particle size of 20-30 nm.

[0033] The surface conductivity modification of the thermally responsive core-shell calcium carbonate whiskers involves first modifying the surface with cerium ions to improve the core-matrix bonding performance of the calcium carbonate whiskers, followed by in-situ oxidative polymerization to coat the surface with a polyaniline conductive layer. The cerium ion loading is 0.8% of the mass of the thermally responsive core-shell calcium carbonate whiskers, the polyaniline conductive layer thickness is 90-110 nm, and the volume resistivity is ≤10 Ω·cm. The cerium ion surface modification steps are as follows: the thermally responsive core-shell calcium carbonate whiskers are immersed in a 0.1 mol / L cerium nitrate-ethanol solution for 60 minutes, then filtered and placed in a constant temperature environment of 60°C for drying and curing for 2 hours to form a uniform and stable cerium ion modified layer.

[0034] The method for preparing the polyaniline conductive layer includes the following steps: Thermally responsive core-shell calcium carbonate whiskers were dispersed at a mass ratio of 1% in an 80% volume ethanol solution. Aniline monomer was added to the solution to make the aniline monomer concentration 0.15 mol / L. A 0.1 mol / L cerium nitrate ethanol solution was added dropwise at 5°C, with a molar ratio of cerium nitrate to aniline monomer of 1.25:1. The reaction was carried out under stirring for 1.5 hours to allow the aniline monomer to polymerize in situ on the surface of the whiskers to form a polyaniline conductive layer. After the reaction was completed, the whiskers were washed multiple times with ethanol and dried at 60°C for 2 hours to obtain thermally responsive core-shell whiskers with a polyaniline conductive layer. The thickness of the polyaniline conductive layer should be 90-110 nm.

[0035] In this embodiment, the directional arrangement of thermally responsive core-shell calcium carbonate whiskers is achieved through the application of an embedded vertical electric field. Specifically, in step two, the electric field of the mold is applied by upper and lower copper electrode plates. The electrodes are fixed in the embedded groove structure of the mold base plate and cover plate, respectively, and are completely isolated from the concrete slurry by a polytetrafluoroethylene (PTFE) insulating layer to prevent conductive contact and interfacial polarization reaction. The effective distance between the upper and lower electrodes is 10 mm. A programmable DC power supply with an output range of 0-5 kV (preferably Chroma61612) is used to apply a stable electric field strength of 2.0 kV / cm for 8 minutes, achieving directional alignment of the whiskers along the electric field direction (i.e., the heat flux direction). During the application of the electric field, the ambient temperature is controlled at 25°C to prevent mechanical disturbance and ensure stable whisker orientation.

[0036] The final test items for the specimens include: (1) Compressive strength under standard curing at room temperature: According to GB / T 17671-2021, the basic compressive strength of the material was tested after curing under standard curing conditions (normal temperature and humidity) for 28 days; (2) High geothermal curing compressive strength: Curing for 28 days in a high temperature and low humidity environment of 105℃ and relative humidity ≤10% to evaluate the material’s ability to retain compressive strength in the target service environment; (3) Porosity: The porosity of the material was determined by mercury intrusion porosimetry (MIP). (4) Mass loss rate after humidity cycling: The mass stability of the material was tested by 100 humidity cycles (simulating alternating dry and wet environment); (5) Elastic modulus: Tested after curing at 105℃ for 28 days; (6) Poisson's ratio: Tested according to GB / T 50081-2019, after treatment at 105℃; (7) Chloride ion permeability coefficient: The chloride ion permeability of the material at 105℃ was determined by the RCM method (rapid chloride ion migration coefficient method); (8) Self-healing capability: Self-healing evaluation was conducted on specimens with pre-fabricated 200μm wide cracks, and the repair efficiency was measured by measuring the change in porosity before and after repair. (9) Crack resistance toughness perpendicular to the crack direction: The crack resistance toughness value perpendicular to the crack propagation direction is obtained by testing the material's resistance to fracture perpendicular to the crack propagation direction (K). IC ).

[0037] The test results are as follows: The compressive strength after standard curing at room temperature is 58.1 MPa, and after high geothermal curing, it is 44.7 MPa; the porosity is 22.3%; the mass loss rate after 100 humidity cycles is 2.1%; the elastic modulus is 30.9 GPa, the Poisson's ratio is 0.19; and the chloride ion permeability coefficient is 4.5 × 10⁻⁶. -12 m 2 / s; The self-healing ability results show that the surface porosity decreased from 22.3% to 12.7%, the repair rate was 43.05%, and the crack resistance value perpendicular to the crack direction was 9.9 MPa·m. 1 / 2 .

[0038] Application examples The composite material of the present invention was used in a geothermal well lining simulation experiment to verify the working performance of the composite material of the present invention in actual jetting application scenarios.

[0039] Preparation and Experiment: Use the sprayed concrete specification with a water-cement ratio of 0.43; The grout used for lining the simulated geothermal well includes: cement (P·O52.5 ordinary Portland cement + 2% surface conductive modified thermally responsive core-shell calcium carbonate whiskers + 0.5% basalt fiber) 420 kg / m³, crushed stone (5-10 mm continuous gradation) 1050 kg / m³, sand 680 kg / m³, water-reducing agent 6.8 kg / m³, and water 182 kg / m³. The prepared slurry jet is poured into the simulated wellbore lining brick mold. To simulate the deep well section jetting construction environment, a modular combination structure can also be used, with each section 50cm high, spliced ​​together to form a simulated wellbore tower, with a total height of 150cm, for continuous jetting and temperature and humidity cycle aging tests. During the jetting process, a detachable electrode array structure is embedded axially inside the wellbore brick mold to form a vertical application system, with the electrode spacing controlled at 10cm and the electric field direction set along the wellbore axis. Before the initial setting of the concrete, an external DC regulated power supply system is activated to apply a stable vertical DC electric field with an intensity of 2.0kV / cm for 8 minutes, causing the surface conductive modified core-shell calcium carbonate whiskers to achieve collimation along the axial heat flux direction, thereby forming a structurally reinforced self-healing concrete lining structure.

[0040] After the lining was completed, a curing and aging environment simulation treatment was carried out sequentially: the aging environment was set at 105℃ and ≤10%RH in a constant temperature and dry environment for 365 days of high temperature and low humidity aging curing. After the curing period, core sampling technology was used to characterize the microstructure and macroscopic mechanical properties. The analysis results showed that the wellbore concrete structure was stable and dense, with obvious whisker orientation, significant internal crack healing, and low chloride ion permeability, meeting the long-term service performance requirements of the wellbore.

[0041] Based on core sampling and microstructure characterization analysis, the core results are as follows: One year later, the compressive strength retention rate was greater than 96%, with no macrocracks visible, but traces of local repair were observed in the microcracks; the chloride ion diffusion coefficient at 105℃ was 4.6 × 10⁻⁶. - With a density of 1²m² / s, a porosity of 23.2%, an elastic modulus of 30.1 GPa, a Poisson's ratio of 0.20, and a mass loss rate of only 2.3% after humidity cycling, this product exhibits a self-healing rate of over 85% for 200 μm cracks, with the porosity reduced to 12-13% after repair. After curing at 105℃ for 28 days, its compressive strength is 44.7 MPa ± 0.3, and its vertical crack toughness is ≥9.8 MPa·m¹. / ², all other indicators meet stringent standards. It satisfies the performance targets of geothermal well lining simulation experiments (porosity ≤25%, mass loss after humidity or thermal cycling ≤2.3%, chloride ion diffusion coefficient at 105℃ ≤4.8×10⁻⁶). -12 m 2 / s, elastic modulus 30-31GPa, Poisson's ratio 0.18-0.20).

[0042] in conclusion: Compared with traditional materials, this technical solution significantly improves the structural density, thermal cracking inhibition, and self-healing ability under high temperature and dry conditions, and is practical and feasible. It is suitable for harsh construction and service environments, and is applicable to shotcrete components such as geothermal wells and deep-buried tunnels, and has outstanding engineering application value.

[0043] Comparative Example 1 The only difference between Comparative Example 1 and Example 1 is that no surface-conductively modified thermally responsive core-shell calcium carbonate whiskers were added to the high-temperature triggered directional self-healing cementitious composite material.

[0044] Comparative Example 2 The only difference between Comparative Example 2 and Example 1 is that no electric field was applied in step two.

[0045] Comparative Example 3 The only difference between Comparative Example 3 and Example 1 is that the surface-conductive modified thermal-responsive core-shell calcium carbonate whiskers in the high-temperature triggered directional self-healing cement-based composite material in step one were not modified with cerium ions, but were coated with a polyaniline conductive layer by in-situ oxidative polymerization to form the core of the calcium carbonate whiskers.

[0046] Comparative Example 4 The only difference between Comparative Example 4 and Example 1 is that in step three, the high wind speed drying pretreatment of 1.5-2.0 m / s was not carried out. Instead, the sample was directly placed in a drying oven for constant temperature static curing. Therefore, the "structural condition adjustment period" brought about by the high wind speed drying pretreatment stage was missing.

[0047] Comparative Examples 1-4 are comparative experiments on the effects of missing key parameters. The test results of Example 1 and Comparative Examples 1-4 are shown in the table below.

[0048] Table 1. Test results of Example 1 and Comparative Examples 1-4

[0049] As shown in Table 1, the surface conductive modification of thermally responsive core-shell calcium carbonate whiskers, the application of an electric field, and the high-speed drying pretreatment all contribute to the performance improvement of the product of the present invention, and the synergistic effect of each component is indispensable.

[0050] Comparative Example 4, although subsequently cured in the same 105℃, ≤10%RH environment for 28 days, lacked a high-velocity drying pretreatment stage. Therefore, it missed the early structural adjustment and whisker interface stabilization process, failing to effectively activate the early self-healing reaction mechanism and enter full-cycle drying before initial structural stabilization. This resulted in insufficient microstructure control, a sharp increase in high-temperature water loss shrinkage stress, and uncontrolled whisker arrangement and nucleation orientation, thus affecting the final performance. This was manifested in its compressive strength (43.5MPa), porosity (25.3%), and chloride ion permeability coefficient (5.9×10⁻⁶). -12 m 2 The self-healing efficiency (%) and elastic modulus (28.7 GPa) were lower than those of Example 1. Although the self-healing efficiency still reached about 86.2%, it was slightly lower than that of Example 1.

[0051] Comparative Example 5 The only difference between Comparative Example 4 and Example 1 is that the whiskers in Example 1 are cylindrical structures with a uniform diameter. The average diameter of the whiskers is 0.5±0.05μm, the length is 24±2μm, the surface is still a PNIPAM / nano-silica sol composite shell with a shell thickness of 200-500nm, and an outer coating of polyaniline conductive layer with a volume resistivity ≤10Ω·cm.

[0052] The test results are as follows: Crack resistance toughness: 7.1 MPa·m¹ / ², compressive strength 42.9 MPa, elastic modulus 28.6 GPa, Poisson's ratio 0.22, porosity 24.8%, self-healing rate 78%, chloride ion permeability coefficient 6.3 × 10⁻⁶. -12 m 2 / s.

[0053] in conclusion: Variable diameter whiskers generate multiple energy-dissipating mechanisms during crack propagation, such as path deflection, bridging, and bifurcation, forming gradient anchoring and enhancing crack propagation resistance.

[0054] Preliminary Experiment 1 This preliminary experiment aims to optimize the effect of electric field strength on the orientation of thermally responsive core-shell calcium carbonate whiskers, evaluate the reinforcing properties of the composite material in the direction perpendicular to the heat flow under different electric field strengths, and determine the optimal electric field parameters suitable for actual spraying construction conditions.

[0055] The specimen formulation and preparation process are consistent with those of Example 1, except for the electric field strength. The slurry used to prepare the specimen includes the following components: The cementitious material is pure P·O52.5 ordinary silicate cement; the whisker core is modified with cerium nitrate ethanol solution (0.1 mol / L), with a cerium ion loading of 0.8%; a 100 nm thick polyaniline conductive layer is applied, and the whisker volume resistivity is ≤10 Ω·cm; the whisker type is a three-segment variable diameter structure, surface conductive modified thermally responsive core-shell calcium carbonate whisker, with a core diameter of 1.0 μm at the root, 0.5 μm in the middle, and 0.2 μm at the tip. Basalt fiber is added at 0.5% of the cementitious material mass, with a length of 100 μm and a diameter of approximately 1 μm; polycarboxylate superplasticizer accounts for 1.0% of the cementitious material mass; the slurry water-cement ratio is 0.45. The mold dimensions are: 40 mm × 40 mm × 160 mm.

[0056] Environmental conditions: The room temperature was controlled at 25℃ during electric field loading, with no vibration. The DC electric field was set to four levels: 0, 1.0, 2.0, and 3.0 kV / cm. The application direction was consistent with the heat flow direction. The electric field application time was uniformly 8 minutes, and the electrode spacing was 10 mm. The goal was to achieve directional alignment of whiskers in the direction of the electric field at the solidification front of the slurry.

[0057] The whisker arrangement was quantitatively evaluated using scanning electron microscopy (SEM) combined with image analysis. The evaluation method involved extracting the principal axis direction of the whiskers from a cross-sectional image, calculating the orientation deviation angle between the whiskers and the electric field direction, and statistically analyzing the standard deviation σ (unit: °) for classification. The evaluation criteria for the arrangement of 100 whiskers, based on sample analysis, are shown in Table 2. Table 2 Evaluation Criteria for Whisker Arrangement Status

[0058] The whisker arrangement and performance results under different electric field strengths are shown in Table 3.

[0059] Table 3. Whisker arrangement and performance results under different electric field intensities.

[0060] The test results of different electric field strengths show that when the vertical electric field strength reaches above 2.0 kV / cm, the whisker orientation dispersion can be significantly reduced. The whiskers are basically arranged perpendicularly along the electric field direction. The crack propagation in the heat flow direction will be significantly interfered with by the gradient arrangement of whiskers, thereby increasing the probability of crack bifurcation and deflection, and improving the crack resistance toughness in the vertical heat flow direction by about 48% (compared to the group without electric field).

[0061] To further evaluate its macroscopic performance, representative specimens prepared under different electric field intensities were selected for the following tests, and the results are shown in the table below: Table 4 Test results of specimens under different electric field strengths

[0062] This preliminary experiment verified the significant influence of electric field strength on whisker orientation and composite material properties. The results show that an electric field strength ≥2.0 kV / cm can effectively drive the whiskers to align in the vertical direction, significantly enhancing crack resistance, structural density, and durability in the heat flow direction.

[0063] Further increasing the electric field strength to 3.0 kV / cm leads to saturation of performance improvement. Considering the safety and energy consumption control of the existing spraying process, it is recommended to select 2.0 kV / cm as the preferred electric field strength parameter value in actual engineering, which balances performance enhancement and on-site feasibility.

[0064] Preliminary Experiment 2 This preliminary experiment aims to evaluate the impact of high-wind-velocity drying pretreatment time on the mechanical properties, thermal cycling durability, and pore structure of self-healing cement-based composite materials, thereby determining the optimal drying pretreatment time and optimizing the balance between construction adaptability and material structural performance.

[0065] The test specimens used in the experiment were completely consistent with those in Example 1 in terms of formulation, preparation process, and electric field loading conditions (2.0 kV / cm, applied for 8 minutes). The only difference was that the "high-speed drying pretreatment" time in step three was set to 0, 12, 24, and 48 hours, while the other conditions remained the same, including a drying temperature of 105±2℃, a relative humidity of 10%, and an environment controlled by a closed constant-temperature drying oven. The test results are shown in Table 5.

[0066] Table 5. Test Results of Preliminary Experiment 2

[0067] Although the high-wind-speed drying pretreatment stage and the subsequent long-term windless curing are under the same temperature and humidity conditions (105℃, 10%RH), the high-wind-speed drying pretreatment stage involves flowing air within the drying chamber, resulting in distinct functional positioning and structural evolution stages. The high-wind-speed drying pretreatment stage occurs within the unstable structural formation window after the initial setting of the concrete slurry. At this stage, the whiskers induced by the applied electric field are not yet fixed, the hydration reaction is active in its early stages, and the interfacial composite is still stabilizing. Implementing timed high-wind-speed, high-temperature drying at this point effectively induces the initial thermal response release of the PNIPAM shell, initiates the penetration and filling behavior of the nano-silica sol, and simultaneously fixes the whisker orientation and surface cross-linking layer, promoting the overall structure's evolution from a plastic state to a microstructure stationary state, ultimately significantly improving material properties. Without the high-wind-speed drying pretreatment stage, this structural activation window will be missed, leading to a loose interfacial layer, unsealed channels, and consequently, increased porosity, decreased self-healing efficiency, and delayed strength development.

[0068] Test results show that as the high-speed drying pretreatment time is extended, the material properties gradually improve, mainly in the following ways: as the high-speed drying pretreatment time increases, the compressive strength increases with the increase of pretreatment time, the whisker-matrix structure becomes more compact, the material porosity and permeability decrease, the structural stability is enhanced after thermal cycling, the elastic modulus increases, and the structural toughness is enhanced.

[0069] When the processing time exceeds 24 hours, the performance improvement tends to plateau, and the differences in various indicators between the 48-hour and 24-hour groups are not significant, but rather slightly reversed. Therefore, to balance construction efficiency and performance optimization, it is recommended to set the high-velocity drying pretreatment time to 24 hours in the process configuration.

[0070] This application enables high-temperature automatic triggering of PNIPAM shell shrinkage to release nano-silica sol, effectively repairing microcracks with a repair rate >85%, manifested as a significant reduction in porosity after repair. The directional effect of the electric field causes the whiskers to align vertically in the heat flow direction. Combined with the gradient anchoring effect of variable-diameter whiskers, this significantly improves the crack resistance (≥9.8 MPa·m) perpendicular to the heat flow direction. 1 / 2 After undergoing high-temperature drying curing treatment simulating service conditions, the material exhibits stable structure, low porosity (≤25%), and a low proportion of large pores (>1000nm pores <8%). After 100 severe thermal cycles, the compressive strength loss rate is extremely low (<3%). This invention is suitable for shotcrete applications in high geothermal and dry environments, such as for geothermal well lining.

[0071] The above description is merely a preferred embodiment of this application and is not intended to limit this application. Various modifications and variations can be made to this application by those skilled in the art. Any modifications, equivalent substitutions, improvements, etc., made within the spirit and principles of this application should be included within the protection scope of this application.

Claims

1. A method for preparing a high-temperature triggered directional self-healing cement-based composite material, characterized in that: Includes the following steps: Step 1: Mix the surface-conductive modified thermally responsive core-shell calcium carbonate whiskers with the cement system at a mass ratio of (1-3):100, and add water and water-reducing agent according to the requirements of the shotcrete process to obtain mixture A; the thermally responsive core-shell calcium carbonate whiskers are composed of a calcium carbonate whisker core and a PNIPAM / nano silica sol composite shell. Step 2: During the shotcrete forming process, a vertical electric field system is set up in the spray trajectory path or lining support surface. Before the initial setting of the concrete, a vertical DC electric field of 1-3kV / cm is applied for 8-12 minutes to induce PNIPAM / nano silica sol core-shell whiskers to oriented along the heat flux path to form product A. Step 3: Place product A in a low-humidity, high-temperature environment of 103℃-107℃ and relative humidity ≤10%, and first perform a high-speed drying pretreatment for 18-24 hours with a wind speed of 1.5-2.0m / s to enhance the stability of the interface structure and shorten the curing start-up period. Then maintain the same temperature and humidity conditions and cure without wind until the age of 28 days to complete the molding and performance maturation of the high-temperature self-healing material, and obtain the final composite material product B.

2. The preparation method of the high-temperature triggered directional self-healing cement-based composite material according to claim 1, characterized in that: In step one, basalt fiber is also added to mixture A. The amount of basalt fiber is 0.3%-0.8% of the mass of the cement system. The length of the basalt fiber is 50-200μm and the diameter is 0.5-2μm.

3. The preparation method of the high-temperature triggered directional self-healing cement-based composite material according to claim 1, characterized in that: The cement system is pure silicate cement, and the silicate cement is P·O52.5 ordinary silicate cement or high-temperature resistant silicate cement.

4. The preparation method of the high-temperature triggered directional self-healing cement-based composite material according to claim 1, characterized in that: The thickness of the PNIPAM / nano-silica sol composite shell of the thermally responsive core-shell calcium carbonate whiskers is 200-500 nm, and the whisker length is 20-30 μm.

5. The preparation method of the high-temperature triggered directional self-healing cement-based composite material according to claim 1, characterized in that: The thermally responsive core-shell calcium carbonate whisker has a three-segment variable diameter structure: the length of the core of the calcium carbonate whisker is 20-30 μm, and the axial length ratio of the root, middle and tip is (3-2):(2-1):1; the root diameter of the core of the calcium carbonate whisker is 1.0±0.2 μm, the middle diameter is 0.5±0.1 μm, and the tip diameter is 0.2±0.05 μm.

6. The preparation method of the high-temperature triggered directional self-healing cement-based composite material according to claim 5, characterized in that: The three-segment variable diameter structure of the calcium carbonate whisker core is obtained by controlling the whisker growth conditions.

7. The preparation method of the high-temperature triggered directional self-healing cement-based composite material according to claim 1, characterized in that: In step one, the thermally responsive core-shell calcium carbonate whiskers are prepared using the following process: the calcium carbonate whisker cores are dispersed in the following premixed liquid under 365nm ultraviolet light irradiation, so that their surfaces are coated to form a thermally responsive shell layer; The premixed liquid has the following composition by mass: 12% PNIPAM prepolymer, 68% anhydrous ethanol, and 20% nano-silica sol; wherein the nano-silica sol contains 10 wt% silica and has a particle size of 20-30 nm.

8. The preparation method of the high-temperature triggered directional self-healing cement-based composite material according to claim 1, characterized in that: In step one, the surface conductive modification of the thermally responsive core-shell calcium carbonate whiskers involves first performing cerium ion surface modification to improve the core-matrix bonding performance of the calcium carbonate whiskers, and then coating the surface with a polyaniline conductive layer through in-situ oxidative polymerization. The cerium ion loading is 0.5%-1.2% of the mass of the thermally responsive core-shell calcium carbonate whiskers, the thickness of the polyaniline conductive layer is 80-120 nm, and the volume resistivity is ≤10 Ω·cm.

9. The preparation method of the high-temperature triggered directional self-healing cement-based composite material according to claim 8, characterized in that: In step one, the cerium ion surface modification involves immersing the thermally responsive core-shell calcium carbonate whiskers in a 0.05-0.15 mol / L cerium nitrate-ethanol solution for 30-90 minutes, followed by filtration and drying at 60°C for 1-3 hours. The in-situ oxidative polymerization step of the polyaniline conductive layer involves dispersing the thermally responsive core-shell calcium carbonate whiskers in an ethanol-water solution, then adding aniline monomer to a concentration of 0.05-0.2 mol / L, followed by the addition of an oxidant. The mixture is then stirred for 1-2 hours to allow the aniline monomer to polymerize in situ on the whisker surface, forming a polyaniline conductive layer. After washing with alcohol, the mixture is dried at 50-60°C to obtain thermally responsive core-shell calcium carbonate whiskers coated with a polyaniline conductive layer.

10. A high-temperature triggered directional self-healing cement-based composite material, characterized in that, It is prepared by the preparation method of high temperature triggered directional self-healing cement-based composite material as described in any one of claims 1 to 9.

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