A hydrogel-modified magnesium phosphate cement-based material, a preparation method and application in piezoresistive coating sensors

Abstract

The application belongs to the technical field of sensors, and particularly relates to a hydrogel modified magnesium phosphate cement-based material, a preparation method and application in a piezoresistive coating sensor. A new type of triple-crosslinking intelligent material is constructed by introducing sodium acrylate monomers into a magnesium phosphate cement matrix through an in-situ radical polymerization method and combining a functionalized sandwich structure BGC composite material. The material utilizes the synergistic effect of covalent bonds, ionic bonds and hydrogen bonds to achieve a balance between mechanical strength, toughness and piezoresistive sensitivity. The hydration exothermicity of MPC triggers the polymerization reaction, and the three-dimensional conductive network of BGC and the ionic channel of poly-sodium acrylate form a percolation structure, which endows the material with excellent pressure-resistance response characteristics. The material can be coated on the surface of a building structure or a flexible substrate, and is used for real-time monitoring of cracks, impacts and other damages, and has a wide application prospect in the fields of civil engineering health monitoring and wearable sensing.

Description

Technical Field

[0001] The present invention belongs to the field of sensor technology, and in particular to a highly durable and tough hydrogel-modified cement-based sensor for monitoring large-span and large-area construction projects, capable of accurately monitoring stress, strain, and crack locations. The present invention specifically relates to a hydrogel-modified magnesium phosphate cement-based material, a preparation method, and its application in piezoresistive coating sensors. Background Art

[0002] The information disclosed in the background of the invention is only intended to enhance understanding of the overall background of the invention and should not necessarily be regarded as an admission or any form of suggestion that the information constitutes the prior art already known to a person skilled in the art.

[0003] With the acceleration of global urbanization, the number of large-scale construction projects such as high-rise buildings, large bridges, and tunnels continues to increase. These large-span, large-area concrete structures are subject to a variety of complex factors over their long-term service life, such as environmental erosion, loads, and temperature fluctuations. This can cause changes in their internal stress and strain states, and even lead to damage such as cracks, seriously affecting the safety and service life of the structures. Therefore, real-time and accurate health monitoring of concrete structures is crucial to promptly identify potential problems and implement appropriate maintenance measures to ensure the safety and reliability of buildings.

[0004] Sensor technology plays a key role in monitoring the health of concrete structures. However, most sensors currently used in the concrete industry are imported from other disciplines, such as fiber optic sensors, photoelectric sensors, magnetic sensors, and piezoresistive sensors. While these sensors have achieved relatively mature research results in their respective fields, they face significant challenges regarding compatibility with concrete. Due to concrete's complex pore structure and highly alkaline environment, the performance of these sensors in concrete is severely affected, resulting in reduced sensor accuracy and even signal errors, making it impossible to accurately reflect the actual health of the concrete structure.

[0005] In addition, the sensors currently used in the field of concrete are mostly embedded sensors, that is, the sensors are pre-embedded in the structure during the concrete pouring process. Although this embedded installation method can achieve internal monitoring of the concrete structure to some extent, it also brings a series of problems. First, the installation process of the embedded sensor is relatively complex, and accurate positioning and fixing need to be performed before the concrete pouring, which increases the construction difficulty and cost. Second, the embedded sensor occupies a certain space, thereby reducing the bearing capacity of the base body and affecting the overall performance of the structure. Third, during long-term service, due to the complexity and unpredictability of the internal environment of the concrete, the embedded sensor is prone to damage or failure. Once the sensor is damaged, it is difficult to replace and maintain due to its embedding in the concrete, which greatly reduces the reliability and sustainability of the monitoring system.

[0006] In summary, the existing concrete sensors have many deficiencies in terms of compatibility with concrete materials, installation methods, and long-term service performance, and are difficult to meet the high requirements of large-span and large-area construction projects for concrete structure health monitoring. Therefore, developing a sensor with good compatibility with concrete materials, convenient installation, and reliable long-term service performance has become an urgent problem in the field of concrete structure health monitoring. SUMMARY

[0007] In view of the needs of the prior art, the purpose of the present application is to provide a hydrogel modified magnesium phosphate cement-based material, a preparation method and an application in a piezoresistive coating sensor. The present application provides a triple-crosslinked cement-based coating sensor, which uses magnesium phosphate cement as the main material. The cement-based material is modified by in-situ polymerization of sodium polyacrylate, so that the cement-based material has high mechanical properties, durability and freeze-thaw resistance. The BGC conductive framework filler serves as a functional phase. By adjusting the water-cement ratio, the polymer-cement ratio and the conductive filler content, the composite material has good workability, durability, toughness and electrical stability.

[0008] Specifically, the present application provides the following technical solutions:

[0009] In the first aspect of the present application, a hydrogel modified magnesium phosphate cement-based material is provided, which comprises the following raw materials in mass fraction: 1-8 parts of sodium acrylate, 90-100 parts of magnesium phosphate cement, 1.5-2 parts of BGC conductive material and 20-30 parts of water.

[0010] In the preparation of the BGC conductive material, the raw materials include hydroxylated carbon nanotubes, bis-hydroxyethyl terephthalate and graphene oxide. The mass ratio of the hydroxylated carbon nanotubes, bis-hydroxyethyl terephthalate and graphene oxide is 1-2:1:1-2.

[0011] Preferably, the hydrogel-modified magnesium phosphate cement-based material includes the following raw materials in parts by mass: 4 parts of sodium acrylate, 100 parts of magnesium phosphate cement, 1.5 parts of BGC conductive material, and 20 parts of water; the mass ratio of the hydroxylated carbon nanotubes, bis(hydroxyethyl) terephthalate, and graphene oxide is 1:1:1.

[0012] Preferably, the preparation process of the BGC conductive material comprises the following steps:

[0013] S1, mixing a graphene oxide aqueous dispersion, 1-(3-dimethylaminopropyl)-3-ethylcarbodiimide hydrochloride, and N-hydroxysuccinimide for reaction in an ice bath, post-treating to obtain a graphene oxide-DMF dispersion, then adding bis(hydroxyethyl) terephthalate and p-toluenesulfonic acid thereto, and heating the mixture to obtain a GO-BHET composite;

[0014] S2, mixing the alcohol solution of hydroxylated carbon nanotubes with γ-aminopropyltriethoxysilane for reflux reaction, dispersing the obtained product in an organic solvent, then adding bis(hydroxyethyl) terephthalate and potassium carbonate thereto, and heating the mixture to obtain a HCNT-BHET composite;

[0015] S3, mixing the GO-BHET complex and the HCNT-BHET complex, forming a gel through a hydrothermal reaction, and then immersing the gel in an ascorbic acid solution and heating the solution to obtain a BGC precursor;

[0016] S4. The BGC precursor is washed with DMF, ethanol, and tert-butanol in a gradient manner in sequence, and a porous BGC aerogel is obtained after freeze-drying. After grinding, a BGC conductive material is obtained.

[0017] Further preferably, in step S1, the mass ratio of the graphene oxide aqueous dispersion, 1-(3-dimethylaminopropyl)-3-ethylcarbodiimide hydrochloride and N-hydroxysuccinimide is 3-5:2-3:1-2; the ice bath reaction time is 3-5 hours; the mass ratio of the graphene oxide aqueous dispersion, bis(hydroxyethyl) terephthalate and p-toluenesulfonic acid is 10-15:20-25:1-2; the heating reaction temperature is 75-90°C, and the time is 5-8 hours.

[0018] Further preferably, in step S2, the dosage ratio of the hydroxylated carbon nanotubes to γ-aminopropyltriethoxysilane is 100 mg:1 mL; the temperature of the reflux reaction is 75-90°C, and the time is 5-8 hours; the mass ratio of the hydroxylated carbon nanotubes, bis(hydroxyethyl terephthalate) and potassium carbonate is 15-20:15-20:1; the temperature of the heating reaction is 55-70°C, and the time is 3-5 hours.

[0019] Further preferably, in step S3, the mass ratio of the GO-BHET complex to the HCNT-BHET complex is 2:1; the temperature of the hydrothermal reaction is 120-150°C, and the time is 4-7 hours; the concentration of the ascorbic acid solution is 0.1 M; the temperature of the heating reaction is 85-100°C, and the time is 3-5 hours.

[0020] Further preferably, in step S4, during the freeze-drying process, the pre-freezing temperature is -70 to -85°C, the pre-freezing time is 10 to 15 hours, and the drying time is 20 to 30 hours; and the particle size of the BGC conductive material obtained after grinding is 1 to 5 μm.

[0021] The second aspect of the present invention provides a method for preparing the above-mentioned hydrogel-modified magnesium phosphate cement-based material, specifically: mixing magnesium phosphate cement dry powder, sodium acrylate powder and BGC conductive material to obtain a mixed dry powder; mixing a mixed solution of N,N'-methylenebisacrylamide and ammonium persulfate with water, adding the mixed solution to the mixed dry powder, stirring, and simultaneously causing hydration reaction and polymerization reaction to obtain.

[0022] Preferably, the magnesium phosphate cement dry powder is obtained by mixing ammonium dihydrogen phosphate and dead-burned magnesium oxide, and the mass ratio of the ammonium dihydrogen phosphate to the dead-burned magnesium oxide is 1:1 to 1.5.

[0023] Preferably, the volume ratio of the mixed solution of N,N'-methylenebisacrylamide and ammonium persulfate to water is 1:0.15-0.25.

[0024] A fourth aspect of the present invention provides a hydrogel-modified magnesium phosphate cement-based piezoresistive coating, comprising the hydrogel-modified magnesium phosphate cement-based material described in the first aspect.

[0025] The fifth aspect of the present invention provides a method for preparing the hydrogel-modified magnesium phosphate cement-based piezoresistive coating described in the fourth aspect, specifically comprising: mixing sodium acrylate, magnesium phosphate cement, BGC conductive material and water, stirring the obtained slurry and applying it to the surface of a concrete test block.

[0026] Preferably, the coating thickness is 0.8 to 1.2 mm, and the coating methods include brushing and spraying.

[0027] A sixth aspect of the present invention provides a use of the hydrogel-modified magnesium phosphate cement-based material described in the first aspect and / or the hydrogel-modified magnesium phosphate cement-based piezoresistive coating described in the fourth aspect in a concrete sensor.

[0028] The beneficial effects achieved by one or more of the above technical solutions of the present invention are as follows:

[0029] (1) Breakthrough Improvement in Comprehensive Material Performance: Traditional cement-based piezoresistive materials rely on a single conductive phase, resulting in low conductivity and limited sensitivity. This invention achieves a significant improvement in conductivity and piezoresistive sensitivity through the collaborative construction of a three-dimensional conductive network and ionic conduction paths. The comprehensive sensing performance far exceeds that of conventional graphene composite systems. Traditional inorganic cementitious materials generally suffer from high-strength and brittle defects. This invention overcomes the bottleneck of material toughening through the synergistic effect of a triple cross-linked network of covalent bonds, ionic bonds, and hydrogen bonds, significantly improving crack resistance and fracture toughness while maintaining high load-bearing capacity.

[0030] (2) Intelligent response and functional integration innovation: The performance of traditional materials is irreversible after damage and requires external sensing equipment. The present invention gives the material self-repairing ability through a dynamic bonding network and ion migration mechanism, which can achieve functional recovery after multiple damage cycles. At the same time, the sensing function is embedded in the material body to construct an integrated system of structural load-bearing and health monitoring, significantly reducing the complexity of engineering monitoring.

[0031] (3) Innovation in damage monitoring accuracy and timeliness: Traditional piezoresistive coatings rely on single-point resistance signals, resulting in insufficient positioning accuracy. This invention integrates multi-mode sensing and distributed network technology, combined with interface impedance optimization strategy, to simultaneously improve the damage spatial positioning accuracy and dynamic response speed, meeting the real-time monitoring needs of complex structures.

[0032] (4) Breakthrough in adaptability to extreme environments: Traditional polymer-based sensors are prone to failure in high-temperature and corrosive environments. The present invention constructs an environmental corrosion-resistant system through matrix modification design and chemical network optimization. Its freeze-thaw and chemical stability are significantly better than conventional materials, ensuring long-term monitoring reliability.

[0033] (5) Innovation in green and efficient preparation process: Abandoning the high energy consumption defect of the traditional step-by-step synthesis process, the self-reaction characteristics of the material are used to realize the in-situ synchronous construction of the conductive network and the matrix, combined with the green synthesis technology of non-toxic reducing agents, to form an efficient and environmentally friendly preparation system, which greatly shortens the production cycle.

[0034] (6) Fundamental Improvement in Engineering Compatibility: This technology overcomes the technical barriers of incompatibility between traditional sensors and the concrete matrix interface. Through component matching and structural design, the material is highly integrated with existing buildings. It can be customized as an external coating with both protective and monitoring functions, resolving the problem of signal distortion from external sensors. Furthermore, the material of the present invention is cement-based and highly plastic. It can be manufactured into shapes that conform to actual engineering needs, depending on the monitoring environment and location, meeting the requirements of all-round detection. BRIEF DESCRIPTION OF THE DRAWINGS

[0035] The accompanying drawings, which constitute a part of the present invention, are used to provide a further understanding of the present invention. The exemplary embodiments of the present invention and their descriptions are used to explain the present invention and do not constitute improper limitations on the present invention.

[0036] Figure 1 This is a schematic structural diagram of the BGC conductive material prepared in Example 1 of the present invention;

[0037] Figure 2 This is a SEM image of the BGC conductive material prepared in Example 1 of the present invention;

[0038] Figure 3 This is an SEM image of the hydrogel-modified magnesium phosphate cement-based material prepared in Example 1 of the present invention;

[0039] Figure 4 This is a dispersed super-depth image of the BGC conductive material prepared in Example 1 of the present invention;

[0040] Figure 5 This is a dispersed super-depth image of the BGC conductive material prepared in Comparative Example 1 of the present invention;

[0041] Figure 6 This is a dispersed super-depth image of the BGC conductive material prepared in Comparative Example 2 of the present invention;

[0042] Figure 7 This is a test chart of the tensile strength performance of the hydrogel-modified magnesium phosphate cement-based materials prepared in Examples 1 to 4 and Comparative Examples 1 to 2 of the present invention;

[0043] Figure 8 The hydrogel-modified magnesium phosphate cement-based material prepared in Example 1 of the present invention was coated on the side of a concrete test block and loaded using a universal press. The coating experienced the same strain as the concrete test block, and the measured resistance under load and strain changes was tested.

[0044] Figure 9 The hydrogel-modified magnesium phosphate cement-based material prepared in Example 2 of the present invention was coated on the side of a concrete test block and loaded using a universal press. The coating experienced the same strain as the concrete test block, and the measured resistance data under load and strain changes were tested.

[0045] Figure 10 The hydrogel-modified magnesium phosphate cement-based material prepared in Example 3 of the present invention was coated on the side of a concrete test block and loaded using a universal press. The coating experienced the same strain as the concrete test block, and the measured resistance under load and strain changes was tested.

[0046] Figure 11The hydrogel-modified magnesium phosphate cement-based material prepared in Example 4 of the present invention was coated on the side of a concrete test block and loaded using a universal press. The coating experienced the same strain as the concrete test block, and the measured resistance data under load and strain changes were tested.

[0047] Figure 12 The hydrogel-modified magnesium phosphate cement-based material prepared in Example 5 of the present invention was coated on the side of a concrete test block and loaded using a universal press. The coating experienced the same strain as the concrete test block, and the measured resistance data under load and strain changes were tested.

[0048] Figure 13 The composite material prepared in Comparative Example 1 of the present invention was coated on the side of a concrete test block and loaded using a universal press. The coating experienced the same strain as the concrete test block, and the measured resistance under load and strain changes was shown in the test graph.

[0049] Figure 14 The composite material prepared in comparative example 2 of the present invention was coated on the side of a concrete test block and loaded using a universal press. The coating experienced the same strain as the concrete test block, and the measured resistance under load and strain changes was shown in the test graph. DETAILED DESCRIPTION

[0050] It should be noted that the following detailed descriptions are exemplary and intended to provide further explanation of the present invention. Unless otherwise specified, all technical and scientific terms used herein have the same meaning as commonly understood by those skilled in the art to which the present invention belongs.

[0051] In order to enable those skilled in the art to more clearly understand the technical solution of the present invention, the technical solution of the present invention will be described in detail below with reference to specific embodiments.

[0052] Sodium polyacrylate is a common hydrogel polymer with very strong water absorption, and the water absorption rate (pure water) of conventional sodium polyacrylate is hundreds of times of itself, and the improved product can reach thousands of times. Often used as water treatment agent, salt water refining and latex thickening, it can also be used as food tackifying, emulsification. The present invention introduces sodium polyacrylate into magnesium phosphate cement in the mode of in-situ polymerization, and the polymerization reaction of polymer occurs while cement hydration, forming an organic-inorganic cross-linked network structure, thereby improving the toughness and frost resistance, corrosion resistance, anti-dry-wet cycle durability of cement-based materials. Magnesium phosphate cement has the disadvantages of setting speed fast, early strength high, and can make cement coating reduce the interference generated by the outside world after spraying or brushing, and rapid hardening, rapid forming are conducive to the stability of coating. But setting time can not be too fast, and enough time will be reserved for construction operation, due to the water absorption of sodium polyacrylate, part of water can be stored when cement hydration, thereby delaying the generation of hydration reaction.

[0053] While sodium polyacrylate, when introduced into cement, imparts a certain degree of conductivity, this conductivity through internal moisture is unstable and lacks the conductive stability required for a piezoresistive coating. Therefore, a carbon-based conductive material must be introduced. Currently, the most commonly used conductive materials include carbon nanotubes, graphite, and graphene. However, these materials share a common problem: due to van der Waals forces, nanomaterials naturally aggregate, making perfect dispersion difficult. Failure to disperse the conductive material within the cement matrix can severely impact the accuracy and stability of sensor monitoring.

[0054] To this end, the present invention introduces sodium acrylate monomer into magnesium phosphate cement (MPC) matrix by in-situ free radical polymerization, and combines it with functional sandwich structure BGC (hydroxylated carbon nanotubes / bis(hydroxyethyl) terephthalate BHET / graphene oxide GO) composite material to construct a new triple cross-linked smart material. 2+ The synergistic effects of coordination with the carboxylate groups of sodium polyacrylate and the phosphate groups of BGC and hydrogen bonding (at the interface between sodium polyacrylate-BGC hydroxyl groups and Mg(OH)2) achieve a balance between mechanical strength, toughness, and piezoresistive sensitivity. Furthermore, the sandwich structure of BGC creates a three-dimensional conductive network and the ionic conductive pathway of PAAS, achieving a dual conductive mechanism. This gives the material excellent pressure-resistance response characteristics and greatly enhances the sensitivity of the sensor.

[0055] The cement-based sensor fabricated using this method can be tailored to the desired shape, thickness, and length, depending on the application. Compared to embedded sensors, this sensor does not affect the load-bearing capacity of the main structure and can be installed on any surface. Furthermore, the present invention protects the monitoring system from erosion by harmful media such as carbon dioxide and moisture, significantly extending the life of the structure.

[0056] The abbreviations in the present invention have the following meanings: MPC: magnesium phosphate cement; PAAS: sodium polyacrylate; GO: graphene oxide; HCNT: hydroxylated carbon nanotube; BHET: bis(hydroxyethyl) terephthalate; APS: ammonium persulfate; MBA: N,N'-methylenebisacrylamide; BGC is a three-dimensional composite material of BHET, GO, and CNT.

[0057] In the following examples, 1 g corresponds to one portion.

[0058] Example 1 :This embodiment provides a hydrogel modified magnesium phosphate cement-based coating and its preparation method

[0059] The hydrogel-modified magnesium phosphate cement-based material specifically includes the following raw materials by weight: 4 g of sodium acrylate, 100 g of magnesium phosphate cement, 1.5 g of BGC conductive material, and 20 g of water.

[0060] The specific preparation method is as follows:

[0061] 1. Preparation of BGC conductive materials

[0062] (1) Carboxyl activation of GO and BHET bonding:

[0063] To 50 mL of GO (2 mg / mL) aqueous dispersion was added 50 mg of 1-(3-dimethylaminopropyl)-3-ethylcarbodiimide hydrochloride (EDC) and 25 mg of N-hydroxysuccinimide (NHS). The mixture was stirred in an ice bath for 4 hours to activate the carboxyl groups to NHS esters. The mixture was centrifuged (8000 rpm for 15 minutes), washed three times with deionized water, and redispersed in 50 mL of DMF. To the GO-DMF dispersion was added 200 mg of bis(hydroxyethyl)terephthalate (BHET) and 10 mg of p-toluenesulfonic acid (PTSA) (catalyst). The mixture was refluxed at 80°C under nitrogen for 6 hours. The mixture was centrifuged (5000 rpm for 10 minutes) and washed three times with DMF to obtain the GO-BHET complex.

[0064] (2) Silanization modification of HCNT and BHET bonding

[0065] 100 mg of HCNT was added to 50 mL of ethanol and subjected to ultrasonication (300 W for 30 minutes). 1 mL of γ-aminopropyltriethoxysilane (APTES) was added, and the mixture was refluxed at 80°C for 6 hours. The mixture was then centrifuged and washed to obtain HCNT-NH2. The HCNT-NH2 was then dispersed in 30 mL of DMF, and 100 mg of BHET and 5 mg of K2CO3 (catalyst) were added. The mixture was stirred at 60°C for 4 hours. The mixture was then centrifuged and washed to obtain the HCNT-BHET complex.

[0066] (3) Sandwich structure assembly and GO reduction: GO-BHET and HCNT-BHET were mixed in a mass ratio of 2:1, added to 100 mL of deionized water, and sonicated for 30 minutes. The mixture was transferred to an autoclave and hydrothermally reacted at 140°C for 6 hours to form a gel. The gel was immersed in a 0.1 M ascorbic acid solution (pH = 10, adjusted with ammonia water) and stirred at 90°C for 4 hours. The mixture was centrifuged and washed until neutral to obtain the BGC precursor.

[0067] (4) Solvent replacement and drying: Wash with DMF, ethanol, and tert-butanol gradient (centrifugation at 8000 rpm for 15 minutes each time) to replace the solvent. -80℃ pre-frozen 12 hours, transferred to freeze dryer (cold trap-50℃, vacuum degree <10Pa), dried for 24 hours, and porous BGC aerogel was obtained. The aerogel was ground in a planetary ball mill (300rpm, zirconia balls) for 2 hours to obtain 1-5μm particles.

[0068] As shown in Figure 1 , the principle diagram shows that the prepared BGC conductive material presents a sandwich structure;

[0069] As shown in Figure 2 , the SEM image shows that the BGC conductive filler is magnified 10,000 times (the right middle image), and the connection of carbon nanotubes and graphene can be found. Although the carbon nanotubes have a slight aggregation phenomenon, they can be perfectly dispersed at the micron level, which provides a strong foundation guarantee for the resistance stability monitoring of the cement-based coating after the conductive filler is incorporated into the cement. Figure 2

[0070] 2. Preparation of hydrogel modified magnesium phosphate cement-based material:

[0071] (1) Pre-mixed dry powder: mix MPC dry powder, sodium acrylate powder, and BGC conductive material uniformly.

[0072] (2) Solution preparation: dissolve APS and MBA in mixed water (water-cement ratio 0.2).

[0073] (3) Mixing and polymerization: add the solution to the dry powder, stir quickly (30 seconds), use the MPC hydration exothermic to trigger the decomposition of APS, and initiate the radical polymerization of sodium acrylate. The polymerization reaction is synchronized with the MPC hydration to form an MPC-PAAS-BGC interpenetrating network.

[0074] As shown in Figure 3 , the SEM image shows that the cement-based material with a mixing ratio of 4% generates a new needle-like ordered arrangement material in the cement pores. This material can offset the large fracture energy of the cement-based material, increase the toughness and ductility of the material, and ensure that the cement-based coating can deform and extend with the surface deformation of the structure during service, and monitor a larger range of strain;

[0075] which is not generated in Examples 2-5, it is speculated that the Mg 2+ of the magnesium phosphate cement forms an ionic bond with the -COO - of the hydrogel to generate a new compound of magnesium acrylate. This ionic bond has stronger fracture energy than the ionic bond of sodium acrylate. (Mg 2+ vs. Na + ​The greater the ionic charge, the stronger the ionic bond. Because the greater the charge, the stronger the charge interaction. Because magnesium ions carry two positive charges while sodium ions carry only one positive charge, it provides a theoretical basis for the results of Example 1 to show the best performance in subsequent tests.

[0076] 3. A method for preparing a hydrogel-modified magnesium phosphate cement-based piezoresistive coating, comprising the following steps:

[0077] According to the formula, sodium acrylate, magnesium phosphate cement, BGC conductive material and water are mixed and stirred until uniform, the obtained slurry is brushed or sprayed onto the surface of the concrete test block, and standard curing is carried out for 3 days, the thickness of the cement conductive coating is 1 mm, and the cement conductive coating is obtained.

[0078] Example 2 The embodiment provides a hydrogel-modified magnesium phosphate cement-based coating and a preparation method thereof

[0079] In the embodiment, the specific components of the hydrogel-modified magnesium phosphate cement-based material are changed, and other method steps are the same as those in Example 1.

[0080] The hydrogel-modified magnesium phosphate cement-based material specifically comprises the following mass of raw materials: 2 g of sodium acrylate, 100 g of magnesium phosphate cement, 1.5 g of BGC conductive material and 20 g of water.

[0081] Example 3 The embodiment provides a hydrogel-modified magnesium phosphate cement-based coating and a preparation method thereof

[0082] In the embodiment, the specific components of the hydrogel-modified magnesium phosphate cement-based material are changed, and other method steps are the same as those in Example 1.

[0083] The hydrogel-modified magnesium phosphate cement-based material specifically comprises the following mass of raw materials: 1 g of sodium acrylate, 100 g of magnesium phosphate cement, 1.5 g of BGC conductive material and 20 g of water.

[0084] Example 4 The embodiment provides a hydrogel-modified magnesium phosphate cement-based coating and a preparation method thereof

[0085] In the embodiment, the specific components of the hydrogel-modified magnesium phosphate cement-based material are changed, and other method steps are the same as those in Example 1.

[0086] The hydrogel-modified magnesium phosphate cement-based material specifically comprises the following mass of raw materials: 8 g of sodium acrylate, 100 g of magnesium phosphate cement, 1.5 g of BGC conductive material and 20 g of water.

[0087] Example 5 The embodiment provides a hydrogel-modified magnesium phosphate cement-based coating and a preparation method thereof

[0088] In this embodiment, the specific composition of the hydrogel-modified magnesium phosphate cement-based material was changed, and the other method steps were the same as those in Example 1.

[0089] The hydrogel-modified magnesium phosphate cement-based material specifically includes the following raw materials by mass: 4 g of sodium acrylate, 100 g of magnesium phosphate cement, 0.5 g of BGC conductive material, and 20 g of water.

[0090] Comparative Example 1 :

[0091] Compared with the above examples, this comparative example is different in that this comparative example does not contain sodium polyacrylate hydrogel, and only prepares the magnesium phosphate cement conductive coating, which specifically includes the following raw materials by mass: 100g magnesium phosphate cement, 1.5g BGC conductive material, and 20g water.

[0092] The preparation method is as follows:

[0093] Add the pre-synthesized BGC conductive material to 20g of water and stir thoroughly to create a slurry. Then, slowly add magnesium phosphate cement and stir thoroughly. Apply the slurry by brushing or spraying it onto the surface of the concrete test block and allow it to cure for three days. The thickness of the cement conductive coating is 1mm.

[0094] Comparative Example 2 :

[0095] Compared with Example 1, this comparative example differs in that the synthetic BGC conductive material is not used in this comparative example. Instead, carbon nanotubes and graphene (GO-CNT mass ratio 1:1) are used to prepare a cement-based composite conductive coating, specifically including the following raw materials by mass: 4 g sodium acrylate, 100 g magnesium phosphate cement, 1.5 g GO-CNT, and 20 g water.

[0096] The preparation method is as follows:

[0097] GO-CNTs were added to a previously prepared sodium acrylate solution and stirred to form a slurry. Magnesium phosphate cement was then slowly added and stirred to form a uniform slurry. The slurry was brushed or sprayed onto the surface of the concrete specimens and cured for 3 days. The thickness of the cement conductive coating was 1 mm.

[0098] Example 6 : This embodiment conducts dispersion super depth of field image test on the BGC conductive materials prepared in the embodiment and comparative example in different solutions.

[0099] like Figure 4 As shown, the materials prepared in Examples 1 to 5 all showed uniform dispersion, that is, sodium acrylate was a polar solution, and the dispersion of the conductive filler at the micron level was achieved by simple stirring without other complicated operations, providing a favorable basis for the realization of stable piezoresistive monitoring of the sensor.

[0100] like Figure 5 As shown in the figure, the dispersion effect of the material prepared in Comparative Example 1 is poor. This is because although water is also a polar solution, sodium acrylate will be completely ionized after being dissolved in water to produce acrylic acid ions (C3H3O2 - ) and sodium ions (Na + ), the polarity of the solution is greater than that of water, so the dispersion of BGC in sodium acrylate solution is poorer than that in the sodium acrylate solution. This also leads to the fact that the resistance cannot change synchronously with the load during the subsequent resistance stability monitoring process, and cannot meet the requirements of coating piezoresistive monitoring.

[0101] like Figure 6 As shown, the dispersion effect of the material prepared in Comparative Example 2 is poor, which is due to the fact that the carbon nanotubes aggregate into clumps and are difficult to disperse. The main reason why carbon nanotubes are easy to aggregate in water is that their surface is composed of non-polar carbon atoms, which does not match the polarity of water molecules, resulting in strong hydrophobicity and difficulty in forming a stable dispersion; at the same time, there are significant van der Waals forces between carbon nanotubes, and the surface energy brought by the high specific surface area prompts them to reduce the energy of the system by aggregation. In addition, the surface charge density of unmodified carbon nanotubes is low and they lack hydrophilic groups, and they cannot resist aggregation through electrostatic repulsion or steric hindrance. Conductive fillers cannot be dispersed in water, resulting in resistance drift, and the change of resistance with pressure is irregular, so they cannot be directly used for sensing coatings.

[0102] Test Example 1 : This test example tests the tensile strength performance of the coating materials prepared in Examples 1 to 4 and Comparative Examples 1 to 2.

[0103] like Figure 7 As shown, according to the standard ASTM D4541, the material was made into a dumbbell shape and subjected to a tensile fracture strength test. The poly-cement ratio refers to the mass ratio of sodium acrylate to magnesium phosphate cement. It can be seen that when the poly-cement ratio is 0% (Comparative Example 1), the fracture strength of the pure cement system is only 3.38 MPa, which is much lower than the fracture strength of the cement system modified by the hydrogel; when the poly-cement ratio is in the range of 0-4%, the tensile fracture strength of the sample gradually increases. At this time, the tensile fracture strength corresponding to Example 2 is 4.29 MPa, and the tensile fracture strength corresponding to Example 3 is 3.85 MPa; this is due to the interpenetrating network structure formed by the hydrogel and the magnesium phosphate system in the embodiments of the present invention;

[0104] When the cement ratio is 4% (Example 1), the tensile strength of the sample is the highest, which is 5.07 MPa. This is because the Mg content of magnesium phosphate cement is 2+ -COO with hydrogel - An ionic bond is formed to generate a new compound of magnesium acrylate, which has a stronger breaking energy than the ionic bond of sodium acrylate;

[0105] When the poly-cement ratio is 8% (Example 4), its tensile strength at break is lower than that when the poly-cement ratio is 4%, reaching 4.67 MPa. This shows that the higher the content of hydrogel introduced, the better. Beyond a certain limit, the hydrogel has a negative impact on the system. This may be because too much hydrogel blocks the interconnection of the cement, causing the cement "skeleton" to separate, and only the hydrogel resists tensile strength, or because too many pores are introduced, making the system looser (after drying, the hydrogel shrinks and introduces pores in the hydrogel cement system).

[0106] When the coating material was prepared without using the synthesized BGC conductive material (Comparative Example 2), its tensile strength at break was only 4.53 MPa, indicating that the cross-linking of the BGC conductive material and PAAS can consume the fracture energy of the material. This is because the carboxylate groups (-COO - ) can form hydrogen bonds with the hydroxyl (-OH) groups of BGC. Compared to the synthesized BGC conductive material, the unmodified conductive filler exhibits significant agglomeration distribution in the cement matrix due to insufficient interfacial compatibility, resulting in a poor dispersion uniformity of the conductive network. Under tensile loading, local stress concentration effects form around the agglomerates, causing cracks to preferentially initiate and propagate, ultimately resulting in an 11% decrease in the material's tensile strength.

[0107] Test Example 2 : This test example performs xenon lamp aging test and freeze-thaw resistance durability test on the coating materials prepared in Examples 1 to 5 and Comparative Examples 1 to 2.

[0108] In this test example, conductive copper foil is distributed on the edges of the coatings prepared in Examples 1 to 5 and Comparative Examples 1 to 2 to connect a digital bridge to form a current loop, and the digital bridge is connected to a computer to collect monitoring signals.

[0109] The xenon arc aging test for the cement-based conductive coating in this test example was conducted in accordance with ISO 4892-3:2006. The freeze-thaw durability test for concrete was conducted in accordance with GB / T 50082-2024, "Standard for Test Methods for Long-term Properties and Durability of Concrete."

[0110] For Example 1: The cement-based composite conductive material showed no cracks after being irradiated with a xenon arc lamp (simulating the solar spectrum and radiation intensity of sunlight) for 28 days. The freeze-thaw durability index of the concrete test block was DF=98%.

[0111] For Example 2: The cement-based composite conductive material prepared in this example was subjected to a performance test according to the performance test standard in Example 1. The performance test results are as follows:

[0112] The cement-based composite conductive material exhibited a few fine cracks after 28 days of xenon arc lamp irradiation, but no cracks were observed in indoor environments. This coating can be used in relatively good indoor environments. The freeze-thaw durability index of the concrete specimen was DF = 96%.

[0113] For Example 3: The cement-based composite conductive material prepared in this example was subjected to a performance test according to the performance test standard in Example 1. The performance test results are as follows:

[0114] The cement-based composite conductive material exhibited numerous fine cracks after 28 days of xenon arc lamp irradiation, but no cracks were observed in indoor environments. This coating can be used in relatively good indoor environments. The freeze-thaw durability index of the concrete specimen was DF = 92%.

[0115] For Example 4: The cement-based composite conductive material prepared in this example was subjected to a performance test according to the performance test standard in Example 1. The performance test results are as follows:

[0116] The cement-based composite conductive material developed numerous wide cracks after 28 days of xenon arc lamp irradiation, but no cracks developed in indoor environments. This coating can be used in relatively good indoor environments. The freeze-thaw durability index of the concrete specimen was DF = 90%.

[0117] For Example 5: The cement-based composite conductive material prepared in this example was subjected to a performance test according to the performance test standard in Example 1. The performance test results are as follows:

[0118] The cement-based composite conductive material showed no cracks after being irradiated by a xenon arc lamp for 28 days. The freeze-thaw durability index of the concrete specimen was DF=97%.

[0119] For Comparative Example 1: The cement-based composite conductive material prepared in this example was subjected to a performance test according to the performance test standard in Example 1. The performance test results are as follows:

[0120] After 28 days of xenon arc lamp irradiation, the cement-based composite conductive material began to fall off, with wide cracks almost penetrating the film. The freeze-thaw durability index of the concrete specimen was DF = 89%.

[0121] For Comparative Example 2: GO-CNT is untreated graphene oxide and carbon nanotubes. These two materials are naturally aggregated due to van der Waals forces, and conventional stirring cannot break up the aggregates. The cement-based composite conductive material prepared in this example was tested for performance according to the performance test standards in Example 1. The performance test results are as follows:

[0122] The cement-based composite conductive material showed no cracks after being irradiated by a xenon arc lamp for 28 days. The freeze-thaw durability index of the concrete specimen was DF=96%.

[0123] Test Example 3 : This test example performs strain load performance tests on the coating materials prepared in Examples 1 to 5 and Comparative Examples 1 to 2.

[0124] When monitoring the piezoresistive stability of the coating, strain gauges are pasted on both sides of the coating at equal distances to monitor the stress-strain changes during the process.

[0125] For Example 1: Figure 8 It can be seen that under graded cyclic loads of 50kN, 100kN, and 150kN, the resistance change of the cement-based composite conductive material coating is consistent with the load change trend, and the conductive stability is excellent. In addition, the strain monitored by the strain gauge also has a consistent trend with the load change, demonstrating that the cement-based composite conductive material coating has both conductive stability and the coating's usability for strain monitoring.

[0126] For Example 2: Figure 9 It can be seen that under the loading of graded cyclic load, the load, strain and resistance of the composite conductive coating have the same change trend, but the resistance change curve fluctuates within a controllable range at the second and third peaks, and the resistance change trend is not affected. That is, although the collected resistance data fluctuates, it is within the controllable range.

[0127] For Example 3: Figure 3 It can be seen that when the composite conductive coating is loaded with graded cyclic loads, the strain of the concrete specimen has a slight upward increasing trend with the increase in the number of loading times, and the peak of the resistance change curve also has a slight upward increasing trend, which proves that the resistance and strain changes are consistent.

[0128] For Example 4: Figure 4 As can be seen, under graded cyclic loading, the composite coating exhibits significant fluctuations in the second and third peaks of the resistance curve, with the second peak gradually approaching the third peak. This indicates that the strain reversibility of this coating is poor, and that resistance changes can only be monitored under a narrow load range. Furthermore, the resistance ranges between 500 and 1000 kilo-ohms, suggesting that excessive hydrogel incorporation affects the stability of resistance monitoring.

[0129] For Example 5: Figure 5 It can be seen that under graded cyclic loading, the resistance change fluctuation of the composite material coating gradually becomes gentle, but the third peak of the resistance change curve decreases with the increase of the number of loading times, which is contrary to the law of gradually increasing strain. That is, as the number of load cycles increases, the resistance change curve cannot accurately reflect the load borne by the structure and the strain on the surface of the structure, but it can be used for applications under monotonic load conditions.

[0130] For comparative example 1: Figure 6As can be seen, under graded cyclic loading, the third peak of the composite coating without hydrogel fluctuates at the height of the second peak and does not rise in parallel with the strain. Due to the lack of hydrogel, the coating has poor toughness and can only be used to monitor strain changes within a small load range.

[0131] For comparative example 2: Figure 7 It can be seen that under graded loads, the resistance change curve of the composite conductive coating fluctuates irregularly, the conductive stability of the coating is extremely poor, and it cannot be used for strain monitoring.

[0132] In summary, the present invention provides a hydrogel-modified magnesium phosphate cement-based piezoresistive coating sensor, which uses magnesium phosphate cement as the main raw material, is mixed with sodium acrylate, pre-synthesized BGC three-dimensional conductive filler, and water, and is applied to the surface of a test block to form a strip. The sensor monitors the resistance change of the composite conductive coating under multi-stage cyclic loading. Similarly, the composite coating can be made into a desired shape according to actual conditions and applied to an appropriate location to monitor data such as strain changes in existing building structures. When cracks occur on the surface of a concrete test block or building structure, the resistance of the composite conductive coating will simultaneously undergo a cliff-like change, providing an early warning for structural health detection and providing guarantees for the safe and long-term service of existing building structures.

[0133] The composite conductive cement-based piezoresistive coating prepared by the present invention constructs a strain-resistance relationship through the strain of the building structure and the change in the resistance of the coating, indirectly reflects the load and cracking conditions borne by the building structure, and provides strong data support for structural health monitoring. The sensor works in the form of a coating, which can shield the internal structure from electromagnetic interference, and can block the invasion of harmful gases and liquids on the building structure, protect the internal structure, and extend the service life of the building structure. In addition, the main raw material of the coating is magnesium phosphate cement, which has the characteristics of early strength, fast hardening, and high bonding strength. It can reduce the impact of the external environment on the construction quality, and can be applied to the bottom and side surfaces of the structure without causing surface slippage and uneven thickness due to the weight of the coating.

[0134] The foregoing description is merely a preferred embodiment of the present invention and is not intended to limit the present invention. Those skilled in the art will readily appreciate that various modifications and variations of the present invention are possible. Any modifications, equivalent substitutions, or improvements made within the spirit and principles of the present invention are intended to be within the scope of protection of the present invention.

Claims

1. A hydrogel-modified magnesium phosphate cement-based material, characterized in that: The hydrogel-modified magnesium phosphate cement-based material includes the following raw materials in parts by weight: 1-8 parts of sodium acrylate, 90-100 parts of magnesium phosphate cement, 1.5-2 parts of BGC conductive material, and 20-30 parts of water; The raw materials used in the preparation of the BGC conductive material include hydroxylated carbon nanotubes, bis(hydroxyethyl) terephthalate, and graphene oxide; the mass ratio of the hydroxylated carbon nanotubes, bis(hydroxyethyl) terephthalate, and graphene oxide is 1-2:1:1-2; The preparation process of the BGC conductive material includes the following steps: S1, mixing a graphene oxide aqueous dispersion, 1-(3-dimethylaminopropyl)-3-ethylcarbodiimide hydrochloride, and N-hydroxysuccinimide for reaction in an ice bath, post-treating to obtain a graphene oxide-DMF dispersion, then adding bis(hydroxyethyl) terephthalate and p-toluenesulfonic acid thereto, and heating the mixture to obtain a GO-BHET composite; S2, mixing the alcohol solution of hydroxylated carbon nanotubes with γ-aminopropyltriethoxysilane for reflux reaction, dispersing the obtained product in an organic solvent, then adding bis(hydroxyethyl) terephthalate and potassium carbonate thereto, and heating the mixture to obtain a HCNT-BHET composite; S3, mixing the GO-BHET complex and the HCNT-BHET complex, forming a gel through a hydrothermal reaction, and then immersing the gel in an ascorbic acid solution and heating the solution to obtain a BGC precursor; S4. The BGC precursor is washed with DMF, ethanol, and tert-butanol in a gradient manner in sequence, and a porous BGC aerogel is obtained after freeze-drying. After grinding, a BGC conductive material is obtained.

2. The hydrogel-modified magnesium phosphate cement-based material according to claim 1, wherein The hydrogel-modified magnesium phosphate cement-based material includes the following raw materials in parts by mass: 4 parts of sodium acrylate, 100 parts of magnesium phosphate cement, 1.5 parts of BGC conductive material, and 20 parts of water; The mass ratio of the hydroxylated carbon nanotubes, bis(hydroxyethyl) terephthalate and graphene oxide is 1:1:

1.

3. The hydrogel-modified magnesium phosphate cement-based material according to claim 1, wherein In step S1, the mass ratio of the graphene oxide aqueous dispersion, 1-(3-dimethylaminopropyl)-3-ethylcarbodiimide hydrochloride and N-hydroxysuccinimide is 3~5:2~3:1~2; the ice bath reaction time is 3~5 h; the mass ratio of the graphene oxide aqueous dispersion, bis(hydroxyethyl) terephthalate and p-toluenesulfonic acid is 10~15:20~25:1~2; the heating reaction temperature is 75~90°C and the time is 5~8 h.

4. The hydrogel-modified magnesium phosphate cement-based material according to claim 1, wherein In step S2, the amount ratio of the hydroxylated carbon nanotubes to γ-aminopropyltriethoxysilane is 100 mg:1 mL; the temperature of the reflux reaction is 75-90°C, and the time is 5-8 h; the mass ratio of the hydroxylated carbon nanotubes, bis(hydroxyethyl) terephthalate, and potassium carbonate is 15-20:15-20:1; the temperature of the heating reaction is 55-70°C, and the time is 3-5 h.

5. The hydrogel-modified magnesium phosphate cement-based material according to claim 1, wherein: In step S3, the mass ratio of the GO-BHET complex to the HCNT-BHET complex is 2:1; the temperature of the hydrothermal reaction is 120-150° C., and the time is 4-7 h; the concentration of the ascorbic acid solution is 0.1 M; and the temperature of the heating reaction is 85-100° C., and the time is 3-5 h.

6. A method for preparing the hydrogel-modified magnesium phosphate cement-based material according to any one of claims 1 to 5, characterized in that: Specifically, magnesium phosphate cement dry powder, sodium acrylate powder and BGC conductive material are mixed to obtain a mixed dry powder; a mixed solution of N,N'-methylenebisacrylamide and ammonium persulfate is mixed with water, and the mixed solution is added to the mixed dry powder, and after stirring, hydration reaction and polymerization reaction occur simultaneously to obtain the product.

7. The preparation method according to claim 6, wherein The magnesium phosphate cement dry powder is obtained by mixing ammonium dihydrogen phosphate and dead-burned magnesium oxide, and the mass ratio of the ammonium dihydrogen phosphate to the dead-burned magnesium oxide is 1:1-1.

5.

8. The preparation method according to claim 6, wherein The volume ratio of the mixed solution of N,N'-methylenebisacrylamide and ammonium persulfate to water is 1:0.15-0.

25.

9. A hydrogel-modified magnesium phosphate cement-based piezoresistive coating, characterized in that: The invention relates to a hydrogel-modified magnesium phosphate cement-based material according to any one of claims 1 to 5.

10. A method for preparing the hydrogel-modified magnesium phosphate cement-based piezoresistive coating according to claim 9, characterized in that: Sodium acrylate, magnesium phosphate cement, BGC conductive material and water are mixed, and the obtained slurry is applied to the surface of the concrete test block.

11. Use of the hydrogel-modified magnesium phosphate cement-based material according to any one of claims 1 to 5 and / or the hydrogel-modified magnesium phosphate cement-based piezoresistive coating according to claim 9 in a concrete sensor.

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