Resistive adaptive low-voltage electric heating cement mortar, preparation method and application

Abstract

The application provides a resistance-adapting low-voltage electric heating cement mortar, a preparation method and application. The cement mortar comprises cement, silica fume, fine aggregate, water, polycarboxylate superplasticizer and a composite conductive phase in terms of mass parts. The composite conductive phase comprises carbon nanotubes and short carbon fibers. The total content of the composite conductive phase is 5%-6% of the mass of the cement. The mass ratio of the carbon nanotubes to the short carbon fibers is 1:0.5. The application of the resistance-adapting low-voltage electric heating cement mortar, the preparation method and application can generate Joule heat through the conductive network of the mortar matrix, can reduce the interface thermal resistance and the risk of local hot spots, and can balance the low-voltage heating adaptability, temperature uniformity and mechanical strength. The cement mortar can be used to prepare low-voltage electric heating components such as road snow-melting and deicing, building heating, bridge anti-icing, tunnel anti-icing, low-temperature curing and prefabricated component electric heating curing.

Description

Technical Field

[0001] This invention relates to the field of cement mortar, and in particular to a resistance-adaptive low-voltage electrothermal cement mortar, its preparation method, and its application. Background Technology

[0002] Against the backdrop of "dual carbon" goals and the development of low-carbon building energy, distributed photovoltaic (PV), building energy storage, and "PV-storage-DC-flexible" systems are increasingly being applied to green buildings and near-zero energy buildings. PV panels and energy storage batteries can output low-voltage power supplies such as 24V, 36V, or 48V, offering advantages such as high safety, convenient system integration, and suitability for distributed energy supply. If this low-voltage electricity can be directly used for building heating, road snow melting and de-icing, bridge deck anti-icing, tunnel lining anti-icing, low-temperature environment component curing, or prefabricated component electrothermal curing, the on-site consumption efficiency of PV power in buildings and infrastructure can be improved.

[0003] Cement mortar and concrete are primary materials in construction and infrastructure, possessing a certain degree of thermal stability and heat storage capacity. If their composition could generate Joule heat under low-voltage electrical stimulation, they could simultaneously function as structural load-bearing, heat-generating, and heat-storing materials. However, ordinary cement mortar and concrete are typically high-resistivity materials, making it difficult to generate effective heat using a safe low-voltage power source under normal conditions.

[0004] Existing electric heating components mostly employ methods such as externally attached heating films, adhesive heating elements, brushed conductive coatings, embedded metal heating wires, carbon fiber heating wires, or other independent heating elements to achieve heating. While these methods can produce a certain heating effect, the heating element is usually independent of the cement matrix or mainly located on the surface of the component, which easily leads to interfacial thermal resistance. During long-term service, problems such as adhesion failure, hollowing and detachment, wear damage, electrode interface deterioration, or localized heat source failure may occur. Furthermore, the heat from linear, sheet-like, or surface heat sources needs to be transferred into the mortar or concrete, easily causing localized hot spots, delayed heating, and uneven temperature distribution.

[0005] In recent years, conductive phases such as carbon fiber, carbon nanotubes, carbon black, graphene, and graphite have been used to prepare conductive cement-based composite materials. However, existing research and patents are mostly focused on piezoresistive sensing, stress-strain monitoring, crack identification, traffic load monitoring, or structural health monitoring. Their design goals are usually to improve the sensitivity to resistance changes and signal stability, rather than to provide continuous, uniform, and safe Joule heating output under a fixed low-voltage power supply.

[0006] For safe low-voltage electric heating applications such as 24V, lower material resistance is not always better. If the resistance is too high, the input power is insufficient, resulting in slow heating; if the resistance is too low, the input power is too high, easily leading to localized overheating, dehydration and cracking, electrode interface damage, and deterioration of mechanical properties. Therefore, the key to low-voltage electric heating of cement mortar or concrete is not simply to improve conductivity, but to determine a reasonable resistance range that matches the low-voltage power supply system based on the supply voltage, component dimensions, electrode spacing, current-carrying cross-sectional area, and target input power. Summary of the Invention

[0007] The purpose of this invention is to provide a resistance-adaptive low-pressure electrothermal cement mortar, its preparation method, and its application. This invention enables the cement-based material to generate stable, uniform, and controllable heat under safe low-pressure conditions, and meets the application requirements of building heating, road snow melting and de-icing, low-temperature anti-icing, and electrothermal curing.

[0008] To achieve the above objectives, the present invention provides a resistance-adaptive low-voltage electrothermal cement mortar, comprising cementitious material, silica fume, fine aggregate, water, polycarboxylate superplasticizer and composite conductive phase, wherein the composite conductive phase comprises carbon nanotubes and short-cut carbon fibers. The mass ratio of cementitious material, silica fume, fine aggregate, and water is (95~105):(9.5~10.5):(95~105):(52.25~57.75). The mass ratio of polycarboxylate superplasticizer to composite conductive phase is (0.95~1.05):(9.5~10.5). Preferably, the mass ratio of cementitious material, silica fume, fine aggregate, and water is 100:10:100:55; the mass ratio of polycarboxylate superplasticizer to composite conductive phase is 1:10. The total content of the composite conductive phase is 5%-6% of the mass of cementitious material, and the mass ratio of the carbon nanotubes to the short-cut carbon fibers is 1:0.5.

[0009] Preferably, the particle size of silica fume is 0.05–0.5 μm, and the particle size of fine aggregate is 0.08–2 mm; Carbon nanotubes have a length of 10-30 μm, a diameter of 5-15 nm, and a purity of >95%; Short-cut carbon fibers have a length of 3 mm, a diameter of 2-10 µm, a carbon content of 99%, and a resistivity of 5 × 10⁻⁶. -6 -13×10 -6 Ω·m.

[0010] Preferably, carbon nanotubes and short-cut carbon fibers work together to form a multi-scale conductive network.

[0011] Preferably, the mass ratio of fine aggregate to cementitious material is (0.9-1.1):1; and the mass ratio of water to cementitious material is (0.53-0.56):1.

[0012] A method for preparing a resistance-adaptive low-voltage electrothermal cement mortar includes the following steps: S1. Dispersion preparation: Polycarboxylate superplasticizer is added to the mixing water, and then dispersed by stirring and ultrasonication to obtain a dispersion. S2. Carbon material pre-dispersion: Add silica fume, chopped carbon fibers, carbon nanotubes and cementitious materials to a mixer for pre-mixing, so that the carbon materials are initially dispersed in solid particles to obtain carbon cement pre-dispersion material; S3. Mortar mixing: Add dispersion liquid to carbon cement predispersant, stir, add fine aggregate and continue stirring to obtain low-pressure electric heating cement mortar.

[0013] Preferably, the stirring speed is 400±20 r / min and the stirring time is 2.5~3.5 min; The ultrasonic dispersion frequency is 40 kHz.

[0014] The application of a resistance-adaptive low-voltage electrothermal cement mortar includes the following steps: S1A, Molding and Electrode Setting: Cement mortar is poured into the mold, and current collecting electrodes are set at both ends or opposite sides of the specimen or component, and vibration molding is performed; S1B, Curing: After molding, demold and perform standard curing to obtain resistance-adaptive low-voltage electrothermal cement mortar specimens or components.

[0015] Preferably, it is suitable for use under safe low-voltage electric heating conditions of 24V or below.

[0016] Therefore, the present invention employs the above-mentioned resistance-adaptive low-voltage electrothermal cement mortar, its preparation method, and its application, with the following technical effects: 1. Good adaptability to low-pressure heating and self-heating of the mortar matrix. Under safe low-pressure conditions of 24V or below, this invention enables the mortar or component to have moderate input power under low-pressure power supply. The multi-scale conductive network inside the mortar matrix directly generates Joule heating, which can achieve stable Joule heating and avoid local overheating, cracking, electrode interface damage and strength reduction caused by excessively low resistance.

[0017] 2. Wide range of applications. This invention can be used in snow melting and de-icing, building heating, bridge deck icing, tunnel icing, low-temperature curing, and electrothermal curing of precast components, among other applications.

[0018] 3. To meet the conductivity requirements under low voltage, the conductive network of electrothermal cement needs to be optimized. Existing solutions mostly rely on increasing the amount of carbon materials to reduce resistance, but excessive carbon is prone to agglomeration, which impairs workability, increases defects, and weakens strength. This invention improves the dry dispersion of carbon by introducing silica fume, eliminating the need for separate ultrasonic, oxidation, or solvent pretreatment of carbon, and also avoiding reliance on special aqueous dispersants, thereby simplifying the process and reducing costs.

[0019] 4. Although carbon nanotubes have excellent electrical conductivity, their high specific surface energy makes them prone to entanglement and flocculation in cement. Therefore, large quantities need to be incorporated to construct a complete conductive network, which is uneconomical. Thus, carbon fibers are used to replace some carbon nanotubes. By utilizing the long-range conductivity of fibers combined with the short-range bridging effect of nanotubes, the two work synergistically to construct a more continuous and efficient conductive pathway, significantly improving connectivity and stability under low pressure. Attached Figure Description

[0020] Figure 1 This is a schematic diagram of a multi-scale conductive network. Figure 2 The 24V power-on temperature rise curve and infrared thermogram of Example 1 are shown. Figure 2 (a) is the temperature rise curve of Example 1 under 24V power-on; Figure 2 (b) is an infrared thermogram of Example 1; Figure 3 The temperature rise curve and infrared thermogram of Comparative Example 1 when powered on at 24V are shown. Figure 3 (a) is the temperature rise curve of Comparative Example 1 when energized at 24V; Figure 3 (b) is the infrared thermal image of Comparative Example 1; Figure 4 The temperature rise curve and infrared thermogram of Comparative Example 2 when powered on at 24V; Figure 4 (a) is the temperature rise curve of Comparative Example 2 when powered on at 24V; Figure 4 (b) is the infrared thermal image of Comparative Example 2; Figure 5 The temperature rise curve and infrared thermogram of Comparative Example 3 when powered on at 24V are shown. Figure 5 (a) is the temperature rise curve of Comparative Example 3 when powered on at 24V; Figure 5 (b) is the infrared thermal image of Comparative Example 3; Figure 6 This is a SEM microstructure image; Figure 6 (a) shows the composite adhesion structure of carbon fiber, carbon nanotube and hydration products (SEM, 30kX). Figure 6 (b) The conductive network formed by the synergistic interaction of carbon fibers, carbon nanotubes and hydration products (SEM, 10kX). Figure 7 This is a schematic diagram of resistivity-electrode spacing-cross-sectional area matching; Figure 8The distribution morphology of carbon materials in the cement matrix of Example 1; Figure 8 (a) The distribution morphology of carbon fibers in the cement matrix of Example 1 (SEM, 500X). Figure 8 (b) The distribution morphology of carbon fibers in the cement matrix of Example 1 (SEM, 500X). Figure 8 (c) shows the distribution morphology of carbon nanotubes in the cement matrix of Example 1 (SEM, 30kX). Figure 8 (d) shows the distribution morphology of carbon nanotubes in the cement matrix of Example 1 (SEM, 30kX). Figure 9 The distribution morphology of carbon materials in the cement matrix of Example 2; Figure 9 (a) The distribution morphology of carbon fibers in the cement matrix of Example 2 (SEM, 500X). Figure 9 (b) The distribution morphology of carbon fibers in the cement matrix of Example 2 (SEM, 500X). Figure 9 (c) shows the distribution morphology of carbon nanotubes in the cement matrix of Example 2 (SEM, 30kX). Figure 9 (d) shows the distribution morphology of carbon nanotubes in the cement matrix of Example 2 (SEM, 30kX). Figure 10 The distribution morphology of carbon materials in the cement matrix of Example 3; Figure 10 (a) The distribution morphology of carbon fibers in the cement matrix of Example 3 (SEM, 500X). Figure 10 (b) The distribution morphology of carbon fibers in the cement matrix of Example 3 (SEM, 500X). Figure 10 (c) shows the distribution morphology of carbon nanotubes in the cement matrix of Example 3 (SEM, 30kX). Figure 10 (d) shows the distribution morphology of carbon nanotubes in the cement matrix of Example 3 (SEM, 30kX). Detailed Implementation

[0021] The technical solution of the present invention will be further described below with reference to the accompanying drawings and embodiments.

[0022] Unless otherwise defined, the technical or scientific terms used in this invention shall have the ordinary meaning as understood by one of ordinary skill in the art to which this invention pertains.

[0023] Silica fume particles can penetrate between carbon nanotube aggregates and chopped carbon fiber bundles, acting as particle wedging and mechanical isolation agents to promote the unbundling of carbon materials. Simultaneously, silica fume fills the larger voids between cement particles, limiting the migration and re-agglomeration of dispersed carbon materials in the freshly mixed slurry, thus facilitating the formation of a more uniformly distributed multi-scale conductive network. During hardening, the active SiO2 in silica fume forms CSH gel with Ca(OH)2 generated during cement hydration; this filling effect also improves the interfacial bonding between carbon materials and hydration products.

[0024] Polycarboxylate superplasticizers include ether-based polycarboxylate superplasticizers, ester-based polycarboxylate superplasticizers, and their blends.

[0025] Example 1: Cement mortar within the resistance range of low-voltage electric heating adapter S1. Dispersion preparation: Add 0.5 parts of polycarboxylate superplasticizer to 55 parts of mixing water, stir mechanically at 400 r / min for 3 min, and then ultrasonically disperse for 10 min to obtain the dispersion. S2. Carbon material pre-dispersion: 1.67 parts of short-cut carbon fiber, 3.33 parts of carbon nanotubes, 10 parts of silica fume and 100 parts of cementitious material are added to a mixer and pre-mixed at 140 r / min for 3 min and 285 r / min for 2 min. The cement and silica fume particles are used to isolate the composite carbon material and pre-disperde it in a solid state. S3. Mortar mixing: Mix 115 parts of carbon cement pre-dispersed material obtained in step S2 and 55.5 parts of dispersion liquid obtained in step S1. Stir at 285 r / min for 2 min, then add 100 parts of standard sand and continue stirring for 2 min to obtain low-pressure electric heating cement mortar.

[0026] S4. Pour the low-pressure electric heating cement mortar into a 4cm×4cm×16cm mold, set electrodes at both ends, set a temperature measuring point at the center to measure the internal temperature, vibrate and compact for 60 times to form, demold 24 hours after forming and perform standard curing for 28 days, then adjust the humidity according to uniform environmental conditions or let it stand at room temperature for 30 days to adjust the humidity.

[0027] Tests showed that Example 1 had a 28-day compressive strength of 32.65 MPa, an equivalent resistivity of 33.99 Ω after humidification, an input power of 16.95 W under 24V power supply, an average temperature rise rate of 1℃ / min from 0 to 10 minutes, and an internal center temperature of 40℃ at 15.8 minutes. At this point, the thermal imager showed a maximum temperature difference of 2.8℃ between the highest surface temperature and the maximum temperature at the surface center of the specimen. Example 1 operates within the low-voltage electrothermal adapter resistance range, achieving stable temperature rise under 24V low-voltage conditions while maintaining good mechanical properties.

[0028] like Figure 8 As shown, Figure 8 (a) and Figure 8 (b) is a low-magnification CF electron microscope image. Figure 8 (c) and Figure 8 (d) is a high-magnification CNT electron microscope image.

[0029] Figure 8 (a) and Figure 8 (b) In the low-magnification SEM image, CF is relatively uniformly distributed in the cement matrix, and no obvious bundles, entanglements or large-scale local enrichment phenomena were observed. Figure 8 (c) and Figure 8 (d) No obvious large-scale CNT agglomerates were observed in the high-magnification image, indicating that CNTs have a good dispersion state in the matrix. Fine silica fume particles may reduce the direct contact and agglomeration tendency of CNT / CF during the mixing process through particle isolation, micro-filling, and improved powder gradation, thereby promoting the uniform distribution of conductive components.

[0030] Example 2 The difference from Example 1 is that no silica fume is added.

[0031] like Figure 9 The image shown is an electron microscope image of the prepared cement matrix. Figure 9 (a) and Figure 9 (b) The low-magnification SEM image shows obvious local enrichment of carbon fibers (CF), with some fibers arranged in parallel and bundled, failing to be fully dispersed in the cement matrix. Figure 9 (c) and Figure 9 (d) The high-magnification SEM image further reveals that carbon nanotubes (CNTs) exhibit significant entanglement and aggregation in the porous regions or on the surface of hydration products, forming localized network clusters and large-scale aggregates. This phenomenon indicates that ordinary dry mixing alone is insufficient to effectively overcome the entanglement of carbon nanotube bundles and the aggregation problems caused by the high specific surface area and van der Waals interactions of CNTs, easily leading to local enrichment of the conductive phase, increased porosity defects, and uneven distribution of the conductive network.

[0032] Example 3 S1. Dispersion preparation: Add 0.5 parts of polycarboxylate superplasticizer to 55 parts of mixing water, stir mechanically at 400 r / min for 3 min, and then ultrasonically disperse for 10 min to obtain the dispersion. S2. Add 1.67 parts of short-cut carbon fiber, 3.33 parts of carbon nanotubes and 10 parts of silica fume to the dispersion obtained in S1, and continue ultrasonic dispersion at 40 kHz for 10 min to obtain carbon-water-reducing agent dispersion.

[0033] S3. Add 100 parts of cement to the carbon-water-reducing agent dispersion prepared in S2, stir at 285 r / min for 2 min, then add 100 parts of standard sand and continue stirring for 2 min to obtain low-pressure electric heating cement mortar.

[0034] S4. Pour the low-pressure electric heating cement mortar into a 4cm×4cm×16cm mold, set electrodes at both ends, set a temperature measuring point at the center to measure the internal temperature, vibrate and compact for 60 times to form, demold 24 hours after forming and perform standard curing for 28 days, then adjust the humidity according to uniform environmental conditions or let it stand at room temperature for 30 days to adjust the humidity.

[0035] like Figure 10 As shown, Figure 10 (a) and Figure 10 (b) The low-magnification SEM image shows obvious bundle and cluster aggregation of carbon fibers (CF). Some fibers are arranged in parallel and concentrated in local areas, indicating that the pre-water dispersion did not effectively break up the entanglement and overlap between long fibers, and the distribution of CF in the cement matrix is ​​still uneven. Figure 10 (c) and Figure 10 (d) High-magnification SEM images did not show very obvious large-scale CNT agglomerates, but local network clusters were observed, indicating that the dispersion of CNTs was improved with the addition of silica fume compared to the dry-mix group without silica fume. The fine silica fume particles may have reduced the direct contact and secondary agglomeration tendency between CNTs to some extent through particle isolation and micro-filling effects. However, since CF still showed obvious clustering and local enrichment, the overall distribution of conductive components was still not uniform enough. Therefore, although the overall dispersion effect of this group was better than that of the dry-mix group without silica fume, it was still weaker than that of the dry-mix group with silica fume.

[0036] The SEM results of Examples 1, 2, and 3 show different distribution characteristics of carbon materials. In Example 2, the silica fume-free dry-mix group simultaneously observed localized CF bundles and localized CNT network agglomerations, indicating that simply relying on dry mixing of cement particles is insufficient to simultaneously disperse micron-sized fibers and nano-sized tubular materials. In Example 3, the carbon material liquid-phase ultrasonic group showed a reduction in large-scale CNT agglomeration compared to the silica fume-free dry-mix group, but CF still exhibited obvious bundle and cluster aggregations, indicating that the liquid-phase ultrasonic treatment used in this experiment had a certain effect on CNT dispersion, but did not simultaneously solve the problem of unbundling and uniform distribution of longer CF. In contrast, Example 1, which involved solid-state premixing of silica fume, cement, CNT, and CF, did not show obvious large-scale CF bundles or CNT agglomerations in the observed area. The morphological differences among the three groups indicate that the process effect of Example 1 is not limited to CNT or CF, but utilizes cement and silica fume particles as solid-state dispersion media to simultaneously reduce the tendency of CNT agglomeration and CF bundle formation. This constitutes the process effect that distinguishes it from the carbon material liquid-phase ultrasonic predispersion method.

[0037] Comparative Example 1: Low Carbon Content The difference from Example 1 is that the chopped carbon fiber was adjusted to 0.67 parts and the carbon nanotubes were adjusted to 1.33 parts.

[0038] Testing revealed that Comparative Example 1 had a 28-day compressive strength of 66.7 MPa, an equivalent resistance of 469.1 Ω after humidification, an input power of 1.23 W under 24V power supply, an average temperature rise rate of 0.04℃ / min from 0 to 10 minutes, and a maximum internal center temperature of 23.5℃, failing to reach 40℃. While Comparative Example 1 exhibited high strength, its excessively high resistance resulted in insufficient input power under low-voltage conditions, hindering effective electrothermal heating.

[0039] Comparative Example 2: High Carbon Content Group The difference from Example 1 is that the chopped carbon fiber was adjusted to 3.34 parts and the carbon nanotubes were adjusted to 6.66 parts.

[0040] Testing revealed that Comparative Example 2 had a 28-day compressive strength of 14 MPa, an equivalent resistivity of 28.68 Ω after humidification, an input power of 20.08 W under 24V power supply, an average temperature rise rate of 4.30℃ / min from 0 to 10 minutes, and an internal center temperature reaching 40℃ at 4.4 minutes. At this point, the thermal imager showed a maximum temperature difference of 22℃ between the highest surface temperature and the surface center temperature of Comparative Example 2, indicating a sudden temperature rise and the appearance of obvious local hot spots. Further observation after heating revealed electrode interface damage in the comparative example. Although Comparative Example 2 had a lower resistivity, its excessive input power and significantly reduced strength posed risks of localized overheating, cracking, and decreased service stability.

[0041] Comparative Example 3: Carbon nanotube group only The difference from Example 1 is that the carbon nanotubes were adjusted to 5 parts, and no carbon fibers were added.

[0042] Tests showed that Comparative Example 3 had a 28-day compressive strength of 21.55 MPa, an equivalent resistivity of 30.58 Ω after humidification, an input power of 18.8 W under 24V power supply, an average temperature rise rate of 1.02℃ / min from 0 to 10 min, and an internal center temperature of 40℃ at 22.5 min. At this point, the thermal imager showed a maximum temperature difference of 1.8℃ between the highest surface temperature and the surface center temperature of Comparative Example 3. Comparative Example 3 exhibited a high input power but a slow response, indicating that simply incorporating carbon nanotubes cannot achieve the comprehensive performance suitable for low-pressure electric heating components.

[0043] The initial equivalent resistance and nominal power of Examples 1, 2, and 3 are similar, but their overall performance differs significantly. In Comparative Example 2, increasing the conductive carbon material doping to 10% only slightly reduced the resistance, but resulted in a 22°C surface temperature difference, electrode interface damage, and a decrease in compressive strength to 14 MPa. Comparative Example 3, with the same total carbon doping of 5% as Example 1, had a lower initial resistance and higher nominal power, but the time required to reach 40°C increased from 15.8 min to 22.5 min, and the compressive strength decreased from 32.65 MPa to 21.55 MPa. These results indicate that Example 1 does not simply pursue a lower initial equivalent resistance or a higher initial nominal power, but rather maintains an effective low-pressure heating response while avoiding the rapid local overheating and significant strength decrease observed in Comparative Example 2. Therefore, the technical effect of Example 1 lies in the comprehensive balance between heating response, surface temperature distribution, and mechanical properties, rather than minimizing the single indicator of initial resistance.

[0044] Figure 1 The cement mortar on display uses short-cut carbon fibers as a long-range skeleton, with CNT nano-bridges connecting the fiber gaps and interface breaks.

[0045] Depend on Figure 2 , Figure 3 , Figure 4 , Figure 5 The low-pressure heating response and stable temperature, temperature uniformity, and local hot spots of the low-resistance group described in the above embodiments and comparative examples are demonstrated.

[0046] like Figure 6 As shown, the CNT / CF multi-scale network is demonstrated.

[0047] like Figure 7 As shown, when electrothermal mortar is applied to components of different sizes, the total resistance of the component depends not only on the conductivity of the material itself, but also on the electrode spacing, the current-carrying cross-sectional area, and the mortar layer thickness. Therefore, the resistance range measured by standard specimens cannot be simply applied; instead, a matching design should be performed based on the actual component dimensions. Under low-voltage power supply conditions, if the electrode spacing is too large, the input power density will be significantly reduced, leading to insufficient heating; if the electrode spacing is too small or the mortar resistivity is too low, it may cause excessive current and localized overheating. Therefore, it is necessary to adjust the equivalent volume resistivity of the electrothermal mortar, the electrode spacing, and the mortar layer thickness to ensure that the component obtains a suitable input power density under 24V or lower power supply conditions.

[0048] The equivalent resistance R of the component satisfies the following relationship with the equivalent volume resistivity ρ of the electrothermal mortar, the electrode spacing L, and the current-carrying cross-sectional area A: R = ρL / A. Combined with the input power relationship P = U... 2 / R, we can get P=U 2A / (ρL). Therefore, by adjusting the equivalent volume resistivity ρ of the electrothermal mortar, the electrode spacing L, the mortar layer thickness, the effective length of the electrode, or the current-carrying cross-sectional area A, the component can be kept within the preset input power range under a power supply of 24V or below.

[0049] For a plate-shaped component, if the electrodes are positioned on opposite sides of a low-pressure electrothermal mortar layer, and the current flows along the plate surface, assuming the electrode spacing is L, the electrode length is W, and the electrothermal mortar layer thickness is t, then the current-carrying cross-sectional area A = W × t, and the component's equivalent resistance R = ρL / (Wt). When the component's heating area is S = L × W, the input power per unit area q = P / S = U. 2 t / (ρL 2 ).

[0050] The above formula illustrates the electrical matching relationship when a component is enlarged or its dimensions change: Insufficient electrode spacing, excessively thick mortar layer, or excessively low resistivity of the electrothermal mortar can significantly increase the input power density, leading to excessive current or localized overheating; conversely, excessively large electrode spacing, excessively thin mortar layer, or excessively high resistivity of the electrothermal mortar will decrease the input power density, resulting in insufficient heating. Therefore, the above formula can be used to determine whether changes in component size, electrode arrangement, and mortar layer thickness will cause excessive or insufficient input power, and accordingly determine the matching range of the equivalent volume resistivity of the electrothermal mortar, electrode spacing, and mortar layer thickness. The structural design focus of this invention is not simply setting up electrodes, but rather establishing a matching relationship between resistivity, geometric dimensions, and input power.

[0051] For low-pressure electrically heated cement mortar, under a given operating voltage U, its input power P and the equivalent resistance R of the material or component satisfy the following relationship: P=U 2 Therefore, when the resistance is too high, the input power is insufficient; when the resistance is too low, the input power is excessive.

[0052] This invention uses a 24V safe low-voltage power supply as the preferred operating condition, and takes 10-30W as the target input power range for a 4cm×4cm×16cm mortar specimen. Therefore, the range of the adaptor resistance can be determined by R=U. 2 / P is determined in reverse. When U=24V, P=10⁻³⁰W, R=24 2 / (10-30), corresponding to 19.2-57.6Ω, and 20-60Ω is used in engineering.

[0053] A 4cm×4cm×16cm specimen with two electrodes, an electrode spacing of 16cm, and a cross-sectional area of ​​16cm² was used. 2 Under these conditions, 20-60Ω corresponds to an equivalent volume resistivity of approximately 0.20-0.60Ω·m. This equivalent resistivity range is not simply about pursuing low resistance, but rather about the power range suitable for stable 24V low-voltage heating.

[0054] Therefore, this invention employs the aforementioned resistance-adaptive low-voltage electrothermal cement mortar, its preparation method, and its application. A multi-scale conductive network is constructed using carbon nanotubes and chopped carbon fibers. By adjusting the total content of the composite conductive phase, the mass ratio of carbon nanotubes to chopped carbon fibers, the mortar-binder ratio, and the water-cement ratio, the cement mortar, after 28 days of curing and humidification, possesses an equivalent resistance range suitable for 24V low-voltage heating. Furthermore, when applied to low-voltage electrothermal components, matching designs can be implemented based on the material's equivalent volume resistivity, electrode spacing, and current-carrying cross-sectional area, ensuring the component operates within a preset input power range under low-voltage power supply conditions. Unlike externally applied heating films, conductive coatings, or embedded heating wires, the Joule heating generated by the mortar's own conductive network reduces interfacial thermal resistance and the risk of localized hot spots, while also considering low-voltage heating adaptability, temperature uniformity, and mechanical strength. This cement mortar can be used to prepare low-voltage electrothermal components for road snow melting and de-icing, building heating, bridge anti-icing, tunnel anti-icing, low-temperature curing, and electrothermal curing of precast components.

[0055] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention and not to limit them. Although the present invention has been described in detail with reference to preferred embodiments, those skilled in the art should understand that modifications or equivalent substitutions can still be made to the technical solutions of the present invention, and these modifications or equivalent substitutions cannot cause the modified technical solutions to deviate from the spirit and scope of the technical solutions of the present invention.

Claims

1. A resistance-adaptive low-voltage electrothermal cement mortar, characterized in that, It includes cementitious materials, silica fume, fine aggregate, water, polycarboxylate superplasticizer, and a composite conductive phase, wherein the composite conductive phase includes carbon nanotubes and chopped carbon fibers; The mass ratio of cementitious material, silica fume, fine aggregate, and water is (95~105):(9.5~10.5):(95~105):(52.25~57.75). The mass ratio of polycarboxylate superplasticizer to composite conductive phase is (0.95~1.05):(9.5~10.5). The total content of the composite conductive phase is 5%-6% of the mass of cementitious material, and the mass ratio of the carbon nanotubes to the short-cut carbon fibers is 1:0.

5.

2. The resistance-adaptive low-voltage electrothermal cement mortar according to claim 1, characterized in that, The particle size of silica fume is 0.05-0.5μm, and the particle size of fine aggregate is 0.08-2mm; Carbon nanotubes, 10-30 μm in length, 5-15 nm in diameter, and >95% purity; Short-cut carbon fibers are 3mm in length, 2-10µm in diameter, 99% carbon content, and have a resistivity of 5×10⁻⁶. -6 -13×10 -6 Ω·m.

3. The resistance-adaptive low-voltage electrothermal cement mortar according to claim 1, characterized in that, Carbon nanotubes and short-cut carbon fibers work together to form a multi-scale conductive network.

4. The resistance-adaptive low-voltage electrothermal cement mortar according to claim 1, characterized in that, The mass ratio of fine aggregate to cementitious material is (0.9-1.1):1; the mass ratio of water to cementitious material is (0.53-0.56):

1.

5. The method for preparing a resistance-adaptive low-voltage electrothermal cement mortar according to claim 1, characterized in that, Includes the following steps: S1. Dispersion preparation: Polycarboxylate superplasticizer is added to the mixing water, and then dispersed by stirring and ultrasonication to obtain a dispersion. S2. Carbon material pre-dispersion: Add silica fume, chopped carbon fibers, carbon nanotubes and cementitious materials to a mixer for pre-mixing, so that the carbon materials are initially dispersed in solid particles to obtain carbon cement pre-dispersion material; S3. Mortar mixing: Add dispersion liquid to carbon cement predispersant, stir, add fine aggregate and continue stirring to obtain low-pressure electric heating cement mortar.

6. The method for preparing a resistance-adaptive low-voltage electrothermal cement mortar according to claim 5, characterized in that, The stirring speed is 400±20 r / min, and the stirring time is 2.5~3.5 min; The ultrasonic dispersion frequency is 40 kHz.

7. The application of the resistance-adaptive low-voltage electrothermal cement mortar according to claim 1, characterized in that, Includes the following steps: S1A, Molding and Electrode Setting: Cement mortar is poured into the mold, and current collecting electrodes are set at both ends or opposite sides of the specimen or component, and vibration molding is performed; S1B, Curing: After molding, demold and perform standard curing to obtain resistance-adaptive low-voltage electrothermal cement mortar specimens or components.

8. The application of the resistance-adaptive low-voltage electrothermal cement mortar according to claim 7, characterized in that, Applicable under safe low-voltage electric heating conditions of 24V or below.

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