Electromagnetic shielding cementitious composite material and method of making same

Abstract

The application discloses an electromagnetic shielding cement-based composite material and a preparation method thereof, and the preparation method comprises the following steps: preparing composite modified biochar powder; soaking the biochar aggregate in a calcium hydroxide saturated solution for treatment, drying to obtain modified biochar aggregate, and performing continuous dense gradation design; preparing Fe3O4 coating carbon fiber; preparing a cement-based material, pouring into a mold, continuously applying a static magnetic field, cooperating with mechanical vibration to remove air bubbles, ensuring directional arrangement of the Fe3O4 coating carbon fiber, and curing. The electromagnetic shielding cement-based composite material obtained by the application has an electromagnetic shielding efficiency of 40 dB or more in a wide frequency band of 8-12 GHz, a 28-day compressive strength of greater than 55 MPa, and a CO2 solidification amount of greater than 35 kg / m 3 The electromagnetic shielding efficiency and mechanical properties of the cement-based composite material are synergistically improved, and the cement-based composite material can be widely applied in the fields of military facilities, precision instrument laboratories, 5G communication base stations and the like.

Description

Technical Field

[0001] This invention pertains to cement-based composite materials and their preparation methods, specifically an electromagnetic shielding cement-based composite material and its preparation method. Background Technology

[0002] With the rapid development of electronic communication technology, electromagnetic radiation pollution has become an increasingly prominent problem. Electromagnetic radiation not only interferes with the normal operation of precision electronic equipment but may also pose potential hazards to human health. Therefore, developing efficient, broadband, and low-cost electromagnetic shielding materials has become an important research direction in the field of building materials.

[0003] Cement-based materials, as the most widely used building materials, possess excellent mechanical properties, durability, and formability. However, they are inherently electromagnetically transparent (with extremely low electrical conductivity, typically <10). -5 With a density of S / m, it has almost no electromagnetic shielding capability. Traditional methods to improve the electromagnetic shielding effectiveness of cement-based materials mainly include the following two categories: 1) Directly adding conductive / magnetic fillers to the cement matrix: Adding carbon-based materials (such as graphite, carbon nanotubes, carbon fibers), metallic materials (such as iron powder, copper powder, nickel powder), or magnetic materials (such as Fe3O4, ferrite) to the cement matrix. Although this method can improve the electromagnetic shielding effectiveness to a certain extent, it still has the following problems: carbon-based fillers have poor dispersibility and are prone to agglomeration, making it difficult to form a continuous conductive network; metallic fillers are costly, dense, and prone to corrosion, and have poor compatibility with the cement matrix; magnetic fillers often require a high dosage (>10wt%) to produce a significant effect, but a high dosage will seriously degrade the mechanical properties of cement-based materials. 2) Multi-layer or sandwich structure design method: By constructing a multi-layer gradient structure or embedding metal mesh, conductive fabric, etc., a reflection-absorption composite shielding layer is formed. Although this type of method has high shielding effectiveness, the preparation process is complex, the interlayer interface bonding is poor, the cost is high, and the thickness is large, making it difficult to meet the requirements of thin-walled components and on-site casting construction.

[0004] In recent years, biochar, a porous carbon material prepared from agricultural and forestry waste through pyrolysis, has begun to be explored for use in cement-based electromagnetic shielding materials due to its wide availability, low cost, low density, large specific surface area, and certain conductivity. However, existing biochar-modified cement technologies still suffer from technical defects such as difficulty in simultaneously achieving mechanical properties and shielding effectiveness, insufficient broadband shielding capability, and weak interfacial bonding with the cement matrix. While the porous structure of biochar improves the electromagnetic shielding effectiveness of cement-based composites, it often leads to a significant decrease in mechanical properties; for example, strength loss can exceed 30% when the biochar content is >10 wt%. In existing technologies, conductive fillers such as carbon fibers are randomly distributed in the cement matrix, resulting in isotropic electromagnetic shielding effectiveness, which cannot be optimized according to actual needs. Therefore, it is urgent to overcome the technical defects in existing technologies and develop an electromagnetic shielding cement-based composite material and its preparation method that can achieve high electromagnetic shielding effectiveness, excellent mechanical properties, and significant carbon fixation capability. Summary of the Invention

[0005] Purpose of the invention: In order to overcome the shortcomings of the existing technology, the purpose of this invention is to provide a method for preparing an electromagnetic shielding cement-based composite material with good mechanical properties and strong carbon fixation ability. Another purpose of this invention is to provide a convenient and controllable electromagnetic shielding cement-based composite material.

[0006] Technical solution: The present invention provides a method for preparing an electromagnetic shielding cement-based composite material, comprising the following steps:

[0007] Step 1: Biochar powder is pre-adsorbed in a CO2 atmosphere and then combined with magnesium oxide by ball milling to obtain composite modified biochar powder. Biochar aggregate is soaked in a saturated calcium hydroxide solution and dried to obtain modified biochar aggregate, which is then subjected to continuous dense gradation design. Carbon fiber is surface oxidized, dried, and then immersed in Fe3O4 precursor solution. Fe3O4 nanoparticles are uniformly grown on the surface of carbon fiber by in-situ co-precipitation. After the reaction is completed, the carbon fiber is removed, cleaned, and dried to obtain Fe3O4 coated carbon fiber.

[0008] Step 2: The composite modified biochar powder, modified biochar aggregate, Fe3O4 coated carbon fiber, cement, and water-reducing agent are mixed evenly in a mass ratio of 5~30:75~100:0.2~0.4:100:0.5~1 to obtain a cement-based material. The material is then poured into a mold. During and after the pouring process, a static magnetic field is continuously applied, and mechanical vibration is used to remove air bubbles and ensure the directional alignment of the Fe3O4 coated carbon fiber. After curing, an electromagnetic shielding cement-based composite material is obtained.

[0009] Further, in step one, the length of the carbon fiber is 3~15 mm, and the surface oxidation treatment is to immerse it in a hydrogen peroxide solution with a mass fraction of 10~30wt% and treat it in a water bath at 60~80℃ for 2~6 hours. After the oxidation treatment, the carbon fiber is taken out and washed.

[0010] Furthermore, in step one, the Fe3O4 precursor solution contains Fe... 3+ The concentration was 0.1~0.5 mol / L, the thickness of the Fe3O4 coating was 10~200 nm, and the Fe3O4 content accounted for 5~30% of the total mass of the coated carbon fibers. The Fe3O4 precursor solution was prepared by mixing FeCl3·6H2O and FeCl2·4H2O according to Fe... 3+ :Fe 2+ Dissolve in deionized water at a molar ratio of 2~3:1, and add 5~10wt% polyethylene glycol (PEG-4000) as a dispersant.

[0011] Further, in step one, the in-situ co-precipitation method involves stirring in a water bath at 50-80°C for 30-60 min, adding ammonia dropwise under N2 protection to adjust the pH of the solution to 9-11, and reacting at 70-90°C for 1-3 h, allowing Fe3O4 nanoparticles to nucleate and grow in situ on the carbon fiber surface. Preferably, the mass fraction of ammonia (NH3·H2O) is 25-28 wt%.

[0012] Furthermore, in step one, the biochar powder has a particle size of 0.1~50 μm and a specific surface area ≥200 m². 2 / g, porosity ≥60%, and the mass percentage of graphitized carbon in the biochar powder is 75~78%. Preferably, the biochar powder is obtained by grinding wood after pyrolysis at 900~950℃.

[0013] Furthermore, in step one, the pressure of the pre-adsorption treatment is 0.1~0.5MPa, the temperature is 25~60℃, the treatment time is 2~8h, and the mass ratio of biochar powder to magnesium oxide is 1~2:1.

[0014] Furthermore, in step one, the ball milling speed is 200~400 rpm, and the ball milling time is 0.5~5h.

[0015] Furthermore, in step one, the drying temperature is 60~80℃ and the time is 12~24h.

[0016] Furthermore, the purity of magnesium oxide is ≥98%, and the particle size is ≤10μm.

[0017] Furthermore, in step one, the pH of the saturated calcium hydroxide solution is 12.2~13.6, the soaking time is 24~48 h, and the treatment temperature is 20~40℃.

[0018] Furthermore, in step one, the continuous dense gradation design of the modified biochar aggregate meets the following cumulative sieve residue percentages: 4.75mm: 0~10%, 2.36mm: 0~25%, 1.18mm: 10~50%, 0.60mm: 40~70%, 0.30mm: 70~90%, 0.15mm: 90~100%.

[0019] Furthermore, in step one, the biochar aggregate is obtained by pyrolyzing wood at 900~950℃ followed by crushing and sieving, with a particle size of 0.15~4.75 mm and an apparent density of 0.4~1.0 g / cm³. 3 Porosity ≥ 50%.

[0020] Furthermore, in step two, the magnetic field strength of the static magnetic field is 100~300 mT, the direction of the magnetic field is set according to the main direction of the target electromagnetic shielding, and the static magnetic field is continuously applied for 2~8 hours.

[0021] Furthermore, in step two, the vibration frequency of mechanical tamping is 50~100 Hz, the time is 1~3 min; the curing temperature is 20±2℃, and the relative humidity is 95~100%.

[0022] Furthermore, the water-cement ratio of the cement-based material is 0.35~0.50.

[0023] The electromagnetic shielding cement-based composite material obtained by the preparation method described in this invention achieves an electromagnetic shielding effectiveness of over 40 dB in the 8–12 GHz wide frequency band, a 28-day compressive strength greater than 55 MPa, and a CO2 curing amount greater than 35 kg / m³. 3 It can be widely used in military facilities, precision instrument laboratories, 5G communication base stations and other fields.

[0024] Preparation principle: This invention constructs a three-in-one multi-level electromagnetic wave dissipation system consisting of "functional cement matrix continuous phase - functional skeleton structure - anisotropic conductive / magnetic network", realizing multi-mechanism and multi-scale synergistic dissipation from dielectric loss, magnetic loss to interface polarization loss, and synergistically improving the mechanical properties and electromagnetic shielding effectiveness of cement-based composite materials.

[0025] By replacing a portion of the cement with biochar powder of equal mass, and through synergistic modification via CO2 pre-adsorption and magnesium oxide chemical activation, a "functional cement matrix continuous phase" with both electrical conductivity and carbon fixation activity is formed. This continuous phase not only provides a dielectric loss matrix for electromagnetic waves, but also generates CaCO3 and basic magnesium carbonate through a CO2 slow-release-carbonation reaction, achieving matrix densification and enhanced mechanical properties. This transforms biochar from an inert filler into a "dual-functional" component of "enhanced activity and carbon fixation."

[0026] Ca in high pH environments 2+ and OH- Ions adsorb onto the oxygen-containing functional groups on the surface of biochar aggregates, reducing their hydrophobicity. Furthermore, the active sites on the biochar surface act as nucleation templates, inducing the in-situ nucleation and growth of the hydration product CSH on the aggregate surface, forming a CSH interface layer. This interface layer constructs a gradient transition structure of "biochar-CSH-cement paste," enhancing the mechanical interlocking and chemical bonding between the aggregate and the matrix, and improving the interfacial bonding strength.

[0027] By replacing natural aggregates with biochar aggregates modified with calcium hydroxide, and employing a continuous dense-gradation design, a "functional skeleton structure" is constructed within the composite material. This skeleton, through the stepwise filling of large, medium, and small-sized aggregates, forms a continuous, interpenetrating multi-level porous network, significantly extending the propagation path of electromagnetic waves, enhancing multiple reflections and scattering dissipation, and simultaneously providing abundant interfacial polarization sites for electromagnetic wave absorption.

[0028] Short-cut carbon fibers, after hydrothermal oxidation with H2O2, develop polar functional groups such as -COOH, -C=O, and -OH on their surface, providing chemical anchoring sites for subsequent coating growth. Immersing the oxidized carbon fibers in a precursor solution allows for a co-precipitation reaction, where Fe3O4 nanoparticles preferentially nucleate and grow on the fiber surface, forming a uniform and dense coating with Fe-OC chemical bonds. This structure endows the carbon fibers with dual functions: the carbon fibers provide high conductivity (conductive network), and the Fe3O4 coating provides ferromagnetism, achieving a dual-loss mechanism of "electromagnetism".

[0029] Fe3O4-coated carbon fibers, acting as functional reinforcements, are subjected to an external static magnetic field before the cement-based material is poured and initially set. This causes the Fe3O4-coated carbon fibers to be highly oriented along the magnetic field direction, constructing an anisotropic conductive-magnetic network: continuous conductive channels are formed along the oriented direction, contributing to reflection loss; the Fe3O4 coating provides magnetic loss (natural resonance, eddy current loss); at the same time, this network effectively connects the "functional cement matrix continuous phase" and the "functional skeleton structure," realizing multi-path, multi-mechanism synergistic dissipation of electromagnetic waves within the material.

[0030] The three functional units mentioned above work together to form a multi-mechanism, multi-scale electromagnetic wave dissipation system, encompassing dielectric loss (continuous biochar phase), magnetic loss (Fe3O4 coating), and interfacial polarization loss (multi-level interfaces of the graded framework). After entering the material, electromagnetic waves sequentially undergo reflection loss induced by the directional conductive network, absorption loss generated by the magnetic coating, and multiple internal reflection losses within the multi-level porous framework, ultimately achieving a high shielding efficiency of 41–55 dB over a wide frequency band of 8–12 GHz, consistent with the advanced electromagnetic shielding design concept of "absorption as the primary factor and reflection as a secondary factor." This invention constructs a complete integrated theoretical system of "material-structure-function" preparation, encompassing chemical activation (CO2 pre-adsorption-MgO excitation), interfacial modification (calcium hydroxide-induced CSH interfacial layer), graded design (continuous dense-graded aggregate filling), magnetic coating (Fe3O4 in-situ co-precipitation), and external field control (magnetic field directional alignment).

[0031] Beneficial effects: Compared with the prior art, the present invention has the following significant features:

[0032] 1. A multi-mechanism, multi-scale electromagnetic wave dissipation system for interfacial polarization loss was constructed through the synergistic effect of highly graphitized biochar powder, graded biochar aggregate, and anisotropic Fe3O4 coated carbon fiber magnetic field orientation. After entering the material, the electromagnetic wave successively experiences reflection loss of the oriented conductive network, absorption loss of the magnetic coating, and multiple internal reflection losses of the multi-level porous framework, ultimately achieving a high shielding efficiency of over 55 dB in the 8~12 GHz wide frequency band (optimal example 8), which is significantly better than existing biochar modified cement-based materials (typically <20 dB, mainly due to reflection loss), with absorption as the main factor and reflection as a secondary factor.

[0033] 2. The CaCO3 and basic magnesium carbonate generated by CO2 pre-adsorption and magnesium oxide chemical excitation fill the pores of the matrix, the CSH interface layer enhances the aggregate-slurry interface bonding, and the continuous dense-graded aggregate maximizes the bulk density, thus achieving a synergistic improvement in electromagnetic shielding effectiveness and mechanical properties.

[0034] 3. The biochar powder and biochar aggregate used are all derived from the pyrolysis products of agricultural and forestry waste such as wood, achieving partial or complete replacement of natural aggregates; at the same time, the pre-adsorbed CO2 biochar slowly releases CO2 during the cement hydration process, reacts with Ca(OH)2 and Mg(OH)2 to form carbonate minerals, permanently solidifying CO2 in the cement matrix, greatly increasing the amount of CO2 solidified, turning waste into treasure, actively fixing carbon, and making the product of this invention a green and low-carbon building material with significant carbon sequestration function;

[0035] 4. An external static magnetic field of 100~300 mT is used to orient the Fe3O4 coated carbon fiber, forming a continuous conductive channel along the orientation direction. The direction of the magnetic field can be flexibly set according to the main direction of the target electromagnetic shielding, realizing the customization of shielding effectiveness as needed.

[0036] 5. From chemical activation (CO2 pre-adsorption-MgO activation), interface modification (calcium hydroxide-induced CSH interface layer), gradation design (continuous dense-graded aggregate filling), magnetic coating (Fe3O4 in-situ co-precipitation) to external field control (magnetic field directional alignment), a complete integrated theoretical system of "material-structure-function" preparation was constructed. The process parameters of each step are clear and the operation is controllable, with good consistency and repeatability, which is convenient for industrial production and promotion. Attached Figure Description

[0037] Figure 1 This is a flowchart of the preparation process of the present invention;

[0038] Figure 2 The images are (a) a physical image of carbon fiber, (b) a SEM image of carbon fiber, (c) a physical image of Fe3O4 coated carbon fiber, and (d) a SEM image of Fe3O4 coated carbon fiber of the present invention.

[0039] Figure 3 This is a photograph of the cement-based material of the present invention being poured into a mold;

[0040] Figure 4 This is a schematic diagram of the electromagnetic shielding mechanism of the high-performance electromagnetic shielding cement-based composite material of the present invention;

[0041] Figure 5 This is a microscope image of the high-performance electromagnetic shielding cement-based composite material of the present invention;

[0042] Figure 6 This is a scanning electron microscope (SEM) image of the high-performance electromagnetic shielding cement-based composite material of the present invention.

[0043] Figure 7 This is a physical image of the conductivity test sample of the high-performance electromagnetic shielding cement-based composite material of the present invention;

[0044] Figure 8 The results are the electromagnetic shielding capability test results of the high-performance electromagnetic shielding cement-based composite material of Embodiment 1 of the present invention. Detailed Implementation

[0045] In the following examples, all materials and reagents used are commercially available unless otherwise specified. Experimental methods not specifically described in the examples are generally performed under standard conditions or as recommended by the manufacturer. The magnesium oxide has a purity of approximately 98% and a particle size of approximately 8 μm. The water-reducing agent is a general-purpose PCA-Ⅰ series polycarboxylate high-performance water-reducing agent manufactured by Jiangsu Subote New Material Co., Ltd.

[0046] Example 1

[0047] like Figure 1 A method for preparing an electromagnetic shielding cement-based composite material includes the following steps:

[0048] Step 1: Biochar powder is obtained by grinding wood after pyrolysis at 900℃. The particle size is 0.1~50μm, the graphitized carbon content is about 76%, and the specific surface area is about 220m². 2 / g, with a porosity of approximately 65%. Biochar powder was pre-adsorbed in a CO2 atmosphere at a pressure of 0.3 MPa, a temperature of 50 °C, and a treatment time of 5 h. The mass ratio of the pre-adsorbed CO2 biochar powder to magnesium oxide was 1:1. Subsequently, it was compounded with magnesium oxide via planetary ball milling at a speed of 200 rpm for 0.5 h to obtain composite modified biochar powder.

[0049] Step two: Biochar aggregate is obtained by pyrolyzing wood at 900℃, followed by crushing and sieving. The particle size is 0.15~4.75mm, and the apparent density is 0.6g / cm³. 3 The porosity was 55%. Modified biochar aggregate was obtained by soaking the biochar aggregate in a saturated calcium hydroxide solution with a pH of 12.4 at 30℃ for 24 hours and drying it at 20℃ for 72 hours. A continuous dense gradation design was then implemented, with the following cumulative sieve residue percentages: 4.75mm (0%), 2.36mm (0%), 1.18mm (10%), 0.60mm (40%), 0.30mm (70%), and 0.15mm (90%).

[0050] Step 3, mix FeCl3·6H2O and FeCl2·4H2O according to Fe 3+ :Fe 2+ A Fe3O4 precursor solution was prepared by dissolving Fe in deionized water at a molar ratio of 2:1 and adding polyethylene glycol PEG-4000 as a dispersant at a concentration of 5 wt% of the total solution mass. 3+The concentration was 0.1 mol / L. Carbon fibers with a length of 3 mm were immersed in a 20 wt% hydrogen peroxide solution and treated with a 60℃ water bath for 2 hours for surface oxidation. After oxidation, the carbon fibers were removed, washed three times with deionized water, and dried in a 60℃ oven for 12 hours. After drying, they were immersed in a Fe3O4 precursor solution and stirred in a 50℃ water bath for 30 minutes. Under N2 protection, 25 wt% ammonia was added dropwise to adjust the pH to 10, and the reaction was carried out at 70℃ for 1 hour. Fe3O4 nanoparticles were uniformly grown on the carbon fiber surface by in-situ co-precipitation. After the reaction, the carbon fibers were removed, washed repeatedly with deionized water and anhydrous ethanol, and then vacuum dried at 60℃ to obtain Fe3O4 coated carbon fibers (see...). Figure 2 The thickness of the Fe3O4 coating is approximately 150 nm, and the Fe3O4 content accounts for approximately 18% of the total mass of the coated carbon fibers.

[0051] Step 4: Weigh out the composite modified biochar powder, modified biochar aggregate, Fe3O4 coated carbon fiber, cement, and water-reducing agent in a mass ratio of 5:75:0.2:100:0.5. Pour the well-mixed cement-based material with a water-cement ratio of 0.35 into a mold (see...). Figure 3 During and after pouring, a static magnetic field with a strength of 100mT is continuously applied for 8 hours. The direction of the magnetic field is set according to the main direction of the target electromagnetic shielding. At the same time, mechanical vibration at 100Hz for 3 minutes is used to remove air bubbles and ensure the directional arrangement of the Fe3O4 coating carbon fiber. After curing at 20±2℃ and 95% relative humidity, a high-performance electromagnetic shielding cement-based composite material is obtained.

[0052] like Figure 4As shown, (1) Biochar powder replaces part of the cement by mass, and after CO2 pre-adsorption and synergistic modification by magnesium oxide chemical excitation, a "functional cement matrix continuous phase" with both conductivity and carbon fixation activity is formed, providing a dielectric loss matrix for electromagnetic waves; (2) Biochar aggregate modified with calcium hydroxide surface replaces natural aggregate by mass, and adopts a continuous dense gradation design to construct a "functional skeleton structure" inside the composite material, forming a continuous, interpenetrating multi-level pore network, significantly extending the propagation path of electromagnetic waves, enhancing multiple reflections and scattering dissipation, and providing abundant interface polarization sites for electromagnetic wave absorption; (3) Fe3O4 coated carbon fibers are highly oriented along the magnetic field direction to construct an anisotropic conductive-magnetic network: a continuous conductive channel is formed along the oriented direction to contribute reflection loss, and the Fe3O4 coating provides magnetic loss (natural resonance, eddy current loss). At the same time, the network effectively connects the "functional cement matrix continuous phase" and the "functional skeleton structure" to achieve multi-path and multi-mechanism synergistic dissipation of electromagnetic waves inside the material. The three functional units mentioned above work together to form a multi-mechanism, multi-scale electromagnetic wave dissipation system, encompassing dielectric loss (continuous biochar phase), magnetic loss (Fe3O4 coating), and interfacial polarization loss (multi-level interfaces of the gradation framework). (See...) Figure 5 and Figure 6 ).

[0053] like Figure 7 The high-performance electromagnetic shielding cement-based composite material was inserted parallel to four electrode plates for conductivity testing. The electromagnetic shielding effectiveness (SE) of the high-performance electromagnetic shielding cement-based composite material of this invention is 42.3 dB (see...). Figure 8 ).

[0054] Example 2

[0055] The remaining steps in this embodiment are the same as in Embodiment 1, except that: in step one, the biochar powder is obtained by grinding wood after pyrolysis at 950°C; the mass ratio of pre-adsorbed CO2 biochar powder to magnesium oxide is 2:1; the planetary ball mill used has a rotation speed of 400 rpm and a milling time of 5 h.

[0056] Example 3

[0057] The remaining steps in this embodiment are the same as in Embodiment 1, except that in step two, the biochar aggregate is obtained by pyrolyzing wood at 950°C, followed by crushing and sieving, with a particle size of 0.15~4.75mm and an apparent density of 0.8g / cm³. 3 The porosity is 51%; the modified biochar aggregate adopts a continuous dense gradation design, and the cumulative sieve residue mass percentage is controlled as follows: 4.75mm (5%), 2.36mm (20%), 1.18mm (30%), 0.60mm (50%), 0.30mm (80%), 0.15mm (95%).

[0058] Example 4

[0059] The remaining steps in this embodiment are the same as in Embodiment 1, except that in step two, the biochar aggregate is obtained by pyrolyzing wood at 950°C, followed by crushing and sieving, with an apparent density of 0.8 g / cm³. 3 The porosity is 51%; the modified biochar aggregate adopts a continuous dense gradation design, and the cumulative sieve residue mass percentage is controlled as follows: 4.75mm (10%), 2.36mm (25%), 1.18mm (50%), 0.60mm (70%), 0.30mm (90%), 0.15mm (100%).

[0060] Example 5

[0061] The remaining steps in this embodiment are the same as in Embodiment 1, except that in step three, the length of the carbon fiber is 10mm.

[0062] Example 6

[0063] The remaining steps in this embodiment are the same as in Embodiment 1, except that in step three, the carbon fiber length is 10mm, and FeCl3·6H2O and FeCl2·4H2O are mixed according to Fe... 3+ :Fe 2+ The Fe3O4 precursor solution was dissolved in deionized water at a molar ratio of 3:1. The amount of polyethylene glycol PEG-4000 dispersant added was 10 wt% of the total solution mass. 3+ The concentration is 0.5 mol / L.

[0064] Example 7

[0065] The remaining steps in this embodiment are the same as in Embodiment 1, except that in step four, the mass ratio of the composite modified biochar powder, modified biochar aggregate, Fe3O4 coated carbon fiber, cement, and water-reducing agent is 30:100:0.4:100:0.5.

[0066] Example 8

[0067] The remaining steps in this embodiment are the same as in Embodiment 1, except that in step four, the mass ratio of composite modified biochar powder, modified biochar aggregate, Fe3O4 coated carbon fiber, cement, and water-reducing agent is 30:100:0.4:100:0.5, and the magnetic field strength is 300 mT.

[0068] Example 9

[0069] A method for preparing an electromagnetic shielding cement-based composite material includes the following steps:

[0070] Step 1: Biochar powder is obtained by grinding wood after pyrolysis at 910℃. The particle size is 0.1~30μm, the graphitized carbon content is about 75%, and the specific surface area is about 200m². 2 / g, with a porosity of approximately 60%. Biochar powder was pre-adsorbed in a CO2 atmosphere at a pressure of 0.1 MPa and a temperature of 25°C for 5 hours, with a CO2 pre-adsorbed biochar powder to magnesium oxide mass ratio of 1:1. Subsequently, it was compounded with magnesium oxide via planetary ball milling at 300 rpm for 3 hours to obtain composite modified biochar powder.

[0071] Step two: Biochar aggregate is obtained by pyrolyzing wood at 920℃, followed by crushing and sieving. The particle size is 0.15~4.75mm, and the apparent density is 0.4g / cm³. 3 The porosity was 50%. Modified biochar aggregate was obtained by soaking the biochar aggregate in a saturated calcium hydroxide solution with a pH of 12.2 at 20℃ for 48 hours and drying it at 20℃ for 72 hours. A continuous dense gradation design was then implemented, with the following cumulative sieve residue percentages: 4.75mm (0%), 2.36mm (0%), 1.18mm (10%), 0.60mm (40%), 0.30mm (70%), and 0.15mm (90%).

[0072] Step 3, mix FeCl3·6H2O and FeCl2·4H2O according to Fe 3+ :Fe 2+ A Fe3O4 precursor solution was prepared by dissolving Fe in deionized water at a molar ratio of 2:1 and adding polyethylene glycol PEG-4000 as a dispersant at a concentration of 7 wt% of the total solution mass. 3+ The concentration was 0.2 mol / L. Carbon fibers with a length of 15 mm were immersed in a 10 wt% hydrogen peroxide solution and treated with an 80℃ water bath for 6 h for surface oxidation. After oxidation, the carbon fibers were removed, washed three times with deionized water, and dried in an 80℃ oven for 24 h. After drying, they were immersed in a Fe3O4 precursor solution and stirred in an 80℃ water bath for 40 min. Under N2 protection, 28 wt% ammonia was added dropwise to adjust the pH to 9, and the reaction was carried out at 90℃ for 2 h. Fe3O4 nanoparticles were uniformly grown on the carbon fiber surface by in-situ co-precipitation. After the reaction, the carbon fibers were removed and repeatedly washed with deionized water and anhydrous ethanol, and then vacuum dried at 60℃ to obtain Fe3O4-coated carbon fibers. The thickness of the Fe3O4 coating was 10 nm, and the Fe3O4 content accounted for 5% of the total mass of the coated carbon fibers.

[0073] Step 4: Weigh out the composite modified biochar powder, modified biochar aggregate, Fe3O4 coated carbon fiber, cement, and water-reducing agent in a mass ratio of 20:85:0.3:100:1. Pour the well-mixed cement-based material with a water-cement ratio of 0.50 into a mold. During and after pouring, continuously apply a static magnetic field with a magnetic field strength of 200mT for 2 hours. The direction of the magnetic field is set according to the main direction of the target electromagnetic shielding. At the same time, mechanically vibrate at 50Hz for 3 minutes to remove air bubbles and ensure the directional alignment of the Fe3O4 coated carbon fiber. After curing at 20±2℃ and 100% relative humidity, a high-performance electromagnetic shielding cement-based composite material is obtained.

[0074] Example 10

[0075] A method for preparing an electromagnetic shielding cement-based composite material includes the following steps:

[0076] Step 1: Biochar powder is obtained by grinding wood after pyrolysis at 930℃. The particle size is 1~40μm, the graphitized carbon content is about 78%, and the specific surface area is about 250m². 2 / g, with a porosity of approximately 70%. Biochar powder was pre-adsorbed in a CO2 atmosphere at a pressure of 0.5 MPa and a temperature of 60 °C for 2 hours, with a CO2 pre-adsorbed biochar powder to magnesium oxide mass ratio of 2:1. Subsequently, it was compounded with magnesium oxide via planetary ball milling at 250 rpm for 2 hours to obtain composite modified biochar powder.

[0077] Step two: Biochar aggregate is obtained by pyrolyzing wood at 930℃, followed by crushing and sieving. The particle size is 0.15~4.75mm, and the apparent density is 1.0g / cm³. 3 The porosity was 60%. Modified biochar aggregate was obtained by soaking the biochar aggregate in a saturated calcium hydroxide solution with a pH of 13.6 at 40℃ for 36 h and drying it at 20℃ for 72 h. A continuous dense gradation design was then implemented, with the following cumulative sieve residue percentages: 4.75 mm (0%), 2.36 mm (0%), 1.18 mm (10%), 0.60 mm (40%), 0.30 mm (70%), and 0.15 mm (90%).

[0078] Step 3, mix FeCl3·6H2O and FeCl2·4H2O according to Fe 3+ :Fe 2+ A Fe3O4 precursor solution was prepared by dissolving Fe in deionized water at a molar ratio of 2:1 and adding polyethylene glycol PEG-4000 as a dispersant at an amount of 8 wt% of the total solution mass. 3+The concentration was 0.3 mol / L. 8 mm long carbon fibers were immersed in a 30 wt% hydrogen peroxide solution and treated with a 70°C water bath for 4 h for surface oxidation. After oxidation, the carbon fibers were removed, washed three times with deionized water, and dried in a 70°C oven for 18 h. After drying, they were immersed in a Fe3O4 precursor solution and stirred in a 70°C water bath for 60 min. Under N2 protection, 27 wt% ammonia was added dropwise to adjust the pH to 11, and the reaction was carried out at 80°C for 3 h. Fe3O4 nanoparticles were uniformly grown on the carbon fiber surface via in-situ co-precipitation. After the reaction, the carbon fibers were removed, washed repeatedly with deionized water and anhydrous ethanol, and then vacuum dried at 60°C to obtain Fe3O4-coated carbon fibers. The thickness of the Fe3O4 coating was 200 nm, and the Fe3O4 content accounted for 30% of the total mass of the coated carbon fibers.

[0079] Step 4: Weigh out the composite modified biochar powder, modified biochar aggregate, Fe3O4 coated carbon fiber, cement, and water-reducing agent in a mass ratio of 25:90:0.3:100:0.8. Pour the well-mixed cement-based material with a water-cement ratio of 0.4 into a mold. During and after pouring, apply a static magnetic field with a strength of 250mT for 5 hours. The direction of the magnetic field is set according to the main direction of the target electromagnetic shielding. At the same time, use 80Hz mechanical vibration for 1 minute to remove air bubbles and ensure the directional alignment of the Fe3O4 coated carbon fiber. After curing at 20±2℃ and 98% relative humidity, a high-performance electromagnetic shielding cement-based composite material is obtained.

[0080] Comparative Example 1

[0081] The remaining steps of this comparative example are the same as those of Example 1, except that in step one, the biochar powder is not subjected to CO2 pre-adsorption treatment, and the biochar powder is directly ball-milled and compounded with magnesium oxide.

[0082] Comparative Example 2

[0083] The remaining steps of this comparative example are the same as those of Example 1, except that in step two, the biochar aggregate is not soaked in a saturated calcium hydroxide solution, nor is a continuous dense gradation design performed (a single particle size is used).

[0084] Comparative Example 3

[0085] The remaining steps of this comparative example are the same as those of Example 1, except that in step three, the carbon fibers are not subjected to surface oxidation treatment, but are directly immersed in the Fe3O4 precursor solution for in-situ co-precipitation.

[0086] Comparative Example 4

[0087] The remaining steps of this comparative example are the same as those of Example 1, except that in step four, no static magnetic field is applied, and only mechanical vibration is used.

[0088] Comparative Example 5

[0089] The remaining steps of this comparative example are the same as those of Example 1, except that in step four, the magnetic field strength is continuously applied at 50mT during and after the pouring process.

[0090] Comparative Example 6

[0091] The remaining steps of this comparative example are the same as those of Example 1, except that in step four, the magnetic field strength is continuously applied at 400mT during and after the pouring process.

[0092] The cement-based composite materials prepared in Examples 1-10 and Comparative Examples 1-6 were tested for electromagnetic shielding effectiveness (SE, test frequency range 30 MHz-1.5 GHz, using the coaxial flange method), compressive strength (28 days, tested according to GB / T 17671), and electrical conductivity (four-probe method). The average values ​​of the results are listed in Table 1. The CO2 curing amount was calculated with reference to ASTM C1910 / C1910M.

[0093] The properties of the materials obtained in Examples 1-10 and Comparative Examples 1-6 are shown in Table 1. It can be seen that, compared to Example 1, in Example 2, ball milling for 5 hours excessively damages the pore structure of the biochar, reducing CO2 adsorption and the integrity of the conductive network, leading to a decrease in electromagnetic shielding, mechanical properties, and CO2 absorption performance. In Examples 3 and 4, pyrolysis at 950℃ improves the graphitization degree and pore development of the biochar. Combined with optimized gradation, this enhances conductive continuity, mechanical strength, and CO2 adsorption capacity, thereby improving electromagnetic shielding effectiveness. In Example 5, increasing the carbon fiber length, under the same mass condition, means a reduction in quantity, which reduces the fiber dispersion uniformity and overlap density in the matrix, leading to a decrease in conductive and electromagnetic shielding effectiveness, fluctuations in mechanical properties (possible agglomeration), and a relatively small impact on CO2 absorption. In Example 6, it facilitates the formation of nano-Fe3O4 particles on the fiber surface, significantly improving electromagnetic shielding capability. In Examples 7 and 8, the reduced number of long fibers is detrimental to overlap, but increasing the Fe3O4-coated carbon fiber content and magnetic field can promote directional alignment. Combined with a higher biochar content, electromagnetic shielding and conductivity are significantly improved, as are mechanical properties and CO2 absorption. In Examples 9 and 10, a higher water-cement ratio and increased porous biochar material introduce more pores, weakening the matrix density. Therefore, the compressive strength is significantly reduced compared to Example 1.

[0094] Table 1. Properties of the materials obtained in Examples 1-10 and Comparative Examples 1-6

[0095]

[0096] Comparing Examples 1, 2, 3, 4, 5, and 6, it is evident that in Comparative Example 1, the lack of CO2 pre-adsorption treatment of the biochar powder, followed by direct ball milling and compounding of the biochar powder with magnesium oxide, reduces carbonation products and the continuity of the conductive network, resulting in a decrease in the electromagnetic shielding, mechanical properties, electrical conductivity, and CO2 absorption capacity of the concrete. In Comparative Example 2, the single aggregate gradation leads to increased porosity and a less dense matrix, reducing the electromagnetic wave reflection path and lowering shielding effectiveness; weakened interfacial bonding results in poor adhesion between the aggregate and cement paste; and the insufficient alkaline sites on the surface of the untreated biochar limit the carbonization reaction, thus reducing CO2 absorption capacity. In Comparative Example 3, the absence of surface oxidation treatment on the carbon fibers results in weaker interfacial bonding between the fibers and the cement matrix, slightly reducing mechanical properties but having little impact on electromagnetic shielding effectiveness, electrical conductivity, and CO2 absorption capacity. In Comparative Example 4, the absence of a magnetic field causes the Fe3O4-coated carbon fibers to distribute randomly, failing to form a directional conductive network. This results in a significant reduction in electromagnetic shielding effectiveness and conductivity, but has little impact on mechanical properties and CO2 absorption. In Comparative Example 5, the magnetic field strength is lower than 100 mT in Example 1, leading to poor fiber orientation and insufficient conductive network integrity, thus significantly reducing shielding effectiveness. In Comparative Example 6, the magnetic field strength is much higher than in Example 1. Excessively high magnetic fields can cause fiber agglomeration or over-orientation, disrupting uniformity and resulting in waste.

[0097] In summary, the optimal embodiment is Embodiment 8.

Claims

1. A method for preparing an electromagnetically shielded cement-based composite material, characterized in that, Includes the following steps: Step 1: Biochar powder is pre-adsorbed in a CO2 atmosphere and then combined with magnesium oxide by ball milling to obtain composite modified biochar powder. Biochar aggregate is soaked in a saturated calcium hydroxide solution and dried to obtain modified biochar aggregate, which is then subjected to continuous dense gradation design. Carbon fiber is surface oxidized, dried, and then immersed in Fe3O4 precursor solution. Fe3O4 nanoparticles are uniformly grown on the surface of carbon fiber by in-situ co-precipitation. After the reaction is completed, the carbon fiber is removed, cleaned, and dried to obtain Fe3O4 coated carbon fiber. Step 2: The composite modified biochar powder, modified biochar aggregate, Fe3O4 coated carbon fiber, cement, and water-reducing agent are mixed evenly in a mass ratio of 5~30:75~100:0.2~0.4:100:0.5~1, and poured into a mold. During and after pouring, a static magnetic field is continuously applied, and mechanical vibration is used to remove air bubbles and ensure that the Fe3O4 coated carbon fiber is oriented. After curing, an electromagnetic shielding cement-based composite material is obtained.

2. The method for preparing an electromagnetic shielding cement-based composite material according to claim 1, characterized in that: In step one, the length of the carbon fiber is 3~15 mm. The surface oxidation treatment is to immerse it in a hydrogen peroxide solution with a mass fraction of 10~30wt% and treat it in a water bath at 60~80℃ for 2~6 hours. After the oxidation treatment, the carbon fiber is taken out and washed.

3. The method for preparing an electromagnetic shielding cement-based composite material according to claim 1, characterized in that: In step one, the Fe3O4 precursor solution contains Fe 3+ The concentration is 0.1~0.5 mol / L, the thickness of the Fe3O4 coating is 10~200nm, and the Fe3O4 content accounts for 5~30% of the total mass of the coated carbon fiber.

4. The method for preparing an electromagnetic shielding cement-based composite material according to claim 1, characterized in that: In step one, the in-situ coprecipitation method involves stirring in a water bath at 50-80°C for 30-60 min, adding ammonia dropwise under N2 protection to adjust the pH of the solution to 9-11, and reacting at 70-90°C for 1-3 h, so that Fe3O4 nanoparticles nucleate and grow in situ on the carbon fiber surface.

5. The method for preparing an electromagnetic shielding cement-based composite material according to claim 1, characterized in that: In step one, the biochar powder has a particle size of 0.1~50 μm, a specific surface area of ​​≥200m² / g, a porosity of ≥60%, and a mass percentage of graphitized carbon in the biochar powder of 75~78%.

6. The method for preparing an electromagnetic shielding cement-based composite material according to claim 1, characterized in that: In step one, the pressure of the pre-adsorption treatment is 0.1~0.5MPa, the temperature is 25~60℃, the treatment time is 2~8h, and the mass ratio of biochar powder to magnesium oxide is 1~2:

1.

7. The method for preparing an electromagnetic shielding cement-based composite material according to claim 1, characterized in that: In step one, the pH of the saturated calcium hydroxide solution is 12.2~13.6, the soaking time is 24~48 h, and the treatment temperature is 20~40℃.

8. The method for preparing an electromagnetic shielding cement-based composite material according to claim 1, characterized in that: In step one, the continuous dense gradation design of the modified biochar aggregate satisfies the following cumulative sieve residue percentages: 4.75mm: 0~10%, 2.36mm: 0~25%, 1.18mm: 10~50%, 0.60mm: 40~70%, 0.30mm: 70~90%, 0.15mm: 90~100%.

9. The method for preparing an electromagnetic shielding cement-based composite material according to claim 1, characterized in that: In step two, the magnetic field strength of the static magnetic field is 100~300 mT, the direction of the magnetic field is set according to the main direction of the target electromagnetic shielding, and the static magnetic field is continuously applied for 2~8 hours.

10. An electromagnetic shielding cement-based composite material obtained by the preparation method according to any one of claims 1 to 9, characterized in that: The electromagnetic shielding effectiveness reaches over 40dB in the 8~12 GHz wide frequency band, the 28-day compressive strength is greater than 55MPa, and the CO2 curing amount is greater than 35 kg / m³. 3 .

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