Preparation method of cement surface layer nanoparticle self-assembled transition layer and double-layer cement-based radiation refrigeration material

Abstract

This invention belongs to the field of cement-based materials technology, and discloses a method for preparing a self-assembled transition layer of cement surface nanoparticles and a double-layer cement-based radiation cooling material. The preparation method includes: Step 1, charge modification of nanoparticles: pretreatment is used to obtain initially dispersed high-reflectivity nanoparticles, followed by charge modification to prepare a modified nanoparticle suspension with a uniform charge; Step 2, preparation of cement-based slurry and incorporation of modified particles: a predetermined amount of white cement, mixing water, and the modified nanoparticle suspension are weighed and mixed evenly to obtain a cement-based slurry; Step 3, electric field-driven nanoparticle layering and self-assembly: an electric field driving device is set up, and the electric field parameters are adjusted according to the electric field driving control method to apply an electric field to complete the self-assembly process of nanoparticles. This invention also overcomes the technical problems of high coupling between the electric field application time window and the initial setting process in electric field-driven self-assembly and the compression of the double electric layer of nanoparticles by high-concentration calcium hydroxide.

Description

Technical Field

[0001] This invention belongs to the field of cement-based materials technology, specifically relating to a method for preparing a self-assembled transition layer of cement surface nanoparticles and a double-layer cement-based radiation cooling material. Background Technology

[0002] The cooling principle of cement-based radiative cooling materials is mainly achieved by incorporating high-reflectivity nanoparticles into cement slurry. The high reflectivity of the nanoparticles to sunlight and their high emission characteristics to infrared radiation combine to achieve a dual cooling effect of reflecting sunlight and emitting infrared radiation.

[0003] At present, common cement-based radiative cooling materials are mainly prepared by directly incorporating high reflectivity nanoparticles into cement paste. This method has the following key technical bottlenecks, which seriously restrict the improvement of the cooling performance and industrial application of the material: (1) High reflectivity nanoparticles are easily encapsulated by cement hydration products: CSH gel, calcium hydroxide and other products generated during cement hydration will encapsulate the nanoparticles, resulting in insufficient exposure of the particle surface, significantly reduced solar reflectivity and infrared emissivity, and particle exposure rate of only 20%-40%, resulting in a significant decrease in cooling performance; (2) Nanoparticles are prone to agglomeration: Nanoparticles have a large specific surface area and high surface energy, and direct incorporation Cement paste is prone to agglomeration, which not only reduces the reflection efficiency, but also affects the fluidity of cement paste and the mechanical properties of materials; (3) Uncontrollable layered structure: The layered structure achieved by natural or mechanical layering in the existing technology is difficult to control precisely. The thickness and distribution density of the surface nanoparticles cannot be adjusted as needed, and the interface between the surface and the bottom layer is poor, which easily leads to problems such as peeling and cracking; (4) Poor performance synergy: Increasing the amount of nanoparticles can enhance the cooling performance, but it will hinder cement hydration and reduce mechanical properties; Although reducing the amount of nanoparticles is beneficial to mechanical properties, it is difficult to meet the cooling requirements and cannot achieve a balance between cooling performance and mechanical properties.

[0004] Existing technologies include electric field-driven self-assembly techniques, which can drive uniformly charged nanoparticles to migrate directionally, arrange themselves in an orderly manner, and achieve surface self-assembly in a liquid matrix. However, applying this technology to precisely control the distribution and thickness of high-reflectivity nanoparticles in cement-based materials to overcome the shortcomings of common cement-based radiative cooling materials still faces the following technical challenges: the rapid hydration and setting rate of cement leads to a high degree of coupling between the electric field application time window and the initial setting process, making precise matching difficult; and the high concentration of calcium hydroxide in cement compresses the double layer of nanoparticles, resulting in a decrease in zeta potential and difficulty in stabilizing electrophoresis. Summary of the Invention

[0005] The purpose of this invention is to provide a method for preparing a self-assembled transition layer of nanoparticles on the surface of cement, in order to solve the technical problems of the high coupling between the electric field application time window and the initial setting process and the compression of the double electric layer of nanoparticles by high concentration of calcium hydroxide in the application of electric field driven self-assembly technology in cement-based materials. This provides a cement-based radiative cooling material with precise and controllable layering, a strong interface, and both cooling and mechanical properties.

[0006] The method for preparing the self-assembled transition layer of cement surface nanoparticles comprises the following steps: Step 1: Charge modification of nanoparticles. Pre-treatment is used to obtain initially dispersed nanoparticles, which are then charged to prepare a modified nanoparticle suspension with uniform charge. The nanoparticles are high reflectivity nanoparticles. Step 2: Preparation of cement-based slurry and incorporation of modified particles. Weigh out the set amount of white cement, mixing water and modified nanoparticle suspension and mix evenly to obtain cement-based slurry. Step 3: Electric field-driven nanoparticle layering and self-assembly. An electric field driving device is set up and the electric field parameters are adjusted according to the electric field driving control method. Based on the electric field parameters, an electric field is applied to complete the self-assembly process of nanoparticles. In step three, the electric field-driven control method includes: S1. Based on the theoretical ion concentration and electric field parameters of cement-based materials, determine the feasible range of electric field strength according to the cement hydration differential equation; S2. Based on the feasible range of electric field strength determined in the previous step, select electric field parameters, conduct experiments under a defined electric field, perform real-time tracking and detection, and determine the time t at which the CSH gel network forms a critical point. c Detect the initial setting time t0 and calculate the time difference. S3. Conduct on-site actual operation, determine the corresponding initial setting time using standard methods, calculate the corresponding electric field application time based on the extended time difference obtained in the previous step, and determine the required electric field parameters.

[0007] Preferably, step one includes: 1) pretreatment; 2) charge modification; 3) preparation of modified nanoparticle suspension; Step 2) includes: charge modification by adding a polycarboxylate superplasticizer to a suspension of nanoparticles, adjusting the pH of the mixture to 8.0-9.0 with NaOH solution, and then stirring the mixture under constant temperature. The polycarboxylate superplasticizer is a polycarboxylate superplasticizer with polyoxyethylene ether side chain and a molecular weight of 20,000-50,000.

[0008] Preferably, in step S1, the lower limit of the electric field strength is determined by the hydration reaction as the main factor in preventing the reaction process from being dominated by the hydration reaction, and the upper limit of the electric field strength is determined by the photocatalytic side reaction of the nanoparticles. In step S2, the initial setting time is first determined by calculating the integral result of the cement hydration differential equation within the feasible range of the electric field strength to satisfy the solution of the CSH nucleation critical concentration, thereby initially determining the theoretical initial setting time t0. Then, it is verified by experiments, thereby calculating the time difference. And the time difference corresponding to cement-based materials with different material components is determined by multiple experiments.

[0009] Preferably, the electric field voltage is set to 7–8V, the anode spacing range is 5–10mm, the electric field current is 0.1–0.2A, the extension time difference range is 5–10min, and the anode spacing is the distance between the anode and the nanoparticle cement composite slurry.

[0010] Preferably, step 1) includes: drying the nanoparticles in a vacuum drying oven, followed by ball milling to obtain preliminarily dispersed nanoparticles; step 3) includes: centrifuging the mixture obtained in step 2) at high speed, discarding the supernatant, washing the precipitate 1-3 times with deionized water to remove free polycarboxylate superplasticizer, and redispersing the precipitate in deionized water to obtain a modified nanoparticle suspension.

[0011] Preferably, step two includes the following steps: 1) Weigh a certain amount of white cement, add mixing water according to the water-cement ratio, and stir at low speed to obtain cement paste. The water-cement ratio is 0.35~0.50. 2) Add the modified nanoparticle suspension obtained in step one to the cement slurry, add enough mixing water, and stir at high speed to obtain a uniform nanoparticle cement composite slurry. The dosage of the modified nanoparticle suspension is 1.0-5.0% of the cement mass.

[0012] Preferably, step three includes: 1) Setting up an electric field driving device: The bottom of the mold into which the nanoparticle cement composite slurry is injected is a stainless steel conductive template, which serves as the cathode; an anode is set above the nanoparticle cement composite slurry, and a stable anode distance is maintained between the anode and the surface of the nanoparticle cement composite slurry; the cathode and anode are respectively connected to the negative and positive terminals of a DC regulated power supply to form a complete electric field circuit. 2) Electric field parameter control and application: The electric field parameters are controlled according to the electric field drive control method to apply an electric field to the nanoparticle cement composite slurry in the stainless steel conductive template.

[0013] Preferably, in step two, the cement hydration differential equation is: d[CSH] / dt=k1[Ca² + ][OH - ] n k2E, where E is the electric field strength, k2 is the inhibition coefficient of the electric field on the hydration rate; d[CSH] / dt is the formation rate of CSH gel, and k1 is the intrinsic rate constant of cement hydration, which is determined by the mineral composition of the cement itself, [Ca² + [OH] indicates the calcium ion concentration in the slurry. - [] indicates the hydroxide ion concentration in the slurry, and n indicates the OH group concentration. - The reaction order; the integral result of the cement hydration differential equation is calculated as follows:

[0014] Where C* is the critical concentration for CSH nucleation.

[0015] The present invention also provides a double-layer cement-based radiative cooling material having a double-layer transition structure, the double-layer transition structure comprising: The bottom cement substrate layer; Surface nanoparticle self-assembled transition layer: Located above the bottom cement matrix layer, it is prepared by the method for preparing a cement surface nanoparticle self-assembled transition layer according to any one of claims 1-8; the surface nanoparticle self-assembled transition layer includes nanoparticles, cement hydration products and surface modifiers, and the nanoparticles are gradient distributed within the surface nanoparticle self-assembled transition layer.

[0016] Preferably, the double-layer cement-based radiative cooling material comprises the following components by mass: 312-320 parts white cement, 101-108 parts deionized water, and 30-33 parts modified nanoparticle suspension. The raw materials used to prepare the modified nanoparticle suspension include the following components by mass parts: 10-20 parts nanoparticles, 96-105 parts deionized water, and 2-4 parts polycarboxylate superplasticizer.

[0017] The technical advantages of this invention are as follows: Addressing the technical problems existing in the application of electric field-driven self-assembly technology in cement-based materials, this invention utilizes an electric field-driven control method based on the cement hydration calculus equation. Combined with experimental verification and measurement, it effectively adjusts electric field parameters such as electric field voltage, application time, and electric field current. By selecting a polycarboxylate superplasticizer to charge-modify nanoparticles, it can precisely control the thickness and distribution density of the surface nanoparticle transition layer, achieving controllable improvement in cooling performance. It also effectively inhibits nanoparticle aggregation, ensuring uniformity of nanoparticles on the surface, and resists ion compression, maintaining the zeta potential. Thus, it simultaneously overcomes the technical problems of high coupling between the electric field application time window and the initial setting process, and the compression of the nanoparticle double layer by high-concentration calcium hydroxide, making the application of electric field-driven self-assembly technology in cement-based materials feasible.

[0018] This invention is the first to apply electric field-driven self-assembly technology to cement-based materials. By driving charged nanoparticles to migrate directionally to the surface of the slurry and self-assemble through electric field force, the nanoparticles are moved away from the cement hydration region, which completely solves the problem of nanoparticles being encapsulated and fully utilizes the light reflection and infrared emission properties of the nanoparticles. The cooling efficiency is significantly improved compared with the existing technology. Attached Figure Description

[0019] Figure 1 This is a schematic diagram comparing the differences in internal nanoparticle distribution between existing cement-based radiative cooling materials and the double-layer cement-based radiative cooling material of this invention. Detailed Implementation

[0020] The following detailed description of the embodiments, with reference to the accompanying drawings, will further illustrate the specific implementation of the present invention, in order to help those skilled in the art to have a more complete, accurate, and in-depth understanding of the inventive concept and technical solution of the present invention.

[0021] High-reflectivity nanoparticles, such as anatase TiO2 nanoparticles, exhibit poor adhesion in traditional physical layering processes in cement-based materials. The surface TiO2 layer tends to randomly accumulate due to a lack of directional driving force and is not chemically anchored to the underlying layers. However, in the case of uniform mixing without an applied electric field, the nanoparticles are completely encapsulated by CSH gel, resulting in a sharp drop in reflectivity. Therefore, controlling the directional migration and ordered arrangement of nanoparticles in a liquid matrix and achieving surface self-assembly is a key technological direction for addressing the shortcomings of existing cement-based radiation-cooled materials.

[0022] Electric field-driven self-assembly technology is a novel technique applied in the fields of nano-coatings, flexible electronics, and biomedical materials. This technology utilizes the electric field force of a weak external electric field to drive the directional migration and orderly arrangement of uniformly charged nanoparticles in a liquid matrix, achieving surface self-assembly. This effectively prevents nanoparticles from being encapsulated by the matrix or agglomerating, while also allowing precise control over the thickness and distribution density of the particle-rich layer. However, this technology is currently only applied to stable liquid systems such as polymer solutions and sols, and is rarely used in cement-based materials.

[0023] Specifically, the rapid hydration and setting rate of cement leads to a high degree of coupling between the electric field application time window and the initial setting process. Current technologies lack research specifically addressing this particular application area. How to adjust the electric field voltage and application time, and how to control the cement composition to obtain cement-based radiative cooling materials that meet construction requirements, remains a research blind spot in this field. Since the critical point for CSH gel network formation corresponds to the moment when the surface CSH evolves from discrete clusters into a continuous, dense film sufficient to physically block TiO2 migration, conventional electric field-driven self-assembly cannot solve the problem of inaccurately controllable thickness and distribution density of the surface nanoparticle transition layer due to the high coupling between the electric field application time window and the initial setting process caused by the rapid hydration and setting rate of cement in the cement-based material field.

[0024] Meanwhile, the application of electric field-driven self-assembly technology in cement-based materials also faces challenges related to the high concentration of Ca²⁺ in cement slurry. + The problem caused by double-layer compression, high concentration of Ca² + This reduces the thickness of the diffused electric double layer around the colloidal particles, thereby lowering the Zeta potential and affecting the electrophoresis speed, making it difficult for electrophoresis to proceed stably. This also results in poor application of electric field-driven self-assembly technology in cement-based materials, failing to achieve the expected results.

[0025] To solve the above technical problems, such as Figure 1 As shown, this invention provides a method for preparing a self-assembled transition layer of nanoparticles on the surface of cement, comprising the following steps: Step 1: Charge modification of nanoparticles. Pre-treatment is used to obtain initially dispersed nanoparticles, which are then charged to prepare a modified nanoparticle suspension with uniform charge.

[0026] This step specifically includes the following sub-steps: 1) Pretreatment: The nanoparticles are dried in a vacuum drying oven and then ball-milled to obtain pre-dispersed nanoparticles.

[0027] In this embodiment, anatase TiO2 nanoparticles, a type of high-reflectivity nanoparticle, with a particle size of 150-500 nm, are selected. In this step, the nanoparticles are dried in a vacuum drying oven at 50-80°C for 1-2 hours. A planetary ball mill is used for ball milling at 100-300 r / min for 30 minutes.

[0028] 2) Charge modification: The pretreated nanoparticles are added to deionized water to prepare a suspension of nanoparticles. Polycarboxylate superplasticizer is added to the suspension. The pH of the mixture is adjusted to 8.0-9.0 with NaOH solution. The mixture is then stirred at a constant temperature to allow the polycarboxylate superplasticizer molecules to be uniformly adsorbed onto the surface of the nanoparticles through the carboxyl groups of the main chain.

[0029] In this step, the mass concentration of nanoparticles in the suspension is 5%~10%; the NaOH solution used is a low-concentration solution. The specific method of stirring under constant temperature is as follows: the mixture is placed in a constant temperature water bath stirrer and stirred at a speed of 300~500 r / min for 1~2 h at room temperature (20±2℃).

[0030] The polycarboxylate superplasticizer (PCE) is a polycarboxylate superplasticizer with polyoxyethylene ether side chains, a molecular weight of 20,000-50,000, and a solid content of 40%-50%. The dosage of PCE is 5.0%-20.0% of the nanoparticle mass. This allows the PCE molecules to be uniformly adsorbed onto the surface of TiO2 nanoparticles via the carboxyl groups (-COOH) in the main chain. Subsequently, the carboxyl groups (-COOH) can stably dissociate into -COO in the alkaline environment of cement. - This provides a stable negative charge to the nanoparticles, while the polyoxyethylene ether groups on the side chains provide steric hindrance, thus doubly inhibiting the aggregation of nanoparticles.

[0031] The mechanism of polycarboxylate superplasticizer (PCE) in the cement-based material of this scheme is as follows: (1) The main chain carboxyl group dissociates into -COO at pH 8.0–9.0. - To ensure the zeta potential remains stable at 40~ 60mV, to resist Ca² + The resulting double-layer compression; (2) The side-chain polyoxyethylene ether provides a steric hindrance layer with a thickness of ≈4.2 nm, which is calculated from the molecular weight and just covers the charge shielding region on the TiO2 surface caused by ion adsorption; (3) This steric layer also serves as a 'reaction buffer', which will reduce the charge shielding region on the TiO2 surface caused by ion adsorption. - With Ca² in CSH + The rate constant k of the coordination reaction was adjusted to 1.2 × 10⁻⁶. - ³–3.5×10 - ³s - ¹ This interval precisely covers the kinetic stages of CSH's transformation from a loose colloid to a dense gel, ensuring that the Ca–O–C bonds (XPS binding energy reaches 531.2 eV) are constructed in situ before initial solidification; if the PCE molecular weight is <15000, the space layer is too thin, and Ca² + Direct attack on the TiO2 surface triggers uncontrollable precipitation; if pH < 8.0, –COOH does not dissociate, and the zeta potential is ineffective; if pH > 9.0, OH… - Excessive amounts accelerate the catalytic precipitation of Ca(OH)2.

[0032] 3) Preparation of modified nanoparticle suspension: The mixture obtained in step 2) is centrifuged at high speed, the supernatant is discarded, the precipitate is washed with deionized water 1-3 times to remove free polycarboxylate superplasticizer, and the precipitate is redispersed in deionized water to obtain modified nanoparticle suspension.

[0033] In this step, the high-speed centrifugation speed is 8000-12000 rpm, and the centrifugation time is 10-20 minutes.

[0034] Step 2: Preparation of cement-based slurry and incorporation of modified particles. Weigh out the set amount of white cement, mixing water and modified nanoparticle suspension and mix evenly to obtain cement-based slurry.

[0035] 1) Weigh a certain amount of white cement, add mixing water according to the water-cement ratio, and stir at low speed to obtain cement slurry. The water-cement ratio is 0.35~0.50, and the low-speed stirring time is 1 minute. 42.5 grade white cement is used.

[0036] 2) Add the modified nanoparticle suspension obtained in step one to the cement slurry, add enough mixing water, and stir at high speed to obtain a uniform nanoparticle cement composite slurry. The dosage of the modified nanoparticle suspension is 1.0-5.0% of the cement mass, and the high-speed stirring time is 2-3 minutes.

[0037] The fluidity of the nanoparticle cement composite slurry is 150-220 mm (measured by the jump table method), thus ensuring that the composite slurry has good fluidity.

[0038] Step 3: Electric field-driven nanoparticle layering and self-assembly. An electric field driving device is set up and the electric field parameters are adjusted according to the electric field driving control method. Based on the electric field parameters, an electric field is applied to complete the self-assembly process of nanoparticles.

[0039] This step includes the following sub-steps: 1) Setting up an electric field driving device: The bottom of the mold into which the nanoparticle cement composite slurry is injected is a stainless steel conductive template, which serves as the cathode; an anode is set above the nanoparticle cement composite slurry, and a stable anode distance is maintained between the anode and the surface of the nanoparticle cement composite slurry; the cathode and anode are respectively connected to the negative and positive terminals of a DC regulated power supply to form a complete electric field circuit.

[0040] The anode is a platinum electrode, which is stably fixed on the support. The anode spacing between the anode and the surface of the nanoparticle cement composite slurry is 5-10 mm. A spacing of 7-8 mm results in better control.

[0041] 2) Electric field parameter control and application: The electric field parameters are controlled according to the electric field drive control method to apply an electric field to the nanoparticle cement composite slurry in the stainless steel conductive template.

[0042] The electric field-driven control method includes: S1. Based on the theoretical ion concentration and electric field parameters of cement-based materials, determine the feasible range of electric field strength according to the cement hydration differential equation.

[0043] The cement-based material used in this scheme is a dynamically coupled body governed by the cement hydration differential equation. The cement hydration differential equation is d[CSH] / dt=k1[Ca²] + ][OH - ] n k2E, where E is the electric field strength, k2 is the inhibition coefficient of the electric field on the hydration rate; d[CSH] / dt is the formation rate of CSH gel, and k1 is the intrinsic rate constant of cement hydration, which is determined by the mineral composition of the cement itself, [Ca² + [OH] indicates the calcium ion concentration in the slurry. - [] indicates the hydroxide ion concentration in the slurry, and n indicates the OH group concentration. - The reaction order.

[0044] When the electric field strength E is too low (<0.88 kV / m), the k2E term can be ignored, and the reaction process is dominated by hydration. The nanoparticles are encapsulated, which prevents directional migration from playing an effective role, corresponding to the lower limit of the electric field strength. When the electric field strength E is too high (>1.14 kV / m), the k2E term significantly accelerates the local OH reaction. - Enrichment induces photocatalytic side reactions in TiO2 nanoparticles, leading to premature catalytic oxidation of substances in the slurry and adversely affecting material properties, corresponding to the upper limit of the electric field strength. Therefore, only when the electric field strength E falls precisely within a specific range can the directional migration of nanoparticles be effectively driven without inducing photocatalytic side reactions in TiO2 nanoparticles.

[0045] S2. Based on the feasible range of electric field strength determined in the previous step, select the electric field parameters and conduct experiments under a defined electric field, performing real-time tracking and detection (such as in-situ resistivity method or in-situ ATR-FTIR infrared spectroscopy) to determine the critical point t for CSH gel network formation. c At this point, the applied electric field is stopped, the initial setting time t0 is detected, and the time difference Δt is calculated. c , Δt c =t0 t c Through multiple experiments, the time difference Δt corresponding to different material components of cement-based materials was determined. c .

[0046] Then, an electric field is applied based on the electric field parameters, and experimental verification is conducted in the electric field environment. This is to ensure the electric force F... e=qE acts on the migration velocity of charged nanoparticles and the propulsion velocity v of the CSH gel front. h To achieve dynamic equilibrium and synchronize particle migration with the hydration front in time and space, the integral result of the cement hydration differential equation needs to satisfy the following within the feasible range of electric field strength:

[0047] Where C* is the critical concentration for CSH nucleation. The theoretical initial setting time t0 is obtained by solving the above integral equation.

[0048] The time t when the CSH gel network forms a critical point c The corresponding time when the surface CSH evolves from discrete clusters into a continuous dense film, thus physically blocking TiO2 migration, is observed. When CSH nucleation is inhibited under the influence of an electric field, its local concentration growth rate decreases, leading to a delay in gel network formation. This time t was determined experimentally by tracking and detecting the process. c Arrival time t c Afterwards, the applied electric field is stopped, and the initial setting time of the cement-based material is awaited. The corresponding initial setting time is then determined using standard methods. Specifically, the initial setting time of white cement paste at different water-cement ratios is determined using a Vicat apparatus according to the national standard GB / T 1346-2011 "Test Methods for Standard Consistency Water Requirement, Setting Time and Soundness of Cement". If the experimental verification fails, the relevant material composition, electric field application equipment, and testing instruments are checked and adjusted to complete the verification.

[0049] Finally, the time difference Δt was calculated. c , Δt c =t0 t c This step involves determining the time difference Δt corresponding to different material compositions of cement-based materials through multiple experiments. c .

[0050] S3. Conduct on-site actual operation, determine the corresponding initial setting time using standard methods, calculate the corresponding electric field application time based on the extended time difference obtained in the previous step, and determine the required electric field parameters.

[0051] Based on the aforementioned calculation results and experimental verification, the specific selection of electric field parameters for the cement-based material in this scheme is as follows: A. The electric field voltage is set to 7–8V. An optimal anode spacing of 7–8mm is selected. The electric field strength, calculated using the previous formula, falls within a specific range of 0.88–1.14 kV / m. This value precisely results in the electric field force F... e =qE and CSH gel leading edge propulsion velocity v h (Actual measurement 2.5×10) -7 Achieving a dynamic balance (m / s) ensures that migration and hydration frontiers are synchronized.

[0052] B. The electric field current is 0.1–0.2A. To match the conductivity of the slurry (σ≈0.05S / m) under this electric field strength value, ensure stable electric field energy input, and avoid local electrolysis; C. The electric field is applied for a time t, t=t0 Δt c , Δt c The values ​​obtained from multiple experiments fall within the range of 5–10 min; the median value of 7.5 min is taken as Δt. c It is compatible with most cement-based materials with varying water-cement ratios. The electric field application time t can be calculated by determining the corresponding initial setting time using standard methods.

[0053] Finally, based on the determined voltage, anode spacing, electric field current, and electric field application time (i.e., the electric field parameters selected in step S2), an electric field is applied to the nanoparticle cement composite slurry in the stainless steel conductive template. Under the action of the electric field force, the negatively charged TiO2 nanoparticles migrate directionally to the cement-based surface layer. After migrating to the surface, they arrange themselves in an orderly manner and self-assemble through the mutual repulsion of surface charges, forming a dense and uniform nanoparticle transition layer (the nanoparticles are semi-embedded inside the cement matrix). At the same time, the polycarboxylate superplasticizer (PCE) molecules adsorbed on the surface of the nanoparticles can form chemical bonds with the CSH gel generated by the hydration of the underlying cement slurry surface layer, further achieving a seamless connection between the surface and the underlying layer. Moreover, the polycarboxylate superplasticizer (PCE) is stable in the alkaline environment of cement, and the self-assembled structure is strong. Finally, it is cured to the specified age to obtain the product.

[0054] In step one, if the charged nanoparticle suspension needs to be stored for a long time, a polycarboxylate dispersant of 0.5% of the nanoparticle mass can be added to inhibit agglomeration; the suspension should be prepared and used immediately to ensure the dispersion effect.

[0055] In step three, a current protector can be connected in series in the electric field circuit of the electric field drive equipment to prevent short circuits due to excessive current. At the same time, an electric field strength tester is used for real-time monitoring to ensure that the uniformity deviation of the electric field strength on the surface of the cement slurry is ≤5%. Meanwhile, the ambient temperature during the electric field application process should be controlled at 15-30℃ and the relative humidity at 60%-80% to avoid excessively low temperatures causing the cement to set too quickly or excessively high temperatures causing the suspension to evaporate moisture, which would affect particle migration and self-assembly.

[0056] The present invention also provides a double-layer cement-based radiative cooling material having a double-layer transition structure, comprising: The bottom cement base layer is bonded to the stainless steel conductive template during pouring, and its thickness is 10-15mm.

[0057] Surface nanoparticle self-assembled transition layer: Located above the underlying cement matrix layer, it is prepared by the above-described method. The surface nanoparticle self-assembled transition layer comprises nanoparticles, cement hydration products, and a surface modifier. The nanoparticles are gradient-distributed within the surface nanoparticle self-assembled transition layer. The nanoparticles are high-reflectivity nanoparticles, such as TiO2 nanoparticles, with a reflectivity ≥90% in the 200–2500 nm solar light band and an infrared emissivity ≥92% in the 8–13 μm atmospheric window. The small amount of cement hydration products mainly consists of CSH and Ca(OH)2. The surface modifier is a polycarboxylate superplasticizer (PCE).

[0058] The double-layer cement-based radiative cooling material comprises the following components by mass: 312-320 parts white cement, 101-108 parts deionized water, and 30-33 parts modified nanoparticle suspension.

[0059] The raw materials used to prepare the modified nanoparticle suspension include the following components by mass parts: 10-20 parts nanoparticles, 96-105 parts deionized water, and 2-4 parts polycarboxylate superplasticizer.

[0060] The specific implementation method is as follows: Example 1 Example 1 provides a method for preparing a self-assembled transition layer of nanoparticles on the surface of cement, comprising the following steps: Step 1: Charged modification of nanoparticles: Take 10 parts of TiO2 nanoparticles, 96 parts of deionized water, 2 parts of polycarboxylate superplasticizer (PCE), and an appropriate amount of NaOH solution, and place them in a constant temperature water bath stirrer. Stir at 300 r / min for 1 h at room temperature (20±2℃). After stirring, centrifuge at 8000 rpm for 15 minutes, discard the supernatant, wash the precipitate 3 times with deionized water to remove free polycarboxylate superplasticizer, and redisperse the precipitate in deionized water to obtain a modified nanoparticle suspension with a solid content of 20%. The precipitate is the modified TiO2 nanoparticles.

[0061] Step 2: Take 312 parts white cement, 101 parts deionized water, and 31 parts modified nanoparticle suspension, mix them at 300 rpm for 3 minutes, then pour the resulting nanoparticle cement composite slurry into a mold and gently shake to eliminate air bubbles. The bottom of the mold has a stainless steel conductive template.

[0062] Step 3: Set up the electric field driving device and adjust the electric field parameters according to the electric field driving control method. Based on the above calculation results and experimental verification, select the electric field parameters for the cement-based material in this scheme, including electric field voltage, electric field current, anode spacing and extension time difference.

[0063] According to standard methods, the initial setting time of the cement-based material (water-cement ratio of 0.403) used in this embodiment was 42 min; the median extension time difference was taken as 7.5 min, and the electric field application time was calculated as initial setting time – extension time difference, i.e., 42 min. 7.5 = 34.5 min, rounded to 35 min. Therefore, the electric field parameters are determined as follows: electric field voltage 7V, electric field current 0.1A, anode spacing of 7-8 mm from the anode to the nanoparticle cement composite slurry, and DC electric field application time of 35 min. Applying an electric field to the nanoparticle cement composite slurry according to these parameters causes the TiO2 nanoparticles to complete directional migration and self-assembly, forming a double-layer transition structure.

[0064] The resulting double-layer cement-based radiative cooling material comprises the following components by mass: 312 parts white cement, 101 parts deionized water, and 31 parts modified nanoparticle suspension.

[0065] The raw materials used to prepare the modified nanoparticle suspension include the following components by mass parts: 10 parts nanoparticles, 96 parts deionized water, and 2 parts polycarboxylate superplasticizer.

[0066] Example 2 Compared with Example 1, Example 2 has a basically the same technical solution, but with the following technical differences: In Example 2, the obtained double-layer cement-based radiative cooling material, by mass parts, includes the following components: 315 parts white cement, 105 parts deionized water, and 30 parts modified nanoparticle suspension, with a solid content of 30% in the modified nanoparticle suspension.

[0067] The raw materials used to prepare the modified nanoparticle suspension include the following components by mass parts: 15 parts TiO2 nanoparticles, 101 parts deionized water, and 3 parts polycarboxylate superplasticizer.

[0068] In step three of the preparation method provided in Example 2, the initial setting time of the cement-based material (water-cement ratio of 0.400) used in this example is 43 min; the median of the extension time difference is taken as 7.5 min, and the electric field application time is calculated as initial setting time – extension time difference, i.e., 43 min. 7.5 = 35.5 min, rounded to 36 min. Therefore, the electric field parameters are determined as follows: electric field voltage 7V, electric field current 0.1A, anode-to-anode distance of 7-8 mm, and DC electric field application time 36 min. Applying an electric field to the nanoparticle cement composite slurry according to these parameters causes the TiO2 nanoparticles to complete directional migration and self-assembly, forming a bilayer transition structure.

[0069] Example 3 Compared with Example 2, Example 3 has a basically the same technical solution, but has the following technical differences: In Example 3, the obtained double-layer cement-based radiative cooling material, by mass parts, includes the following components: 320 parts white cement, 108 parts deionized water, and 33 parts modified nanoparticle suspension, with a solid content of 40% in the modified nanoparticle suspension.

[0070] The raw materials used to prepare the modified nanoparticle suspension include the following components by mass parts: 20 parts TiO2 nanoparticles, 105 parts deionized water, and 4 parts polycarboxylate superplasticizer.

[0071] The comparison is provided below, and the details are as follows.

[0072] Comparative Example 1 Comparative Example 1 provides a cement-based radiative cooling material, comprising the following components by mass: 20 parts TiO2 nanoparticles, 105 parts deionized water, and 312 parts white cement. The corresponding preparation method is as follows: 20 parts TiO2 nanoparticles, 1-2 parts polycarboxylate superplasticizer (PCE), 312 parts white cement, and 125 parts deionized water are mixed and stirred at 300 r / min for 3 min, then poured into a mold and gently shaken to eliminate air bubbles.

[0073] No additional electric field was applied in the above preparation method, so most of the nanoparticles were encapsulated inside the cement matrix, failing to form an effective double-layer transition structure. Finally, water curing was performed to a specified age to obtain a cement-based radiative cooling material with ordinary added nano-TiO2 particles.

[0074] Comparative Example 2 Comparative Example 2 provides a common two-layer cement-based radiative cooling material, comprising a base matrix and a high-reflectivity coating. The high-reflectivity coating is prepared using a TiO2 nanoparticle surface slurry. The base matrix, by mass, comprises the following components: 300 parts white cement and 120 parts deionized water. The preparation method is as follows: 300 parts white cement and 120 parts deionized water are mixed at room temperature (20±2℃) and 300 r / min for 3 min. The mixture is poured into a mold and gently shaken to eliminate air bubbles. It is then allowed to stand naturally to solidify without applying an electric field or controlling particle self-assembly. After the base matrix has initially solidified, a surface slurry containing 20 parts TiO2 nanoparticles is directly coated onto the surface of the solidified matrix, forming a simple two-layer structure through physical layering. Finally, it is water-cured to a specified age to obtain the common two-layer cement-based radiative cooling material. The two-layer structure of this material is prepared using a physical layering method, rather than a gradient transition structure formed by electric field-driven self-assembly.

[0075] The performance of cement-based radiative cooling materials of the products obtained in Examples 1-3 and Comparative Examples 1-2 was tested, and the results are shown in Table 1.

[0076] Table 1. Performance Comparison of Products Obtained in Examples 1-3 and Comparative Examples 1-2

[0077] Table 1 highlights the synergistic effect of electric field-driven self-assembly on improving interfacial bonding strength and optical performance. A comparison of the various embodiments with Comparative Example 1 shows that the reflectivity of each embodiment is 87.8%–91.3%, a significant improvement over 76.4% in Comparative Example 1. Similarly, the emissivity of each embodiment is 92.3%–95.1%, also a significant improvement over 87.5% in Comparative Example 1. This indicates that by adopting the scheme of this invention, electric field driving causes TiO2 nanoparticles to migrate to the surface, avoiding encapsulation by CSH, significantly increasing the exposure rate and substantially improving the optical performance of the cement-based radiation-cooled material.

[0078] Meanwhile, the changes in the optical properties of each embodiment steadily increased with the increase of TiO2 nanoparticle dosage, indicating that the improved electric field self-assembly technology applied to cement-based materials can enable uniform distribution of nanoparticles without agglomeration, and the optical properties can be stably controlled. Furthermore, the compressive strength of the embodiments is higher than that of Comparative Example 1, indicating that the use of a double-layer structure avoids the influence of nanoparticles on the compressive strength of the bottom substrate, thus ensuring that the product of this invention more easily meets construction requirements in terms of strength performance.

[0079] The bonding strength of each embodiment is 1.31–1.40 MPa, while that of Comparative Example 2, which uses a physical delamination method to prepare a double-layer structure, is only 0.45 MPa. The bonding strength of the embodiments is increased by 191.1%, which proves that the present invention achieves a leapfrog breakthrough in interfacial bonding strength through innovation. The reason is that the present invention achieves delamination based on electric field driven self-assembly technology. The layers are bonded in situ by Ca–O–C chemical bonding, rather than physical bonding, which completely solves the problem of delamination and detachment.

[0080] As can be seen from the comparison of the above performance indicators, the present invention combines electric field-driven self-assembly technology with improvements to specific electric field parameters and material composition, thereby overcoming the technical problems of high coupling between the electric field application time window and the initial solidification process, and high concentration of calcium hydroxide compressing the double electric layer of nanoparticles in similar existing technologies.

[0081] Although the embodiment is not as strong as Comparative Example 2 in terms of compressive strength and not as strong as Comparative Example 1 in terms of bonding strength, the difference is small. On the contrary, the optical performance of Embodiment 1 is seriously defective, while the bonding strength of Comparative Example 2 is much lower than that of Comparative Example 1 and other embodiments. Therefore, it can be seen that the product of the present invention simultaneously achieves high reflectivity, high emission and high interfacial bonding strength, thereby meeting the performance requirements of such materials.

[0082] The present invention has been described above by way of example with reference to the accompanying drawings. Obviously, the specific implementation of the present invention is not limited to the above-described manner. Any non-substantial improvements made using the inventive concept and technical solution of the present invention, or the direct application of the inventive concept and technical solution of the present invention to other occasions without modification, are all within the protection scope of the present invention.

Claims

1. A method for preparing a self-assembled transition layer of nanoparticles on the surface of cement, characterized in that, Includes the following steps: Step 1: Charge modification of nanoparticles. Pre-treatment is used to obtain initially dispersed nanoparticles, which are then charged to prepare a modified nanoparticle suspension with uniform charge. The nanoparticles are high reflectivity nanoparticles. Step 2: Preparation of cement-based slurry and incorporation of modified particles. Weigh out the set amount of white cement, mixing water and modified nanoparticle suspension and mix evenly to obtain cement-based slurry. Step 3: Electric field-driven nanoparticle layering and self-assembly. An electric field driving device is set up and the electric field parameters are adjusted according to the electric field driving control method. Based on the electric field parameters, an electric field is applied to complete the self-assembly process of nanoparticles. In step three, the electric field-driven control method includes: S1. Based on the theoretical ion concentration and electric field parameters of cement-based materials, determine the feasible range of electric field strength according to the cement hydration differential equation; S2. Based on the feasible range of electric field strength determined in the previous step, select electric field parameters, conduct experiments under a defined electric field, perform real-time tracking and detection, and determine the time t at which the CSH gel network forms a critical point. c Detect the initial setting time t0 and calculate the time difference Δt. c = t0 t c ; S3. Conduct on-site actual operation, determine the corresponding initial setting time using standard methods, calculate the corresponding electric field application time based on the extended time difference obtained in the previous step, and determine the required electric field parameters.

2. The method for preparing a self-assembled transition layer of cement surface nanoparticles according to claim 1, characterized in that, Step one includes: 1) pretreatment; 2) charge modification; 3) preparation of modified nanoparticle suspension; Step 2) includes: charge modification by adding a polycarboxylate superplasticizer to a suspension of nanoparticles, adjusting the pH of the mixture to 8.0-9.0 with NaOH solution, and then stirring the mixture under constant temperature. The polycarboxylate superplasticizer is a polycarboxylate superplasticizer with polyoxyethylene ether side chain and a molecular weight of 20,000-50,000.

3. The method for preparing a self-assembled transition layer of cement surface nanoparticles according to claim 1, characterized in that, In step S1, the lower limit of the electric field strength is determined by preventing the reaction process from being dominated by the hydration reaction, and the upper limit of the electric field strength is determined by inducing photocatalytic side reactions of nanoparticles. In step S2, the initial setting time is first determined by calculating the integral result of the cement hydration differential equation within the feasible range of electric field strength to satisfy the CSH nucleation critical concentration, thereby initially determining the theoretical initial setting time t0. Then, it is verified by experiments, thereby calculating the time difference. Furthermore, the time difference corresponding to cement-based materials with different material components is determined through multiple experiments.

4. The method for preparing a self-assembled transition layer of cement surface nanoparticles according to claim 1, characterized in that, The electric field voltage is set to 7–8V, the anode spacing range is 5–10mm, the electric field current is 0.1–0.2A, the extension time difference range is 5–10min, and the anode spacing is the distance from the anode to the surface of the nanoparticle cement composite slurry.

5. The method for preparing a self-assembled transition layer of cement surface nanoparticles according to claim 2, characterized in that, Step 1) includes: drying the nanoparticles in a vacuum drying oven, followed by ball milling to obtain preliminarily dispersed nanoparticles; Step 3) includes: centrifuging the mixture obtained in Step 2) at high speed, discarding the supernatant, washing the precipitate with deionized water 1-3 times to remove free polycarboxylate superplasticizer, redispersing the precipitate in deionized water to obtain a modified nanoparticle suspension.

6. The method for preparing a self-assembled transition layer of cement surface nanoparticles according to claim 1, characterized in that, Step two includes the following steps: 1) Weigh a certain amount of white cement, add mixing water according to the water-cement ratio, and stir at low speed to obtain cement paste. The water-cement ratio is 0.35~0.

50. 2) Add the modified nanoparticle suspension obtained in step one to the cement slurry, add enough mixing water, and stir at high speed to obtain a uniform nanoparticle cement composite slurry. The dosage of the modified nanoparticle suspension is 1.0-5.0% of the cement mass.

7. The method for preparing a self-assembled transition layer of cement surface nanoparticles according to claim 1, characterized in that, Step three includes: 1) Setting up an electric field driving device: The bottom of the mold into which the nanoparticle cement composite slurry is injected is the cathode; an anode is set above the nanoparticle cement composite slurry, and a stable anode distance is maintained between the anode and the surface of the nanoparticle cement composite slurry; the cathode and anode are respectively connected to the negative and positive terminals of a DC regulated power supply to form a complete electric field circuit; 2) Electric field parameter control and application: The electric field parameters are controlled according to the electric field drive control method to apply an electric field to the nanoparticle cement composite slurry in the stainless steel conductive template.

8. The method for preparing a self-assembled transition layer of cement surface nanoparticles according to claim 3, characterized in that, In step two, the differential equation for cement hydration is: d[CSH] / dt=k1[Ca² + ][OH - ] n k2E, where E is the electric field strength, k2 is the inhibition coefficient of the electric field on the hydration rate; d[CSH] / dt is the formation rate of CSH gel, and k1 is the intrinsic rate constant of cement hydration, which is determined by the mineral composition of the cement itself, [Ca² + [OH] indicates the calcium ion concentration in the slurry. - [] indicates the hydroxide ion concentration in the slurry, and n indicates the OH group concentration. - The reaction order; the integral result of the cement hydration differential equation is calculated as follows: Where C* is the critical concentration for CSH nucleation.

9. A double-layer cement-based radiative cooling material, characterized in that, It has a double-layer transition structure, the double-layer transition structure comprising: The bottom cement substrate layer; Surface nanoparticle self-assembled transition layer: Located above the bottom cement matrix layer, it is prepared by the method for preparing a cement surface nanoparticle self-assembled transition layer according to any one of claims 1-8; the surface nanoparticle self-assembled transition layer includes nanoparticles, cement hydration products and surface modifiers, and the nanoparticles are gradient distributed within the surface nanoparticle self-assembled transition layer.

10. A double-layer cement-based radiative cooling material according to claim 9, characterized in that, The double-layer cement-based radiative cooling material comprises the following components by mass: 312-320 parts white cement, 101-108 parts deionized water, and 30-33 parts modified nanoparticle suspension. The raw materials used to prepare the modified nanoparticle suspension include the following components by mass parts: 10-20 parts nanoparticles, 96-105 parts deionized water, and 2-4 parts polycarboxylate superplasticizer.

Images