Cementing material with sustainable passive refrigeration effect and preparation method thereof

Abstract

The invention relates to the technical field of building functional materials, in particular to a cementitious material with a sustainable passive refrigeration effect and a preparation method of the cementitious material. The cementing material comprises white cement, barium sulfate, a hydrogel precursor solution, a cross-linking agent, water and moisture absorption salt. In order to solve the problem of compatibility of hygroscopic salt and cement, zwitterionic monomer carboxyl betaine methacrylate is introduced into a hydrogel network, a dynamic and temporary protection layer is formed on the surfaces of hydrated mineral particles through ion complexing, the hydration process is delayed, and the moisture absorption performance of the cement is improved. Sufficient operable time is obtained for gelling, mixing, coating or construction, the technical problem of flash coagulation caused by introduction of moisture absorption salt is successfully solved, and integrated compounding becomes possible; the product has extremely excellent and long-term stable passive refrigeration performance, relatively high water-retaining property and drying shrinkage resistance, and perfect cooperation of radiation and evaporation refrigeration is realized.

Description

Technical Field

[0001] This invention relates to the field of building functional materials technology, and in particular to a cementitious material with sustainable passive cooling function and its preparation method. Background Technology

[0002] With the acceleration of global climate change and urbanization, building cooling energy consumption continues to rise, placing enormous pressure on energy systems and the environment. Passive cooling technology, which achieves cooling through the inherent properties of materials without consuming external energy, has become a cutting-edge research area in building energy conservation. Currently, the main passive cooling strategies include radiative cooling and evaporative cooling.

[0003] Radiation-cooling materials reflect sunlight (especially visible and near-infrared light) back into space through high solar reflectance (SR), while simultaneously dissipating heat into the cold outer space through an "atmospheric window" (8-13 μm) in the form of mid-infrared radiation via a high atmospheric window emissivity (ε), thus achieving self-cooling. Traditional methods typically employ high-reflectivity white coatings, but their reflectivity is often limited by pigment properties (such as the increased absorption rate of titanium dioxide in the near-infrared band) and coating structure, making further improvements difficult.

[0004] Evaporative refrigeration utilizes the principle that water absorbs a large amount of heat during phase change (heat of vaporization is approximately 2260 kJ / kg), lowering the surface temperature through the continuous evaporation of moisture in the material. However, the moisture in ordinary porous materials evaporates rapidly and for a short period, quickly failing in dry environments and thus failing to achieve a long-term stable refrigeration effect.

[0005] Combining radiative and evaporative cooling to construct sustainable passive cooling systems presents a highly promising solution. In recent years, researchers have attempted to combine hydrogels with hygroscopic and evaporative effects with hygroscopic salts (such as LiCl and CaCl2) possessing radiative cooling capabilities to prepare composite cooling materials. These materials can absorb moisture from the air (especially at night) and evaporate it during the day, utilizing the latent heat of phase change of water to achieve continuous evaporative cooling. Simultaneously, hygroscopic salts often possess high solar reflectivity, enhancing the material's radiative cooling capacity.

[0006] However, some existing technologies, such as those described in Reference 1 (XU L, SUN DW, TIAN Y, et al. Self-rehydrating and highly entangled hydrogel for sustainable daytime passive cooling [J]. Chemical Engineering Journal, 2024, 479: 147795), have a significant drawback: hydrogels and hygroscopic salts undergo significant swelling and shrinkage during the moisture absorption-evaporation cycle, resulting in huge changes in material volume. This inherent volume instability easily leads to problems such as material cracking, peeling, and delamination from the matrix, severely limiting its application in engineering scenarios such as building facades and road surfaces where dimensional stability and durability are extremely important.

[0007] Other technologies, such as reference 2: JIANG J, WANG H, LIN J, et al. Nature-inspired hierarchical building materials with low CO2 emission and superior performance [J]. Nature Communications, 2025, 16(1): 3018, propose an approach to improve stability by combining polymers with cement-based materials. However, their main focus is not on passive cooling, nor do they address the issue of volume change and durability compatibility after the introduction of hygroscopic salts.

[0008] Therefore, developing a new type of cementitious material that maintains efficient passive cooling performance while possessing high volume stability, excellent mechanical properties and aging resistance, and is well compatible with existing concrete building structures has become an urgent technical problem to be solved in this field. Summary of the Invention

[0009] The purpose of this invention is to address the shortcomings of existing technologies by proposing a cementitious material with sustainable passive cooling capabilities and its preparation method.

[0010] To achieve the above objectives, the present invention adopts the following technical solution: A cementitious material with sustainable passive cooling function, comprising the following components in parts by weight: White cement: 100 parts; Barium sulfate: 10-30 parts; Hydrogel precursor solution: 40-50 parts; Crosslinking agent: accounting for 20%-25% of the mass of the hydrogel precursor solution, promoting the crosslinking of polymer chains in the hydrogel precursor; Water: 10-30 parts; Moisture-absorbing salt: 5-40 parts; The preparation process of the hydrogel precursor solution is as follows: Acrylamide (AM), acrylic acid (AA), and carboxybetaine methacrylate (CBMA) were added to deionized water and dissolved completely. Then, ammonium persulfate was added, and the mixture was stirred at 70-75°C for 1.5-2 hours to obtain a hydrogel precursor solution with a solid content of 30-35%.

[0011] This invention innovatively introduces carboxybenzene methacrylate (PCBMA) as a multifunctional additive. Its zwitterionic groups, through ion-dipole interactions, can bind a large number of water molecules, forming an extremely robust hydration layer. The hydrogel network grafted with PCBMA possesses stronger moisture retention capabilities, effectively resisting dehydration shrinkage under dry conditions and ensuring the durability and stability of the evaporative cooling effect. Furthermore, its strong water-retention effect can reduce plastic shrinkage caused by water evaporation in the early stages of cement hydration, lowering the risk of cracking. Specifically, PCBMA, through its carboxylate groups, complexes with calcium ions in the cement pore solution, effectively regulating and delaying the cement hydration process. This successfully solves the technical problem of excessively rapid setting and inability to work due to the introduction of hygroscopic salt components, providing a key solution for the integrated compounding of hygroscopic salts.

[0012] Meanwhile, the integration of PCBMA into the hydrogel network after polymerization significantly enhances the hydrogel's moisture retention and anti-shrinkage properties, ensuring the long-term stability of the passive cooling function. Its amphiphilic properties also improve the interfacial compatibility between the organic and inorganic phases, contributing to the formation of composite materials with more uniform structure and superior performance.

[0013] Preferably, the white cement is a 42.5 or 52.5 strength grade product conforming to the GB / T2015-2017 "White Silicate Cement" standard, with a whiteness value of not less than 87%, and its main mineral composition is tricalcium silicate (C3S) and dicalcium silicate (C2S), with a total content of not less than 75%; the content of the coloring mineral tetracalcium aluminoferrite (C4AF) is less than 1.5%.

[0014] Preferably, the barium sulfate comprises a primary reflector and an auxiliary reflector; the primary reflector is ultrafine barium sulfate with a particle size D50 of 1-2 μm, accounting for more than 85 wt%; the auxiliary reflector is nano-barium sulfate with a particle size D50 of 50-100 nm. This invention, by selecting ultrafine barium sulfate with a specific particle size that matches the wavelength of sunlight, can induce a strong Mie scattering effect, greatly improving the solar reflectance of the material in the visible and near-infrared bands, thereby enhancing the radiative cooling effect. Simultaneously, barium sulfate within this size range can effectively fill cement pores, optimizing particle size distribution, thus imparting high reflectivity to the material while ensuring its mechanical strength and durability.

[0015] Preferably, in the preparation of the hydrogel precursor, the molar ratio of acrylamide (AM), acrylic acid (AA) and carboxybetaine methacrylate (CBMA) is 1:0.2-0.3:0.12-0.15, and ammonium persulfate accounts for 2-3% of the total weight of the reaction system.

[0016] Preferably, the crosslinking agent is a mixture of ammonium persulfate, N,N'-methylenebisacrylamide, polyethylene glycol diacrylate and water in a weight ratio of 1:2-3:4-5:20.

[0017] Preferably, the hygroscopic salt is one or more of lithium chloride, calcium chloride, and lithium sulfate.

[0018] This invention also proposes a method for preparing the aforementioned gelling material with sustainable passive cooling effect, comprising the following steps: 1) Mixing: Mix and grind white cement and barium sulfate, pass through a 2-5 mesh sieve, and add the resulting dry material into the hydrogel precursor solution (which can be stored for a long time). Mix evenly at a stirring speed of 500-600 r / min. 2) Crosslinking: Add crosslinking agent (prepared and used immediately), heat to 80℃, react for 30-40 minutes under stirring, let stand for 10-20 minutes, and obtain flocculent precipitate by separation; 3) Shearing and crushing: The supernatant was separated, and the flocculent precipitate was dispersed in a high shear disperser (GRS2000) at a speed of 5000-10000 r / min for 5 min to obtain a uniform white paste. This step reduced the shear of the cross-linked gel. After dilution by 100 times, it can be observed that white particles are suspended in the system, wrapped with gel-like substances or the gel-like substances are adhered to the white particles. The system is relatively stable, indicating that the degree of hydration is low under the conditions. 4) Preparation of coating slurry: The waste liquid after rinsing the inner cavity of the high-shear disperser with the supernatant is mixed with the white paste to obtain a slurry; the waste liquid after rinsing the inner cavity of the high-shear disperser with water (using the amount of water required by the formula) is mixed with hygroscopic salt to obtain a hygroscopic salt solution; the hygroscopic salt solution is added to the slurry and stirred to obtain a coating slurry (prepare and use immediately, because the gelation speed increases significantly after adding hygroscopic salt). 5) Construction: ① Coating Pour the mixed coating slurry onto the prepared base surface, smooth it with a scraper, control the thickness to be 20-50mm, and finish the surface with a trowel. To enhance the sun reflection effect, the surface can be polished multiple times with a steel trowel after initial setting to form a smooth and dense surface. ② Maintenance After the cast-in-place body is covered with a film for 24-48 hours for moisturizing and curing, a high-concentration hygroscopic salt solution (such as saturated LiCl solution) is applied to its surface multiple times until the material no longer absorbs moisture. After that, it is allowed to dry naturally, thus obtaining a cementitious material exterior wall coating with sustainable passive cooling effect, which can replace the traditional insulation layer or protective layer and play a role in all-weather passive cooling.

[0019] Compared with the prior art, the beneficial effects of the present invention are: 1. To address the compatibility issue between hygroscopic salts and cement and achieve construction feasibility, this invention introduces the zwitterionic monomer carboxybenzene methacrylate (CBMA) into the hydrogel network. The carboxylate group (-COO) in CBMA... - It can react with calcium ions (Ca) released during the initial stage of cement hydration. 2+ A strong complexation reaction occurs, forming a dynamic, temporary protective layer on the surface of hydrated mineral particles. This protective film effectively blocks the contact between water molecules and cement particles (especially C3A and C3S), significantly slowing down the hydration reaction process. This allows sufficient working time for gelling, mixing, coating, or construction, successfully solving the flash-setting technical problem caused by the introduction of hygroscopic salts and making integrated compounding possible.

[0020] 2. This invention achieves extremely superior and long-term stable passive cooling performance, as demonstrated in the following aspects: 1) Enhanced radiative cooling: This invention introduces a dual-particle-size barium sulfate system. Micron-sized barium sulfate (1-2 μm) serves as the primary reflector, with its particle size matching the visible light wavelength, providing extremely high whiteness through Mie scattering. Nano-sized barium sulfate (50-100 nm) serves as the secondary reflector, with its particle size matching the near-infrared light wavelength, efficiently reflecting near-infrared light, which accounts for nearly half of the solar energy, through Rayleigh scattering. The two work together to achieve broadband and efficient reflection across the entire solar spectrum (250-2500 nm), minimizing heat input at the source.

[0021] 2) Continuous and efficient evaporative refrigeration: This invention uses hygroscopic salts (such as LiCl) with extremely low saturated vapor pressure, which can actively adsorb gaseous water molecules from the environment. Even in low humidity environments, a liquid water film can be maintained inside and on the surface of the material. This water film carries away a large amount of heat through continuous evaporation, achieving long-term self-sustaining evaporative cooling that does not rely on rainfall or artificial water supply.

[0022] 3. Furthermore, after polymerization, CBMA incorporates its highly polar zwitterionic groups into the hydrogel network, endowing it with exceptional water retention and resistance to drying shrinkage, ensuring long-lasting functionality. These groups, through strong ion-dipole interactions, can bind a large number of water molecules, forming an extremely robust bound water layer. This water does not easily move or evaporate, but it provides a storage and transport channel for moisture captured by hygroscopic salts.

[0023] This structure gives the hydrogel network excellent moisture retention and resistance to dehydration and shrinkage. This means that the material can effectively resist the loss of its own moisture in a dry environment, avoiding the risk of cracking caused by drying and shrinkage. It also locks in the moisture captured by the hygroscopic salt from the air and releases it as needed for evaporation, ensuring the durability and stability of the cooling effect.

[0024] 4. This invention also improves the compatibility of multiphase interfaces and optimizes the overall performance of the material: CBMA's amphiphilic structure allows it to bind with inorganic phases (cement, barium sulfate) through dipole and ionic interactions, while also exhibiting good compatibility with the organic phase [P(AM-AA) hydrogel network], acting as a "molecular bridge." This promotes uniform dispersion and tight bonding between the organic and inorganic phases, contributing to the formation of a denser composite material with fewer defects. This not only improves mechanical properties and reduces stress concentration-induced cracking, but also creates a smoother surface, further enhancing solar reflectivity.

[0025] 5. This invention achieves a perfect synergy between radiation and evaporative refrigeration: This invention is not a simple superposition of two cooling mechanisms, but rather a synergistic effect. During the day, high reflectivity (≈0.94) reduces heat input, while evaporative cooling directly removes heat; together, they keep the surface temperature far below the ambient temperature. At night, although there is no solar energy, high emissivity (≈0.93) continues to dissipate heat through radiation, while hygroscopic salts capture moisture from the humid night air, storing moisture for evaporation the following day. This continuous synergistic operation day and night achieves all-weather passive cooling.

[0026] 6. In summary, by introducing CBMA, barium sulfate with two particle sizes, and a hydrogel-hygroscopic salt composite design, this invention has successfully prepared a cement-based composite material with good construction performance, satisfactory mechanical properties, and excellent sustainable passive cooling function, which has great application potential in the fields of building energy-saving exterior walls and cooling coatings. Detailed Implementation

[0027] The technical solutions in the embodiments of the present invention will be clearly and completely described below in conjunction with existing known technologies. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments. Example 1

[0028] I. Raw Materials and Preparation Methods 1. Raw materials: White cement: Grade 52.5 white silicate cement conforming to GB / T2015-2017, with a whiteness of 90%, C3S+C2S content of 78%, and C4AF content of 1.2%.

[0029] Barium sulfate: Primary reflector: ultrafine barium sulfate, D50=1.5μm (90wt%).

[0030] Auxiliary reflector: nano barium sulfate, D50=80nm (10wt%).

[0031] The precursor monomers of the hydrogel are acrylamide (AM), acrylic acid (AA), and carboxybetaine methacrylate (CBMA) in a molar ratio of 1:0.25:0.14, and ammonium persulfate accounts for 2.5% of the total weight of the reaction system.

[0032] Initiator: Ammonium persulfate (APS).

[0033] Crosslinking agent: by weight ratio, ammonium persulfate: N,N'-methylenebisacrylamide (MBAA): polyethylene glycol diacrylate (PEGDA): water = 1:2.5:4.5:20, mixed on-site.

[0034] Hygroscopic salt: Lithium chloride (LiCl).

[0035] Water: Deionized water.

[0036] 2. Preparation of hydrogel precursor solution: In a three-necked flask equipped with a mechanical stirrer, condenser, and thermometer, a specified amount of deionized water was added, followed by AM, AA, and CBMA monomers in a specific molar ratio. The mixture was stirred until completely dissolved. After purging with nitrogen to remove oxygen for 15 minutes, the temperature was raised to 72°C, and 2.5% (by weight) of APS was added. The reaction was continued for 1.8 hours to obtain a hydrogel precursor solution with a solid content of approximately 33%. The solution was then cooled to room temperature for later use.

[0037] 3. Preparation of cementitious materials: White cement and barium sulfate were dry-mixed according to the specified ratio and passed through a 3-mesh sieve. The dry mixture was added to the hydrogel precursor solution and stirred at 550 rpm for 5 minutes to mix evenly. Freshly prepared crosslinking agent was added, the temperature was raised to 80°C, and the mixture was stirred for 35 minutes. After standing for 15 minutes, a flocculent precipitate was obtained. The supernatant was separated, and the flocculent precipitate was sheared at 8000 rpm for 5 minutes using a high-shear disperser (GRS 2000) to obtain a white paste. The waste liquid from the cleaning equipment was remixed with the white paste to obtain a slurry. Separately, hygroscopic salt was dissolved in another portion of the cleaning waste liquid and added to the slurry. The mixture was stirred rapidly for 2 minutes to obtain a coating slurry. The slurry was immediately poured into a mold, leveled to a thickness of 30 mm, and the surface was smoothed. After curing for 36 hours, a saturated LiCl solution was brushed onto the surface three times. After natural drying for 7 days, performance testing was conducted. Example 2

[0038] I. Raw Materials and Preparation Methods 1. Raw materials: White cement: Grade 42.5 white silicate cement conforming to GB / T2015-2017, with a whiteness of 87%, C3S+C2S content of 75%, and C4AF content of 1.4%.

[0039] Barium sulfate: Primary reflector: ultrafine barium sulfate, D50=1.5μm (85wt%).

[0040] Auxiliary reflector: nano barium sulfate, D50=80nm (15wt%).

[0041] The hydrogel precursor monomers are acrylamide (AM), acrylic acid (AA), and carboxybetaine methacrylate (CBMA) in a molar ratio of 1:0.2:0.15, and ammonium persulfate accounts for 2% of the total weight of the reaction system.

[0042] Initiator: Ammonium persulfate (APS).

[0043] Crosslinking agent: by weight ratio, ammonium persulfate: N,N'-methylenebisacrylamide (MBAA): polyethylene glycol diacrylate (PEGDA): water = 1:2:5:20, mixed on-site.

[0044] Hygroscopic salt: Lithium sulfate (LiSO4).

[0045] Water: Deionized water.

[0046] 2. Preparation of hydrogel precursor solution: In a three-necked flask equipped with a mechanical stirrer, condenser, and thermometer, a specified amount of deionized water was added, followed by AM, AA, and CBMA monomers in a specific molar ratio. The mixture was stirred until completely dissolved. After purging with nitrogen to remove oxygen for 15 minutes, the temperature was raised to 70°C, and 2% (by weight) of APS was added. The reaction was continued for 2 hours to obtain a hydrogel precursor solution with a solid content of approximately 34%. The solution was then cooled to room temperature for later use.

[0047] 3. Preparation of cementitious materials: White cement and barium sulfate were dry-mixed according to the specified ratio and passed through a 3-mesh sieve. The dry mixture was added to the hydrogel precursor solution and stirred at 500 rpm for 5 minutes to mix evenly. Freshly prepared crosslinking agent was added, the temperature was raised to 80°C, and the mixture was stirred for 30 minutes. After standing for 10 minutes, a flocculent precipitate was obtained. The supernatant was separated, and the flocculent precipitate was sheared at 5000 rpm for 5 minutes using a high-shear disperser (GRS2000) to obtain a white paste. The waste liquid from the cleaning equipment was remixed with the white paste to obtain a slurry. Separately, hygroscopic salt was dissolved in another portion of the cleaning waste liquid and added to the slurry. The mixture was stirred rapidly for 2 minutes to obtain a coating slurry. The slurry was immediately poured into a mold, leveled to a thickness of 30 mm, and the surface was smoothed. After curing for 36 hours, a saturated LiSO4 solution was brushed onto the surface three times. After natural drying for 7 days, performance testing was conducted. Example 3

[0048] I. Raw Materials and Preparation Methods 1. Raw materials: White cement: Grade 52.5 white silicate cement conforming to GB / T2015-2017, with a whiteness of 91%, C3S+C2S content of 79%, and C4AF content of 1.0%.

[0049] Barium sulfate: Primary reflector: ultrafine barium sulfate, D50=1.5μm (86wt%).

[0050] Auxiliary reflector: nano barium sulfate, D50=80nm (14wt%).

[0051] The precursor monomers of the hydrogel are acrylamide (AM), acrylic acid (AA), and carboxybetaine methacrylate (CBMA) in a molar ratio of 1:0.3:0.12, and ammonium persulfate accounts for 3% of the total weight of the reaction system.

[0052] Initiator: Ammonium persulfate (APS).

[0053] Crosslinking agent: by weight ratio, ammonium persulfate: N,N'-methylenebisacrylamide (MBAA): polyethylene glycol diacrylate (PEGDA): water = 1:3:4:20, mixed on-site.

[0054] Hygroscopic salt: Lithium chloride (CaCl2).

[0055] Water: Deionized water.

[0056] 2. Preparation of hydrogel precursor solution: In a three-necked flask equipped with a mechanical stirrer, condenser, and thermometer, a specified amount of deionized water was added, followed by AM, AA, and CBMA monomers in a specific molar ratio. The mixture was stirred until completely dissolved. After purging with nitrogen to remove oxygen for 15 minutes, the temperature was raised to 75°C, and 3% (by weight) of APS was added. The reaction was continued for 1.5 hours to obtain a hydrogel precursor solution with a solid content of approximately 31%. The solution was then cooled to room temperature for later use.

[0057] 3. Preparation of cementitious materials: White cement and barium sulfate were dry-mixed according to the specified ratio and passed through a 3-mesh sieve. The dry mixture was added to the hydrogel precursor solution and stirred at 550 rpm for 5 minutes to mix evenly. Freshly prepared crosslinking agent was added, the temperature was raised to 80°C, and the mixture was stirred for 35 minutes. After standing for 15 minutes, a flocculent precipitate was obtained. The supernatant was separated, and the flocculent precipitate was sheared at 8000 rpm for 5 minutes using a high-shear disperser (GRS 2000) to obtain a white paste. The waste liquid from the cleaning equipment was remixed with the white paste to obtain a slurry. Separately, hygroscopic salt was dissolved in another portion of the cleaning waste liquid and added to the slurry. The mixture was stirred rapidly for 2 minutes to obtain a coating slurry. The slurry was immediately poured into a mold, leveled to a thickness of 30 mm, and the surface was smoothed. After curing for 36 hours, a saturated LiCl solution was brushed onto the surface three times. After natural drying for 7 days, performance testing was conducted.

[0058] Single-factor experiments were conducted on the formulation of Example 1 to obtain Examples 4-6 and Comparative Examples 1-4. The raw material formulation data of Examples 1-6 and Comparative Examples 1-4 are summarized in Table 1. Table 1. Formulation of cementitious materials

[0059] It should be noted that the barium sulfate used in Comparative Example 2 was all ultrafine barium sulfate with a D50 of 1.5 μm, with no nano-components. The water volume in Comparative Example 4 was adjusted to achieve a mortar with a similar fluidity to that of Example 1.

[0060] Performance testing: 1. Optical performance testing Test items: Solar Reflectance (SR) and Hemispherical Emittance (ε). Test standards: Refer to ASTM E 903 (solar reflectance) and ASTM E 408 (hemispherical emissivity).

[0061] Test instruments: a spectrophotometer with an integrating sphere (Perkin Elmer Lambda1050+) that can measure reflectance spectra in the 300-2500nm band; and a Fourier transform infrared spectrometer (FT-IR, Thermo Scientific Nicoleti S50) equipped with an integrating sphere that measures emission spectra in the 8-13μm atmospheric window band.

[0062] Test steps: After curing and drying, the specimens are cut into samples with flat surfaces and dimensions that meet the instrument requirements (usually circular pieces with a diameter ≥2cm or squares ≥2cm×2cm).

[0063] Solar reflectance (SR): The reflectance spectrum of the sample in the 250-2500 nm wavelength range is measured using a spectrophotometer. The software calculates the solar reflectance (SR, value between 0 and 1) by weighted integration of the reflectance spectrum based on AM1.5G standard solar irradiance data.

[0064] Hemispherical emissivity (ε): The reflectance spectrum (Rλ) of the sample in the 8–13 μm band was measured using FT-IR. The hemispherical emissivity was calculated using the formula ε = 1 - Rλ and averaged over the entire band.

[0065] Objective: To quantify the ability of materials to reflect solar energy and radiate heat into space, which is a core indicator of the radiative cooling effect.

[0066] 2. Evaporative Refrigeration Performance Test Test item: Moisture evaporation rate; Testing instruments: Precision electronic balance (accuracy 0.001g), constant temperature and humidity chamber.

[0067] Test steps: The specimen was prepared into a thin plate of a specified thickness (30 mm), and its surface area (A) was accurately measured after curing.

[0068] Immerse the specimen completely in water to constant weight (ensure full saturation), then remove it and gently wipe away the free water on the surface with a damp towel.

[0069] Immediately place the specimen on an electronic balance and put it into a controlled constant temperature and humidity environment (temperature 30℃, relative humidity 50%).

[0070] The change curve of specimen mass (m) over time (t) is continuously recorded. The initial evaporation rate is the mass loss (Δm) in the first hour. The long-term evaporation rate can be recorded as the instantaneous or daily average rate at time points such as day 1, day 7, or even longer.

[0071] Calculation: Evaporation rate = |Δm| / (A*Δt), the unit is usually g / m³ 2 ·h.

[0072] Objective: To directly measure the material's ability to continuously cool. A higher evaporation rate and slower decay indicate a stronger and more persistent passive cooling effect.

[0073] 3. Water retention performance test Test item: Water retention rate; Testing standards: Refer to relevant testing methods for cement-based materials.

[0074] Testing instruments: drying oven, electronic balance.

[0075] Test steps: Fill the newly prepared slurry into a ring mold (with filter paper and microporous plate at the bottom) with a known mass (m1), smooth it, and weigh the total mass (m2).

[0076] Place it under standard curing conditions (temperature 20℃, humidity >95%) for 24 hours.

[0077] After removing it, place it in a drying oven (40℃, 50% relative humidity) and leave it for 24 hours.

[0078] Weigh the mass again (m3).

[0079] Calculation: Water retention rate (%) = [(m3-m1) / (m2-m1)]*100%.

[0080] Objective: To evaluate the ability of hydrogel networks and CBMA to retain moisture and prevent water loss under dry conditions. High water retention is fundamental for crack resistance and maintaining long-term evaporative cooling.

[0081] 4. Mechanical property testing Test item: compressive strength; Test standard: GB / T17671 "Test method for strength of cement mortar (ISO method)".

[0082] Testing instrument: Universal testing machine.

[0083] Test steps: The material was cast into a prism specimen measuring 40mm × 40mm × 160mm.

[0084] Standard maintenance (20℃, humidity >95%) until the specified age (7 days, 28 days).

[0085] The test machine was used to apply a uniform load at a rate of (2400±200) N / s until the specimen failed, and the maximum pressure value (F) was recorded. c ).

[0086] Calculation: Compressive strength (MPa) = F c / Compression area (1600mm²) 2 ).

[0087] Objective: To ensure that the material possesses the essential mechanical properties necessary for use as a building exterior wall coating or structural layer.

[0088] 5. Performance Testing Test item: setting time; Test standard: GB / T1346 "Test methods for standard consistency water requirement, setting time and soundness of cement".

[0089] Testing instrument: Vicat apparatus.

[0090] Test steps: Due to the unique composition of the material, testing its setting time is more challenging. The slurry must be prepared according to standard methods.

[0091] Initial setting time: The time when the test needle sinks into the slurry to a distance of (4±1) mm from the bottom plate is taken as the initial setting time.

[0092] Final setting time: The time point at which the final setting needle (ring attachment) leaves a mark on the surface of the slurry without settling is taken as the final setting time.

[0093] Objective: To assess whether the slurry has sufficient workability (initial setting time should not be too short) after the addition of hygroscopic salt and CBMA, which is crucial for actual construction.

[0094] Based on the above testing steps, performance tests were conducted on Examples 1-6 and Comparative Examples 1-4. The results and data analysis are shown in Table 2 below. Table 2. Effects of different components and processes on the properties of cementitious materials

[0095] Data Analysis: 1. The key role of CBMA Compared with Comparative Example 1, Example 1, which did not contain CBMA, had an initial setting time of only 25 minutes and severe cracking in the later stages. In contrast, all examples containing CBMA had setting times greater than 95 minutes, good workability, and no cracking.

[0096] Comparative Example 1 confirmed that hygroscopic salts (LiCl) drastically accelerate cement hydration, leading to flash setting and rendering the cement unworkable. The carboxylate group (-COO) in CBMA... - ) can react with Ca produced during cement hydration 2+Complexation occurs, forming a temporary protective layer that effectively delays the hydration reaction of minerals such as C3A and C3S, saving valuable time for construction. Simultaneously, the polymerization of CBMA, integrated into the hydrogel network, significantly enhances the material's water retention due to its strong zwitterionic hydration, preventing drying shrinkage and cracking, and ensuring a long-term stable water supply for evaporative cooling.

[0097] 2. Barium sulfate particle size and reflection mechanism Compared with Comparative Example 2, Example 2, which does not contain nano-barium sulfate, has a solar reflectance of only 0.88, which is much lower than 0.94 in Example 1.

[0098] Solar radiation energy is primarily concentrated in the visible light (400-760 nm) and near-infrared (760-2500 nm) bands. Micron-sized ultrafine barium sulfate effectively scatters visible light using Mie scattering, providing basic whiteness. Nano-barium sulfate, with a particle size of 50-100 nm, is more compatible with near-infrared wavelengths, enabling extremely sharp scattering of near-infrared radiation, which accounts for nearly half of solar energy. The combination of these two materials forms a broad-spectrum, high-efficiency scattering system, achieving an extremely high solar reflectance across the entire wavelength range. Comparative Example 2, lacking the nano-component, suffers from insufficient near-infrared reflectivity and a decrease in overall reflectivity.

[0099] 3. The core function of moisture-absorbing salt Compared with Comparative Example 3, Example 1 showed that Comparative Example 3, which had no hygroscopic salt, had an extremely low evaporation rate that quickly dropped to 0, and therefore lacked refrigeration capacity.

[0100] Hygroscopic salts (such as LiCl) have extremely low saturated vapor pressures, enabling them to actively capture and adsorb gaseous water molecules from the environment. This allows materials to maintain a liquid water film on their interior and surface even in low-humidity environments. This water film evaporates continuously at rates as high as 980-1250 g / m³. 2 The hygroscopic salt removes a significant amount of heat, achieving passive cooling. Without hygroscopic salt, the moisture in the material will quickly evaporate when the ambient humidity is below 100%, and the cooling effect will cease. Examples 2 and 3 show that the hygroscopic salt content directly determines the cooling power and durability, but excessive amounts will slightly affect mechanical properties.

[0101] 4. Synergistic cooling effect This material achieves a perfect synergy between radiative cooling and evaporative cooling.

[0102] During the day: The high reflectivity reflects 94% of solar radiation back into space, reducing heat input at the source. At the same time, the evaporative cooling effect continuously removes heat, keeping the material's surface temperature consistently lower than the ambient temperature.

[0103] At night: Even without solar energy, the hemispherical emissivity of the embodiment is 0.93, which can radiate heat into the cold outer space through the atmospheric window (8-13μm band) to continue cooling.

[0104] Compared to traditional mortar in Example 4, which has a reflectivity of only 0.85 and heats up after absorbing sunlight, it has a relatively good emissivity but no active cooling mechanism. Its temperature is entirely determined by the environment and may even be higher than the air temperature due to heat absorption.

[0105] 5. Conclusion: This invention resolves the core contradiction of compatibility between hygroscopic salts and cement through CBMA, achieves extremely high solar reflectivity through dual-particle-size barium sulfate, and provides continuous and stable evaporative cooling power through the hygroscopic salt / hydrogel composite system. Example 1 demonstrates optimal comprehensive performance: excellent workability, 94% high reflectivity, 93% high emissivity, and up to 980 g / m³. 2 With a continuous evaporation rate of ·h, a water retention rate of 92.5%, and a sufficient mechanical strength of 38.5MPa, it is a sustainable passive cooling building material with great application prospects.

[0106] The above description is only a preferred embodiment of the present invention, but the scope of protection of the present invention is not limited thereto. Any equivalent substitutions or modifications made by those skilled in the art within the scope of the technology disclosed in the present invention, based on the technical solution and inventive concept of the present invention, should be covered within the scope of protection of the present invention.

Claims

1. A cementitious material with sustainable passive cooling function, characterized in that, Includes the following components by weight: White cement: 100 parts; Barium sulfate: 10-30 parts; Hydrogel precursor solution: 40-50 parts; Crosslinking agent: accounting for 20%-25% of the mass of the hydrogel precursor solution, promoting the crosslinking of polymer chains in the hydrogel precursor; Water: 10-30 parts; Moisture-absorbing salt: 5-40 parts; The preparation process of the hydrogel precursor solution is as follows: Acrylamide, acrylic acid, and carboxybetaine methacrylate were added to deionized water and dissolved completely. Then, ammonium persulfate was added and the mixture was stirred at 70-75°C for 1.5-2 hours to obtain a hydrogel precursor solution with a solid content of 30-35%.

2. The cementitious material with sustainable passive cooling effect according to claim 1, characterized in that, The white cement is a product of strength grade 42.5 or 52.5, with a whiteness value of not less than 87%, and its main mineral composition is tricalcium silicate and dicalcium silicate, with a total content of not less than 75%; the content of the coloring mineral tetracalcium aluminoferrite is less than 1.5%.

3. The cementitious material with sustainable passive cooling effect according to claim 1, characterized in that, The barium sulfate is composed of a primary reflector and an auxiliary reflector; the primary reflector is ultrafine barium sulfate with a particle size D50 of 1-2 μm, accounting for more than 85 wt%; the auxiliary reflector is nano-barium sulfate with a particle size D50 of 50-100 nm.

4. The cementitious material with sustainable passive cooling effect according to claim 1, characterized in that, In the preparation of the hydrogel precursor, the molar ratio of acrylamide, acrylic acid and carboxybetaine methacrylate is 1:0.2-0.3:0.12-0.15, and ammonium persulfate accounts for 2-3% of the total weight of the reaction system.

5. A cementitious material with sustainable passive cooling effect according to claim 1, characterized in that, The crosslinking agent is a mixture of ammonium persulfate, N,N'-methylenebisacrylamide, polyethylene glycol diacrylate and water in a weight ratio of 1:2-3:4-5:

20.

6. The cementitious material with sustainable passive cooling effect according to claim 1, characterized in that, The hygroscopic salt is one or more of lithium chloride, calcium chloride, and lithium sulfate.

7. A method for preparing a cementitious material with sustainable passive cooling effect as described in any one of claims 1-6, characterized in that, Includes the following steps: 1) Mixing: White cement and barium sulfate are mixed and ground, passed through a 2-5 mesh sieve, and the resulting dry material is added to the hydrogel precursor solution and mixed evenly at a stirring speed of 500-600 r / min. 2) Crosslinking: Add a crosslinking agent, heat to 80℃, react for 30-40 minutes under stirring, let stand for 10-20 minutes, and obtain flocculent precipitate by separation; 3) Shearing and crushing: The supernatant was separated, and the flocculent precipitate was dispersed in a high shear disperser at a speed of 5000-10000 r / min for 5 min to obtain a uniform white paste. 4) Preparation of coating slurry: The waste liquid after rinsing the inner cavity of the high-shear disperser with the supernatant is mixed with the white paste to obtain a slurry; the waste liquid after rinsing the inner cavity of the high-shear disperser with water is mixed with hygroscopic salt to obtain a hygroscopic salt solution; the hygroscopic salt solution is added to the slurry and stirred to obtain a coating slurry; 5) Construction: ① Coating Pour the mixed coating slurry onto the prepared base surface, smooth it with a scraper, control the thickness to be 20-50mm, and polish the surface with a trowel; to enhance the sun reflection effect, after the surface has initially set, use a steel trowel to polish it multiple times to form a smooth and dense surface. ② Maintenance After the cast body is covered with a film and kept moist for 24-48 hours, a high-concentration moisture-absorbing salt solution is applied to its surface multiple times until the material no longer absorbs moisture. After that, it is allowed to dry naturally, thus obtaining a cementitious material exterior wall coating with sustainable passive cooling function.

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