Cement-based cooling mortar for building external walls and preparation method of cement-based cooling mortar

Abstract

The invention relates to the technical field of building materials, in particular to cementitious cooling mortar for a building exterior wall and a preparation method of the cementitious cooling mortar, the cementitious cooling mortar comprises geopolymer cement, modified linen fibers, a water-absorbing polymer and water, the geopolymer cement comprises solid powder, toughening fibers and an alkali activator, the solid powder contains Ca < 2 + >, and the modified linen fibers are added into the alkali activator. The modified linen fiber is obtained by loading calcium sulphoaluminate and water glass on linen fiber and packaging, and the mass percent of the modified linen fiber in the geopolymer cement is 0.3%-0.7%, the mass percent of the water-absorbing polymer in the geopolymer cement is 0.3%-1.8%, and the mass percent of the water in the geopolymer cement is 5%-15%. According to laboratory determination, the cement-based cooling mortar for the building outer wall, provided by the invention, realizes surface cooling through evaporation and heat dissipation, and meanwhile, the cement-based cooling mortar plays a self-repairing function, prevents extension and recovers strength through modified linen fiber toughening and crack resistance when cracks appear, so that the durability is remarkably improved.

Description

Technical Field

[0001] This invention relates to the field of building materials technology, and in particular to a cement-based cooling mortar for building exterior walls and its preparation method. Background Technology

[0002] Cement mortar is the most common protective structural layer for building exterior walls. When applied to building exteriors, it forms a physical barrier against wind and rain erosion, ultraviolet radiation, and temperature fluctuations. This prevents wall cracking, provides waterproofing, reduces steel corrosion and freeze-thaw damage caused by moisture penetration, and extends the service life of the wall. Against the backdrop of global warming, the demand for energy-saving cement cooling mortar with cooling properties has surged to address extreme weather conditions.

[0003] In existing technologies, cement mortar that can continuously cool the surface and achieve energy-saving purposes mainly adopts the following principles for thermal regulation: (1) heat radiation heat dissipation, by incorporating SiC or Al2O3 nanoparticles, heat is radiated to space through atmospheric windows to achieve passive cooling; (2) solar radiation reflection cooling, by incorporating anatase titanium dioxide or infrared reflective pigments to selectively reflect sunlight in the 400-2500nm band; (3) evaporative cooling, by incorporating media that can hold a large amount of water, such as zeolite or modified diatomaceous earth, to achieve slow release of water through capillary action, achieving continuous cooling of 4-7℃ in an environment with RH≤50%; (4) phase change energy storage temperature regulation, by incorporating phase change materials to achieve reversible phase change in the 28-32℃ range, thereby buffering heat flow. Among these, evaporative cooling has low cost, strong adaptability to extreme environments, and dynamic responsiveness, and has therefore received widespread attention.

[0004] However, the evaporative cooling method of circulating drying is prone to causing micro-cracks in the mortar, resulting in significant strength loss and poor durability, which is significantly inferior to systems such as reflective cooling and phase change energy storage temperature regulation.

[0005] Therefore, this application is submitted. Summary of the Invention

[0006] In view of the above-mentioned shortcomings of the prior art, the present invention provides a cement-based cooling mortar for building exterior walls and its preparation method, which solves the problems of cyclic drying easily causing mortar micro-cracks, significant strength loss, and poor durability in the prior art.

[0007] To achieve the above objectives, the main technical solutions adopted by the present invention include: On one hand, this invention proposes a cementitious cooling mortar for building exterior walls, comprising geopolymer cement, modified flax fiber, water-absorbing polymer, and water. The geopolymer cement comprises solid powder, toughening fibers, and an alkali activator, wherein the solid powder contains Ca. 2+Modified flax fiber is obtained by loading calcium sulfoaluminate and water glass onto flax fiber and then encapsulating it. The mass percentages of modified flax fiber, water-absorbing polymer and water in geopolymer cement are 0.3%-0.7% for modified flax fiber, 0.3%-1.8% for water-absorbing polymer and 5%-15% for water, respectively.

[0008] Modified flax fiber utilizes its natural macroporous structure to conduct water while providing an evaporation surface. The water-absorbing polymer absorbs water under high humidity and releases water under low humidity. The modified flax fiber and the water-absorbing polymer work together to construct a water storage-conducting-evaporation system, which plays an active evaporative cooling role. Geopolymer cement is rich in microporous structure, which can delay water loss. Moreover, its low thermal conductivity can reduce the transfer of external heat, which helps to maintain the cooling effect of active evaporative cooling. The synergistic effect constructs a dual temperature regulation mechanism, which can achieve the purpose of cooling the building surface and meet the requirements of green building materials and zero-energy cooling technology.

[0009] More importantly, when modified flax fibers are incorporated into geopolymer cement in a predetermined ratio, they work in conjunction with the water-absorbing polymer to reduce surface temperature through evaporation. Simultaneously, the fibers exert their inherent bridging and toughening effect, preventing the formation of drying shrinkage cracks. Furthermore, when cracks occur, they break down, exposing calcium sulfoaluminate and water glass. At this point, water-absorbing polymers or moisture from the environment can enter, and the calcium provided by the geopolymer cement... 2+ A strongly alkaline environment is precisely the optimal condition for the reaction between water glass and calcium sulfoaluminate. Calcium sulfoaluminate reacts rapidly with Ca upon contact with water. 2+ The process involves the formation of ettringite, which expands significantly in volume to fill cracks. Water glass reacts with geopolymer cement to rapidly generate NASH gel (sodium aluminum silicate gel, also known as geopolymer gel). The ettringite and NASH gel work synergistically to fill the cracks and restore strength. Therefore, the resulting cementitious cooling mortar not only possesses excellent cooling performance, but the synergistic effect of the composite repair agent calcium sulfoaluminate and water glass also gives it a self-healing function. Crack repair does not lead to significant strength loss and also helps improve durability.

[0010] In summary, the cement-based cooling mortar for building exterior walls proposed in this invention absorbs, stores, and slowly releases water through water-absorbing polymers and modified flax fibers to achieve evaporative cooling. At the same time, the modified flax fibers toughen and prevent cracking, and when cracks appear, they play a self-repairing role and prevent the expansion of cracks. Therefore, it has both good evaporative cooling function and durability.

[0011] Furthermore, the mass ratio of calcium sulfoaluminate to water glass is 1:1, and the solid powder contains at least 30% by mass of slag powder, which is a key component. Within this proportion range, the slag powder can provide sufficient Ca... 2+ To promote the formation of the expanded filler ettringite; Preferably, the solid powder comprises the following components by mass percentage: 50%-60% fly ash, 30%-40% slag powder, 5%-10% limestone powder, and 0.1%-10% metakaolin. Preferably, the particle size of the solid powder is 0.1-2 μm.

[0012] In this technical solution, if the mass ratio of calcium sulfoaluminate to water glass is greater than 1:1, the chemical bonding effect is too weak, causing the expandable filler to be unable to bond sufficiently and easily disperse. If the mass ratio of calcium sulfoaluminate to water glass is less than 1:1, insufficient expandable filler leads to excessive shrinkage. This invention unexpectedly discovered that a mass ratio of calcium sulfoaluminate to water glass of 1:1 is most conducive to maximizing the synergistic effect. At this ratio, the expansion and bonding of the expandable filler, ettringite, are balanced under the chemical bonding effect of NASH gel, resulting in the best repair effect.

[0013] In this technical solution, the solid powder is limited to the above-mentioned components with a particle size of 0.1-2μm, so that the overall appearance of the cement-based cooling mortar after curing is close to that of fair-faced concrete, and it has low roughness, smooth surface and good decorative effect.

[0014] Furthermore, the flax fiber has a cellulose content of ≥70%, a length of 10-20 mm, a diameter of 15-25 μm, and a BET of 3-6 m. 2 / g, the modified flax fiber has a cellulose content ≥80% and a BET of 8-12m. 2 / g.

[0015] In this technical solution, flax fibers themselves possess a natural porous structure—hollow cell cavities and micropores in the cell walls—providing an excellent skeletal basis for the loading of calcium sulfoaluminate and water glass. Considering both the dispersibility and loading capacity of flax fibers, fibers with a cellulose content ≥70%, a length of 10-20 mm, a diameter of 15-25 μm, and a BET of 3-6 m are preferred. 2 / g of flax fiber, after being alkali-dissolved to create pores, can increase porosity without damaging the main structure, and provide more loading sites, thus enhancing load stability.

[0016] Furthermore, the modified flax fiber is obtained by loading flax fiber with calcium sulfoaluminate and water glass and then encapsulating it with sodium alginate, calcium chloride, and aluminum chloride through composite cross-linking.

[0017] In this technical solution, calcium chloride is the main crosslinking agent, aluminum chloride is the auxiliary crosslinking agent, and Ca... 2+ Provides fast, flexible primary networks, AI 3+The slow, rigid reinforcement nodes, working synergistically, contribute to the formation of a three-dimensional gel network with high toughness, appropriate mechanical strength, and resistance to ion exchange. This three-dimensional gel network is particularly suitable for the high-alkali, high-sodium environment of geopolymer cement. If sodium alginate is used alone, its water-absorbing properties may lead to partial dissolution or swelling, causing unplanned leakage of calcium sulfoaluminate or premature hydration due to direct contact with pore fluid. Once cracks form, it cannot provide sufficient expansive products to fill them. Furthermore, geopolymer cement is highly alkaline and has a high sodium content. + Concentration, easily in gel Ca 2+ Ion exchange occurs, leading to a decrease in gel strength and an increase in solubility.

[0018] Furthermore, the loading of calcium sulfoaluminate and water glass on the flax fiber is 28%-40%, and the thickness of the cross-linked layer obtained after composite cross-linking and encapsulation is 0.5-3μm.

[0019] In this technical solution, the present invention unexpectedly discovered that the optimal loading of calcium sulfoaluminate and water glass onto the flax fiber is between 28% and 40%. If the loading is below 28%, on the one hand, the total amount of the composite repair agent calcium sulfoaluminate and water glass will be too low, resulting in insufficient expansion filler and bonding strength, making it unable to repair larger cracks. On the other hand, because the flax fiber pores are not fully utilized, excessive water retention and excessive evaporation will lead to high surface humidity. If the loading is above 40%, the flax fiber cavity will be overfilled, and the cell walls will expand and rupture, which will not only significantly reduce fiber strength and make it brittle, but also easily block water channels, making it difficult to release the composite repair agent and significantly reducing repair efficiency. Ultimately, this will adversely affect the product's cooling performance, toughness, and self-healing properties.

[0020] In this technical solution, the thickness of the cross-linking layer should preferably be designed to be 0.5-3μm. If the thickness is less than 0.5μm, the cross-linking layer is too thin, which can easily lead to incomplete encapsulation, poor adhesion, insufficient cross-linking network, and overly sensitive triggering, making it very easy for the composite repair agent to prematurely break and release even without cracks. If the thickness is greater than 3μm, the cross-linking layer is too thick, which can easily dry and crack, degrading the encapsulation effect, and can also harden, increasing brittleness and making the fiber as a whole prone to breakage, thus creating an unplanned triggering scenario. In addition, an excessively thick cross-linking layer can also make it difficult for water to enter quickly when cracks occur, resulting in hindered hydration of calcium sulfoaluminate and stagnation of the repair reaction.

[0021] Furthermore, a hydrophobic coating is provided outside the crosslinking layer, specifically a three-dimensional Si-O-Si network formed by heating and curing sodium methylsilicate.

[0022] In this technical solution, a hydrophobic coating is additionally applied outside the crosslinking layer. The hydrophobic coating and the crosslinking layer form a double-layer encapsulation, further reducing water absorption and enhancing Na resistance. +Exchangeability, with fracture release during cracking, further enhances the reliability of repair.

[0023] Furthermore, the volume percentage of the toughening fiber to the solid powder is 1.5%-5%, and the mass percentage of the alkali activator to the solid powder is 20%-40%.

[0024] In this technical solution, the volume percentage of the toughening fiber to the solid powder can specifically be 1.5%, 1.6%, 1.7%, 1.8%, 1.9%, 2.0%, 2.1%, 2.2%, 2.3%, 2.4%, 2.5%, 2.6%, 2.7%, 2.8%, 2.9%, 3.0%, 3.1%, 3.2%, 3.3%, 3.4%, 3.5%, 3.6%, 3.7%, 3.8%, 3.9%, 4.0%, 4.1%, 4.2%, 4.3%, 4.4%, 4.5%, 4.6%, 4.7%, 4.8%, 4.9%, 5.0%, etc.

[0025] In this technical solution, the mass percentage of the alkali activator to the solid powder can be 20%, 21%, 22%, 23%, 24%, 25%, 26%, 27%, 28%, 29%, 30%, 31%, 32%, 33%, 34%, 35%, 36%, 37%, 38%, 39%, 40%, 41%, 42%, 43%, 44%, 45%, 46%, 47%, 48%, 49%, 50%, etc.

[0026] Furthermore, the toughening fiber is selected from at least one of polyvinyl alcohol fiber, polypropylene fiber, and steel fiber; preferably, the toughening fiber has a single fiber length of 10-50 mm, a single fiber diameter of 10-20 μm, and a tensile strength of 100-5000 MPa.

[0027] Furthermore, the alkaline activator is prepared by mixing sodium hydroxide, water glass and water, and has a modulus of 1-1.2 and a solid content of 42%-58%.

[0028] Furthermore, the water-absorbing polymer is selected from at least one of sodium alginate, hydroxypropyl methylcellulose, and sodium carboxymethyl cellulose. These can all form hydrogen bond networks with flax fibers and chemically crosslink with geopolymer cement to improve durability and evaporative cooling performance. Preferably, the water-absorbing polymer is sodium alginate, as sodium alginate has the most significant crosslinking effect with geopolymer and synergistic effect with flax fibers. While exerting the evaporative cooling effect, it also helps to reduce shrinkage and enhance crack resistance. Moreover, the synergistic effect is most significant when the mass percentage of sodium alginate in geopolymer cement is 0.5%-1.5%.

[0029] On the other hand, the present invention proposes a method for preparing cement-based cooling mortar for building exterior walls, comprising the following steps: S1. Preparation of modified flax fiber: First, the flax fiber is alkali-dissolved to create pores, expanding and unblocking the original cavities (i.e., natural macropores) of the flax fiber without damaging the fiber integrity, so as to enhance its connectivity. Then, calcium sulfoaluminate and water glass are loaded. Calcium sulfoaluminate is mainly loaded in the natural macropores of the flax fiber after unblocking. Water glass not only fills the natural macropores and forms a slurry with calcium sulfoaluminate, but also penetrates into the cell wall micropores, forming a dual structure of "macropore reservoir-micropore channel". After drying and curing, it is encapsulated and dried again after encapsulation. S2. Mix the solid components of solid powder, toughening fiber and alkali activator evenly in proportion, then add water-absorbing polymer and mix evenly again to obtain dry mixture. S3. Mix the liquid component of the alkali activator with water, then slowly pour it into the dry mixture, stir evenly to form a slurry, and finally add the modified flax fiber. Note: Modified flax fiber should be added in multiple batches to avoid clumping and segregation of the mortar due to excessive amount added at once.

[0030] Furthermore, in step S1, before loading calcium sulfoaluminate and water glass, the flax fibers are dried to remove moisture from the cavity and increase the slurry absorption. The specific loading operation is as follows: the dried flax fibers are vacuum-immersed in a slurry with a mass ratio of calcium sulfoaluminate and water glass of 1:1, and after 20-40 minutes, they are removed and drained. In order to ensure that the calcium sulfoaluminate mainly enters the cavity without blocking the micropore channels, the calcium sulfoaluminate is calcium sulfoaluminate powder with a D50 of no more than 5 μm. Water glass with a modulus of 1 and a solid content of no more than 35% has strong permeability and can fully penetrate into the cell wall micropores. Under the premise that the mass ratio of calcium sulfoaluminate and water glass is 1:1, the viscosity of the slurry is controlled at 100-200 mPa•s, which helps to balance macropore filling and micropore permeation. Preferably, in step S1, the encapsulation includes composite crosslinking. The specific operation of the composite crosslinking is as follows: the obtained flax fiber is immersed in a mixed solution of sodium alginate, calcium chloride, and aluminum chloride for 20-35 seconds, taken out and drained, and washed with water, so as to obtain a gel layer with a thickness of 1-2 μm; the mass ratio of sodium alginate, calcium chloride, and aluminum chloride is 4:10:1. The high concentration of calcium chloride ensures rapid coverage and provides a basic network, while the low concentration of aluminum chloride avoids instantaneous gelation and allows penetration to the internal nodes. The composite crosslinking of sodium alginate, calcium chloride, and aluminum chloride in a mass ratio of 4:10:1 can obtain a gel layer with moderate crosslinking degree and maintain hydrophilicity. Preferably, in step S1, the specific operation of alkali dissolution pore formation is as follows: first, soak the flax fiber in a sodium hydroxide solution of 3%-5% by mass at 50-65℃ for at least 3 hours, and then wash and dry it with deionized water.

[0031] In this technical solution, the specific operation of vacuum impregnation is as follows: First, the dried flax fibers are loosely piled up, and slurry is added until the liquid surface is about 2 cm above the flax fibers. Then, a vacuum is drawn to -0.08 MPa and maintained for 30 min. During this period, water glass with low viscosity preferentially enters all pores, while calcium sulfoaluminate particles selectively enter macropores. Finally, the medium cavity and some large cell wall pores are filled. Then, the pressure is slowly released, and the pressure release rate is preferably controlled to be no more than 0.01 MPa / s. The slurry is further "drawn" into the fiber interior, calcium sulfoaluminate is deposited in the medium cavity, and water glass permeates into the micropores. Then, a vacuum is drawn again to -0.08 MPa and maintained for 10 min.

[0032] In this technical solution, the drying and curing temperature after vacuum impregnation is 60℃. The drying and curing causes the water glass to partially gel and fix the calcium sulfoaluminate particles, forming a dual structure of "macroporous reservoir-microporous channel".

[0033] Furthermore, in step S1, the encapsulation also includes setting a hydrophobic coating. Specifically, the cross-linked flax fibers are immersed in a 5% sodium methylsilicate solution for 8-15 seconds, then drained and cured at 80°C for 10-15 minutes. Preferably, the drying process in step S1 is as follows: drying under vacuum at 60°C for 4-6 hours.

[0034] In this technical solution, to further avoid partial dissolution or swelling of the gel layer formed by composite cross-linking during the water absorption-release cycle, which would lead to premature exposure of calcium sulfoaluminate and improve the reliability of the repair, the present invention impregnates the composite cross-linked flax fibers (dried before impregnation) in a sodium methylsilicate solution. After draining, the fibers are heated to solidify and dehydrate, forming a hydrophobic three-dimensional Si-O-Si network. This network not only has good hydrophobicity but also strong alkali resistance, moderate mechanical strength, and the ability to block water molecules, thus effectively blocking Na+. + It allows moisture to penetrate and releases calcium sulfoaluminate when cracks form, ensuring more reliable repairs.

[0035] It should be noted that during impregnation, the flax fibers are completely immersed in the sodium methyl silicate solution to ensure that the gel layer is covered. When draining, the flax fibers are washed and placed to drain naturally until a liquid film forms on the surface and no liquid drips. Before curing and dehydration, it is preferable to place them at room temperature for a period of time or blow them gently with a fan to initially fix them. After curing and dehydration, they are naturally cooled to below 40°C to avoid moisture absorption during high-temperature discharge.

[0036] Compared with existing technologies, the cement-based cooling mortar for building exterior walls proposed in this invention has at least the following advantages: (1) Through the synergistic effect of modified flax fiber and water-absorbing polymer, this invention simultaneously possesses cooling and self-repairing functions, which helps to improve durability; (2) It has high and stable repair efficiency. The alkaline environment provided by the geopolymer cement provides the best reaction conditions for the composite repair agent. Laboratory tests show that the reaction between calcium sulfoaluminate and water glass results in a self-repair rate of over 80% for the material; moreover, the generated ettringite and NASH gel have good chemical compatibility and will not cause secondary cracking due to expansion stress, resulting in a stable repair effect; (3) Compared with traditional silicate cement and synthetic fibers, the geopolymer cement and natural flax fiber used in this invention are more green, environmentally friendly, and sustainable. Therefore, the resulting cement-based cooling mortar for building exterior walls has good application prospects in the field of energy-saving building materials. Attached Figure Description

[0037] To more clearly illustrate the technical solutions in the embodiments of this application, the accompanying drawings used in the description of the embodiments will be briefly introduced below. Obviously, the accompanying drawings described below are only some embodiments of this application. For those skilled in the art, other drawings can be obtained based on these drawings without creative effort.

[0038] Figure 1 This is a photograph of the surface of the cementitious cooling mortar used for building exterior walls after curing, obtained in Example 1 of the present invention.

[0039] Figure 2 This is a photograph of the cement-based cooling mortar for building exterior walls obtained in Embodiment 1 of the present invention.

[0040] Figure 3 This is a photograph of the surface of the product obtained in Comparative Example 10 of the present invention after curing. Detailed Implementation

[0041] The technical solutions of the embodiments of this application will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only some embodiments of this application, and not all embodiments. Based on the embodiments of this application, all other embodiments obtained by those of ordinary skill in the art without creative effort are within the scope of protection of this application.

[0042] It should be noted that the following materials are all conventional materials in this field and were obtained commercially. Technical details not described in detail below employ conventional techniques in this field, which can be combined and used by those skilled in the art without creative effort.

[0043] Example 1 like Figure 2As shown: A cement-based cooling mortar for building exterior walls, comprising geopolymer cement, modified flax fiber, water-absorbing polymer sodium alginate and water; The geopolymer cement comprises solid powder, toughening fibers, and an alkali activator. The solid powder has a particle size of 0.1-2 μm and comprises the following components by mass percentage: 55% fly ash, 35% slag powder, 7% limestone powder, and 3% metakaolin. The toughening fibers are polyvinyl alcohol fibers with a single fiber length of 10-50 mm, a single fiber diameter of 10-20 μm, and a tensile strength of 100-5000 MPa. The alkali activator is prepared by mixing sodium hydroxide, water glass, and water, with a modulus of 1.1 and a solid content of 50%. The volume percentage of polyvinyl alcohol fibers to solid powder is 3%, and the mass percentage of the alkali activator to solid powder is 30%. Modified flax fiber is obtained by encapsulating flax fiber with calcium sulfoaluminate and water glass in a 1:1 mass ratio (calcium sulfoaluminate and water glass loading is 35%) through composite crosslinking (crosslinking layer thickness is 2μm). The flax fiber has a cellulose content ≥70%, a length of 10-20mm, a diameter of 15-25μm, and a BET of 3-6m. 2 / g, the modified flax fiber has a cellulose content ≥80% and a BET of 8-12m. 2 / g; The mass percentages of modified flax fiber, sodium alginate, and water in the geopolymer cement are, in order, 0.5% modified flax fiber, 1% sodium alginate, and 10% water.

[0044] The preparation method of the above-mentioned cement-based cooling mortar for building exterior walls is as follows: S1. Preparation of modified flax fiber Alkali dissolution for pore formation: First, soak the flax fibers in a 4% sodium hydroxide solution at 60℃ for 3.5 hours, then wash with deionized water and dry. Loading calcium sulfoaluminate and water glass: Dry flax fibers are vacuum impregnated in a slurry (viscosity 140 mPa•s) with a mass ratio of calcium sulfoaluminate and water glass of 1:1. After 30 min, the fibers are removed, drained, and dried and cured. The calcium sulfoaluminate is calcium sulfoaluminate powder with a D50 of no more than 5 μm, and the water glass is water glass with a modulus of 1 and a solid content of 30%. Composite crosslinking: The obtained flax fibers were immersed in a mixed solution of sodium alginate, calcium chloride, and aluminum chloride for 30 seconds, then removed, drained, and washed with water; the mass ratio of sodium alginate, calcium chloride, and aluminum chloride was 4:10:1. The hydrophobic coating is set by immersing the composite cross-linked flax fibers in a 5% sodium methylsilicate solution for 12 seconds, then draining and curing at 80°C for 13 minutes. Drying: The modified flax fiber was dried in a vacuum at 60℃ for 5 hours. The resulting modified flax fiber had a dry and smooth feel, no stickiness, and a glossy appearance. The strength of the single fiber was greater than that of 60% of the flax fiber strength through a tensile test. The preparation method of the slurry with a mass ratio of calcium sulfoaluminate and water glass of 1:1 is as follows: (1) 15g of calcium sulfoaluminate with a D50 of not more than 5μm is uniformly dispersed in 15g of water to obtain a calcium sulfoaluminate suspension; (2) 15g of water is slowly added to 15g of water glass with a modulus of 1 and a solid content of 30%, and stirred for 5min to obtain diluted water glass; (3) The calcium sulfoaluminate suspension is mixed with the diluted water glass and mechanically stirred at a stirring speed of 300rpm for 10min, and then allowed to stand for 30min. n, its viscosity was tested to be 160 mPa•s; the preparation method of the mixed solution of sodium alginate, calcium chloride and aluminum chloride is as follows: (1) 5g calcium chloride and 0.5g aluminum chloride are put into 50g deionized water, stirred at room temperature until completely dissolved, and filtered to remove insoluble matter to obtain salt solution; (2) 2g sodium alginate is added to 50g deionized water, stirred at room temperature for 30min to obtain sodium alginate solution; (3) the salt solution is slowly added to sodium alginate solution and stirred evenly, and then left to stand for at least 15min; S2. Mix the solid components of solid powder, toughening fiber and alkali activator evenly in proportion, then add water-absorbing polymer and mix evenly again to obtain dry mixture. S3. Mix the liquid component of the alkali activator with water, then slowly pour it into the dry mixture, stir evenly to form a slurry, and finally add the modified flax fiber in multiple batches and stir evenly.

[0045] The aforementioned cementitious cooling mortar for building exterior walls is applied to the brick wall to form a cementitious mortar layer approximately 0.5-1 cm thick. After curing, the surface of the cementitious mortar layer appears as follows: Figure 1 As shown: the color is close to that of fair-faced concrete, and the surface is smooth with little roughness.

[0046] Example 2 Compared with Example 1, the amount of modified flax fiber in the geopolymer cement was adjusted from 0.5% to 0.3%, while the rest remained the same as in Example 1.

[0047] Example 3 Compared with Example 1, the amount of modified flax fiber in the geopolymer cement was adjusted from 0.5% to 0.4%, while the rest remained the same as in Example 1.

[0048] Example 4 Compared with Example 1, the amount of modified flax fiber in the geopolymer cement was adjusted from 0.5% to 0.6%, while the rest remained the same as in Example 1.

[0049] Example 5 Compared with Example 1, the amount of modified flax fiber in the geopolymer cement was adjusted from 0.5% to 0.7%, while the rest remained the same as in Example 1.

[0050] Example 6 Compared with Example 1, in step S1 of preparing cementitious cooling mortar: the time for vacuum impregnating flax fibers in the slurry was adjusted from 30 min to 20 min; the impregnation time during composite crosslinking was adjusted from 30 s to 35 s; the rest remained the same as in Example 1.

[0051] Tests showed that the loading of calcium sulfoaluminate and water glass on flax fibers was 28% (by weighing method, vs. oven-dry flax fibers), and the cross-linking layer thickness was 3 μm (by SEM).

[0052] Example 7 Compared with Example 1, in step S1 of preparing cementitious cooling mortar: the time for vacuum impregnating flax fibers in the slurry was adjusted from 30 min to 40 min; the impregnation time during composite crosslinking was adjusted from 30 s to 20 s; the rest remained the same as in Example 1.

[0053] Tests showed that the loading of calcium sulfoaluminate and water glass on flax fibers was 40% (by weighing method, vs. oven-dry flax fibers), and the cross-linking layer thickness was 0.5 μm (by SEM).

[0054] Example 8 Compared with Example 1, the modified flax fiber used in the cement-based cooling mortar does not have a hydrophobic coating, and no hydrophobic coating-related operations are performed during preparation. All other aspects are consistent with Example 1.

[0055] Example 9 Compared with Example 1, the toughening fiber in the geopolymer cement was changed from polyvinyl alcohol fiber to polypropylene fiber, the modulus of the alkali activator was changed from 1.1 to 1 and the solid content was 58%, the volume percentage of polypropylene fiber to solid powder was 1.5%, and the mass percentage of alkali activator to solid powder was 40%; the mass percentage of water-absorbing polymer sodium alginate in the geopolymer cement was changed from 1% to 1.5%; the amount of water added was changed from 10% to 15%; the rest remained the same as in Example 1.

[0056] Example 10 Compared with Example 1, the toughening fiber in the geopolymer cement was changed from polyvinyl alcohol fiber to steel fiber, the modulus of the alkali activator was changed from 1.1 to 1.2 and the solid content was 42%, the volume percentage of polypropylene fiber to solid powder was 5%, and the mass percentage of alkali activator to solid powder was 20%; the mass percentage of water-absorbing polymer sodium alginate in the geopolymer cement was changed from 1% to 0.5%; the amount of water added was changed from 10% to 5%; the rest remained the same as in Example 1.

[0057] Comparative Example 1 Compared with Example 1, the modified flax fiber was replaced with flax fiber of the same specification. The flax fiber was only subjected to alkali dissolution and pore-forming treatment, without loading any other substances or encapsulation; everything else was the same as in Example 1.

[0058] Comparative Example 2 Compared with Example 1, the modified flax fiber was loaded with only calcium sulfoaluminate and not water glass. The specific loading operation was as follows: the dried flax fiber was vacuum impregnated in the calcium sulfoaluminate suspension, and after 30 minutes it was taken out and drained; the rest was the same as in Example 1.

[0059] Comparative Example 3 Compared with Example 1, the modified flax fiber was loaded with water glass only and not with calcium sulfoaluminate. The specific loading operation was as follows: the dried flax fiber was vacuum impregnated in diluted water glass, and after 30 minutes it was taken out and drained; the rest was the same as in Example 1.

[0060] Comparative Example 4 Compared with Example 1, the mass ratio of calcium sulfoaluminate and water glass loaded on the modified flax fiber was adjusted from 1:1 to 2:1. Specifically, the mass of calcium sulfoaluminate in the calcium sulfoaluminate suspension prepared in Example 1 was adjusted from 15g to 30g; all other aspects remained the same as in Example 1.

[0061] Comparative Example 5 Compared with Example 1, the mass ratio of calcium sulfoaluminate and water glass loaded on the modified flax fiber was adjusted from 1:1 to 1:2. Specifically, the mass of water glass in the preparation of diluted water glass in Example 1 was adjusted from 15g to 30g; all other aspects remained the same as in Example 1.

[0062] Comparative Example 6 Compared with Example 1, the amount of modified flax fiber in the geopolymer cement was adjusted from 0.5% to 0.2%, while the rest remained the same as in Example 1.

[0063] Comparative Example 7 Compared with Example 1, the amount of modified flax fiber in the geopolymer cement was adjusted from 0.5% to 0.8%, while the rest remained the same as in Example 1.

[0064] Comparative Example 8 Compared with Example 1, in step S1 of preparing cementitious cooling mortar: the time for vacuum impregnating flax fibers in the slurry was adjusted from 30 min to 15 min; the impregnation time during composite crosslinking was adjusted from 30 s to 45 s; the rest remained the same as in Example 1.

[0065] Tests showed that the loading of calcium sulfoaluminate and water glass on flax fibers was 22% (by weighing method, vs. oven-dry flax fibers), and the cross-linking layer thickness was 4 μm (by SEM).

[0066] Comparative Example 9 Compared with Example 1, in step S1 of preparing cementitious cooling mortar: the time for vacuum impregnating flax fibers in the slurry was adjusted from 30 min to 50 min; the impregnation time during composite crosslinking was adjusted from 30 s to 15 s; the rest remained the same as in Example 1.

[0067] Tests showed that the loading of calcium sulfoaluminate and water glass on flax fibers was 45% (by weighing method, vs. oven-dry flax fibers), and the cross-linking layer thickness was 0.2 μm (by SEM).

[0068] Comparative Example 10 Compared to Example 1, no modified flax fiber was added and the geopolymer cement was adjusted to an equal amount of silicate cement, while all other aspects remained the same as in Example 1.

[0069] The crack closure rate of the cement-based cooling mortars obtained in Examples 1-10 and Comparative Examples 1-10 was tested by visual crack healing method. The test operation was as follows: (1) Prepare mortar specimens of 40mm×40mm×160mm, cure for 28 days, and then load them with three-point bending until the crack width is controlled to be 0.2-0.5mm, with a span of 100mm and a loading speed of 0.5mm / min. Record the initial crack width w0; (2) Reassemble the fractured specimens and wrap them with a damp cloth. Cure for 14 days; (3) Take out the specimens and wipe the surface dry. Observe the crack closure and measure the crack width w1 after healing; (4) Calculate the crack closure rate = (w0-w1) / w0×100%.

[0070] The freeze-thaw damage self-healing performance of cementitious cooling mortars obtained in Examples 1-10 and Comparative Examples 1-10 were tested according to T / CECS 913-2021 "Standard for Test Method of Self-Healing Performance of Cement Concrete".

[0071] The cooling performance of the cement-based cooling mortars obtained in Examples 1-10 and Comparative Examples 1-10 was tested using the constant temperature and humidity-mass loss method. The specific operation was as follows: (1) Prepare flat plate specimens with a size of 100mm×100mm×20mm; (2) First, vacuum immerse the flat plate specimens until they are saturated with water, then wipe off the free water on the surface and place them in an open test environment with a temperature of 35℃, humidity of 50%RH and wind speed of 0.5m / s. Heat is removed by water evaporation, and the maximum surface temperature drop ΔT of the flat plate specimens within 6 hours is measured. max (ΔT) max =T0−T min T0 is the initial temperature of the plate specimen, i.e., the ambient temperature. min (The lowest temperature on the surface of the plate specimen within 6 hours) and calculate the average evaporation rate E. avg (E) avg =Δm / 6A, Δm=m0−m1, m0 is the weight of the plate specimen at 0h (kg), m1 is the weight of the plate specimen at 6h (kg), and A is the evaporation area of ​​the plate specimen (m²). 2 E avg The unit is kg / m 2 •h). The specific test results are shown in Table 1 (for ease of listing, Example 1 is abbreviated as "S1", Comparative Example 1 is abbreviated as "D1", and so on).

[0072] Table 1 Performance test results of each embodiment and comparative example

[0073] From Table 1, Figures 1-3 It can be known that: (1) such as Figure 1 and Figure 3 As shown, a comparison of the cured photos of Example 1 and Comparative Example 10 reveals that: Comparative Example 10, which does not contain modified flax fiber and has a silicate cement matrix, appears "harder," with some areas showing shine, obvious texture, and deep grooves and significant height differences on its surface, giving it a strong sense of fragmentation; Example 1, which contains modified flax fiber and has a geopolymer cement matrix, has a dull, matte surface with no obvious directional texture, a smooth surface with small height differences, and better overall integrity; visually, Example 1 exhibits superior toughness compared to Comparative Example 10. Photos of the cured cementitious cooling mortars obtained in Examples 2-10 are also shown. Figure 1 near.

[0074] Furthermore, after crack closure rate testing of the specimens prepared in Examples 1-10, the surface cracks were almost invisible, with only minor traces remaining; while the cracks in the specimen of Comparative Example 10 showed no significant change upon visual inspection, indicating that it had almost no self-healing ability. Combined with the crack closure rate test results, it can be concluded that, through the synergistic effect of modified flax fiber and geopolymer cement, the cementitious cooling mortar proposed in this invention possesses excellent self-healing properties. The maximum surface temperature drop ΔT of the flat plate specimens prepared in Examples 1-10 within 6 hours... max All above 5℃ and with an average evaporation rate E avg Maintain at 0.45-0.48 kg / m 2 •h, Compared with Comparative Example 10, the product obtained by the present invention has excellent evaporative cooling performance. Moreover, further testing revealed that the freeze-thaw damage strength recovery rate of the products obtained in Examples 1-10 is at least 75% and can reach up to 81%, demonstrating good durability.

[0075] Obviously, the cement-based cooling mortar prepared by this invention has both excellent evaporative cooling performance and self-healing performance, and also has good durability.

[0076] (2) Comparison of the test results of Example 1, Comparative Example 1, Comparative Example 2, and Comparative Example 3 shows that: flax fibers without a repair agent, or with calcium sulfoaluminate alone, or with water glass alone, cannot achieve good repair capabilities. While calcium sulfoaluminate alone can provide expansion filler, it is easily dispersed because it has no bonding effect with the matrix. Water glass alone can provide bonding, but it shrinks significantly due to the lack of expansion filler. When both are used in combination, calcium sulfoaluminate reacts with water and Ca... 2+ The reaction generates ettringite to fill crack pores and provide expansion and early strength. Water glass reacts with the geopolymer matrix to generate a gel that provides adhesion to promote the bond strength between ettringite and the matrix, thereby providing long-term strength. The synergistic effect of these two functions significantly improves the repair effect.

[0077] (3) Comparison of the test results of Example 1, Comparative Example 4 and Comparative Example 5 shows that when the mass ratio of calcium sulfoaluminate to water glass is 1:1, the dual repair and functional synergy result in the best repair effect; while when the mass ratio of calcium sulfoaluminate to water glass is 2:1 or 1:2, the repair efficiency of cement-based cooling mortar is not as good as that of mass ratio 1:1. Tests revealed that when the mass ratio of calcium sulfoaluminate to water glass was 2:1, the viscosity of the mixed slurry was 750 mPa•s. This slurry was too thick and lacked sufficient alkalinity, resulting in slow hydration of calcium sulfoaluminate and insufficient early expansion. When the mass ratio was 1:2, the viscosity was 70 mPa•s. This slurry was too thin and had excessively high alkalinity, with water glass inhibiting calcium sulfoaluminate hydration and causing the gel to become too brittle. However, when the mass ratio was 1:1, the corresponding mixed slurry had a moderate viscosity of 160 mPa•s, indicating a harmonious reaction and a balance between expansion and adhesion, thus achieving the best repair effect.

[0078] (4) Comparison of the test results of Examples 1-5, Comparative Examples 6 and 7 shows that the adjustment of the modified flax fiber doping amount has no significant effect on the mortar cooling function. However, when the doping amount is lower than 0.3% (e.g., 0.2% as shown in Comparative Example 6) or higher than 0.7% (e.g., 0.8% as shown in Comparative Example 7), the durability of the product deteriorates significantly. When the doping amount of modified flax fiber is controlled between 0.3% and 0.7%, durability can be maintained while achieving evaporative cooling.

[0079] Although embodiments of the present invention have been shown and described above, it is understood that the above embodiments are exemplary and should not be construed as limiting the present invention. Those skilled in the art can make modifications, alterations, substitutions, and variations to the above embodiments within the scope of the present invention. Furthermore, without contradiction, those skilled in the art can combine and integrate the different embodiments or examples described in this specification, as well as the features of the different embodiments or examples.

Claims

1. A cement-based cooling mortar for building exterior walls, characterized in that: The product comprises geopolymer cement, modified flax fiber, water-absorbing polymer, and water. The geopolymer cement includes solid powder, toughening fibers, and an alkali activator, wherein the solid powder contains Ca. 2+ Modified flax fiber is obtained by loading calcium sulfoaluminate and water glass onto flax fiber and then encapsulating it. The mass percentages of modified flax fiber, water-absorbing polymer and water in geopolymer cement are 0.3%-0.7% for modified flax fiber, 0.3%-1.8% for water-absorbing polymer and 5%-15% for water, respectively.

2. The cement-based cooling mortar for building exterior walls according to claim 1, characterized in that: The mass ratio of calcium sulfoaluminate to water glass is 1:1, and the solid powder contains not less than 30% by mass of slag powder. Preferably, the solid powder comprises the following components by mass percentage: 50%-60% fly ash, 30%-40% slag powder, 5%-10% limestone powder, and 0.1%-10% metakaolin. Preferably, the particle size of the solid powder is 0.1-2 μm.

3. The cement-based cooling mortar for building exterior walls according to claim 1, characterized in that: The flax fiber has a cellulose content of ≥70%, a length of 10-20 mm, a diameter of 15-25 μm, and a BET of 3-6 m. 2 / g, the modified flax fiber has a cellulose content ≥80% and a BET of 8-12m. 2 / g; Preferably, the modified flax fiber is obtained by loading calcium sulfoaluminate and water glass onto flax fiber and then encapsulating it through a composite crosslinking process using sodium alginate, calcium chloride, and aluminum chloride; more preferably, the loading amount of calcium sulfoaluminate and water glass on the flax fiber is 28%-40%, and the thickness of the crosslinked layer obtained after composite crosslinking encapsulation is 0.5-3 μm. Preferably, a hydrophobic coating is further provided outside the crosslinking layer, which is formed by heating and curing sodium methylsilicate.

4. The cement-based cooling mortar for building exterior walls according to claim 1, characterized in that: The toughening fiber has a volume percentage of 1.5%-5% to the solid powder, and the alkali activator has a mass percentage of 20%-40% to the solid powder.

5. The cement-based cooling mortar for building exterior walls according to claim 4, characterized in that: The toughening fiber is selected from at least one of polyvinyl alcohol fiber, polypropylene fiber, and steel fiber; Preferably, the toughening fiber has a single fiber length of 10-50 mm, a single fiber diameter of 10-20 μm, and a tensile strength of 100-5000 MPa.

6. The cement-based cooling mortar for building exterior walls according to claim 4, characterized in that: The alkaline activator is prepared by mixing sodium hydroxide, water glass and water, and has a modulus of 1-1.2 and a solid content of 42%-58%.

7. The cement-based cooling mortar for building exterior walls according to claim 1, characterized in that: The water-absorbing polymer is selected from at least one of sodium alginate, hydroxypropyl methylcellulose, and sodium carboxymethyl cellulose. Preferably, the water-absorbing polymer is sodium alginate, and the mass percentage of sodium alginate in the geopolymer cement is 0.5%-1.5%.

8. A method for preparing cement-based cooling mortar for building exterior walls as described in any one of claims 1-7, characterized in that: The following steps are included: S1. Preparation of modified flax fiber: First, the flax fiber is alkali-dissolved to create pores, then loaded with calcium sulfoaluminate and water glass, dried and cured, then encapsulated, and dried again. S2. Mix the solid components of solid powder, toughening fiber and alkali activator evenly in proportion, then add water-absorbing polymer and mix evenly again to obtain dry mixture. S3. Mix the liquid component of the alkali activator with water, then slowly pour it into the dry mixture, stir evenly to form a slurry, and finally add modified flax fiber.

9. The method for preparing cement-based cooling mortar for building exterior walls according to claim 8, characterized in that: In step S1, the flax fibers are dried before loading calcium sulfoaluminate and water glass. The specific loading operation is as follows: the dried flax fibers are vacuum impregnated in a slurry with a mass ratio of calcium sulfoaluminate and water glass of 1:1, and after 20-40 minutes, they are taken out and drained. The viscosity of the slurry is 100-200 mPa•s, the calcium sulfoaluminate is calcium sulfoaluminate powder with a D50 of not more than 5 μm, and the water glass is water glass with a modulus of 1 and a solid content of not more than 35%. Preferably, in step S1, the encapsulation includes composite crosslinking, and the specific operation of the composite crosslinking is as follows: immersing the obtained flax fiber in a mixed solution of sodium alginate, calcium chloride, and aluminum chloride for 20-35 seconds, taking it out and draining it, and washing it with water; the mass ratio of sodium alginate, calcium chloride, and aluminum chloride is 4:10:

1. Preferably, in step S1, the specific operation of alkali dissolution pore formation is as follows: first, soak the flax fiber in a sodium hydroxide solution of 3%-5% by mass at 50-65℃ for at least 3 hours, and then wash and dry it with deionized water.

10. The method for preparing cement-based cooling mortar for building exterior walls according to claim 9, characterized in that: In step S1, the encapsulation also includes setting a hydrophobic coating. Specifically, the cross-linked flax fibers are immersed in a 5% sodium methylsilicate solution for 8-15 seconds, then drained and cured at 80°C for 10-15 minutes. Preferably, the drying process in step S1 is as follows: drying under vacuum at 60°C for 4-6 hours.

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