An admixture for ferrocement

Abstract

The present invention discloses an admixture for architectural formwork concrete, belonging to the technical field of concrete materials. The admixture for architectural formwork concrete of the present invention comprises the following components in parts by weight: chitosan-modified metakaolin: 65 - 70 parts; microencapsulated latex powder: 10 - 15 parts; calcium sulfoaluminate clinker: 5 - 10 parts; modified polypropylene chopped fibers: 3 - 5 parts. By the reasonable proportioning of each component, the present invention gives full play to the synergistic effect among different components, aiming to provide an admixture for architectural formwork concrete which can significantly improve the thixotropic property, interfacial bonding force and crack resistance of architectural formwork concrete, and simultaneously effectively prevent the corrosion of steel formwork. It is applicable to the architectural formwork concrete project with complex grid structures and has significant application value.

Description

Technical Field

[0001] The invention relates to the technical field of concrete materials, in particular to an admixture for building formwork concrete. Background Art

[0002] The building formwork concrete structure is a space grid composed of steel plate mesh, vertical stiffening ribs and horizontal hook tie bars. It is a new type of composite structure formed by pouring self-compacting concrete inside. It has significant advantages such as convenient construction, strong integrity and material saving. It is widely used in various construction fields. However, although the construction technology of building formwork is becoming more and more mature, there are still some key technical problems that need to be solved.

[0003] First of all, during the construction process, the fluidity of building formwork concrete is crucial to the quality of the structure. Insufficient fluidity will increase pumping resistance, making it difficult to fill complex formwork structures, forming voids or loose areas; while excessive fluidity will easily cause slurry to seep out of the holes in the formwork, resulting in material waste and reduced concrete density and mechanical properties. Therefore, concrete must meet the requirements of good pumping performance and rapid viscosity recovery at the same time to ensure sufficient filling of complex structures and inhibit slurry seepage. To achieve this goal, it is necessary to optimize the thixotropy of concrete so that it exhibits adaptive rheological properties at different construction stages.

[0004] Secondly, as the core of the composite structure, the steel mesh and concrete need to form an integrated load-bearing system through interface bonding. The steel mesh not only serves as a permanent formwork, but also participates in the overall force to enhance the structural performance. If the interface bonding is insufficient, it may cause the concrete and steel mesh to partially separate or slip, weakening the structural stiffness and load-bearing capacity, and affecting the mechanical properties of the solid structure.

[0005] In addition, the amount of adhesive used in building formwork concrete is relatively high, and the high amount of adhesive used exacerbates the shrinkage effect, especially in the case of insufficient maintenance, which is very likely to cause shrinkage cracks. If no measures are taken, the concrete surface will crack seriously. This cracking phenomenon will bring a series of hidden dangers, including reducing the durability of the structure, accelerating the corrosion of the steel formwork and steel bars, weakening the bearing capacity, and may even affect the overall safety and service life of the building. Especially in harsh environments or long-term loads, these problems may be further magnified, thereby increasing maintenance costs and safety hazards.

[0006] At present, the solutions to the above technical requirements mostly rely on the addition of specific admixtures, but the existing technologies generally fail to make targeted designs for the performance requirements of building formwork concrete. Although some patents have proposed admixtures that can meet some performance requirements, there are still obvious shortcomings:

[0007] As disclosed in the Chinese invention patent (CN107200500A), a special admixture for self-compacting and non-shrinking steel tube concrete is provided. The admixture contains, by weight, 30 - 50% of lightly burned calcium oxide clinker, 10 - 30% of lightly burned magnesium oxide clinker, 10 - 30% of gypsum, 10 - 30% of fly ash, 1 - 3% of polycarboxylate superplasticizer, 0.1 - 1.0% of thixotropic agent, 0.2 - 2.0% of plastic foaming agent, 0.2 - 1.0% of retarder, and 0.5 - 2.0% of defoamer. Although the patent mentions that the addition of this admixture can significantly improve the workability of concrete, enhance thixotropy, compensate for shrinkage deformation, and ensure its dense filling in the steel tube structure, enabling the concrete and the steel tube wall to work together under stress. However, the expansion agent component in this patent is too high and the expansion effect is too strong. When applied in architectural mesh concrete, it will cause the concrete paste to overflow from the holes in the mesh during the hardening process, thereby affecting the quality of the solid structure.

[0008] Again, as disclosed in the Chinese invention patent (CN110937842A), an admixture for improving the crack resistance and thixotropic properties of ultra-high-rise pumped concrete is provided. The mass percentages of its various components are as follows: silica fume 49% - 52%, fly ash microspheres 24% - 29%, ultra-fine limestone powder 5% - 7%, modified montmorillonite 10% - 12%, and microcrystalline cellulose-modified superabsorbent resin fiber 3% - 5%. It can significantly improve the pumpability, thixotropy, and crack resistance of high-rise pumped concrete, reducing the construction difficulty of special structures and the risk of pipe blockage. This admixture is composed of multiple high-performance materials, resulting in too high a cost, which limits its application in large-scale projects. At the same time, the patent does not clearly define the optimal dosage range and its sensitivity to the properties of concrete. Since the admixture contains multiple functional components, slight changes in its dosage may significantly affect the workability, thixotropy, and crack resistance of concrete, increasing the control difficulty in practical applications.

[0009] For another example, a thixotropic agent for ballastless track base slab concrete is disclosed in a Chinese invention patent (CN119100650A). Its components are as follows (by weight): attapulgite 0.5 - 4 parts, sepiolite 0.5 - 5 parts, fumed silica 0.02 - 0.38 parts, silica fume 2 - 30 parts, triterpenoid saponin 0.06 - 0.5 parts, cellulose ether 0.03 - 0.5 parts, fiber 0 - 2.5 parts. This thixotropic agent can significantly improve the static form stability of fresh concrete and can form a fixed slope after pouring. At the same time, it does not increase the dynamic deformation energy and has good rheological properties during construction such as pumping, pouring, and vibrating, facilitating construction. This thixotropic agent achieves slope setting by increasing the static yield stress of fresh concrete and reduces the dynamic yield stress to ensure rheological properties during construction. However, the requirement for the static form retention ability of architectural formwork concrete is not so strict because the concrete is fixed by the formwork during construction and does not need to maintain the slope by its own characteristics like the ballastless track base slab. The design goal of this thixotropic agent does not match the actual needs of architectural formwork concrete, and the functionality is not compatible. Summary of the Invention

[0010] To solve the above technical problems, the purpose of the present invention is to provide an admixture for architectural formwork concrete, which can improve the thixotropy and crack resistance of the concrete used for architectural formwork, enhance the interfacial bonding force between the concrete and the steel formwork, and effectively prevent the corrosion of the steel formwork.

[0011] The purpose of the present invention is achieved through the following technical solutions:

[0012] The present invention provides an admixture for architectural formwork concrete, comprising the following components in parts by weight:

[0013] Chitosan-modified metakaolin: 65 - 70 parts,

[0014] Microencapsulated latex powder: 10 - 15 parts,

[0015] Calcium sulfoaluminate clinker: 5 - 10 parts,

[0016] Modified polypropylene short fiber: 3 - 5 parts.

[0017] As some specific embodiments of the present invention, the preparation method of the chitosan-modified metakaolin includes:

[0018] S1. Mix metakaolin with hydrochloric acid solution, stir and activate, then filter, wash with deionized water until neutral, and dry to obtain acidified metakaolin;

[0019] S2. Add the acidified metakaolin, chitosan solution, and cationic surfactant prepared in step S1 into a reaction kettle, seal the system, and heat and react under nitrogen protection;

[0020] S3. After the reaction is completed, cool down, wash the product, and obtain chitosan-modified metakaolin after vacuum drying.

[0021] As some specific embodiments of the present invention, step S1 includes at least one of the following technical features:

[0022] (1) By weight, take 40 - 60 parts of the metakaolin, 140 - 160 parts of hydrochloric acid solution, and the concentration of the hydrochloric acid solution is 3% - 5%;

[0023] (2) The temperature of the stirring activation is 50 - 70 °C, and the time is 1 - 3 hours;

[0024] (3) The temperature of the drying is 70 - 90 °C, and the time is 3 - 5 hours.

[0025] As some specific embodiments of the present invention, step S2 includes at least one of the following technical features:

[0026] (1) By weight, take 20 - 40 parts of the acidified metakaolin, 90 - 110 parts of chitosan solution, and 6 - 10 parts of cationic surfactant;

[0027] (2) The cationic surfactant is selected from at least one of octadecyl dimethyl benzyl ammonium chloride, cetylpyridinium bromide, and benzyl triethyl ammonium chloride; preferably octadecyl dimethyl benzyl ammonium chloride;

[0028] (3) The temperature of the temperature-raising reaction is 60 - 80 °C, and the time is 100 - 140 minutes, and continuous stirring reaction is carried out in a reaction kettle.

[0029] As some specific embodiments of the present invention, in step S2, the concentration of the chitosan solution is 3% - 5%; its preparation method is: by weight, dissolve 6 - 10 parts of chitosan in 186 - 198 parts of lactic acid solution with a concentration of 2% - 3%, and stir at 35 - 45 °C until completely dissolved.

[0030] As some specific embodiments of the present invention, step S3 includes at least one of the following technical features:

[0031] (1) The temperature of the cooling is 20 - 30 °C;

[0032] (2) The detergent used for washing includes 60 - 80% ethanol;

[0033] (3) The temperature of the vacuum drying is 50 - 70 °C, and the time is 5 - 7 hours.

[0034] As some specific embodiments of the present invention, the preparation method of the chitosan-modified metakaolin is as follows: By weight, 50 parts of metakaolin are mixed with 150 parts of a hydrochloric acid solution with a concentration of 4%, stirred and activated at 60°C for 2 hours, filtered, repeatedly washed with deionized water until neutral, and dried at 80°C for 4 hours to obtain acidified metakaolin. The acidification treatment enhances the active sites on the surface of metakaolin, promoting its combination with chitosan and cationic surfactants; 30 parts of acidified metakaolin are added to a reaction kettle, and at the same time, 100 parts of a chitosan solution with a concentration of 4% and 8 parts of a cationic surfactant octadecyl dimethyl benzyl ammonium chloride are added. Then the system is sealed, heated to 70°C under nitrogen protection, and continuously stirred and reacted for 120 minutes. The cationic surfactant bridges chitosan and metakaolin through electrostatic interaction, enhancing the adsorption ability of chitosan on the surface of metakaolin, thereby improving the loading efficiency; after the reaction is completed, it is cooled to room temperature, and the product is washed with a 70% ethanol solution, and finally vacuum dried at 60°C for 6 hours to obtain chitosan-modified metakaolin. Chitosan is a natural polymer polysaccharide with good film-forming property and adhesiveness, which can significantly improve the thixotropy of concrete. Metakaolin plays a lubricating and filling role in concrete, optimizing the particle size distribution and reducing the porosity. By modifying metakaolin with chitosan, the dispersibility and stability of metakaolin in cement-based materials can be significantly improved. The chitosan-modified metakaolin can significantly enhance the thixotropic performance of concrete, preventing segregation and bleeding phenomena; the preparation method of the chitosan solution is: 8 parts of chitosan are dissolved in 192 parts of a lactic acid solution with a concentration of 3%, and stirred at 40°C until completely dissolved.

[0035] As some specific embodiments of the present invention, the preparation method of the microencapsulated redispersible latex powder includes:

[0036] A1. Taking polyurethane as the polymer wall material, adding it to deionized water, heating and stirring to dissolve, and then adding an emulsifier and stirring to form an emulsion system;

[0037] A2. Slowly adding the redispersible latex powder to the emulsion system and stirring to form a coated emulsion;

[0038] A3. Spray-drying the coated emulsion and collecting to obtain the microencapsulated redispersible latex powder.

[0039] As some specific embodiments of the present invention, by weight, 40 - 60 parts of redispersible latex powder, 15 - 25 parts of polyurethane, 8 - 12 parts of emulsifier, and 180 - 220 parts of deionized water are taken;

[0040] The emulsifier is selected from at least one of Tween - 80, Tween - 60, and Tween - 40;

[0041] The redispersible latex powder is selected from at least one of polyvinyl acetate latex powder, polyvinyl alcohol latex powder, and acrylate latex powder.

[0042] As some specific embodiments of the present invention, in step A1, the heating temperature for heating and stirring to dissolve is 50 - 70 °C.

[0043] As some specific embodiments of the present invention, in step A2, the rotation speed of the stirring is 1000 - 2000 rpm, and the time is 25 - 30 minutes.

[0044] As some specific embodiments of the present invention, in step A3, the spray drying is carried out by a spray drying device, the inlet air temperature is 110 - 130 °C, and the outlet air temperature is 50 - 70 °C.

[0045] As some specific embodiments of the present invention, after the microencapsulated latex powder is prepared, it is vacuum dried, and then an antioxidant is added to improve the storage stability.

[0046] Furthermore, the temperature of the vacuum drying is 35 - 45 °C, and the time is 1.5 - 2.5 hours.

[0047] Furthermore, the antioxidant is dibutylhydroxytoluene.

[0048] As some specific embodiments of the present invention, the preparation method of the microencapsulated latex powder is as follows: By weight, 50 parts of redispersible latex powder, 20 parts of polymer wall material polyurethane, 10 parts of emulsifier Tween-80 and 200 parts of deionized water are weighed; the polymer wall material is added to the deionized water and stirred to dissolve at 60°C. After adding the emulsifier, it is stirred for 10 minutes to form a stable emulsion system. The polymer wall material polyurethane can gradually dissolve and release the redispersible latex powder components during the cement hydration process, and the emulsifier ensures the stability of the wall material solution during the subsequent coating process to avoid agglomeration of latex powder particles; the redispersible latex powder is slowly added to the emulsion system and stirred at a speed of 1500 rpm for 30 minutes; the coated emulsion is dried by a spray drying device, with an inlet air temperature of 120°C and an outlet air temperature of 60°C. The microencapsulated latex powder is collected. The spray drying technology can quickly solidify the microcapsule structure, avoid the decomposition of the latex powder at high temperature, and at the same time ensure the sphericity and fluidity of the particles; the dried microencapsulated latex powder is vacuum dried at 40°C for 2 hours, and antioxidant dibutylhydroxytoluene is added to improve the storage stability; finally, the particle size range of the microencapsulated latex powder product is 10 - 50 μm, the wall thickness is 1 - 5 μm, and the latex powder components can be gradually released under the conditions of pH>12 or temperature>60°C. The release of the latex powder is delayed through the microencapsulation technology, so as to ensure that the redispersible latex powder will not significantly increase the initial viscosity of the concrete during the mixing process, enable the latex powder to gradually play a role during the cement hydration process, provide long-term interfacial adhesion and anti-corrosion protection, and effectively reduce the leakage of the paste and improve the overall performance of the concrete.

[0049] As some specific embodiments of the present invention, the specific surface area of the calcium sulfoaluminate clinker is 430 - 490 m 2 / kg. The calcium sulfoaluminate clinker can generate moderate volume expansion during the hardening process of the concrete, effectively compensate for the shrinkage of the concrete caused by dry shrinkage and temperature changes, thereby reducing the generation of cracks and improving the integrity and durability of the structure.

[0050] As some specific embodiments of the present invention, the preparation method of the silane-modified polypropylene short fibers is as follows:

[0051] The polypropylene short fibers are immersed in a KH-570 silane coupling agent solution with a concentration of 0.5 - 1% for soaking treatment, taken out and rinsed with deionized water, and dried to constant weight to obtain the silane-modified polypropylene short fibers.

[0052] As some specific embodiments of the present invention, the length of the polypropylene short fibers is 6 - 12 mm, and the diameter is 10 - 20 μm;

[0053] And / or, the soaking treatment time is 30 - 60 minutes;

[0054] And / or, the drying temperature is 70-90°C.

[0055] As some specific embodiments of the present invention, the preparation method of the silane-modified polypropylene short fibers is as follows: Polypropylene short fibers with a length of 6-12 mm and a diameter of 10-20 μm are immersed in a KH-570 silane coupling agent solution with a concentration of 0.5-1% for 30-60 minutes, taken out, rinsed with deionized water, and dried to constant weight at 80°C to obtain surface-modified polypropylene short fibers. The polypropylene short fibers play a role of "bridging" cracks in concrete and can effectively inhibit the expansion of microcracks. After surface treatment of the fibers with KH-570 silane coupling agent, the hydrophilicity of the fiber surface is improved, making it easier to disperse uniformly in the cement paste. This uniform dispersion helps to optimize the microstructure of concrete and further improve the strength and toughness of the material.

[0056] As some specific embodiments of the present invention, when the admixture is incorporated into the architectural formwork concrete, by weight percentage, it accounts for 3-6% of the total amount of the cementitious materials in the architectural formwork concrete.

[0057] Compared with the prior art, the present invention has the following beneficial effects:

[0058] 1) Through acidification treatment and treatment with a cationic surfactant, the present invention enhances the adsorption capacity of chitosan on the surface of metakaolin and improves the chitosan loading efficiency. Metakaolin as a carrier can provide stable attachment points for chitosan, and the loaded chitosan can be more evenly distributed throughout the system. The chitosan molecules are attached to the surface of metakaolin particles through physical adsorption or chemical bonding, which not only changes the surface charge characteristics of metakaolin particles but also can prevent particle aggregation through steric hindrance effects, thereby improving the dispersibility and stability of metakaolin in fresh concrete. Chitosan increases the viscosity of the paste, forms a dynamic network structure, and improves particle dispersibility; metakaolin reacts with cement hydration products through its pozzolanic activity to generate more C-S-H gels, enhancing the cohesion and structural stability of the paste. By means of modifying metakaolin with chitosan, the ability of both to improve the rheological properties of concrete can be fully exerted, and the thixotropy of concrete can be significantly improved.

[0059] 2) The present invention uses the microencapsulation technology to incorporate redispersible latex powder, which can gradually release the latex powder components during the cement hydration process. This can not only avoid the problem of excessive initial viscosity of fresh concrete caused by premature dissolution of the latex powder but also improve the interfacial bonding force between the concrete and the steel formwork during the hardening process of the concrete, reduce the leakage of the paste during the process of architectural formwork concrete from pouring to hardening, and improve the density of the solid structure of the architectural formwork concrete.

[0060] 3) The present invention uses KH-570 silane coupling agent to modify polypropylene short fibers, thereby improving the hydrophilicity and dispersibility of the fiber surface, forming a uniformly distributed "bridging" network structure in the cement matrix, effectively inhibiting the expansion of microcracks, and enhancing the crack resistance of concrete. At the same time, calcium sulfoaluminate clinker generates a moderate volume of expansion during the hardening process of concrete, compensating for the shrinkage caused by dry shrinkage, further reducing the generation of cracks, and filling the voids between the steel formwork mesh and concrete more tightly. Through the synergistic effect of fibers and expansive agents, the technical problems of excessive dry shrinkage and easy cracking of concrete caused by excessive consumption of building formwork concrete cementitious materials are solved. Detailed implementation manners

[0061] The present invention will be described in detail below in conjunction with specific embodiments. The following embodiments will help those skilled in the art to further understand the present invention, but do not limit the present invention in any form. It should be noted that for those of ordinary skill in the art, without departing from the concept of the present invention, several modifications and improvements can still be made. These all belong to the protection scope of the present invention.

[0062] The raw materials used in the following examples, comparative examples and application tests are as follows:

[0063] (1) Cement: 42.5 grade ordinary Portland cement;

[0064] (2) Fly ash: Class C Grade II fly ash;

[0065] (3) Metakaolin: Specific surface area 672 m 2 / kg, mainly composed of silicon and aluminum oxides, with SiO2 and Al2O3 contents of 50.67% and 44.34% respectively;

[0066] (4) Coarse aggregate: Particle size 5 - 25 mm, continuous grading;

[0067] (5) Manufactured sand: Fineness modulus 2.7, MB 0.8, stone powder content 6.4%;

[0068] (6) Water reducing agent: Polycarboxylate high-performance water reducing agent with a water reducing rate of 25%, solid content 15%;

[0069] (7) Redispersible latex powder: Polyvinyl acetate latex powder, solid content 99%, ash content 12%, bulk density 500g / l, average particle size d 50 is 70 μm;

[0070] (8) Calcium sulfoaluminate clinker: Specific surface area 460 m 2 / kg;

[0071] (9) Calcium oxide clinker: Specific surface area 300 m 2 / kg;

[0072] (9)Polypropylene short-cut fibers: length 6 - 12 mm, diameter 10 - 20 μm, tensile strength ≥ 500 MPa;

[0073] (10)Mixing water: ordinary tap water.

[0074] The chitosan-modified metakaolin used in each example and comparative example of the present invention is prepared through the following steps:

[0075] By weight, mix 50 parts of metakaolin with 150 parts of hydrochloric acid solution with a concentration of 4%, stir and activate at 60 °C for 2 hours, filter and wash repeatedly with deionized water until neutral, and dry at 80 °C for 4 hours to obtain acidified metakaolin; add 30 parts of acidified metakaolin into a reaction kettle, and at the same time add 100 parts of chitosan solution with a concentration of 4% and 8 parts of cationic surfactant octadecyl dimethyl benzyl ammonium chloride, then seal the system, heat up to 70 °C under nitrogen protection, and continuously stir and react for 120 minutes; after the reaction is completed, cool to room temperature, wash the product with 70% ethanol solution, and finally vacuum dry at 60 °C for 6 hours to obtain metakaolin supported chitosan; the preparation method of the chitosan solution is: dissolve 8 parts of chitosan in 192 parts of lactic acid solution with a concentration of 3%, and stir at 40 °C until completely dissolved.

[0076] The microencapsulated latex powder used in each example and comparative example of the present invention is prepared through the following steps:

[0077] By weight, weigh 50 parts of redispersible latex powder, 20 parts of polymer wall material polyurethane, 10 parts of emulsifier Tween-80 and 200 parts of deionized water; add the polymer wall material into deionized water, stir and dissolve at 60 °C, add the emulsifier and stir for 10 minutes to form a stable emulsion system; slowly add the redispersible latex powder into the emulsion system, and stir at a speed of 1500 rpm for 30 minutes; dry the coated emulsion through a spray drying device, with an inlet air temperature of 120 °C and an outlet air temperature of 60 °C, and collect the microencapsulated latex powder; vacuum dry the dried microencapsulated latex powder at 40 °C for 2 hours, and add antioxidant dibutylhydroxytoluene to improve storage stability; finally obtain the microencapsulated latex powder product, whose particle size range is 10 - 50 μm and the wall material thickness is 1 - 5 μm.

[0078] The silane-modified polypropylene short-cut fibers used in each example and comparative example of the present invention are prepared through the following steps:

[0079] The preparation method of the silane-modified polypropylene short fibers is as follows: Polypropylene short fibers with a length of 6 - 12 mm and a diameter of 10 - 20 μm are immersed in a 1% KH-570 silane coupling agent solution for 40 minutes, taken out, rinsed with deionized water, and dried to a constant weight at 80 °C to obtain surface-modified polypropylene short fibers.

[0080] Example 1

[0081] Provide an admixture for architectural formwork concrete, including the following components in parts by weight:

[0082] Chitosan-modified metakaolin: 70 parts,

[0083] Microencapsulated latex powder: 10 parts,

[0084] Calcium sulfoaluminate clinker: 5 parts,

[0085] Modified polypropylene short fibers: 5 parts.

[0086] After the admixture is incorporated, it accounts for 6% by weight of the total amount of cementitious materials in the concrete.

[0087] Example 2

[0088] Provide an admixture for architectural formwork concrete, including the following components in parts by weight:

[0089] Chitosan-modified metakaolin: 65 parts,

[0090] Microencapsulated latex powder: 15 parts,

[0091] Calcium sulfoaluminate clinker: 10 parts,

[0092] Modified polypropylene short fibers: 3 parts.

[0093] After the admixture is incorporated, it accounts for 3% by weight of the total amount of cementitious materials in the concrete.

[0094] Example 3

[0095] Provide an admixture for architectural formwork concrete, including the following components in parts by weight:

[0096] Chitosan-modified metakaolin: 68 parts,

[0097] Microencapsulated latex powder: 12 parts,

[0098] Calcium sulfoaluminate clinker: 8 parts,

[0099] Modified polypropylene short fibers: 4 parts.

[0100] After the admixture is incorporated, it accounts for 4% by weight of the total amount of cementitious materials in the concrete.

[0101] Comparative Example 1

[0102] An admixture for concrete, compared with Example 3, is different in that the chitosan-modified metakaolin is replaced with ordinary metakaolin, and it includes the following components in parts by weight:

[0103] Metakaolin: 68 parts,

[0104] Microencapsulated latex powder: 12 parts,

[0105] Calcium sulfoaluminate clinker: 8 parts,

[0106] Modified polypropylene short fibers: 4 parts.

[0107] After the admixture is incorporated, it accounts for 4% by weight of the total amount of cementitious materials in the concrete.

[0108] Comparative Example 2

[0109] An admixture for concrete, compared with Example 3, is different in that the microencapsulated latex powder is replaced with ordinary redispersible latex powder, and it includes the following components in parts by weight:

[0110] Chitosan-modified metakaolin: 68 parts,

[0111] Redispersible latex powder: 12 parts,

[0112] Calcium sulfoaluminate clinker: 8 parts,

[0113] Modified polypropylene short fibers: 4 parts.

[0114] After the admixture is incorporated, it accounts for 4% by weight of the total amount of cementitious materials in the concrete.

[0115] Comparative Example 3

[0116] An admixture for concrete, compared with Example 3, is different in that the calcium sulfoaluminate clinker is replaced with calcium oxide clinker, and it includes the following components in parts by weight:

[0117] Chitosan-modified metakaolin: 68 parts,

[0118] Microencapsulated latex powder: 12 parts,

[0119] Calcium oxide clinker: 8 parts,

[0120] Modified polypropylene short fibers: 4 parts.

[0121] After the admixture is incorporated, it accounts for 4% by weight of the total amount of cementitious materials in the concrete.

[0122] Comparative Example 4

[0123] An admixture for concrete, which is different from Example 3 in that the modified polypropylene short fibers are replaced with ordinary polypropylene short fibers, and it comprises the following components in parts by weight:

[0124] Chitosan-modified metakaolin: 68 parts,

[0125] Redispersible latex powder: 12 parts,

[0126] Calcium sulfoaluminate clinker: 8 parts,

[0127] Short polypropylene fibers: 4 parts.

[0128] After the admixture is incorporated, it accounts for 4% of the total amount of cementitious materials in the concrete by weight.

[0129] Effect Example 1

[0130] The concrete admixtures prepared in Examples 1, 2, 3 and Comparative Examples 1, 2, 3, 4 were used to replace fly ash in the cementitious materials of the concrete in an equal-mass internal addition method to prepare building formwork concrete for performance comparison. The mix ratios of different building formwork concretes are shown in Table 1, where Blank Example 1 is the building formwork concrete without admixture.

[0131] Table 1 Concrete mix ratio (kg / m 3 )

[0132]

[0133] The properties of the concretes in Examples 1 - 3, Comparative Examples 1 - 4 and Blank Example 1 were tested, and the test results are shown in Table 2. The slump flow was tested according to GB / T 50080 - 2016 "Standard Test Method for Performance of Ordinary Concrete Mixtures"; the 28-day compressive strength was tested according to GB / T 50081 - 2019 "Standard Test Method for Physical and Mechanical Properties of Concrete"; the 28-day drying shrinkage rate was tested according to "Test Regulations for Cement and Cement Concrete in Highway Engineering"; the 28-day unit cracking area of the concrete was tested according to the early-age cracking test of concrete in GB / T 50082 - 2024 "Standard Test Method for Long-Term Performance and Durability of Concrete".

[0134] The thixotropy test method is as follows: The Brookfield RST-SST rheometer is used to test the thixotropic properties of fresh concrete paste. After the paste is thoroughly mixed evenly in the mixer, it is immediately transferred to the rheometer for testing. Within 0 - 90 seconds, the rotational speed linearly increases from 0 to 60 r / min, during which the flocculated structure inside the paste is gradually destroyed; subsequently, the rotational speed linearly decreases from 60 r / min to 0, and the internal structure of the paste is gradually rebuilt. Through the above test process, the torque-rotation speed hysteresis curve (i.e., thixotropic loop) of the paste can be obtained, and the thixotropic properties of the paste can be quantitatively evaluated based on the area enclosed by the thixotropic loop.

[0135] The test method for the bond strength of the formwork-net interface is as follows: Cubic concrete specimens with dimensions of 150 mm×150 mm×150 mm are prepared, and a formwork-net with dimensions of 200 mm×150 mm and a formwork-net pore size of 10.0 mm×4.5 mm is embedded in them. After curing for 28 days under standard curing conditions, the specimen is fixed on a pull-out testing machine. The exposed part of the steel formwork-net is clamped using a fixture, and a uniaxial pull-out test is carried out with a constant loading rate of 1 mm / min. Record the maximum pull-out load value F at failure and calculate the interface bond strength P based on the effective bond area S. The calculation formula is as follows:

[0136]

[0137] The test method for the paste bleeding rate is as follows: A formwork-net with a designed length×width×height = 600 mm×150 mm×400 mm is used, and the formwork-net pore size is 10.0 mm×4.5 mm. The specific test method is to pour the trial-mixed fresh mixture into the formwork-net at different heights and test the bleeding situation of the paste in the formwork-net. Calculation method: Weigh the mass m1 of the mixture, pour it into the formwork-net, and let it freely flow into the steel formwork-net. After waiting for 5 minutes, the paste that bleeds out from the formwork-net is weighed, and the mass is m2. Calculate the paste bleeding rate. The calculation formula for the paste bleeding rate is as follows:

[0138]

[0139] Table 2 Test Results of Concrete Properties

[0140]

[0141] Combined with Blank Example 1 and Examples 1-3, it can be seen that after adding the admixture for architectural formwork concrete of the present invention, the mechanical properties, thixotropy, workability, interfacial bonding strength, 28-day dry shrinkage rate, crack resistance and paste bleeding rate of the concrete have been significantly improved. This shows that by adding the admixture for architectural formwork concrete of the present invention, the thixotropy of the architectural formwork concrete and its interfacial bonding force with the steel formwork can be effectively enhanced, and at the same time, the dry shrinkage cracking problem caused by the excessive amount of binder in the architectural formwork concrete can be alleviated. Moreover, through the reasonable proportioning of each component in the present invention, the thixotropy, interfacial bonding strength and dry shrinkage cracking resistance of the architectural formwork concrete can be targeted and regulated according to the actual performance requirements of the architectural formwork concrete.

[0142] Combined with Example 3 and Comparative Examples 1-4, it can be seen that when ordinary metakaolin is used instead of chitosan-modified metakaolin in the proportion of the present invention, the improvement effect of the admixture on the thixotropy of the concrete decreases; when ordinary redispersible latex powder is used instead of microencapsulated latex powder in the proportion of the present invention, the fluidity of the concrete decreases significantly after adding the admixture. When the fluidity of the concrete is insufficient, the pumping and filling of the formwork cannot be smoothly completed during construction, so the concrete cannot be used in the construction of architectural formwork at this time; when calcium oxide clinker is used instead of calcium sulfoaluminate clinker in the proportion of the present invention, the 28-day dry shrinkage rate of the concrete decreases after adding the admixture, but the paste bleeding rate increases significantly, which will affect the density of the solid structure of the architectural formwork concrete during actual construction application, thereby reducing the performance of the solid structure; when ordinary polypropylene chopped fibers are used instead of modified polypropylene chopped fibers in the proportion of the present invention, the crack resistance of the concrete decreases after adding the admixture. This is because the unmodified polypropylene chopped fibers cannot be well dispersed in the concrete, so they cannot effectively form a "bridging" network structure in the concrete to inhibit crack growth and enhance the crack resistance of the concrete.

[0143] The specific embodiments of the present invention have been described above. It should be understood that the present invention is not limited to the above specific embodiments, and those skilled in the art can make various deformations or modifications within the scope of the claims, which does not affect the essence of the present invention.

Claims

1. An admixture for architectural formwork concrete, characterized in that, Comprising the following components in parts by weight: Chitosan-modified metakaolin: 65 - 70 parts, Microencapsulated latex powder: 10 - 15 parts, Calcium sulfoaluminate clinker: 5 - 10 parts, Modified polypropylene short fibers: 3 - 5 parts; The preparation method of the chitosan-modified metakaolin includes: S1. Mix metakaolin with hydrochloric acid solution, stir and activate, then filter, wash with deionized water until neutral, and dry to obtain acidified metakaolin; S2. Add the acidified metakaolin, chitosan solution, and cationic surfactant prepared in step S1 into a reaction kettle, seal the system, and heat and react under nitrogen protection; S3. After the reaction is completed, cool, wash the product, and vacuum dry to obtain chitosan-modified metakaolin; The preparation method of the microencapsulated latex powder includes: A1. Take polyurethane as the polymer wall material, add it to deionized water, heat and stir to dissolve, and then add an emulsifier and stir to form an emulsification system; A2. Slowly add the redispersible latex powder into the emulsification system and stir to form a coated emulsion; A3. Spray-dry the coated emulsion and collect to obtain microencapsulated latex powder.

2. The admixture for building formwork concrete according to claim 1, characterized in that, Step S1 includes at least one of the following technical features: (1) By weight, take 40 - 60 parts of the metakaolin and 140 - 160 parts of the hydrochloric acid solution, and the concentration of the hydrochloric acid solution is 3% - 5%; (2) The temperature of the stirring and activation is 50 - 70 °C, and the time is 1 - 3 hours; (3) The temperature of the drying is 70 - 90 °C, and the time is 3 - 5 hours.

3. The admixture for architectural formwork concrete according to claim 1, characterized in that, Step S2 includes at least one of the following technical features: (1) By weight, take 20 - 40 parts of the acidified metakaolin, 90 - 110 parts of the chitosan solution, and 6 - 10 parts of the cationic surfactant; (2) The cationic surfactant is selected from at least one of octadecyl dimethyl benzyl ammonium chloride, cetylpyridinium bromide, and benzyl triethyl ammonium chloride; (3) The temperature of the heating and reaction is 60 - 80 °C, and the time is 100 - 140 minutes; (4) The concentration of the chitosan solution is 3% - 5%; its preparation method is: by weight, dissolve 6 - 10 parts of chitosan in 186 - 198 parts of lactic acid solution with a concentration of 2% - 3%, and stir at 35 - 45 °C until completely dissolved.

4. The admixture for architectural formwork concrete according to claim 1, characterized in that, Step S3 includes at least one of the following technical features: (1) The temperature of the cooling is 20 - 30 °C; (2) The detergent used for washing includes 60 - 80% ethanol; (3) The temperature of the vacuum drying is 50 - 70 °C, and the time is 5 - 7 hours.

5. The admixture for architectural formwork concrete according to claim 1, characterized in that, When preparing the microencapsulated latex powder, by weight, take 40 - 60 parts of the redispersible latex powder, 15 - 25 parts of polyurethane, 8 - 12 parts of emulsifier, and 180 - 220 parts of deionized water; The emulsifier is selected from at least one of Tween - 80, Tween - 60, and Tween - 40; The redispersible latex powder is selected from at least one of polyvinyl acetate latex powder, polyvinyl alcohol latex powder, and acrylate latex powder.

6. The admixture for architectural formwork concrete according to claim 1, characterized in that, When preparing the microencapsulated latex powder, it includes at least one of the following technical features: (1) In step A1, the heating temperature for heating and stirring to dissolve is 50 - 70 °C; (2) In step A2, the rotation speed of the stirring is 1000 - 2000 rpm, and the time is 25 - 30 minutes; (3) In step A3, the spray drying is carried out by a spray drying device, the inlet air temperature is 110 - 130 °C, and the outlet air temperature is 50 - 70 °C; (4) After the microencapsulated latex powder is prepared, it is vacuum dried, and then an antioxidant is added. The temperature of the vacuum drying is 35 - 45 °C, and the time is 1.5 - 2.5 hours; the antioxidant is dibutylhydroxytoluene.

7. The admixture for building formwork concrete according to claim 1, characterized in that, The specific surface area of the calcium sulfoaluminate clinker is 430 - 490 m 2 / kg.

8. The admixture for architectural formwork concrete according to claim 1, characterized in that, The modified polypropylene short fiber is a silane-modified polypropylene short fiber, and its preparation method is: immersing the polypropylene short fiber in a KH-570 silane coupling agent solution with a concentration of 0.5 - 1% for soaking treatment, taking it out, rinsing with deionized water, and drying to constant weight to obtain the silane-modified polypropylene short fiber; The length of the polypropylene short fiber is 6 - 12 mm, and the diameter is 10 - 20 μm; And / or, the time of the soaking treatment is 30 - 60 minutes; And / or, the temperature of the drying is 70 - 90 °C.

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