Additive for building formwork concrete

Abstract

The invention discloses an admixture for building formwork concrete, and belongs to the technical field of concrete materials. The admixture for the building formwork concrete, disclosed by the invention, comprises the following components in parts by weight: 65-70 parts of chitosan modified metakaolin; 10 to 15 parts of microencapsulated latex powder; 5 to 10 parts of calcium sulphoaluminate clinker; and 3-5 parts of modified polypropylene chopped fiber. Through reasonable proportioning of the components, the synergistic effect of the different components is brought into full play, and the invention aims to provide the additive for the building formwork concrete, which can significantly improve the thixotropic property, the interface bonding force and the crack resistance of the building formwork concrete and effectively prevent the corrosion of a steel formwork at the same time. The method is suitable for building formwork concrete engineering of a complex grid structure and has remarkable 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: For example, the Chinese invention patent (CN107200500A) discloses a special admixture for self-compacting, shrink-free steel tube concrete, which contains 30-50% light-burned calcium oxide clinker, 10-30% light-burned magnesium oxide clinker, 10-30% gypsum, 10-30% fly ash, 1-3% polycarboxylic acid high-performance water reducer, 0.1-1.0% thixotropic agent, 0.2-2.0% plastic gasifier, 0.2-1.0% retarder, and 0.5-2.0% defoamer. Although the patent mentions that the admixture can significantly improve the workability of concrete, enhance thixotropy, compensate for shrinkage deformation, and ensure its dense filling in the steel tube structure, so that the concrete and the steel tube wall are synergistically stressed. However, the admixture expansion agent component in this patent is too high and the expansion effect is too strong. Its application in building formwork concrete will cause the concrete slurry to overflow from the formwork holes during the hardening process, thereby affecting the quality of the physical structure.

[0007] Another example is the Chinese invention patent (CN110937842A) which discloses an admixture for improving the crack resistance and thixotropy of super-high-rise pumped concrete. The mass percentage of each component is: silica fume 49% to 52%, fly ash microspheres 24% to 29%, ultrafine limestone powder 5% to 7%, modified montmorillonite 10% to 12%, and microcrystalline cellulose modified super absorbent resin fiber 3% to 5%. It can significantly improve the pumpability, thixotropy and crack resistance of high-rise pumped concrete, reduce the construction difficulty of special structures and the risk of pipe blockage. The admixture is composed of a variety of high-performance materials and is too expensive, which limits its application in large-scale projects. At the same time, the patent does not specify the optimal dosage range and its sensitivity to concrete performance. Since the admixture contains a variety of functional components, slight changes in its dosage may significantly affect the working performance, thixotropy and crack resistance of concrete, increasing the difficulty of control in practical applications.

[0008] Another example is a Chinese invention patent (CN119100650A) that discloses a thixotropic agent for ballastless track base plate concrete. Its components are (by weight): 0.5-4 parts of attapulgite, 0.5-5 parts of sepiolite, 0.02-0.38 parts of fumed silica, 2-30 parts of silica fume, 0.06-0.5 parts of triterpenoid saponin, 0.03-0.5 parts of cellulose ether, and 0-2.5 parts of fiber. The thixotropic agent can greatly improve the static morphological stability of fresh concrete, and it can be formed into a fixed slope after pouring; at the same time, it does not increase the dynamic deformation energy, and has good rheological properties during pumping, pouring, vibrating and other constructions, making it easy to construct. This thixotropic agent achieves slope shaping by increasing the static yield stress of fresh concrete, while reducing the dynamic yield stress to ensure the rheological properties during construction. The requirements for static shape retention ability of building formwork concrete are not so strict, because the concrete is fixed by the formwork during construction and does not need to rely on its own characteristics to maintain the slope like the ballastless track base plate. The design goal of this thixotropic agent does not meet the actual needs of building formwork concrete, and the functionality does not match. Summary of the invention

[0009] In order to solve the above technical problems, the purpose of the present invention is to provide an admixture for building formwork concrete, which can improve the thixotropy and crack resistance of building formwork concrete, enhance the interfacial adhesion between concrete and steel formwork, and effectively prevent corrosion of steel formwork.

[0010] The objective of the present invention is achieved through the following technical solutions: The present invention provides an admixture for building formwork concrete, comprising the following components in parts by weight: Chitosan modified metakaolin: 65-70 parts, Microencapsulated latex powder: 10-15 parts, Calcium sulphoaluminate clinker: 5-10 parts, Modified polypropylene chopped fibers: 3-5 parts.

[0011] As some specific embodiments of the present invention, the preparation method of the chitosan-modified metakaolin includes: S1. Mixing metakaolin with hydrochloric acid solution, stirring to activate, filtering, washing with deionized water until neutral, and drying to obtain acidified metakaolin; S2, adding the acidified metakaolin, chitosan solution, and cationic surfactant prepared in step S1 into a reaction kettle, sealing the system and heating the reaction under nitrogen protection; S3. After the reaction is completed, the product is cooled, washed, and vacuum dried to obtain chitosan-modified metakaolin.

[0012] As some specific implementations of the present invention, 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 a hydrochloric acid solution, wherein the concentration of the hydrochloric acid solution is 3%-5%; (2) The stirring activation temperature is 50-70°C and the time is 1-3 hours; (3) The drying temperature is 70-90°C and the drying time is 3-5 hours.

[0013] As some specific implementations of the present invention, 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 chitosan solution, and 6-10 parts of a cationic surfactant; (2) The cationic surfactant is at least one selected from octadecyl dimethyl benzyl ammonium chloride, cetyl pyridinium bromide, and benzyl triethyl ammonium chloride; preferably octadecyl dimethyl benzyl ammonium chloride; (3) The temperature of the temperature-raising reaction is 60-80°C, the time is 100-140 minutes, and the reaction is continuously stirred in the reactor.

[0014] 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, 6-10 parts of chitosan are dissolved in 186-198 parts of 2-3% lactic acid solution, and stirred at 35-45°C until completely dissolved.

[0015] As some specific implementations of the present invention, step S3 includes at least one of the following technical features: (1) The cooling temperature is 20-30°C; (2) The detergent used for washing includes 60-80% ethanol; (3) The vacuum drying temperature is 50-70°C and the time is 5-7 hours.

[0016] As some specific embodiments of the present invention, the preparation method of the chitosan-modified kaolin is as follows: by weight, 50 parts of kaolin are mixed with 150 parts of a 4% hydrochloric acid solution, stirred and activated at 60°C for 2 hours, filtered and repeatedly washed with deionized water until neutral, and dried at 80°C for 4 hours to obtain acidified kaolin, and the active sites on the surface of the kaolin are enhanced by acidification treatment to promote its combination with chitosan and cationic surfactants; 30 parts of acidified kaolin are added to a reactor, and 100 parts of a 4% chitosan solution and 8 parts of a cationic surfactant octadecyl dimethyl benzyl ammonium chloride are added at the same time, and then the system is sealed, heated to 70°C under nitrogen protection, and the reaction is stirred for 120 minutes, and the cationic surfactant is used to bridge the chitosan and kaolin through electrostatic action to enhance the shell The adsorption capacity of chitosan on the surface of metakaolin is improved, thereby improving the loading efficiency; after the reaction is completed, the product is cooled to room temperature, washed with 70% ethanol solution, and finally vacuum dried at 60°C for 6 hours to obtain chitosan-modified metakaolin. Chitosan is a natural high-molecular polysaccharide with good film-forming and adhesion properties, and can significantly improve the thixotropy of concrete. Metakaolin plays a lubricating and filling role in concrete, optimizes particle grading, and reduces porosity. Modification of metakaolin by chitosan can significantly improve the dispersibility and stability of metakaolin in cement-based materials. The metakaolin modified by chitosan can significantly enhance the thixotropic properties of concrete and prevent segregation and bleeding; the preparation method of the chitosan solution is: 8 parts of chitosan are dissolved in 192 parts of 3% lactic acid solution, and stirred at 40°C until completely dissolved.

[0017] As some specific embodiments of the present invention, the preparation method of the microencapsulated latex powder includes: A1. Add polyurethane as the polymer wall material into deionized water, heat and stir to dissolve, then add emulsifier and stir to form an emulsified system; A2. Slowly add the redispersible latex powder into the emulsification system and stir to form a coating emulsion; A3. Spray-dry the coated latex and collect to obtain microencapsulated latex powder.

[0018] 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; 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.

[0019] As some specific embodiments of the present invention, in step A1, the heating temperature for heating, stirring and dissolving is 50-70°C.

[0020] As some specific embodiments of the present invention, in step A2, the stirring speed is 1000-2000 rpm, and the time is 25-30 minutes.

[0021] As some specific embodiments of the present invention, in step A3, the spray drying is performed by a spray drying device, the inlet air temperature is 110-130°C, and the outlet air temperature is 50-70°C.

[0022] 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 storage stability.

[0023] Furthermore, the vacuum drying is carried out at a temperature of 35-45° C. and for a time of 1.5-2.5 hours.

[0024] Furthermore, the antioxidant is butylated hydroxytoluene.

[0025] 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, stirred and dissolved at 60°C, and stirred for 10 minutes after the emulsifier is added to form a stable emulsified system, the polymer wall material polyurethane can gradually dissolve and release the redispersible latex powder component during the cement hydration process, and the emulsifier ensures the stability of the wall material solution in the subsequent coating process to avoid agglomeration of the latex powder particles; the redispersible latex powder is slowly added to the emulsified system at 1500°C. The mixture was stirred at a speed of rpm for 30 minutes; the coated latex was dried by a spray drying device with an inlet air temperature of 120°C and an outlet air temperature of 60°C, and microencapsulated latex powder was collected. The spray drying technology can quickly solidify the microcapsule structure, avoid decomposition of the latex powder at high temperature, and ensure the sphericity and fluidity of the particles; the dried microencapsulated latex powder was vacuum dried at 40°C for 2 hours, and the antioxidant butylated hydroxytoluene was added to improve the storage stability; the final microencapsulated latex powder product had a particle size range of 10-50 μm and a wall material thickness of 1-5 μm, and the latex powder component could be gradually released under the conditions of pH>12 or temperature>60°C. The release of the latex powder was delayed by the microencapsulation technology, thereby ensuring that the redispersible latex powder would not significantly increase the initial viscosity of the concrete during the mixing process, so that the latex powder would gradually play a role in the cement hydration process, provide long-term interface adhesion and anti-corrosion protection, and effectively reduce slurry leakage, thereby improving the overall performance of the concrete.

[0026] As some specific embodiments of the present invention, the calcium sulphoaluminate clinker has a specific surface area of ​​430-490m 2 / kg. Calcium sulphoaluminate clinker can produce moderate volume expansion during the hardening process of concrete, effectively compensating for the shrinkage of concrete caused by drying shrinkage and temperature changes, thereby reducing the occurrence of cracks and improving the integrity and durability of the structure.

[0027] As some specific embodiments of the present invention, the preparation method of the silane-modified polypropylene chopped fibers is: The polypropylene short fibers are immersed in a 0.5-1% KH-570 silane coupling agent solution, taken out, rinsed with deionized water, and dried to constant weight to obtain silane-modified polypropylene short fibers.

[0028] As some specific embodiments of the present invention, the length of the polypropylene chopped fibers is 6-12 mm and the diameter is 10-20 μm; And / or, the soaking time is 30-60 minutes; And / or, the drying temperature is 70-90°C.

[0029] As some specific embodiments of the present invention, the preparation method of the silane-modified polypropylene short fibers is as follows: immerse the polypropylene short fibers with a length of 6-12 mm and a diameter of 10-20 μm in a KH-570 silane coupling agent solution with a concentration of 0.5-1% for 30-60 minutes, rinse with deionized water after taking out, and dry to constant weight at 80°C to obtain surface-modified polypropylene short fibers, which play the role of "bridging" cracks in concrete and can effectively inhibit the expansion of microcracks. After the fiber is surface treated with KH-570 silane coupling agent, the hydrophilicity of the fiber surface is improved, making it easier to be evenly dispersed in cement paste. This uniform dispersion helps to optimize the microstructure of concrete and further improve the strength and toughness of the material.

[0030] As some specific embodiments of the present invention, when the admixture is added to the building formwork concrete, it accounts for 3-6% of the total amount of cementitious materials in the building formwork concrete by weight percentage.

[0031] Compared with the prior art, the present invention has the following beneficial effects: 1) The present invention enhances the adsorption capacity of chitosan on the surface of metakaolin and improves the chitosan loading efficiency by acidification treatment and treatment with cationic surfactants. Metakaolin as a carrier can provide stable attachment points for chitosan. The loaded chitosan can be more evenly distributed in the entire system. Chitosan molecules are attached to the surface of metakaolin particles by physical adsorption or chemical bonding, which not only changes the surface charge characteristics of metakaolin particles, but also prevents particle agglomeration through steric hindrance effect, thereby improving the dispersibility and stability of metakaolin in fresh concrete. Chitosan increases the viscosity of the slurry, forms a dynamic network structure, and improves the dispersibility of particles; metakaolin reacts with cement hydration products through its volcanic ash activity to generate more CSH gel, enhances the cohesion and structural stability of the slurry, and the ability of both to improve the rheological properties of concrete can be fully utilized by the means of chitosan modification of metakaolin, significantly improving the thixotropy of concrete.

[0032] 2) The present invention adopts microencapsulation technology to add redispersible latex powder, which can gradually release the latex powder components during the cement hydration process, which can not only avoid the problem of excessive initial viscosity of fresh concrete due to premature dissolution of latex powder, but also improve the interfacial adhesion between concrete and steel formwork during the hardening process of concrete, reduce slurry leakage from the casting to the hardening process of building formwork concrete, and improve the density of the building formwork concrete entity structure.

[0033] 3) The present invention uses KH-570 silane coupling agent to modify polypropylene chopped fibers, thereby improving the hydrophilicity and dispersibility of the fiber surface, so that it forms a uniformly distributed "bridge" 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 produces a moderate volume expansion during the concrete hardening process, compensating for the shrinkage caused by shrinkage, further reducing the generation of cracks, and more tightly filling the gap between the steel formwork and concrete. Through the synergistic effect of fiber and expansion agent, the technical problem of concrete shrinkage and easy cracking caused by excessive use of cementitious materials in building formwork concrete is solved. DETAILED DESCRIPTION

[0034] The present invention is 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 are not intended to limit the present invention in any form. It should be noted that, for those of ordinary skill in the art, several variations and improvements may be made without departing from the concept of the present invention. These all belong to the protection scope of the present invention.

[0035] The raw materials used in the following examples, comparative examples and application tests are as follows: (1) Cement: 42.5 grade ordinary Portland cement; (2) Fly ash: Class C, Grade II fly ash; (3) Metakaolin: specific surface area 672 m 2 / kg, the main component is silicon aluminum oxide, its SiO 2 and Al 2 O 3 The contents were 50.67% and 44.34% respectively; (4) Coarse aggregate: particle size is 5-25 mm, with continuous grading; (5) Artificial sand: fineness modulus is 2.7, MB is 0.8, and stone powder content is 6.4%; (6) Water reducing agent: polycarboxylic acid high performance water reducing agent with a water reducing rate of 25% and a solid content of 15%; (7) Redispersible latex powder: polyvinyl acetate latex powder, solid content 99%, ash content 12%, bulk density 500g / l, average particle size d 50 70 μm; (8) Calcium sulphoaluminate clinker: specific surface area 460 m 2 / kg; (9) Calcium oxide clinker: specific surface area is 300 m 2 / kg; (9) Polypropylene chopped fibers: length 6-12 mm, diameter 10-20 μm, tensile strength ≥500 MPa; (10) Mixing water: ordinary tap water.

[0036] The chitosan-modified metakaolin used in the embodiments and comparative examples of the present invention is prepared by the following steps: According to weight, 50 parts of metakaolin were mixed with 150 parts of 4% hydrochloric acid solution, stirred and activated at 60°C for 2 hours, filtered and repeatedly washed with deionized water until neutral, and dried at 80°C for 4 hours to obtain acidified metakaolin; 30 parts of acidified metakaolin were added to a reactor, and 100 parts of 4% chitosan solution and 8 parts of cationic surfactant octadecyl dimethyl benzyl ammonium chloride were added at the same time, and then the system was sealed, heated to 70°C under nitrogen protection, and stirred for 120 minutes; after the reaction was completed, it was cooled to room temperature, the product was washed with 70% ethanol solution, and finally vacuum dried at 60°C for 6 hours to obtain metakaolin loaded chitosan; the preparation method of chitosan solution was as follows: 8 parts of chitosan was dissolved in 192 parts of 3% lactic acid solution, and stirred at 40°C until completely dissolved.

[0037] The microencapsulated latex powder used in the embodiments and comparative examples of the present invention is prepared by the following steps: The invention relates to a method for preparing a microcapsule-encapsulated latex powder. The method comprises the following steps: weighing 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 by weight; adding the polymer wall material into the deionized water, stirring and dissolving the polymer wall material at 60°C, adding the emulsifier and stirring for 10 minutes to form a stable emulsified system; slowly adding the redispersible latex powder into the emulsified system, stirring at a speed of 1500 rpm for 30 minutes; drying the coated latex by a spray drying device, with an inlet air temperature of 120°C and an outlet air temperature of 60°C, and collecting the microcapsulated latex powder; vacuum drying the dried microcapsulated latex powder at 40°C for 2 hours, and adding an antioxidant dibutylhydroxytoluene to improve the storage stability; finally obtaining a microcapsulated latex powder product, wherein the particle size ranges from 10 to 50 μm, and the wall material thickness is from 1 to 5 μm.

[0038] The silane-modified polypropylene chopped fibers used in the embodiments and comparative examples of the present invention are prepared by the following steps: The preparation method of the silane-modified polypropylene chopped fibers is as follows: immersing polypropylene chopped fibers with a length of 6 to 12 mm and a diameter of 10 to 20 μm in a 1% KH-570 silane coupling agent solution for 40 minutes, taking them out and rinsing them with deionized water, and drying them at 80° C. to constant weight to obtain surface-modified polypropylene chopped fibers.

[0039] Example 1 Provided is an admixture for building formwork concrete, comprising the following components in parts by weight: Chitosan modified metakaolin: 70 parts, Microencapsulated latex powder: 10 parts, Calcium sulphoaluminate clinker: 5 parts, Modified polypropylene chopped fibers: 5 parts.

[0040] After the admixture is added, it accounts for 6% of the total amount of cementitious materials in the concrete by weight.

[0041] Example 2 Provided is an admixture for building formwork concrete, comprising the following components in parts by weight: Chitosan modified metakaolin: 65 parts, Microencapsulated latex powder: 15 parts, Calcium sulphoaluminate clinker: 10 parts, Modified polypropylene chopped fibers: 3 parts.

[0042] After the admixture is added, it accounts for 3% of the total amount of cementitious materials in the concrete by weight.

[0043] Example 3 Provided is an admixture for building formwork concrete, comprising the following components in parts by weight: Chitosan modified metakaolin: 68 parts, Microencapsulated latex powder: 12 parts, Calcium sulphoaluminate clinker: 8 parts, Modified polypropylene chopped fibers: 4 parts.

[0044] After the admixture is added, it accounts for 4% of the total amount of cementitious materials in the concrete by weight.

[0045] Comparative Example 1 A concrete admixture, compared with Example 3, the difference is that the chitosan-modified metakaolin is replaced with ordinary metakaolin, comprising the following components in parts by weight: Metakaolin: 68 parts, Microencapsulated latex powder: 12 parts, Calcium sulphoaluminate clinker: 8 parts, Modified polypropylene chopped fibers: 4 parts.

[0046] After the admixture is added, it accounts for 4% of the total amount of cementitious materials in the concrete by weight.

[0047] Comparative Example 2 A concrete admixture, compared with Example 3, differs in that the microencapsulated latex powder is replaced with ordinary redispersible latex powder, and comprises the following components in parts by weight: Chitosan modified metakaolin: 68 parts, Redispersible latex powder: 12 parts, Calcium sulphoaluminate clinker: 8 parts, Modified polypropylene chopped fibers: 4 parts.

[0048] After the admixture is added, it accounts for 4% of the total amount of cementitious materials in the concrete by weight.

[0049] Comparative Example 3 A concrete admixture, compared with Example 3, the difference is that the calcium sulfoaluminate clinker is replaced by calcium oxide clinker, comprising the following components in parts by weight: Chitosan modified metakaolin: 68 parts, Microencapsulated latex powder: 12 parts, Calcium oxide clinker: 8 parts, Modified polypropylene chopped fibers: 4 parts.

[0050] After the admixture is added, it accounts for 4% of the total amount of cementitious materials in the concrete by weight.

[0051] Comparative Example 4 A concrete admixture, compared with Example 3, differs in that the modified polypropylene chopped fibers are replaced with ordinary polypropylene chopped fibers, and comprises the following components in parts by weight: Chitosan modified metakaolin: 68 parts, Redispersible latex powder: 12 parts, Calcium sulphoaluminate clinker: 8 parts, Polypropylene chopped fibers: 4 parts.

[0052] After the admixture is added, it accounts for 4% of the total amount of cementitious materials in the concrete by weight.

[0053] Effect Example 1 The concrete admixtures prepared in Examples 1, 2, 3 and Comparative Examples 1, 2, 3, and 4 were used to replace the fly ash in the concrete cementitious material in an internal mixing manner to prepare building formwork concrete for performance comparison. The mix proportions of different building formwork concretes are shown in Table 1, wherein Blank Example 1 is the building formwork concrete without the admixture.

[0054] Table 1 Concrete mix ratio (kg / m 3 )

[0055] The concrete of Examples 1-3, Comparative Examples 1-4 and Blank Example 1 was tested for performance, and the test results are shown in Table 2. The expansion was tested with reference to GB / T 50080-2016 "Standard for Test Methods for Performance of Ordinary Concrete Mixtures"; the 28d compressive strength was tested with reference to GB / T 50081-2019 "Standard for Test Methods for Physical and Mechanical Properties of Concrete"; the 28d drying shrinkage was tested with reference to the "Test Procedures for Cement and Cement Concrete for Highway Engineering"; the 28d unit cracking area of ​​concrete was tested with reference to the early crack resistance test of concrete in the standard GB / T 50082-2024 "Standard for Test Methods for Long-term Performance and Durability of Concrete".

[0056] The thixotropy test method is as follows: The thixotropy of the fresh concrete paste is tested using a Brookfield RST-SST rheometer. After the paste is fully mixed in the mixer, it is immediately transferred to the rheometer for testing. Within 0 to 90 seconds, the speed increases linearly from 0 to 60 r / min, during which the flocculation structure inside the paste is gradually destroyed; then, the speed decreases linearly from 60 r / min to 0, and the internal structure of the paste is gradually rebuilt. Through the above test process, the torque-speed hysteresis curve (i.e., thixotropic ring) 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 ring.

[0057] The test method of the interface bonding strength of the mold mesh is as follows: prepare a cubic concrete test block with a size of 150 mm×150 mm×150 mm, and embed a mold mesh with a size of 200 mm×150 mm and a mold mesh aperture of 10.0 mm×4.5 mm in it. After curing for 28 days under standard curing conditions, fix the sample on a pull-out test machine, use a clamp to clamp the exposed part of the steel mold mesh, and set a constant loading rate of 1 mm / min for a uniaxial pull-out test. Record the maximum pull-out load value F at the time of failure and calculate the interface bonding strength P based on the effective bonding area S. The calculation formula is as follows:

[0058] The test method for slurry seepage rate is as follows: a mesh with a design length × width × height = 600 mm × 150 mm × 400 mm is used, and the mesh aperture size is 10.0 mm × 4.5 mm. The specific test method is to pour the newly prepared mixture into the mesh at different heights to test the slurry seepage of the mesh. Calculation method: weigh the mass m1 of the mixture, pour it into the mesh, let it flow freely into the steel mesh, wait for 5 minutes, and the slurry will seep out of the mesh, weigh the mass m2, and calculate the slurry seepage rate. The slurry seepage rate calculation formula is as follows:

[0059] Table 2 Concrete performance test results

[0060] Combining Blank Example 1 with Examples 1-3, it can be seen that after adding the admixture for building formwork concrete of the present invention, the mechanical properties, thixotropy, workability, interface bonding strength, 28d shrinkage rate, crack resistance and slurry exudation rate of concrete have been significantly improved, indicating that by adding the admixture for building formwork concrete of the present invention, the thixotropy of building formwork concrete and the interface bonding force between it and the steel formwork can be effectively enhanced, while reducing the shrinkage cracking problem caused by the excessive amount of adhesive used in building formwork concrete. And through the reasonable proportion of each component in the present invention, targeted regulation can be carried out according to the actual performance requirements of building formwork concrete to control the thixotropy, interface bonding strength and shrinkage cracking resistance of building formwork concrete.

[0061] Combining Example 3 with Comparative Examples 1-4, it can be seen that when the chitosan-modified metakaolin in the ratio of the present invention is not used, but ordinary metakaolin is used, the effect of the admixture on improving the thixotropy of the concrete is reduced; when the microencapsulated latex powder in the ratio of the present invention is not used, but ordinary redispersible latex powder is used, the fluidity of the concrete after adding the admixture is significantly reduced, and when the concrete fluidity is insufficient, it is impossible to successfully complete the construction pumping and fill the formwork, so the concrete cannot be used in the construction of the building formwork; when the calcium sulfoaluminate clinker in the ratio of the present invention is not used, but calcium oxide clinker is used When the modified polypropylene chopped fibers in the present invention are not used but ordinary polypropylene chopped fibers are used, the crack resistance of the concrete is reduced after the admixture is added. This is because the unmodified polypropylene chopped fibers cannot be well dispersed in the concrete, and therefore cannot effectively form a "bridging" network structure in the concrete, thereby inhibiting crack growth and enhancing the crack resistance of the concrete.

[0062] The above describes the specific embodiments of the present invention. It should be understood that the present invention is not limited to the above specific embodiments, and those skilled in the art may make various modifications or variations within the scope of the claims, which do not affect the essence of the present invention.

Claims

1. An admixture for building formwork concrete, characterized in that: The composition comprises the following components in parts by weight: Chitosan modified metakaolin: 65-70 parts, Microencapsulated latex powder: 10-15 parts, Calcium sulphoaluminate clinker: 5-10 parts, Modified polypropylene chopped fibers: 3-5 parts; The preparation method of the chitosan-modified metakaolin comprises: S1. Mixing metakaolin with hydrochloric acid solution, stirring to activate, filtering, washing with deionized water until neutral, and drying to obtain acidified metakaolin; S2, adding the acidified metakaolin, chitosan solution, and cationic surfactant prepared in step S1 into a reaction kettle, sealing the system and heating the reaction under nitrogen protection; S3. After the reaction is completed, the product is cooled, washed, and vacuum dried to obtain chitosan-modified metakaolin.

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 a hydrochloric acid solution, wherein the concentration of the hydrochloric acid solution is 3%-5%; (2) The stirring activation temperature is 50-70°C and the time is 1-3 hours; (3) The drying temperature is 70-90°C and the drying time is 3-5 hours.

3. The admixture for building 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 chitosan solution, and 6-10 parts of a cationic surfactant; (2) The cationic surfactant is at least one selected from octadecyl dimethyl benzyl ammonium chloride, cetyl pyridinium bromide, and benzyl triethyl ammonium chloride; (3) The temperature of the temperature-raising reaction is 60-80°C and the time is 100-140 minutes; (4) The concentration of the chitosan solution is 3-5%. The preparation method is as follows: by weight, 6-10 parts of chitosan are dissolved in 186-198 parts of a 2-3% lactic acid solution, and stirred at 35-45°C until completely dissolved.

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

5. The admixture for building formwork concrete according to claim 1, characterized in that: The preparation method of the microencapsulated latex powder comprises: A1. Add polyurethane as the polymer wall material into deionized water, heat and stir to dissolve, then add emulsifier and stir to form an emulsified system; A2. Slowly add the redispersible latex powder into the emulsification system and stir to form a coating emulsion; A3. Spray-dry the coated latex and collect to obtain microencapsulated latex powder.

6. The admixture for building formwork concrete according to claim 5, characterized in that: By weight, take 40-60 parts of 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.

7. The admixture for building formwork concrete according to claim 5, characterized in that: Include at least one of the following technical features: (1) In step A1, the heating temperature for heating, stirring and dissolving is 50-70°C; (2) In step A2, the stirring speed 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, with an inlet air temperature of 110-130°C and an outlet air temperature of 50-70°C; (4) After the microencapsulated latex powder is obtained, it is vacuum dried and an antioxidant is added. The vacuum drying temperature is 35-45° C. and the time is 1.5-2.5 hours. The antioxidant is butylated hydroxytoluene.

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

9. The admixture for building formwork concrete according to claim 1, characterized in that: The modified polypropylene chopped fibers are silane-modified polypropylene chopped fibers, and the preparation method thereof is as follows: immersing the polypropylene chopped fibers in a 0.5-1% KH-570 silane coupling agent solution, taking them out, rinsing them with deionized water, and drying them to constant weight to obtain the silane-modified polypropylene chopped fibers; The length of the polypropylene chopped fibers is 6-12 mm and the diameter is 10-20 μm; And / or, the soaking time is 30-60 minutes; And / or, the drying temperature is 70-90°C.

10. The admixture for building formwork concrete according to claim 1, characterized in that: When the admixture is added into the building formwork concrete, the admixture for the building formwork concrete accounts for 3-6% of the total amount of the cementitious material in the building formwork concrete in terms of weight percentage.

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