A method for improving the rheological properties of PVA fiber and nano-silicon dioxide cement-based composite materials

Abstract

This invention discloses a method for improving the rheological properties of PVA fiber and nano-silica cement-based composite materials, belonging to the field of cement preparation technology. The improvement method involves adding PVA fibers with an aspect ratio of 100 to 300. By adjusting the aspect ratio of the PVA fibers, the rheological properties of the PVA fiber and nano-SiO2 cement-based composite materials are improved, which can effectively increase the strength of cement. It has the functions of adsorption, air entrainment, dispersion, wetting and solubilization, which can maximize the efficiency of cement use and ensure good cement fluidity.

Description

Technical Field

[0001] This invention relates to the field of cement preparation technology, and in particular to a method for improving the rheological properties of PVA fiber and nano-silica cement-based composite materials. Background Technology

[0002] Adding fibers is one of the most effective methods to improve the ductility and toughness of traditional cement-based materials and enhance their mechanical and durability properties. A new type of building material, using cement-based materials as a matrix and uniformly dispersing discontinuous fibers within it, is called fiber-reinforced cementitious composites (FRCC). Currently, the most commonly used fibers in engineering include steel fibers, polyvinyl alcohol fibers (PVA), basalt fibers, carbon fibers, polypropylene fibers, and glass fibers. Among these, PVA fibers have good dispersibility, can be uniformly distributed in cement-based materials, and possess excellent properties such as high strength, high elastic modulus, non-toxicity, good hydrophilicity, and excellent acid and alkali resistance. Therefore, they are often used as reinforcing materials to prepare fiber-reinforced cement-based composites (PVA-FRCC). Previous studies have shown that PVA-FRCC not only has high compressive strength, ductility, and toughness but also excellent durability. Meanwhile, PVA-FRCC can not only meet the requirements of large span, lightweight and long service life of building structures, but also significantly improve the service life of hydraulic structures and reduce the maintenance cost during service when used as a structural material, thus realizing the rational use of resources and protection of the ecological environment.

[0003] Due to their small particle size and large specific surface area, nanomaterials possess unique effects such as macroscopic quantum tunneling, surface effects, and small size effects. These superior properties have made nanomaterials highly sought after by scholars and engineers in the field of building materials, demonstrating a promising future. Currently, nanoparticles commonly used to improve the performance of cement-based composite materials mainly include nano-SiO2, nano-TiO2, nano-Fe2O3, and nano-CaCO3. Compared to other particles, nano-SiO2 not only acts as a filler but also reacts with Ca(OH)2 in cement-based composite materials to form hydrated calcium silicate gel (CSH). This reaction is exothermic, further accelerating the hydration process. Furthermore, nano-SiO2 acts as a nucleus in the matrix, enabling CSH gel to bond to its surface, forming a three-dimensional network structure. This significantly improves the microstructure of cement-based materials, enhancing their mechanical strength and durability. Therefore, incorporating nano-SiO2 into cement-based composite materials has become an important means of improving the performance of building materials.

[0004] Although the addition of PVA fibers and nano-SiO2 can bridge cracks and transfer loads in cement-based materials, resulting in higher ductility, toughness, and flexural strength compared to traditional cement-based materials, the pursuit of ultra-high mechanical properties in fiber-reinforced cementitious materials has led to increasingly higher fiber content and varied fiber mixing methods in recent years. These practices result in uneven fiber dispersion within the matrix, poor rheological properties, and difficulties in molding the mixture, thus limiting the application of fiber-reinforced cementitious composites. Therefore, it is necessary to provide a method for improving the rheological properties of PVA fiber and nano-SiO2 cementitious composites. Summary of the Invention

[0005] The purpose of this invention is to provide a method for improving the rheological properties of PVA fiber and nano-SiO2 cement-based composite materials.

[0006] To achieve the above object, the present invention provides the following solutions:

[0007] This invention provides a method for improving the rheological properties of PVA fiber and nano-silica cement-based composite materials by adding PVA fibers with an aspect ratio of 100 to 300.

[0008] Preferably, PVA fibers with an aspect ratio of 200 to 300 are added.

[0009] Preferably, the volumetric doping of the PVA fiber is 0.3% to 1%.

[0010] Preferably, the volumetric doping of the PVA fiber is 0.3% to 0.8%.

[0011] Preferably, the volumetric content of the PVA fiber is 0.3% to 0.5%.

[0012] The yield stress and plastic viscosity of PVA fiber and nano-SiO2 cement-based composites increase with increasing fiber length and aspect ratio, while the flow spread and flow rate decrease in the opposite direction. The effect of different particle shapes on the plastic viscosity of the slurry is as follows: needle-like > lamellar > cubic / granular > spherical. PVA fibers, as typical needle-like particles, easily overlap in suspensions containing uniform spherical particles. The greater their length and aspect ratio, the greater the probability and quantity of them forming a network structure. This network structure provides resistance to the slurry flow in the opposite direction, thus macroscopically resulting in increased plastic viscosity and decreased flow rate. Furthermore, studies based on the particle surface water thickness (WFT) model show that the yield stress and plastic viscosity of PVA fiber and nano-SiO2 cement-based composites exhibit an exponential relationship with WFT. As the interparticle water thickness gradually increases, both the plastic viscosity and yield stress of the PVA fiber and nano-SiO2 cement-based composites decrease significantly, while the flow rate and flow spread are linearly correlated with WFT. With increasing interparticle water thickness, the flow rate and flow spread of the slurry increase. In PVA fiber and nano-SiO2 cement-based composites, fibers with larger aspect ratios and longer lengths have larger specific surface areas, making them more prone to absorbing free water. This leads to reduced water content around the particles, decreased interparticle water film thickness, and increased interparticle friction due to the lack of free water lubrication. This increases the force required to overcome for the slurry to flow, thus increasing the yield stress and decreasing the flow spread. Therefore, this invention limits the aspect ratio of PVA fibers to 100–300.

[0013] When only the PVA fiber content is increased, the flow spread and flow rate of the PVA fiber and nano-SiO2 cement-based composite material decrease, while the yield stress plastic viscosity increases. This is because, under the premise of a constant water-cement ratio, the amount of free water inside the mixture is also constant. Increasing the fiber content makes it insufficient for the free water to wet the surface of solid particles, leading to increased friction between particles. In addition, with the increase of fiber content, the concentration of solid particles in the suspended phase increases, and particles are more likely to collide with each other, generating additional energy loss. Under the combined effect of these two factors, the flowability of the PVA fiber and nano-SiO2 cement-based composite material deteriorates and the rheological parameters increase. However, the rheological parameters do not increase indefinitely; there is a critical value for the fiber content. When this critical value is exceeded, the change in the rheological parameters of the mixture will tend to level off until the mixture stops flowing. This may be because at the critical content, the fibers agglomerate or clump, hindering the flow of the mixture. Furthermore, the critical content decreases with the increase of the aspect ratio. When PVA fibers are added, the flocculated structure inside the composite material will be broken and reorganized under shear action, resulting in a sharp decrease in the increment of a certain rheological parameter. Under certain fiber content and good fiber dispersion conditions, strain hardening may also occur.

[0014] The introduction of fibers increases the number of interfacial defects. Due to the enrichment of free water in this area, the bonding force at the boundary weakens, and the pores left after water evaporation also weaken the strength of the PVA fiber and nano-SiO2 cement-based composite. Therefore, optimization of the interfacial region is crucial. The pozzolanic active blend has small diameters (micrometer and nanometer scale). Besides filling the pores in the interfacial region, it can also undergo secondary hydration reactions to improve the interfacial bonding force and strength of the PVA fiber and nano-SiO2 cement-based composite. The blend also has a large specific surface area and particle characteristics. After adding the slurry, the concentration of solid particles increases significantly. The free water released from filling the pores is insufficient to wet the particle surface, leading to a decrease in the flow parameters of the PVA fiber and nano-SiO2 cement-based composite and an improvement in its anti-segregation ability. Therefore, the volumetric content of PVA fibers is limited to 0.3%–1%, and the amount of nano-SiO2 added is 3–5 parts.

[0015] Preferably, the mixture further includes adding a blend of materials, which includes copper slag, gypsum, volcanic ash, metakaolin, and fly ash. Preferably, 200-300 parts of copper slag, 40-80 parts of gypsum, 30-60 parts of volcanic ash, 50-100 parts of metakaolin, and 150-200 parts of fly ash are added.

[0016] The addition of admixtures has a certain impact on the rheological properties of cement, but the degree of influence on standard consistency water requirement and setting time varies among different admixtures. The addition of copper slag slightly increases the standard consistency water requirement of cement, prolongs the setting time, and slightly reduces the mortar fluidity. The addition of metakaolin and gypsum reduces the water requirement of cement, prolongs both initial and final setting times, and increases mortar fluidity. This is because metakaolin has very low or virtually no activity, resulting in a relative reduction in the amount of cement clinker. Although it can undergo hydration, the amount of hydration products generated is reduced. Gypsum, being a filler admixture, basically does not participate in the hydration reaction. Its addition reduces the clinker content in the cement, thus decreasing the amount of hydration products generated, leading to a prolonged setting time. Simultaneously, gypsum particles do not absorb water on their surface; they fill the voids in the flocculated structure, releasing free water, thus reducing the standard consistency water requirement of cement. The addition of volcanic ash and fly ash increases the standard consistency water requirement of cement, shortens the initial and final setting times, and reduces the fluidity of the mortar. This is because both of these admixtures have a certain degree of volcanic ash activity, and more importantly, they are both porous materials with a large specific surface area, making them prone to absorbing water. The water filling the spaces between cement particles is absorbed by the admixtures, inevitably leading to an increase in water demand. Furthermore, the addition of these materials with large specific surface areas accelerates cement hydration, increasing the amount of hydration products and thus shortening the initial and final setting times.

[0017] The addition of different admixtures significantly affects the penetration of cement paste. The addition of active substances such as copper slag and fly ash increases the hydration rate of cement and the amount of hydration products, thus reducing the penetration of the paste within the same time frame. Fly ash shows the largest reduction in penetration, indicating that its activity is significantly higher than that of the other admixtures. The addition of metakaolin and gypsum reduces the relative clinker content in the cement, greatly delaying the hydration of the cement paste and reducing the amount of hydrated calcium silicate and other products generated in the early stages of cement hydration. Therefore, the reduction in penetration is relatively small, indicating that metakaolin and gypsum have virtually no activity or very low activity.

[0018] The type and amount of admixtures have a significant impact on the rheological properties of cement paste. The addition of admixtures such as copper slag, fly ash, and metakaolin increases the standard consistency water requirement of cement, shortens the setting time, reduces the mortar fluidity, and increases the yield value and plastic viscosity of cement paste to a certain extent. The effect is enhanced with the increase of the amount added, especially with kiln ash activated coal gangue. The addition of gypsum and metakaolin improves the fluidity of cement and delays setting, and the more they are added, the stronger the delaying effect.

[0019] Preferably, the method for improving the rheological properties of PVA fiber and nano-silica cement-based composite materials specifically includes the following steps:

[0020] S1. Mix cement with admixtures, add 1 / 2 water, and premix for 20-30 seconds to obtain mortar;

[0021] S2. Mix the nano-SiO2 particles with the water-reducing agent and add them to the remaining water. Stir and disperse the mixture. After it is evenly dispersed, add it to the mortar and stir for 1 to 2 minutes to obtain the mixture.

[0022] S3. Add PVA fiber to the mixture obtained in S2 and stir until homogeneous.

[0023] The present invention discloses the following technical effects:

[0024] 3D printing requires materials with rapid forming properties. After exiting the printer, the material must immediately "stand upright" and not continue to flow, exhibiting excellent thixotropic properties (high plastic viscosity, low ultimate shear stress, rapid rheological change upon agitation, and quick standing upright after exiting). It should also possess rapid setting time and high early strength. Otherwise, the printed structure's dimensions will deviate significantly from the initial design, and the lower part will not deform during the continuous layering process. This invention improves the rheological properties of PVA fiber and nano-SiO2 cement-based composite materials by adjusting the aspect ratio of PVA fibers. This effectively increases cement strength and provides adsorption, air entrainment, dispersion, wetting, and solubilization functions, maximizing cement utilization efficiency and ensuring good cement flowability, making it suitable for 3D printing. Detailed Implementation

[0025] Various exemplary embodiments of the present invention will now be described in detail. This detailed description should not be considered as a limitation of the present invention, but rather as a more detailed description of certain aspects, features, and embodiments of the present invention.

[0026] It should be understood that the terminology used in this invention is merely for describing particular embodiments and is not intended to limit the invention. Furthermore, with respect to numerical ranges in this invention, it should be understood that each intermediate value between the upper and lower limits of the range is also specifically disclosed. Every smaller range between any stated value or intermediate value within a stated range, and any other stated value or intermediate value within said range, is also included in this invention. The upper and lower limits of these smaller ranges may be independently included or excluded from the range.

[0027] Unless otherwise stated, all technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the art. While only preferred methods and materials have been described herein, any methods and materials similar or equivalent to those described herein may be used in the implementation or testing of this invention. All references to this specification are incorporated by way of citation to disclose and describe methods and / or materials associated with those references. In the event of any conflict with any incorporated reference, the content of this specification shall prevail.

[0028] Various modifications and variations can be made to the specific embodiments described in this specification without departing from the scope or spirit of the invention, as will be apparent to those skilled in the art. Other embodiments derived from this specification will also be readily apparent to those skilled in the art. This specification and embodiments are merely exemplary.

[0029] The terms “include,” “including,” “have,” “contain,” etc., used in this article are all open-ended terms, meaning that they include but are not limited to.

[0030] In this embodiment of the invention, the cement used is P.O42.5 type ordinary Portland cement produced by Henan Xinxiang Mengdian Group, with a density of 3160 kg / m³. 3 Its technical specifications conform to the national standard GB175-2007. Grade I fly ash produced by Luoyang Power Plant is used, and tap water supplied by Zhengzhou City is used. High-strength, high-elasticity polyvinyl alcohol fiber (PVA fiber) produced by Kuraray Co., Ltd. of Japan is used. Nano-SiO2, a white powder, produced by Hangzhou Wanjing New Materials Co., Ltd., is used. The water-reducing agent is a polycarboxylate-based high-performance water-reducing agent, a pale yellow liquid with a water reduction rate of 25%.

[0031] A method for improving the rheological properties of PVA fiber and nano-silica cement-based composite materials specifically includes the following steps:

[0032] S1. Mix cement with admixtures, add 1 / 2 mass of water, premix for 25 seconds to obtain mortar;

[0033] S2. Mix the nano-SiO2 particles with the water-reducing agent and add them to the remaining water. Stir and disperse the mixture. After it is evenly dispersed, add it to the mortar and stir for 2 minutes to obtain the mixture.

[0034] S3. Add PVA fiber to the mixture obtained in S2 and stir until homogeneous.

[0035] Examples 1-9 all used the above steps to prepare PVA fiber and nano-silica cement-based composite materials, the difference being the mixing ratio of the added raw materials. Specifically, the aspect ratio of the PVA fibers in Examples 1-2 was 100, the aspect ratio of the PVA fibers in Examples 3-4 was 200, the aspect ratio of the PVA fibers in Examples 5-6 was 300, and the aspect ratio of the PVA fibers in Examples 7-9 was 230, as shown in Table 1.

[0036] Table 1

[0037]

[0038]

[0039] Comparative Example 1

[0040] Same as Example 7, except that the aspect ratio of the PVA fiber is 400.

[0041] Comparative Example 2

[0042] Same as Example 7, except that the aspect ratio of the PVA fiber is 75.

[0043] Comparative Example 3

[0044] Same as Example 7, except that the volume fraction of PVA fiber is 3%.

[0045] Comparative Example 4

[0046] Same as Example 1, except that the preparation method is as follows:

[0047] S1. Mix cement with the admixture, add 1 / 2 water, and premix for 25 seconds to obtain mortar;

[0048] S2. Mix the nano-SiO2 particles, PVA fibers and water-reducing agent, add them to the remaining water, stir and disperse. After they are evenly dispersed, add them to the mortar and stir for 2 minutes.

[0049] According to the national standards "Standard for Test Methods of Performance of Ordinary Concrete Mixtures" (GB / T 50080-2016), "Test Method for Flowability of Cement Mortar" (GB / T 2419-2005), and "Standard for Test Methods of Performance of Ordinary Concrete Mixtures" (GB / T 50080-2002), the cement-based composite materials prepared in the examples and comparative proportions were subjected to micro-slump tests, flowability tests, and 1-day compressive and flexural strength tests. The test results are shown in Table 2.

[0050] Table 2

[0051]

[0052]

[0053] As shown in Table 2, the composite material of the present invention has good flowability, which meets the requirements of 3D printing.

[0054] The rheological properties were tested using a TR-CRI fully automatic concrete rheometer manufactured by Shanghai Tongrui Instrument Equipment Co., Ltd. The specific test method is as follows:

[0055] (1) Fill a test barrel with an inner diameter of 300 mm and a height of 310 mm with 2 / 3 of the volume of fresh cement-based composite material, install a cross rotor, control the test tube to rise until it is submerged to 150 mm of the rotor, test the torque at a speed of 0.1 (rps), and calculate the static yield stress.

[0056] (2) After the static test, keeping the immersion depth of the cross rotor constant, the torque generated at rotational speeds of 0.6, 0.55, 0.5, 0.45, 0.4, 0.35, 0.3, 0.25, 0.2, and 0.15 (rpm) was measured sequentially. The dynamic yield stress and plastic viscosity of the fresh cement-based composite material were then calculated. Due to the complexity of the impeller rotation, the shear stress and shear rate were calculated using the measured torque and impeller speed, as shown in the following formula:

[0057] T = G + H × N

[0058] In the formula: T—torque, the unit is Newton-meter (N·m);

[0059] G—the y-intercept of the extension of the linear segment of the curve;

[0060] H—Slope of the linear segment of the curve;

[0061] N—Impeller speed, measured in revolutions per second (rps).

[0062] (3) Replace the cross rotor and install the cylindrical rotor (φ200x200mm). Control the test barrel to rise until it is submerged 150mm below the cylindrical rotor. Test the torque generated at speeds of 0.6, 0.55, 0.5, 0.45, 0.4, 0.35, 0.3, 0.25, 0.2, and 0.15 (rps) in sequence, and calculate the viscosity of the pumped cement-based composite material lubricating layer.

[0063] The test results are shown in Table 3.

[0064] Table 3

[0065]

[0066] The embodiments described above are merely descriptions of preferred embodiments of the present invention and are not intended to limit the scope of the present invention. Without departing from the spirit of the present invention, various modifications and improvements made to the technical solutions of the present invention by persons skilled in the art should fall within the scope of protection defined by the claims of the present invention.

Claims

1. A method for improving the rheological properties of PVA fiber and nano-silica cement-based composite materials, characterized in that, Specifically, the steps include the following: S1. Mix cement with admixtures, add 1 / 2 mass of water, premix for 25 seconds to obtain mortar; S2. Mix the nano-SiO2 particles with the water-reducing agent and add them to the remaining water. Stir and disperse the mixture. After it is evenly dispersed, add it to the mortar and stir for 2 minutes to obtain the mixture. S3. Add PVA fibers to the mixture obtained in S2 and stir until homogeneous; The volumetric doping of the PVA fiber is 1%, and the aspect ratio is 300. The amount of nano-SiO2 particles added is 3 parts; The mixed material, by weight, consists of 200 parts copper slag, 40 parts gypsum, 35 parts volcanic ash, 60 parts metakaolin, and 180 parts fly ash. The cement is 600 parts; the water is 300 parts.

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