Low-cost and air-corrosion-resistant steel fiber modification method and application thereof in UHPC (Ultra High Performance Concrete)

Abstract

The invention relates to a low-cost and air-corrosion-resistant steel fiber modification method and application thereof in UHPC, and the steel fiber modification method specifically comprises the following steps: mixing a saturated calcium hydroxide solution with glycerol to obtain a modification liquid, then soaking steel fibers in the obtained modification liquid, standing and modifying, and then drying to obtain the steel fibers. And finally taking out the steel fiber and drying to obtain the modified steel fiber. According to the steel fiber modification method, common industrial raw materials and by-products can be adopted as raw materials, the cost is low, the modification process is non-toxic and harmless, the steps are simple, and industrial application is easy.

Description

Technical Field

[0001] The present invention belongs to the treatment of metal fiber materials specifically used to enhance their filling performance in mortar, concrete or artificial stone, and particularly relates to a low-cost steel fiber modification method resistant to air corrosion and its application in UHPC. Background Art

[0002] Ultra-high performance concrete (UHPC) overcomes the disadvantages of ordinary concrete such as poor durability and low strength, and is widely used in many fields such as bridges, building components, and water conservancy projects. Incorporating steel fibers into the UHPC system builds a three-dimensional network structure inside, effectively improving its own brittleness and achieving the effect of enhancing toughness. However, steel fibers are prone to oxidation and rust during the service period of concrete, and the bonding ability of the interfacial transition zone between the steel fibers and the concrete matrix is weak. When UHPC is in a stress state, the internal steel fibers are prone to slip failure, and the stress on the steel fibers is much less than their own yield stress, seriously reducing the use efficiency of the steel fibers; in addition, when the internal steel fibers of UHPC are electrochemically corroded under complex environments, not only does it greatly reduce the yield stress of the steel fibers, but the corrosion on the surface of the steel fibers destroys the original interfacial bond between the steel fibers and the UHPC matrix, greatly affecting the long-term service performance of UHPC.

[0003] Therefore, modifying the surface of steel fibers, optimizing the interfacial transition zone between the steel fibers and the matrix, increasing their bonding strength, and improving the corrosion resistance of the steel fibers in UHPC are beneficial to improving the performance of ultra-high performance concrete in all aspects. CN107721327A discloses a method for enhancing the durability of steel fibers, using polyethyleneimine solution, polyurethane acrylate, geranyl acetate, etc. to perform ultraviolet curing on the surface of the steel fibers, inhibiting the rusting rate of the steel fibers in acidic and alkaline environments. CN114656181A discloses a preparation method for surface hyperbranched modified steel fibers, using organic reagents such as silane coupling agent and tetraethyl orthosilicate to perform hyperbranched modification on the surface of the steel fibers to strengthen the adhesion and improve the interfacial transition zone.

[0004] However, although the surface of the steel fibers is plated with a copper-zinc alloy layer, after being modified by the above water-soluble solutions, a loose oxide layer easily appears on the surface of the steel fibers when placed in the air, resulting in obvious peeling of the original modified layer. Therefore, generally, it needs to be repeatedly rinsed several times with deionized water or distilled water after modification, seriously wasting water resources; in addition, the above methods have a large number of expensive modification raw materials, and the surface treatment process is cumbersome, and the operability is not strong in the large-scale modification treatment of steel fibers. Summary of the Invention

[0005] The technical problem to be solved by the present invention is to provide a low-cost steel fiber modification method resistant to air corrosion and its application in UHPC in view of the above deficiencies in the prior art. By using common industrial raw materials to modify steel fibers, the air corrosion resistance of steel fibers is improved, the chemical bonding force with the concrete matrix is increased at the same time, the interfacial transition zone between the fiber and the matrix is improved, the bonding performance with the concrete is improved, and the mechanical properties and durability of the ultra-high performance concrete material are effectively improved.

[0006] To solve the above technical problems, the technical solution provided by the present invention is as follows:

[0007] Provide a low-cost steel fiber modification method resistant to air corrosion. The specific steps are as follows: Mix saturated calcium hydroxide solution and glycerol to obtain a modification liquid, then soak the steel fibers in the obtained modification liquid and let them stand for modification treatment, and finally take out the steel fibers and dry them to obtain modified steel fibers.

[0008] According to the above scheme, the saturated calcium hydroxide is the saturated supernatant of carbide slag dissolved in water or saturated lime water.

[0009] According to the above scheme, the glycerol is industrial glycerol with a glycerol content of ≥95wt%.

[0010] According to the above scheme, the mass ratio of the saturated calcium hydroxide solution to glycerol is 1:0.005 - 0.05.

[0011] According to the above scheme, the diameter of the steel fiber is 0.04 - 0.2mm, the length is 5 - 30mm, and the tensile strength ≥

[0012] 2000MPa. According to the shape, it can be straight steel fibers, end-hooked steel fibers, corrugated steel fibers or other special-shaped steel fibers.

[0013] According to the above scheme, the standing modification treatment time is 0.5 - 10min.

[0014] According to the above scheme, the drying conditions are: heating at 50 - 80°C for 10 - 60min in an air atmosphere or a 5 - 15vol% carbon dioxide atmosphere.

[0015] The present invention also includes the modified steel fibers obtained according to the above modification method. Uniformly dispersed sub-micron calcium carbonate particles are deposited on the surface of the modified steel fibers, and a glycerol film covers the surface of the modified steel fibers. The attached calcium carbonate particles make the steel fibers have a higher roughness, and in addition, a thin glycerol film completely covers the surface of the steel fibers to form a protective film to prevent the steel fibers from contacting air and oxidizing.

[0016] The present invention also includes the application of the above modified steel fibers in UHPC. The specific usage method includes the following steps:

[0017] 1) Add PO42.5 cement, ultra-fine mineral powder, fly ash, river sand, and water reducer into the mixing pan and mix dry evenly.

[0018] 2) Add water into the mixing pan and stir until a slurry state appears.

[0019] 3) Add the modified steel fiber into the mixing pan and stir evenly to obtain a slurry mixture.

[0020] 4) Pour the slurry mixture obtained in step 3) into a mold, vibrate to remove air bubbles, and cure after molding to obtain UHPC.

[0021] According to the above scheme, the D50 of the ultra-fine mineral powder in step 1) is 3 - 10 μm.

[0022] According to the above scheme, the specific surface area of the fly ash in step 1) is 830 - 870 m 2 / kg, and the content of SiO2

[0023] ≥50 wt%.

[0024] According to the above scheme, the particle size of the river sand in step 1) is 150 - 400 μm.

[0025] According to the above scheme, the curing conditions in step 4) are: first cure for 24 h at a temperature of 25°C and a relative humidity ≥95% and then demold, and then cure until the age of 28 d at a temperature of 25°C and a relative humidity ≥95%.

[0026] During the modification process of the steel fiber of the present invention, the saturated calcium hydroxide solution has a mild electrochemical corrosion effect on the surface of the steel fiber, increasing the surface roughness of the steel fiber. The calcium hydroxide solution adheres to the surface of the steel fiber. After drying under appropriate conditions, the calcium hydroxide on the surface of the steel fiber absorbs carbon and generates sub-micron amorphous calcium carbonate particles attached to the surface of the steel fiber, further increasing the surface roughness of the steel fiber. At the same time, the sub-micron amorphous calcium carbonate particles have high activity, providing nucleation sites for cement hydration, accelerating the early hydration of cement, and improving the early strength. At the same time, a very thin lubricating film is formed by a small amount of glycerol adhering to the surface of the steel fiber. Glycerol can also form complexes with iron, copper, and zinc ions on the surface of the steel fiber, preventing oxidation, corrosion, and rusting on the surface of the steel fiber, so that the modified steel fiber can be stored in the air for a long time. On the other hand, the hydroxyl groups in glycerol can form hydrogen bonds with the C-S-H gel of the cement hydration products, improving the bonding property between the steel fiber and the cement-based material. The above two effects can enhance the bonding ability between the steel fiber and the matrix, improve the interfacial transition zone, and exert the excellent intrinsic mechanical properties of the steel fiber, thereby improving the compressive, flexural and durability of ultra-high performance concrete.

[0027] The beneficial effects of the present invention are as follows: 1. The steel fiber modification method of the present invention can use common industrial raw materials and by-products as raw materials, with low cost, non-toxic and harmless in the modification process, simple steps, and easy to industrialize. 2. The modified steel fibers provided by the present invention are resistant to air corrosion, can be stored for a long time, and can effectively improve the mechanical properties and durability of concrete. BRIEF DESCRIPTION OF THE DRAWINGS

[0028] Figure 1 Photographs of the untreated steel fibers (a) of Comparative Example 1 of the present invention, the steel fibers (b) modified by the method of Example 1, and the steel fibers (c) modified by the method of Comparative Example 2 after standing in air for 30 days;

[0029] Figure 2 Optical microscope image (scale bar 20 μm) of the unmodified steel fibers of Comparative Example 1 after being placed in air for 30 days;

[0030] Figure 3 Optical microscope image (scale bar 20 μm) of the steel fibers modified by the method of Example 1 after being placed in air for 30 days;

[0031] Figure 4 Scanning electron microscope image (scale bar 1 μm) of the steel fibers modified by the method of Example 1 after being placed in air for 30 days;

[0032] Figure 5 Elemental analysis spectrogram of the surface particles of the steel fibers modified by the method of Example 1;

[0033] Figure 6 Scanning electron microscope image (scale bar 100 μm) of the steel fibers modified by the method of Comparative Example 2 after being placed in air for 30 days. DETAILED DESCRIPTION OF THE EMBODIMENTS

[0034] To enable those skilled in the art to better understand the technical solutions of the present invention, the present invention will be further described in detail below with reference to the accompanying drawings.

[0035] The steel fibers used in the embodiments of the present invention are straight steel fibers with a diameter of 0.2 mm, a length of 13 mm, a length-diameter ratio of 65, and a tensile strength of 2850 MPa; the glycerol used is industrial glycerol with a glycerol content of ≥ 95 wt%; the cement used is 42.5-grade ordinary Portland cement; the fly ash used is Class I fly ash with a specific surface area of 830 - 870 m 2 / kg and a SiO2 content of ≥ 50 wt%; the ultrafine mineral powder has a D50 of 3 - 10 μm; the aggregate used is river sand with a particle size of 150 - 400 μm; the water reducer used is a solid high-performance polycarboxylate water reducer with a water reduction rate of 40%.

[0036] Examples 1 - 6

[0037] The steel fibers were immersed in a modified solution obtained by mixing saturated calcium hydroxide solution and glycerol for static modification treatment, and then taken out and dried to obtain modified steel fibers. The mass parts of each raw material are shown in Table 1; the process parameters of the soaking time of the steel fibers, the drying atmosphere, the drying temperature, and the drying time are shown in Table 2.

[0038] Six kinds of UHPC were prepared by respectively taking the above-prepared modified steel fibers. The specific method is as follows:

[0039] 1) Add PO42.5 cement, ultra-fine mineral powder, fly ash, river sand, and water reducer into the mixing pot and dry mix for 3 min until evenly mixed;

[0040] 2) Add water into the mixing pot and stir until a paste state appears;

[0041] 3) Add the modified steel fibers into the mixing pot and stir for 3 min until a uniform paste mixture is obtained;

[0042] 4) Pour the paste mixture obtained in step 3) into a mold, vibrate to defoam, and after forming, cure it under the conditions of a temperature of 25 °C and a relative humidity ≥ 95% for 24 h and then demold, and then cure it under the conditions of a temperature of 25 °C and a relative humidity ≥ 95% until the age of 28 d to obtain UHPC. The mass parts of each raw material used for preparing UHPC are shown in Table 3.

[0043] Comparative Example 1

[0044] A kind of UHPC, the difference in its preparation method from Example 1 is that an equal amount of unmodified steel fibers is added.

[0045] Comparative Example 2

[0046] A kind of UHPC, the difference in its preparation method from Example 1 is that the steel fiber modified solution is saturated lime water.

[0047] Comparative Example 3

[0048] A kind of UHPC, the difference in its preparation method from Example 1 is that the saturated lime water in the steel fiber modified solution is replaced with an equal amount of tap water.

[0049] Table 1 Mass parts of raw materials in Examples 1-6

[0050]

[0051] Table 2 Soaking time of steel fibers, drying atmosphere, drying temperature, and drying time in Examples 1-6

[0052]

[0053]

[0054] Table 3 Parts by mass of raw materials of UHPC prepared in Examples 1-6

[0055]

[0056] Table 4 Test results of slump flow and 28-day performance of concrete prepared in Examples 1-6 and Comparative Examples 1-3

[0057] Number Spreading degree mm Compressive strength MPa Flexural strength MPa Tensile strength MPa Example 1 240 143.6 30.6 8.92 Example 2 250 138.9 29.5 7.17 Example 3 250 136.3 26.4 6.99 Example 4 245 156.4 35.6 9.44 Example 5 250 133.8 25.1 6.88 Example 6 240 146.1 32.0 8.51 Comparative example 1 240 124.3 22.8 6.11 Comparative example 2 230 130.6 24.7 6.73 Comparative example 3 240 128.5 24.0 6.27

[0058] As can be seen from Table 4, compared with Comparative Example 1 with untreated steel fibers added, the workability of the concrete in Example 1 is comparable, and the 28-day compressive, flexural and tensile strengths are increased by 15.5%, 34.2% and 45.9% respectively, indicating that the modified steel fibers of the present invention can significantly improve the strength and toughness of UHPC. Comparing Comparative Example 1, Comparative Example 2 and Comparative Example 3, it shows that the single modified solution can slightly improve the performance of UHPC to a certain extent, but the improvement effect is limited. In addition, the 28-day compressive, flexural and tensile properties of the UHPC prepared in Examples 1-6 are improved to varying degrees compared with those in Comparative Examples 1-3.

[0059] Figure 1 In (a), (b), and (c) are photos of untreated steel fibers, steel fibers modified by the method of Example 1, and steel fibers modified by the method of Comparative Example 2 after standing in air for 30 days. From Figure 1 it can be seen that neither the untreated steel fibers nor the steel fibers modified in Example 1 show obvious rusting. However, for the steel fibers treated only with saturated lime water in Comparative Example 2, after being placed in air for 30 days, the surface shows a rusty color and rust particles flake off, and the rusting situation is obvious.

[0060] Figure 2 is an optical microscope image of the unmodified steel fibers of Comparative Example 1 after standing in air for 30 days. It can be seen that the surface of the unmodified steel fibers is relatively smooth.

[0061] Figure 3 is an optical microscope image of the steel fibers modified by the method of Example 1 after standing in air for 30 days. It can be seen that obvious interference colors appear on the surface of the steel fibers in the optical microscope image, indicating that a thin film is formed on the surface of the steel fibers by glycerol.

[0062] Figure 4 is an electron scanning microscope image of the steel fibers modified by the method of Example 1 after standing in air for 30 days. It can be observed that submicron-sized particles are evenly attached to the fiber surface. Further energy spectrum elemental analysis ( Figure 5 ) infers that the submicron particles are amorphous calcium carbonate particles.

[0063] Figure 6 The electron scanning microscope image (scale bar 100 μm) of the steel fibers modified by the method of Comparative Example 2 after being placed in air for 30 days. It can be seen that after the steel fibers treated only with a single saturated lime water treatment are placed in air for 30 days, there are obvious protruding patchy rust morphologies on the surface. The comparison shows that modifying the steel fibers by the method of the present invention can improve the air corrosion resistance of the steel fibers.

[0064] The present invention uses common industrial products or by-products to modify steel fibers, which can effectively solve the problems in traditional steel fiber modification, such as using expensive raw materials, having a cumbersome treatment process, and being prone to a loose oxide layer on the surface after treatment, resulting in serious rust peeling of the original modified layer. While effectively improving the mechanical properties and toughness of ultra-high performance concrete materials, the modified products can be stored for a long time, are easy to promote, and can be effectively applied industrially.

[0065] It should be noted that the above are only the preferred embodiments of the present invention and are not used to limit the protection scope of the present invention. Any combination or equivalent transformation made on the basis of the above embodiments belongs to the protection scope of the present invention.

Claims

1. A low-cost method for modifying steel fibers resistant to air corrosion, characterized in that The specific steps are as follows: Mix a saturated calcium hydroxide solution with glycerol to obtain a modified solution, then soak steel fibers in the obtained modified solution and let them stand for modification treatment, and finally take out the steel fibers and dry them to obtain modified steel fibers.

2. The low-cost steel fiber modification method resistant to air corrosion according to claim 1, characterized in that, The saturated calcium hydroxide is the saturated supernatant of carbide slag dissolved in water or saturated lime water.

3. The low-cost steel fiber modification method resistant to air corrosion according to claim 1, characterized in that The glycerol is industrial glycerol with a glycerol content of ≥95 wt%.

4. The low-cost steel fiber modification method resistant to air corrosion according to claim 1, characterized in that The mass ratio of the saturated calcium hydroxide solution to glycerol is 1:0.005 - 0.

05.

5. The low-cost and air-corrosion-resistant steel fiber modification method according to claim 1, characterized in that The diameter of the steel fibers is 0.04 - 0.2 mm, the length is 5 - 30 mm, and the tensile strength is ≥2000 MPa.

6. The low-cost steel fiber modification method resistant to air corrosion according to claim 1, wherein, The standing modification treatment time is 0.5 - 10 min.

7. The low-cost steel fiber modification method resistant to air corrosion according to claim 1, wherein The drying conditions are: heating at 50 - 80 °C for 10 - 60 min in an air atmosphere or a 5 - 15 vol% carbon dioxide atmosphere.

8. The modified steel fiber obtained by the modification method according to any one of claims 1-7, characterized in that, The surface of the modified steel fibers is deposited with uniformly dispersed sub-micron-sized calcium carbonate particles, and a glycerol film covers the surface of the modified steel fibers.

9. Use of the modified steel fiber according to claim 8 in UHPC, characterized in that, The specific usage method includes the following steps: 1) Add PO42.5 cement, ultrafine mineral powder, fly ash, river sand, and water reducer into a mixing pan and mix them evenly in dry state; 2) Add water into the mixing pan and stir until a paste state appears; 3) Add the modified steel fibers into the mixing pan and stir evenly to obtain a paste mixture; 4) Pour the paste mixture obtained in step 3) into a mold, vibrate to remove air bubbles, and cure it after forming to obtain UHPC.

10. The application of the modified steel fiber in UHPC according to claim 9, wherein, Step The curing conditions in step 4) are: first cure for 24 h at a temperature of 25 °C and a relative humidity of ≥95% and then demold, and then cure until the 28-day age at a temperature of 25 °C and a relative humidity of ≥95%.

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