A special reinforcing steel fiber for solid waste concrete and its modification method

Abstract

This invention discloses a special reinforcing steel fiber for all-solid waste concrete and its modification method, belonging to the field of building materials technology. The reinforcing steel fiber uses recycled steel fiber from automobile tires as the core raw material. Through a graded process of raw material pretreatment, dopamine coating, and ettringite deposition modification, a chemical bonding layer and a microstructure reinforcing layer are formed on the fiber surface, significantly improving its interfacial bonding performance with the all-solid waste concrete matrix. The suitable all-solid waste concrete system consists of granulated blast furnace slag powder, industrial by-product gypsum, alkaline industrial by-products, and iron tailings sand, achieving 100% solid waste utilization. When the modified steel fiber content is 1-2%, the 28-day compressive strength of the concrete increases by over 10%, and the flexural strength increases by over 100%, combining environmental friendliness and high performance, providing an innovative solution for solid waste resource utilization and building material upgrading.

Description

Technical Field

[0001] This invention belongs to the field of building materials technology, specifically relating to a concrete-specific reinforcing steel fiber based on a whole solid waste system and its modification method, particularly to the surface functionalization modification technology of recycled steel fibers from automobile tires and its application in a whole solid waste concrete system. Background Technology

[0002] Traditional ultra-high performance concrete (UHPC) primarily uses cement, silica fume, and quartz sand as raw materials and relies on virgin steel fibers as reinforcement, resulting in significant issues such as high raw material costs and large carbon emissions. Its cementitious system is mainly composed of silicate cement, and the hydration product is primarily CSH gel. Traditional steel fiber surface modification technologies (such as acid washing and silane coupling) are all designed specifically for the chemical properties of CSH gel.

[0003] However, with the development of solid waste resource utilization technology, the all-solid waste UHPC system has gradually become a research hotspot. It mainly uses granulated blast furnace slag, industrial by-product gypsum, and industrial alkaline solid waste as cementing systems, and the core hydration product is ettringite (AFt). This makes the application of traditional modified steel fibers in this system significantly limited: on the one hand, the surface modification layer of traditional modified steel fibers (such as silane coating) lacks chemical bonding with the ettringite matrix of all-solid waste UHPC, and stress concentration is easily formed in the interface transition zone; on the other hand, the surface of traditional modified fibers is smooth, and it is difficult to form an effective mechanical interlock with the needle-rod structure of ettringite crystals, resulting in a chemical-physical double mismatch problem at the fiber-matrix interface, which seriously weakens the reinforcement effect.

[0004] In recent years, recycled steel fibers from waste tires have attracted attention due to their potential mechanical properties and environmental value, but they face two major technical bottlenecks in practical applications: (1) insufficient interfacial chemical compatibility, the modified layer on the surface of recycled steel fibers cannot form a stable chemical bond with the ettringite matrix of all-solid waste UHPC; (2) microstructure mismatch, the fiber surface morphology and ettringite crystal structure are difficult to mechanically anchor. In existing technologies, acid washing can only remove impurities on the fiber surface and cannot solve the chemical bonding problem; although silane coupling agent modification can increase the number of active groups on the fiber surface, the coating has low chemical affinity with the ettringite matrix and is easily degraded in a high-alkali environment. In addition, the modification process of traditional UHPC reinforcing fibers is mostly based on silicate system design and has not been optimized for compatibility with the ettringite main crystal phase of all-solid waste UHPC, which further limits the application of recycled steel fibers in all-solid waste UHPC.

[0005] Therefore, developing a recycled steel fiber modification technology that is compatible with the UHPC system for all solid waste and can simultaneously solve the problems of chemical compatibility and microstructure matching has significant engineering value and environmental significance. Summary of the Invention

[0006] To achieve the above objectives, the present invention provides a method for modifying solid waste concrete with reinforcing steel fibers, characterized by comprising the following core steps performed in sequence: (1) Raw material pretreatment: The recycled steel fibers from automobile tires are subjected to the following steps in sequence: stripping of surface rubber residue, magnetic separation purification, and oxidation modification of surface residual rubber. (2) Dopamine coating: The steel fibers that have been pretreated with raw materials are immersed in dopamine-Tris buffer solution and a polydopamine (PDA) coating is formed through surface polymerization reaction; (3) Erythrite deposition modification: Erythrite crystal layer is directionally deposited on the surface of steel fiber coated with PDA to obtain special reinforced steel fiber.

[0007] Furthermore, the specific steps for removing surface rubber residue in step (1) are as follows: using industrial-grade ultrasonic cleaning equipment, with a NaOH solution of 0.5-1.5% by mass as the cleaning medium, and continuously cleaning for 30-60 minutes at 50-60℃ and 40kHz to remove residual rubber particles attached to the surface of the steel fiber.

[0008] Furthermore, the magnetic separation purification in step (1) specifically involves: using an industrial-grade magnetic separation equipment, setting the magnetic field strength to 0.3T-1T, and performing magnetic separation on the recycled steel fibers that have completed surface rubber peeling, so that the purity of the recycled steel fibers after separation is >95%.

[0009] Furthermore, the surface residual rubber oxidation modification in step (1) specifically involves immersing the magnetically purified recycled steel fibers in a 1-5% H2O2 and H2SO4 mixture, wherein the volume ratio of H2O2 to H2SO4 in the mixture is 3:1, and treating it at a constant temperature of 50-80℃ for 0.5-5 hours to remove residual rubber and activate the fiber surface.

[0010] Furthermore, in step (2), the preparation of dopamine-Tris buffer solution is as follows: dissolve dopamine powder in Tris buffer solution with a pH of 8-9, stir until completely dissolved, and form a dopamine-Tris solution with a concentration of 1-3 mg / mL.

[0011] Furthermore, in step (2), the PDA coating preparation is specifically as follows: the recycled steel fibers after raw material pretreatment are added to the solution at a volume ratio of 1:2-4 with the dopamine-Tris solution, and the polymerization reaction is carried out in a shaking manner at an ambient temperature of 20-35℃ for 0.5-2 hours, so that the dopamine is oxidized and polymerized on the fiber surface to form a PDA coating.

[0012] Furthermore, step (2) also includes a PDA coating post-treatment step: take out the steel fiber that has completed the polymerization reaction, rinse it repeatedly with deionized water 3-5 times to remove unreacted dopamine on the surface, and then dry it in a forced-air drying oven at 20-50℃ to constant weight to ensure stable adhesion of the PDA coating.

[0013] Furthermore, the ettringite deposition modification in step (3) specifically includes: 3.1 Preparation of reaction solutions: Prepare a 0.05 mol / L Al2(SO4)3·18H2O solution, a 0.3 mol / L Ca(OH)2 solution, and a 10 mol / L NaOH solution; 3.2 Electrite Deposition: Recycled steel fibers are added to a Ca(OH)2 solution, with the fiber volume to solution volume ratio fixed at 1:4-6. Al2(SO4)3·18H2O and 0.1-0.3% of polycarboxylate superplasticizer (total solution volume) are added under a stirring rate of 100-300 rpm. The pH of the system is controlled at 11.0-12.5 by adding NaOH or H2SO4. The reaction temperature is controlled at 30-50℃, and the reaction time is 4-6 hours. 3.2 Post-treatment: The reaction was terminated by rinsing with ethanol and then dried under vacuum at 60°C for 1 day.

[0014] The present invention also provides an all-solid-waste ultra-high performance concrete, comprising reinforcing steel fibers prepared by the above modification method, wherein the dosage is 0.5-2% of the total mass of the concrete.

[0015] Furthermore, the cementitious system of the concrete is composed of granulated blast furnace slag powder, industrial by-product gypsum, and alkaline industrial by-products, with iron tailings sand as the aggregate; compared with the use of unmodified fibers, the 28-day flexural and compressive strengths of the concrete are improved respectively.

[0016] The beneficial effects of this invention are: (1) Efficient and synergistic utilization of solid waste resources: This invention uses recycled steel fibers from automobile tires as the core reinforcing material, which is compatible with a solid waste concrete system composed of granulated blast furnace slag powder, industrial by-product gypsum, alkaline industrial by-products and iron tailings sand, to achieve 100% solid waste utilization of concrete and its fiber-reinforced system, significantly reducing resource consumption and environmental pressure; (2) Synergistic enhancement of interface chemistry and physics: The ettringite layer deposited on the surface of the steel fiber is homogeneous with the hydration products of the solid waste matrix, and can form a continuous interface through ion bridging and crystal epitaxial growth, eliminating chemical heterogeneity. At the same time, the needle-shaped ettringite crystals can be embedded in the matrix hydration products to form a three-dimensional mechanical anchoring effect. Combined with the hydrogen bond binding sites provided by the dopamine intermediate layer, the fiber-matrix interface bonding strength is improved, effectively reducing the risk of interface stress concentration; (3) Environmentally friendly modification process: The ettringite deposition reaction is carried out under alkaline and mild conditions, and the process wastewater can be recycled. Compared with the traditional electroplating / chemical plating process, it reduces energy consumption and has no harmful pollutant emissions, which is in line with the concept of green production. (4) Excellent process compatibility: The graded modification steps can be implemented in a modular manner, seamlessly connecting with existing steel fiber production lines, with a single batch processing time of ≤8 hours, meeting the needs of continuous industrial production. Detailed Implementation

[0017] The following examples illustrate specific implementations of the present invention. However, these examples are merely for illustrative purposes and do not limit the scope of the invention in any way.

[0018] The pre-cleaning method for recycling steel fibers is as follows: (1) Using industrial-grade ultrasonic cleaning equipment, with 1% NaOH solution as the cleaning medium, clean for 30-60 min at 50-60 ℃ and 40 kHz to remove residual rubber particles from the fiber surface; (2) Immerse the separated recycled steel fibers in a 2-5% H2O2 and H2SO4 mixture (volume ratio 3:1) at 60 ℃ for 1 h; (3) Using industrial-grade magnetic separation equipment, separate the recycled steel fibers at a magnetic field strength of 0.5 T to achieve a purity >95%. The relevant processes and properties of the steel fiber series products (S1-S7) are shown in Table 1. The fiber purity is determined by the ratio of its mass after constant weight after calcination at 1000 ℃ to its mass before calcination.

[0019] Table 1. Process parameters and fiber purity for cleaning recycled steel fibers

[0020] The dopamine pre-coating modification method for recycled steel fibers is as follows: (1) Dopamine was dissolved in Tris buffer solution with pH 8-9 to form a dopamine-Tris solution with a concentration of 1-3 mg / mL; (2) Recycled steel fibers were added to the dopamine-Tris buffer solution at a volume ratio of 1:3, and polymerized by shaking at 20-35 °C for 0.5-2 h; (3) The fibers were rinsed with deionized water and dried at 30 °C to constant weight. A series of products (PDA1-PDA7) with a coating thickness of 50-200 nm were obtained. The process and performance are shown in Table 2. The dopamine pre-coating effect of the recycled steel fibers was determined by transmission electron microscopy (TEM).

[0021] Table 2. Dopamine pre-coating process and product performance

[0022] The following are the methods for recovering the ettringite deposition on the surface of steel fibers: A specific volume concentration of 0.05 mol / L Al2(SO4)3·18H2O solution, 0.3 mol / L Ca(OH)2 solution, and 10 mol / L NaOH solution were prepared. Recycled steel fibers were added to the Ca(OH)2 solution (fiber volume to solution volume fixed at 1:5). Under stirring at 200 rpm, Al2(SO4)3·18H2O and 0.2 parts of polycarboxylate superplasticizer (total solution volume) were added, and NaOH solution was added to control the pH of the system to 11-12.5. The reaction temperature was controlled at 30-50 ℃, and the reaction time was 4-6 hours. Specialized reinforcing steel fiber series products (ASF1-ASF7) were obtained. Specific processes and ettringite coating amounts are shown in Table 3.

[0023] Table 3. Process and Erythrite Coating Thickness of Special Reinforced Steel Fiber Series Products

[0024] This embodiment (1-7) describes a special reinforcing steel fiber for ultra-high performance concrete made entirely from solid waste and its modification method, which consists of the aforementioned recycled steel fiber cleaning process, PDA coating process, and ettringite deposition process. The ultra-high performance concrete material made entirely from solid waste used to test the fiber reinforcement effect consists of 84 parts granulated blast furnace slag powder, 15 parts calcium sulfate industrial solid waste, and 1 part alkaline industrial solid waste. The effects of different embodiments are shown in Table 4.

[0025] Table 4. Examples and their performance evaluation tables

[0026] Overall description of Examples 1-7 Examples 1-7 all employed the three-stage modification process proposed in this invention—pre-cleaning, PDA intermediate layer, and ettringite directional deposition—to prepare reinforcing steel fibers. These fibers were then incorporated into ultra-high performance concrete composed entirely of solid waste, including granulated blast furnace slag powder, calcium sulfate industrial solid waste, and alkaline industrial solid waste, to verify the effects of different modification parameters and fiber content on mechanical properties. Comparative Examples 1-4 correspond to no fiber addition, only cleaned fibers, only PDA-coated fibers, and only ettringite-deposited fibers, respectively, to illustrate the necessity and synergistic effects of each modification step.

[0027] Examples 1–3: The Influence of PDA Coating Conditions on Enhancement Effect In Examples 1-3, the steel fibers were all cleaned using the same process (S1) and ettringite deposition process (ASF2). Only the pH value of the Tris buffer solution during the dopamine coating stage was changed (PDA1-PDA3) to regulate the PDA coating structure.

[0028] The results showed that as the pH of the Tris buffer solution increased from 8 to 9, the 28-day flexural strength of concrete increased from 30 MPa to 37 MPa, with the growth rate increasing from 100% to 147%, and the compressive strength growth rate also increased from 42% to 54%. This indicates that under higher pH conditions, the dopamine oxidative polymerization reaction is more complete, resulting in a more continuous and denser PDA coating, which provides more stable nucleation sites for the subsequent directional deposition of ettringite crystals, thereby significantly improving the interfacial bonding strength between the fiber and the solid waste matrix.

[0029] Examples 2, 4, and 5: Effects of ettringite deposition parameters on performance Examples 2, 4, and 5 use the same cleaning and PDA coating processes, only changing the ettringite deposition process parameters.

[0030] The results showed that when the pH of the ettringite deposition system increased from 11 to 12.5, the thickness of the ettringite coating on the steel fiber surface increased, and the flexural strength of the concrete increased from 32 MPa to 38 MPa, while the compressive strength increased from 139 MPa to 148 MPa. This indicates that under suitable alkalinity conditions, ettringite crystals undergo more complete epitaxial growth on the PDA coating surface. Their needle-like crystal structure is highly homogeneous in crystal form and chemical composition with the ettringite hydration products generated in situ in the solid waste concrete, allowing for the formation of continuous mineral phase connections at the interface, thereby significantly enhancing the anchoring effect of the fibers.

[0031] Examples 1, 6, and 7: The effect of fiber content on reinforcement effect Examples 1, 6, and 7 use the same fiber modification process, only the fiber content is changed.

[0032] The results show that both the flexural and compressive strengths of concrete increase significantly with increasing fiber content. When the fiber content is 2%, the flexural strength growth rate reaches 100%, while when the content decreases to 0.5%, the flexural strength growth rate drops to 33%. This indicates that in the all-solid-waste UHPC system, the reinforcing effect of modified steel fibers depends not only on the interfacial properties but also on the spatial distribution density of the fibers in the matrix. Appropriately increasing the fiber content is beneficial for forming a more complete crack bridging network.

[0033] Comparative analysis: Comparative Example 1, which did not contain any fibers, had the lowest flexural and compressive strength in its concrete and was used as the reference sample.

[0034] Comparative Example 2, which only used cleaned recycled steel fibers, showed limited improvement in mechanical properties, indicating that simply removing surface impurities cannot significantly improve the interfacial bonding between the fibers and the solid waste matrix.

[0035] Comparative Example 3 introduced PDA coating on the cleaned fiber, which slightly improved the strength, indicating that the PDA coating can improve the surface activity of the fiber, but it lacks structural homogeneity with the ettringite matrix, so the reinforcing effect is still limited.

[0036] Comparative Example 4 only underwent ettringite deposition treatment, and its flexural and compressive strengths were improved compared to Comparative Examples 2 and 3, but were still significantly lower than those of the Example, indicating that the bonding stability of ettringite crystals on the fiber surface is insufficient in the absence of a PDA interlayer.

[0037] Overall conclusion: The results from the combined examples and comparative studies demonstrate that the cleaning, PDA pre-coating, and ettringite deposition-graded modification process proposed in this invention exhibits a significant synergistic enhancement effect in the all-solid-waste ultra-high-performance concrete system. The PDA coating provides a stable nucleation and adhesion base for ettringite crystals, while the ettringite coating layer is highly consistent with the hydration products of the all-solid-waste matrix in terms of chemical composition and crystal structure. The synergistic effect of these two processes significantly improves the fiber-matrix interfacial bonding strength, thereby achieving a simultaneous improvement in flexural and compressive strength.

[0038] The embodiments described above are some, but not all, embodiments of the present invention. The detailed description of the embodiments of the present invention is not intended to limit the scope of the claimed invention, but merely to illustrate selected embodiments. All other embodiments obtained by those skilled in the art based on the embodiments of the present invention without inventive effort are within the scope of protection of the present invention.

Claims

1. A method for modifying solid waste concrete with reinforcing steel fibers, characterized in that, The core steps are performed in the following order: (1) Raw material pretreatment: The recycled steel fibers from automobile tires are subjected to the following steps in sequence: stripping of surface rubber residue, magnetic separation purification, and oxidation modification of surface residual rubber; (2) Dopamine coating: The steel fibers after raw material pretreatment are immersed in dopamine-Tris buffer solution and a polydopamine (PDA) coating is formed through surface polymerization reaction; (3) Erythrite deposition modification: Erythrite crystal layer is directionally deposited on the surface of steel fiber coated with PDA to obtain special reinforced steel fiber.

2. The method according to claim 1, characterized in that, The specific steps for removing surface rubber residue in step (1) are as follows: using industrial-grade ultrasonic cleaning equipment, with a NaOH solution of 0.5-1.5% by mass as the cleaning medium, and continuously cleaning for 30-60 minutes at 50-60℃ and 40kHz to remove residual rubber particles attached to the surface of the steel fiber.

3. The method according to claim 1, characterized in that, In step (1), the magnetic separation purification is specifically carried out by using an industrial-grade magnetic separation equipment, setting the magnetic field strength to 0.3T-1T, and performing magnetic separation on the recycled steel fibers that have completed surface rubber peeling, so that the purity of the recycled steel fibers after separation is >95%.

4. The method according to claim 1, characterized in that, The surface residual rubber oxidation modification in step (1) specifically involves immersing the magnetically purified recycled steel fibers in a mixture of H2O2 and H2SO4 with a mass concentration of 1-5%, wherein the volume ratio of H2O2 to H2SO4 in the mixture is 3:1, and treating it at a constant temperature of 50-80℃ for 0.5-5 hours to remove residual rubber and activate the fiber surface.

5. The method according to claim 1, characterized in that, In step (2), the dopamine-Tris buffer solution is prepared by dissolving dopamine powder in Tris buffer solution with a pH of 8-9 and stirring until completely dissolved to form a dopamine-Tris solution with a concentration of 1-3 mg / mL.

6. The method according to claim 1, characterized in that, In step (2), the PDA coating is prepared by adding the pretreated recycled steel fibers to the solution at a volume ratio of 1:2-4. The polymerization reaction is carried out in a shaking manner at an ambient temperature of 20-35℃ for 0.5-2 hours, so that the dopamine is oxidized and polymerized on the fiber surface to form a PDA coating.

7. The method according to claim 1, characterized in that, Step (2) also includes a PDA coating post-treatment step: take out the steel fiber that has completed the polymerization reaction, rinse it repeatedly with deionized water 3-5 times to remove unreacted dopamine on the surface, and then dry it in a forced-air drying oven at 20-50℃ to constant weight to ensure stable adhesion of the PDA coating.

8. The method according to claim 1, characterized in that, Step (3) of the ettringite deposition modification specifically includes: 3.1 Preparation of reaction solutions: Prepare a 0.05 mol / L Al2(SO4)3·18H2O solution, a 0.3 mol / L Ca(OH)2 solution, and a 10 mol / L NaOH solution; 3.2 Electrite Deposition: Recycled steel fibers are added to a Ca(OH)2 solution, with the fiber volume to solution volume ratio fixed at 1:4-6. Al2(SO4)3·18H2O and 0.1-0.3% of polycarboxylate superplasticizer (total solution volume) are added under a stirring rate of 100-300 rpm. The pH of the system is controlled at 11.0-12.5 by adding NaOH or H2SO4. The reaction temperature is controlled at 30-50℃, and the reaction time is 4-6 hours. 3.3 Post-treatment: The reaction was terminated by rinsing with ethanol and then dried under vacuum at 60°C for 1 day.

9. A type of ultra-high performance concrete made entirely from solid waste, characterized in that, The concrete contains reinforcing steel fibers prepared by the modification method described in claims 1-8, with a dosage of 0.5-2% of the total mass of the concrete.

10. The all-solid-waste ultra-high-performance concrete according to claim 9, characterized in that, The cementitious system of the concrete consists of granulated blast furnace slag powder, industrial by-product gypsum, and alkaline industrial by-products, with iron tailings sand as the aggregate. Compared with the use of unmodified fibers, the 28-day flexural and compressive strengths of the concrete are improved.

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