An ultra-early-strength, high-toughness shotcrete and a preparation method thereof

Abstract

The application discloses an ultra-early-strength and high-toughness shotcrete and a preparation method thereof, and belongs to the technical field of concrete. The shotcrete comprises the following components: ordinary Portland cement, first-class fly ash, fly ash microbeads, machine-made sand, coarse aggregate, a calcium-based accelerator, first steel fibers, modified composite fibers, a composite admixture, a quick-setting agent and water. The modified composite fibers are prepared by compounding second steel fibers and polypropylene fibers in a mass ratio of 3-5:1, and then performing plasma surface activation treatment. The first steel fibers and the second steel fibers are both end-hook type steel fibers, and the polypropylene fibers are bundle-shaped monofilament polypropylene fibers. The application improves the mechanical occlusion capacity of the cement hydration product by adding the steel fibers and performing plasma modification treatment on the compounding mixture of the end-hook type steel fibers and the bundle-shaped monofilament polypropylene fibers, thereby improving the ultra-early toughness of the shotcrete and solving the problem of easy agglomeration of high-surface-energy fibers.

Description

Technical Field

[0001] This invention belongs to the field of concrete technology, specifically relating to an ultra-early strength, high-toughness shotcrete and its preparation method. Background Technology

[0002] In tunnel construction in high-stress, high-rockburst sections, rockburst hazards exhibit significant time-sensitivity, primarily occurring within the first 12 hours of excavation, with a peak frequency occurring 3-5 hours after excavation. This unique condition demands that the initial shotcrete support possess both extremely high compressive strength and excellent flexural toughness within its early-age period, enabling it to withstand the immense energy released by the surrounding rock through its ductile deformation. However, existing shotcrete technologies struggle to meet this demanding requirement. To achieve early strength and toughening, traditional techniques primarily rely on adjusting material proportions and using admixtures. For early strength, methods such as incorporating large amounts of sulfoaluminate cement or accelerators, and reducing the water-cement ratio, are commonly used to accelerate setting and hardening. For toughening, the common approach is to incorporate a single type of fiber (such as steel fiber or synthetic fiber) into the concrete mix through simple physical mixing. The random distribution of the fibers in the matrix bridges cracks, enhancing the material's crack resistance and deformation capacity. However, effective fiber dispersion remains a long-standing technological challenge. Existing technologies typically focus on increasing mechanical anchoring force by optimizing the fiber's own shape, such as end hooks or corrugations, or by relying on the lubrication and dispersing effects of additives to improve workability.

[0003] A search revealed that patent publication number CN118084437A discloses a high-early-strength, high-durability steel fiber shotcrete, which attempts to improve the dispersion of steel fibers by compounding special cement and a viscosity-regulating and quick-setting agent. Another patent publication number, CN119161156A, discloses a recycled aggregate shotcrete that utilizes a chemical activator to enhance cohesion, aiming to achieve lower spray rebound during construction. The core ideas of these solutions mostly focus on optimizing the material formulation. However, in actual mixing and shotcreting processes, especially when pursuing ultra-early strength leads to a rapid increase in the viscosity of the fresh concrete paste, the problem of fiber agglomeration becomes particularly prominent.

[0004] When a large amount of fiber is added to the mixer at once, the fibers easily become entangled due to physical entanglement and surface electrostatic interactions, forming fiber clumps that are difficult to encapsulate in the slurry. These fiber clumps become weak points in the strength and toughness of the concrete, not only failing to provide toughening but also potentially damaging the overall structure. More seriously, the high shear forces during mixing often cause fiber damage or excessive bending when attempting to break up these clumps, further weakening their performance. This uneven dispersion problem directly leads to poor workability and reduced pumpability of the concrete, and causes severe material rebound and waste during spraying, making it impossible to achieve the designed superior material properties in the final support structure. Therefore, there is an urgent need for a fiber introduction method based on the mixing process mechanism, which can improve the ultra-early toughness of shotcrete by adding hybrid fibers while solving the problem of uniform dispersion under high fiber content. Summary of the Invention

[0005] The purpose of this invention is to overcome the shortcomings of the prior art and provide an ultra-early strength, high-toughness shotcrete and its preparation method.

[0006] The objective of this invention is achieved through the following technical solution:

[0007] The first aspect of the present invention is to provide an ultra-early strength, high-toughness shotcrete, comprising, by weight, the following components: 350-400 parts of ordinary silicate cement, 130-150 parts of grade I fly ash, 120-130 parts of fly ash microspheres, 680-710 parts of manufactured sand, 520-550 parts of coarse aggregate, 3-5 parts of calcium-based accelerator, 120-130 parts of first steel fiber, 4-6 parts of modified composite fiber, 7-8 parts of composite admixture, 40-55 parts of quick-setting agent, and 160-170 parts of water; wherein the modified composite fiber is prepared by compounding second steel fiber and polypropylene fiber in a mass ratio of 3-5:1 and then subjecting it to plasma surface activation treatment, wherein both the first steel fiber and the second steel fiber are end-hooked steel fibers, and the polypropylene fiber is a bundle of monofilament polypropylene fiber.

[0008] This application designs a modified composite fiber by adding steel fibers to concrete matrix and then surface-activating a portion of the steel fibers and polypropylene fibers using plasma. All steel fibers used are end-hooked, and all polypropylene fibers are bundled monofilaments. The end-hooked structure increases pull-out resistance, while the bundled monofilament structure facilitates fiber dispersion. Plasma modification of the mixture creates a micro-coarsened interface on the fiber surface, improving mechanical bonding with cement hydration products. In the modified composite fiber, the mass ratio of the second steel fiber to the polypropylene fiber is 3-5:1. This ratio can be adjusted according to specific application requirements. For example, in high-strength applications, the ratio can be adjusted to 4:1 or 5:1, while in applications requiring high crack resistance, it can be adjusted to 2:1 or 1:1. Optimizing the fiber mixing ratio allows for consideration of both the mechanical properties of shotcrete and cost control. Adjusting the ratio according to the construction scenario achieves a balance between performance and cost.

[0009] Secondly, selecting primary fly ash and fly ash microspheres to form a dual-scale filling system can reduce porosity and increase the early densification rate. Primary fly ash provides pozzolanic reactivity, while fly ash microspheres fill the microporous structure of cement paste. The two work synergistically to reduce porosity and increase the early densification rate, and can also work with calcium-based accelerators to regulate hydration kinetics, thereby improving the early strength of concrete while maintaining volume stability. This compounding method forms a dense particle size distribution at the microscopic level, rapidly filling pores and accelerating the early hydration process. Thus, while achieving the ultra-early strength target of ≥5MPa compressive strength at 1 hour, it ensures good volume stability and density of the paste, and provides a superior matrix environment for uniform fiber dispersion and overall mechanical properties.

[0010] It is also understood that the ordinary Portland cement used in this application refers to ordinary Portland cement (P·O) as commonly understood by those skilled in the art, that is, a hydraulic cementitious material made of Portland cement clinker, 5% to 20% admixtures, and an appropriate amount of gypsum. Similarly, the Grade I fly ash used refers to Grade I fly ash as classified according to the national standard GB / T 1596-2017 "Fly Ash for Cement and Concrete".

[0011] Preferably, the modified composite fiber is prepared by plasma surface activation of the second steel fiber and the polypropylene fiber before compounding, wherein the second steel fiber is treated with low-temperature plasma at 30-50°C for 55-60 seconds, and the polypropylene fiber is treated with high-frequency pulsed plasma for 30-120 seconds.

[0012] Before compounding, the second steel fiber and polypropylene fiber were subjected to plasma surface activation treatment. High-energy particles excited the fiber surface, breaking down its molecular structure and increasing surface activity, thereby improving the adhesion between the fiber and the matrix. Different plasma treatment modes were used for different fiber types to accommodate the differences in their properties. Steel fiber, as a metallic material, and polypropylene fiber, as an organic polymer, have different plasma response mechanisms; therefore, using matching treatment methods for each is essential to achieve the best treatment effect after subsequent plasma surface activation.

[0013] The process involves treating steel fibers with low-temperature plasma at 30–50°C for 55–60 seconds, and polypropylene fibers with high-frequency pulsed plasma for 30–120 seconds. For hook-shaped steel fibers, plasma treatment for less than 55 seconds leads to insufficient activation, while treatment for more than 60 seconds causes excessive oxidation and reduces interfacial stability. After treatment, the surface roughness of hook-shaped steel fibers increases, and functional groups such as hydroxyl and carboxyl groups are generated, strengthening the mechanical and chemical bonding with the cement matrix. Excessive treatment time for bundled monofilament polypropylene fibers can damage the fiber surface; a treatment time of 30–45 seconds is preferred to ensure activation while avoiding fiber damage, and can be extended to 90–120 seconds if necessary. Furthermore, by adjusting the frequency and duty cycle of the high-frequency pulses, the surface roughness and polarity requirements of the polypropylene fibers can be further matched. After treatment, the surface activity of bundled monofilament polypropylene fibers is enhanced, along with increased hydrophilicity and reactivity, improving their dispersibility in concrete and preventing fiber agglomeration. Introducing various plasma treatment modes to optimize the activation process further increases fiber surface roughness and the amount of chemical functional groups introduced.

[0014] The activated second steel fiber and polypropylene fiber are mixed and compounded to obtain a composite fiber. A second plasma surface activation is then performed, followed by glow discharge treatment to obtain the modified composite fiber. The treatment time is controlled between 45 and 60 seconds. It is understood that glow discharge treatment is a type of plasma surface treatment, and it is preferred here because it meets the mild treatment conditions of low temperature and low pressure. Specifically, the composite fiber is placed in the reaction chamber of a 13.56 MHz low-temperature radio frequency plasma treatment device or a 2.45 GHz microwave plasma treatment device. The reactor is a cylindrical quartz tube or a parallel plate electrode structure with an electrode spacing of 40–60 mm. Before treatment, the reaction chamber is evacuated to a background vacuum of ≤5 Pa, and then argon gas with a purity ≥99.999% is introduced, i.e., the reaction is carried out under an argon atmosphere. The gas flow rate is controlled between 50 and 200 sccm, and the working pressure inside the chamber is stabilized between 10 and 50 Pa. The plasma power supply was activated at an output power of 15kW to perform glow discharge treatment on the composite fibers. The treatment time was controlled between 45 and 60 seconds, preferably 55 seconds. During the treatment, the sample stage temperature was maintained at 30–50°C using a circulating water cooling system to prevent fiber damage due to overheating. During treatment, the fibers were kept in a state of agitation by mechanical vibration or airflow disturbance to ensure uniform surface activation. After treatment under the above conditions, a micro-coarsened structure was formed on the surface of the second steel fiber, and the surface contact angle was reduced to below 60°, optimizing the surface wettability of the hook-shaped steel fiber. The surface roughness Ra of the monofilament polypropylene fiber reached 0.2–0.5 μm. Simultaneously, active functional groups, mainly hydroxyl and carboxyl groups, were introduced into the surfaces of both types of fibers, constructing a micro-coarsened interface. This significantly enhanced the mechanical interlocking and chemical bonding strength between the fiber and the cement matrix, laying the interfacial foundation for improving the early toughness and ultimate bearing capacity of concrete.

[0015] Preferably, the lengths of the first steel fiber and the second steel fiber are both 12-18 mm, and the length of the polypropylene fiber is 6-9 mm.

[0016] Preferably, the primary fly ash has a particle size of 0.4–60 μm, a D50 of 7–9 μm, and a glass content ≥80 wt%; the fly ash microspheres have a particle size of 0.2–8 μm, a D50 of 2–3 μm, and a hollow microsphere content ≥70 wt%. Controlling the particle size and activity of the primary fly ash and fly ash microspheres can better form a dual-filler system compatible with concrete materials.

[0017] Preferably, the manufactured sand has a continuous gradation of 0-5mm particle size, a fineness modulus of 2.5-2.8, and an MB value ≤1.4. The continuous gradation of manufactured sand results in a uniform particle distribution, which can effectively fill the gaps between coarse aggregates. Combined with the dual-scale filling of fly ash microspheres and grade I fly ash, it further optimizes the internal structure of concrete and improves its density and mechanical properties.

[0018] Preferably, the coarse aggregate has a particle size of 5-10 mm, an ultra-small particle size of ≤5.0%, a mud powder content of ≤0.5 wt%, and a needle-like and flaky particle content of ≤5.0 wt%. The coarse aggregate in this application can be a common coarse aggregate in the art, preferably crushed stone or pebbles, whose gaps can be effectively filled.

[0019] Preferably, the calcium-based accelerator is one or more of calcium formate, calcium nitrite, and amorphous calcium aluminate powder.

[0020] Preferably, the composite admixture includes one or two of early-strength polycarboxylate superplasticizers and viscosity modifiers.

[0021] A second aspect of the present invention is to provide a method for preparing the aforementioned ultra-early strength, high-toughness shotcrete, comprising the following steps:

[0022] S1 Base Material Premix: The ordinary silicate cement, the grade I fly ash, the fly ash microspheres, the manufactured sand, the coarse aggregate and the calcium-based accelerator are put into a planetary mixer according to the corresponding weight parts, and dry-mixed at a speed of 45r / min for 100~120s to obtain a base material with uniform composition and stable gradation.

[0023] S2 Plasma Surface Activation Modification: The second steel fiber and the polypropylene fiber are respectively activated by plasma surface activation, and then compounded at a mass ratio of 3~5:1 to form a composite fiber. The composite fiber is placed in a low-temperature radio frequency plasma or microwave plasma treatment device and subjected to plasma surface activation treatment under an argon atmosphere to obtain the modified composite fiber.

[0024] S3 Three-stage gradient dispersion addition: After surface dust removal, the first steel fiber is combined with the modified composite fiber to obtain a hybrid fiber. The hybrid fiber is added to the base material obtained in S1 in three stages to obtain a premix.

[0025] S4 Water and Admixture Molding: Under continuous stirring, the composite admixture and mixing water at 15±2℃ are added to the premix within 30s, and then stirred at 65r / min for 180s to obtain the mixture of ultra-early strength and high toughness shotcrete, and the quick-setting agent is added before spraying.

[0026] Specifically, in step S1, fly ash microspheres preferentially fill the gaps between coarse aggregates, and primary fly ash is uniformly dispersed on the surface of cement particles, optimizing the gradation stability and density of the matrix. Step S2 is performed using the plasma surface activation method described above.

[0027] Step S3 involves adding the hybrid fibers obtained by mixing the plasma-activated modified composite fibers and the first steel fiber to the base material obtained in step S1 in three stages, combined with the mechanical shearing action of 5-10mm coarse aggregate. This addition method is based on optimal parameter verification, combined with optimized design of fiber ratio, plasma treatment mode, and treatment time. It forces the fiber mixture to undergo the processes of spreading, cross-attachment, and final dispersion and encapsulation in the mixing chamber, thereby achieving uniform and cross-distributed fibers at the microscopic level. The mechanical shearing action of the coarse aggregate further enhances the uniform distribution effect, effectively controlling the diameter of single fiber agglomerates to below 5mm, fundamentally ensuring the full realization of the fiber reinforcement and toughening effect, and systematically solving the problem of easy agglomeration of high surface energy fibers. In the three-stage addition strategy, the first addition is used to form a basic fiber network, the second addition establishes stable isolation points around the existing network, and the third addition completes gradient spreading in a confined flow field. The three additions, combined with the shearing of coarse aggregate and particle collision field respectively, allow the fibers to spread out in space step by step, avoiding entanglement and agglomeration caused by a one-time high-concentration addition.

[0028] In step S4, the mixing water temperature is controlled at 15±2℃ to suppress bleeding and localized condensation caused by excessively rapid hydration reaction, while ensuring fluidity and pumpability. Too low a temperature will reduce the efficiency of early strength enhancement, while too high a temperature will exacerbate fiber agglomeration and spray rebound. This temperature range, together with the accelerator, optimizes the coagulation kinetics.

[0029] Preferably, in step S3, the amount of the mixed fiber added each time is 1 / 3 of the total amount of the mixed fiber. The first addition is carried out with stirring at a speed of 45 r / min, the second addition is carried out with stirring at a speed of 45 r / min, and the third addition is carried out with stirring at a speed of 65 r / min for 180 s.

[0030] By controlling the specific addition and mixing parameters, one-third of the total mixed fiber was added for the first time, and the fibers were spread along the surface of the coarse aggregate and between the particles under low-speed mixing at 45 r / min, forming a primary dispersion structure. The second addition of one-third of the total fiber was carried out while maintaining low-speed mixing at 45 r / min, allowing the newly added fibers to preferentially adhere to and cross-distribute around the already dispersed fibers. For the third addition of the remaining fibers, the mixing speed was increased to 65 r / min and maintained for 180 s, allowing the fibers to further disperse in the shear flow field and be fully coated by the slurry. The 45 r / min premixing and 65 r / min final mixing constituted a staged shear regime. The low shear in the initial stage facilitated particle size distribution stability and initial fiber spreading, while the high shear in the later stage facilitated further fiber dispersion and slurry coating. This shear gradient, synergistically with the three-stage addition, significantly reduced the probability of secondary agglomeration of the plasma-activated fibers. Finally, under microscopic observation, the fibers exhibited a uniform cross-distribution, with individual agglomerate diameters ≤5 mm.

[0031] Preferably, the accelerator is added before the ultra-early strength, high toughness shotcrete is sprayed, forming a synergistic regulation with the composite admixture, so that the slurry can quickly establish structural strength at the moment of spraying without damaging the fiber distribution.

[0032] The beneficial effects of this invention are:

[0033] (1) This application first selects to add a small amount of a compound mixture of end-hooked steel fibers and bundled monofilament polypropylene fibers on the basis of adding end-hooked steel fibers. The mechanical interlocking ability between the fiber materials and cement hydration products can be greatly improved by plasma surface activation technology. Among the remaining raw materials, a "dual-scale filling system" is composed of primary fly ash and fly ash microspheres. The former provides pozzolanic activity, and the latter efficiently fills the micropores. In conjunction with calcium-based accelerators, the early hydration process and microstructure density are optimized. This design enables concrete to quickly establish a structural strength of more than 5 MPa within 1 hour, meeting the requirements of rapid support, while avoiding problems such as increased shrinkage and easy cracking caused by solely pursuing early strength.

[0034] (2) The plasma treatment in this application includes differentiated treatment of steel fibers and polypropylene fibers respectively. For steel fibers, low-temperature plasma treatment is used to roughen their surface and generate active groups such as hydroxyl and carboxyl groups. For polypropylene fibers, high-frequency pulsed plasma treatment is used to improve their surface roughness and hydrophilicity. This treatment creates a micro-interface on the fiber surface that is more conducive to mechanical interlocking and chemical bonding.

[0035] (3) This application proposes a phased, gradient fiber addition process, coupled with a shear regime from low speed to high speed, so that the plasma-activated high surface energy fibers can be orderly guided and dispersed during the stirring process. This process forces the modified composite fibers to enter the stirring system in batches and in layers. The first addition forms a primary skeleton, and subsequent additions are cross-distributed with the front fibers as isolation points. Finally, under high shear, they are fully wrapped by the slurry, effectively avoiding the formation of clusters due to physical entanglement and electrostatic adsorption of the fibers.

[0036] (4) This application integrates and coordinates the process of plasma activation of the above-mentioned fibers, sets up a three-stage gradient addition, and combines it with a dual-scale filling system, along with key process parameters such as mixing water temperature control at 15±2℃ and online addition of accelerator, to form a systematic process integration and synergistic control of ultra-early strength concrete. It achieves an excellent balance among multiple requirements, satisfying the engineering needs of rapid support through ultra-early strength, and achieving high toughness and low rebound rate (the rebound rate can be reduced to below 10%) through the extreme dispersion of fibers. Finally, it forms a replicable, controllable, and high-performance industrial preparation solution for ultra-early strength high-toughness shotcrete. Attached Figure Description

[0037] Figure 1 This is a flowchart of the preparation method of this application. Detailed Implementation

[0038] The technical solution of the present invention will be clearly and completely described below with reference to the embodiments. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention.

[0039] See Figure 1 The present invention provides a technical solution:

[0040] An ultra-early strength, high-toughness shotcrete comprises, by weight, the following components: 350-400 parts ordinary Portland cement, 130-150 parts grade I fly ash, 120-130 parts fly ash microspheres, 680-710 parts manufactured sand, 520-550 parts coarse aggregate, 3-5 parts calcium-based accelerator, 120-130 parts first steel fiber, 4-6 parts modified composite fiber, 7-8 parts composite admixture, 40-55 parts quick-setting agent, and 160-170 parts water; the modified composite fiber is prepared by compounding second steel fiber and polypropylene fiber in a mass ratio of 3-5:1 and then undergoing plasma surface activation treatment; both the first and second steel fibers are end-hooked steel fibers, and the polypropylene fiber is a bundle of monofilament polypropylene fiber.

[0041] Before compounding, the modified composite fibers were subjected to plasma surface activation of the second steel fiber and the polypropylene fiber, respectively. The second steel fiber was treated with low-temperature plasma at 30-50°C for 55-60 seconds, and the polypropylene fiber was treated with high-frequency pulsed plasma for 30-120 seconds.

[0042] The composite fiber is obtained by compounding the second steel fiber and the polypropylene fiber. The modified composite fiber is then subjected to glow discharge treatment for 45–60 seconds. In the modified composite fiber after plasma surface activation treatment, the surface contact angle of the second steel fiber is ≤60°, the surface roughness Ra of the monofilament polypropylene fiber reaches 0.2–0.5 μm, and hydroxyl and carboxyl active functional groups are introduced onto the surfaces of both the second steel fiber and the polypropylene fiber.

[0043] The lengths of the first and second steel fibers are both 12–18 mm, and the lengths of the polypropylene fibers are 6–9 mm.

[0044] The particle size of the aforementioned Grade I fly ash is 0.4–60 μm, D50 is 7–9 μm, and glass content is ≥80 wt%. The particle size of the aforementioned fly ash microspheres is 0.2–8 μm, D50 is 2–3 μm, and hollow microsphere content is ≥70 wt%. The particle size of the aforementioned manufactured sand is 0–5 mm continuously graded, fineness modulus is 2.5–2.8, and MB value is ≤1.4. The particle size of the aforementioned coarse aggregate is 5–10 mm, oversize ≤5.0%, mud powder content ≤0.5 wt%, and needle-like and flaky particle content ≤5.0 wt%.

[0045] The aforementioned calcium-based accelerator is one or more of calcium formate, calcium nitrite, and amorphous calcium aluminate powder. The aforementioned composite additive includes one or two of early-strength polycarboxylate superplasticizer and viscosity modifier.

[0046] A method for preparing ultra-early strength, high-toughness shotcrete includes the following steps:

[0047] S1 Base Material Premix: The above-mentioned ordinary silicate cement, the above-mentioned grade I fly ash, the above-mentioned fly ash microspheres, the above-mentioned manufactured sand, the above-mentioned coarse aggregate and the above-mentioned calcium-based accelerator are put into a planetary mixer according to the corresponding weight parts, and dry-mixed at a speed of 45r / min for 100~120s to obtain a base material with uniform composition and stable gradation.

[0048] S2 Plasma Surface Activation Modification: The above-mentioned second steel fiber and the above-mentioned polypropylene fiber are respectively activated by plasma surface, and then compounded at a mass ratio of 3~5:1 to form a composite fiber. The above-mentioned composite fiber is placed in a low-temperature radio frequency plasma or microwave plasma treatment device and subjected to plasma surface activation treatment under an argon atmosphere to obtain modified composite fiber.

[0049] S3 Three-stage gradient dispersion addition: After surface dust removal, the first steel fiber is combined with the modified composite fiber to obtain a mixed fiber. The mixed fiber is added to the base material obtained in S1 in three stages, with each addition amount being 1 / 3 of the total amount of the mixed fiber. The first addition is carried out under stirring at a speed of 45 r / min, the second addition is carried out while maintaining a stirring speed of 45 r / min, and the third addition is carried out by increasing the stirring speed to 65 r / min and continuing for 180 s to obtain a premixed material.

[0050] S4 Water and Admixture Molding: Under continuous stirring, the composite admixture and mixing water at 15±2℃ are added to the above premix within 30s, and then stirred at 65r / min for 180s to obtain the above ultra-early strength, high toughness shotcrete mixture, and the above quick-setting agent is added before spraying. Example 1

[0051] Reference Figure 1 , Figure 1This is a flowchart of the preparation method of this application. The implementation methods of Embodiment 1 of this invention are all based on the specific implementation methods described below.

[0052] The composition of ultra-early strength, high-toughness shotcrete by weight is as follows:

[0053] 370 parts ordinary Portland cement, 140 parts grade I fly ash (particle size 0.4–60 μm, D50 = 8 μm, glass content ≥ 80%), 130 parts fly ash microspheres (particle size 0.2–8 μm, D50 = 2.5 μm, hollow microspheres ≥ 70%), 695 parts manufactured sand (0–5 mm continuous gradation, fineness modulus 2.65, MB ≤ 1.4), 535 parts 5–10 mm coarse aggregate (oversize ≤ 5 mm). 0.0%, mud powder ≤0.5%, needle-like and flaky ≤5.0%), 4 parts calcium-based accelerator (calcium formate + amorphous calcium aluminate compound), 125 parts first steel fiber (12-18mm end hook type), 4 parts modified composite fiber (end hook type second steel fiber: bundled monofilament polypropylene fiber = 3:1), 7.5 parts composite admixture (early strength polycarboxylate superplasticizer + viscosity modifier), 48 parts quick-setting agent, 165 parts water (15±2℃).

[0054] The fibers were modified by plasma surface activation: 12–18 mm end-hooked second steel fibers were treated with low-temperature plasma at 40°C for 55 s, and 6–9 mm bundled monofilament polypropylene fibers were treated with high-frequency pulsed plasma for 40 s. The modified second steel fibers and modified polypropylene fibers were then mixed at a mass ratio of 3:1 and treated in a low-temperature radio frequency plasma device with an argon atmosphere and a total power of 15 kW. The contact angle of the treated second steel fibers was approximately 55°, and the Ra of the polypropylene fibers was approximately 0.32 μm.

[0055] Preparation of ultra-early strength, high-toughness shotcrete:

[0056] S1 base material premix: Ordinary silicate cement, grade 1 fly ash, fly ash microspheres, manufactured sand, coarse aggregate, and calcium-based accelerator are dry-mixed in a planetary mixer at 45 r / min for 120 s according to the above weight proportions to obtain a uniform base material.

[0057] S3 Three-Stage Gradient Dispersion Addition: After surface dust removal, the first steel fiber is combined with the modified composite fiber to obtain a hybrid fiber. The hybrid fiber is added in three stages. The first stage adds 1 / 3 of the total fiber volume and stirs at 45 r / min for 60 s to form a primary fiber skeleton. The second stage adds 1 / 3 of the total fiber volume and stirs at 45 r / min for 60 s to form an isolation point network. The third stage adds 1 / 3 of the total fiber volume and stirs at 65 r / min for 180 s to complete the gradient spreading.

[0058] S4 Water and Admixture Molding: Add the composite admixture and water at 15±2℃ at a uniform speed within 30s, and continue stirring at 65r / min for 180s to obtain the mixture of ultra-early strength and high toughness shotcrete of this application. When using, add 48 parts of quick-setting agent online before spraying.

[0059] The prepared sprayed concrete mixture was sprayed and its performance was tested. The test results are shown in Table 1. Table 1 shows the performance test results of the sprayed concrete. The concrete mixture rapidly developed a structural strength exceeding 5 MPa within 1 hour, and its compressive strength was 13.1 MPa at 3 hours, 22.5 MPa at 8 hours, and 47.8 MPa at 1 day; the flexural toughness index I at 3 hours was also measured. 20 The bending toughness index I is 15.9 at 8h. 20 The bending toughness index I is 18.4, 1d. 20 The bending toughness index I is 21.0 and 28d. 20 The strength was 28.2; microscopic observation showed that the fibers were uniformly and cross-distributed, with individual agglomerates having a diameter of ≤4mm. This demonstrated excellent early strength and toughness properties. Example 2

[0060] Reference Figure 1 , Figure 1 This is a flowchart of the preparation method of this application. The implementation method of Example 2 of this invention is basically the same as that of Example 1, and the specific implementation is as follows.

[0061] The physical property parameters of the raw material formulation for ultra-early strength and high-toughness shotcrete are consistent with those in Example 1. The formulation composition of ultra-early strength and high-toughness shotcrete, by weight, is as follows:

[0062] 390 parts of ordinary silicate cement, 135 parts of grade I fly ash, 125 parts of fly ash microspheres, 705 parts of manufactured sand, 545 parts of coarse aggregate, 3.5 parts of calcium-based accelerator, 130 parts of first steel fiber, 5 parts of modified composite fiber (end-hooked second steel fiber: bundled monofilament polypropylene fiber = 4:1), 8 parts of composite admixture, 52 parts of quick-setting agent, and 160 parts of water.

[0063] The fibers were modified by plasma surface activation: 12–18 mm hook-shaped second steel fibers were treated with low-temperature plasma at 50°C for 60 s, and 6–9 mm bundled monofilament polypropylene fibers were treated with high-frequency pulsed plasma for 45 s. The modified second steel fibers and modified polypropylene fibers were then mixed at a mass ratio of 4:1 and treated in a low-temperature radio frequency plasma device with an argon atmosphere and a total power of 15 kW. The contact angle of the treated second steel fibers was approximately 50°, and the Ra of the polypropylene fibers was approximately 0.45 μm.

[0064] The preparation process and parameters of ultra-early strength and high toughness shotcrete are consistent with those in Example 1.

[0065] The prepared sprayed concrete mixture was sprayed and its performance was tested. The test results are shown in Table 1. The obtained concrete mixture also rapidly established a structural strength exceeding 5 MPa within 1 hour, and its compressive strength was 12.9 MPa at 3 hours, 23.1 MPa at 8 hours, and 44.6 MPa at 1 day; the flexural toughness index I at 3 hours was also measured. 20 The bending toughness index I is 15.6 and 8h. 20 The bending toughness index I is 19.1, 1d. 20 The bending toughness index I is 22.0 and 28d. 20 The strength was 27.6; microscopic observation showed that the fibers were uniformly cross-distributed, and the diameter of individual agglomerates was ≤4mm. This demonstrated excellent early strength and toughness properties. Example 3

[0066] Reference Figure 1 , Figure 1 This is a flowchart of the preparation method of this application. The implementation method of Example 3 of the present invention is basically the same as that of Example 1, and the specific implementation is as follows.

[0067] The physical property parameters of the raw material formulation for ultra-early strength and high-toughness shotcrete are consistent with those in Example 1. The formulation composition of ultra-early strength and high-toughness shotcrete, by weight, is as follows:

[0068] 360 parts of ordinary silicate cement, 150 parts of grade I fly ash, 140 parts of fly ash microspheres, 685 parts of manufactured sand, 520 parts of coarse aggregate, 5 parts of calcium-based accelerator, 120 parts of first steel fiber, 3 parts of modified composite fiber (end-hooked second steel fiber: bundled monofilament polypropylene fiber = 2.5:1, biased towards crack resistance), 7 parts of composite admixture, 45 parts of quick-setting agent, and 170 parts of water.

[0069] The fibers were modified by plasma surface activation: 12–18 mm end-hooked second steel fibers were treated with low-temperature plasma at 35°C for 56 s, and 6–9 mm bundled monofilament polypropylene fibers were treated with high-frequency pulsed plasma for 90 s (extended activation). The modified second steel fibers and modified polypropylene fibers were then mixed at a mass ratio of 4:1 and treated in a low-temperature radio frequency plasma device with an argon atmosphere and a total power of 15 kW. The contact angle of the treated second steel fibers was approximately 58°, and the Ra of the polypropylene fibers was approximately 0.28 μm.

[0070] The preparation process and parameters of ultra-early strength and high toughness shotcrete are consistent with those of Example 1. The only difference is that a 30-second steady-state mixing window is added after the second addition to enhance the stability of the "isolation point".

[0071] The prepared sprayed concrete mixture was sprayed and its performance was tested. The test results are shown in Table 1. The obtained concrete mixture also rapidly developed a structural strength exceeding 5 MPa within 1 hour, with a 3-hour compressive strength of 13.8 MPa, an 8-hour compressive strength of 24.5 MPa, and a 1-day compressive strength of 47.5 MPa; the 3-hour flexural toughness index I... 20 The bending toughness index I is 16.4, 8h. 20 The bending toughness index I is 21.0, 1d. 20 The bending toughness index I is 23.2 and 28 days. 20 The strength was 29.9; microscopic observation showed that the fibers were uniformly and cross-distributed, with individual agglomerates having a diameter of ≤4mm. This demonstrated excellent early strength and toughness properties. Comparative Example 1

[0072] The early-strength shotcrete prepared in Comparative Example 1 of this invention has a formula and preparation method that are basically the same as those in Example 1. It also adopts a three-stage addition method. The only difference is that the fibers used are not subjected to any plasma modification treatment.

[0073] The prepared sprayed concrete mixture was sprayed and its performance was tested. The test results are shown in Table 1. The compressive strength of the obtained concrete mixture was 9.5 MPa at 3 hours, 12.2 MPa at 8 hours, and 31.2 MPa at 1 day; the flexural toughness index I at 3 hours was [missing value]. 20 The bending toughness index I is 8.8 and 8h. 20 The bending toughness index I is 12.9, 1d. 20 The bending toughness index I is 18.2 and 28 days. 20 The strength was 21.7; microscopic observation revealed local fiber entanglement, with agglomerated clumps reaching 10–15 mm in diameter. These test results indicate that, due to the lack of plasma-activated modification, the fiber interface exhibits poor adhesion and dispersion stability, leading to fiber agglomeration and entanglement, ultimately resulting in lower strength and toughness of the concrete. Comparative Example 2

[0074] Comparative Example 2 of this invention prepares early-strength shotcrete. The formula and preparation method are basically the same as those in Example 1. The plasma modification process and parameters of Example 1 are also used. The only difference is that the fiber is added at one time instead of using a three-stage gradient addition.

[0075] The prepared sprayed concrete mixture was sprayed and its performance was tested. The test results are shown in Table 1. The compressive strength of the obtained concrete mixture was 8.8 MPa at 3 hours, 14.2 MPa at 8 hours, and 33.2 MPa at 1 day; the flexural toughness index I at 3 hours was [missing value]. 20 The bending toughness index I is 9.0 and 8h.20 The bending toughness index I is 11.4, 1d. 20 The bending toughness index I is 17.8 and 28 days. 20 The value was 21.1; the diameter of the agglomerated clumps was generally >12mm, with obvious fiber "clumps". The test results show that, due to the three-stage gradient addition design in the example, and using the first added fiber as an isolation point, the problem of easy agglomeration of high surface energy fibers after plasma activation was solved, thus avoiding the fiber agglomeration phenomenon seen in Comparative Example 2, and preventing a deterioration in the strength and toughness of the concrete.

[0076] Table 1 Performance Test Results of Shotcrete

[0077]

[0078] The above description is merely a preferred embodiment of the present invention. It should be understood that the present invention is not limited to the forms disclosed herein and should not be construed as excluding other embodiments. It can be used in various other combinations, modifications, and environments, and can be altered within the scope of the concept described herein through the above teachings or related technologies or knowledge. Modifications and variations made by those skilled in the art that do not depart from the spirit and scope of the present invention should be within the protection scope of the appended claims.

Claims

1. An ultra-high early strength, high toughness shotcrete, characterized by: By weight, it comprises the following components: 350-400 parts of ordinary Portland cement, 130-150 parts of Grade I fly ash, 120-130 parts of fly ash microspheres, 680-710 parts of manufactured sand, 520-550 parts of coarse aggregate, 3-5 parts of calcium-based accelerator, 120-130 parts of first-grade steel fiber, 4-6 parts of modified composite fiber, 7-8 parts of composite admixture, 40-55 parts of quick-setting agent, and 160-170 parts of water; The modified composite fiber is obtained by compounding a second steel fiber and a polypropylene fiber in a mass ratio of 3 to 5:1, and then subjecting the composite fiber to glow discharge treatment for a treatment time of 45 to 60 seconds. The first steel fiber and the second steel fiber are both end-hooked steel fibers, and the polypropylene fiber is a bundle of monofilament polypropylene fiber. Before compounding, the modified composite fiber is used to perform plasma surface activation on the second steel fiber and the polypropylene fiber respectively. The second steel fiber is treated with low-temperature plasma at 30-50°C for 55-60 seconds, and the polypropylene fiber is treated with high-frequency pulsed plasma for 30-120 seconds.

2. The ultra-high early strength, high ductility shotcrete according to claim 1, characterized in that: In the modified composite fiber after plasma surface activation treatment, the surface contact angle of the second steel fiber is ≤60°, the surface roughness Ra of the polypropylene fiber reaches 0.2~0.5μm, and hydroxyl and carboxyl active functional groups are introduced into the surface of both the second steel fiber and the polypropylene fiber.

3. The ultra-high early strength, high ductility shotcrete according to claim 1, characterized in that: The lengths of the first and second steel fibers are both 12–18 mm, and the length of the polypropylene fiber is 6–9 mm.

4. The ultra-high early strength, high ductility, shotcrete according to claim 1, characterized in that: The grade I fly ash has a particle size of 0.4–60 μm, a D50 of 7–9 μm, and a glass content of ≥80 wt%. The fly ash microspheres have a particle size of 0.2–8 μm, a D50 of 2–3 μm, and a hollow microsphere content of ≥70 wt%. The manufactured sand has a particle size of 0~5mm with continuous gradation, a fineness modulus of 2.5~2.8, and an MB value ≤1.4; The coarse aggregate has a particle size of 5~10mm, an oversize of ≤5.0%, a mud powder content of ≤0.5wt%, and a needle-like and flaky particle content of ≤5.0wt%.

5. The ultra-early strength, high-toughness shotcrete according to claim 1, characterized in that: The calcium-based accelerator is one or more of calcium formate, calcium nitrite, and amorphous calcium aluminate powder.

6. The ultra-early strength, high-toughness shotcrete according to claim 1, characterized in that: The composite admixture includes one or two of the following: early-strength polycarboxylate superplasticizer and viscosity modifier.

7. A method for preparing ultra-early strength, high-toughness shotcrete as described in any one of claims 1-6, characterized in that: Includes the following steps: S1 Base Material Premix: The ordinary silicate cement, the grade 1 fly ash, the fly ash microspheres, the manufactured sand, the coarse aggregate and the calcium-based accelerator are put into a planetary mixer according to the corresponding weight parts, and dry-mixed at a speed of 45r / min for 100~120s to obtain a base material with uniform composition and stable gradation. S2 Plasma Surface Activation Modification: The second steel fiber and the polypropylene fiber are respectively activated by plasma surface, and then compounded at a mass ratio of 3~5:1 to form a composite fiber. The composite fiber is placed in a low-temperature radio frequency plasma or microwave plasma treatment device and subjected to plasma surface activation treatment under an argon atmosphere to obtain the modified composite fiber. S3 Three-stage gradient dispersion addition: After surface dust removal, the first steel fiber is combined with the modified composite fiber to obtain a hybrid fiber. The hybrid fiber is added to the base material obtained in S1 in three stages to obtain a premix. S4 Water and Admixture Molding: Under continuous stirring, the composite admixture and mixing water at 15±2℃ are added to the premix within 30s, and then stirred at 65r / min for 180s to obtain the mixture of ultra-early strength and high toughness shotcrete, and the quick-setting agent is added before spraying.

8. The method for preparing ultra-early strength, high-toughness shotcrete according to claim 7, characterized in that: In step S3, the amount of the mixed fiber added each time is 1 / 3 of the total amount of the mixed fiber. The first addition is carried out with stirring at a speed of 45 r / min, the second addition is carried out with stirring at a speed of 45 r / min, and the third addition is carried out with stirring at a speed of 65 r / min for 180 s.

Images