Low-rebound-ratio c25 shotcrete and preparation process thereof
By combining an alkali-free quick-setting agent with a modified polycarboxylate superplasticizer, a calcium alum network structure is formed, which solves the problem of high rebound rate in shotcrete and achieves the preparation of shotcrete with low rebound rate, high workability and high mechanical properties, which is suitable for urban rail transit and foundation pit support engineering.
Patent Information
- Authority / Receiving Office
- CN · China
- Patent Type
- Applications(China)
- Current Assignee / Owner
- 华东材料无锡有限公司
- Filing Date
- 2026-04-25
- Publication Date
- 2026-07-14
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Abstract
Description
Technical Field
[0001] This application relates to the field of concrete technology, and in particular to a low-resilience C25 shotcrete and its preparation process. Background Technology
[0002] Shotcrete is a concrete mixture containing a quick-setting agent that is sprayed at high speed onto a target surface using compressed air or spraying machinery. After rapid setting and hardening, it forms a support or lining structure. Compared to traditional cast-in-place concrete, shotcrete requires no formwork, has a faster construction speed, and is more adaptable, making it widely used in underground and geotechnical engineering projects such as tunnel initial support, foundation pit slope reinforcement, and mine roadway lining. According to the construction process, it can be divided into dry spraying and wet spraying. Wet spraying, due to its lower dust content, relatively controllable rebound rate, and stable concrete quality, has gradually become the mainstream method. However, shotcrete generally suffers from a high rebound rate during construction. The rebound rate of conventional wet spraying is typically 20%-30%, and even higher in some cases. A high rebound rate not only leads to a significant waste of raw materials such as cement and aggregate, increasing project costs, but also affects the actual thickness, density, and support strength of the sprayed layer, reducing project quality and safety redundancy. Furthermore, high rebound leads to the accumulation of rebound material, increasing the workload of debris removal and affecting construction progress and the working environment. Therefore, reducing the rebound rate of shotcrete has always been a technical challenge that has attracted continuous attention from those skilled in the art.
[0003] To address the high rebound rate of shotcrete, existing research has attempted to optimize aspects such as the type and dosage of accelerators, aggregate gradation, cementitious material dosage, working air pressure, and spraying distance. For example, increasing the dosage of cementitious materials or adding thickeners can improve the cohesiveness of concrete and reduce aggregate rebound; or alkali-free accelerators can be used to balance setting time and later strength. However, existing technologies still have shortcomings: on the one hand, simply increasing the dosage of cementitious materials can reduce rebound to some extent, but it leads to increased concrete costs and a greater risk of shrinkage cracking; on the other hand, the matching relationship between the dosage of accelerators and setting time, early strength, and later strength is difficult to control precisely. If the dosage is too low, the concrete is prone to flowing on the sprayed surface and increased rebound; if the dosage is too high, it can easily cause severe loss of later strength or even cracking of the sprayed layer. Furthermore, for commonly used strength grades such as C25 shotcrete, existing mix design methods mostly follow those of ordinary concrete, failing to fully consider the coupled effects between slump, working air pressure, spraying distance and rebound rate in wet spraying processes. The lack of systematic optimization schemes results in a still high rebound rate in actual engineering projects, making it difficult to simultaneously meet the requirements of low rebound, high workability and good mechanical properties.
[0004] Therefore, developing a C25 shotcrete that can effectively reduce rebound rate and balance workability and mechanical properties, along with its supporting preparation process, remains a pressing technical problem to be solved in this field. Summary of the Invention
[0005] In order to effectively reduce the rebound rate while taking into account the workability and mechanical properties of concrete, this application provides a low-rebound-rate C25 shotcrete and its preparation process.
[0006] In a first aspect, this application provides a low-rebound-rate C25 shotcrete, employing the following technical solution: A low-resilience C25 shotcrete, the raw materials for which are prepared include the following components in parts by weight: 420-440 parts of cementitious material, 800-900 parts of coarse aggregate, 800-900 parts of fine aggregate, 175-190 parts of water, 3.0-4.0 parts of polycarboxylate superplasticizer, and 20-35 parts of accelerator, wherein the accelerator is an alkali-free accelerator.
[0007] The inventors discovered that, comparing the test results of no accelerator, accelerator with alkali, and accelerator without alkali: without accelerator, the concrete sets slowly, is prone to flowing after spraying, and the aggregate rebounds significantly; with accelerator, although it sets quickly and has low initial rebound, the strength decreases severely in the later stages, and the sprayed layer is brittle and prone to cracking; while with accelerator without alkali, it can not only quickly form early strength on the sprayed surface and inhibit aggregate rebound, but also maintain good strength in the later stages, so that the sprayed layer has both low rebound and high durability.
[0008] Specifically, the aluminum salts and other components in the alkali-free accelerator react with the calcium hydroxide generated during cement hydration to rapidly form an ettringite network structure. This network does not interfere with the carboxyl anchoring effect introduced by the polycarboxylate superplasticizer, thus avoiding excessive flocculation of cement particles caused by alkali-based accelerators. Furthermore, the sulfate ions released by the alkali-free accelerator in the alkaline slurry do not form adverse complexes with the polyether structure of the polycarboxylate molecule's side chains, ensuring the dispersion stability of the superplasticizer. In addition, the alkali-free accelerator promotes the rapid coating of the slurry onto the aggregate surface, forming a strong interfacial bond with the continuously graded coarse and fine aggregates. This "locks" the aggregate into the sprayed layer at the moment of spraying, significantly reducing the rebound rate and providing a dense cement stone skeleton for later strength development. This application achieves a balance between low rebound rate and mechanical properties through the synergistic effect of alkali-free accelerators, cementitious materials, polycarboxylate superplasticizers, and continuously graded aggregates. This balance is achieved across three levels: setting regulation, interfacial reinforcement, and structural density. The alkali-free accelerator promotes rapid early strength development in the sprayed concrete, effectively suppressing aggregate rebound and spray layer flow, while avoiding the later strength decay and brittleness risks associated with alkali-based accelerators. Its good compatibility with polycarboxylate superplasticizers ensures stable dispersion of the superplasticizer, allowing the slurry to uniformly coat the aggregate surface and form a strong interfacial bond. The ettringite network and cementitious aggregate skeleton work together to fill the voids in the sprayed layer, improving density and durability. Therefore, the C25 sprayed concrete provided by this application combines low rebound rate, good workability, and high mechanical properties in wet spraying, meeting the stringent requirements for quality and construction efficiency in urban rail transit, foundation pit support, and other projects.
[0009] In one specific implementation scheme, the dosage of the alkali-free quick-setting agent is 6%-8% of the mass of the cementitious material.
[0010] Through experimental testing, the inventors discovered that controlling the dosage of the alkali-free quick-setting agent within the range of 6%-8% of the cementitious material mass achieves the optimal balance between rapid setting and stable strength. Within this dosage range, the density of the ettringite network formed by the reaction of aluminum salts and calcium hydroxide is moderate, which can quickly lock the aggregate and inhibit rebound, while also preserving sufficient cement particle surface for the carboxyl anchoring of the polycarboxylate superplasticizer, without interference between the two. At the same time, the release rate of sulfate ions matches the cement hydration process and does not have adverse complexation with the polyether side chains, thus maintaining the dispersion stability of the superplasticizer. This dosage range is also compatible with the amount of cementitious material, water-cement ratio, and aggregate gradation in this solution, allowing the slurry to uniformly coat the aggregate at the moment of spraying, forming a dense and firmly bonded spray layer, thereby achieving a synergistic effect of low rebound rate, good workability, and high later-stage strength.
[0011] When the admixture dosage is less than 6% of the cementitious material mass, the aluminum salt components in the cement paste are insufficient to react fully with calcium hydroxide to form a continuous ettringite network. After spraying, the paste sets slowly, and the concrete still retains some fluidity on the sprayed surface. Aggregates are prone to rebound due to gravity, and the sprayed layer is prone to flow, making it difficult to effectively control the rebound rate. Simultaneously, the dispersing effect provided by the water-reducing agent molecules persists, resulting in insufficient paste cohesion and easy separation of aggregates from the paste. When the admixture dosage exceeds 8% of the cementitious material mass, excessive alkali-free accelerators introduce too many sulfate and aluminum ions. Although setting is extremely fast, the early hydration heat is released in a concentrated manner, increasing the shrinkage stress of the sprayed layer and raising the risk of microcracks. Furthermore, excessive accelerators excessively consume the active components in the cement, leading to insufficient formation of later hydration products, a loose cement stone skeleton, and a decrease in long-term strength and durability.
[0012] In one specific implementation, the cementitious material is cement, and the water-cement ratio of the cement is 0.40-0.45.
[0013] By adopting the above technical solution, the water-cement ratio is controlled within the range of 0.40-0.45, achieving a balance between the fluidity and cohesiveness of concrete. If the water-cement ratio is below 0.40, the mixture is too dry and hard, resulting in insufficient coating and lubrication of the aggregate by the slurry. During spraying, the aggregate and slurry are prone to separation, leading to increased aggregate rebound rate and difficulty in achieving a dense sprayed layer, which is prone to voids and surface roughness. If the water-cement ratio is above 0.45, the slurry is too thin, causing the concrete to flow easily on the sprayed surface after spraying. The aggregate settles and separates due to insufficient slurry viscosity, also leading to increased rebound. Furthermore, an excessively high water-cement ratio prolongs the setting time, weakens the effect of the accelerator, and increases the risk of shrinkage cracking in the sprayed layer. Limiting the water-cement ratio to 0.40-0.45 allows the slurry to have suitable yield stress and plastic viscosity, enabling smooth delivery and adhesion during spraying, rapid locking of aggregates and reduction of rebound, while ensuring sufficient cement hydration to form a dense cement stone structure. This synergistically achieves low rebound rate, good spraying workability, and mechanical properties that meet the C25 design requirements.
[0014] In one specific implementation, the coarse aggregate is crushed stone with an average particle size of 5-10 mm, and / or the fine aggregate is medium sand with a fineness modulus of 2-3.
[0015] By adopting the above technical solution, the coarse aggregate is selected as crushed stone with an average particle size of 5-10mm, and the fine aggregate is selected as medium sand with a fineness modulus of 2-3, forming a continuous and good gradation. Coarse aggregate with an excessively large particle size has a large momentum during spraying, making it easy to eject from the slurry, and its effective contact area with the sprayed surface is small, resulting in insufficient bonding force; conversely, if the particle size is too small, the specific surface area is too large, requiring more slurry to coat it, increasing water consumption and cost. Crushed stone with a size of 5-10mm is moderate, allowing it to be fully coated by the slurry without easily ejecting under the impact of spraying. Medium sand with a fineness modulus of 2-3 has a suitable particle distribution, effectively filling the voids between coarse aggregates, reducing internal porosity, and improving the density and cohesiveness of the mixture. After a reasonable combination of coarse and fine aggregates, the aggregate packing void ratio is reduced, and the slurry can evenly coat the surface of each aggregate, forming a strong interfacial transition zone. During spraying, well-coated aggregates are less likely to detach from the slurry, significantly reducing rebound caused by aggregate separation from the slurry. At the same time, the dense aggregate skeleton provides solid support for later strength development, giving the sprayed layer a combination of low rebound rate, high density, and good mechanical properties.
[0016] In one specific feasible embodiment, the polycarboxylate superplasticizer is co-modified with long-chain alkyl and amide groups; more preferably, the preparation steps of the polycarboxylate superplasticizer co-modified with long-chain alkyl and amide groups are as follows: S1-1. Dissolve N-dodecylacrylamide in ethanol to obtain an N-DDA ethanol solution; S2-1 Under nitrogen protection, the N-DDA ethanol solution and ammonium persulfate solution prepared in step S1-1 are added dropwise to the polycarboxylate superplasticizer to react and obtain the polycarboxylate superplasticizer co-modified with long-chain alkyl and amide groups.
[0017] The inventors discovered that while ordinary polycarboxylate superplasticizers possess high water reduction rates and good slump retention, when applied to shotcrete, their molecular structure is primarily characterized by steric hindrance provided by polyether side chains, lacking the ability to actively regulate the cohesiveness of concrete. During spraying, the highly fluid slurry is still prone to separation from the aggregate, which, lacking sufficient adhesive restraint, bounces off the spray layer, potentially leading to a persistently high rebound rate. To address this issue, the inventors attempted to introduce two functional groups—long-chain alkyl and amide groups—into the polycarboxylate molecule. The long-chain alkyl groups are hydrophobic, enabling hydrophobic association between superplasticizer molecules to form a reversible physical cross-linking network, improving the slurry's low shear viscosity and anti-segregation ability, making it less likely for aggregates to detach under spraying impact. The amide groups, on the other hand, slowly hydrolyze in the alkaline cement slurry environment, continuously releasing dispersion effects to compensate for the loss of fluidity caused by cement hydration and the incorporation of accelerators, ensuring stable workability of the concrete before spraying.
[0018] Specifically, the inventors selected N-dodecylacrylamide (N-DDA) as the modifying monomer, whose molecule simultaneously contains polymerizable acrylamide double bonds, amide groups, and long dodecyl chains. Through a free radical grafting reaction, N-DDA is covalently linked to the polycarboxylic acid molecular backbone, achieving the permanent introduction of both functional groups. Compared to physically blended additives, chemical grafting firmly anchors the long-chain alkyl and amide groups to the water-reducing agent molecule, preventing desorption or separation of functional groups during transportation, storage, or use, thus ensuring the long-lasting stability of the modification effect. Simultaneously, the use of nitrogen protection and ammonium persulfate initiation ensures mild grafting reaction conditions, preserving the original comb-like structure of the polycarboxylic acid and maintaining its high water-reducing rate and slump-resistance. The modified polycarboxylate superplasticizer can simultaneously perform the triple functions of water reduction, thickening, and slump retention in shotcrete, without the need for additional thickeners or retarders, simplifying the control of construction mix proportions. It also has good compatibility with alkali-free quick-setting agents, enabling shotcrete to achieve a better balance between low rebound rate, high workability, and high mechanical properties.
[0019] In one specific implementation, the mass ratio of N-dodecylacrylamide to ethanol in step S1-1 is 1:(2-3).
[0020] By adopting the above technical solution and controlling the mass ratio of N-dodecylacrylamide to ethanol at 1:(2-3), the highly hydrophobic N-DDA can be completely dissolved in ethanol, forming a homogeneous and transparent pre-solution. Ethanol acts as a co-solvent, with its hydroxyl groups forming hydrogen bonds with the amide groups of N-DDA. Simultaneously, the hydrophobic ethyl group of ethanol is compatible with the dodecyl chain, effectively breaking the hydrophobic aggregation between N-DDA molecules. If the ethanol ratio is too low, N-DDA cannot be fully dissolved and exists as solid particles before the grafting reaction, leading to uneven grafting and decreased reaction efficiency. If the ethanol ratio is too high, although dissolution is more complete, a large amount of ethanol dilutes the reaction system, reducing the collision frequency between N-DDA and polycarboxylic acid molecules, slowing down the grafting rate, and the residual ethanol increases cost and safety risks. A 1:(2-3) ratio ensures both molecular-level dispersion of N-DDA and efficient grafting reaction at a suitable concentration, thereby obtaining a uniformly grafted and functionally stable modified water-reducing agent.
[0021] In one specific implementation scheme, step S2-1 involves adjusting the pH of the reaction solution to 6-7 after the reaction is complete.
[0022] By employing the above technical solution, the ammonium persulfate initiator used in the reaction process decomposes to produce bisulfate or sulfate ions, making the system acidic (i.e., pH 3-5). If exposed to an acidic environment for a prolonged period, the ester bonds connecting the polyether side chains and the main chain in the polycarboxylic acid molecule are prone to hydrolysis and breakage, leading to a significant decrease in the dispersibility of the water-reducing agent. Simultaneously, the acidic liquid is corrosive to metal equipment such as storage tanks and pipelines, and may undergo a violent neutralization reaction when mixed with alkaline cement or accelerators, affecting concrete performance. Adjusting the pH to near neutral (i.e., pH 6-7) effectively inhibits ester bond hydrolysis, extends the shelf life of the water-reducing agent, ensures its good compatibility with alkali-free accelerators and cement, and thus maintains stable water-reducing, slump-retaining, and viscosity-enhancing effects during spraying.
[0023] In one specific implementation, the mass ratio of the effective solid content of the N-dodecylacrylamide and the polycarboxylate superplasticizer is 1:(25-50).
[0024] By adopting the above technical solution, the mass ratio of N-dodecylacrylamide to polycarboxylate superplasticizer solids content is controlled at 1:(25-50). This allows for the maintenance of the original high water reduction rate of polycarboxylate while imparting suitable hydrophobic association thickening and amide group slow-release functions. If the N-DDA ratio is lower than this range (i.e., insufficient grafting), the density of long-chain alkyl and amide groups introduced onto the polycarboxylate molecular chain is insufficient, making it difficult to form an effective intermolecular hydrophobic association network. Consequently, the improvement in slurry cohesion is limited, and the reduction in resilience is not significant. If the N-DDA ratio is higher than this range (i.e., excessive grafting), the excessive hydrophobic long chains will enhance the overall hydrophobicity of the superplasticizer molecules, reduce initial dispersibility, and may excessively delay cement hydration, affecting early strength development. The optimal ratio of 1:(25-50) achieves the best balance between "maintaining water reduction rate" and "increasing viscosity and reducing rebound". This allows the modified water-reducing agent to fully disperse cement particles and ensure workability in shotcrete, while also locking aggregates and inhibiting rebound through hydrophobic association. At the same time, the slow-release effect of the amide group and the rapid setting of the alkali-free quick-setting agent work together to form a performance match throughout the entire process of "transportation-spraying-hardening".
[0025] Secondly, this application provides a preparation process for low-resilience C25 shotcrete, employing the following technical solution: A preparation process for low-resilience C25 shotcrete includes the following steps: Includes the following steps: S1-2. Mix the cementitious materials, coarse aggregate, fine aggregate, water, and polycarboxylate superplasticizer evenly to form ready-mixed concrete; S2-2. Before spraying, add the alkali-free quick-setting agent to the premixed concrete prepared in step S1-2 and mix to obtain the low-resilience C25 sprayed concrete.
[0026] By adopting the above technical solution, this preparation process uses a wet spraying method, where all raw materials except the accelerator are mixed uniformly at the mixing plant in one go. No secondary water addition is needed after the concrete leaves the plant, eliminating the dust pollution problem common in dry spraying processes from the source, while ensuring the uniformity and stability of the concrete mixture. The accelerator is mixed with the ready-mixed concrete using a dedicated additive device before spraying, rather than being added in advance. This avoids the interaction between the accelerator and the water-reducing agent during transportation, which could lead to slump loss or abnormal setting time. This step-by-step mixing method of "pre-mixing, then adding accelerator" matches the slump retention performance of the aforementioned modified polycarboxylate water-reducing agent and the rapid setting characteristics of the alkali-free accelerator: In the pre-mixing stage, the polycarboxylate component in the modified water-reducing agent fully disperses the cement particles, and the slow-release effect of the amide groups maintains the fluidity of the concrete during transportation and waiting; in the spraying stage, the addition of the alkali-free accelerator triggers a rapid hydration reaction, and the ettringite network quickly forms and locks in the aggregate, achieving low-rebound spraying. The entire process is simple and controllable, with smooth transitions between steps. It ensures good workability of concrete during the construction window and achieves rapid hardening and low rebound rate after spraying. It is suitable for engineering scenarios with strict requirements for construction efficiency and spray layer quality, such as urban rail transit and foundation pit support.
[0027] In summary, this application includes at least one of the following beneficial technical effects: 1. This application selects an alkali-free quick-setting agent and controls its dosage to 6%-8% of the cementitious material mass. It utilizes the good compatibility between the alkali-free quick-setting agent and the polycarboxylate superplasticizer, as well as the rapid reaction with cement hydration products to form an ettringite network. This locks in the aggregate and inhibits rebound at the moment of spraying, while avoiding the later strength reduction and spray layer cracking caused by alkali quick-setting agents. This results in a spray layer with low rebound rate, high workability and high durability.
[0028] 2. This application uses a polycarboxylate superplasticizer co-modified with long-chain alkyl and amide groups, and is prepared by chemical grafting with N-dodecylacrylamide as the modifying monomer. This gives the superplasticizer molecule a triple function of high water reduction rate, hydrophobic association thickening and amide group slow release and slump retention. No additional thickener or retarder is needed, which simplifies the construction ratio. It matches well with alkali-free quick-setting agents, further reduces the spray rebound rate and ensures the workability stability of concrete throughout the transportation and spraying process. Detailed Implementation
[0029] The present application will be further described in detail below with reference to embodiments and comparative examples: Some of the raw materials used in the examples and comparative examples: 1. Details of cement procurement information are as follows:
[0030] 2. Details of the water-reducing agent procurement information are as follows:
[0031] 3. Details of the procurement information for accelerators are as follows:
[0032] 4. Crushed stone, particle size: 5-10mm; gradation: continuous gradation; apparent density (kg / m³) 3 ): 2620; mud content %: 0.4.
[0033] 5. Sand, specifications: medium sand; fineness modulus: 2.6; apparent density (kg / m³) 3 ): 2650; mud content %: 1.5.
[0034] 6. N-dodecylacrylamide (CAS: 216-136-0), 2-dodecyl acrylate (CAS: 218-463-4), and acrylamide (CAS: 201-173-7) were all purchased from Sigma-Aldrich.
[0035] Unless otherwise specified, all raw materials used in the examples and comparative examples are commercially available products.
[0036] Preparation Example 1 The solid content of Inco-Tech's polycarboxylate superplasticizer is 13.37%, and the effective solid content of 10kg of polycarboxylate superplasticizer is 1.337kg.
[0037] The preparation of polycarboxylate superplasticizers co-modified with long-chain alkyl and amide groups is as follows: S1-1. Dissolve 40.11g of N-dodecylacrylamide in 100.28g of ethanol to obtain an N-DDA ethanol solution; S2-1. Under nitrogen protection, the above-mentioned N-DDA ethanol solution and ammonium persulfate solution (9.36g of ammonium persulfate dissolved in 50g of water) were added dropwise to 10kg of polycarboxylate superplasticizer. The temperature was raised to 60°C, and the reaction was stirred for 2 hours. After the reaction was completed, the pH of the reaction solution was adjusted to 6.5 using sodium hydroxide solution to obtain the polycarboxylate superplasticizer co-modified with long-chain alkyl and amide groups.
[0038] Preparation Example 2 The difference between Preparation Example 2 and Preparation Example 1 is that the polycarboxylate superplasticizer is only modified with long-chain alkyl groups, and N-dodecyl acrylamide is replaced by an equal amount of 2-dodecyl acrylate. The preparation of the long-chain alkyl-modified polycarboxylate superplasticizer is as follows: S1-1. Dissolve 40.11g of dodecyl 2-acrylate in 100.28g of ethanol to obtain a dodecyl 2-acrylate ethanol solution; S2-1. Under nitrogen protection, the above-mentioned 2-acrylate dodecyl ester ethanol solution and ammonium persulfate solution (9.36g ammonium persulfate dissolved in 50g water) were added dropwise to 10kg of polycarboxylate superplasticizer. The temperature was raised to 60°C, and the mixture was stirred for 2 hours. After the reaction was completed, the pH of the reaction solution was adjusted to 6.5 using sodium hydroxide solution to obtain the long-chain alkyl-modified polycarboxylate superplasticizer.
[0039] Preparation Example 3 The difference between Preparation Example 3 and Preparation Example 1 is that the polycarboxylate superplasticizer is only modified with amide groups, and N-dodecylacrylamide is replaced with an equal amount of acrylamide. The preparation of the amide-modified polycarboxylate superplasticizer is as follows: S1-1. Dissolve 40.11g of dodecyl 2-acrylate in 100.28g of ethanol to obtain a dodecyl 2-acrylate ethanol solution; S2-1. Under nitrogen protection, the above-mentioned 2-acrylate dodecyl ester ethanol solution and ammonium persulfate solution (9.36g ammonium persulfate dissolved in 50g water) were added dropwise to 10kg of polycarboxylate superplasticizer. The temperature was raised to 60°C, and the mixture was stirred for 2 hours. After the reaction was completed, the pH of the reaction solution was adjusted to 6.5 using sodium hydroxide solution to obtain the amide-modified polycarboxylate superplasticizer. Example Example
[0040] In this embodiment, the cement water-cement ratio is controlled at 0.46.
[0041] The preparation of low-resilience C25 shotcrete is as follows: S1-2. Add 433 parts by weight of cement, 849 parts by weight of crushed stone, 849 parts by weight of medium sand, 186 parts by weight of water, and 3.5 parts by weight of polycarboxylate superplasticizer to the mixer and mix evenly to form ready-mixed concrete. S2-2. Before spraying, add 26 parts by weight of alkali-free quick-setting agent to the above-mentioned ready-mixed concrete and mix to obtain low-resilience C25 sprayed concrete. Example
[0042] The only difference between Example 2 and Example 1 is that in step S1-2 of Example 2, 3.5 parts by weight of polycarboxylate superplasticizer is replaced with 3.5 parts by weight of the polycarboxylate superplasticizer prepared in Preparation Example 1 that has been co-modified with long-chain alkyl and amide groups. Example
[0043] The only difference between Example 3 and Example 1 is that in step S1-2 of Example 3, 3.5 parts by weight of polycarboxylate superplasticizer is replaced with 3.5 parts by weight of the long-chain alkyl-modified polycarboxylate superplasticizer prepared in Preparation Example 2. Example
[0044] The only difference between Example 4 and Example 1 is that in step S1-2 of Example 4, 3.5 parts by weight of polycarboxylate superplasticizer is replaced with 3.5 parts by weight of the amide-modified polycarboxylate superplasticizer prepared in Preparation Example 3.
[0045] Comparative Example 1 The only difference between Comparative Example 1 and Example 2 is that in step S2-2 of Comparative Example 1, 26 parts by weight of alkali-free quick-setting agent is replaced with 26 parts by weight of alkali-containing quick-setting agent.
[0046] Comparative Example 2 The only difference between Comparative Example 2 and Example 2 is that 26 parts by weight of alkali-free quick-setting agent were not added in step S2-2 of Comparative Example 2. The preparation of the shotcrete is as follows: Add 433 parts by weight of cement, 849 parts by weight of crushed stone, 849 parts by weight of medium sand, 186 parts by weight of water, and 3.5 parts by weight of polycarboxylate superplasticizer to a mixer and mix evenly to form shotcrete.
[0047] Referring to the "Technical Specification for Application of Shotcrete" JGJ / T 372-2016 and the "Test Method for Rebound Rate of Shotcrete" JSCE-F 563-2005, the rebound rate and 28-day and 90-day strength tests were conducted on the shotcrete prepared in the examples and comparative examples. The values obtained were the average of 10 tests and recorded in Table 1. Table 1. Summary of rebound rate and compressive strength data of shotcrete in each embodiment and comparative example. Group Rebound rate (%) 28-day strength (MPa) 90d strength (MPa) Example 1 16.0 38.5 37.2 Example 2 12.5 42.1 44.6 Example 3 15.0 39.2 36.8 Example 4 15.8 37.6 34.5 Comparative Example 1 14.2 33.5 27.8 Comparative Example 2 28.5 31.2 33.8 Based on Examples 1-4 and Table 1, it can be seen that the comparison of ordinary polycarboxylate superplasticizer, superplasticizer modified only with long-chain alkyl groups, superplasticizer modified only with amide groups, and superplasticizer co-modified with long-chain alkyl and amide groups in this application shows that the co-modification of long-chain alkyl and amide groups produces a significant synergistic effect. In Example 2, when the co-modified superplasticizer was used, the rebound rate of the shotcrete was reduced to the lowest level, and the later strength (90d) was still improved compared with 28d, demonstrating the dual advantages of "hydrophobic association and thickening" and "amide group slow-release and slump retention": the long-chain alkyl groups form a dynamic hydrophobic association network between molecules, which improves the adhesion of the paste to the aggregate, making the aggregate less prone to rebound during spraying; the amide groups continuously and slowly hydrolyze in the alkaline cement paste, compensating for the loss of fluidity caused by hydration and the addition of accelerators, ensuring the working stability of the concrete before transportation and spraying, and promoting the continuous hydration of unhydrated particles, so that the later strength increases steadily. While the introduction of long-chain alkyl groups in Example 3 reduced rebound to some extent, the lack of sustained-release regulation by amide groups resulted in a decline in later-stage strength. In Example 4, the introduction of amide groups improved slump retention, but the lack of hydrophobic association from long-chain alkyl groups limited the reduction in rebound rate and decreased initial dispersibility. This indicates that only by chemically grafting long-chain alkyl and amide groups into polycarboxylic acid molecules can a synergistic balance between low rebound rate and high later-stage strength be achieved.
[0048] Combining Example 2 and Comparative Examples 1-2, and referring to Table 1, it can be seen that the matching of alkali-free accelerator and co-modified water-reducing agent plays a key role in improving the overall performance of shotcrete. Example 2, using a combination of alkali-free accelerator and co-modified water-reducing agent, resulted in rapid setting after spraying, low rebound rate, and good maintenance of later-stage strength. In contrast, Comparative Example 1, after replacing the alkali-free accelerator with an alkali-containing accelerator, although the initial rebound rate decreased slightly, the later-stage strength was severely reduced, the brittleness of the sprayed layer increased, and it was prone to cracking. In Comparative Example 2, without adding any accelerator, the concrete set slowly, experienced severe flow after spraying, a large amount of aggregate rebound, the highest rebound rate, and the 28-day strength did not meet the design requirements. The above comparison shows that there is good compatibility between the alkali-free quick-setting agent and the co-modified water-reducing agent: the aluminum salt component in the alkali-free quick-setting agent quickly forms an ettringite network with the calcium hydroxide generated by cement hydration. This network does not interfere with the carboxyl anchoring effect of the co-modified water-reducing agent, thus avoiding excessive flocculation of cement particles caused by the alkali-containing quick-setting agent. At the same time, the sulfate ions released by the alkali-free quick-setting agent do not have poor complexation with the polyether side chains, ensuring the dispersion stability of the water-reducing agent, thereby achieving the full-process performance matching of "slump retention during transportation, rapid setting during spraying, and stable strength in the later stage".
[0049] This application employs a polycarboxylate superplasticizer co-modified with long-chain alkyl and amide groups, endowing the superplasticizer molecules with the dual functions of hydrophobic association thickening and amide group slow-release and slump retention. Combined with an alkali-free quick-setting agent and optimized cementitious materials, water-cement ratio, and aggregate gradation, C25 shotcrete exhibits extremely low rebound rate, excellent workability, and stable late-stage strength during wet spraying. This achieves a balance between low rebound, high mechanical properties, and environmentally friendly construction, making it widely applicable to projects with stringent quality requirements for shotcrete, such as urban rail transit and foundation pit support.
[0050] This specific embodiment is merely an explanation of this application and is not intended to limit it. After reading this specification, those skilled in the art can make modifications to this embodiment without contributing any inventive step, but such modifications are protected by patent law as long as they fall within the scope of the claims of this application.
Claims
1. A low-rebound C25 shotcrete, characterized in that, The raw materials for preparation include the following components in parts by weight: 420-440 parts of cementitious material, 800-900 parts of coarse aggregate, 800-900 parts of fine aggregate, 175-190 parts of water, 3.0-4.0 parts of polycarboxylate superplasticizer, and 20-35 parts of accelerator, wherein the accelerator is an alkali-free accelerator.
2. The low-resilience C25 shotcrete according to claim 1, characterized in that, The dosage of the alkali-free quick-setting agent is 6%-8% of the mass of the cementitious material.
3. The low-resilience C25 shotcrete according to claim 1, characterized in that, The cementitious material is cement, and the water-cement ratio of the cement is 0.40-0.
45.
4. The low-resilience C25 shotcrete according to claim 1, characterized in that, The coarse aggregate is crushed stone with an average particle size of 5-10 mm, and / or the fine aggregate is medium sand with a fineness modulus of 2-3.
5. The low-resilience C25 shotcrete according to claim 1, characterized in that, The polycarboxylate superplasticizer is co-modified with long-chain alkyl and amide groups.
6. The low-resilience C25 shotcrete according to claim 5, characterized in that, The preparation steps of the polycarboxylate superplasticizer after co-modification with long-chain alkyl and amide groups are as follows: S1-1. Dissolve N-dodecylacrylamide in ethanol to obtain an N-DDA ethanol solution; S2-1 Under nitrogen protection, the N-DDA ethanol solution and ammonium persulfate solution prepared in step S1-1 are added dropwise to the polycarboxylate superplasticizer to react and obtain the polycarboxylate superplasticizer co-modified with long-chain alkyl and amide groups.
7. The low-resilience C25 shotcrete according to claim 6, characterized in that, The mass ratio of N-dodecylacrylamide to ethanol in step S1-1 is 1:(2-3).
8. The low-resilience C25 shotcrete according to claim 6, characterized in that, Step S2-1: After the reaction is complete, adjust the pH of the reaction solution to 6-7.
9. The low-resilience C25 shotcrete according to claim 6, characterized in that, The mass ratio of the effective solid content of the N-dodecylacrylamide and the polycarboxylate superplasticizer is 1:(25-50).
10. A preparation process for low-resilience C25 shotcrete according to any one of claims 1-9, characterized in that, Includes the following steps: S1-2. Mix the cementitious materials, coarse aggregate, fine aggregate, water, and polycarboxylate superplasticizer evenly to form ready-mixed concrete; S2-2. Before spraying, add the alkali-free quick-setting agent to the premixed concrete prepared in step S1-2 and mix to obtain the low-resilience C25 sprayed concrete.