Composite admixture for concrete and method for reducing rebound of wet shotcrete

Abstract

The present application relates to the field of building materials and tunnel support construction technology, and discloses a composite admixture for concrete and a method for reducing the rebound rate of wet-sprayed concrete, wherein the composite admixture comprises water-reducing and flow-regulating components, viscosity-increasing and slurry-stabilizing components, and setting-regulating components. The construction method comprises adding the admixture to the mixing system, and then wet-spraying and forming after the admixture enters a spraying adaptation window through standing and control. The spraying adaptation window is determined by at least two of the slump flow change rate, the viscosity value, the initial setting time, the beam diffusion angle, and the post-spraying adhesion thickness. The present application can synergistically improve the fluidity, cohesiveness, segregation resistance, and early structure formation ability of the wet-sprayed concrete mixture, so that the mixture can maintain a pumpable, sprayable, and adherable state. The present application improves the wet-spraying construction adaptability, improves the spraying stability, effectively reduces the spraying rebound rate and reduces material loss, and is suitable for tunnel initial support construction.

Description

Technical Field

[0001] This invention relates to the field of building materials and tunnel support construction technology, specifically to a composite admixture for concrete and a method for reducing the rebound rate of wet-sprayed concrete. Background Technology

[0002] Shotcrete is a support material formed by spraying a concrete mixture at high speed onto a target surface under air pressure, allowing it to quickly solidify and harden. With the development of tunnel and underground engineering construction, wet shotcrete technology has become the main application method for initial support construction due to its low dust content, high production efficiency, and stable forming quality. In actual engineering projects, wet shotcrete requires continuous construction steps, including front-end mixing, long-distance pipeline pumping, high-pressure air jetting, and adhesion and forming on the target surface.

[0003] Currently, existing wet-mix shotcrete typically involves adding water-reducing agents during the mixing process to lower the water-cement ratio and achieve higher fluidity, thus meeting the requirements for smooth pumping during long-distance pipeline transportation. When the material is transported to the spraying nozzle, construction workers connect an external delivery pipeline to the nozzle to inject a liquid accelerator under high pressure and force-mix it with the concrete mixture. The accelerator catalyzes the rapid hydration of the cement, causing the slurry to lose its plasticity and develop early strength shortly after impacting the sprayed surface, thereby completing the adhesion and shaping process on the sprayed surface.

[0004] However, this passive construction method, relying on front-end flow retention and end-end setting acceleration, is ill-suited to the complex jetting dynamics. The highly fluid mixture prepared at the front end, lacking internal cohesion, is prone to dispersion when subjected to high-pressure airflow at the nozzle, leading to increased kinetic energy loss during its flight. When this highly fluid slurry impacts the sprayed surface, it lacks sufficient buffering and aggregate-coating capabilities, making it highly susceptible to slippage and rebound under its own weight and the continuous impact of subsequent jetting aggregates. Furthermore, the extremely short mixing time for the temporary addition of accelerators at the nozzle easily results in uneven mixing between the accelerator and the mixture, leading to variations in the localized setting state of the concrete and limiting the thickness of a single spray application on the sprayed surface. Summary of the Invention

[0005] To address the shortcomings of existing technologies, this invention provides a composite admixture for concrete and a method for reducing the rebound rate of wet-sprayed concrete. Existing technologies often involve temporarily adding accelerators at the nozzle of the spraying equipment. This method is a passive end-stage adjustment and cannot resolve the contradiction between the high fluidity of the mixture during the pumping stage and the high cohesiveness during the jet adhesion stage. As a result, the material disperses after leaving the nozzle and lacks buffering and encapsulation capabilities when impacting the sprayed surface, leading to rebound loss.

[0006] To address the above problems, the present invention provides the following technical solution: In a first aspect, the present invention provides a composite admixture for concrete, which adopts the following technical solution: A composite admixture for concrete, based on 100% of its total weight, comprises the following effective components in the following proportions: 10%–40% water-reducing and flow-regulating component; 0.001%–0.5% thickening and stabilizing component; 0.1%–10% setting regulator component; and the remainder being water and optional auxiliary agents. The water-reducing and flow-regulating component is a water-soluble polycarboxylate-based water-reducing polymer; the thickening and stabilizing component is selected from one or more of cellulose ether derivatives, polyacrylic acid polymers, xanthan gum, and styrax; and the setting regulator component is selected from one or more of organic alcohol amines, inorganic nitrates, and aluminates.

[0007] By employing the above technical solution, the water-reducing and flow-regulating components, the viscosity-enhancing and stabilizing components, and the setting-regulating components undergo physical and chemical reactions at different stages of the mixture's transport and molding process, synergistically regulating the rheological behavior and early hydration kinetics of the cement paste. The specific reaction and action processes are as follows: First, during the pipeline pumping stage, the main molecular chain of the water-reducing and flow-regulating component is adsorbed onto the surface of cement particles, while the side chains extend into the liquid phase to provide steric hindrance, disperse cement particles and release free water, reduce the yield stress of the mixture, and ensure low-resistance pumping.

[0008] Secondly, during the jetting stage after exiting the nozzle, the polymer chains of the thickening and stabilizing components form a network structure in the liquid phase through intermolecular hydrogen bonding and physical entanglement. This network structure exhibits shear-thinning properties, maintaining the flow state under high-shear conditions during pumping, and rapidly recovering its entangled state after jetting when the shear force disappears. This increases the plastic viscosity of the liquid phase, improves the cohesiveness of the mixed material bundles, and prevents the high-pressure air from shearing the slurry.

[0009] Finally, during the adhesion stage on the impact-receiving surface, the inorganic salts in the coagulation regulating components dissociate into metal cations, disrupting the liquid-phase dissolution equilibrium, accelerating the hydration of tricalcium silicate, and promoting the precipitation of hydrated calcium silicate gel. Simultaneously, organic alcohol amines complex hydrated ions, lowering the reaction activation energy and promoting the formation of ettringite crystals. The hydration reaction rapidly consumes free water, transforming the mixture from a plastic fluid state to a solid initial structure with yield stress, completing thixotropic rapid hardening, and thus adhering to subsequent aggregates and preventing their rebound.

[0010] Preferably, based on 100% of the total weight of the composite admixture, the water-reducing and flow-regulating component is 30%, the thickening and stabilizing component is 0.25%–0.36%, the coagulation-regulating component is 1.6%–2.0%, and the remainder is water. The cellulose ether derivative is hydroxypropyl methylcellulose; the polyacrylic acid polymer is polyacrylic acid with a weight-average molecular weight between 3000 and 5000; the organic alcohol amine is triethanolamine; and the inorganic nitrate is calcium nitrate tetrahydrate.

[0011] By adopting the above technical solution, polyacrylic acid and calcium nitrate, within the specified concentration range, are placed in an aqueous phase and a polycarboxylic acid dispersion system, avoiding macroscopic flocculation and cross-linking between carboxyl anions and calcium ions. The components dissociate and disperse in alkaline cement slurry, constructing a rheology control system that meets the requirements of wet spraying technology.

[0012] Preferably, the thickening and stabilizing component is composed of hydroxypropyl methylcellulose and polyacrylic acid, and the weight ratio of the two is 1:4.0 to 8.33; the coagulation regulating component is composed of triethanolamine and calcium nitrate tetrahydrate, and the weight ratio of the two is 1:1.

[0013] By adopting the above technical solution, nonionic hydroxypropyl methylcellulose and anionic polyacrylic acid form a composite thickening system, which takes into account both water retention and dynamic yield stress improvement; triethanolamine and calcium nitrate are compounded in equal proportion to coordinate the early hydration rate of aluminate and silicate minerals, avoiding the later strength reduction caused by excessively rapid hydration of a single mineral phase.

[0014] Preferably, the water-soluble polycarboxylate superplasticizer is a custom-synthesized spray-adaptive polycarboxylate polymer, the preparation method of which includes the following steps: mixing isopentenyl alcohol polyoxyethylene ether macromonomer and deionized water according to the ratio as a base material, and controlling the temperature at 15-40°C; adding hydrogen peroxide solution as an initiator to the system at one time; preparing a first aqueous solution containing acrylic acid and acrylamide, and a second aqueous solution containing ascorbic acid and mercaptopropionic acid, and simultaneously adding the first aqueous solution and the second aqueous solution to the reaction system for copolymerization reaction; after the reaction is completed, keeping the temperature, and adding sodium hydroxide solution to adjust the pH value of the system to 6.0-7.0, thereby obtaining the spray-adaptive polycarboxylate polymer.

[0015] By employing the above technical solution, amide groups are introduced into the polycarboxylic acid backbone. These amide groups increase the rigidity of the polymer chain in an alkaline liquid environment and provide additional hydrogen bonding sites, thereby enhancing the physical entanglement between the water-reducing polymer and the thickening and stabilizing components.

[0016] Preferably, optional auxiliary agents include one or a combination of two of the following: slump-retaining components and defoaming components.

[0017] By adopting the above technical solution, the defoaming component reduces the content of irregular large air bubbles inside the mixture, improves the compactness of the slurry structure and the volume stability of the pumping process.

[0018] Secondly, the present invention provides a method for reducing the rebound rate of wet-sprayed concrete, which adopts the following technical solution: A method for reducing the rebound rate of wet-mixed shotcrete, using the composite admixture for concrete mentioned in the first aspect above, includes the following steps: S1. Provide a wet sprayed concrete mixing system containing cementitious materials, aggregates and mixing water according to the proportion, and add concrete with composite admixture to it for mixing to obtain wet sprayed concrete mixture. S2. Allow the mixed wet-sprayed concrete mixture to stand for adjustment to confirm that the various rheological and setting performance parameters of the mixture are within the spraying adaptation window. S3. The wet-mixed concrete mixture within the spraying adaptation window is transported to the spraying equipment, and wet spraying is carried out under the set mechanical operating parameters to complete the adhesion and formation of the mixture on the sprayed surface.

[0019] By adopting the above technical solution, relying on the premixing of composite admixtures and the monitoring of rheological parameters, overall control of the mixture from stirring, pumping, jetting to wall contact can be achieved. Static adjustment allows the cement particle surface to fully adsorb the admixture components. Spraying is only carried out after confirming that the rheological and setting performance parameters meet the requirements, reducing the impact of material state fluctuations on construction quality and lowering the rebound rate.

[0020] Preferably, in step S1, the amount of concrete composite admixture added is 1.0% to 3.0% of the total weight of cementitious materials in the wet sprayed concrete mixing system; the specific timing of addition and mixing method are as follows: first, dry mix the cementitious materials and aggregates evenly, then add the mixing water containing the concrete composite admixture, and continue wet mixing for 120 to 180 seconds.

[0021] By adopting the above technical solution, the composite additive is pre-dissolved in the mixing water, allowing the polymer chain segments to fully expand in the aqueous phase, and then enters the dry mixture with the water, ensuring the uniform coating and adsorption of the effective components on the surface of the cementitious material.

[0022] Preferably, in step S2, the controlled spray adaptation window is determined by at least two of the following mixture performance parameters: the slump change rate of the mixture after standing for 60 minutes after exiting the machine is 10% to 30%; the viscosity of the mixture is 8 to 25 Pa·s; the initial setting time is 30 to 120 min; the particle diffusion angle is 8° to 25°; and the adhesion thickness after a single spray is 10 to 40 mm.

[0023] By adopting the above technical solution, the physical and mechanical state of the mixture entering the pumping stage is defined by detection indicators. The slump change rate limits the lower limit of the mixture's fluidity decay; the viscosity value defines the anti-segregation ability of the slurry suspended aggregate; the initial setting time ensures the timing matching of the hydration reaction; the particle diffusion angle and the post-spray adhesion thickness provide macroscopic verification indicators for the actual spraying state. The determination of each indicator ensures that the material entering the pumping equipment has both fluidity and thixotropic properties.

[0024] Preferably, in step S3, the specific mechanical operation parameters for wet spraying are as follows: the vertical distance between the nozzle of the spraying equipment and the sprayed surface is controlled to be 0.8 to 1.5 m; the spray angle between the nozzle axis and the normal of the sprayed surface is controlled to be 70° to 90°; and the system spray air pressure entering the nozzle is controlled to be stable at 0.4 to 0.6 MPa.

[0025] By adopting the above technical solution, the operating parameter range of the mechanical equipment is limited, ensuring that the kinetic energy of the mixture after leaving the nozzle is within the tolerance range of the composite admixture control system. Appropriate distance and air pressure prevent excessive aggregate kinetic energy from penetrating the underlying slurry, or insufficient kinetic energy from leading to inadequate compaction.

[0026] Preferably, in step S3, no additional accelerator is added at the nozzle during the jet ejection and adhesion molding process of the mixture; after molding, the adhesion thickness of the final single continuous fixed-point spray is controlled to reach more than 27 mm.

[0027] By adopting the above technical solution, the equipment requirement of adding a accelerator delivery pipeline at the spraying end, as in conventional processes, is eliminated. Relying on the synergistic reaction of the coagulation regulator and thickening components already present in the mixture, the mixture achieves contact wall formation, thus increasing the adhesion thickness.

[0028] This invention provides a composite admixture for concrete and a method for reducing the rebound rate of wet-sprayed concrete. It has the following beneficial effects: 1. This invention achieves synergistic control over the entire process of wet-mixed shotcrete delivery and molding by physically compounding water-reducing and flow-regulating components, thickening and stabilizing components, and setting-regulating components. The water-reducing component ensures low resistance during pumping, the thickening component forms physical entanglement during the material jet stage to reduce the risk of the slurry being sheared and dispersed by high-pressure air, and the setting component accelerates the hydration reaction after the material impacts the sprayed surface, causing the slurry to quickly lose plasticity and establish an initial structure. This synergistic response mechanism for different construction stages improves the adhesion of the material to the sprayed surface and effectively reduces the rebound rate during construction.

[0029] 2. This invention transforms the original experience-based construction process into a standardized, quantifiable process by subjecting the mixture to static conditioning and sampling testing before pumping. This indicator system can screen out mixtures whose rheological properties and setting times do not meet process requirements before the material enters the spraying equipment, reducing the interference of material state fluctuations on molding quality and ensuring the stability of construction quality.

[0030] 3. The construction method of this invention, while retaining the basic requirement of applying an external accelerator to the nozzle in traditional wet spraying processes, replaces conventional admixtures with the composite admixture of this invention in the front-end mixing stage. This pre-mixed composite admixture imparts excellent rheological stability and early thixotropic cohesion to the mixture, resulting in more uniform and better compatibility when the material is forcibly mixed with the high-pressure accelerator at the nozzle. This completely solves the defects of slurry dispersion and slippage caused by poor instantaneous compatibility and uneven mixing between admixtures and accelerators in traditional processes. Under the same accelerator construction conditions, this invention can significantly reduce the overall rebound rate of wet sprayed concrete from approximately 18% to approximately 8%, reducing material waste and greatly improving the single-pass adhesion thickness and overall construction efficiency of the initial tunnel support. Attached Figure Description

[0031] Figure 1 This is a schematic diagram of the wet spraying construction method and testing and evaluation process of the present invention; Figure 2 This is a schematic diagram illustrating the mechanism of action of the composite admixture of the present invention; Figure 3 This is a schematic diagram of the jet adaptation window determination process of the present invention. Detailed Implementation

[0032] The technical solution of the present invention will now be clearly and completely described with reference to the accompanying drawings. 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.

[0033] Current wet-sprayed concrete construction in tunnels and underground engineering projects generally suffers from technical bottlenecks such as high rebound rate, poor slurry adhesion, and delayed early strength formation. Conventional methods often rely on passively adjusting the concrete mix proportion or simply adding commercially available accelerators, making it difficult to achieve a physical and mechanical balance between the high fluidity required for pumping and the high adhesion state after spraying under pressure.

[0034] This invention aims to actively intervene in the rheological behavior and early hydration kinetics of cement paste by physically combining and chemically synergizing polymeric compounds with different functional groups and inorganic salt materials, from the dimensions of steric hindrance dispersion, physical entanglement of liquid-phase macromolecular chains, and induced crystal nucleation, thereby defining and maintaining an optimal spraying adaptation window, changing the kinetic response characteristics of the mixture, and reducing the rebound rate of wet-sprayed concrete.

[0035] To make the objectives, technical solutions, and beneficial effects of this invention clearer, the invention will be further described below with reference to specific embodiments. Those skilled in the art should understand that these embodiments are only used to illustrate the technical concept of this invention and do not limit the scope of protection of this invention.

[0036] The main raw materials and reagents used in the following examples and comparative examples have the following sources and specifications. Reagents not specifically mentioned are all commercially available analytical grade or higher grade products.

[0037] Isopentenyl alcohol polyoxyethylene ether (CAS No. 31497-33-3) has a number-average molecular weight of 2400 and a molecular weight distribution index of less than 1.1; Acrylic acid (CAS No. 79-10-7) has a purity greater than 99.0%; Acrylamide (CAS No. 79-06-1) has a purity greater than 99.0%; Hydroxypropyl methylcellulose (CAS No. 9004-65-3) has a methoxy substitution degree of 19.0% to 24.0% and a hydroxypropoxy substitution degree of 4.0% to 12.0%, and a dynamic viscosity of 40,000 mPa·s in a 2.0% aqueous solution at 20°C. Polyacrylic acid (CAS No. 9003-01-4) has an average weight-average molecular weight between 3,000 and 5,000 and a solid content of 40%. Triethanolamine (CAS No. 102-71-6) has a purity greater than 99.0%; Calcium nitrate tetrahydrate (CAS No. 13477-34-4) has a purity greater than 99.0%. The hydrogen peroxide solution, ascorbic acid, mercaptopropionic acid, and sodium hydroxide required for the synthesis were all commercially available analytical grade products. Ordinary Portland cement uses P.O42.5 grade with a 28-day compressive strength greater than 42.5 MPa; fly ash uses Class F grade II fly ash; fine aggregate uses continuously graded manufactured sand with a fineness modulus of 2.6 to 2.8; coarse aggregate uses continuously graded basalt crushed stone with a particle size range of 5 mm to 10 mm.

[0038] The construction process of wet-mix shotcrete encompasses three distinct rheological and kinetic stages: pipeline pumping, high-pressure jetting, and wall adhesion. To accurately and quantitatively characterize the synergistic regulatory effect of composite admixtures on the mixture throughout these processes, this invention introduces an evaluation system based on the spraying adaptation window. This adaptation window integrates the static workability, dynamic rheological characteristics, early hydration process, and macroscopic construction response of the mixture, specifically composed of core detectable indicators such as the slump change rate, viscosity value, initial setting time, particle diffusion angle, post-spray adhesion thickness, and overall rebound rate. By jointly judging these indicators, it is possible to accurately confirm whether the mixture is in its optimal construction matching state. The specific physical meaning and standard test methods for each indicator are described below.

[0039] The slump change rate of the mixture mainly reflects the slump retention capacity of concrete after it leaves the mixer, during waiting and long-distance transportation. The test method uses a standard slump cone.

[0040] The specific steps are as follows: Immediately after the wet-mixed concrete is discharged from the mixer, its initial slump is measured according to the specifications; then the concrete is poured into a container and the surface is covered with a damp cloth to prevent moisture evaporation. After standing for 60 minutes, its slump is measured again. The slump change rate is calculated by dividing (initial slump minus slump after 60 minutes) by the initial slump, and then multiplying by 100% to obtain the result.

[0041] The viscosity of the mixture reflects the viscosity of the concrete paste itself and its ability to resist segregation. A coaxial cylindrical concrete rheometer was used for testing. After mixing the concrete with water, it was allowed to stand for 30 minutes. A sample was then placed into the test cylinder of the rheometer. During testing, the instrument was set to decrease the shear rate in a stepwise manner from high to low, and the corresponding shear stress data was recorded simultaneously. Finally, the Bingham fluid model was applied to linearly fit the recorded data, and the slope of the resulting line was the plastic viscosity of the mixture, expressed in Pa·s.

[0042] Initial setting time is used to determine how quickly the concrete slurry begins to lose its fluidity and develop early strength after being sprayed onto the surface. The test uses the conventional penetration resistance method. The mixed wet-mixed sprayed concrete is sieved through a 5mm mesh to remove coarse aggregate. The sieved mortar portion is placed in a standard mold and left to stand in a constant temperature and humidity environment. Timing begins when water is added to the concrete. The probe of a penetration resistance meter is periodically and uniformly inserted vertically and at a constant speed into the mortar to a depth of 25mm. The pressure value is read and converted into penetration resistance. When the penetration resistance reaches 3.5MPa, the corresponding time is the initial setting time, expressed in minutes.

[0043] The material jet diffusion angle reflects the degree to which the concrete mixture is dispersed by high-pressure air after being sprayed from the nozzle. A smaller angle indicates a more concentrated material flow, a more stable spraying process, and less energy loss. During testing, under normal wet spraying conditions and with stable material output, a high-speed camera is mounted perpendicular to the spraying direction, 2 meters from the nozzle, to record the jet image from the nozzle to the wall. The image from 1 meter from the nozzle outlet is captured, and the angle between the lines connecting the upper and lower edges of the jet body is measured using image measurement software; this angle is the material jet diffusion angle, expressed in degrees (°).

[0044] Post-spray adhesion thickness is used to test how thick a single spray of concrete can adhere to the sprayed surface without flaking off. The test uses a fine steel probe with millimeter graduations. On a standard test rock surface, a wet spraying machine is used to continuously spray the same area at a fixed point. Construction workers observe the wall surface, and if they find that the concrete begins to slide down or flaking off, spraying is stopped immediately.

[0045] The steel probe is then vertically inserted into the concrete in the center of the sprayed surface until its tip touches the hard rock behind it. The reading on the probe's scale on the concrete surface is then recorded. Three adjacent points in the center area are measured, and the average value is taken as the critical adhesion thickness, expressed in mm.

[0046] Rebound rate is the final measured indicator for evaluating the waste of sprayed material and the overall effect of admixtures. The test uses a base plate collection and weighing method. Before the actual spraying, a clean steel plate or tarpaulin is laid on the ground directly below the wall to be sprayed. The total weight of the wet-mixed sprayed concrete to be sprayed is recorded, and this material is then evenly sprayed onto the wall surface. After the spraying operation is completed, all material that has fallen onto the base plate is collected and weighed. The rebound rate is calculated by dividing the total weight of the collected rebound material by the total weight of the actual sprayed concrete, and then multiplying by 100%.

[0047] Preparation Example 1: This preparation example provides a method for preparing a dedicated polymeric water-reducing and flow-regulating component, namely a spray-adaptive polycarboxylate polymer, including the following steps: (1) Preparation of base material: Add 100 parts by mass of isopentenyl alcohol polyoxyethylene ether and 80 parts by mass of deionized water to a reactor equipped with a constant temperature water bath and a mechanical stirrer. Turn on the stirrer, set the speed to 200 rpm, control the temperature inside the reactor to be constant at 25°C, and stir until the macromonomer is completely dissolved.

[0048] (2) Initiator addition: Add 1.0 part by mass of hydrogen peroxide solution with a mass concentration of 27.5% to the reactor at once, and continue stirring for 15 min.

[0049] (3) Droplet copolymerization: Prepare component A and component B separately. Component A consists of 12 parts by mass of acrylic acid and 3 parts by mass of acrylamide dissolved in 40 parts by mass of deionized water; component B consists of 0.3 parts by mass of ascorbic acid and 0.2 parts by mass of mercaptopropionic acid dissolved in 50 parts by mass of deionized water. Use a dual-channel constant flow pump to simultaneously and uniformly add component A and component B to the reactor. The dropping time of component A is controlled at 3 hours, and the dropping time of component B is controlled at 3.5 hours. The dropping of component B ends 30 minutes later than that of component A to ensure that the residual monomers in the system react fully.

[0050] (4) Heat preservation and neutralization: After the addition is completed, continue to heat and stir at 25°C for 1.5 h. Slowly add a 30% sodium hydroxide solution to adjust the pH of the system to between 6.0 and 7.0, and obtain a spray-adaptive polycarboxylate polymer mother liquor with a solid content of about 40%, for later use.

[0051] Preparation Example 2: This preparation example provides a method for preparing a spray-adaptive composite admixture, including the following steps: (1) Preparation of raw materials: Weigh 30 parts by mass of the polycarboxylic acid polymer mother liquor obtained in Preparation Example 1, 0.05 parts by mass of hydroxypropyl methylcellulose, 0.20 parts by mass of polyacrylic acid, 1.0 part by mass of triethanolamine, 1.0 part by mass of calcium nitrate tetrahydrate, and 67.75 parts by mass of deionized water. The sum of the mass parts of the above components is 100 parts by mass.

[0052] (2) Physical mixing: Under normal temperature and pressure, add 50 parts by weight of deionized water to the compounding vessel, turn on the mechanical stirrer and set the speed to 400 rpm; slowly and uniformly add the weighed hydroxypropyl methylcellulose and polyacrylic acid, and continue high shear stirring for 45 min until the solution is uniform and transparent and there are no agglomerated undissolved particles; then add triethanolamine and calcium nitrate tetrahydrate, and continue stirring for 20 min; then reduce the stirring speed to 150 rpm, add the polycarboxylic acid polymer mother liquor and the remaining 17.75 parts by weight of deionized water, mix at low speed for 30 min, and the finished compound additive is obtained by discharging.

[0053] Preparation Example 3: This preparation example provides another method for preparing a spray-adaptive composite admixture, including the following steps: (1) Preparation of raw materials: Weigh 30 parts by mass of the polycarboxylic acid polymer mother liquor obtained in Preparation Example 1, 0.03 parts by mass of hydroxypropyl methylcellulose, 0.25 parts by mass of polyacrylic acid, 0.8 parts by mass of triethanolamine, 0.8 parts by mass of calcium nitrate tetrahydrate, and 68.12 parts by mass of deionized water. The sum of the mass parts of the above components is 100 parts by mass.

[0054] (2) Physical mixing: Under normal temperature and pressure, add 50 parts by weight of deionized water to the compounding vessel, turn on the mechanical stirrer and set the speed to 400 rpm; slowly and uniformly add the weighed hydroxypropyl methylcellulose and polyacrylic acid, and continue high shear stirring for 45 min until the solution is uniform and transparent; then add triethanolamine and calcium nitrate tetrahydrate, and continue stirring for 20 min; then reduce the stirring speed to 150 rpm, add the polycarboxylic acid polymer mother liquor and the remaining 18.12 parts by weight of deionized water, mix uniformly at low speed for 30 min, and the finished compound additive is obtained by discharging.

[0055] Preparation Example 4: This preparation example provides another method for preparing a spray-adaptive composite admixture, comprising the following steps: (1) Preparation of raw materials: Weigh 30 parts by mass of the polycarboxylic acid polymer mother liquor obtained in Preparation Example 1, 0.06 parts by mass of hydroxypropyl methylcellulose, 0.30 parts by mass of polyacrylic acid, 1.0 part by mass of triethanolamine, 1.0 part by mass of calcium nitrate tetrahydrate, and 67.64 parts by mass of deionized water. The sum of the mass parts of the above components is 100 parts by mass.

[0056] (2) Physical mixing: Under normal temperature and pressure, add 50 parts by weight of deionized water to the compounding vessel, turn on the mechanical stirrer and set the speed to 400 rpm; slowly and uniformly add the weighed hydroxypropyl methylcellulose and polyacrylic acid, and continue high shear stirring for 45 min until the solution is uniform and transparent; then add triethanolamine and calcium nitrate tetrahydrate, and continue stirring for 20 min; then reduce the stirring speed to 150 rpm, add the polycarboxylic acid polymer mother liquor and the remaining 17.64 parts by weight of deionized water, mix uniformly at low speed for 30 min, and the finished compound additive is obtained by discharging.

[0057] To conduct a scientific and consistent evaluation of the performance of the composite admixtures in each embodiment and comparative example, this test used a fixed wet-sprayed concrete foundation mix ratio and followed standardized wet-spraying construction procedures. The specific implementation methods are as follows.

[0058] The design strength grade of the basic wet-mix shotcrete is C25. Based on the material usage per cubic meter of wet-mix shotcrete, the fixed mix proportions are as follows: 400 kg of ordinary Portland cement, 50 kg of fly ash, and 450 kg of total cementitious materials; the water-cement ratio is set at 0.42, and the mixing water usage is 189 kg; the sand ratio is set at 48%, with 820 kg of fine aggregate (manufactured sand) and 890 kg of coarse aggregate (crushed stone).

[0059] In the examples and comparative examples, the dosage of the composite admixture was fixed at 1.5% of the total mass of the cementitious materials, which is equivalent to 6.75 kg of composite admixture solution per cubic meter of concrete. At the same time, the water content in the admixture was deducted from the mixing water accordingly to ensure a constant total water-cement ratio.

[0060] Reference Appendix Figures 1-3 The specific wet spraying construction methods and testing and evaluation procedures for reducing the rebound rate of wet sprayed concrete include the following steps: Step S1 involves providing the mixing system and the material mixing process, using a forced concrete mixer. First, the weighed crushed stone, manufactured sand, cement, and fly ash are added to the mixer according to the mix proportions, and dry-mixed for 60 seconds to ensure the dry materials are uniformly combined. Then, the prepared sprayable composite admixture is added to the weighed mixing water and stirred slightly to disperse it. Next, the mixing water containing the admixture is evenly added to the mixer, and wet-mixing continues for 120 seconds until the concrete mixture exhibits a uniform color, no dry material lumps, and a glossy, paste-like appearance, thus completing the preparation of the wet-mixed concrete system.

[0061] Step S2 is the static conditioning and spraying adaptation window assessment process. The wet sprayed concrete mixture from the mixer is unloaded into a storage container with moisture-retaining measures. The initial slump after discharge is measured, and the mixture is then allowed to stand at room temperature to simulate transportation and waiting time at the construction site. During the 30-60 minute standing period, the slump change rate, viscosity value, and initial setting time of the mortar formed under the same conditions are sampled and tested sequentially according to the aforementioned testing method. When the slump change rate is confirmed to be between 10% and 30%, the viscosity value between 8 and 25 Pa·s, and the estimated initial setting time between 30 and 120 minutes, it can be determined that all test data of the mixture fall within the set spraying adaptation window, at which point the mixture enters the optimal pumping and spraying state.

[0062] Step S3 is the conveying and spraying operation control process. The wet-mixed concrete material, within the spraying adaptation window, is loaded into the hopper of a rotor-type or plunger-type wet spraying machine. The equipment is started to pump and smoothly convey the mixture to the nozzle. During the spraying operation, the following mechanical operating parameters are strictly controlled to ensure optimal kinetic energy transfer and deposition of the material stream: maintaining the vertical distance between the nozzle and the sprayed surface within the range of 0.8 meters to 1.5 meters; controlling the spraying angle between the nozzle axis and the normal to the sprayed surface within the range of 70 degrees to 90 degrees; adjusting the system spraying air pressure entering the nozzle to stably maintain it between 0.4 MPa and 0.6 MPa.

[0063] Step S4 is the synergistic molding and rebound rate evaluation process. Under these specific spraying parameters, the composite admixture exerts a synergistic effect of water reduction, viscosity enhancement, and coagulation promotion. During the material jet phase, the diffusion angle of the material beam is recorded using a high-speed camera. During the material impact and deposition phase on the sprayed surface, the thickness of the material adhered after a single spray is measured using a probe. After the single-point spraying operation is completed, all fallen material on the substrate below the sprayed surface is immediately collected and weighed. This weight is used to calculate and evaluate the actual rebound rate under these conditions, thus directly verifying the actual effect of the composite admixture in synergistically regulating the flowability, cohesiveness, and coagulation behavior of the mixture.

[0064] Example 1: This example provides a spray-compatible composite admixture and its application in reducing the rebound rate of wet-sprayed concrete, including the following steps: Based on mass parts, 30 parts of the polycarboxylic acid polymer mother liquor obtained in Preparation Example 1, 0.05 parts of hydroxypropyl methylcellulose, 0.20 parts of polyacrylic acid, 1.0 part of triethanolamine, 1.0 part of calcium nitrate tetrahydrate, and 67.75 parts of deionized water were weighed and prepared by physical mixing at room temperature to obtain the finished composite additive.

[0065] Weigh cement, fly ash, manufactured sand, crushed stone and mixing water according to the aforementioned basic mix proportion. Add the above-mentioned composite admixture at a dosage of 1.5% of the total mass of cementitious materials with the mixing water. Follow the aforementioned steps S1 to S4 to complete the preparation of wet sprayed concrete mixing system, static conditioning and spray adaptation window parameter detection and evaluation, mechanical spraying molding and rebound rate evaluation.

[0066] Example 2: This example provides a spray-adaptive composite admixture and its application in reducing the rebound rate of wet-sprayed concrete, including the following steps: Based on mass parts, 30 parts by mass of the polycarboxylic acid polymer mother liquor obtained in Preparation Example 1, 0.03 parts by mass of hydroxypropyl methylcellulose, 0.25 parts by mass of polyacrylic acid, 0.8 parts by mass of triethanolamine, 0.8 parts by mass of calcium nitrate tetrahydrate, and 68.12 parts by mass of deionized water were weighed and prepared by physical mixing at room temperature to obtain the finished composite additive.

[0067] Weigh cement, fly ash, manufactured sand, crushed stone and mixing water according to the aforementioned basic mix proportion. Add the above-mentioned composite admixture at a dosage of 1.5% of the total mass of cementitious materials with the mixing water. Follow the aforementioned steps S1 to S4 to complete the preparation of wet sprayed concrete mixing system, static conditioning and spray adaptation window parameter detection and evaluation, mechanical spraying molding and rebound rate evaluation.

[0068] Example 3: This example provides a spray-compatible composite admixture and its application in reducing the rebound rate of wet-sprayed concrete, including the following steps: By weight, 30 parts by weight of the polycarboxylic acid polymer mother liquor obtained in Preparation Example 1, 0.06 parts by weight of hydroxypropyl methylcellulose, 0.30 parts by weight of polyacrylic acid, 1.0 part by weight of triethanolamine, 1.0 part by weight of calcium nitrate tetrahydrate, and 67.64 parts by weight of deionized water were weighed and prepared by physical mixing at room temperature to obtain the finished composite additive.

[0069] Weigh cement, fly ash, manufactured sand, crushed stone and mixing water according to the aforementioned basic mix proportion. Add the above-mentioned composite admixture at a dosage of 1.5% of the total mass of cementitious materials with the mixing water. Follow the aforementioned steps S1 to S4 to complete the preparation of wet sprayed concrete mixing system, static conditioning and spray adaptation window parameter detection and evaluation, mechanical spraying molding and rebound rate evaluation.

[0070] Comparative Example 1: Compared with Example 1, the difference is that hydroxypropyl methylcellulose, polyacrylic acid, triethanolamine and calcium nitrate tetrahydrate were not added to the compound additive formulation, and the amount of deionized water was adjusted to 70 parts by weight. All other aspects are the same.

[0071] Comparative Example 2: Compared with Example 1, the difference is that polyacrylic acid, triethanolamine and calcium nitrate tetrahydrate were not added to the composite additive formulation, and the amount of deionized water was adjusted to 69.95 parts by weight. All other aspects are the same.

[0072] Comparative Example 3: Compared with Example 2, the difference is that hydroxypropyl methylcellulose, triethanolamine and calcium nitrate tetrahydrate were not added to the compound additive formulation, and the amount of deionized water was adjusted to 69.75 parts by mass. All other aspects are the same.

[0073] Comparative Example 4: Compared with Example 1, the difference is that triethanolamine and calcium nitrate tetrahydrate were not added to the compound additive formulation, and the amount of deionized water was adjusted to 69.75 parts by mass. All other aspects are the same.

[0074] Comparative Example 5: Compared with Example 1, the difference is that the composite admixture is made by mixing 30 parts by weight of commercially available conventional polycarboxylate superplasticizer mother liquor with 70 parts by weight of deionized water, without adding thickening and setting-promoting components, and in the spraying operation of step S3, a commercially available liquid alkali-free quick-setting agent with an admixture of 4.0% of the total mass of cementitious material is added at the nozzle according to the conventional construction process, and the rest are the same.

[0075] To provide a clear and objective demonstration of the technical effects of the embodiments and comparative examples, all samples were experimentally measured according to the aforementioned test methods. The following is a summary of the performance test data, along with a technical analysis based on the experimental results.

[0076] Table 1: Summary of Performance Test Results of Composite Admixtures in Each Example and Comparative Example

[0077] Table 2: Comprehensive Evaluation of Construction Adaptability and Spraying Effect of Each Example and Comparative Example

[0078] The first aspect involves an analysis of window indicators and liquidity maintenance characteristics: Test data show that the slump change rate of the mixtures in Examples 1 to 3 was controlled between 15% and 18%, the viscosity value was between 14.5 Pa·s and 16.0 Pa·s, and the initial setting time was maintained between 58 minutes and 72 minutes. All of the above-mentioned pre-rheological parameters meet the spray adaptability window criteria defined in this invention. The dispersion and thickening network composed of polycarboxylic acid, hydroxypropyl methylcellulose, and polyacrylic acid balances the suspension stability and pumpability of the slurry.

[0079] Comparative Example 1, using a single water-reducing agent, lacked thickening and stabilizing components, resulting in a slump change rate of 32% and a viscosity of only 7.8 Pa·s. The slurry was highly prone to segregation and bleeding over time. Comparative Examples 2 to 4, lacking setting regulators, all had initial setting times exceeding 100 minutes, leaving the slurry in a readily flowable plastic state for an extended period, failing to meet the requirements for rapid hardening after spraying. Comparative Example 5, using a conventional water-reducing agent with an external accelerator at the nozzle, lacked a thickening system within the pumping pipeline, resulting in a static slump change rate of 28% and a low viscosity of 9.5 Pa·s, deviating from the suitable spraying window.

[0080] The second aspect is the analysis of rebound rate and macroscopic jetting effect: The rebound rate of Examples 1 to 3 decreased to 7.6% to 8.2%. During the jet flight phase, the material bundle diffusion angle of the examples narrowed to 11 to 13 degrees. The thickening component rapidly restored the polymer network after the shear force disappeared, tightly encapsulating the slurry and aggregate, reducing the risk of material dispersion caused by high-pressure air shearing of the slurry. In Comparative Example 1, the material bundle diffusion angle reached 24 degrees, resulting in severe material dispersion in the air and significant kinetic energy loss.

[0081] During the wall adhesion stage, the adhesion thickness after a single spray in the examples reached 27 mm to 30 mm. The coagulation regulating component played a rapid catalytic hydration role, and the slurry quickly lost its fluidity and established yield stress after impacting the sprayed surface, forming an effective micro-buffer layer. Comparative Example 4, due to the lack of a coagulating component, although the cohesion was acceptable, the hardening of the adhesion surface was slow, and it was prone to slippage under its own weight and impact. Its adhesion thickness was 21 mm, and the rebound rate was 11.9%. Although Comparative Example 5 had a shorter initial setting time, because the diffusion angle of the mixed material bundles reached 22 degrees in the early stage, the aggregate was exposed when it impacted the rock surface and the kinetic energy was greatly reduced. The final adhesion thickness was only 17 mm, and the rebound rate reached 15.8%.

[0082] Experimental results confirm that simply applying a setting accelerator at the nozzle cannot fundamentally solve the rebound problem. By synergistically regulating the rheological and hydration kinetics of the entire process of wet-mixed shotcrete mixing, conveying, jetting, and adhesion formation through water-reducing, viscosity-enhancing, and setting accelerator components, the mixture can be kept within a set spraying adaptation window, which can significantly improve construction stability and reduce the rebound rate.

[0083] Although embodiments of the invention have been shown and described, it will be understood by those skilled in the art that various changes, modifications, substitutions and alterations can be made to these embodiments without departing from the principles and spirit of the invention, the scope of which is defined by the appended claims and their equivalents.

Claims

1. A composite admixture for concrete, characterized in that, Based on 100% of the total weight of the composite admixture, the proportions of each effective component are as follows: water-reducing and flow-regulating component 10%–40%; thickening and stabilizing component 0.001%–0.5%; setting and regulating component 0.1%–10%; the remainder being water and optional auxiliary agents. The water-reducing and flow-regulating component is a water-soluble polycarboxylate-based water-reducing polymer; the thickening and stabilizing component is selected from one or more of cellulose ether derivatives, polyacrylic acid polymers, xanthan gum, and warming gum; and the coagulation regulating component is selected from one or more of organic alcohol amines, inorganic nitrates, and aluminates.

2. The composite admixture for concrete according to claim 1, characterized in that: The cellulose ether derivative is hydroxypropyl methylcellulose; the polyacrylic acid polymer is polyacrylic acid with a weight average molecular weight between 3000 and 5000; the organic alcohol amine is triethanolamine; and the inorganic nitrate is calcium nitrate tetrahydrate.

3. The composite admixture for concrete according to claim 2, characterized in that: The thickening and stabilizing component is composed of the hydroxypropyl methylcellulose and the polyacrylic acid, and the weight ratio of the two is 1:(4.0~8.33). The coagulation regulating component consists of the triethanolamine and the calcium nitrate tetrahydrate, with a weight ratio of 1:

1.

4. The composite admixture for concrete according to claim 1, characterized in that: The water-soluble polycarboxylate superplasticizer is a custom-synthesized spray-adaptive polycarboxylate polymer, and its preparation method includes the following steps: (1) Mix isopentenyl alcohol polyoxyethylene ether macromonomer and deionized water according to the formula as the base material, and control the temperature at 15-40℃; (2) Add hydrogen peroxide solution to the system as an initiator in one step; (3) Prepare a first aqueous solution containing acrylic acid and acrylamide, and a second aqueous solution containing ascorbic acid and mercaptopropionic acid, and add the first aqueous solution and the second aqueous solution dropwise into the reaction system simultaneously to carry out the copolymerization reaction; (4) After the reaction is completed, keep warm and add sodium hydroxide solution to adjust the pH value of the system to 6.0-7.0 to obtain the spray-adapted polycarboxylic acid polymer.

5. A composite admixture for concrete according to claim 1, characterized in that: The optional auxiliary agents include one or a combination of two of the following: slump-retaining components and defoaming components.

6. A method for reducing the rebound rate of wet-sprayed concrete, characterized in that, Using the composite admixture for concrete as described in any one of claims 1-5 includes the following steps: S1. Provide a wet sprayed concrete mixing system containing cementitious materials, aggregates and mixing water according to the proportion, and add the concrete with a composite admixture to it and mix to obtain a wet sprayed concrete mixture. S2. Allow the mixed wet-sprayed concrete mixture to stand for adjustment to confirm that the various rheological and setting performance parameters of the mixture are within the spraying adaptation window. S3. The wet-mixed concrete mixture within the spraying adaptation window is transported to the spraying equipment and wet-sprayed under the set mechanical operating parameters to complete the adhesion and formation of the mixture on the sprayed surface.

7. The method for reducing the rebound rate of wet-sprayed concrete according to claim 6, characterized in that: In step S1, the amount of the composite admixture for concrete added is 1.0% to 3.0% of the total weight of the cementitious materials in the wet-mixed concrete system. The specific timing and mixing method for adding the cementitious material are as follows: first, dry mix the cementitious material and the aggregate evenly, and then add the mixing water containing the concrete composite admixture. Continue wet mixing for 120–180 seconds.

8. The method for reducing the rebound rate of wet-sprayed concrete according to claim 6, characterized in that: In step S2, the controlled spray adaptation window is determined by at least two of the following mixture performance parameters: The slump change rate of the mixture after it has been left to stand for 60 minutes after exiting the mixer is 10% to 30%. The viscosity of the mixture is 8–25 Pa·s; The initial setting time is 30–120 minutes; The diffusion angle of the material bundle is 8° to 25°; The thickness of the coating after a single spray is 10-40 mm.

9. A method for reducing the rebound rate of wet-sprayed concrete according to claim 6, characterized in that: In step S3, the specific mechanical operation parameters for wet spraying are as follows: The vertical distance between the nozzle of the spraying equipment and the surface to be sprayed is controlled to be 0.8–1.5 m; The spray angle between the nozzle axis and the normal to the sprayed surface is controlled to be 70° to 90°. The system jet pressure entering the nozzle is controlled to be stable at 0.4–0.6 MPa.

10. A method for reducing the rebound rate of wet-sprayed concrete according to claim 6, characterized in that: In step S3, no additional accelerator is added at the nozzle during the jet ejection and adhesion molding process of the mixture. After molding, the final adhesion thickness of a single continuous fixed-point spray is controlled to reach more than 27mm.

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