A composite admixture and its preparation method, and a method for preparing concrete pipe piles.

Abstract

This invention discloses a composite admixture and its preparation method, as well as a method for preparing concrete pipe piles. The composite admixture comprises submicron-sized metakaolin, ultrafine quartz sand powder, slag powder, calcium formate, sulfate activator, water-retaining and toughening component, auxiliary setting regulator component, and superplasticizer; wherein, by weight percentage, the submicron-sized metakaolin is 12%-18%, the ultrafine quartz sand powder is 22%-30%, the slag powder is 35%-45%, the calcium formate is 2.5%-4.0%, the sulfate activator is 7%-11%, and the water-retaining and toughening component is 2.0%-4.0%; the median particle size D50 of the submicron-sized metakaolin is 1.0μm-3.0μm, and the specific surface area is ≥1200m² / kg; the water-retaining and toughening component is a mixture of redispersible latex powder and cellulose ether in a mass ratio of (2:1)-(1:1). This invention, through multi-component synergy, significantly improves the early strength, later strength, durability, and crack resistance of concrete under low-temperature steam curing conditions of 40℃-60℃, which can simultaneously meet the differentiated performance requirements of PHC piles and large-diameter pipe piles, and greatly reduce production energy consumption.

Description

Technical Field

[0001] This invention relates to the field of building materials and precast components, and particularly to a composite admixture and its preparation method, and a method for preparing concrete pipe piles. Background Technology

[0002] Prestressed concrete pipe piles, mainly including PHC piles and large-diameter pipe piles, are core load-bearing components in foundation engineering projects such as buildings, bridges, and ports. Although they differ in specifications, reinforcement, and applicable scenarios, their production processes heavily rely on high-temperature steam curing to achieve early high strength and meet the stringent requirements of demolding, hoisting, and prestressing tensioning. Specifically, PHC piles generally employ high-pressure, high-temperature curing (typically 1.0 MPa pressure and ≥ 170 ℃), resulting in enormous steam consumption; while large-diameter pipe piles mostly use normal-pressure, high-temperature curing (e.g., 80 ℃ ~ 90 ℃), which not only consumes more energy but also, due to the large wall thickness and difficulty in controlling the heating and cooling rates, easily induces harmful temperature stress and shrinkage cracks. These two traditional processes directly lead to high production energy consumption and carbon emissions, and the resulting problems such as increased porosity, larger and uneven hydration products in the concrete microstructure also pose a potential threat to the long-term durability of prestressed concrete pipe piles. Summary of the Invention

[0003] The purpose of this invention is to provide a composite admixture and its preparation method, as well as a method for preparing concrete pipe piles.

[0004] An embodiment of the present invention provides a composite admixture for producing concrete pipe piles using a steam curing process. The composite admixture includes submicron-sized metakaolin, ultrafine quartz sand powder, slag powder, calcium formate, sulfate activator, and water-retaining and toughening components.

[0005] The composition of the composite admixture, by weight percentage, is as follows:

[0006] The submicron-sized metakaolinite comprises 12-18 wt%, the ultrafine quartz sand powder comprises 22-30 wt%, the slag powder comprises 35-45 wt%, the calcium formate comprises 2.5-4.0 wt%, the sulfate activator comprises 7-11 wt%, and the water-retaining and toughening component comprises 2.0-4.0 wt%.

[0007] The median particle size D50 of the submicron-sized metakaolin is 1.0 μm - 3.0 μm, and the specific surface area is ≥ 1200 m². 2 / kg;

[0008] The water-retaining and toughening component includes redispersible latex powder and cellulose ether in a mass ratio of (2:1) to (1:1).

[0009] In one embodiment, the ultrafine quartz sand powder is a quartz material with a silica content greater than 90% and a specific surface area of ​​500 m². 2 / kg - 600 m 2 / kg.

[0010] In one embodiment, the ultrafine quartz sand powder is at least one of natural quartzite powder, siliceous tailings powder, or foundry recycled sand powder.

[0011] In one embodiment, the slag powder is grade S95 or higher, with a specific surface area of ​​350 m². 2 / kg - 400 m 2 / kg.

[0012] In one embodiment, the sulfate activator is at least one of gypsum dihydrate, gypsum hemihydrate, or anhydrite.

[0013] In one embodiment, the composite admixture further includes an auxiliary setting regulator and a superplasticizer, wherein the auxiliary setting regulator is 0-1.5 wt% and the superplasticizer is 3.0-4.5 wt%.

[0014] In one embodiment, the auxiliary coagulation component is at least one of sodium gluconate, citric acid, or tartaric acid.

[0015] This invention also provides a method for preparing the above-mentioned composite admixture, comprising the following steps:

[0016] Weigh the submicron-sized metakaolin, the ultrafine quartz sand powder, the slag powder, the calcium formate, the sulfate activator, and the water-retaining and toughening components according to the specified ratio.

[0017] The weighed submicron-sized metakaolin, ultrafine quartz sand powder, slag powder, calcium formate, and sulfate activator are mixed in the first stage for 10 min to 20 min to form an active powder premix.

[0018] Add the weighed water-retaining and toughening components to the premix and perform a second mixing for 15 min - 30 min.

[0019] This invention also provides a method for preparing concrete pipe piles, comprising the following steps:

[0020] The above-mentioned composite admixture is used;

[0021] The composite admixture is added to the concrete mix of prestressed concrete pipe piles at a dosage of 5% to 12% of the total mass of cementitious materials to replace cement.

[0022] Forming concrete pipe piles;

[0023] The formed concrete pipe piles are steam cured under constant temperature conditions of 40℃~60℃.

[0024] In one embodiment, when the concrete pipe pile is a PHC pile, the amount of the composite admixture is 8%-12% of the total mass of the cementitious material, replacing cement in equal amounts;

[0025] The formed concrete pipe piles were then left to stand for 2-3 hours.

[0026] Heat to 50℃-60℃ and maintain constant temperature for 10-12 hours;

[0027] Alternatively, when the concrete pipe pile is a large pipe pile, the amount of the composite admixture is 5%-10% of the total mass of the cementitious material, replacing cement in equal amounts;

[0028] The formed concrete pipe piles were then left to stand for 3-5 hours.

[0029] Raise the temperature to 40℃-50℃ and maintain it at a constant temperature for 12-16 hours.

[0030] The composite admixture provided by this invention fully demonstrates its core value and beneficial effects under the specific process conditions of low-temperature steam curing at 40℃ to 60℃, effectively solving the contradictions of high energy consumption, microstructure deterioration, slow cement hydration, and insufficient performance development under low-temperature conditions in traditional high-temperature steam curing processes.

[0031] In a low-temperature steam curing environment (40℃-60℃), this composite admixture exhibits a unique low-temperature adaptive activation mechanism. The submicron-sized metakaolin in the composite admixture, with its extremely high specific surface area and reactivity, can efficiently initiate the pozzolanic reaction at temperatures far below 60℃, significantly lower than traditional steam curing temperatures. This rapidly consumes the calcium hydroxide produced in the early stages of cement hydration, generating cementitious hydration products that provide a crucial source of early strength for concrete in low-temperature environments. Simultaneously, calcium formate dissolves in the low-temperature aqueous medium, providing a high concentration of calcium ions, while the sulfate activator continuously activates the potential activity of slag powder and other materials under low-temperature and humid conditions. The synergistic effect of these three factors significantly overcomes the inhibitory effect of low temperature on cement hydration kinetics, enabling the concrete piles to achieve a demolding strength of over 40MPa in just 10-18 hours at a curing temperature of 40℃-60℃, successfully achieving rapid production under low-temperature conditions.

[0032] More importantly, the gradient pozzolanic reaction system constructed in this invention exhibits a more optimized long-term performance development path under low-temperature conditions. Under mild thermal activation at 40℃-60℃, ultrafine quartz sand powder and slag powder can continuously and stably undergo deep secondary reactions, avoiding excessively rapid reactions or microstructure coarsening that may occur at high temperatures. This controlled and continuous reaction at low temperatures not only ensures the stable growth of concrete strength to C80 and above in the later stages, but also promotes more uniform and dense hydration products and significantly optimizes pore size distribution. As a result, the concrete retains excellent resistance to chloride ion penetration and durability even after low-temperature steam curing, overturning the traditional understanding that low-temperature curing inevitably leads to performance degradation.

[0033] Specifically addressing the risks of temperature stress and drying shrinkage that are prone to occur in large-diameter pipe piles during low-temperature steam curing, the water-retaining and toughening components in this composite admixture play a crucial role. At lower curing temperatures (e.g., 40℃-50℃), the water-retaining effect of cellulose ethers is more significant, effectively delaying moisture evaporation and reducing plastic shrinkage; the flexible network formed by redispersible latex powder can better buffer internal stress during low-temperature hardening. Combined with the aforementioned chemical shrinkage reduction mechanism, these factors together endow the concrete of large-diameter pipe piles with superior volume stability and crack resistance under low-temperature steam curing.

[0034] In summary, this composite admixture is specifically designed for low-temperature steam curing processes. Through innovation in the material system, it not only overcomes the constraints of low-temperature environments on the development of concrete performance, but also transforms the energy consumption pain points of traditional processes into opportunities to improve microstructure and long-term performance. It achieves a balance of early strength, high strength, high durability, and high crack resistance in concrete pipe piles within a relatively low temperature range of 40℃-60℃, providing a practical material solution for the green and low-carbon production of prestressed concrete pipe piles. Detailed Implementation

[0035] To make the objectives, technical solutions, and advantages of the embodiments of the present invention clearer, the various embodiments of the present invention are described in detail below. However, those skilled in the art will understand that many technical details are presented in the various embodiments of the present invention to enable the reader to better understand this application. However, the technical solutions claimed in this application can be implemented even without these technical details and various changes and modifications based on the following embodiments.

[0036] In the following description, certain specific details are set forth for the purpose of illustrating various disclosed embodiments in order to provide a thorough understanding of the various disclosed embodiments. However, those skilled in the art will recognize that embodiments may be practiced without one or more of these specific details. In other instances, well-known apparatuses, structures, and techniques associated with this application may not have been shown or described in detail to avoid unnecessarily obscuring the description of the embodiments.

[0037] Unless the context requires otherwise, throughout the specification and claims, the word “comprising” and its variations, such as “including” and “having”, shall be understood to have an open, inclusive meaning, that is, to be interpreted as “including, but not limited to”.

[0038] Throughout this specification, references to "an embodiment" or "an embodiment" indicate that a particular feature, structure, or characteristic described in connection with the embodiment is included in at least one embodiment. Therefore, the appearance of "in an embodiment" or "an embodiment" in various places throughout the specification does not necessarily refer to the same embodiment. Furthermore, a particular feature, structure, or characteristic may be combined in any manner in one or more embodiments.

[0039] The singular forms “a” and “the” used in this specification and the appended claims include plural references unless otherwise expressly stated herein. It should be noted that the term “or” is generally used to mean “and / or” unless otherwise expressly stated herein.

[0040] The embodiments of the present invention are described below.

[0041] This invention provides a composite admixture for producing concrete pipe piles using steam curing process. The composite admixture includes submicron metakaolin, ultrafine quartz sand powder, slag powder, calcium formate, sulfate activator, water-retaining and toughening components, auxiliary setting regulator components, and superplasticizer.

[0042] The composition of the composite admixture, by weight percentage, is as follows:

[0043] The composition includes: submicron metakaolin (12-18 wt%), ultrafine quartz sand powder (22-30 wt%), slag powder (35-45 wt%), calcium formate (2.5-4.0 wt%), sulfate activator (7-11 wt%), water-retaining and toughening components (2.0-4.0 wt%), auxiliary setting regulator components (0-1.5 wt%), and superplasticizer (3.0-4.5 wt%).

[0044] Preferably, the median particle size D50 of submicron metakaolinite is 1.0 μm - 3.0 μm, and the specific surface area is ≥1200 m². 2 / kg.

[0045] Preferably, the ultrafine quartz sand powder is a quartz material with a silica content greater than 90% and a specific surface area of ​​500 m². 2 / kg - 600 m 2 / kg.

[0046] Preferably, the ultrafine quartz sand powder is at least one of natural quartzite powder, siliceous tailings powder, or foundry recycled sand powder.

[0047] Preferably, the slag powder is grade S95 or higher, with a specific surface area of ​​350 m². 2 / kg - 400 m 2 / kg.

[0048] Preferably, the sulfate activator is at least one of gypsum dihydrate, gypsum hemihydrate, or anhydrite.

[0049] Preferably, the water-retaining and toughening components include redispersible latex powder and cellulose ether in a mass ratio of (2:1) to (1:1).

[0050] Preferably, the composite admixture further includes an auxiliary setting regulator and a superplasticizer, wherein the auxiliary setting regulator is 0-1.5 wt% and the superplasticizer is 3.0-4.5 wt%.

[0051] Preferably, the auxiliary coagulation component is at least one of sodium gluconate, citric acid or tartaric acid.

[0052] The composite admixture provided by this invention, by introducing components with different reactivity and functional properties, actively intervenes in and optimizes the cement hydration process under low-temperature conditions (40℃-60℃), constructing a continuously densified and high-performance concrete microstructure from early to late stages. The specific roles and synergistic mechanisms of each component are as follows:

[0053] First, in low-temperature environments, the hydration rate of cement decreases significantly, resulting in slow early strength development. This composite admixture effectively overcomes this bottleneck through the synergistic effect of submicron-sized metakaolinite and calcium formate.

[0054] Calcium formate, as a non-chloride salt early-strength agent, dissolves instantly in water, rapidly providing a high concentration of calcium ions (Ca²⁺). + This directly promotes the early nucleation and crystallization of hydration products (such as ettringite and CSH gel), providing an initial driving force for strength development.

[0055] Submicron-sized metakaolin has an extremely high specific surface area (≥1200 m² / kg) and chemical activity, which allows it to undergo a rapid pozzolanic reaction with the large amount of calcium hydroxide (Ca(OH)2) generated in the early stage of cement hydration under low temperature conditions of 40℃-60℃, generating hydrated calcium silicate (CSH) gel and hydrated calcium aluminosilicate (CASH) gel with high cementitious properties.

[0056] Calcium hydroxide is a product with low strength, coarse grains, and poor durability. Therefore, when submicron-sized metakaolin reacts with calcium hydroxide, it not only consumes the harmful calcium hydroxide but also generates in situ highly gelling calcium silicate (CSH) gel and calcium aluminosilicate (CASH) gel.

[0057] In the above reaction, the main active components of submicron-sized metakaolin are amorphous alumina and silica. Submicron-sized metakaolin reacts with calcium hydroxide. However, in the initial low-temperature phase, the calcium hydroxide produced by the cement's own hydration is limited and slow. At this point, calcium formate plays a crucial role, providing Ca²⁺. + It will quickly react with OH in the system - This combination rapidly creates a localized microenvironment with a high concentration of calcium hydroxide. This is equivalent to pre-establishing a sufficient supply station of reaction raw materials for the submicron-sized metakaolin clay awaiting reaction.

[0058] Submicron-sized metakaolinite, with its enormous specific surface area, almost simultaneously captures and consumes these newly generated calcium hydroxides. This process is not a simple sequential one, but a dynamic and continuous coupling process: calcium formate continuously dissociates to supply calcium, rapidly forming calcium hydroxide; submicron-sized metakaolinite immediately captures and reacts with the calcium hydroxide, and finally, after consuming the calcium hydroxide, it promotes further dissociation of calcium formate to maintain the ion concentration gradient.

[0059] Meanwhile, the median particle size D50 of submicron metakaolinite is 1.0 μm - 3.0 μm, and the specific surface area is ≥ 1200 m². 2 / kg, meaning that submicron metakaolin is an ultrafine particle. Submicron metakaolin can act as a physical filler, blocking the gaps between cement particles, so that concrete can obtain a denser microstructure in the early stages of molding.

[0060] A robust early strength framework was rapidly established at low temperatures by combining calcium formate with submicron-sized metakaolin.

[0061] Secondly, in order to solve the problem of excessively rapid reaction and insufficient strength growth in the later stage of single submicron metakaolin, and to achieve a complete and gradient conversion of calcium hydroxide, this composite admixture introduces ultrafine quartz sand powder and slag powder, and forms a synergistic chain from activation to reaction with sulfate activator.

[0062] Sulfate activators are key regulators of the reaction environment; gypsum can be one such activator. Sulfate activators provide sulfate ions (SO4²⁻) in the liquid phase. - It reacts with the aluminum phase in cement and the active components of slag to continuously generate ettringite with micro-expansion and reinforcing effects, and maintains the alkaline environment of the system, providing the necessary conditions for the activation of slag.

[0063] Slag powder (S95 grade and above) reacts slowly at low temperatures, but its potential activity is effectively activated under the alkaline environment and sulfate activation conditions described above. The slag powder continues to react with calcium hydroxide in the system, generating additional CSH and CASH gels, becoming one of the main contributors to the mid-to-late stage strength increase.

[0064] The main component of ultrafine quartz sand powder is crystalline SiO2, which exhibits mild and sustained reactivity at low temperatures. Its role is to follow the rapid reaction of submicron-sized metakaolin, consuming residual calcium hydroxide in the system and initiating a deep and continuous pozzolanic reaction. This process continues throughout the middle and later stages of concrete curing, continuously transforming weak-strength calcium hydroxide crystals into high-strength CSH gel, achieving a gradient and thorough conversion of harmful substances. This not only ensures stable strength growth in the later stages of concrete curing, meeting the requirements of PHC piles above C80, but also significantly refines pores and optimizes pore structure from a chemical perspective, thereby significantly improving the concrete's impermeability and durability.

[0065] Finally, in response to the risks of water loss shrinkage and temperature stress during low-temperature steam curing (especially for large pipe piles), the composite admixture in this application embodiment also integrates a dual crack-resistant mechanism of chemical shrinkage reduction and physical toughening and water retention.

[0066] Regarding chemical shrinkage, as mentioned earlier, the continuous consumption of calcium hydroxide by submicron-sized metakaolin, quartz sand powder, and slag powder directly reduces the chemical shrinkage caused by the carbonization or crystallization of calcium hydroxide. This is the fundamental way to improve volume stability intrinsically.

[0067] In terms of physical toughening and water retention, the cellulose ether in the water-retaining and toughening component can form a water-retaining network inside the concrete, effectively locking in moisture, slowing down the rate of moisture evaporation under low-temperature steam curing conditions, reducing plastic shrinkage, and providing an internal water source for the continuous hydration of cement. Meanwhile, the redispersible latex powder in the water-retaining and toughening component can form a continuous flexible polymer film during the concrete hardening process. This flexible polymer film can bridge microcracks, dissipate fracture energy, and significantly improve the toughness (fracture energy) and crack resistance of concrete. Through the combination of cellulose ether and redispersible latex powder, tensile stress caused by temperature differences and drying is effectively buffered.

[0068] In addition, to ensure that the above-mentioned functional components, especially submicron metakaolin, are uniformly dispersed in concrete and perform their functions, and to adapt to different production rhythms, the composite admixture in the embodiments of this application includes superplasticizers and auxiliary setting-regulating components.

[0069] Superplasticizers can be, for example, polycarboxylate-based superplasticizers. Through their efficient steric hindrance effect, superplasticizers effectively disperse cement and admixture particles during mixing, especially preventing the agglomeration of submicron metakaolin, thus ensuring that fresh concrete has excellent workability and homogeneity.

[0070] Auxiliary setting modifiers such as sodium gluconate can be selectively added. These auxiliary setting modifiers temporarily regulate the concentration of early-stage hydration ions in cement through complexation, fine-tuning the setting time so that this composite admixture can better adapt to different ambient temperatures, cement types, and production process requirements without affecting the final strength.

[0071] In summary, the composite admixture provided in this application relies on a complete synergistic system to achieve its low-temperature steam curing effect: firstly, calcium formate and submicron-sized metakaolin synergistically stimulate early strength under low-temperature conditions; secondly, the gradient pozzolanic reaction formed by submicron-sized metakaolin, ultrafine quartz sand powder, and slag powder achieves continuous strength growth and long-term optimization of microstructure; simultaneously, the chemical consumption mechanism of calcium hydroxide and the physical effects of water-retaining and toughening components jointly ensure the volume stability and crack resistance of concrete; finally, the workability and setting time are precisely controlled by superplasticizers and auxiliary setting-regulating components to ensure that the admixture can stably adapt to the requirements of actual production processes.

[0072] Furthermore, it should be noted that the specific proportion design of the composite admixture of the present invention is strictly designed to achieve the multiple stringent objectives of rapid early strength, high later strength, high durability and high crack resistance of concrete under low temperature steam curing conditions of 40℃-60℃.

[0073] Firstly, to achieve rapid early strength at low temperatures, the proportion of submicron-sized metakaolin is 12%-18%, while the proportion of calcium formate is 2.5%-4.0%. The 12% proportion of submicron-sized metakaolin is the minimum threshold to ensure the formation of an effective reaction network in the system and significantly improve early strength; while the upper limit of 18% avoids potential risks to later volume stability due to its rapid reaction and concentrated exothermic reaction, and maintains balance with other components. The amount of calcium formate ensures that a high calcium ion concentration can be quickly established in the initial stage of mixing, providing immediate raw materials for the rapid pozzolanic reaction of the submicron-sized metakaolin. Too high a calcium formate content may lead to excessively rapid setting, affecting construction; too low a content will fail to effectively activate the low-temperature reaction.

[0074] Secondly, to achieve sustained strength growth and fundamental structural densification, the ratio of ultrafine quartz sand powder (22%-30%) to slag powder (35%-45%) constitutes a relay reaction system. The content of ultrafine quartz sand powder needs to be sufficiently high (≥22%) to ensure that after the rapid reaction of submicron-sized metakaolin, there is sufficient active SiO2 to continuously consume the residual calcium hydroxide in the system, enabling a deep and long-term pozzolanic reaction. This is crucial for ensuring no strength reduction and pore refinement in the later stages. The upper limit of ultrafine quartz sand powder (30%) is limited by its slower contribution to early strength and the influence of water demand. As one of the main matrix materials of the system, the high content (35%-45%) of slag powder ensures the long-term cementing activity and chemical stability of the cementing system and allows it to fully interact with the sulfate activator.

[0075] The proportion of sulfate activator (7%-11%) is crucial. Its dosage must be sufficient to continuously activate the slag and fly ash, and maintain the system to generate an appropriate amount of ettringite to provide micro-expansion to compensate for shrinkage. A dosage below 7% may lead to insufficient activation, preventing the slag from reaching its full potential; a dosage above 11% may introduce excessive sulfate, resulting in later expansion or durability risks.

[0076] The formulation of the water-retaining and toughening component (2.0%-4.0%) is directly designed for low-temperature steam curing characteristics. Sufficient content (≥2.0%) is the basis for forming an effective water-retaining film and a continuous polymer network to improve crack resistance; exceeding 4.0% may excessively delay setting and significantly increase costs.

[0077] The dosage of superplasticizer (3.0%-4.5%) is the key to ensuring the uniform dispersion of all the aforementioned micro-nano and micron-sized powders (especially submicron-sized metakaolin) in concrete and preventing agglomeration. Its ratio needs to strike a balance between achieving the best dispersion effect and avoiding excessive retardation of setting.

[0078] In summary, the formulation of this invention is a highly synergistic and mutually constraining system. Each percentage carries a specific functional mission, collectively ensuring that the entire material system, under low-temperature thermal activation conditions, can evolve along a predetermined path of rapid initiation, gradient response, structural strengthening, and shrinkage inhibition, ultimately achieving a comprehensive performance breakthrough that cannot be achieved by a single material. This formulation system is specifically designed to address the unique characteristics of the low-temperature steam curing process.

[0079] In addition, it should be noted that in some existing schemes, raw materials may also include metakaolin, slag powder, gypsum, etc. However, the metakaolin, quartz sand powder and other materials in the existing schemes can usually only reach the conventional fineness, such as a specific surface area of ​​about 300-500 m² / kg. Their volcanic ash reactivity is limited, making it difficult to achieve rapid and efficient conversion of hydration products under low temperature conditions.

[0080] The submicron-sized metakaolin used in this invention has a specific surface area ≥1200 m² / kg, while the specific surface area of ​​ultrafine quartz sand powder is 500-600 m² / kg, and the specific surface area of ​​slag powder with a certain fineness is 350-400 m² / kg. The submicron-sized metakaolin, with its ultra-high specific surface area, rapidly initiates the reaction at low temperatures; the ultrafine quartz sand powder, with its moderate fineness, continuously consumes calcium hydroxide in the middle stage; and the slag powder contributes to the later-stage strength under sulfate activation.

[0081] Specifically, the specific surface area of ​​conventional fine-grained materials is 300-400 m² / kg, which is similar to the reaction rate of volcanic ash. At low temperatures, both are either slow or fast, making it impossible to form a temporal gradient. This leads to either incomplete consumption of calcium hydroxide or concentrated consumption with incomplete conversion, resulting in unresolved problems such as coarse crystal residue, later strength reduction, and shrinkage cracking.

[0082] The composite admixture in this invention significantly differentiates the specific surface areas of the three materials: submicron-sized metakaolin (≥1200 m² / kg) provides rapid initial conversion, micron-sized quartz sand powder (500-600 m² / kg) provides intermediate conversion, and slag powder (350-400 m² / kg) provides final reinforcement. This achieves, for the first time, a rapid, intermediate, and slow phased, complete conversion of calcium hydroxide under low-temperature steam curing conditions. This time-series control based on fineness gradient is simply impossible to achieve in conventional fineness material systems.

[0083] In addition to the aforementioned beneficial effects, the method described in this application embodiment can also achieve low-carbon benefits from the source of production to the entire product lifecycle.

[0084] Firstly, this invention reduces the curing temperature of PHC piles from ≥170℃ (high pressure, high temperature) to 50℃-60℃ (normal pressure, low temperature), and the curing temperature of large pipe piles from 80℃-90℃ to 40℃-50℃. Calculations show that energy consumption in the steam curing process alone can be reduced by 60%-70%, corresponding to a direct carbon emission reduction of over 50%, achieving deep decarbonization of the production process.

[0085] Secondly, the core components of the composite admixture, such as slag powder and ultrafine quartz sand powder, are all derived from industrial by-products or natural minerals. Their extensive application not only reduces the amount of cement clinker used, but also realizes the resource utilization of bulk solid waste and reduces the environmental burden caused by traditional solid waste storage.

[0086] Furthermore, the significantly improved chloride ion penetration resistance and volume stability of concrete in this invention directly translates into a longer service life and lower maintenance frequency for the pipe piles in harsh environments (such as marine environments and saline-alkali land). According to Life Cycle Assessment (LCA) theory, extending the structural lifespan is one of the most effective strategies for reducing its total life cycle carbon emissions.

[0087] Finally, by using the composite admixture in this embodiment of the invention, while achieving the aforementioned green benefits, the comprehensive performance of prestressed concrete pipe piles surpasses that of traditional products, achieving a unified goal of energy saving, waste utilization, carbon reduction, quality upgrading, and cost control. This provides a solution that combines technological advancement and economic feasibility for the large-scale promotion of green and low-carbon concrete products.

[0088] Finally, it is important to emphasize that the composite admixture in this embodiment of the invention is supplied in the form of a premixed composite admixture, which is fundamentally different from the on-site concrete formulations in some existing technologies. Pipe pile manufacturers do not need to adjust their existing concrete mix proportions or modify their existing production lines; they only need to replace cement with the composite admixture in this embodiment of the invention at the recommended dosage (5%-12%) to achieve a smooth upgrade from traditional high-temperature steam curing to low-temperature steam curing. This plug-and-play approach not only ensures product quality stability and batch consistency but also significantly reduces the barriers and risks associated with applying new technologies, facilitating rapid industry adoption. In contrast, existing technologies often provide a complete on-site concrete formulation, requiring companies to comprehensively adjust raw material procurement, mix design, and production processes, resulting in high application costs and significant promotion difficulties.

[0089] This invention also provides a method for preparing the above-mentioned composite admixture, comprising the following steps:

[0090] Weigh the submicron-sized metakaolin, the ultrafine quartz sand powder, the slag powder, the calcium formate, the sulfate activator, and the water-retaining and toughening component according to the specified ratio; mix the weighed submicron-sized metakaolin, ultrafine quartz sand powder, slag powder, calcium formate, and sulfate activator in a first-stage mixing process for 10 min - 20 min to form an active powder premix; add the weighed water-retaining and toughening component to the premix and mix in a second-stage mixing process for 15 min - 30 min.

[0091] Preferably, if the composite admixture also includes an auxiliary setting regulator and a superplasticizer, the auxiliary setting regulator is 0-1.5 wt% and the superplasticizer is 3.0-4.5 wt%. The auxiliary setting regulator and superplasticizer are weighed according to this ratio, and added during the second-stage mixing process and mixed together.

[0092] This invention also provides a method for preparing concrete pipe piles, comprising the following steps: using the above-mentioned composite admixture; replacing cement with the composite admixture at a mass ratio of 5% to 12% of the total mass of cementitious materials, and adding it to the concrete mix proportion of the prestressed concrete pipe pile; molding to form a concrete pipe pile; and steam curing the molded concrete pipe pile under a constant temperature condition of 40℃ to 60℃.

[0093] When the concrete pipe pile is a PHC pile, the amount of the composite admixture is 8%-12% of the total mass of the cementitious material, replacing the cement in equal amounts; and the formed concrete pipe pile is left to stand still for 2-3 hours; then heated to 50℃-60℃ and kept at a constant temperature for 10-12 hours.

[0094] When the concrete pipe pile is a large pipe pile, the amount of the composite admixture is 5%-10% of the total mass of the cementitious material, replacing cement in equal amounts; and the formed concrete pipe pile is left to stand still for 3-5 hours; then heated to 40℃-50℃ and kept at a constant temperature for 12-16 hours.

[0095] The present invention will be described in detail through the following specific embodiments, which systematically demonstrate the formulation, preparation process, and specific application method of the composite admixture in the production of prestressed concrete pipe piles.

[0096] In Example 1, a specific formulation of the composite admixture was provided for the application of PHC piles: 15.6% submicron metakaolin (D50=1.8μm), 26.0% ultrafine quartz sand powder, 39.0% S95 grade slag powder, 3.1% calcium formate, 9.1% dihydrate gypsum, 2.0% water-retaining and toughening component, 0.8% sodium gluconate, and 3.6% polycarboxylate-based high-performance water-reducing agent powder. The water-retaining and toughening component was compounded with redispersible latex powder and HPMC at a mass ratio of 1.2:0.8.

[0097] The preparation process strictly follows the method of this invention: First, the weighed submicron-sized metakaolin, ultrafine quartz sand powder, slag powder, calcium formate, gypsum dihydrate, and sodium gluconate are dry-mixed for 15 minutes to form an active powder premix; then, water-retaining and toughening components and water-reducing agent powder are added to the premix, and a second mixing is performed for 25 minutes to obtain the finished product. This composite admixture, at a dosage of 10% of the total mass of cementitious materials, replaces cement and is applied to the production of C80 PHC piles, using the recommended low-temperature steam curing regime: after static curing for 3 hours, the temperature is raised to 55℃ and maintained at a constant temperature for 12 hours. Test results show that the resulting concrete has a demolding strength of 44.2 MPa, a 28-day compressive strength of 85.8 MPa, and an electrical flux of 410 C, fully meeting the high standard requirements.

[0098] Example 2 verifies the feasibility of using the lower limit combination of key parameters. The composite admixture formulation is as follows: 12.0% submicron metakaolin (D50=3.0μm), 22.0% ultrafine quartz sand powder, 45.0% S95 slag powder, 2.5% calcium formate, 7.0% anhydrite, 2.0% water-retaining and toughening component, no auxiliary setting regulators added, and 3.0% superplasticizer. During preparation, the first mixing stage lasts 20 minutes, followed by a second mixing stage lasts 15 minutes. The admixture is applied to C80 concrete at a dosage of 8% of the total cementitious material mass, and after a 2-hour static curing period, it is heated to 50℃ and cured for 12 hours. Tests yielded a demolding strength of 38.2 MPa, a 28-day strength of 80.5 MPa, and an electrical flux of 460 C, confirming the effectiveness of the present invention even under the lower limit combination of parameters.

[0099] For large-diameter pipe pile applications, Example 3 used a high-limit combination for verification: 18.0% submicron metakaolin (D50=1.0μm), 30.0% ultrafine quartz sand powder, 35.0% S95 slag powder, 4.0% calcium formate, 11.0% hemihydrate gypsum, 3.0% water-retaining and toughening component (latex powder and cellulose ether compounded at 2.0%:1.0%), 0.5% citric acid, and 4.5% superplasticizer. The first mixing stage was 10 minutes, and the second mixing stage was 30 minutes. C60 large-diameter pipe pile concrete was produced with a cementitious material content of 10% of the total mass. The curing regime was a 3-hour static curing followed by a 16-hour constant temperature curing at 50℃. Performance tests showed a demolding strength of 33.5 MPa, a 28-day strength of 69.2 MPa, an electrical flux of 390°C, and a 28-day drying shrinkage as low as 185×10⁻⁶. -6 It exhibits excellent crack resistance.

[0100] Example 4 further demonstrates the optimized scheme for large-diameter pipe piles: 12.5% ​​submicron metakaolin (D50=2.2μm), 24.8% ultrafine quartz sand powder, 43.0% S95 slag powder, 2.0% calcium formate, 9.8% dihydrate gypsum, 2.5% water-retaining and toughening component (latex powder and HPMC compounded at 1.5%:1.0%), no setting regulator added, and 4.2% superplasticizer. The same preparation process as in Example 1 was used. A 7% admixture was applied to C60 large-diameter pipe pile concrete. After static curing for 4 hours, the temperature was slowly increased to 45℃ and maintained at this temperature for 14 hours. The resulting concrete had a demolding strength of 31.1 MPa, a 28-day strength of 67.7 MPa, an electrical flux of 480°C, and a 28-day drying shrinkage value that was 19% lower than the control group, showing a significant improvement in crack resistance.

[0101] To highlight the advancements of this invention, Comparative Examples 1 and 2 were provided. Comparative Example 1 used conventional C80 concrete without any admixtures. Under the same curing regime as Example 1, its demolding strength was only 30.8 MPa, its 28-day strength was 65.2 MPa, and its electrical flux was as high as 870 C. Comparative Example 2, under the same conditions as Example 4, replaced the composite admixture of this invention with an equal amount of ordinary Class II fly ash. The result was a demolding strength of 30.3 MPa, a 28-day strength of 61.1 MPa, and an electrical flux of 820 C, with significantly insufficient durability and crack resistance.

[0102] As can be seen from the above embodiments, the prestressed concrete pipe piles produced according to the composite admixture formula, preparation method and application process provided by the present invention, under low temperature steam curing conditions of 40℃-60℃, have achieved breakthrough improvements in early strength, later strength, durability and crack resistance.

[0103] The key performance test results of each embodiment and the comparative example are compared in the table below:

[0104]

[0105] As can be seen from the data in the table above, when the present invention is used for PHC piles, its early strength, later strength and resistance to chloride ion penetration are significantly better than the baseline group (Comparative Example 1) without admixtures and the control group (Comparative Example 2) with only ordinary fly ash, successfully achieving high strength and high durability of concrete under low temperature steam curing process.

[0106] Example 2 verified that even with the use of low-limit combinations of composite admixture formulation, dosage, and curing temperature, the quality requirements of PHC pile C80 concrete can still be effectively met.

[0107] Example 3 verified that under the condition of using a high-limit combination of composite admixture formulation and process parameters, large pipe pile concrete exhibits better crack resistance and durability.

[0108] In Example 4, when used for large pipe piles, the concrete drying shrinkage rate was significantly reduced while ensuring the concrete strength, and the crack resistance was significantly improved.

[0109] In summary, within the range of component content defined by this invention, whether it is the lower limit, the upper limit, or a typical intermediate value combination, the prepared composite admixture can enable concrete to obtain early strength, later strength, durability, and crack resistance that meet or even exceed standard requirements under low-temperature steam curing at 40℃-60℃, which is significantly better than traditional solutions.

[0110] PHC piles and large-diameter pipe piles are two main types of prestressed concrete pipe piles. PHC piles, or prestressed high-strength concrete pipe piles, are manufactured using pre-tensioning prestressing technology and centrifugal molding. They require high-pressure steam curing (usually at 1.0 MPa) to achieve a concrete strength of C80 or higher. They are characterized by high pile strength and large single-pile bearing capacity, and are widely used in the foundations of industrial and civil buildings, railways, highways, bridges, and other engineering projects.

[0111] Large-diameter prestressed concrete pipe piles are formed using a composite process combining centrifugal and vibratory rolling. They are typically produced using a post-tensioning process, where multiple pipe sections are manufactured first and then spliced ​​and tensioned. This method can produce pipe piles of considerable length and is mainly used in projects such as ports, wharves, and large bridges where high requirements for pile diameter and bearing capacity are necessary.

[0112] HPMC is an abbreviation for hydroxypropyl methyl cellulose. It is a semi-synthetic, non-ionic cellulose ether, usually produced by modifying natural plant fibers (such as cotton and wood) through processes such as alkalization and etherification. HPMC appears as a white powder and has thickening, water-retaining, film-forming, and binding properties.

[0113] The preferred embodiments of the present invention have been described in detail above, but it should be understood that, if necessary, aspects of the embodiments can be modified to utilize aspects, features, and concepts from various patents, applications, and publications to provide other embodiments.

[0114] In light of the detailed description above, these and other changes can be made to the embodiments. Generally, the terminology used in the claims should not be considered limited to the specific embodiments disclosed in the specification and claims, but should be understood to include all possible embodiments together with the full scope of equivalents enjoyed by these claims.

[0115] Those skilled in the art will understand that the above embodiments are specific embodiments for implementing the present invention, and in practical applications, various changes in form and detail may be made without departing from the spirit and scope of the present invention.

Claims

1. A composite admixture for producing a concrete pipe pile by steam curing process, characterized by, The composite admixture includes submicron-sized metakaolin, ultrafine quartz sand powder, slag powder, calcium formate, sulfate activator, and water-retaining and toughening components. The composition of the composite admixture, by weight percentage, is as follows: The submicron-sized metakaolinite comprises 12-18 wt%, the ultrafine quartz sand powder comprises 22-30 wt%, the slag powder comprises 35-45 wt%, the calcium formate comprises 2.5-4.0 wt%, the sulfate activator comprises 7-11 wt%, and the water-retaining and toughening component comprises 2.0-4.0 wt%. The sub-micron metakaolin has a median particle size D50 of 1.0 μm - 3.0 μm, a specific surface area ≥ 1200 m 2 / kg; The water-retaining and toughening component includes redispersible latex powder and cellulose ether in a mass ratio of (2:1) to (1:1).

2. The composite admixture for producing a concrete pile by steam curing process according to claim 1, wherein The superfine quartz sand powder is a quartz material with a silicon dioxide content of greater than 90%, a specific surface area of 500 m 2 / kg - 600 m 2 / kg.

3. The composite admixture for producing a concrete pile by steam curing process according to claim 2, wherein The ultrafine quartz sand powder is at least one of natural quartzite powder, siliceous tailings powder, or foundry recycled sand powder.

4. The composite admixture for producing a concrete pile by autoclaving according to any one of claims 1 to 3, characterized in that, The slag superfine powder is S95 grade or above, and the specific surface area is 350 m 2 / kg - 400 m 2 / kg.

5. The composite admixture for producing concrete pipe piles using steam curing process according to any one of claims 1 to 3, characterized in that, The sulfate activator is at least one of gypsum dihydrate, gypsum hemihydrate, or anhydrite.

6. The composite admixture for producing concrete pipe piles using steam curing process according to claim 1, characterized in that, The composite admixture also includes auxiliary setting regulator components and superplasticizers, wherein the auxiliary setting regulator components are 0-1.5wt% and the superplasticizers are 3.0-4.5wt%.

7. The composite admixture for producing concrete pipe piles using steam curing process according to claim 6, characterized in that, The auxiliary coagulation component is at least one of sodium gluconate, citric acid, or tartaric acid.

8. A method for preparing a composite admixture as described in any one of claims 1 to 7, characterized in that, Includes the following steps: Weigh the submicron-sized metakaolin, the ultrafine quartz sand powder, the slag powder, the calcium formate, the sulfate activator, and the water-retaining and toughening components according to the specified ratio. The weighed submicron-sized metakaolin, ultrafine quartz sand powder, slag powder, calcium formate, and sulfate activator are mixed in the first stage for 10 min to 20 min to form an active powder premix. Add the weighed water-retaining and toughening components to the premix and perform a second mixing for 15 min - 30 min.

9. A method for preparing concrete pipe piles, characterized in that, Including the following steps: The composite admixture as described in any one of claims 1 to 7 is used; The composite admixture is added to the concrete mix of prestressed concrete pipe piles at a dosage of 5% to 12% of the total mass of cementitious materials to replace cement. Forming concrete pipe piles; The formed concrete pipe piles are steam cured under constant temperature conditions of 40℃~60℃.

10. The method for preparing concrete pipe piles according to claim 9, characterized in that, When the concrete pipe pile is a PHC pile, the amount of the composite admixture is 8%-12% of the total mass of the cementitious material, replacing cement in equal amounts; The formed concrete pipe piles were then left to stand for 2-3 hours. Heat to 50℃-60℃ and maintain constant temperature for 10-12 hours; Alternatively, when the concrete pipe pile is a large pipe pile, the amount of the composite admixture is 5%-10% of the total mass of the cementitious material, replacing cement in equal amounts; The formed concrete pipe piles were then left to stand for 3-5 hours. Raise the temperature to 40℃-50℃ and maintain it at a constant temperature for 12-16 hours.

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