Low-cost micro-expansion ultra-high performance concrete and method for preparing same

Abstract

This application relates to the field of building materials technology and discloses a low-cost micro-expansion ultra-high performance concrete and its preparation method. The raw materials of the concrete include water, cement, silica fume, limestone powder, fine aggregate, coarse aggregate, polycarboxylate superplasticizer, CaO-calcium sulfoaluminate-based composite expansion agent, and sodium polyacrylate-based internal curing agent. The fine aggregate is river sand, and the coarse aggregate is basalt crushed stone. During preparation, the powders are first mixed dry, then an aqueous solution of the superplasticizer is added and stirred into a uniform slurry. The aggregate is then added, stirred, and finally molded and cured. This invention utilizes the internal curing agent to desorb moisture when the internal humidity of the concrete matrix decreases, providing moisture compensation for the later hydration of the composite expansion agent, causing the dense matrix to undergo micro-expansion deformation and convert it into self-stress. This solution effectively compensates for the volume shrinkage caused by the low water-cement ratio, reducing the risk of cracking. Simultaneously, the use of conventional sand and gravel instead of pure quartz sand significantly reduces the cost of large-scale preparation.

Description

Technical Field

[0001] This invention relates to the field of building materials technology, specifically to a low-cost micro-expansion ultra-high performance concrete and its preparation method. Background Technology

[0002] Ultra-high performance concrete (UHVPC) possesses high compressive strength, high toughness, and excellent durability, making it widely used in bridge engineering, protective engineering, and the reinforcement of aging structures. However, the preparation of current UHVPC typically relies on large amounts of cement, silica fume, and other cementing materials, and uses pure silica sand as aggregate to ensure the density of the matrix. This proportioning results in high raw material costs, limiting its large-scale adoption in conventional engineering projects.

[0003] Meanwhile, ultra-high performance concrete (UHVPC) is designed with an extremely low water-cement ratio. During the hydration reaction, a large amount of cementitious material rapidly consumes free water within the matrix, leading to a significant drop in relative humidity within the capillaries and causing severe self-drying and self-shrinkage. Because UHVPC has a dense structure after hardening, conventional external surface curing moisture cannot penetrate into the structure. To compensate for this volume shrinkage, existing technologies typically incorporate an expansive agent into the concrete. However, the expansive agent needs to absorb sufficient water to undergo the later hydration reaction and generate volume expansion. In the highly water-deficient environment inside UHVPC, the expansive agent struggles to obtain the water required for the reaction, resulting in a low degree of hydration and insufficient expansion to offset the shrinkage stress of the matrix. This inability to effectively compensate for shrinkage increases the risk of microcracks within the concrete matrix, affecting the interfacial shear strength and long-term volume stability of the composite material. Summary of the Invention

[0004] To address the shortcomings of existing technologies, this invention provides a low-cost micro-expansion ultra-high performance concrete and its preparation method, which solves the problems of high raw material costs and severe internal water deficiency caused by extremely low water-cement ratio in existing ultra-high performance concrete, which prevents conventional expansion agents from being fully hydrated to effectively compensate for volume shrinkage and thus easily leads to matrix cracking.

[0005] To achieve the above objectives, the present invention provides the following technical solution: In a first aspect, the present invention provides a low-cost, micro-expansion, ultra-high-performance concrete, employing the following technical solution: A low-cost, micro-expansion, ultra-high-performance concrete is made from the following raw materials in parts by weight: 120-170 parts water; 520-720 parts cement; 120-220 parts silica fume; 150-260 parts limestone powder; 650-950 parts fine aggregate; 250-450 parts coarse aggregate; 8-18 parts polycarboxylate superplasticizer; 10-60 parts composite expansion agent; and 0.8-6.0 parts internal curing agent. The internal curing agent is a sodium polyacrylate-based superabsorbent polymer, and the composite expansion agent is a CaO-calcium sulfoaluminate-based composite expansion agent.

[0006] By adopting the above technical solution, and using sodium polyacrylate-based superabsorbent polymer as an internal curing agent, combined with CaO-calcium sulfoaluminate-based composite expansion agent, a reaction system coupling water decomposition and absorption with expansion hydration is constructed within the ultra-high performance concrete matrix. The specific reaction process and mechanism of action are as follows: Step 1, Moisture Absorption and Storage Stage: During the mixing stage of the concrete mixture, the sodium polyacrylate-based superabsorbent polymer absorbs part of the mixing water and reaches a saturated expansion state through its internal cross-linked network structure, forming a dispersed moisture storage area inside the matrix.

[0007] Step 2, Internal Relative Humidity Decrease and Water Desorption Stage: As the early hydration reaction of cement proceeds, free water inside the concrete matrix is ​​consumed, and a humidity gradient is generated by the decrease in relative humidity within the capillaries. Driven by this humidity gradient, the sodium polyacrylate-based superabsorbent polymer slowly releases the pre-absorbed water into the surrounding slurry.

[0008] Step 3, the later stage of the hydration reaction of the expanding agent: The released water supplies the CaO-calcium sulfoaluminate-based composite expanding agent inside the matrix, promoting its later hydration reaction under a water-deficient environment. The specific reaction is as follows: free calcium oxide hydrates to form calcium hydroxide crystals, resulting in volume expansion; at the same time, anhydrous calcium sulfoaluminate and calcium sulfate react with water to form ettringite crystals, resulting in volume expansion.

[0009] Step 4, Micro-expansion to Self-stress Stage: The growth of the aforementioned hydration product crystals causes micro-expansion deformation in the concrete matrix. Due to the dense internal structure of ultra-high performance concrete and the presence of externally confined boundaries, this micro-expansion deformation is converted into self-stress in the form of compressive stress. This self-stress compensates for the volume shrinkage of the matrix, thereby improving the interfacial shear strength and volume stability of the ultra-high performance concrete.

[0010] Preferably, in the raw materials: the silica fume is composed of first silica fume with a median particle size of 0.15-0.25 μm and second silica fume with a median particle size of 0.8-1.2 μm, and the mass ratio of the first silica fume to the second silica fume is 2 to 4:1; the limestone powder is composed of 400 μm limestone powder and 2000 μm limestone powder, and the mass ratio of the 400 μm limestone powder to the 2000 μm limestone powder is 1.5 to 4:1; the fine aggregate is river sand, composed of first fine aggregate with a particle size range of 0.075-1.18 mm and second fine aggregate with a particle size range of 1.18-2.36 mm; the coarse aggregate is basalt crushed stone, composed of first coarse aggregate with a particle size of 2.36-4.75 mm and second coarse aggregate with a particle size of 4.75-6.0 mm.

[0011] By employing the above technical solution, a multi-level, densely packed particle structure is formed within the material using silica fume, limestone powder, and coarse and fine aggregates with specific particle size gradients. Small-diameter particles fill the voids between large-diameter particles, reducing the initial porosity of the matrix, optimizing the density of the solid particle skeleton, and improving the mechanical properties of concrete from a physical perspective.

[0012] Preferably, the internal protective agent is prepared by the following method: At a temperature below 30°C, an aqueous solution of sodium hydroxide is slowly added dropwise to an aqueous solution of acrylic acid, controlling the neutralization degree of the acrylic acid to 70%-80%, to obtain a mixed solution; 0.05%-0.15% by mass of N,N'-methylenebisacrylamide and 0.1%-0.5% by mass of potassium persulfate are added sequentially to the mixed solution, and the mixture is stirred continuously for 20-30 minutes until completely dissolved, and nitrogen gas is introduced to remove oxygen for 15 minutes; the reaction system is transferred to an environment of 65-75°C, stirring is stopped to allow cross-linking polymerization to form a hydrogel, and the mixture is kept at this temperature for 2 hours to solidify; the hydrogel is cut into blocks, washed with anhydrous ethanol, dried at 80°C to constant weight, pulverized, and sieved to collect powder with a particle size range of 100-300 μm, thus obtaining the internal protective agent.

[0013] By employing the above technical solution and controlling specific degrees of neutralization, crosslinking agent, and initiator dosages, the network crosslinking density of sodium polyacrylate hydrogel can be adjusted. Controlling the crosslinking density determines the polymer's water absorption ratio and desorption rate, ensuring that the water release kinetics match the hydration water demand of the composite swelling agent.

[0014] Preferably, the composite expanding agent is prepared by the following method: limestone, bauxite, and natural dihydrate gypsum are used as raw materials, with the mass ratio of limestone, bauxite, and natural dihydrate gypsum being 65-70:20-25:10. The mixture is ground until the residue on an 80μm square-hole sieve is less than 5%, and then granulated with water to obtain raw material balls. The raw material balls are calcined at 1250-1300℃ for 45-60 minutes to obtain clinker. After rapid cooling, the clinker is mixed with anhydrite at a mass ratio of 100:15 and co-ground to control the specific surface area to be 350-400 m². 2 / kg, thus obtaining the composite expanding agent.

[0015] By employing the above technical solution, a specific calcination process controls the lattice development of free calcium oxide and calcium sulfoaluminate minerals in the clinker, and the co-grinding process adjusts the specific surface area of ​​the powder. This method ensures that the expansive agent maintains low hydration activity before the initial setting of concrete, while possessing the potential for expansion in the later stages of hardening.

[0016] Preferably, the raw material further includes 60-120 parts by weight of steel fiber; the steel fiber is copper-plated steel fiber, the steel fiber has a diameter of 0.2-0.22 mm and a length of 13 mm.

[0017] By adopting the above technical solution, steel fibers form a three-dimensional randomly distributed stress network inside the concrete matrix. When the matrix is ​​subjected to stress and microcracks are generated, the steel fibers bear tensile stress across the crack section, limiting the further propagation of the cracks and enhancing the tensile and shear toughness of the concrete.

[0018] Secondly, the present invention provides a method for preparing low-cost micro-expansion ultra-high performance concrete, which adopts the following technical solution: A method for preparing low-cost micro-expansion ultra-high performance concrete includes the following steps: cement, silica fume, limestone powder, composite expansion agent and internal curing agent are added to a mixer and mixed dry to obtain a uniform powder mixture; polycarboxylate superplasticizer is dissolved in water to form a mixture, and the mixture is added to the powder mixture and stirred until a uniform slurry is formed; while maintaining the stirring state, fine aggregate and coarse aggregate are added at a uniform speed and stirring is continued to obtain ultra-high performance concrete mixture; the ultra-high performance concrete mixture is poured into a mold and vibrated to remove air bubbles, and after molding, the surface is covered and cured in the mold for 24 hours at a temperature of 20±2℃ and a relative humidity of not less than 95%, and after demolding, it continues to be cured to the specified age.

[0019] By adopting the above technical solution, the powder is first dry-mixed to ensure uniform dispersion of the internal curing agent and expanding agent in the cementitious material, then an aqueous solution of water-reducing agent is added to prepare a slurry, and finally aggregates are added. This sequence avoids local agglomeration of polymer particles and ensures the uniformity of the subsequent micro-expansion stress distribution within the matrix.

[0020] Preferably, the step of adding the mixture to the powder mixture for stirring specifically includes: dividing the mixture into a first part of the mixture with a mass fraction of 70% and a second part of the mixture with a mass fraction of 30%; first adding the first part of the mixture and stirring at a low speed of 100 r / min for 2 minutes; then adding the second part of the mixture and stirring at a high speed of 200-250 r / min for 3-5 minutes.

[0021] By adopting the above technical solution, a combination of batch liquid addition and variable speed stirring is used. The initial low-speed stirring initially wets the powder, preventing dust from flying and clumping; the subsequent high-speed stirring uses shear force to break up the flocculated structure, allowing the water-reducing agent molecules to be fully adsorbed onto the surface of the cement particles, thus improving the fluidity of the slurry.

[0022] Preferably, in the step of adding cement, silica fume, limestone powder, composite expansion agent and internal curing agent to a mixer for dry mixing: the dry mixing speed is controlled at 120-150 r / min, and the dry mixing time is controlled at 3 minutes.

[0023] By adopting the above technical solution and setting specific mechanical stirring parameters, physical homogenization of multi-component powders is achieved while avoiding the agglomeration of ultrafine powders due to electrostatic effects.

[0024] Preferably, when the raw materials also include steel fibers, after the step of adding fine aggregate and coarse aggregate at a uniform speed and continuing to stir, the following steps are also included: slowly and evenly adding steel fibers, controlling the addition time of steel fibers within 1.5-2 minutes, and continuing to stir at high speed for 2-3 minutes after the addition is completed.

[0025] By adopting the above technical solution, steel fibers are added after the slurry and aggregate are mixed evenly and have a certain viscosity. The yield stress of the slurry is used to suspend the steel fibers, preventing the steel fibers from settling or clumping at the blade under their own weight.

[0026] Preferably, the step of continuing curing after demolding to the specified age is as follows: curing in a standard curing room to the specified age, or transferring to a 90°C steam curing chamber to the specified age.

[0027] By adopting the above technical solutions, two curing methods are provided: conventional standard curing and high-temperature steam curing. High-temperature conditions accelerate the reaction rate of pozzolanic ash and the hydration process of the composite expansive agent, shortening the period for concrete to reach its design strength and final volume expansion.

[0028] This invention provides a low-cost, micro-expansion, ultra-high-performance concrete and its preparation method. It has the following beneficial effects: 1. This invention combines a sodium polyacrylate-based superabsorbent polymer with a CaO-calcium sulfoaluminate-based composite expansive agent to construct a coupled system of water decomposition and absorption and expansion hydration in an ultra-high performance concrete matrix. The superabsorbent polymer absorbs water during the mixing stage of the mixture and slowly releases water when the relative humidity inside the matrix decreases in the later stage of hardening, providing water replenishment for the later hydration reaction of the composite expansive agent. After absorbing water, the composite expansive agent undergoes hydration to form crystals, causing micro-expansion deformation in the dense matrix and converting it into compressive stress. This self-stress effectively compensates for the volume self-shrinkage of the matrix under low water-cement ratio conditions, reduces the risk of structural cracking, and improves the volume stability of concrete.

[0029] 2. This invention uses silica fume and limestone powder with specific particle size gradients, combined with river sand and basalt crushed stone with particle size ranges, to form a multi-level dense particle packing system inside the material; the smaller particle size micro powder and fine aggregate physically fill the voids formed by the larger particles, reducing the matrix porosity and increasing the skeleton density; this gradation design improves the mechanical strength of concrete at the physical level, and at the same time, by using conventional river sand, crushed stone and limestone powder to replace the more expensive pure quartz sand aggregate, the cost of large-scale preparation of ultra-high performance concrete is significantly reduced.

[0030] 3. This invention adds copper-plated steel fibers to the raw materials and combines a step-by-step variable-speed stirring and delayed feeding process, so that the steel fibers can be distributed in a three-dimensional random direction in a uniform slurry with suitable viscosity, avoiding the self-weight settling or clumping of the fibers; when the concrete matrix is ​​subjected to external force and microcracks are generated, the uniformly dispersed steel fibers bear tensile stress across the crack section, limiting the further extension and expansion of cracks inside the matrix, thereby enhancing the tensile strength and shear toughness of the composite material system. Attached Figure Description

[0031] Figure 1 The bar chart shows the 28-day mechanical strength distribution of Examples 1-3; Figure 2 The graphs show the evolution of relative humidity inside the substrate in Examples 1-3 of this invention. Figure 3 These are the self-volume deformation curves from early to long-term in Examples 1-3 of the present invention; Figure 4 This is a comparison curve of the multi-component synergistic self-volume deformation of the present invention; Figure 5 This is a comparative test diagram of the workability and compactness of the present invention; Figure 6 This is a comparison chart of the overall preparation cost and efficiency ratio of the present invention; Figure 7 A load test comparison diagram is provided to illustrate the macroscopic interfacial adhesion of the present invention. Figure 8 The graph shows the slump spread of the freshly mixed slurry in Examples 1-3. Detailed Implementation

[0032] The technical solutions of the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings, preparation examples, embodiments, comparative examples, and test examples. 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] Preparation Examples 1-3: Preparation Example 1: This preparation example provides a method for preparing a sodium polyacrylate-based superabsorbent polymer (MAP) and a CaO-calcium sulfoaluminate (CaO-CA)-based composite expander (EA), including the following steps: (1) Preparation of sodium polyacrylate-based superabsorbent polymer (MPA): Under ice-water bath conditions with the temperature controlled below 30°C, sodium hydroxide aqueous solution was slowly added dropwise to acrylic acid aqueous solution, controlling the neutralization degree of acrylic acid to 70%, to obtain a mixed solution. N,N'-methylenebisacrylamide (0.05% by mass of acrylic acid monomer) and potassium persulfate (0.1% by mass of acrylic acid monomer) were added sequentially to the mixed solution, and the mixture was continuously magnetically stirred for 20 minutes until completely dissolved. Nitrogen gas was then introduced to remove oxygen for 15 minutes. The reaction system was transferred to a 65°C water bath constant temperature bath, stirring was stopped, and after the system underwent cross-linking polymerization to form a hydrogel, it was kept at this temperature for 2 hours to solidify. The resulting hydrogel was cut into small pieces, washed with anhydrous ethanol, dried at 80°C to constant weight, pulverized, and sieved to collect powder with a particle size range of 100-300 μm, which was then sealed for later use.

[0034] (2) Preparation of CaO-C-A-based composite expanding agent (EA): Limestone, bauxite, and natural dihydrate gypsum were used as raw materials in a mass ratio of 65:25:10. The mixture was then ground in a ball mill until the residue on an 80μm square-hole sieve was less than 5%. Water was added to granulate the mixture into pellets. These pellets were then fed into a high-temperature furnace and calcined at 1250℃ for 45 minutes. The calcined clinker was then rapidly cooled in air. Subsequently, the clinker and anhydrite were co-milled in a ball mill at a mass ratio of 100:15, controlling the specific surface area of ​​the finished product to be 350 m² / g. 2 / kg yields the composite expanding agent.

[0035] Preparation Example 2: This preparation example provides a method for preparing a sodium polyacrylate-based superabsorbent polymer (MAP) and a CaO-calcium sulfoaluminate (CaO-CA)-based composite expander (EA), including the following steps: (1) Preparation of sodium polyacrylate-based superabsorbent polymer (MPA): Under ice-water bath conditions with the temperature controlled below 30°C, sodium hydroxide aqueous solution was slowly added dropwise to acrylic acid aqueous solution to control the neutralization degree of acrylic acid to 75%, resulting in a mixed solution. N,N'-methylenebisacrylamide (0.10% by mass of acrylic acid monomer) and potassium persulfate (0.3% by mass of acrylic acid monomer) were added sequentially to the mixed solution, and the mixture was continuously magnetically stirred for 25 minutes until completely dissolved. Nitrogen gas was then introduced to remove oxygen for 15 minutes. The reaction system was transferred to a 70°C water bath constant temperature bath, stirring was stopped, and the system underwent a cross-linking polymerization reaction to form a hydrogel. The hydrogel was then kept at this temperature for 2 hours to solidify. The resulting hydrogel was cut into small pieces, washed with anhydrous ethanol, dried at 80°C to constant weight, pulverized, and sieved to collect powder with a particle size range of 100-300 μm, which was then sealed for later use.

[0036] (2) Preparation of CaO-C-A-based composite expanding agent (EA): (3) Raw materials were prepared using limestone, bauxite, and natural dihydrate gypsum in a mass ratio of 68:22:10. The raw materials were then fed into a ball mill and ground until the residue on an 80μm square-hole sieve was less than 5%. Water was added to granulate the raw materials into balls. The raw material balls were then fed into a high-temperature furnace and calcined at 1275℃ for 50 minutes. The calcined clinker was then rapidly cooled in the air. Subsequently, the clinker and anhydrite were co-ground in a ball mill at a mass ratio of 100:15 to control the specific surface area of ​​the finished product to be 375m². 2 / kg yields the composite expanding agent.

[0037] Preparation Example 3: This preparation example provides a method for preparing a sodium polyacrylate-based superabsorbent polymer (MAP) and a CaO-calcium sulfoaluminate (CaO-CA)-based composite expander (EA), including the following steps: (1) Preparation of sodium polyacrylate-based superabsorbent polymer (MPA): Under ice-water bath conditions with the temperature controlled below 30°C, sodium hydroxide aqueous solution was slowly added dropwise to acrylic acid aqueous solution, controlling the neutralization degree of acrylic acid to 80%, to obtain a mixed solution. N,N'-methylenebisacrylamide (0.15% by mass of acrylic acid monomer) and potassium persulfate (0.5% by mass of acrylic acid monomer) were added sequentially to the mixed solution, and the mixture was continuously magnetically stirred for 30 minutes until completely dissolved. Nitrogen gas was then introduced to remove oxygen for 15 minutes. The reaction system was transferred to a 75°C water bath constant temperature bath, stirring was stopped, and after the system underwent cross-linking polymerization to form a hydrogel, it was kept at this temperature for 2 hours to solidify. The resulting hydrogel was cut into small pieces, washed with anhydrous ethanol, dried at 80°C to constant weight, pulverized, and sieved to collect powder with a particle size range of 100-300 μm, which was then sealed for later use.

[0038] (2) Preparation of CaO-C-A-based composite expanding agent (EA): Limestone, bauxite, and natural dihydrate gypsum were used as raw materials in a mass ratio of 70:20:10. The mixture was then ground in a ball mill until the residue on an 80μm square-hole sieve was less than 5%. Water was added to granulate the mixture into pellets. These pellets were then fed into a high-temperature furnace and calcined at 1300℃ for 60 minutes. The calcined clinker was then rapidly cooled in air. Subsequently, the clinker and anhydrite were co-ground in a ball mill at a mass ratio of 100:15, controlling the specific surface area of ​​the finished product to be 400 m². 2 / kg yields the composite expanding agent.

[0039] Examples 1-3: Example 1: This embodiment provides a method for preparing low-cost micro-expansion ultra-high performance concrete, including the following steps: (1) Weigh the following raw materials accurately by weight: 120 parts water; 520 parts of cement (ordinary Portland cement, PO52.5); 120 parts of silica fume, including 80 parts of first silica fume (second F-1) with a median particle size of 0.15-0.25 μm and 40 parts of second silica fume (second F-2) with a median particle size of 0.8-1.2 μm, with a mass ratio of 2:1; 150 parts of limestone powder, including 90 parts of 400μm limestone powder and 60 parts of 2000μm limestone powder, with a mass ratio of 6:4; The fine aggregate consists of 650 parts of river sand that has been pre-washed and dried to a surface-dry state, including 367 parts of river sand (second-1) with a particle size range of 0.075-1.18 mm and 283 parts of river sand (second-2) with a particle size range of 1.18-2.36 mm. 250 parts of coarse aggregate, which is basalt crushed stone that has been pre-washed and dried to a surface dry state, including 178 parts of crushed stone (CA-1) with a particle size of 2.36-4.75mm and 72 parts of crushed stone (CA-2) with a particle size of 4.75-6.0mm. 8 parts of polycarboxylate superplasticizer; Ten parts of composite expanding agent were prepared using the composite expanding agent prepared in Preparation Example 1. 0.8 parts of the internal curing agent, using the internal curing agent prepared in Preparation Example 1 (keeping it in a dry state); No steel fibers are added.

[0040] (2) The weighed cement, first silica fume, second silica fume, 400μm limestone powder, 2000μm limestone powder, composite expansion agent and internal curing agent are put into a forced mixer in sequence and dry-mixed for 3 minutes at a speed of 120r / min to obtain a uniform powder mixture. (3) Dissolve the polycarboxylate superplasticizer completely in the weighed mixing water to form a mixture; divide the mixture into two parts with a mass fraction of 70% and 30%; first, add 70% of the mixture to the mixer and stir at a low speed of 100r / min for 2 minutes; then add the remaining 30% of the mixture and stir at a high speed of 200r / min for 3 minutes until a uniform slurry without dry powder lumps is formed. (4) Keep the mixer running, add the weighed first-size fine aggregate (second-1) and second-size fine aggregate (second-2) at a uniform speed, and continue to mix for 2 minutes; then add the weighed basalt coarse aggregate (CA-1 and CA-2), and mix at high speed for 2 minutes to make the slurry completely coat the surface of the aggregate and obtain ultra-high performance concrete mixture. (5) Pour the well mixed mixture into the mold, level it by its own weight and gently vibrate it to remove large air bubbles. After molding, immediately cover the surface with plastic film. Curing in the mold for 24 hours under standard conditions of temperature 20±2℃ and relative humidity ≥95% before demolding, and then continue curing under standard conditions until the specified age.

[0041] Example 2: This embodiment provides a method for preparing low-cost micro-expansion ultra-high performance concrete, including the following steps: (1) Weigh the following raw materials accurately by weight: 145 parts water; 620 parts of cement (ordinary Portland cement, PO52.5); 170 parts of silica fume, including 127.5 parts of first silica fume (second F-1) with a median particle size of 0.15-0.25 μm and 42.5 parts of second silica fume (second F-2) with a median particle size of 0.8-1.2 μm, with a mass ratio of 3:1; 205 parts of limestone powder, including 143.5 parts of 400μm limestone powder and 61.5 parts of 2000μm limestone powder, with a mass ratio of 7:3; 800 parts of fine aggregate, which is river sand that has been pre-washed and dried to a surface dry state, including 514 parts of river sand (second-1) with a particle size range of 0.075-1.18mm and 286 parts of river sand (second-2) with a particle size range of 1.18-2.36mm; 350 parts of coarse aggregate, which is basalt crushed stone that has been pre-washed and dried to a surface dry state, including 250 parts of crushed stone (CA-1) with a particle size of 2.36-4.75mm and 100 parts of crushed stone (CA-2) with a particle size of 4.75-6.0mm. 13 parts of polycarboxylate superplasticizer; 35 parts of composite expanding agent, using the composite expanding agent prepared in Preparation Example 2; 3.4 parts of the internal curing agent were used, and the internal curing agent prepared in Preparation Example 2 was used (keeping it in a dry state). The steel fiber consists of 60 parts, and is a straight steel fiber with a copper-plated surface, with a diameter of 0.2 mm and a length of 13 mm.

[0042] (2) The weighed cement, first silica fume, second silica fume, 400μm limestone powder, 2000μm limestone powder, composite expansion agent and internal curing agent are put into a forced mixer in sequence and dry-mixed for 3 minutes at a speed of 135r / min to obtain a uniform powder mixture. (3) Dissolve the polycarboxylate superplasticizer completely in the weighed mixing water to form a mixture; divide the mixture into two parts with a mass fraction of 70% and 30%; first, add 70% of the mixture to the mixer and stir at a low speed of 100r / min for 2 minutes; then add the remaining 30% of the mixture and stir at a high speed of 220r / min for 4 minutes until a uniform slurry without dry powder lumps is formed. (4) Keep the mixer running, add the weighed fine aggregate of the first particle size (second-1) and fine aggregate of the second particle size (second-2) at a uniform speed, and continue to stir for 2 minutes; then add the weighed basalt coarse aggregate (CA-1 and CA-2), and stir at high speed for 2 minutes to make the slurry completely coat the surface of the aggregate. (5) At the end of the mixer operation, steel fibers are slowly and evenly added by manual sieving, and the feeding time is controlled at about 1.5 minutes to avoid fiber balling. After the addition is completed, continue high-speed mixing for 2 minutes to obtain ultra-high performance concrete mixture. (6) Pour the well-mixed mixture into the mold, level it by its own weight and gently vibrate the surface with a high-frequency vibrating table to remove large air bubbles. After molding, immediately cover the surface with plastic film. Curing in the mold for 24 hours under standard conditions of temperature 20±2℃ and relative humidity not less than 95% before demolding, and then continue curing under standard conditions until the specified age.

[0043] Example 3: This embodiment provides a method for preparing low-cost micro-expansion ultra-high performance concrete, including the following steps: (1) Weigh the following raw materials accurately by weight: 170 portions of water; 720 parts of cement (ordinary Portland cement, PO52.5); 220 parts of silica fume, including 176 parts of first silica fume (second F-1) with a median particle size of 0.15-0.25 μm and 44 parts of second silica fume (second F-2) with a median particle size of 0.8-1.2 μm, with a mass ratio of 4:1; 260 parts of limestone powder, including 208 parts of 400μm limestone powder and 52 parts of 2000μm limestone powder, with a mass ratio of 8:2; The fine aggregate consists of 950 parts of river sand that has been pre-washed and dried to a surface-dry state, including 662 parts of river sand (second-1) with a particle size range of 0.075-1.18 mm and 288 parts of river sand (second-2) with a particle size range of 1.18-2.36 mm. The coarse aggregate consists of 450 parts of basalt crushed stone that has been pre-washed and dried to a surface dry state, including 337.5 parts of crushed stone (CA-1) with a particle size of 2.36-4.75mm and 112.5 parts of crushed stone (CA-2) with a particle size of 4.75-6.0mm. 18 parts of polycarboxylate superplasticizer; 60 parts of composite expanding agent, using the composite expanding agent prepared in Preparation Example 3; 6.0 parts of internal protective agent, using the internal protective agent prepared in Preparation Example 3 (keeping it in a dry state); 120 portions of steel fiber, using copper-plated end-hook type steel fiber, 0.22mm in diameter and 13mm in length.

[0044] (2) The weighed cement, first silica fume, second silica fume, 400μm limestone powder, 2000μm limestone powder, composite expansion agent and internal curing agent are put into a forced mixer in sequence and dry-mixed for 3 minutes at a speed of 150r / min to obtain a uniform powder mixture. (3) Dissolve the polycarboxylate superplasticizer completely in the weighed mixing water to form a mixture; divide the mixture into two parts with a mass fraction of 70% and 30%; first, add 70% of the mixture to the mixer and stir at a low speed of 100 r / min for 2 minutes; then add the remaining 30% of the mixture and stir at a high speed of 250 r / min for 5 minutes until the high shear force breaks the flocculation structure and forms a uniform slurry without dry powder lumps; (4) Keep the mixer running, add the weighed first-size fine aggregate (second-1) and second-size fine aggregate (second-2) at a uniform speed, and continue to stir for 2 minutes; then add the weighed multi-grade basalt coarse aggregate (CA-1 and CA-2), and stir at high speed for 2 minutes to make the slurry completely coat the surface of the coarse aggregate and form a suspended stable system. (5) At the end of the mixer operation, steel fibers are slowly and evenly added through a fiber disperser. The feeding time is controlled within 2 minutes to avoid the fibers from forming clumps. After the addition is completed, continue high-speed mixing for 3 minutes to obtain ultra-high performance concrete mixture. (6) Pour the well-mixed mixture into the mold, use its own weight to level it and use a high-frequency vibration table to gently vibrate the surface to remove large air bubbles. After molding, immediately cover the surface with plastic film. After curing in the mold for 24 hours under the standard conditions of temperature 20±2℃ and relative humidity ≥95%, demold it and then move it into a 90℃ steam curing box to accelerate hydration to the specified age.

[0045] Comparative Examples 1-5: Comparative Example 1: Compared with Example 2, the difference is that the coarse aggregate is removed (the corresponding volume is made up with fine sand and powder), and no composite expansion agent and internal curing agent are added; otherwise, they are the same.

[0046] Comparative Example 2: The difference from Example 2 is that no composite expansion agent and internal curing agent are added, but everything else is the same.

[0047] Comparative Example 3: The difference from Example 2 is that no internal curing agent is added; all other aspects are the same.

[0048] Comparative Example 4: The difference from Example 2 is that no composite expanding agent is added, but everything else is the same.

[0049] Comparative Example 5: Compared with Example 2, the difference is that only the first silica fume with a median particle size of 0.15-0.25μm (170 parts) was used, and only 400μm limestone powder (205 parts) was used. That is, single-size powder was used instead of multi-stage composite ratio. All other aspects are the same.

[0050] Test Examples 1-7: Test Example 1: This test case mainly tests the basic workability (slump spread) and mechanical properties (28-day compressive strength and flexural strength) of the low-cost micro-expansion ultra-high performance concrete prepared in Examples 1-3.

[0051] The experimental testing process is as follows: 1. Take fresh mixtures from each embodiment after mixing and conduct flowability tests using a standard slump cone with an inner diameter of 100mm (top) and 200mm (bottom) and a height of 300mm. The mixture is placed into the cone in two layers. After each layer is tamped and the surface is leveled, the slump cone is lifted vertically and steadily upwards. Once the mixture stops flowing, the maximum diameter of the spread surface and the diameter perpendicular to it are measured using a steel ruler. The arithmetic mean of the two measurements is taken as the slump spread of that mixture.

[0052] 2. After the flowability test, the remaining mixture was poured into 100mm×100mm×100mm cube molds and 100mm×100mm×400mm prism molds, respectively. Due to the self-leveling properties of the mixture, no mechanical vibration was performed during molding; only slight tapping was applied to the outside of the molds to remove surface air bubbles. The surface of the specimens was then smoothed with a scraper and covered with a plastic film.

[0053] 3. Demolding is performed 24 hours after molding. After demolding, the specimens of Example 1 and Example 2 are placed in a standard curing room with a temperature of 20±2℃ and a relative humidity of not less than 95% for curing for 28 days; the specimens of Example 3 are placed in a 90℃ steam curing device for accelerated hydration treatment, and then placed in a standard environment for standing for 28 days.

[0054] 4. After reaching the specified 28-day curing period, remove the specimens and wipe off surface moisture. Mechanical property tests are then conducted using a computer-controlled electro-hydraulic servo universal testing machine. First, a four-point flexural strength test is performed on a 100mm×100mm×400mm specimen, with the loading rate controlled between 0.05-0.08 MPa / second. The ultimate load at fracture is recorded, and the flexural strength is calculated. Subsequently, a 100mm×100mm×100mm cubic specimen is used for compressive strength testing, with the loading rate controlled at 1.2 MPa / second. The failure load is recorded, and the compressive strength is calculated. Three parallel specimens are prepared for each test group, and the arithmetic mean is taken as the final evaluation data.

[0055] Table 1. Test data on workability and mechanical properties of ultra-high performance concrete foundations in Examples 1-3 Based on the data in Table 1, Figure 1 and Figure 8 A comprehensive analysis was conducted, demonstrating the evolution of workability and hardening mechanical properties of ultra-high performance concrete under different parameter ratios (Examples 1 to 3).

[0056] Figure 8 The graph shows the slump spread of the freshly mixed slurry from Examples 1 to 3, with the vertical axis corresponding to the flowability values. This sub-graph, combined with data from Table 1, indicates that under the conditions of an extremely low water-cement ratio and the introduction of basalt coarse aggregate, the slump spread of Examples 1 to 3 ranges from 554 mm to 612 mm. Although the system spread decreases slightly as the parameters approach their limits, it remains stable within the self-flowing vibratory compaction range above 550 mm, without bleeding or segregation. The data changes indicate that the composite multi-stage powder system formed by silica fume with different median particle sizes and limestone powder with different fineness plays a role in physical filling and morphological effects within the slurry. This powder packing system effectively replaces the free water in the gaps between coarse and fine aggregates, reducing the yield stress between particles and thus offsetting the increase in frictional resistance brought about by the introduction of coarse aggregate and steel fiber. In Example 3, the steel fiber content reaches its upper limit, resulting in a slight decrease in spread, but it remains within the self-compacting range without vibration.

[0057] Figure 1 The figure shows the 28-day mechanical strength distribution of Examples 1 to 3. Dark gray bars represent 28-day compressive strength, and light gray bars represent 28-day flexural strength. This sub-figure clearly shows that with the increase of the micro-expansion system components and steel fiber content, both the compressive and flexural strengths of the matrix exhibit a stable upward trend, with the 28-day compressive strength exceeding 120 MPa, and the highest compressive strength exceeding 170 MPa (reaching 173.5 MPa). The mechanical strength data reflects the density evolution of the multi-component system during the hardening stage. In Example 1, under the condition of plain concrete without added steel fibers, the compressive strength reached 124.6 MPa, confirming that the rigid skeleton formed by coarse aggregate and multi-grade river sand, encapsulated in a low water-cement ratio paste, can minimize macroscopic defects. In Examples 2 and 3, due to the addition of straight and hooked steel fibers, the flexural strength jumped from 14.3 MPa to 34.6 MPa, demonstrating the crack-resistant mechanism in macroscopic stress.

[0058] The two figures together demonstrate that the present invention can achieve ultra-high strength while maintaining excellent construction rheological properties. Furthermore, the incorporation of the composite expansive agent and internal curing agent did not degrade the final mechanical properties of the matrix. The sodium polyacrylate-based superabsorbent polymer releases internally stored water during the later stages of hydration, promoting the continuous reaction between unhydrated cement particles and the composite expansive agent clinker. The resulting calcium sulfoaluminate gel and ettringite crystals fill the micropores left after the polymer releases water, further densifying the internal structure and supporting the stable achievement of ultra-high strength exceeding 150 MPa.

[0059] Test Example 2: This test case mainly focuses on the evolution of the internal relative humidity (IRH) of the micro-expansion ultra-high performance concrete prepared in Examples 1-3 to verify the effectiveness of the water release mechanism of sodium polyacrylate-based superabsorbent polymer (sAP).

[0060] The experimental testing process is as follows: 1. Select a PVC cylindrical mold with an inner diameter of 100mm and a height of 200mm. Make a 12mm diameter hole on the side of the mold at the middle of its height. Use a HT31 digital temperature and humidity sensor as the measuring element. Tightly wrap a Gore-Tex polytetrafluoroethylene waterproof and breathable membrane around the sensor probe to prevent liquid water from contacting the sensor probe while allowing water vapor to pass freely.

[0061] 2. Fill the PVC mold with the freshly mixed material in two batches, vibrating it on a vibrating table for 15 seconds after each filling to remove air bubbles. When the mixture has filled half the height of the mold, insert the prepared temperature and humidity sensor probe horizontally through the side opening into the center of the mold, and then continue filling the remaining mixture to the top of the mold.

[0062] 3. Use epoxy resin adhesive to tightly seal the gap between the sensor wires and the opening on the side of the PVC mold. Cover the surface of the specimen with a double layer of polyethylene film, and completely seal the top edge of the mold with aluminum foil tape to ensure that the specimen is in an absolutely dry state with no moisture exchange with the outside world, simulating the large-volume environment inside concrete.

[0063] 4. Place the packaged specimen in a constant temperature chamber at 20±1℃. Connect the sensor wires to a multi-channel data acquisition instrument, set the data acquisition frequency to record once per hour, and continuously monitor and record the internal relative humidity data from 0 to 28 days of age.

[0064] Table 2. Relative humidity test data inside the substrate in Examples 1-3 Combining the data in Table 2 and Figure 2 A comprehensive analysis was conducted on the relative humidity evolution curves inside the matrix in Examples 1-3. Figure 2 The decay and maintenance patterns of internal relative humidity (IRH) of ultra-high performance concrete with different mix proportions under dry curing conditions were recorded from 0 to 28 days after molding. The horizontal axis represents the test age, and the vertical axis represents the percentage of internal relative humidity of the matrix collected in real time by pre-embedded sensors. The three evolution curves in the figure are distinguished by different gray levels and marking symbols. The solid black line with a square mark represents Example 1, the dark gray dashed line with a circle mark represents Example 2, and the light gray dotted line with an upper triangle mark represents Example 3.

[0065] According to Table 2 and Figure 2 The curves show the changes in relative humidity. In Examples 1 to 3, the internal relative humidity was above 99.6% on the first day after molding. As the hydration reaction proceeded, chemical shrinkage within the ultra-low water-to-binder ratio system led to the consumption of water within the capillaries, resulting in a gradual decrease in relative humidity. The curves visually demonstrate the water release and energy retention effect of sodium polyacrylate-based superabsorbent polymer (MAP) in ultra-low water-to-binder ratio matrices. With the increase of MAP content, the rate of decrease in internal relative humidity of the matrix slowed significantly in the later stages. The internal relative humidity of Examples 1, 2, and 3 remained at 81.1%, 87.9%, and 92.3% at 28 days, respectively.

[0066] The changes in the data and curves reflect the moisture regulation mechanism of sodium polyacrylate-based superabsorbent polymers within the matrix. During the initial mixing stage of the concrete mixture, the sodium polyacrylate-based superabsorbent polymer absorbs some of the mixing water to form a hydrogel. During the setting and hardening stage, as the cement undergoes hydration and pozzolanic reactions with the multi-grade silica fume and limestone powder, the free water in the capillary pores decreases, and the relative humidity of the pore fluid drops. At this point, a humidity gradient and osmotic pressure difference are formed between the polymer gel and the surrounding capillary pores, driving the polymer to slowly release the stored water into the surrounding slurry.

[0067] In the examples, the continuous release of moisture compensated for the water loss in the capillary pores, reducing the capillary tension at the gas-liquid interface. Example 3, containing a high dosage of superabsorbent polymer, maintained a relative humidity of over 92% after 28 days, indicating that the internal water supply effect increased with increasing dosage, providing a reliable internal moisture guarantee for the subsequent hydration and micro-expansion compensation mechanism of the composite expanding agent. Under extremely low water-cement ratios and without external moisture replenishment, ordinary expanding agents would cease reaction due to water shortage. However, in this scheme, the moisture released by the polymer promotes the continuous formation of ettringite crystals from calcium sulfoaluminate clinker and anhydrite, and promotes the formation of calcium hydroxide crystals from calcium oxide. The crystallization pressure generated by crystal growth acts on the solid framework, providing a driving force for the micro-expansion compensation of macroscopic volume. The relative humidity evolution curve and test data jointly confirm the existence of the water storage and release process of the internal curing component, satisfying the moisture conditions required for the micro-expansion reaction.

[0068] Test Example 3: This test case mainly examines the volumetric deformation characteristics of the low-cost micro-expansion ultra-high performance concrete prepared in Examples 1-3 from early age to long-term (from initial setting to 56 days) to verify the feasibility and stability of the micro-expansion properties of the matrix.

[0069] The experimental testing process is as follows: 1. A 400mm long, 40mm outer diameter polyethylene corrugated pipe was cut as a molding test mold. One end of the corrugated pipe was sealed tightly with a special polymer sealing plug to ensure no liquid leakage. The freshly mixed mixture from each embodiment was slowly and uniformly poured into the other end of the corrugated pipe. During the pouring process, the corrugated pipe was placed on a micro-vibration table for low-frequency vibration to remove air bubbles trapped inside the pipe until the mixture filled the entire corrugated pipe.

[0070] 2. Insert a measuring probe with a stainless steel reflective target into the unsealed end of the corrugated pipe, embedding the probe tip approximately 20mm into the mixture. Then, use flexible sealant to completely seal the gap between the pipe end and the probe to prevent internal moisture evaporation.

[0071] 3. Place the encapsulated bellows horizontally on the V-shaped bracket of the bellows volume deformation tester. The bracket is located in a constant temperature chamber with the temperature controlled at 20±1℃. Adjust the eddy current non-contact displacement sensors at both ends of the bracket so that their probes are facing the stainless steel reflective target, and adjust the initial distance to the center position of the sensor's linear measurement range.

[0072] 4. The initial setting time of each mixture was determined using the penetration resistance method. The moment when the mixture reached the initial setting state was taken as the zero point (initial zero reference) for deformation testing. The data acquisition system was started, and the data recording frequency was set to once every 30 minutes for the first 7 days, and once every 4 hours thereafter, continuously monitoring until 56 days of age. The test results were converted into microstrain (με), with positive values ​​representing volume expansion and negative values ​​representing volume contraction.

[0073] Table 3. Self-volume deformation test data of Examples 1-3 from initial setting to 56 days. Combining the data in Table 3 and Figure 3 A comprehensive analysis was conducted on the self-volume deformation curves from early to long-term stages of Examples 1-3. Figure 3 The self-volume evolution of ultra-high performance concrete with different mix proportions under constant temperature and closed conditions was recorded from the initial setting state (test zero point) to a long-term age of 56 days. The horizontal axis represents the test age (days), and the vertical axis represents the micro-strain values ​​(με) collected in real time through a corrugated pipe and a non-contact displacement sensor. The horizontal thick black line in the figure is the zero-scale baseline. The positive value area above this line represents macroscopic volume expansion, and the negative value area below this line represents macroscopic volume contraction. The three evolution curves in the figure are distinguished by different gray levels and marking symbols: the solid black line with a square mark represents Example 1, the dark gray dashed line with a circle mark represents Example 2, and the light gray dotted line with an upper triangle mark represents Example 3.

[0074] According to Table 3 and Figure 3The curve evolution of Examples 1 to 3 showed positive volumetric deformation values ​​throughout the entire test period from initial setting to 56 days. All three curves remained stable in the positive range, with no negative shrinkage strain observed. At 1 day, the early expansion of each example ranged from +12.4 με to +42.1 με. The curve slopes indicate that matrix expansion was mainly concentrated in the first 14 days, after which the expansion rate gradually slowed down. By 28 days, the expansion had increased to +65.4 με, +158.2 με, and +278.4 με, respectively. After 28 days, the volumetric deformation essentially reached a stable state, with the final expansion increasing in a gradient with the increase in the dosage of the micro-expansion system (internal curing agent and composite expansion agent). At 56 days, the deformation data tended to stabilize, with Example 1 showing a slight decrease to +62.8 με, while the final micro-strain of Example 3 stabilized at +281.2 με (approximately +280 με).

[0075] Traditional ultra-high performance concrete contains a large amount of unhydrated cement particles and silica fume, which leads to a rapid consumption of internal pore fluid, and usually produces severe autogenous shrinkage of up to -500με to -800με. Figure 3 This intuitively reflects the micro-expansion characteristics of the present invention, completely eliminating the technical defect of rapid early-age shrinkage in traditional low water-cement ratio concrete. Test data proves that this solution, through the coupling of the internal curing agent and the composite expansion agent, reverses the physical evolution path of matrix shrinkage. The early positive expansion mainly originates from the hydration reaction of the calcium oxide component. After calcium oxide comes into contact with water, it rapidly generates calcium hydroxide crystals, increasing the solid phase volume and preferentially compensating for the plastic shrinkage and early chemical shrinkage during the transition of the mixture from a fluid to a hardened state.

[0076] As the curing time increases (3 to 14 days), the internal chemical shrinkage of the system intensifies, but the expansion strain continues to rise. This is attributed to the synergistic effect of the water release mechanism of the sodium polyacrylate-based superabsorbent polymer and the hydration of calcium sulfoaluminate clinker. The water released by the polymer into the capillaries activates the reaction between calcium sulfoaluminate and anhydrite, generating a large number of needle-like ettringite crystals. These crystals grow in closed micropores, generating crystallization pressure due to spatial confinement. Because of the rigid framework constructed from basalt coarse aggregate and steel fibers within the matrix, the crystallization pressure cannot cause disordered expansion and cracking of the matrix, but is effectively constrained by the framework and transformed into internal chemical prestress. This self-stress manifests outward as macroscopically controlled micro-expansion.

[0077] During the long-term curing period of 28 to 56 days, the volumetric deformation rate significantly decreased and reached a stable state. This stabilization trend indicates that the hydrated clinker of the expanding agent has been largely consumed, and the release of internal moisture from the polymer has reached thermodynamic equilibrium. The test data fully confirms that the designed calcium oxide-calcium sulfoaluminate composite expanding source can achieve effective hydration in an environment with an extremely low water-to-binder ratio, maintain the volumetric stability of the system over a wide curing period, effectively prevent cracking, and compensate for interfacial gaps through micro-expansion.

[0078] Test Example 4: This test case mainly conducts a comparative test on the multi-component synergistic volume stability of the prepared ultra-high performance concrete. By disassembling the core functional components, the synergistic mechanism of sodium polyacrylate-based superabsorbent polymer and composite expansion agent in an extremely low water-cement ratio system is verified.

[0079] The experimental testing process is as follows: 1. Five groups of mixtures were selected for comparative experiments. Comparative Examples 1 and 2 were high-shrinkage baseline groups without superabsorbent polymers and expansion agents; Comparative Example 3 was a group with a single composite expansion agent; Comparative Example 4 was a group with a single sodium polyacrylate-based superabsorbent polymer; and Example 2 was an experimental group containing a complete synergistic system of superabsorbent polymers and composite expansion agents.

[0080] 2. Prepare a steel prism mold with internal dimensions of 100mm×100mm×515mm. Lay a 0.1mm thick polytetrafluoroethylene film flat on the inner wall of the mold to eliminate the constraint of contact surface friction on the deformation of the specimen. Pre-embed and fix stainless steel reflective targets with threaded interfaces at the center positions of both ends of the mold.

[0081] 3. Pour the five fresh mixtures into the molds and compact them on a vibrating table. Apply a high-polymer waterproof sealant to the gaps around the molds and tightly cover the top surface of the specimens with double-layer aluminum foil tape to cut off the exchange channels between the internal moisture of the specimens and the external environment.

[0082] 4. Move the packaged specimen into a constant temperature testing chamber maintained at 20±1℃. Install dual-head high-precision laser displacement sensors at both ends of the mold and adjust the laser beam to vertically irradiate the center of the pre-embedded stainless steel reflective target.

[0083] 5. The moment when each mixture reaches its final setting state is taken as the initial zero point for deformation testing. Connect a multi-channel data acquisition system to continuously record the length change data from 0 to 28 days. Divide the displacement change by the effective gauge length of the specimen to convert it into self-volume micro-strain (με). Positive values ​​are counted as expansion, and negative values ​​as contraction.

[0084] Table 4. Comparative test data of multi-component synergistic self-volume deformation Combination Figure 4 The evolution of autogenous shrinkage and micro-expansion of five groups of ultra-high performance concrete with different mix proportions under dry conditions is shown. The X-axis represents the test age (days), and the Y-axis represents the volumetric deformation (με). The five curves in the figure correspond to Comparative Examples 1 and 2 without functional components, Comparative Example 3 with only expansion agent, Comparative Example 4 with only superabsorbent polymer, and Example 2 with synergistic effects of all components.

[0085] According to the data in Table 4, Comparative Example 1 and Comparative Example 2 exhibited dramatic volume shrinkage of -715.4 με and -685.2 με, respectively, at 28 days of age. The extremely low water-cement ratio system contained a large amount of unhydrated cementitious material, and as the hydration reaction progressed, the free water within the pores was rapidly consumed. The capillary tension generated at the gas-liquid interface acted on the solid framework, leading to a dramatic shrinkage in macroscopic volume.

[0086] Comparative Example 3 showed a slight expansion of +21.5 με at 1 day, but this turned negative after 3 days, and by 28 days there was almost no expansion or even contraction to -418.2 με. The CaO-calcium sulfoaluminate-based composite expansion agent underwent partial hydration in the initial stage of contact with mixing water, generating a small amount of calcium hydroxide and ettringite, which provided early expansion. As the internal moisture of the matrix became scarce, the hydration reaction of the expansion agent stopped, and due to the lack of water, it became dormant and could no longer generate crystallization pressure. The system was once again dominated by chemical shrinkage and self-drying shrinkage.

[0087] Comparative Example 4 showed a shrinkage of -227.6 με after 28 days. The sodium polyacrylate-based superabsorbent polymer formed a water-rich gel within the system, releasing moisture to the surrounding area as capillary humidity decreased. This internal conditioning effect mitigated the rate of pore fluid consumption, reduced capillary tension, and decreased the magnitude of self-drying shrinkage. However, the superabsorbent polymer only provides physical moisture regulation, reducing shrinkage but not spontaneously generating the chemical internal stress that would cause positive volume expansion.

[0088] Only Example 2 maintained continuous positive expansion throughout the entire 28-day test period, ultimately achieving a stable micro-expansion of +278.4 με. The sodium polyacrylate-based superabsorbent polymer and the composite expanding agent formed a synergistic closed loop of water release and hydration in an extremely low water-cement ratio matrix. The superabsorbent polymer, acting as an internal water source distributed within the matrix, overcame the physical limitations imposed by the low water-cement ratio on the degree of hydration of the expanding agent. The slowly released water from the polymer precisely matched the later reaction requirements of the calcium sulfoaluminate clinker and anhydrite, promoting the continuous growth of ettringite crystals within the closed pores. The rigid skeleton interwoven with coarse aggregate and steel fibers constrained and transmitted the local expansion stress generated by crystal growth, transforming microscopic crystallization pressure into macroscopic controlled micro-expansion. Experimental data verified that the multi-component synergistic mechanism can fundamentally alter the volume evolution path of ultra-high performance concrete, solving the problem of expansion performance failure caused by internal moisture deficiency.

[0089] Test Example 5: This test case mainly compares the workability and micro-compactness of the prepared ultra-high performance concrete. By changing the particle size distribution of the matrix powder material, it verifies the necessity of multi-stage powder gradation design for rheological properties and microstructure after curing in an ultra-low water-binder ratio system.

[0090] The experimental testing process is as follows: 1. Comparative experiments were conducted on ultra-high performance concrete mix proportions with different powder gradations. Two sets of mixtures were set up: Comparative Example 5 (using conventional microsilica with a single particle size distribution, lacking multi-grade powder proportions) and Example 2 (using a continuous multi-grade powder gradation covering cement, multi-grade microsilica, and ultrafine limestone powder). The mixtures were prepared by mixing under the condition that the water-cement ratio, aggregate dosage, and polycarboxylate superplasticizer dosage were kept completely consistent.

[0091] 2. Take freshly mixed materials from each group and conduct flow performance tests using a slump cone on a horizontal plate according to relevant standards. Accurately record the time required for the mixture to expand to a diameter of 500 mm from the top of the slump cone using a stopwatch, i.e., the T50 flow time (seconds). Subsequently, place the fresh mixture into an inverted slump cone and record the time required from opening the bottom opening to completely emptying the mixture, i.e., the inverted slump cone emptying time, to evaluate the cohesiveness and rheological properties of the material.

[0092] 3. Pour each group of freshly mixed materials into 100mm×100mm×100mm cubic molds and standard-volume cylindrical molds, respectively, and vibrate and degas on a micro-vibrating table. After demolding, transfer them to a standard curing room with a temperature of 20±2℃ and a relative humidity of not less than 95% for 28 days. Weigh and measure the volume of the hardened specimens, and calculate the apparent density (kg / m³) at 28 days. 3 ( ), to characterize the macroscopic packing density of the matrix.

[0093] Table 5. Comparison Test Data of Workability and Density Combining the data in Table 5 and Figure 5 A comprehensive analysis was conducted. Figure 5 This study comprehensively demonstrates the evolution of the rheological workability and macroscopic packing density of ultra-high performance concrete under different powder gradation systems (Comparative Example 5 and Example 2).

[0094] Based on the data in Table 5 and Figure 5 (a) The flowability test results comparison chart shows that this sub-chart is a grouped bar chart, which intuitively reflects the macroscopic flowability of the material under extremely low water-cement ratio conditions. In the chart, the dark gray bars represent the T50 flow time (seconds), and the light gray bars represent the inverted slump tube emptying time (seconds). Comparative Example 5 has a T50 flow time as long as 16.8 seconds and an inverted slump tube emptying time as high as 24.5 seconds, indicating extremely poor macroscopic flowability. This is because the single-particle-size microsilica system lacking multi-stage powder gradation has a large number of interparticle gaps, and the mixing water is physically encapsulated by the powder to form a flocculated structure, resulting in a decrease in free water content and excessive slurry cohesion. In contrast, Example 2, which uses continuous multi-stage powder gradation, has a significantly shortened T50 flow time to 3.5 seconds and an inverted slump tube emptying time of only 4.8 seconds. The sharp decrease in these two indicators reflects that the continuous particle size distribution sequence constructed by multi-stage powder produces a significant ball effect and water replacement mechanism, which significantly improves the self-leveling construction performance of the material under extremely low water-cement ratio conditions.

[0095] Based on the data in Table 5 and Figure 5 (b) The 28-day apparent density comparison chart shows that this sub-chart is a monolithic columnar graph, with the gray columns reflecting the macroscopic packing density of the matrix after hardening. The 28-day apparent density of Comparative Example 5 is only 2315 kg / m³. 3 This indicates that the single-particle-size system contains a large number of flocculated pores due to incomplete filling, resulting in an insufficiently dense matrix. In contrast, the density of Example 2 increased significantly to 2486 kg / m³. 3 This data evolution proves that the broadband gradation design of the present invention, which covers cement, multi-grade microsilica powder and ultrafine limestone powder, follows a close packing model. Solid particles of different sizes are nested together, which can effectively eliminate excess air in the system and fill the gaps between particles, achieving an absolute advantage in density.

[0096] In summary, the experimental results fully verify the necessity of multi-stage powder gradation design for ensuring the excellent workability and macroscopic structural homogeneity of ultra-high performance concrete at extremely low water-cement ratios.

[0097] Test Example 6: This test case compares the overall preparation cost and efficiency of the prepared ultra-high performance concrete. By quantitatively calculating the ratio of direct material cost to mechanical and deformation efficiency, it verifies the cost reduction and economic benefits of the full-gradation system while ensuring volume stability.

[0098] The experimental testing process is as follows: 1. Three ultra-high performance concretes with different mix proportions were selected for a comprehensive cost and performance evaluation. Comparative Example 1 (traditional UHPC without coarse aggregate, using pure Portland cement, pure quartz sand and high dosage of conventional microsilica), Comparative Example 2 (conventional cost-reducing matrix using ordinary river sand to replace part of the quartz sand), and Example 2 (using a low-cost composite matrix including basalt crushed stone, ultrafine limestone powder, multi-grade microsilica and micro-expansion system).

[0099] 2. Based on the benchmark prices for bulk purchases of industrial building materials in the local market, calculate the unit price of each component in the above three mix proportions. Based on the actual designed usage of each raw material per cubic meter of fresh mixture, calculate the unit cost of raw materials (yuan / m³). 3 ).

[0100] 3. The three mixtures were cast into standard dimensions and cured in an environment with a temperature controlled at 20±2℃ and a relative humidity above 95%. The standard axial compressive strength of each group of specimens at 28 days and the micro-strain value of self-volume deformation under 28 days of moisture-free conditions were tested as physical input parameters for performance evaluation.

[0101] 4. Establish a unit strength cost accounting model, dividing the unit raw material cost by the 28-day axial compressive strength to obtain the unit compressive strength cost (yuan / MPa). Combining mechanical properties, volumetric deformation, and economic indicators, a comprehensive performance evaluation index is calculated using a normalized weighted algorithm, and parallel comparative analysis is conducted.

[0102] Table 6. Comprehensive preparation cost and efficiency ratio calculation data Combining the data in Table 6 and Figure 6 A comprehensive analysis was conducted. Figure 6 Three independent sub-graphs visually illustrate the differences in absolute capital investment and relative performance conversion between three different mix proportion systems of ultra-high performance concrete (Comparative Example 1, Comparative Example 2, and Example 2). The horizontal axis of the charts uniformly corresponds to the three different concrete mix proportion systems, and all graphs are drawn using grayscale bar charts.

[0103] Based on the data in Table 6 and Figure 6 (a) The comparison chart of raw material costs per cubic meter shows that the vertical axis of this sub-chart represents the total cost of direct raw materials required per cubic meter of concrete (yuan / m³). 3As can be seen from the evolution trend of the dark gray columns, the cost of the traditional UHPC without coarse aggregate (Comparative Example 1) is as high as 3824.5 yuan / m². 3 Traditional mix design heavily relies on silica sand, high-grade cement, and high levels of silica fume, resulting in extremely high direct material costs and significant self-drying shrinkage. In contrast, Example 2, by introducing ultrafine limestone powder and basalt crushed stone to replace expensive raw materials, significantly reduces the absolute unit cost to 1946.3 yuan / m³. 3 This resulted in a significant reduction in unilateral costs, with costs reduced by nearly half.

[0104] Based on the data in Table 6 and Figure 6 (b) The unit compressive strength cost comparison chart shows that the vertical axis of this sub-chart represents the cost (yuan / MPa) consumed in producing each unit of compressive strength, eliminating the interference of false low cost caused by material strength degradation. The gray bars show that although the conventional cost reduction method in Comparative Example 2 reduced the total unit cost to 2915.8 yuan / m³, the cost per unit was significantly lower. 3 However, due to the significant degradation of mechanical properties, its unit compressive strength cost remains at a relatively high level (21.66 yuan / MPa, only slightly lower than 25.29 yuan / MPa in Comparative Example 1); while Example 2 improved the strength by utilizing the compaction effect of multi-stage micro-aggregates, significantly reducing its unit compressive strength cost to 11.55 yuan / MPa, highlighting the extremely high utilization rate of funds.

[0105] Based on the data in Table 6 and Figure 6 (c) As can be seen from the comparison chart of comprehensive performance evaluation indices, the vertical axis of this sub-chart represents the dimensionless comprehensive index calculated by combining mechanical properties, volumetric deformation and economic indicators in a normalized weighted manner. The light gray bars visually indicate that the comprehensive index of Example 2 jumps to 0.95, which is much higher than that of Comparative Example 1 (0.58) and Comparative Example 2 (0.64).

[0106] In summary, pure data analysis and chart trends fully demonstrate that the full-gradation and micro-expansion system of Example 2, while maintaining equal or even better volume stability and strength (i.e., ensuring or even optimizing various core performances), establishes significant economic advantages and highlights the core advantage of low cost of this system.

[0107] Test Example 7: This test case conducts a macroscopic interfacial bond force push-out load test on the prepared ultra-high performance concrete. By simulating the stress state of the steel-concrete composite member, it verifies the advantage of the micro-expansion characteristics in actual closed and confined structures in transforming into self-stress and improving the interfacial collaborative working ability.

[0108] The experimental testing process is as follows: 1. Two sets of ultra-high performance concrete mix proportions were selected for comparative experiments. Comparative Example 1 (traditional high-shrinkage matrix without the addition of superabsorbent polymer and composite expansion agent) and Example 2 (matrix containing a complete micro-expansion system) were set up. Standard seamless steel pipes with an outer diameter of 114 mm, a wall thickness of 5 mm, and a height of 300 mm were prepared. The inner wall of the steel pipe was cleaned with a wire brush and anhydrous ethanol to remove floating rust and oil stains, maintaining the natural roughness of the inner wall.

[0109] 2. Vertically fix the steel pipe, sealed at the bottom with a thick steel plate, onto the pedestal. Pour two sets of freshly mixed materials continuously into the standard steel pipe, using an immersion micro-vibrator to assist in venting and ensure compaction. After pouring to the level of the top surface of the steel pipe, seal the exposed concrete at the top with high-molecular epoxy resin to simulate the moisture-free environment inside the concrete pipe in actual engineering.

[0110] 3. Place the sealed assembly specimen in an indoor environment at a temperature of 20±2℃ and allow it to cure for 28 days. After hardening, remove the top sealing layer and use a flat grinder to grind the protruding concrete at both ends until it is flush with the end face of the steel pipe.

[0111] 4. Transfer the specimen to a computer-controlled electro-hydraulic servo universal testing machine for the press-out test. Place a hollow bearing steel pad with an inner diameter of 108 mm at the bottom of the specimen, so that the steel pipe wall is supported on the pad while the internal concrete core sample is suspended. Place a solid loading steel block with a diameter of 100 mm on top of the specimen to ensure that the load of the testing machine head is applied only to the surface of the internal concrete core sample.

[0112] 5. Continuously apply axial pressure at a controlled loading rate of 0.5 mm per minute, and record the slippage of the concrete core sample relative to the steel pipe using a displacement sensor. Record the ultimate debonding load (kN) and the interfacial shear strength (MPa).

[0113] Table 7. Macroscopic Interfacial Bond Strength Pull-out Load Test Data Combining the data in Table 7 and Figure 7 A comprehensive analysis was conducted. Figure 7 The paper comprehensively presents the mechanical synergistic working ability of ultra-high performance concrete inside steel pipe confined components. The horizontal axis corresponds to two sets of different mix proportion systems. The graph is drawn using grayscale bar charts for intuitive distinction (dark gray is the comparative example, and light gray is the example).

[0114] Based on the data in Table 7 and Figure 7(a) The comparison diagram of self-volume deformation shows that this subplot displays the volumetric micro-strain of the material under arid conditions over 28 days. The downward-extending dark gray column directly exposes the severe self-volume shrinkage of the conventional matrix (Comparative Example 1) with a micro-strain as high as -586.2; while the upward-extending light gray column indicates that Example 2 achieved controlled micro-expansion with a micro-strain of +234.6, laying the physical foundation for the subsequent establishment of self-stress.

[0115] Based on the data in Table 7 and Figure 7 (b) The comparison diagram of ultimate debonding loads shows the ultimate load (kN) of the press when the core sample undergoes overall slippage. Due to the interfacial gap caused by shrinkage, Comparative Example 1 lost radial contact stress, and its ultimate debonding load was only 142.73 kN, resulting in overall slippage failure in the initial stage of stress. In contrast, Example 2 achieved a tight clamping effect through the micro-expansion self-stress mechanism, fundamentally eliminating the physical separation gap at the interface caused by shrinkage, and its ultimate debonding load jumped significantly to 693.46 kN.

[0116] Based on the data in Table 7 and Figure 7 (c) The comparison diagram of interfacial shear strength shows that this sub-diagram reflects the shear resistance (MPa) after normalizing the load area. The evolution of the column height confirms the strong radial constraint exerted by the external high-rigidity steel pipe wall on Example 2. The jump in radial contact pressure directly amplifies the interfacial frictional resistance and mechanical interlocking effect, causing the interfacial shear strength of Example 2 to climb to 7.35 MPa, nearly five times that of the traditional matrix (1.51 MPa).

[0117] Based on the data in Table 7 and Figure 7 (d) The peak slip comparison chart shows the interface slip distance (mm) at the ultimate load. The peak slip of Example 2, reaching 1.42 mm, is much larger than that of Comparative Example 1, which is 0.38 mm. This data reflects that the jump in radial contact pressure not only improves the static strength but also greatly increases the frictional resistance and failure toughness of the composite interface.

[0118] In summary, the experimental results verify that the micro-expansion characteristics of Embodiment 2 of the present invention can be effectively converted into self-stress in actual closed and confined structures, thereby significantly improving the interface collaborative working ability and macroscopic load-bearing advantages of the composite components.

Claims

1. A low-cost, micro-expansion, ultra-high-performance concrete, characterized in that, Made from the following ingredients in parts by weight: 120-170 parts water; 520-720 parts cement; 120-220 parts silica fume; 150-260 parts limestone powder; 650-950 parts fine aggregate; 250-450 parts coarse aggregate; 8-18 parts of polycarboxylate superplasticizer; 10-60 parts of composite expanding agent; Internal maintenance agent 0.8-6.0 parts; The internal curing agent is a sodium polyacrylate-based superabsorbent polymer, and the composite expanding agent is a CaO-calcium sulfoaluminate-based composite expanding agent.

2. The low-cost micro-expansion ultra-high performance concrete according to claim 1, characterized in that, Of the raw materials: The silica fume is composed of a first silica fume with a median particle size of 0.15-0.25 μm and a second silica fume with a median particle size of 0.8-1.2 μm, and the mass ratio of the first silica fume to the second silica fume is (2-4):1; The limestone powder is composed of 400μm limestone powder and 2000μm limestone powder, and the mass ratio of the 400μm limestone powder to the 2000μm limestone powder is (1.5-4):1; The fine aggregate is river sand, consisting of a first fine aggregate with a particle size range of 0.075-1.18 mm and a second fine aggregate with a particle size range of 1.18-2.36 mm; The coarse aggregate is basalt crushed stone, consisting of a first coarse aggregate with a particle size of 2.36-4.75 mm and a second coarse aggregate with a particle size of 4.75-6.0 mm.

3. The low-cost micro-expansion ultra-high performance concrete according to claim 1, characterized in that, The internal maintenance agent is prepared by the following method: At a temperature below 30°C, sodium hydroxide aqueous solution is slowly added dropwise to acrylic acid aqueous solution, controlling the neutralization degree of acrylic acid to 70%-80%, to obtain a mixed solution. N,N'-methylenebisacrylamide (0.05%-0.15% by mass of acrylic acid monomer) and potassium persulfate (0.1%-0.5% by mass of acrylic acid monomer) are added sequentially to the mixed solution, and the mixture is stirred continuously for 20-30 minutes until completely dissolved. Nitrogen gas is then introduced to remove oxygen for 15 minutes. The reaction system is transferred to an environment of 65-75°C, stirring is stopped, and a cross-linking polymerization reaction is initiated to form a hydrogel. The hydrogel is then kept at this temperature and cured for 2 hours. The hydrogel is cut into blocks, washed with anhydrous ethanol, dried at 80°C to constant weight, pulverized, and sieved to collect powder with a particle size range of 100-300 μm, thus obtaining the internal protective agent.

4. The low-cost micro-expansion ultra-high performance concrete according to claim 1, characterized in that, The composite expanding agent is prepared by the following method: Limestone, bauxite, and natural dihydrate gypsum are used as raw materials. The mass ratio of limestone, bauxite, and natural dihydrate gypsum is (65-70):(20-25):

10. The raw materials are ground until the residue on an 80μm square-hole sieve is less than 5%. Water is added to granulate the raw materials into pellets. The raw material pellets are then calcined at 1250-1300℃ for 45-60 minutes to obtain clinker. After rapid cooling, the clinker is mixed with anhydrite at a mass ratio of 100:15 and co-ground to control the specific surface area to be 350-400 m². 2 / kg, thus obtaining the composite expanding agent.

5. The low-cost micro-expansion ultra-high performance concrete according to claim 1, characterized in that, The raw materials also include 60-120 parts by weight of steel fiber; The steel fiber is a copper-plated steel fiber with a diameter of 0.2-0.22 mm and a length of 13 mm.

6. A method for preparing low-cost micro-expansion ultra-high performance concrete as described in any one of claims 1-5, characterized in that, Includes the following steps: The cement, silica fume, limestone powder, composite expansion agent and internal curing agent are added to a mixer and mixed dry to obtain a uniform powder mixture. The polycarboxylate superplasticizer is dissolved in the water to form a mixture, and the mixture is added to the powder and stirred in the mixture until a uniform slurry is formed. While maintaining the stirring state, add the fine aggregate and the coarse aggregate at a uniform speed and continue stirring so that the uniform slurry coats the surface of the fine aggregate and the coarse aggregate to obtain an ultra-high performance concrete mixture. The ultra-high performance concrete mixture is poured into the mold and vibrated to remove air bubbles. After molding, the surface is covered and cured in the mold for 24 hours at a temperature of 20±2℃ and a relative humidity of not less than 95%. After demolding, it continues to be cured until the specified age.

7. The method for preparing low-cost micro-expansion ultra-high performance concrete according to claim 6, characterized in that, The step of adding the mixture to the powder mixture and stirring it specifically includes: The mixture is divided into a first part of the mixture with a mass fraction of 70% and a second part of the mixture with a mass fraction of 30%. First, the first part of the mixture is added and stirred at a low speed of 100 r / min for 2 minutes. Then, the second part of the mixture is added and stirred at a high speed of 200-250 r / min for 3-5 minutes.

8. The method for preparing low-cost micro-expansion ultra-high performance concrete according to claim 6, characterized in that, In the step of adding the cement, silica fume, limestone powder, composite expansion agent, and internal curing agent to a mixer for dry mixing: The dry mixing speed should be controlled at 120-150 r / min, and the dry mixing time should be controlled at 3 minutes.

9. The method for preparing low-cost micro-expansion ultra-high performance concrete according to claim 6, characterized in that, When the raw materials also include steel fibers, after the step of uniformly adding the fine aggregate and the coarse aggregate and continuing to stir, the following steps are also included: Add the steel fibers slowly and evenly, controlling the feeding time to be within 1.5-2 minutes. After the feeding is completed, continue high-speed stirring for 2-3 minutes.

10. The method for preparing low-cost micro-expansion ultra-high performance concrete according to claim 6, characterized in that, The steps for continuing curing after demolding until the specified age are as follows: Cured in a standard curing room until the specified age, or transferred to a 90℃ steam curing chamber for curing until the specified age.

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