Composite low-shrinkage high-ductility concrete and preparation method thereof

Abstract

The application relates to the field of building materials, and particularly discloses a composite low-shrinkage high-ductility concrete and a preparation method thereof. The concrete comprises cement, fly ash, river sand, ultra-high molecular weight polyethylene fiber, composite internal curing aggregate, composite expansion agent, water reducing agent and water; the composite internal curing aggregate is made by loading water-absorbing resin in a porous carrier, and the water-absorbing resin is composed of cross-linked 2-acrylamido-2-methylpropanesulfonic acid sodium water-absorbing resin and cross-linked polycarboxylic acid ether-acrylic acid block water-absorbing resin. The preparation method comprises dry mixing, water adding and mixing, fiber dispersing, molding and double-coating curing. Through the synergy of the water-absorbing resin, the expansion agent compensation and the double-coating curing, the application realizes the full-chain inhibition of the drying shrinkage of the high-ductility concrete, and the application has low shrinkage, high ductility and good mechanical properties, and is suitable for masonry reinforcement, bridge deck joint filling and other engineering.

Description

Technical Field

[0001] This invention belongs to the field of building materials technology, specifically relating to high-ductility concrete production technology. Background Technology

[0002] High-ductility concrete (Engineering Cementitious Composite, ECC) has attracted widespread attention due to its excellent toughness and crack control capabilities. By incorporating short fibers into the cement matrix, the inherent defects of ordinary concrete, such as high brittleness and easy cracking, can be overcome, achieving multi-crack cracking and strain hardening behavior.

[0003] However, high-ductility concrete has long faced a prominent technical challenge in its widespread application—large drying shrinkage. Because ECC uses a lower water-cement ratio and contains no coarse aggregate, its autogenous shrinkage and drying shrinkage are far higher than those of ordinary concrete. The 28-day drying shrinkage of ordinary ECC is approximately 900 × 10⁻⁻⁻⁶. 6 In dry environments, the shrinkage can be even higher. Excessive drying shrinkage not only makes the material more prone to cracking during the hardening process, but also weakens the interfacial bond between concrete and the existing structure, greatly limiting its application in scenarios with stringent requirements for volume stability, such as masonry reinforcement and repair, expansion joint filling, and bridge deck paving.

[0004] Extensive research has been conducted by scholars both domestically and internationally on the problem of shrinkage inhibition in high-ductility concrete. Current main technical directions include: using shrinkage-reducing agents to lower the surface tension of capillary solutions, adding expansion agents to compensate for volume shrinkage, introducing pre-hygroscopic internal curing materials to maintain internal humidity, and surface film curing. However, existing technologies mostly rely on a simple combination of single or two techniques, lacking a systematic design for a low-shrinkage system covering the entire lifecycle of "early internal curing - mid-term expansion compensation - late-stage shrinkage reduction protection." Furthermore, superabsorbent polymer (SAP) internal curing materials are prone to particle agglomeration in concrete, and the pores formed after water release can adversely affect strength.

[0005] Therefore, developing a high-ductility concrete that can systematically suppress full-cycle drying shrinkage while maintaining good mechanical properties is of great engineering significance and economic benefit. Summary of the Invention

[0006] This invention aims to solve the problems of large drying shrinkage in existing high-ductility concrete and the difficulty of using a single shrinkage reduction method to take into account different shrinkage stages. It provides a composite low-shrinkage high-ductility concrete and its preparation method. Through a full-cycle synergistic inhibition mechanism, the drying shrinkage value is significantly reduced and the volume stability is improved without sacrificing the high ductility characteristics of the material.

[0007] To solve the above-mentioned technical problems, the present invention provides the following technical solution:

[0008] A composite low-shrinkage high-ductility concrete comprises the following components in parts by weight: 400-600 parts cement, 200-500 parts fly ash, 400-700 parts river sand, 15-30 parts ultra-high molecular weight polyethylene fiber, 5-45 parts composite internal curing aggregate, 40-75 parts composite expansion agent, 4-7 parts water-reducing agent, and 180-260 parts water.

[0009] The composite internal curing aggregate is made by loading a water-absorbing resin onto a porous carrier. The water-absorbing resin is composed of a first component water-absorbing resin and a second component water-absorbing resin. The first component water-absorbing resin is a cross-linked sodium 2-acrylamido-2-methylpropanesulfonate water-absorbing resin, and the second component water-absorbing resin is a cross-linked polycarboxylic acid ether-acrylic acid block water-absorbing resin. The mass ratio of the first component water-absorbing resin to the second component water-absorbing resin is 3 to 8:2.

[0010] The composite expanding agent comprises a sulfoaluminate expanding agent, a magnesium-based expanding agent, and a polycarboxylate ether shrinkage reducing agent, wherein the mass ratio of the sulfoaluminate expanding agent, the magnesium-based expanding agent, and the polycarboxylate ether shrinkage reducing agent is 4-6: 2-4: 1.

[0011] Partial component descriptions:

[0012] In this invention, the cross-linked sodium 2-acrylamido-2-methylpropanesulfonate superabsorbent resin (i.e., the first component superabsorbent resin) refers to a superabsorbent resin formed by polymerization of sodium 2-acrylamido-2-methylpropanesulfonate (AMPS-Na) as a monomer, initiated by a cross-linking agent (such as N,N'-methylenebisacrylamide). This resin molecular chain contains strongly hydrophilic sulfonic acid groups (-SO3⁻) and amide groups (-CONH2), forming a water-insoluble, swellable gel after absorbing water, capable of absorbing and retaining tens to hundreds of times its own weight in water. This resin is commercially available (e.g., similar products are available from Sumitomo Chemical Co., Ltd. of Japan and Essen of France), or it can be prepared by the method provided in the specific embodiments of this invention. It should be noted that in this invention, the resin specifically refers to a cross-linked, water-insoluble but water-absorbing and swelling superabsorbent resin, not a water-soluble polyelectrolyte thickener.

[0013] In this invention, the cross-linked polycarboxylate ether-acrylic acid block superabsorbent resin (i.e., the second component superabsorbent resin) refers to a block copolymer formed by controlled free radical polymerization of acrylic acid or sodium acrylate as the main monomer and methyl allyl polyoxyethylene ether (HPEG, molecular weight typically 1200-4000) or isopentenyl polyoxyethylene ether (TPEG) as the polycarboxylate ether macromolecular segments, and then cross-linked with a cross-linking agent (such as ethylene glycol dimethacrylate) to form a three-dimensional network structure. The molecular structure of this resin simultaneously contains polyacrylic acid (or sodium polyacrylate) segments and polycarboxylate ether side chains. This resin is not a physical mixture of commercially available conventional polycarboxylate superplasticizers and ordinary superabsorbent resins, but rather a polymeric material that integrates water absorption and retention functions with water reduction and dispersion functions into the same molecular structure through the aforementioned chemical bonding. Those skilled in the art can prepare it themselves according to the methods provided in the inventive description or specific embodiments of this invention, or obtain it through other custom synthesis methods of the prior art. It should be noted that if an uncrosslinked water-soluble polycarboxylic acid-acrylic acid copolymer (such as a conventional water-reducing agent) is used, the water absorption and retention function cannot be achieved; if a common crosslinked sodium polyacrylate resin without polycarboxylic acid ether side chains is used, the dispersion improvement and water reduction effect cannot be obtained, and the synergistic effect with the crosslinked 2-acrylamido-2-methylpropanesulfonate sodium water-absorbing resin will be lost.

[0014] As an improvement of the present invention, the cross-linked polycarboxylate ether-acrylic acid block absorbent resin is a block copolymer resin formed by free radical polymerization and cross-linking, with acrylic acid or sodium acrylate as the main monomer and methyl allyl polyoxyethylene ether or isopentenyl polyoxyethylene ether as the polycarboxylate ether macromolecular segment. The molar ratio of acrylic acid or sodium acrylate to methyl allyl polyoxyethylene ether or isopentenyl polyoxyethylene ether in the block copolymer is 2 to 4:1.

[0015] This invention, through optimization of polymerization process parameters, determined the optimal molar ratio range of acrylic acid / sodium acrylate and polycarboxylate macromonomer. When the molar ratio is below 2:1, the proportion of polycarboxylate segments is too high, resulting in a significant decrease in water absorption ratio; when the molar ratio is above 4:1, the proportion of polycarboxylate segments is too low, and the improvement in dispersibility is not significant. Therefore, the molar ratio is limited to 2–4:1.

[0016] As an improvement of the present invention, the porous carrier is expanded glass or coal bottom ash; the loading is achieved by vacuum impregnation: after the porous carrier is dried, it is placed in a vacuum impregnation device, and a vacuum is drawn to -0.08 to -0.1 MPa. Then, the water-absorbing resin is prepared into a suspension of 5 to 10 wt% and injected for impregnation. After impregnation, it is taken out, drained, and dried at low temperature.

[0017] The skeletal effect of the porous carrier can mitigate the weakening of structural strength caused by the pores after SAP releases water. The expanded glass carrier itself also exhibits micro-expansion characteristics after water absorption, providing additional early volume compensation. The vacuum impregnation process ensures that the SAP solution fully penetrates the pores of the carrier, improving loading efficiency and uniformity.

[0018] As a further improvement of the present invention, the mass ratio of the water-absorbing resin to the porous carrier is 1:3 to 5. Through extensive experiments, the inventors discovered that when the mass ratio of the water-absorbing resin to the porous carrier is less than 1:5, the proportion of the porous carrier is too high, and the resin loading is insufficient to provide adequate internal curing, making it difficult to effectively inhibit drying shrinkage. When the mass ratio is higher than 1:3, the resin content on the carrier surface and in the pores is too high, resulting in too many pores after water release, which adversely affects the concrete strength. Limiting the mass ratio to the range of 1:3 to 5 allows the resin to be evenly loaded inside and on the surface of the carrier pores, utilizing the skeletal effect of the carrier to share the weakening effect of pores on structural strength after water release, while ensuring sufficient resin dosage to provide continuous water replenishment required for internal curing. Furthermore, the expanded glass carrier itself exhibits micro-expansion characteristics after water absorption, providing additional volume compensation in the early stages and further improving the shrinkage inhibition effect.

[0019] As a further improvement of the present invention, the cement is P·O 42.5 grade ordinary Portland cement; the fly ash is Grade I fly ash with a fineness ≤12%, water requirement ratio ≤95%, and 28-day activity index ≥70%; the river sand has a fineness modulus of 1.8 to 2.5 and a mud content ≤1.5%. We chose P·O 42.5 grade ordinary Portland cement as the main component of the cementitious material based on its stable strength development and good compatibility with water-reducing agents, which can provide a reliable foundation for the early and later strength of high-ductility concrete. The pozzolanic reaction of fly ash can reduce the heat of hydration and improve workability, while its micro-aggregate filling effect can enhance the matrix density and help reduce drying shrinkage. Limiting the fineness of Grade I fly ash to ≤12%, water requirement ratio ≤95%, and 28-day activity index ≥70% ensures that the fly ash has sufficient particle fineness and cementitious activity, effectively replacing part of the cement and improving the overall performance of concrete. The fineness modulus of river sand should be controlled within the range of 1.8 to 2.5 (medium sand), and the mud content should be controlled below 1.5%, which helps to obtain good workability and stable interfacial bonding performance.

[0020] As a further improvement of the present invention, the ultra-high molecular weight polyethylene fiber has a tensile strength ≥3000 MPa, an elastic modulus ≥95 GPa, an elongation at break of 3%–5%, a diameter of 15–20 μm, and a length of 8–12 mm. The PE fiber used in this invention, with a tensile strength ≥3000 MPa, can provide sufficient bridging stress after the concrete matrix cracks, preventing excessive crack propagation; the elastic modulus ≥95 GPa ensures that the fiber can effectively transfer loads without excessive deformation under stress; the elongation at break of 3%–5% allows the fiber to continuously withstand tensile stress without brittle fracture after the matrix cracks, providing protection for multi-crack cracking and strain hardening behavior; the selection of a diameter of 15–20 μm and a length of 8–12 mm ensures sufficient bonding area between the fiber and the matrix while avoiding the problems of excessively long fibers leading to dispersion difficulties and excessively short fibers leading to insufficient bridging effect. The synergistic effect of the above parameter ranges enables the high-ductility concrete of the present invention to achieve significant strain hardening behavior and multi-crack cracking characteristics.

[0021] As a further improvement of the present invention, the water-reducing agent is a polycarboxylate-type water-reducing agent with a solid content of 15-25% and a dosage of 4-7 parts. The polyoxyethylene ether side chains in the molecular structure of the polycarboxylate-type water-reducing agent provide excellent dispersion performance through steric hindrance. The inventors selected a polycarboxylate-type water-reducing agent with a solid content of 15-25% and a dosage range of 4-7 parts by weight. This dosage ensures that the fresh concrete has sufficient fluidity to meet construction requirements under low water-cement ratio conditions, while avoiding excessive retardation or segregation problems that may result from excessive use of the water-reducing agent. Within this dosage range, the water-reducing agent can effectively reduce water consumption and increase matrix density, thereby improving mechanical properties and impermeability without sacrificing workability.

[0022] The present invention also provides a method for preparing the above-mentioned composite low-shrinkage and high-ductility concrete, comprising the following steps:

[0023] (1) Put cement, fly ash, river sand, composite internal curing aggregate, and composite expansion agent dry powder into a mixer and dry mix for 3-5 minutes until the mixture is uniform to obtain dry mix;

[0024] (2) Add water and water-reducing agent to the dry mixture and continue stirring for 3-5 minutes until the slurry is uniform to obtain the slurry;

[0025] (3) Add ultra-high molecular weight polyethylene fiber slowly into the slurry in two batches, stirring for 3 to 5 minutes each time, until the fiber is evenly dispersed and there are no visible agglomerates, to obtain the mixture;

[0026] (4) The mixture is poured into a mold, vibrated to compact it, and cured under standard curing conditions for 20-30 hours before demolding.

[0027] The mixer mentioned in step (1) above is preferably a forced mixer. The purpose of dry material premixing is to ensure that all powder components are fully and evenly mixed, avoiding uneven local material concentration that could affect the consistency of performance after hardening. In step (2), the water-reducing agent is premixed with water before being added together, which helps the water-reducing agent to be more evenly dispersed in the slurry and improves the water-reducing efficiency. In step (3), the fiber is sprinkled in two batches and stirred in two batches, which can effectively prevent the fiber from clumping due to being added all at once, ensuring the uniform dispersion of the fiber in the matrix, thereby ensuring the toughness performance of the high-ductility concrete.

[0028] As a further improvement to the preparation method of the present invention, a coating curing step is also included: applying two coatings to the concrete after demolding for curing: first, applying an ethylene glycol-based shrinkage reducing agent solution with a mass concentration of 15% to 25% and a coating amount of 150 to 250 mL / m², and allowing it to dry naturally for 30 to 60 minutes; then applying a styrene-acrylate-based emulsion curing agent with a solid content of 30% to 45% and a coating amount of 200 to 300 mL / m², with the interval between the two coatings not exceeding 2 hours.

[0029] This double-coat curing is a key measure for shrinkage protection in the later stages of this invention. The bottom layer of ethylene glycol-based shrinkage reducer reduces shrinkage driving force by lowering the surface tension of the capillary solution, while the top layer of styrene-acrylate-based emulsion curing agent forms a dense polymer film on the concrete surface, which not only effectively blocks the evaporation of internal moisture but also prevents the volatilization of the inner layer shrinkage reducer, ensuring that the shrinkage reducer continues to perform its shrinkage reduction function during long-term curing.

[0030] As a further improvement to the preparation method of the present invention, the mixer in step (1) is a forced mixer; the standard curing conditions in step (4) are a temperature of 20±2℃ and a relative humidity of ≥95%; the surface tension of the ethylene glycol-based shrinkage reducing agent solution in the coating curing step is ≤45 mN / m at 25℃, and the 24h water retention rate of the styrene-acrylate-based emulsion curing agent is ≥80%.

[0031] Compared with the prior art, the present invention has the following beneficial effects:

[0032] (1) Synergistic inhibition throughout the entire cycle: The composite internal curing aggregate provides continuous water replenishment to inhibit auto-shrinkage in the early stage, the composite expansion agent compensates for drying shrinkage through volume expansion in the middle stage, and the double-coating curing provides surface shrinkage reduction and sealing protection in the later stage, thus realizing a closed-loop inhibition of shrinkage throughout the entire chain. As shown in the performance verification experiment in the specific embodiment, the 28-day drying shrinkage value of the high ductility concrete prepared by this invention can be as low as 342×10⁻ 6 ~385×10⁻ 6 The shrinkage rate is as high as 57.8% or more, which is far superior to existing technologies.

[0033] (2) Synergistic effect of two-component composite water-absorbing resin: The combination of the first component water-absorbing resin (crosslinked 2-acrylamido-2-methylpropanesulfonate sodium water-absorbing resin) and the second component water-absorbing resin (crosslinked polycarboxylic acid ether-acrylic acid block water-absorbing resin) not only improves the dispersibility of water-absorbing resin in concrete, but also shows from the performance verification experiment that, under the same total water-absorbing resin dosage, the 28-day drying shrinkage value of the two-component composite system is significantly lower than that of the single-component water-absorbing resin system.

[0034] (3) Loading technology reduces strength loss: The composite water-absorbing resin is loaded into an expanded glass porous carrier. The skeletal effect of the carrier shares the weakening effect of the pores on the structural strength after the resin releases water. Combined with the micro-expansion characteristics of the carrier itself, the strength loss caused by internal curing is significantly reduced. As shown in the performance verification experiment, the 60-day compressive strength of the loaded two-component composite resin (Example 1) is 58.6 MPa, while that of the unloaded single resin (Comparative Example 2) is only 53.7 MPa. The loading technology increases the compressive strength by about 9%.

[0035] (4) The expansion agent compounding achieves full-cycle expansion compensation: sulfoaluminate expansion agent provides rapid expansion in the early stage, magnesium-based expansion agent provides continuous expansion in the middle and late stages, and polycarboxylic acid ether shrinkage agent provides shrinkage reduction and hydration regulation functions. The close connection of the three components on the time axis forms a gradient compensation for shrinkage.

[0036] (5) Good mechanical properties are maintained: By optimizing the mix proportions of each component, the 60-day compressive strength of the high ductility concrete of the present invention is ≥50 MPa, the equivalent flexural strength is ≥11 MPa, and the equivalent flexural toughness is ≥180 kJ / m³, which meets the relevant standard requirements. Attached Figure Description

[0037] Figure 1 This is a process flow diagram of Embodiment 1 of the specific implementation method.

[0038] Figure 2 This is an FTIR spectrum analysis of the cross-linked sodium 2-acrylamido-2-methylpropanesulfonate water-absorbing resin in a specific embodiment.

[0039] Figure 3 This is an FTIR spectrum analysis of the cross-linked polycarboxylate ether-acrylic acid block superabsorbent resin in a specific embodiment. Detailed Implementation

[0040] To enable those skilled in the art to better implement the present invention, the present invention will be further described below with reference to embodiments. However, it should be understood that the present invention is not limited to the following embodiments.

[0041] For ease of comparison, the raw materials used in the following examples and comparative examples are all from the same batch, and the source and specific parameters of the raw materials are as follows:

[0042] Cement: P·O 42.5 grade ordinary Portland cement, specific surface area 320 m² / kg, conforming to GB 175 standard, commercially available.

[0043] Fly ash: Grade I fly ash, fineness (45 μm square hole sieve residue) 8.5%, water requirement ratio 92%, 28-day activity index 75%, loss on ignition 2.5%, commercially available.

[0044] River sand: fineness modulus 2.2, mud content 0.8%, commercially available.

[0045] Ultra-high molecular weight polyethylene fiber: tensile strength 3200 MPa, elastic modulus 105 GPa, elongation at break 3.5%, diameter 18 μm, length 10 mm, commercially available.

[0046] Water-reducing agent: Polycarboxylate type high-efficiency water-reducing agent, solid content 20%, water reduction rate 28%, commercially available.

[0047] Expanded glass: Porous expanded glass particles, particle size 2-3 mm, apparent density 580 kg / m³, 24h water absorption rate 22%, cylinder compressive strength 1.8 MPa, commercially available.

[0048] Sulfoaluminate expansion agent: Al2(SO4)3 and CaSO4 compound, with a limited expansion rate (7 days in water) of 0.030% and (21 days in air) of -0.015%, commercially available.

[0049] Magnesium-based expanding agent (MgO): 92% active MgO content, 180 s activity value (citric acid neutralization method), 3.5% fineness (45 μm square hole sieve residue), commercially available.

[0050] Polycarboxylic acid ether shrinkage reducer: polycarboxylic acid ether and citric acid are compounded in a mass ratio of 10:1, with a solid content of 32% and a pH of 7.2.

[0051] Ethylene glycol-based shrinkage reducer: 20% ethylene glycol mass concentration, surface tension of 42 mN / m at 25℃, commercially available.

[0052] Styrene-acrylate emulsion conditioner: Styrene-acrylate copolymer emulsion, solid content 35%, film forming time 25 min (20℃), 24h water retention rate 85%, commercially available.

[0053] I. Preparation of Water-Absorbent Resin

[0054] The water-absorbing resin used in this invention is prepared according to the following method:

[0055] (1) Preparation of the first component, the water-absorbing resin (crosslinked sodium 2-acrylamido-2-methylpropanesulfonate water-absorbing resin):

[0056] 50 g of sodium 2-acrylamido-2-methylpropanesulfonate monomer was weighed and dissolved in 200 mL of deionized water. 0.10 g of crosslinking agent MBA (0.2 mol% of total monomer) and 0.40 g of initiator APS (0.8 mol% of total monomer) were added. The mixture was deoxygenated by purging with nitrogen for 30 min and reacted in a water bath at 65℃ for 5 h to form a crosslinked network hydrogel. The gel was washed five times with deionized water to remove unreacted monomers, dried in a vacuum drying oven at 60℃ for 12 h, pulverized, and sieved to a particle size of 200–300 μm. The water absorption ratio (deionized water) of the obtained resin was determined to be 112 g / g, and the water retention rate (24 h) was 86%.

[0057] (2) Preparation of the second component, water-absorbing resin (crosslinked polycarboxylic acid ether-acrylic acid block water-absorbing resin):

[0058] Methyl allyl polyoxyethylene ether (HPEG, molecular weight 2400) was dissolved in 200 mL of deionized water. Acrylic acid was added (molar ratio of acrylic acid:HPEG = 3:1), and the mixture was stirred until dissolved. The pH was adjusted to 6.5 with 1 mol / L sodium hydroxide solution. 0.12 g of crosslinking agent EGDMA (0.3 mol% of the total monomer content) and 0.40 g of chain transfer agent mercaptoacetic acid were added. The mixture was purged with nitrogen for 30 min, heated to 65 °C, and an initiator solution (0.35 g ammonium persulfate dissolved in 10 mL of deionized water) was added dropwise over 30 min. The reaction was continued for 6 h. After the reaction was completed, the polymer solution was poured into 500 mL of anhydrous ethanol for precipitation. The solution was filtered, washed three times with anhydrous ethanol, dried in a vacuum drying oven at 50 °C for 12 h, pulverized, and sieved to a particle size of 200–300 μm. The water absorption ratio (deionized water) of the obtained resin was measured to be 86 g / g, the water retention rate (24 h) was 82%, and there was no obvious agglomeration under stirring conditions.

[0059] (3) Two-component composite:

[0060] The two resins were mechanically mixed for 5 minutes at a mass ratio of first component water-absorbing resin: second component water-absorbing resin = 7:2 (i.e. 3.5:1, falling within the range of 3 to 8:2) to obtain a two-component composite water-absorbing resin powder.

[0061] (4) Loading steps:

[0062] After drying the expanded glass porous carrier material to constant weight at 105℃ (approximately 6 h), it was measured according to the proportions in the examples and placed into a vacuum impregnation apparatus. A vacuum was drawn to -0.09 MPa and maintained for 15 min. A two-component composite water-absorbing resin powder was prepared into an 8 wt% suspension and injected into the impregnation tank, maintained under vacuum for 45 min. After releasing the vacuum, the material was soaked for another 8 h. Excess solution was drained from the surface, and the material was dried in a 50℃ low-temperature drying oven for 12 h to obtain the composite internal curing aggregate.

[0063] (5) Structural characterization of the resin:

[0064] Fourier transform infrared (FTIR) spectroscopy analysis was performed on the first and second component water-absorbing resins prepared above. In the FTIR spectrum of the first component resin, characteristic absorption peaks of amide I and amide II bands appeared at approximately 1650 cm⁻¹ and 1550 cm⁻¹, respectively, and characteristic absorption peaks of asymmetric and symmetric stretching vibrations of the sulfonic acid group (-SO₃⁻) appeared at approximately 1040 cm⁻¹ and 1180 cm⁻¹, confirming the presence of the 2-acrylamido-2-methylpropanesulfonate sodium structural unit. The FTIR spectrum analysis is shown below. Figure 2 .

[0065] In the infrared spectrum of the second component resin, a characteristic absorption peak for the stretching vibration of C=O in the carboxyl group (-COOH) appeared at approximately 1720 cm⁻¹, and a characteristic absorption peak for the stretching vibration of COC in the polyoxyethylene ether segment (-CH₂-CH₂-O-) appeared at approximately 1100 cm⁻¹, confirming the existence of the polycarboxylic acid ether-acrylic acid block copolymer structure. These infrared spectral analysis results further confirmed the successful synthesis of the two target resins. The FTIR spectra are shown below. Figure 3 .

[0066] II. Preparation of Composite Low-Shrinkage High-Ductility Concrete

[0067] Example 1

[0068] This embodiment provides a composite low-shrinkage high-ductility concrete and its preparation method.

[0069] Mixing ratio:

[0070] Cement: 500 kg

[0071] Grade I fly ash: 400 kg

[0072] River sand: 550 kg

[0073] Ultra-high molecular weight polyethylene fiber: 25 kg

[0074] Composite internal curing aggregate: 35 kg (including 7 kg of two-component composite water-absorbing resin and 28 kg of expanded glass carrier, with a mass ratio of 1:4).

[0075] Composite expanding agent: 28 kg of sulfoaluminate expanding agent, 21 kg of magnesium-based expanding agent, and 7 kg of polycarboxylic acid ether shrinkage reducing agent, totaling 56 kg.

[0076] Water-reducing agent (polycarboxylate type, solid content 20%): 5 kg

[0077] Water: 230 kg

[0078] Preparation steps:

[0079] (1) Put 500 kg of cement, 400 kg of fly ash, 550 kg of river sand, 35 kg of composite internal curing aggregate, and 56 kg of composite expansion agent into a forced mixer and dry mix at a low speed of 35 r / min for 3 min to obtain dry mix.

[0080] (2) Mix 230 kg of water and 5 kg of water-reducing agent evenly, add them to the dry mixture, and continue stirring at 35 r / min for 4 min to obtain slurry.

[0081] (3) Slowly add 25 kg of ultra-high molecular weight polyethylene fiber to the slurry in two batches. First, add 12.5 kg and stir at a medium speed of 45 r / min for 4 min. Then, add the remaining 12.5 kg and continue stirring at 45 r / min for 4 min. The total stirring time is controlled at 11 min. Visually inspect the fiber for uniform dispersion and ensure that there are no visible agglomerates when touched to obtain the mixture.

[0082] (4) Pour the mixture into steel molds of 100 mm × 100 mm × 100 mm and 40 mm × 40 mm × 160 mm, vibrate on a vibrating table for 30 s, smooth the surface with a trowel, and remove the mold after curing under standard curing conditions (temperature 20 ± 1℃, relative humidity ≥ 95%) for 24 h.

[0083] (5) Curing steps: Apply two coats of curing agent to the concrete after demolding. First, apply a 20% glycol-based shrinkage reducer solution using a brush, applying 200 mL / m², and allow it to dry naturally for 45 minutes (until there is no free liquid on the surface). Then, apply a 35% styrene-acrylate emulsion curing agent, applying 250 mL / m², with a 1.5-hour interval between the two coats. After application, continue curing under standard conditions until the specified age of 60 days.

[0084] Example 2:

[0085] The method is basically the same as in Example 1, except that the amount of composite internal curing aggregate is adjusted to 45 kg (including 9 kg of two-component composite water-absorbing resin and 36 kg of expanded glass carrier, with a mass ratio still of 1:4). The amounts of the remaining components remain unchanged: 500 kg of cement, 400 kg of fly ash, 550 kg of river sand, 25 kg of ultra-high molecular weight polyethylene fiber, 56 kg of composite expanding agent, 5 kg of water-reducing agent, and 230 kg of water.

[0086] Preparation steps: exactly the same as in Example 1.

[0087] Example 3

[0088] The method is basically the same as in Example 1, except that the amount of composite internal curing aggregate is adjusted to 21 kg (including 4.2 kg of two-component composite water-absorbing resin and 16.8 kg of expanded glass carrier, with the mass ratio still being 1:4). The amounts of the other components remain unchanged: 500 kg of cement, 400 kg of fly ash, 550 kg of river sand, 25 kg of ultra-high molecular weight polyethylene fiber, 56 kg of composite expanding agent, 5 kg of water-reducing agent, and 230 kg of water.

[0089] Preparation steps: exactly the same as in Example 1.

[0090] Comparative Example 1 (Blank Group)

[0091] The difference between this comparative example and Example 1 is that no composite internal curing aggregate is added, and the application of the bottom shrinkage reducer is omitted in the curing step; only the top layer of styrene-acrylate emulsion curing agent is applied.

[0092] Mix proportions: 500 kg cement, 400 kg fly ash, 550 kg river sand, 25 kg ultra-high molecular weight polyethylene fiber, 230 kg water, 5 kg water-reducing agent, and 56 kg composite expansion agent. No composite internal curing aggregate is added.

[0093] Preparation steps:

[0094] (1) to (4) are the same as in Example 1 (without adding composite internal curing aggregate in step (1)).

[0095] (5) Curing steps: Apply only the styrene-acrylate emulsion curing agent (35% solids content, 250 mL / m²), without applying the underlying ethylene glycol-based shrinkage reducer solution. The remaining curing conditions are the same as in Example 1.

[0096] Comparative Example 2 (Single first component water-absorbing resin, non-loaded group)

[0097] The difference between this comparative example and Example 1 is that the composite internal curing aggregate is replaced by direct addition of unloaded first component water-absorbing resin powder (i.e., the cross-linked 2-acrylamido-2-methylpropanesulfonate sodium water-absorbing resin prepared in Example 1, with a particle size of 200-300 μm), at a dosage of 7 kg (the total resin dosage remains the same as in Example 1), without the expanded glass loading step. The second component water-absorbing resin is not used. The composite expansion agent and curing steps are the same as in Example 1.

[0098] Mix proportions: 500 kg cement, 400 kg fly ash, 550 kg river sand, 25 kg ultra-high molecular weight polyethylene fiber, 7 kg first component water-absorbing resin powder, 56 kg composite expanding agent, 5 kg water-reducing agent, and 230 kg water. No porous carrier, no second component water-absorbing resin.

[0099] Preparation steps: basically the same as in Example 1, except that in step (1), the first component of water-absorbing resin powder (replacing the composite internal curing aggregate) is added to the dry material part. Step (5) curing steps are the same as in Example 1 (double coating curing).

[0100] Comparative Example 3 (Single First Component Water-Absorbent Resin Loading Group)

[0101] The difference between this comparative example and Example 1 is that the composite internal curing aggregate is changed to use only the first component, water-absorbing resin, loaded in the expanded glass, without using the second component, water-absorbing resin. The loading ratio is first component water-absorbing resin: expanded glass = 1:4 (i.e., 7 kg of resin, 28 kg of carrier), which is the same as the loading ratio in Example 1.

[0102] Mix proportions: 500 kg cement, 400 kg fly ash, 550 kg river sand, 25 kg ultra-high molecular weight polyethylene fiber, 35 kg composite internal curing aggregate (first component: 7 kg water-absorbing resin + 28 kg expanded glass), 56 kg composite expansion agent, 5 kg water-reducing agent, and 230 kg water.

[0103] Preparation steps: exactly the same as in Example 1.

[0104] Comparative Example 4 (Single Second Component Water-Absorbent Resin Loading Group)

[0105] The difference between this comparative example and Example 1 is that the composite internal curing aggregate is replaced by using only the second component of the water-absorbing resin (crosslinked polycarboxylate ether-acrylic acid block water-absorbing resin) loaded in the expanded glass, without using the first component of the water-absorbing resin. The loading ratio is second component water-absorbing resin:expanded glass = 1:4 (i.e., 7 kg of resin, 28 kg of carrier), which is the same as the loading ratio in Example 1.

[0106] Mix proportions: 500 kg cement, 400 kg fly ash, 550 kg river sand, 25 kg ultra-high molecular weight polyethylene fiber, 35 kg composite internal curing aggregate (7 kg water-absorbing resin + 28 kg expanded glass as the second component), 56 kg composite expansion agent, 5 kg water-reducing agent, and 230 kg water.

[0107] Preparation steps: exactly the same as in Example 1.

[0108] III. Performance Verification Experiment:

[0109] 3.1 Test Method

[0110] (1) Compressive strength: The test was conducted in accordance with GB / T 50081 "Standard for Test Methods of Physical and Mechanical Properties of Concrete". The specimen size was 100 mm × 100 mm × 100 mm, and the loading rate was 0.6 MPa / s. Five specimens were prepared for each group, and the compressive strength at 60 days was tested. The arithmetic mean was taken.

[0111] (2) Equivalent flexural strength and equivalent flexural toughness: The tests were conducted in accordance with the Sichuan Provincial Local Standard DBJ51 / T 289-2025 "Technical Standard for High-Ductility Concrete-Reinforced Masonry Structures in Sichuan Province". The specimen size was 40 mm × 40 mm × 160 mm. The four-point bending loading method was used, with a span of 150 mm and a loading rate of 0.5 mm / min. The equivalent flexural strength and equivalent flexural toughness were calculated according to the formula in the standard. Three specimens were prepared for each group, and the arithmetic mean was taken.

[0112] (3) Drying shrinkage value: The test was conducted in accordance with JGJ / T 70 "Standard for Test Methods of Basic Performance of Building Mortar". The specimen size was 40 mm × 40 mm × 160 mm. The specimens were cured at a temperature of 20 ± 1℃ and a relative humidity of 50 ± 2%. The drying shrinkage strain at 28 days was measured using a length comparator. Five specimens were prepared for each group, and the arithmetic mean was taken.

[0113] (4) Flowability: The flowability was determined according to JGJ / T 70. A truncated cone mold (upper diameter φ50 mm, lower diameter φ100 mm, height 60 mm) was used to fill the fresh concrete in two batches. Each batch was tamped 15 times with a tamping rod. After leveling, the mold was lifted vertically, and the flow diameter in two vertical directions was measured. The average value was taken as the initial flowability. The test was repeated after 60 min, and the flowability retention rate was calculated as (60 min flowability / initial flowability × 100%).

[0114] 3.2 Mechanical property test results

[0115] The concrete specimens prepared in Examples 1 to 3 and Comparative Examples 1 to 4 were subjected to mechanical property tests according to the above method. The test results at 60 days are shown in Table 1.

[0116] Table 1. Mechanical property test results of each embodiment and comparative example.

[0117] Example 1 Two-component composite loading (7:2, 1.0%) 58.6 12.2 198.5 Example 2 Two-component composite loading (high doping level 1.3%) 53.2 11.8 191.8 Example 3 Two-component composite loading (low doping level 0.6%) 62.1 12.6 205.2 Comparative Example 1 Blank group (no inner curing, no underlayer shrinkage reducer) 55.3 11.9 194.6 Comparative Example 2 Single first-component water-absorbing resin, unloaded 53.7 11.8 192.9 Comparative Example 3 Single first-component water-absorbing resin, loaded 57.8 12.1 196.4 Comparative Example 4 Single second-component water-absorbing resin, loaded 58.2 12.0 195.8

[0118] As shown in Table 1, the compressive strength of Example 1 (58.6 MPa) is higher than that of Comparative Example 2 (53.7 MPa), Comparative Example 3 (57.8 MPa), and Comparative Example 4 (58.2 MPa), and is also higher than that of the blank control Comparative Example 1 (55.3 MPa). This indicates that the specimen of Example 1 has better mechanical properties than the comparative examples. The compressive strength of Comparative Example 4 (single second-component water-absorbing resin) (58.2 MPa) is slightly lower than that of Example 1 (58.6 MPa), but higher than that of Comparative Example 3 (57.8 MPa). This may be because the addition of the second-component water-absorbing resin has a water-reducing function, which can improve the density of the matrix. However, when used alone, it lacks sufficient internal curing and water replenishment, which may lead to microcracks due to excessive self-shrinkage, thus slightly weakening its strength advantage.

[0119] 3.3 Shrinkage performance test results

[0120] The concrete specimens prepared in each embodiment and comparative example were subjected to drying shrinkage tests according to the above method. The test results at 28 days are shown in Table 2.

[0121] Table 2. Drying shrinkage test results for each example and comparative example.

[0122] Example 1 Two-component composite loading (7:2, 1.0%) 385 57.8 Example 2 Two-component composite loading (high doping level 1.3%) 342 62.5 Example 3 Two-component composite loading (low doping level 0.6%) 418 54.2 Comparative Example 1 Blank group (no inner curing, no underlayer shrinkage reducer) 912 — Comparative Example 2 Single first-component water-absorbing resin, unloaded 568 37.7 Comparative Example 3 Single first-component water-absorbing resin, loaded 496 45.6 Comparative Example 4 Single second-component water-absorbing resin, loaded 452 50.4

[0123] The following conclusions can be drawn from Table 2:

[0124] (1) The shrinkage value of Comparative Example 4 (single second component water-absorbing resin loading group) was 452×10⁻ 6 The shrinkage rate was 50.4%. This value is lower than that of Comparative Example 2 (568×10⁻⁻⁶). 6 ) and Comparative Example 3 (496×10⁻ 6 ), but higher than Example 1 (385×10⁻ 6 This indicates that the single second-component water-absorbing resin, due to its water-reducing function, can reduce the porosity of the matrix, thereby reducing drying shrinkage to some extent. However, lacking the compounding effect of the first-component water-absorbing resin, its shrinkage reduction effect is still inferior to that of the two-component composite system.

[0125] (2) When the amount of resin used is exactly the same, the shrinkage value of Example 1 (two-component composite load group) is 385×10⁻ 6The shrinkage rate was 57.8%, which was 22.4% lower than that of Comparative Example 3 (single first-component water-absorbing resin loading group) and 14.8% lower than that of Comparative Example 4 (single second-component water-absorbing resin loading group). This fully demonstrates that the shrinkage reduction effect of the two-component composite system is not a simple superposition of the effects of the two single components, but rather produces a significant synergistic effect. The inventors believe that the reason may be that the dynamic hydrogel microdomains formed by the first-component water-absorbing resin after water absorption and swelling not only provide continuous internal curing moisture to the matrix, but also undergo volume shrinkage during water release. This forms a space-occupancy-release competition mechanism with the cement particles adsorbed by the polycarboxylate ether segments in the second-component water-absorbing resin. As the first-component water-absorbing resin gradually shrinks, the cement particles pre-dispersed by the second-component water-absorbing resin can be more tightly packed, thereby constructing a gradient structure of "filling pores first and then densifying" in situ at the interface transition zone of the two resins. Meanwhile, the expanded glass porous carrier loaded with the two resins, due to its rigid framework and micro-expansion characteristics after water absorption, acts as a mechanical anchor in the resin swelling-shrinkage cycle, forcing the aforementioned gradient structure to arrange itself in an orderly manner along the carrier surface, avoiding defects caused by random pore interconnection. The synergistic result of these three factors is that the water release channels are effectively constrained by the carrier pore walls, and the water reduction-densification effect is preferentially completed in the micro-regions around the resin, resulting in the entire matrix exhibiting low shrinkage characteristics of suppressed local self-shrinkage, defect-free interfaces, and coordinated overall deformation on a macroscopic scale.

[0126] (3) From Comparative Example 2 (single unloaded first component water-absorbing resin) to Comparative Example 3 (single loaded first component water-absorbing resin) and then to Comparative Example 4 (single loaded second component water-absorbing resin), the shrinkage rates were 37.7%, 45.6%, and 50.4%, respectively, while Example 1 (two-component loaded) reached 57.8%. Even compared with the best single-component system (Comparative Example 4), the two-component system still increased the absolute value of the shrinkage rate by 7.4 percentage points, with a relative increase of 14.7%. This difference has significant engineering implications in the field of concrete shrinkage control.

[0127] 3.4 Performance Test Results

[0128] The fresh concrete prepared in Example 1, Comparative Example 2, Comparative Example 3 and Comparative Example 4 were subjected to flowability tests according to the above method, and the results are shown in Table 3.

[0129] Table 3 Performance test results of each embodiment and comparative example

[0130] Example 1 Two-component composite loading (7:2, 1.0%) 185 170 92 Comparative Example 2 Single first-component water-absorbing resin, unloaded 170 133 78 Comparative Example 3 Single first-component water-absorbing resin, loaded 178 151 85 Comparative Example 4 Single second-component water-absorbing resin, loaded 182 164 90

[0131] As shown in Table 3, the initial flowability of Comparative Example 4 (single second-component water-absorbing resin loading group) was 182 mm, and the flowability retention rate at 60 min was 90%, which was better than Comparative Example 3 (85%) and Comparative Example 2 (78%), but slightly lower than Example 1 (92%). This may be because when the second-component water-absorbing resin is used alone, the steric hindrance effect of its polycarboxylate ether segments can significantly improve the dispersibility of the resin and the flowability retention of the slurry; while in the two-component composite system (Example 1), the synergistic effect of the first-component water-absorbing resin and the second-component water-absorbing resin achieves the optimal dispersion effect, which further improves the flowability retention rate.

Claims

1. A composite low-shrinkage, high-ductility concrete, characterized in that, It contains the following components in the indicated weight proportions: 400-600 parts cement, 200-500 parts fly ash, 400-700 parts river sand, 15-30 parts ultra-high molecular weight polyethylene fiber, 5-45 parts composite internal curing aggregate, 40-75 parts composite expansion agent, 4-7 parts water-reducing agent, and 180-260 parts water. The composite internal curing aggregate is made by loading a water-absorbing resin onto a porous carrier. The water-absorbing resin is composed of a first component water-absorbing resin and a second component water-absorbing resin. The first component water-absorbing resin is a cross-linked sodium 2-acrylamido-2-methylpropanesulfonate water-absorbing resin, and the second component water-absorbing resin is a cross-linked polycarboxylic acid ether-acrylic acid block water-absorbing resin. The mass ratio of the first component water-absorbing resin to the second component water-absorbing resin is 3-8:

2. The cross-linked polycarboxylate ether-acrylic acid block superabsorbent resin is a block copolymer resin formed by free radical polymerization and cross-linking, with acrylic acid or sodium acrylate as the main monomer and methallyl polyoxyethylene ether or isopentenyl alcohol polyoxyethylene ether as the polycarboxylate macromolecular segment. The molar ratio of acrylic acid or sodium acrylate to methallyl polyoxyethylene ether or isopentenyl alcohol polyoxyethylene ether in the block copolymer is 2-4:

1. The cross-linked sodium 2-acrylamido-2-methylpropanesulfonate superabsorbent resin is a superabsorbent resin formed by polymerization initiated by a cross-linking agent with sodium 2-acrylamido-2-methylpropanesulfonate as the monomer. The composite expanding agent comprises a sulfoaluminate expanding agent, a magnesium-based expanding agent, and a polycarboxylate ether shrinkage reducing agent, wherein the mass ratio of the sulfoaluminate expanding agent, the magnesium-based expanding agent, and the polycarboxylate ether shrinkage reducing agent is 4-6: 2-4:

1.

2. The composite low-shrinkage high-ductility concrete according to claim 1, characterized in that, The porous carrier is expanded glass or coal bottom ash; the loading is achieved by vacuum impregnation: after the porous carrier is dried, it is placed in a vacuum impregnation device, and a vacuum is drawn to -0.08 to -0.1 MPa. Then, the water-absorbing resin is prepared into a suspension of 5 to 10 wt% and injected for impregnation. After impregnation, it is taken out, drained, and dried at low temperature.

3. The composite low-shrinkage, high-ductility concrete according to claim 2, characterized in that, The mass ratio of the water-absorbing resin to the porous carrier is 1:3 to 5.

4. The composite low-shrinkage high-ductility concrete according to claim 1, characterized in that, The cement is P·O42.5 grade ordinary Portland cement; the fly ash is grade I fly ash with a fineness ≤12%, water requirement ratio ≤95%, and 28-day activity index ≥70%; the river sand has a fineness modulus of 1.8 to 2.5 and a mud content ≤1.5%.

5. The composite low-shrinkage high-ductility concrete according to claim 1, characterized in that, The ultra-high molecular weight polyethylene fiber has a tensile strength ≥3000 MPa, an elastic modulus ≥95 GPa, an elongation at break of 3% to 5%, a diameter of 15 to 20 μm, and a length of 8 to 12 mm.

6. The composite low-shrinkage high-ductility concrete according to claim 1, characterized in that, The water-reducing agent is a polycarboxylate-type water-reducing agent with a solid content of 15-25%.

7. The method for preparing composite low-shrinkage high-ductility concrete according to any one of claims 1 to 6, characterized in that, Includes the following steps: (1) Put cement, fly ash, river sand, composite internal curing aggregate and composite expansion agent into a mixer and dry mix for 3 to 5 minutes until the mixture is uniform to obtain dry mix; (2) Add water and water-reducing agent to the dry mixture and continue stirring for 3-5 minutes until the slurry is uniform to obtain the slurry; (3) Slowly sprinkle ultra-high molecular weight polyethylene fiber into the slurry in two batches, stirring for 3 to 5 minutes each time, until the fiber is evenly dispersed and there are no visible agglomerates, to obtain the mixture; (4) The mixture is poured into a mold, vibrated to compact it, and cured under standard curing conditions for 20-30 hours before demolding.

8. The preparation method according to claim 7, characterized in that, It also includes a coating curing step: after demolding, the concrete is coated with two layers of curing agent in sequence: first, apply ethylene glycol-based shrinkage reducing agent solution with a mass concentration of 15% to 25% and a coating amount of 150 to 250 mL / m², and let it dry naturally for 30 to 60 minutes; then apply styrene-acrylate-based emulsion curing agent with a solid content of 30% to 45% and a coating amount of 200 to 300 mL / m², with the interval between the two coatings not exceeding 2 hours.

9. The preparation method according to claim 8, characterized in that, The mixer mentioned in step (1) is a forced mixer; the standard curing conditions mentioned in step (4) are a temperature of 20±2℃ and a relative humidity of ≥95%; the surface tension of the ethylene glycol-based shrinkage reducer solution in the coating curing step is ≤45 mN / m at 25℃, and the 24h water retention rate of the styrene-acrylate-based emulsion curing agent is ≥80%.

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