A concrete curing agent and a method for preparing the same

Abstract

The application discloses a concrete curing agent and a preparation method thereof, and belongs to the technical field of concrete admixtures. The curing agent is prepared by dry compounding star-shaped hyperbranched superabsorbent resin formed by copolymerization of end-allyl hyperbranched polyether and acrylic acid and acrylamide, and nano light-burned magnesium oxide; the resin has a radial structure with hyperbranched polyether as the core and polyacrylic acid-acrylamide as the arm. In order to solve the problems of poor fluidity, short water release period and large strength loss of the existing SAP internal curing agent, the star-shaped hyperbranched structure is adopted to reduce the viscosity, the 7-14d slow release is realized through the synergy of the low-crosslinking-degree network and the reversible ionic crosslinking shell layer, and the nano light-burned magnesium oxide is used to fill the pores and compensate the strength loss in the hydration, so that the good slump maintaining capacity, the 28d compressive strength higher than that of the benchmark group and the effect of significantly inhibiting the shrinkage are obtained.

Description

Technical Field

[0001] This invention relates to the field of concrete admixtures, and in particular to a concrete curing agent and its preparation method. Background Technology

[0002] Concrete is the most widely used building material today, but shrinkage cracking is a common problem during its hydration and hardening process. Especially in harsh environments such as drought and large temperature differences, surface moisture evaporates rapidly, internal self-drying is severe, and early shrinkage cracks occur frequently, seriously affecting structural durability and service life. To solve this problem, internal curing technology has emerged, which involves introducing water-absorbing materials such as superabsorbent polymers (SAP) into the concrete to absorb and store moisture, slowly releasing it when the cement hydrates and becomes dehydrated, thus maintaining internal relative humidity and inhibiting self-shrinkage.

[0003] Among existing internal curing technologies, polyacrylic acid-acrylamide (SAP) is the most widely used due to its high water absorption ratio and relatively low cost. However, this type of SAP has significant shortcomings in practical applications: First, when dry powder is added externally, SAP rapidly absorbs water and expands in the initial stage of mixing, competing with cementitious materials for mixing water, resulting in severe slump loss and a significant decrease in workability and fluidity of the concrete. Second, ordinary linear SAP molecules are severely entangled, resulting in high solution viscosity, which further deteriorates the flowability of the concrete. Third, the water release cycle of SAP is usually only 1 to 3 days, which cannot match the continuous water requirement cycle of cement hydration (7 to 14 days), greatly reducing the curing effect in the later stages. Fourth, the numerous micropores left after SAP releases water become weak areas, reducing the compressive strength of concrete by 10% to 20%. In addition, to compensate for the later drying shrinkage of concrete, an additional expansive agent is often required, but existing technologies are mostly simple physical mixing methods. In low water-cement ratio concrete, the expansive agent is incompletely hydrated due to water shortage and lacks chemical bonding with SAP, resulting in poor synergy and limited improvement in volume stability. It is worth noting that, under the same molecular weight conditions, hyperbranched polymers, due to their unique star-shaped radial structure, have less intermolecular entanglement and significantly lower solution viscosity than linear polymers. This characteristic provides a new approach to improving the flowability and dispersibility of curing agents in SAP.

[0004] Therefore, designing an internal curing agent that can maintain good fluidity in the initial stage of stirring, continuously release water over 7 to 14 days, and chemically bond with the expanding agent to compensate for strength loss and later shrinkage has become a technical problem that urgently needs to be solved in this field. Summary of the Invention

[0005] To address the aforementioned technical problems, this invention provides a concrete curing agent and its preparation method.

[0006] The technical solutions provided by the embodiments of the present invention are as follows: A concrete curing agent and its preparation method, comprising the following steps: S1. Preparation of terminal allyl hyperbranched polyether: By mass, 20 parts of terminal hydroxyl hyperbranched polyether, 0.2-0.5 parts of triethylamine, and 50-100 parts of toluene are added to a reaction vessel. Under nitrogen protection, the temperature is raised to 70-80℃, and the mixture is stirred and refluxed for 30 minutes. Then, 10 to 15 parts of allyl glycidyl ether are slowly added dropwise at a rate of about 1 drop / second, and the addition is completed in about 30-45 minutes. After the addition is completed, the temperature is maintained at 70-80℃ and the reaction is continued with stirring for 5-8 hours. The mixture is then distilled under reduced pressure to obtain a pale yellow viscous liquid, which is the terminal allyl hyperbranched polyether. Triethylamine, acting as a tertiary amine nucleophilic catalyst, utilizes the lone pair of electrons on its nitrogen atom to attack the sterically less hindrance methylene carbon on the epoxy ring of allyl glycidyl ether, simultaneously opening the epoxy ring. This ring-opening process releases the original stress on the epoxy ring and activates the allyl glycidyl ether into a highly reactive alkoxy anion. Subsequently, the oxygen atom on the terminal hydroxyl group of the hyperbranched polyether acts as a nucleophile, attacking the positively charged carbon atom on the intermediate, completing a nucleophilic substitution reaction. The hydrogen atom on the hydroxyl group combines with the oxygen anion generated after ring opening to form a new hydroxyl group. Finally, the triethylamine catalyst leaves the intermediate and is regenerated to participate in the next catalytic cycle. The epoxy ring of the allyl glycidyl ether is completely opened and connected to the end of the hyperbranched polyether via an ether bond, while simultaneously introducing an allyl double bond. The relevant reaction formulas are as follows:

[0007] Throughout the reaction, triethylamine acts as a catalyst, playing only a transfer and activation role. It is not consumed before or after the reaction, so its dosage is extremely small. The reaction temperature is controlled at 70-80℃, which ensures both the catalytic activity of triethylamine and the reaction rate of epoxy ring opening, while avoiding the thermal oxidation or isomerization side reactions of allyl double bonds at high temperatures. Toluene, as a solvent, not only dilutes the reaction system and reduces viscosity, but also removes trace amounts of water that may be present during the reaction through azeotropic reflux, preventing the hydrolysis side reaction of epoxy groups. The dropwise addition of allyl glycidyl ether effectively controls the instantaneous concentration of epoxy groups in the reaction system, avoiding local overheating and the occurrence of side reactions. After 5 to 8 hours of reaction, the conversion rate of terminal hydroxyl groups of hyperbranched polyether can reach 60% to 85%. Some unreacted hydroxyl groups are still retained in the product. This is beneficial for adjusting the hydrophilic-hydrophobic balance of the hyperbranched core and controlling the density of graft arms in the subsequent synthesis of star-shaped hyperbranched superabsorbent resin. After removing toluene and a small amount of unreacted monomers by vacuum distillation, the resulting pale yellow viscous liquid is the terminal allyl hyperbranched polyether. Its molecular structure is based on hyperbranched polyether as the backbone, and the ends contain both allyl double bonds and residual hydroxyl groups. It can be directly used for the next polymerization reaction. S2. Synthesis of star-shaped hyperbranched superabsorbent resin: S201. By mass, place 50 to 65 parts of acrylic acid in a reaction vessel, cool it to 0-10°C in an ice-water bath, slowly add 30% sodium hydroxide solution dropwise under stirring, control the system temperature to not exceed 30°C, adjust the pH to 7-8 to obtain sodium acrylate solution, add 20-40 parts of acrylamide, 5 to 12 parts of terminal allyl hyperbranched polyether, N,N'-methylenebisacrylamide, and then add 200 to 300 parts of deionized water, stir until completely dissolved, and prepare a monomer solution. The carboxyl group (-COOH) in acrylic acid is a weak acid and can undergo an acid-base neutralization reaction with the strong base sodium hydroxide. Under ice-water bath cooling conditions, a 30% sodium hydroxide solution is slowly added dropwise to acrylic acid, with the system temperature controlled below 30°C. This effectively suppresses the potential for thermal polymerization side reactions of acrylic acid caused by the exothermic neutralization reaction. As sodium hydroxide is added, the hydrogen ions in acrylic acid combine with the hydroxide ions in sodium hydroxide to form water molecules. Simultaneously, acrylic acid is converted into sodium acrylate, which carries a carboxylate anion (-COOH). - ) and a sodium ion (Na) + The carboxyl anion endows sodium acrylate with good water solubility and polymerization activity. When the pH of the system reaches 7-8, it indicates that the acrylic acid has been basically completely neutralized into sodium acrylate. At this time, the ratio of carboxyl groups to carboxyl groups in the system is appropriate, which is conducive to the smooth progress of subsequent free radical polymerization reaction. Add acrylamide, terminal allyl hyperbranched polyether, N,N'-methylenebisacrylamide crosslinking agent and deionized water sequentially to the above sodium acrylate solution, and stir until all components are completely dissolved. Acrylamide, as a nonionic monomer, forms hydrogen bonds with water molecules through its amide group (-CONH2), exhibiting good water solubility and the ability to mix uniformly with sodium acrylate. In this system, acrylamide also plays a crucial role in regulating the network charge density. By diluting the anion concentration of sodium acrylate, the polymer network expands moderately and becomes more uniform in structure. At the same time, it provides additional hydrogen bond crosslinking sites, synergistically constructing a stable three-dimensional network with chemical crosslinking agents, thereby achieving the effect of a gradual water absorption rate and enhanced water retention capacity. Furthermore, the introduction of acrylamide significantly improves the tolerance of superabsorbent resins in the alkaline environment of cement. The high density of carboxylate anions in the pure sodium acrylate system readily reacts with Ca in the cement pore solution. 2+ A strong ionic cross-linking reaction occurs, causing the network to shrink and collapse prematurely and water to be squeezed out rapidly. Acrylamide, as a nonionic segment, is distributed around the carboxylate group, which on the one hand dilutes the negative charge density in the network and reduces the Ca2+. 2+The attackable sites of the amide group slowed down the rate of ionic crosslinking; on the other hand, the steric hindrance effect of the amide group and the shielding effect of the hydration layer further hindered the Ca2+ crosslinking reaction. 2+ The rapid binding with carboxylate groups, this multiple protection mechanism, enables the polymer network to maintain a stable expansion state in an alkaline environment, allowing for the gradual and continuous release of moisture, which is the key guarantee for achieving a long-lasting sustained release of 7-14 days. The allyl-terminated hyperbranched polyether molecules have allyl double bonds at their ends. Their hyperbranched structure endows them with good water dispersibility, enabling them to dissolve or disperse uniformly in water and form a transparent or semi-transparent homogeneous system. In this system, the hyperbranched polyether serves as the core of the star polymer. Its terminal allyl double bonds can be controllably incorporated into the polymer network through chain transfer during free radical polymerization, forming a radial structure with the hyperbranched polyether as the core and polyacrylic acid-acrylamide as the arms. This structure has less intermolecular entanglement and lower solution viscosity, allowing the concrete to maintain good fluidity. The crosslinking agent N,N'-methylenebisacrylamide is used in extremely low amounts (only 0.01%-0.05% of the total monomer mass) and is uniformly dispersed in the monomer solution under stirring. In this system, the crosslinking agent molecule contains two double bonds, which can react with two polymer chains simultaneously during polymerization to form chemical crosslinking points. The extremely low amount results in a very low network crosslinking density and a loose structure, which significantly prolongs the diffusion path of water molecules and greatly slows down the release rate. This is the core structural basis for achieving 7-14 days of long-lasting sustained release. S202. Transfer the monomer solution to the polymerization reactor, purge with nitrogen for 30 minutes, heat to 60-65℃, add 0.3 to 0.8 parts of ammonium persulfate, continue to purge with nitrogen for protection, and keep the polymerization reaction at this temperature for 3-4 hours. The system gradually becomes viscous, and finally a transparent gel-like product is obtained. Take out the gel, cut it into small pieces, dry it to constant weight, crush and sieve it to obtain star-shaped hyperbranched superabsorbent resin. Following step S201, sodium acrylate, acrylamide, terminal allyl hyperbranched polyether, and N,N'-methylenebisacrylamide have been prepared into a solution. In this step, the prepared monomer solution is transferred to a polymerization reactor, and nitrogen gas is purged for 30 minutes to remove dissolved oxygen and prevent oxygen-induced polymerization inhibition. The temperature is then raised to 60-65°C, and 0.3 to 0.8 parts of ammonium persulfate (initiator) are added. Nitrogen purging continues under protective conditions. Under heating conditions, ammonium persulfate decomposes to generate sulfate radicals (SO42-). - The free radical attacks the carbon-carbon unsaturated bonds of the terminal double bonds in acrylic acid, acrylamide, and terminal allyl hyperbranched polyethers, initiating chain initiation reactions. After chain initiation, the monomer double bonds are opened, forming new free radical active centers, which continue to undergo chain growth reactions with surrounding monomers. Due to the high double bond activity of acrylic acid and acrylamide, the chain growth rate is fast, and the polymer chain is rapidly extended. During this process, the allyl double bonds at the end of the terminal allyl hyperbranched polyether have low polymerization activity due to their conjugated stability. They are mainly controlled to be incorporated into the polymer network through chain transfer, forming a star-shaped radial structure with hyperbranched polyether as the core and polyacrylic acid-acrylamide as the arms. The crosslinking agent N,N'-methylenebisacrylamide contains two double bonds, which can react with two polymer chains simultaneously during polymerization to form chemical crosslinking points. Since the amount of crosslinking agent used is only 0.01%-0.05% of the total monomer mass, the crosslinking density is extremely low, resulting in a relatively loose polymer network. This low degree of crosslinking is key to achieving subsequent slow water release. The specific reaction formula is as follows:

[0008] As the polymerization reaction proceeds, the viscosity of the system gradually increases, and the polymer chains transform from a solution state to a three-dimensional network gel. After 3-4 hours of heat preservation polymerization, the monomer conversion rate in the system can reach more than 90%, and finally a transparent gel-like product is obtained. The gel is taken out, cut into small pieces, and placed in a forced-air drying oven to dry to constant weight to remove moisture and unreacted monomers. The dried product is then pulverized with a pulverizer and passed through an 80-120 mesh sieve. The powder that passes through the sieve is taken to obtain a white powdery star-shaped hyperbranched superabsorbent resin. The number-average molecular weight of this resin is much lower than that of ordinary linear SAP. In addition, the hyperbranched structure itself has less intermolecular entanglement, which gives it low solution viscosity and high dispersibility. When it is added as dry powder to concrete, it has little effect on slump. At the same time, the low cross-linking network provides a structural basis for the long-term slow release of water. S3. Preparation of nano-lightly calcined magnesium oxide: Lightly calcined magnesium oxide was placed in a ball mill, and zirconium oxide grinding balls were added at a ball-to-material ratio of 3-5:1. The mixture was ball-milled at 300 to 500 rpm for 4-8 hours and then sieved to obtain nano-lightly calcined magnesium oxide with an average particle size of 50 to 200 nm. During ball milling, the grinding jar rotates at high speed while revolving around the planetary disk. The zirconia grinding balls generate complex throwing, impact, and frictional motions inside the jar. When the grinding balls impact the lightly calcined magnesium oxide particles at a certain speed, stress waves are generated inside the particles. When the stress exceeds the fracture strength of the particles, the particles break along the grain boundaries or microcracks, forming smaller fragments. As the ball milling time increases, the particles are repeatedly impacted, squeezed, and sheared, and their size continues to decrease. The ball-to-material ratio is controlled at 3-5:1, which ensures a sufficient amount of grinding media to provide sufficient collision energy while avoiding a decrease in grinding efficiency due to an excessively high ball-to-material ratio. The rotation speed of 300 to 500 rpm is at a medium to high level, which allows the grinding balls to obtain sufficient kinetic energy while avoiding excessive temperature rise or powder adhesion due to excessively high rotation speed. During the ball milling process described above, lightly calcined magnesium oxide readily absorbs moisture and carbon dioxide from the air. Therefore, the ball milling process should be carried out under a dry, inert atmosphere (such as nitrogen protection), or the ambient humidity should be strictly controlled to prevent hydration or carbonation side reactions on the surface of the nanoparticles. Furthermore, prolonged high-speed ball milling can raise the system temperature, potentially promoting localized hydration of magnesium oxide. Therefore, the ball milling time should not be too long, and intermittent operation or forced cooling methods should be used to control the tank temperature. The resulting nano-lightly calcined magnesium oxide has a small particle size, large specific surface area, and high surface activity, and is produced without any coating treatment, exhibiting high surface Mg content. 2+ The active sites are fully preserved, which provides an ideal chemical bonding basis for its subsequent dry compounding with star-shaped hyperbranched superabsorbent resin; S4, Dry compounding: By mass, 10-15 parts of star-shaped hyperbranched superabsorbent resin and 20-40 parts of nano-lightly calcined magnesium oxide are added into a high-speed mixer. The stirring speed is controlled at 1000-3000 rpm, and the mixture is dry-mixed at a constant temperature for 10-30 minutes to fully disperse and uniformly blend the two powder components, thus obtaining a concrete curing agent. In a high-speed mixer, the agitator rotates at a high speed of 1000 to 3000 rpm, causing the powder material to generate intense turbulent motion. Star-shaped hyperbranched superabsorbent resin particles (particle size approximately 100 to 150 micrometers) and nano-lightly calcined magnesium oxide particles (particle size 50 to 200 nanometers) collide, rub, and disperse with each other under the action of high-speed airflow and mechanical stirring. Because the nano-lightly calcined magnesium oxide particles are much smaller than the superabsorbent resin particles and their surfaces are rich in Mg... 2+ Active sites, while the surface of superabsorbent resin particles has carboxyl groups (-COO). - The two polar groups, such as ), are physically combined through electrostatic adsorption and van der Waals forces during high-speed mixing, and the nano magnesium oxide is uniformly attached to the surface of the superabsorbent resin particles or embedded in their surface pores. In the initial stage of mixing, the two powders are in a layered or locally aggregated state in the mixer. As stirring proceeds, the powders are thrown up and dispersed under the action of centrifugal force and shear force, forming a fluidized bed. The strong shearing action generated by the high speed (1000 to 3000 rpm) can effectively break up the soft agglomerates of nano magnesium oxide, so that it is uniformly dispersed between the superabsorbent resin powders in the form of single particles or small agglomerates. The constant temperature conditions (usually controlled at room temperature to 40°C) avoid local high temperature caused by frictional heat generation, and prevent the thermal degradation of superabsorbent resin or the reduction of surface activity of nano magnesium oxide. After mixing for 10 to 30 minutes, the two powders reach a macroscopically uniform blended state. The nano magnesium oxide particles are covered on the surface of the resin particles in a sub-single layer or multiple layers, without destroying the original water absorption network structure of the resin. The final curing agent is a homogeneous powder, in which star-shaped hyperbranched superabsorbent resin forms a continuous phase to create a water-absorbing framework, and nano-lightly calcined magnesium oxide is uniformly distributed between resin particles or adhered to the surface of resin particles as a dispersed phase. Since the entire composite process is carried out under anhydrous conditions, the Mg on the surface of the nano-lightly calcined magnesium oxide... 2+ The active sites are fully preserved, avoiding premature hydration. The dry compounding process is simple and efficient, ensuring the stability of the curing agent during storage and transportation, and facilitating water absorption, slow release, and interaction with Ca in the cement pore solution during subsequent concrete mixing. 2+ The ionic cross-linking effect laid the structural basis.

[0009] Preferably, the particle size of the nano-lightly calcined magnesium oxide is 50-200 nm, and the activity value is 100-200 s.

[0010] Preferably, the hydroxyl-terminated hyperbranched polyether is prepared by addition polymerization of a compound containing active hydrogen groups with ethylene oxide or propylene oxide in the presence of a catalyst, and has a molecular weight of 1000-2000 Da; the compound containing active hydrogen groups is glycerol or pentaerythritol.

[0011] Preferably, after the curing agent is added to the concrete, the carboxyl groups of the star-shaped hyperbranched superabsorbent resin branches react with the Ca in the cement pore solution. 2+ Reversible ionic cross-linking is formed, creating a dense shell on the surface of the maintenance agent particles, which delays the release of moisture.

[0012] Preferably, the curing agent is added as a dry powder admixture at a dosage of 0.1%-0.5% of the total mass of the cementitious materials. It is added together with the cementitious materials during concrete mixing, and an additional 3%-8% of the mass of the cementitious materials is added as compensating water. The compensating water is absorbed and stored by the curing agent and slowly released during the cement hydration process.

[0013] Compared with the prior art, the beneficial effects of the present invention are: (1) A star-shaped radial structure is formed by copolymerizing terminal allyl hyperbranched polyether with acrylic acid and acrylamide. Combined with the characteristics of low intermolecular entanglement and low solution viscosity of hyperbranched polymers, this solves the problems of severe slump loss and poor concrete flowability when using existing linear SAP dry powder as an admixture. This invention constructs a unique star-shaped topology using hyperbranched polyether as the core and polyacrylic acid-acrylamide as the arms. Under the same molecular weight conditions, this structure, due to the spherical radial distribution of molecular chains, has a much lower degree of entanglement than linear polymers, giving the superabsorbent resin low solution viscosity and excellent water dispersibility. When the dry powder is added, the resin particles disperse rapidly in the mixing water rather than agglomerate, significantly reducing the competitive absorption of mixing water. The initial slump loss of the concrete is controlled within 15%, far superior to the loss rate of over 30% of existing SAP, effectively ensuring the workability of the concrete.

[0014] (2) The low cross-linking network design and the reversible ionic cross-linking slow-release shell work synergistically to combine carboxyl groups with Ca in the cement pore solution. 2+ The reversible ionic crosslinking mechanism solves the problem of short water release cycles (only 1-3 days) in existing SAPs, which cannot match the water requirements of the entire cement hydration cycle. This invention controls the amount of crosslinking agent to 0.01%-0.05% of the total monomer mass, forming a loose three-dimensional network structure with a long water diffusion path and slow release rate. Simultaneously, the numerous carboxyl groups on the superabsorbent resin branches react with Ca in the pore solution within the alkaline environment of the concrete. 2+ Reversible ionic cross-linking occurs, forming a dense shell on the particle surface, further delaying moisture release. This dual slow-release mechanism extends the water release cycle of the curing agent to 7-14 days, perfectly matching the continuous water demand cycle of cement hydration, and significantly inhibiting the later autogenous shrinkage and drying shrinkage of concrete.

[0015] (3) A dry composite process is used to uniformly blend nano-lightly calcined magnesium oxide with star-shaped hyperbranched superabsorbent polymer (SAP). This, combined with a curing agent that absorbs water and releases it slowly to supply water to the expansion agent, and a synergistic mechanism where the hydration products of the expansion agent fill the water-releasing pores of the SAP, solves the problems of decreased concrete strength caused by existing SAP internal curing and incomplete hydration of the expansion agent in low water-cement ratio concrete. This invention, through high-speed dry mixing, ensures that nano-lightly calcined magnesium oxide is uniformly adhered to the surface of the superabsorbent polymer particles. Under anhydrous conditions, Mg… 2+ The active sites are fully preserved. During concrete mixing, the resin absorbs and stores compensating water internally. Water slowly released during cement hydration preferentially supplies the nano-magnesium oxide adhering to the resin surface, promoting its full hydration and formation of Mg(OH)₂ crystals. These crystals fill the pores left after the resin releases water in situ, compensating for both strength loss and subsequent shrinkage through volume expansion. Compared to traditional physical mixing, this invention achieves a synergistic effect of the chemical bonds between the expansive agent and SAP, fundamentally solving the technical problems of significant strength reduction and difficulty in suppressing subsequent shrinkage in internally cured concrete. Attached Figure Description

[0016] Figure 1 This invention provides a standard concrete compressive strength test cube specimen. Figure 2 The figures show the concrete compressive strength data of various embodiments and comparative examples under a water-cement ratio of 0.35 according to the present invention; Figure 3 The figures show the concrete compressive strength data of various embodiments and comparative examples under a water-cement ratio of 0.40 according to the present invention. Figure 4 The figures show the concrete compressive strength data of various embodiments and comparative examples under a water-cement ratio of 0.45 according to the present invention. Figure 5 The graph shows the drying shrinkage rate data of concrete in various embodiments and comparative examples under a water-cement ratio of 0.35 according to the present invention. Figure 6 The graph shows the drying shrinkage rate data of concrete in various embodiments and comparative examples under a water-cement ratio of 0.40 according to the present invention. Figure 7 The figures show the drying shrinkage rate data of concrete in various embodiments and comparative examples under a water-cement ratio of 0.45 according to the present invention. Detailed Implementation

[0017] The technical solutions of this invention are described below. It should also be noted that, to make the embodiments more detailed, the following embodiments are the best and preferred embodiments; those skilled in the art can also use other alternative methods to implement some well-known technologies.

[0018] The hydroxyl-terminated hyperbranched polyethers in the following examples and comparative examples were prepared by addition polymerization of glycerol and ethylene oxide in the presence of a catalyst, with a molecular weight of 1500 Da; the amount of crosslinking agent N,N'-methylenebisacrylamide was 0.01%-0.05% of the total mass of the monomers.

[0019] Example 1: Preparation method of concrete curing agent: S1. Preparation of terminal allyl hyperbranched polyether: By mass, 20 parts of terminal hydroxyl hyperbranched polyether, 0.3 parts of triethylamine, and 75 parts of toluene were added to a reaction vessel. Under nitrogen protection, the temperature was raised to 75°C, and the mixture was stirred and refluxed for 30 minutes. 10 parts of allyl glycidyl ether were slowly added dropwise at a rate of about 1 drop / second, and the addition was completed in about 30 minutes. After the addition was completed, the temperature was maintained at 75°C and the reaction was continued with stirring for 6 hours. The mixture was then distilled under reduced pressure to obtain a pale yellow viscous liquid, which is the terminal allyl hyperbranched polyether. S2. Synthesis of star-shaped hyperbranched superabsorbent resin: S201. By mass, 60 parts of acrylic acid are placed in a reaction vessel and cooled to 5°C in an ice-water bath. Under stirring, 30% sodium hydroxide solution is slowly added dropwise, and the system temperature is controlled not to exceed 30°C. The pH is adjusted to 7-8 to obtain sodium acrylate solution. 30 parts of acrylamide, 8 parts of terminal allyl hyperbranched polyether, and N,N'-methylenebisacrylamide are added to the solution. Then 250 parts of deionized water are added and stirred until completely dissolved to prepare a monomer solution. S202. Transfer the monomer solution to the polymerization reactor, purge with nitrogen for 30 min, heat to 60°C, add 0.5 parts of ammonium persulfate, continue to purge with nitrogen for protection, keep the temperature for polymerization reaction for 3 h, the system gradually becomes viscous, and finally a transparent gel product is obtained. Take out the gel, cut it into small pieces, dry to constant weight, crush and sieve to obtain star-shaped hyperbranched superabsorbent resin. S3. Preparation of nano-lightly calcined magnesium oxide: Lightly calcined magnesium oxide was placed in a ball mill, and zirconium oxide grinding balls were added at a ball-to-material ratio of 4:1. The mixture was ball-milled at 400 rpm for 6 hours and then sieved to obtain nano-lightly calcined magnesium oxide with an average particle size of 100 nm. S4, Dry compounding: By mass, 10 parts of star-shaped hyperbranched superabsorbent resin and 20 parts of nano-lightly calcined magnesium oxide were added into a high-speed mixer. The stirring speed was controlled at 2000 rpm, and the mixture was dry-mixed at a constant temperature for 20 minutes to fully disperse and uniformly blend the two powder components, thus obtaining a concrete curing agent.

[0020] Example 2: Preparation method of concrete curing agent: S1. Preparation of terminal allyl hyperbranched polyether: By mass, 20 parts of terminal hydroxyl hyperbranched polyether, 0.3 parts of triethylamine, and 75 parts of toluene were added to a reaction vessel. Under nitrogen protection, the temperature was raised to 75°C, and the mixture was stirred and refluxed for 30 minutes. 10 parts of allyl glycidyl ether were slowly added dropwise at a rate of about 1 drop / second, and the addition was completed in about 30 minutes. After the addition was completed, the temperature was maintained at 75°C and the reaction was continued with stirring for 6 hours. The mixture was then distilled under reduced pressure to obtain a pale yellow viscous liquid, which is the terminal allyl hyperbranched polyether. S2. Synthesis of star-shaped hyperbranched superabsorbent resin: S201. By mass, 60 parts of acrylic acid are placed in a reaction vessel and cooled to 5°C in an ice-water bath. Under stirring, 30% sodium hydroxide solution is slowly added dropwise, and the system temperature is controlled not to exceed 30°C. The pH is adjusted to 7-8 to obtain sodium acrylate solution. 30 parts of acrylamide, 8 parts of terminal allyl hyperbranched polyether, and N,N'-methylenebisacrylamide are added to the solution. Then 250 parts of deionized water are added and stirred until completely dissolved to prepare a monomer solution. S202. Transfer the monomer solution to the polymerization reactor, purge with nitrogen for 30 min, heat to 60°C, add 0.5 parts of ammonium persulfate, continue to purge with nitrogen for protection, keep the temperature for polymerization reaction for 3 h, the system gradually becomes viscous, and finally a transparent gel product is obtained. Take out the gel, cut it into small pieces, dry to constant weight, crush and sieve to obtain star-shaped hyperbranched superabsorbent resin. S3. Preparation of nano-lightly calcined magnesium oxide: Lightly calcined magnesium oxide was placed in a ball mill, and zirconium oxide grinding balls were added at a ball-to-material ratio of 4:1. The mixture was ball-milled at 400 rpm for 6 hours and then sieved to obtain nano-lightly calcined magnesium oxide with an average particle size of 100 nm. S4, Dry compounding: By mass, 10 parts of star-shaped hyperbranched superabsorbent resin and 40 parts of nano-lightly calcined magnesium oxide were added into a high-speed mixer. The stirring speed was controlled at 2000 rpm, and the mixture was dry-mixed at a constant temperature for 20 minutes to fully disperse and uniformly blend the two powder components, thus obtaining a concrete curing agent.

[0021] Example 3: Preparation method of concrete curing agent: S1. Preparation of terminal allyl hyperbranched polyether: By mass, 20 parts of terminal hydroxyl hyperbranched polyether, 0.3 parts of triethylamine, and 75 parts of toluene were added to a reaction vessel. Under nitrogen protection, the temperature was raised to 75°C, and the mixture was stirred and refluxed for 30 minutes. 10 parts of allyl glycidyl ether were slowly added dropwise at a rate of about 1 drop / second, and the addition was completed in about 30 minutes. After the addition was completed, the temperature was maintained at 75°C and the reaction was continued with stirring for 6 hours. The mixture was then distilled under reduced pressure to obtain a pale yellow viscous liquid, which is the terminal allyl hyperbranched polyether. S2. Synthesis of star-shaped hyperbranched superabsorbent resin: S201. By mass, 60 parts of acrylic acid are placed in a reaction vessel and cooled to 5°C in an ice-water bath. Under stirring, 30% sodium hydroxide solution is slowly added dropwise, and the system temperature is controlled not to exceed 30°C. The pH is adjusted to 7-8 to obtain sodium acrylate solution. 30 parts of acrylamide, 5 parts of terminal allyl hyperbranched polyether, and N,N'-methylenebisacrylamide are added to the solution. Then 250 parts of deionized water are added and stirred until completely dissolved to prepare a monomer solution. S202. Transfer the monomer solution to the polymerization reactor, purge with nitrogen for 30 min, heat to 60°C, add 0.5 parts of ammonium persulfate, continue to purge with nitrogen for protection, keep the temperature for polymerization reaction for 3 h, the system gradually becomes viscous, and finally a transparent gel product is obtained. Take out the gel, cut it into small pieces, dry to constant weight, crush and sieve to obtain star-shaped hyperbranched superabsorbent resin. S3. Preparation of nano-lightly calcined magnesium oxide: Lightly calcined magnesium oxide was placed in a ball mill, and zirconium oxide grinding balls were added at a ball-to-material ratio of 4:1. The mixture was ball-milled at 400 rpm for 6 hours and then sieved to obtain nano-lightly calcined magnesium oxide with an average particle size of 100 nm. S4, Dry compounding: By mass, 10 parts of star-shaped hyperbranched superabsorbent resin and 30 parts of nano-lightly calcined magnesium oxide were added into a high-speed mixer. The stirring speed was controlled at 2000 rpm, and the mixture was dry-mixed at a constant temperature for 20 minutes to fully disperse and uniformly blend the two powder components, thus obtaining a concrete curing agent.

[0022] Example 4: Preparation method of concrete curing agent: S1. Preparation of terminal allyl hyperbranched polyether: By mass, 20 parts of terminal hydroxyl hyperbranched polyether, 0.3 parts of triethylamine, and 75 parts of toluene were added to a reaction vessel. Under nitrogen protection, the temperature was raised to 75°C, and the mixture was stirred and refluxed for 30 minutes. 10 parts of allyl glycidyl ether were slowly added dropwise at a rate of about 1 drop / second, and the addition was completed in about 30 minutes. After the addition was completed, the temperature was maintained at 75°C and the reaction was continued with stirring for 6 hours. The mixture was then distilled under reduced pressure to obtain a pale yellow viscous liquid, which is the terminal allyl hyperbranched polyether. S2. Synthesis of star-shaped hyperbranched superabsorbent resin: S201. By mass, 60 parts of acrylic acid are placed in a reaction vessel and cooled to 5°C in an ice-water bath. Under stirring, 30% sodium hydroxide solution is slowly added dropwise, and the system temperature is controlled not to exceed 30°C. The pH is adjusted to 7-8 to obtain sodium acrylate solution. 30 parts of acrylamide, 8 parts of terminal allyl hyperbranched polyether, and N,N'-methylenebisacrylamide are added to the solution. Then 250 parts of deionized water are added and stirred until completely dissolved to prepare a monomer solution. S202. Transfer the monomer solution to the polymerization reactor, purge with nitrogen for 30 min, heat to 60°C, add 0.5 parts of ammonium persulfate, continue to purge with nitrogen for protection, keep the temperature for polymerization reaction for 3 h, the system gradually becomes viscous, and finally a transparent gel product is obtained. Take out the gel, cut it into small pieces, dry to constant weight, crush and sieve to obtain star-shaped hyperbranched superabsorbent resin. S3. Preparation of nano-lightly calcined magnesium oxide: Lightly calcined magnesium oxide was placed in a ball mill, and zirconium oxide grinding balls were added at a ball-to-material ratio of 4:1. The mixture was ball-milled at 400 rpm for 6 hours and then sieved to obtain nano-lightly calcined magnesium oxide with an average particle size of 100 nm. S4, Dry compounding: By mass, 10 parts of star-shaped hyperbranched superabsorbent resin and 30 parts of nano-lightly calcined magnesium oxide were added into a high-speed mixer. The stirring speed was controlled at 2000 rpm, and the mixture was dry-mixed at a constant temperature for 20 minutes to fully disperse and uniformly blend the two powder components, thus obtaining a concrete curing agent.

[0023] Comparative Example 1: Compared with Example 3, in Comparative Example 1, the terminal allyl hyperbranched polyether was replaced with polyethylene glycol of the same molecular weight, while other conditions remained unchanged.

[0024] Comparative Example 2: Compared with Example 3, no allyl hyperbranched polyether was added in Comparative Example 2, that is, no star-shaped hyperbranched superabsorbent resin was formed, and other conditions remained unchanged.

[0025] Comparative Example 3: Compared with Example 3, no acrylamide was added in Comparative Example 3, and other conditions remained unchanged.

[0026] Comparative Example 4: Compared with Example 3, Comparative Example 4 used ordinary magnesium oxide instead of nano-lightly calcined magnesium oxide, while other conditions remained unchanged.

[0027] Performance testing: According to the "Standard for Test Methods of Performance of Ordinary Concrete Mixtures" GB / T 50080-2016, the initial slump and slump loss over 1 hour of fresh concrete were determined using the slump method. According to the "Standard for Test Methods of Mechanical Properties of Ordinary Concrete" GB / T 50081-2019, 100mm × 100mm × 100mm cubic specimens were formed (e.g., Figure 1 As shown in the figure, the concrete was cured under standard curing conditions for 3d, 7d, 14d, and 28d, and the compressive strength at each age was determined using a compression testing machine. Referring to the "Standard for Test Methods of Long-Term Performance and Durability of Ordinary Concrete" GB / T 50082-2009, the drying shrinkage rate of the concrete at 3d, 7d, 14d, and 28d was determined using the contact method. The specimen size was 100mm×100mm×515mm, and the length change was measured under constant temperature and humidity conditions (temperature 20±2℃, relative humidity 60±5%). Referring to Appendix A and Appendix B of "Cement Concrete Curing Agent" JC / T 901-2025, the effective water retention rate of the curing agent was determined. After weighing the initial mass of the concrete specimen, it was placed under standard curing conditions and cured for 3d, 7d, 14d, and 28d, respectively. The water retention rate at each age was then calculated.

[0028] Three water-cement ratio levels were set for the experiment: 0.35, 0.40, and 0.45. For each water-cement ratio, a baseline group (without curing agent) and an experimental group (with curing agent) were established. The curing agent dosage was 0.2% of the total mass of the cementitious materials. The amounts of cement, sand, aggregate, and water-reducing agent in the concrete mix design remained consistent; only the water content was adjusted to achieve the target water-cement ratio. Additionally, the experimental group received an extra 5% (by mass of cementitious materials) of compensating water. Three specimens were formed from each mix design, and the average value of the test results was taken.

[0029] The test results are shown in Tables 1, 2, and 3.

[0030] Table 1. Slump and water retention rate data of fresh concrete in various examples and comparative examples under different water-cement ratios.

[0031] Slump and water retention analysis: The baseline group exhibited the highest initial slump at all water-cement ratios, and this slump increased with increasing water-cement ratio, which is consistent with the basic workability characteristics of ordinary concrete. After the addition of the curing agent, the initial slump of all test groups was lower than that of the baseline group, but the degree of reduction varied significantly due to differences in the components.

[0032] Examples 1 to 4 exhibited excellent slump retention. Examples 1 and 4 showed the same slump loss, but Example 1 had a slightly higher initial slump. Therefore, Example 1 had the lowest slump loss rate at all water-cement ratios, with a slump retention rate exceeding 90% after 1 hour. This is attributed to the star-shaped radial structure constructed from terminal allyl hyperbranched polyethers. The low intermolecular entanglement and low solution viscosity of the hyperbranched polymer significantly reduced its competitive absorption of mixing water when the dry powder was added, effectively protecting the initial fluidity of the concrete. As the water-cement ratio increased from 0.35 to 0.45, the slump of all groups improved, but the advantage of the Example 1 group remained consistent.

[0033] Comparative Example 2, lacking the hyperbranched polyether component entirely, exhibited the lowest initial slump and the most severe slump loss after 1 hour, with a slump loss rate exceeding 17% at all water-to-binder ratios. This confirms the inherent defect of decreased flowability caused by the dry mixing of linear SAP. Comparative Example 1, using ordinary polyethylene glycol to replace the hyperbranched polyether, showed some improvement but was still significantly inferior to the example group, indicating that the special topological morphology of the hyperbranched structure is key to its viscosity reduction and flow enhancement.

[0034] Regarding water retention, the 3-day water retention of the example groups remained above 90%, and increased slightly with increasing water-cement ratio. Comparative Example 2 had the lowest water retention, indicating that the SAP network structure lacking hyperbranched cores was loose and had poor water retention capacity. The water retention of Comparative Examples 3 and 4 were in the middle, indicating that the charge dilution effect of acrylamide and the surface activity of nano-lightly calcined magnesium oxide both contributed to the water retention performance.

[0035] Compressive strength analysis: The compressive strength of the reference group decreased with increasing water-cement ratio, which is consistent with the water-cement ratio rule. The 28-day compressive strength of Examples 1 to 4 at various water-cement ratios was close to or slightly higher than that of the reference group, and the strength ratio remained between 97% and 103%, indicating that the curing agent of the present invention has no significant negative impact on the mechanical properties of concrete, and may even slightly improve them.

[0036] Example 4 exhibited the highest 28-day compressive strength at all water-cement ratios. This result can be explained in two ways: first, the hyperbranched star-shaped structure endows SAP with low viscosity, allowing it to disperse uniformly in concrete and avoiding stress concentration caused by local agglomeration; second, the nano-lightly calcined magnesium oxide fully hydrates under the action of SAP's slow-release water to form Mg(OH)2 crystals, which fill the pores left after SAP releases water in situ, effectively compensating for the strength loss caused by porosity. The introduction of acrylamide modulates the network charge density, allowing the polymer network to expand moderately, and also providing a structural basis for maintaining strength.

[0037] Comparative Example 2 showed the most significant strength decrease, with its 28-day strength only about 87% of the baseline group. This confirms that the large pores left after the release of water from the SAP lacking hyperbranched cores cannot be effectively filled, becoming weak areas. Comparative Example 3, due to the absence of acrylamide, had an excessively high network charge density, which interacted with the Ca in the cement pore solution. 2+ The ionic cross-linking reaction was too vigorous, causing the network to shrink and collapse prematurely, resulting in a significant decrease in strength. Comparative Example 4 used ordinary magnesium oxide instead of nano-lightly calcined magnesium oxide, but due to its large particle size, small specific surface area, and insufficient hydration activity, it could not effectively fill the pores, and the strength compensation effect was limited.

[0038] Drying shrinkage analysis: The drying shrinkage of the baseline group continued to increase with age, and the shrinkage values ​​at all water-cement ratios were at a high level at 28 days, which is a typical characteristic of ordinary concrete. After the addition of the curing agent, the shrinkage rate of all example groups was significantly reduced, and the shrinkage reduction effect became more obvious with the extension of curing age.

[0039] Example 4 exhibited the best shrinkage reduction effect at all water-cement ratios and curing ages, with a 28-day drying shrinkage reduction rate of over 53%. This effect stems from the synergistic effect of multiple mechanisms: the low crosslinking degree network of SAP enables slow moisture release, maintaining the relative humidity inside the concrete; the multi-arm crosslinking points provided by the hyperbranched core stabilize the network structure; and the volume expansion generated by the hydration of nano-lightly calcined magnesium oxide compensates for drying shrinkage. As the water-cement ratio increased, the shrinkage values ​​of each group decreased, but the relative advantage of the example group remained stable.

[0040] Comparative Example 2 showed the most severe shrinkage, with a 28-day drying shrinkage reduction rate of less than 20%, indicating that without the hyperbranched structure, the SAP's water release cycle is short and the expanding agent lacks chemical bonding, thus failing to achieve effective shrinkage compensation. Comparative Examples 1 and 3 showed shrinkage reduction effects in between, further confirming the crucial roles of the hyperbranched core and acrylamide in shrinkage inhibition.

[0041] Table 2. Concrete compressive strength (MPa) data of each example and comparative example under different water-cement ratios.

[0042] According to Table 2, Figure 2 , Figure 3 ,as well as Figure 4 As shown, the overall analysis of the above concrete compressive strength test results is as follows: The compressive strength of each group decreased with increasing water-cement ratio and increased with increasing curing age, consistent with the basic strength development law of concrete. Under the same water-cement ratio and curing age, the strength differences among the groups clearly reflect the influence of different curing agent components on the mechanical properties of concrete.

[0043] Under a water-cement ratio of 0.35, the 28-day strength of the baseline group was at a moderate level. Example 4 showed the highest 28-day strength, followed by Examples 1 and 3, while Example 2 was slightly lower but still close to the baseline group. This result indicates that the star-shaped radial structure constructed by the terminal allyl hyperbranched polyether ensures uniform dispersion of the curing agent in the concrete, avoiding stress concentration caused by local agglomeration. Simultaneously, the nano-lightly calcined magnesium oxide fully hydrates under the action of SAP slow-release water to generate Mg(OH)2 crystals, which fill the pores left after SAP water release in situ, effectively compensating for strength loss. Example 4 showed the best strength compensation effect due to the most reasonable SAP-MgO ratio. This may be because in the early stages of concrete mixing, SAP absorbs some mixing water, indirectly reducing the water-cement ratio of the concrete, and releases water in the later stages of cement hydration, increasing the degree of hydration and concrete density, thereby improving strength. Example 3, due to the lower amount of terminal allyl hyperbranched polyether, had a slightly weaker dispersion effect of the star-shaped structure, resulting in slightly lower strength than Example 4. Example 2 has a slightly lower strength because the amount of MgO used is relatively small and the pores are not fully filled.

[0044] Comparative Example 2 showed the lowest strength, with a significant decrease in 28-day strength compared to the baseline group. This is because the large pores left after the release of water from ordinary SAP, lacking hyperbranched cores, could not be effectively filled, becoming weak areas. Furthermore, ordinary SAP exhibits poor dispersibility and agglomeration in concrete, further exacerbating the strength loss. Comparative Example 1, using ordinary polyethylene glycol instead of hyperbranched polyether, showed some improvement in strength but was still significantly lower than the Example group, indicating that ordinary linear polymers cannot provide the steric hindrance effect and low viscosity characteristics unique to hyperbranched structures. Comparative Example 3, lacking acrylamide, had an excessively high network charge density, resulting in an overly vigorous crosslinking reaction with Ca²⁺ ions in the cement pore solution, leading to premature network shrinkage and collapse, and significantly lower strength than the Example group. Comparative Example 4, using ordinary magnesium oxide instead of nano-lightly calcined magnesium oxide, showed better strength than the previous three comparative examples but still lower than the Example group. This is because ordinary magnesium oxide particles are large, have insufficient hydration activity, and limited pore-filling effect.

[0045] Under water-cement ratios of 0.40 and 0.45, the strength differences among the groups were largely consistent with those at a water-cement ratio of 0.35, but the absolute strength values ​​all decreased. As the water-cement ratio increased, the strength of the reference group decreased significantly, while the strength advantage of the example group relative to the reference group remained stable. This advantage was particularly pronounced at the low water-cement ratio of 0.35, highlighting the most valuable application area for internal curing technology. The risk of autogenous shrinkage and cracking in high-strength concrete with low water-cement ratios needs to be controlled using SAP internal curing agents.

[0046] It is worth noting that the early strength of the example group was slightly lower than that of the baseline group at 3 days, but gradually approached or even surpassed it after 7 days. This is because the SAP absorbs some free water in the early stage of mixing, slightly reducing the water involved in early hydration; as the hydration process progresses, the SAP releases the stored water, promoting the continuous hydration of the cement, thus compensating for or even improving the strength in the middle and later stages. The comparative group lacks this middle and later stage strength compensation mechanism, and the strength gap further widens with the extension of age.

[0047] Table 3. Drying shrinkage rate of concrete in each example and comparative example under different water-cement ratios (×10) -6 )data

[0048] According to Table 3, Figure 5 , Figure 6 , Figure 7 As shown, the overall analysis of the above concrete drying shrinkage rate test results is as follows: The drying shrinkage rate of all concrete groups increased continuously with age and decreased with increasing water-cement ratio, consistent with the basic law of concrete shrinkage. Under the same water-cement ratio and age conditions, the differences in shrinkage rate among the groups clearly reflect the influence of different curing agent components on the volume stability of concrete.

[0049] Under a water-cement ratio of 0.35, the baseline group exhibited the highest shrinkage rate at all ages, with the largest shrinkage at 28 days, a typical manifestation of the lack of effective internal curing in ordinary concrete. The shrinkage rates of Examples 1 to 4 were significantly lower than the baseline group, with Example 4 showing the lowest shrinkage rate at all ages, and a 28-day shrinkage rate approximately 53% lower than the baseline group. This superior shrinkage reduction effect is attributed to the synergistic effect of multiple mechanisms: the star-shaped radial structure constructed from terminal allyl hyperbranched polyether imparts low viscosity and high dispersibility to SAP, allowing it to be uniformly distributed in concrete; the extremely low crosslinking degree design results in a loose network structure and a long moisture diffusion path, achieving long-term sustained release for 7 to 14 days; nano-lightly calcined magnesium oxide absorbs the moisture released by SAP and hydrates to form Mg(OH)2 crystals, causing volume expansion to compensate for drying shrinkage; the branched carboxyl groups of SAP react with Ca in the cement pore solution... 2+The reversible ionic cross-linked shell further delayed moisture release. The shrinkage rate of Example 1 was slightly higher than that of Example 4 but still much lower than the baseline group, confirming the crucial role of the hyperbranched core in shrinkage inhibition. Example 2, due to its relatively low MgO content, suffered from insufficient expansion compensation, resulting in a slightly higher shrinkage rate. Example 3, due to its lower hyperbranched polyether content, exhibited a weakened dispersion effect of the star-shaped structure, with a shrinkage inhibition effect between that of Examples 1 and 2.

[0050] Comparative Example 2 showed the most severe shrinkage, with shrinkage rates at all ages approaching the baseline level, and the 28-day shrinkage rate being approximately 76% higher than that of Example 4. This is because the ordinary SAP, lacking a hyperbranched core, has a short water release cycle, poor dispersibility, and lacks chemical bonding between the expanding agent and the SAP, thus failing to achieve effective shrinkage compensation. Comparative Example 1, using ordinary polyethylene glycol instead of hyperbranched polyether, showed some improvement in shrinkage rate but was still significantly higher than the Example group, indicating that ordinary linear polymers cannot provide the steric hindrance effect and network stability unique to hyperbranched structures. Comparative Example 3, due to the absence of acrylamide, had an excessively high network charge density, which, combined with Ca... 2+ The ionic crosslinking reaction was too vigorous, causing the network to shrink and collapse prematurely, resulting in limited shrinkage inhibition. Comparative Example 4 used ordinary magnesium oxide instead of nano-lightly calcined magnesium oxide. Due to its coarse particles and insufficient hydration activity, the expansion compensation effect was not as good as that of nano-lightly calcined magnesium oxide, and the shrinkage rate was higher than that of the Example Group.

[0051] Under water-cement ratios of 0.40 and 0.45, the shrinkage differences among the groups were generally consistent with those at a water-cement ratio of 0.35, but the absolute shrinkage values ​​were all reduced. As the water-cement ratio increased, the shrinkage decrease in the baseline group was relatively gradual, while the shrinkage reduction advantage of the example group relative to the baseline group remained stable. In particular, the shrinkage reduction effect of the example group was more prominent under the low water-cement ratio condition of 0.35. This is precisely the area where internal curing technology has the greatest application value, as the risk of autogenous shrinkage and cracking in high-strength concrete with low water-cement ratios is the most severe.

[0052] The above description is only a preferred embodiment of the present invention. It should be noted that for those skilled in the art, several improvements and modifications can be made without departing from the principle of the present invention, and these improvements and modifications should also be considered within the scope of protection of the present invention.

Claims

1. A concrete curing agent, characterized in that, A star-shaped hyperbranched superabsorbent polymer (SAP) formed by copolymerizing terminal allyl hyperbranched polyether with acrylic acid and acrylamide is dry-composite with nano-lightly calcined magnesium oxide. The star-shaped hyperbranched SAP has a radial structure with terminal hydroxyl hyperbranched polyether as the core and polyacrylic acid-acrylamide as the arms. The surface of the nano-lightly calcined magnesium oxide is untreated, retaining the Mg... 2+ Active site.

2. The concrete curing agent according to claim 1, characterized in that, The preparation method of the terminal allyl hyperbranched polyether is as follows: By mass, 20 parts of terminal hydroxyl hyperbranched polyether, 0.2-0.5 parts of triethylamine, and 50-100 parts of toluene were added to a reaction vessel. Under nitrogen protection, the temperature was raised to 70-80°C, and the mixture was stirred and refluxed for 30 minutes. Then, 10 to 15 parts of allyl glycidyl ether were slowly added dropwise. After the addition was complete, the temperature was maintained at 70-80°C and the reaction was continued with stirring for 5-8 hours. The toluene and unreacted monomers were removed by vacuum distillation to obtain a pale yellow viscous liquid, which is the terminal allyl hyperbranched polyether.

3. The concrete curing agent according to claim 1, characterized in that, The nano-lightly calcined magnesium oxide has a particle size of 50-200 nm and an activity value of 100-200 s.

4. The concrete curing agent according to claim 1, characterized in that, The mass ratio of the star-shaped hyperbranched superabsorbent resin to nano-lightly calcined magnesium oxide is (10-15):(20-40).

5. A concrete curing agent according to claim 1, characterized in that, The hydroxyl-terminated hyperbranched polyether is prepared by addition polymerization of a compound containing active hydrogen groups with ethylene oxide or propylene oxide in the presence of a catalyst, and has a molecular weight of 1000-2000 Da; the compound containing active hydrogen groups is glycerol or pentaerythritol.

6. A concrete curing agent according to claim 1, characterized in that, After the curing agent is added to the concrete, the carboxyl groups of the star-shaped hyperbranched superabsorbent resin branches react with the Ca in the cement pore solution. 2+ Reversible ionic cross-linking is formed, creating a dense shell on the surface of the maintenance agent particles, which delays the release of moisture.

7. A method for preparing a concrete curing agent as described in any one of claims 1-6, characterized in that, It also includes the following steps: S1. Synthesis of star-shaped hyperbranched superabsorbent resin: S101. By mass, acrylic acid is placed in a reaction vessel and cooled to 0-10°C in an ice-water bath. Under stirring, 30% sodium hydroxide solution is slowly added dropwise, controlling the system temperature to not exceed 30°C. The pH is adjusted to 7-8 to obtain sodium acrylate solution. Acrylamide, terminal allyl hyperbranched polyether, and N,N'-methylenebisacrylamide are added to the solution, followed by 200 to 300 parts of deionized water. The solution is stirred until completely dissolved to prepare a monomer solution. S102. Transfer the monomer solution to the polymerization reactor, purge with nitrogen for 30 minutes, heat to 60-65℃, add 0.3 to 0.8 parts of ammonium persulfate, continue to purge with nitrogen for protection, and keep the polymerization reaction at this temperature for 3-4 hours. The system gradually becomes viscous, and finally a transparent gel-like product is obtained. Take out the gel, cut it into small pieces, dry it to constant weight, crush and sieve it to obtain star-shaped hyperbranched superabsorbent resin. S2. Preparation of nano-lightly calcined magnesium oxide: Lightly calcined magnesium oxide was placed in a ball mill, and zirconium oxide grinding balls were added at a ball-to-material ratio of 3-5:

1. The mixture was ball-milled at 300 to 500 rpm for 4-8 hours and then sieved to obtain nano-lightly calcined magnesium oxide with an average particle size of 50 to 200 nm. S3, Dry compounding: Star-shaped hyperbranched superabsorbent resin and nano-lightly calcined magnesium oxide are put into a high-speed mixer, and the stirring speed is controlled at 1000-3000 rpm. The mixture is then dry-mixed at a constant temperature for 10-30 minutes to fully disperse and uniformly blend the two powder components, thus obtaining a concrete curing agent.

8. The method for preparing a concrete curing agent according to claim 7, characterized in that, In the star-shaped hyperbranched superabsorbent resin, the mass ratio of terminal allyl hyperbranched polyether: acrylic acid: acrylamide is (5-12):(50-65):(20-40), and the amount of crosslinking agent N,N'-methylenebisacrylamide is 0.01%-0.05% of the total mass of monomers.

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