Anti-shrinkage and reinforcing dual-purpose concrete internal curing agent and preparation method thereof

Abstract

This invention discloses a dual-effect concrete internal curing agent that combines shrinkage resistance and reinforcement, and its preparation method. Belonging to the field of building materials technology, the internal curing agent is composed of 100 parts phosphoric acid, 100 parts calcium hydroxide, 15-30 parts anionic or nonionic polyacrylamide, 20-30 parts sodium polyacrylate, 20-40 parts maleic anhydride (MA), 20-40 parts hydroxypropyl methylcellulose, 50 parts amorphous nano-silica particles with an average particle size of 20-50 nm, 5 parts defoamer, and 800 parts deionized water. In preparation, calcium hydroxide is first dissolved in water, then phosphoric acid is added and reacted at 60-80℃ until the pH value reaches 5.5-6.5. Then, nano-silica particles are added sequentially for dispersion, followed by HPMC and PAM, and stirring to obtain the final product. This invention effectively inhibits concrete shrinkage through the synergistic effect of organic polymers, inorganic minerals, and nanomaterials. The 60-day drying shrinkage rate is reduced by up to 77.25%, and the compressive strength is increased by 9.57%. As a water-based preparation, it can be directly incorporated into concrete and is suitable for natural curing in high-altitude and plateau regions.

Description

Technical Field

[0001] This invention belongs to the technical field of building materials and concrete admixtures, specifically relating to a dual-effect concrete internal curing agent that provides both shrinkage resistance and reinforcement, its preparation method, and its application. Background Technology

[0002] Concrete, as the most widely used material in modern construction engineering, has a significant impact on its workability and physical and mechanical properties during the curing process. With the advancement of high-strength and high-performance concrete preparation technology, high-performance concrete is showing a trend towards lower water-cement ratios and higher cementitious material content. This leads to a rapid decrease in the relative humidity inside the concrete during cement hydration, and the dense structure makes it difficult to effectively replenish the lost moisture. This easily causes the concrete to undergo autogenous shrinkage and drying shrinkage, which in turn induces early cracking and seriously affects the durability and safety of concrete structures.

[0003] Traditional curing methods mainly include water spraying, covering with film, or spraying curing agents. However, in large-scale projects such as super high-rise buildings, bridge piers, and dam construction, traditional water spraying is difficult to penetrate into the interior of the concrete structure, and manual high-altitude operations pose significant safety risks; covering curing is complicated to operate and has the problem of ineffective curing in blind spots.

[0004] Existing internal curing technologies mostly employ superabsorbent polymer (SAP) or lightweight aggregate (LWA). While SAP can alleviate the water requirement for hardening in high-performance concrete, its expansion after absorbing water leaves large pores within the concrete. Different materials also affect its water absorption rate, making it difficult to ensure sufficient cement hydration. LWA, due to its low strength, can negatively impact the mechanical properties of concrete, reducing its overall strength. Therefore, there is an urgent need to develop an internal curing agent that can effectively inhibit concrete autogenous shrinkage while simultaneously filling micropores and improving mechanical properties. Summary of the Invention

[0005] To address the shortcomings of the existing technology, the purpose of this invention is to provide a dual-effect concrete internal curing agent that combines anti-shrinkage and strengthening properties, and its preparation method. This curing agent, through the synergistic effect of organic polymers and modified inorganic materials, prevents early drying shrinkage of concrete and the generation of microcracks. It improves the water retention capacity and the density of concrete, thereby reducing the risk of early shrinkage and promoting strength development.

[0006] The technical solution of the present invention is as follows: A dual-effect concrete internal curing agent that provides both shrinkage resistance and reinforcement is made from the following raw materials in parts by weight: 100 parts phosphoric acid, 100 parts calcium hydroxide, 15-30 parts polyacrylamide, 20-30 parts sodium polyacrylate, 20-40 parts maleic anhydride, 20-40 parts hydroxypropyl methylcellulose, 50 parts amorphous nano-silica particles, 800 parts deionized water, and 5 parts defoamer; wherein the polyacrylamide is anionic or nonionic with a molecular weight of 5 million to 12 million; the sodium polyacrylate has a molecular weight of 8 million to 12 million; and the amorphous nano-silica particles have an average particle size of 20-50 nm.

[0007] The preparation method of the above-mentioned internal maintenance agent includes the following steps: Step 1: Add 800 parts of deionized water to the reaction vessel, slowly add 100 parts of calcium hydroxide powder to the deionized water, stir for 30 minutes to obtain a calcium hydroxide suspension; Step 2: Slowly add 100 parts of phosphoric acid into the reaction vessel, control the reaction temperature at 60-80℃, and stir continuously for 60 minutes to allow the phosphoric acid to react with calcium hydroxide to form calcium phosphate precipitate, resulting in a mixed solution with a pH of 5.5-6.5. Step 3: Add 50 parts of amorphous nano silica particles to the mixed solution and disperse them at a high speed of 2000 r / min for 30 minutes. Step 4: Keep the solution temperature at 60-80℃, add 20-40 parts of maleic anhydride to the reaction vessel, stir for 60 minutes, then lower the temperature to 30-40℃, slowly add 20-40 parts of hydroxypropyl methylcellulose, and stir for 60 minutes. Step 5: Control the reaction temperature to 20-50℃, add 15-30 parts of polyacrylamide and 20-30 parts of sodium polyacrylate to the reactor, stir continuously for 150 minutes, cool to room temperature, add defoamer and stir for 30 minutes to obtain the anti-shrinkage and reinforcing dual-effect concrete internal curing agent.

[0008] Furthermore, the phosphoric acid is provided in the form of an 85% phosphoric acid solution, and the calcium hydroxide is in powder form with a fineness ≥300 mesh, and the weight ratio of the phosphoric acid solution to the calcium hydroxide is 1:1.

[0009] Furthermore, the specific surface area of ​​the amorphous nano-silica particles is 120–400 m². 2 / g, silica content ≥98wt%.

[0010] Furthermore, the maleic anhydride is in the form of a white crystalline powder with a purity ≥99% and a molecular weight of 98.06 g / mol; the defoamer is a polyether-modified silicone defoamer or a polyether-based defoamer.

[0011] Furthermore, the hydroxypropyl methylcellulose is prepared by alkalization and etherification of refined cotton as cellulose raw material, and contains methoxy and hydroxypropoxy groups on the molecular chain, with a moisture content of ≤5%.

[0012] Furthermore, a concrete material containing the aforementioned internal curing agent comprises cement, fly ash, aggregate, water, and the aforementioned internal curing agent, wherein the mass ratio of the internal curing agent to the cementitious material is 5:100.

[0013] Furthermore, the concrete material comprises, by weight: 168 parts water, 280 parts cement, 120 parts fly ash, 843 parts sand, 1013 parts crushed stone coarse aggregate, 20 parts internal curing agent, and 5 parts polycarboxylate superplasticizer.

[0014] Hydroxypropyl methylcellulose (HPMC) molecules contain numerous hydroxyl and ether groups. These polar groups can form hydrogen bonds with water molecules, binding free water around the molecular chain, reducing water loss, and ensuring sufficient moisture around cement particles for complete cement hydration. The methoxy and hydroxypropyl groups on the HPMC molecular chain interact with calcium and aluminum ions in the mortar to form a viscous gel that fills the voids in the cement mortar. When the internal humidity of the concrete decreases, this gel layer slowly releases the stored water, ensuring the cement's hydration, hardening, and full strength development, improving the mortar's bond strength, and effectively reducing plastic shrinkage cracks. Furthermore, after hydration, HPMC forms a thin latex film between itself and the cement particles. This film has a sealing effect, improving the mortar's fluidity during mixing, giving it good plasticity and flexibility, and reducing shrinkage deformation.

[0015] Maleic anhydride (MA) contains a dicarboxyl group structure, which hydrolyzes to produce maleic acid, providing strong adsorption capacity. The anhydride groups on the maleic anhydride molecule can undergo esterification reactions with alcohols, offering abundant molecular design space for the synthesis of novel admixtures. It can enhance the water retention of concrete, effectively slow down internal moisture evaporation, and ensure complete cement hydration. Furthermore, maleic anhydride can reduce concrete viscosity and shrinkage stress, and through synergistic effects with other components, slow down the rate of water loss from concrete.

[0016] This invention introduces polyacrylamide (PAM) composite hydrogel as an internal curing agent for concrete because it is less prone to ionization in alkaline environments, exhibiting more stable and ideal swelling behavior. The silica contained in the polyacrylamide composite hydrogel enhances its ability to absorb liquids, promoting water absorption and release, and effectively inhibiting the self-drying of concrete. Furthermore, incorporating polyacrylamide hydrogel into concrete with the same water-cement ratio helps generate CSH and CASH gel products. The generated CSH gel and expansive ettringite (AFt) products effectively fill internal pores, compensate for voids left after gel dehydration, improve the microstructure of the cement paste, and enhance the mechanical and durability properties of the concrete.

[0017] The beneficial effects of this invention are as follows: 1. Significantly Improved Shrinkage Resistance: Polyacrylamide (PAM), sodium polyacrylate, and hydroxypropyl methylcellulose (HPMC) form a high-viscosity three-dimensional network structure in water, capable of absorbing a large amount of water. During the concrete hardening process, as the ambient humidity decreases, the water in this network is slowly released, compensating for the water consumed during concrete hydration, maintaining a high humidity environment inside the concrete, and effectively inhibiting autogenous shrinkage and drying shrinkage. Experimental results show that after incorporating the internal curing agent of this invention, the 60-day drying shrinkage rate of concrete can be reduced by up to approximately 77.25% compared to the control group (Comparative Example 1) without the curing agent.

[0018] 2. Enhanced Mechanical Properties: Nano-silica can fill the tiny pores inside concrete and react with the hydration product calcium hydroxide to form CSH gel, increasing the density of concrete. Simultaneously, the calcium phosphate precipitate formed by the reaction of phosphoric acid and calcium hydroxide further fills the pores left after the polymer releases moisture. Experimental results show that after incorporating the internal curing agent of this invention, the 60-day compressive strength of concrete can be increased by up to approximately 9.57% compared to the control group (Comparative Example 1).

[0019] 3. Excellent water retention performance: The internal curing agent of this invention, through the synergistic effect of slow-release water supply from the polymer network structure and pore filling by inorganic components, effectively slows down the evaporation and loss of moisture from the concrete to the external environment. Experimental results show that after incorporating the internal curing agent of this invention, the water loss of concrete after 60 days can be reduced by up to approximately 34.78% compared to the control group (Comparative Example 1).

[0020] 4. Convenient and safe construction: This invention is a water-based preparation that will not pollute the environment. It can be directly mixed into concrete mixtures and then cured naturally without the need for manual watering or covering. This eliminates the tedious later curing process and is especially suitable for tall structures and other parts that are difficult to cure manually, as well as cold and dry plateau areas, which greatly reduces the safety risks of personnel working at heights.

[0021] 5. Improved microstructure: SEM observation revealed that after adding the internal curing agent of this invention, the CSH gel in the concrete hydration products changed from a loose network to a more compact felt-like or honeycomb structure, the ettringite (AFt) crystals became shorter and thicker and more evenly distributed, the internal cracks of the concrete were significantly reduced, and the matrix became more compact.

[0022] 6. Significant synergistic effect: Comparative analysis shows that the reduction in shrinkage, increase in compressive strength, and reduction in water loss of the full-component formulation of this invention are 2.13 times, 2.69 times, and 1.95 times the theoretical upper limit of the superposition effect of each default component formulation, respectively. This proves that there is a significant synergistic effect among the components of calcium phosphate, nano silica, polyacrylamide, sodium polyacrylate, maleic anhydride, and hydroxypropyl methylcellulose. The overall effect far exceeds the simple superposition of the individual contributions of each component. Attached Figure Description

[0023] To make the technical solutions of the embodiments of the present invention clearer and easier to understand, the accompanying drawings will be briefly described below in conjunction with the accompanying drawings.

[0024] Figure 1 This is a photograph of the internal maintenance agent prepared in Example 5 of the present invention.

[0025] Figure 2 The images shown are SEM micrographs of the concrete hydration products prepared in Comparative Example 1 and Example 5 of the present invention; where (a) is Comparative Example 1 and (b) is Example 5.

[0026] Figure 3 These are test diagrams of the compressive strength of concrete prepared in Comparative Examples 1-4 and Examples 1-5 of the present invention.

[0027] Figure 4 The figures show the drying shrinkage performance test results of concrete prepared in Comparative Examples 1-4 and Examples 1-5 of this invention.

[0028] Figure 5 The figures show the water loss rate test results of concrete prepared in Comparative Examples 1-4 and Examples 1-5 of this invention.

[0029] Figure 6 The above are the concrete workability rating charts prepared in Comparative Examples 1-4 and Examples 1-5 of the present invention. Detailed Implementation

[0030] The present invention will be clearly and completely described below with reference to specific embodiments. The embodiments described below are only some embodiments of the present invention, not all embodiments, and are used only to illustrate the present invention, and should not be considered as limiting the scope of the present invention. All other embodiments obtained by those skilled in the art based on the embodiments of the present invention without creative effort are within the scope of protection of the present invention.

[0031] The concrete in this invention comprises the following components by weight: 168 parts water, 280 parts cement, 120 parts fly ash, 843 parts sand, 1013 parts crushed stone coarse aggregate, 0-20 parts internal curing agent, and 5 parts polycarboxylate superplasticizer.

[0032] The hydroxypropyl methylcellulose (HPMC) used in the various embodiments of the present invention is a commercially available cellulose ether product, which is obtained by alkalization and etherification processes of refined cotton. The molecular chain contains methoxy and hydroxypropoxy groups, and the moisture content is ≤5%.

[0033] The amorphous nano-silica particles used in the various embodiments of this invention are prepared by chemical precipitation. Using sodium silicate solution and sulfuric acid solution as raw materials, the particles undergo precipitation reaction, aging, pressure filtration and washing, spray drying, and air jet milling. The average particle size is 20–50 nm, and the specific surface area is 120–400 m². 2 / g, silica content ≥98wt%.

[0034] The following are five specific embodiments of the anti-shrinkage and reinforcing dual-effect concrete internal curing agent of the present invention, each in parts by weight: Example 1:

[0035] The internal maintenance agent consists of: 100 parts phosphoric acid, 100 parts calcium hydroxide, 15 parts polyacrylamide, 20 parts sodium polyacrylate, 20 parts maleic anhydride (MA), 20 parts hydroxypropyl methylcellulose, 50 parts amorphous nano silica particles, 800 parts deionized water, and 5 parts defoamer.

[0036] The preparation method includes the following steps: (1) Pour 800 parts of deionized water into the reactor, slowly add 100 parts of calcium hydroxide powder into the deionized water, stir for 30 minutes to obtain calcium hydroxide suspension; (2) Slowly add phosphoric acid into the reaction vessel, control the reaction temperature at 60°C, and stir continuously for 60 minutes to obtain a mixed solution; (3) Add amorphous nano silica particles to the mixed solution and shear and disperse them at a high speed of 2000 r / min for 30 minutes; (4) Keep the solution temperature at 60-80℃, add maleic anhydride (MA) to the reaction vessel, stir for 60 minutes, then lower the temperature to 30-40℃, slowly add hydroxypropyl methylcellulose, and stir for 60 minutes; (5) Control the reaction temperature to 30°C, add polyacrylamide and sodium polyacrylate to the reactor, stir continuously for 150 minutes, cool to room temperature, add defoamer and stir for 30 minutes to obtain the anti-shrinkage and reinforcing dual-effect concrete internal curing agent.

[0037] Concrete contains the following components by weight: 168 parts water, 280 parts cement, 120 parts fly ash, 843 parts sand, 1013 parts crushed stone coarse aggregate, 20 parts internal curing agent, and 5 parts polycarboxylate superplasticizer. Example 2:

[0038] The internal maintenance agent consists of: 100 parts phosphoric acid, 100 parts calcium hydroxide, 20 parts polyacrylamide, 20 parts sodium polyacrylate, 20 parts maleic anhydride (MA), 25 parts hydroxypropyl methylcellulose, 50 parts amorphous nano silica particles, 800 parts deionized water, and 5 parts defoamer.

[0039] The preparation method includes the following steps: (1) Pour 800 parts of deionized water into the reactor, slowly add 100 parts of calcium hydroxide powder into the deionized water, stir for 30 minutes to obtain calcium hydroxide suspension; (2) Slowly add phosphoric acid into the reaction vessel, control the reaction temperature at 60°C, and stir continuously for 60 minutes to obtain a mixed solution; (3) Add amorphous nano silica particles to the mixed solution and shear and disperse them at a high speed of 2000 r / min for 30 minutes; (4) Keep the solution temperature at 60-80℃, add maleic anhydride (MA) to the reaction vessel, stir for 60 minutes, then lower the temperature to 30-40℃, slowly add hydroxypropyl methylcellulose, and stir for 60 minutes; (5) Control the reaction temperature to 30°C, add polyacrylamide and sodium polyacrylate to the reactor, stir continuously for 150 minutes, cool to room temperature, add defoamer and stir for 30 minutes to obtain the anti-shrinkage and reinforcing dual-effect concrete internal curing agent.

[0040] Concrete contains the following components by weight: 168 parts water, 280 parts cement, 120 parts fly ash, 843 parts sand, 1013 parts crushed stone coarse aggregate, 20 parts internal curing agent, and 5 parts polycarboxylate superplasticizer. Example 3:

[0041] The internal maintenance agent consists of: 100 parts phosphoric acid, 100 parts calcium hydroxide, 25 parts polyacrylamide, 25 parts sodium polyacrylate, 30 parts maleic anhydride (MA), 30 parts hydroxypropyl methylcellulose, 50 parts amorphous nano silica particles, 800 parts deionized water, and 5 parts defoamer.

[0042] The preparation method includes the following steps: (1) Pour 800 parts of deionized water into the reactor, slowly add 100 parts of calcium hydroxide powder into the deionized water, stir for 30 minutes to obtain calcium hydroxide suspension; (2) Slowly add phosphoric acid into the reaction vessel, control the reaction temperature at 70°C, and stir continuously for 60 minutes to obtain a mixed solution; (3) Add amorphous nano silica particles to the mixed solution and shear and disperse them at a high speed of 2000 r / min for 30 minutes; (4) Keep the solution temperature at 60-80℃, add maleic anhydride (MA) to the reaction vessel, stir for 60 minutes, then lower the temperature to 30-40℃, slowly add hydroxypropyl methylcellulose, and stir for 60 minutes; (5) Control the reaction temperature to 40°C, add polyacrylamide and sodium polyacrylate to the reactor, stir continuously for 150 minutes, cool to room temperature, add defoamer and stir for 30 minutes to obtain the anti-shrinkage and reinforcing dual-effect concrete internal curing agent.

[0043] Concrete contains the following components by weight: 168 parts water, 280 parts cement, 120 parts fly ash, 843 parts sand, 1013 parts crushed stone coarse aggregate, 20 parts internal curing agent, and 5 parts polycarboxylate superplasticizer. Example 4:

[0044] The internal maintenance agent consists of: 100 parts phosphoric acid, 100 parts calcium hydroxide, 25 parts polyacrylamide, 30 parts sodium polyacrylate, 30 parts maleic anhydride (MA), 40 parts hydroxypropyl methylcellulose, 50 parts amorphous nano silica particles, 800 parts deionized water, and 5 parts defoamer.

[0045] The preparation method includes the following steps: (1) Pour 800 parts of deionized water into the reactor, slowly add 100 parts of calcium hydroxide powder into the deionized water, stir for 30 minutes to obtain calcium hydroxide suspension; (2) Slowly add phosphoric acid into the reaction vessel, control the reaction temperature at 70°C, and stir continuously for 60 minutes to obtain a mixed solution; (3) Add amorphous nano silica particles to the mixed solution and shear and disperse them at a high speed of 2000 r / min for 30 minutes; (4) Keep the solution temperature at 60-80℃, add maleic anhydride (MA) to the reaction vessel, stir for 60 minutes, then lower the temperature to 30-40℃, slowly add hydroxypropyl methylcellulose, and stir for 60 minutes; (5) Control the reaction temperature to 45°C, add polyacrylamide and sodium polyacrylate to the reactor, stir continuously for 150 minutes, cool to room temperature, add defoamer and stir for 30 minutes to obtain the anti-shrinkage and reinforcing dual-effect concrete internal curing agent.

[0046] Concrete contains the following components by weight: 168 parts water, 280 parts cement, 120 parts fly ash, 843 parts sand, 1013 parts crushed stone coarse aggregate, 20 parts internal curing agent, and 5 parts polycarboxylate superplasticizer. Example 5:

[0047] The internal maintenance agent consists of: 100 parts phosphoric acid, 100 parts calcium hydroxide, 30 parts polyacrylamide, 30 parts sodium polyacrylate, 40 parts maleic anhydride (MA), 40 parts hydroxypropyl methylcellulose, 50 parts amorphous nano silica particles, 800 parts deionized water, and 5 parts defoamer.

[0048] The preparation method includes the following steps: (1) Pour 800 parts of deionized water into the reactor, slowly add 100 parts of calcium hydroxide powder into the deionized water, stir for 30 minutes to obtain calcium hydroxide suspension; (2) Slowly add phosphoric acid into the reaction vessel, control the reaction temperature at 80°C, and stir continuously for 60 minutes to obtain a mixed solution; (3) Add amorphous nano silica particles to the mixed solution and shear and disperse them at a high speed of 2000 r / min for 30 minutes; (4) Keep the solution temperature at 60-80℃, add maleic anhydride (MA) to the reaction vessel, stir for 60 minutes, then lower the temperature to 30-40℃, slowly add hydroxypropyl methylcellulose, and stir for 60 minutes; (5) Control the reaction temperature to 50°C, add polyacrylamide and sodium polyacrylate to the reactor, stir continuously for 150 minutes, cool to room temperature, add defoamer and stir for 30 minutes to obtain the anti-shrinkage and reinforcing dual-effect concrete internal curing agent.

[0049] The internal conditioning agent prepared by the above method is as follows: Figure 1 As shown, the concrete contains the following components by weight: 168 parts water, 280 parts cement, 120 parts fly ash, 843 parts sand, 1013 parts crushed stone coarse aggregate, 20 parts internal curing agent, and 5 parts polycarboxylate superplasticizer.

[0050] Comparative Example 1: The concrete mix design without any internal curing agent contains the following components by weight: 168 parts water, 280 parts cement, 120 parts fly ash, 843 parts sand, 1013 parts crushed stone coarse aggregate, 0 parts internal curing agent, and 5 parts polycarboxylate superplasticizer.

[0051] Comparative Example 2: The concrete, using a conventional superabsorbent polymer (SAP) resin instead of the internal curing agent of this invention, comprises the following components by weight: 168 parts water, 280 parts cement, 120 parts fly ash, 843 parts sand, 1013 parts crushed stone coarse aggregate, 20 parts SAP internal curing agent, and 5 parts polycarboxylate superplasticizer. Comparative Example 3: The formula for an internal curing agent that does not contain phosphoric acid or calcium hydroxide consists of: 30 parts polyacrylamide, 40 parts hydroxypropyl methylcellulose, 50 parts amorphous nano silica particles, 800 parts deionized water, and 5 parts defoamer.

[0052] Preparation method: Deionized water was added to the reactor, and nano-silica particles were added and dispersed by high-speed shearing for 30 minutes; after cooling to 30°C, hydroxypropyl methylcellulose was added and stirred for 60 minutes; polyacrylamide was added and stirred for 150 minutes while maintaining the temperature at 50°C, and then cooled to room temperature. The concrete mix proportions were the same as in Example 5.

[0053] Comparative Example 4: The formula for an internal curing agent that does not contain nano-silica consists of: 100 parts phosphoric acid, 100 parts calcium hydroxide, 30 parts polyacrylamide, 30 parts sodium polyacrylate, 20 parts maleic anhydride (MA), 40 parts hydroxypropyl methylcellulose, 800 parts deionized water, and 5 parts defoamer.

[0054] Preparation method: Deionized water was added to a reactor, calcium hydroxide was added and stirred for 30 minutes; phosphoric acid was added and stirred at 80°C for 60 minutes; the solution temperature was maintained at 60-80°C, maleic anhydride (MA) was added to the reactor and stirred for 60 minutes, then the temperature was lowered to 30-40°C, hydroxypropyl methylcellulose was slowly added and stirred for 60 minutes; polyacrylamide and sodium polyacrylate were added at 50°C and stirred for 150 minutes, cooled to room temperature, and then defoamer was added and stirred for 30 minutes to obtain the anti-shrinkage and reinforcing dual-effect concrete internal curing agent. The concrete mix proportions were the same as in Example 5.

[0055] Application testing: The mechanical properties of concrete were tested according to GB / T 50081-2019, and the results are shown in Tables 1, 2 and 3.

[0056] Table 1. Compressive strength (MPa) of concrete prepared in the examples and comparative examples. Table 2. Drying shrinkage properties of concrete prepared in the examples and comparative examples (drying shrinkage rate / ×10) -6 ) Table 3. Water loss of concrete prepared in the examples and comparative examples (g / m³) 2 ) From Table 1 and Figure 3 It can be seen that the compressive strength of Examples 1-5 was basically the same as that of the comparative examples in the early stage. However, after 28 days of curing, the compressive strength of Examples 1-5 was significantly higher than that of Comparative Examples 1-4. Taking 60 days as an example, compared with Comparative Example 1, Examples 1-5 showed increases of approximately 3.24%, 4.80%, 5.73%, 6.58%, and 9.57%, respectively. The increase in compressive strength of the examples indicates that within the dosage range of this invention, the internal curing agent can effectively play a role in slow-release water supply, promote the uniform generation of hydration products, and thus ensure the continuous increase of concrete strength in the later stage.

[0057] It should be noted that the compressive strength of Example 5 at 7 days was 32.73 MPa, lower than that of Comparative Example 1 (33.81 MPa), showing a temporary decrease. This is because the total polymer content in Example 5 was the highest among all examples, with 30 parts polyacrylamide, 40 parts hydroxypropyl methylcellulose, 30 parts sodium polyacrylate, and 40 parts maleic anhydride (MA). The higher content of polyacrylamide, hydroxypropyl methylcellulose, and maleic anhydride formed a thicker polymer gel coating layer in the early stage of cement hydration. This coating layer blocked the direct contact between water and cement particles in the early stage of hydration, resulting in a small difference in the internal humidity gradient of the concrete and a slower water release rate. This delayed the early hydration rate of tricalcium silicate (C3S) and tricalcium aluminate (C3A), leading to a relatively low degree of hydration and a temporary decrease in strength at 7 days.

[0058] However, as the curing period lengthens, the aforementioned polymer gel network, driven by the gradual decrease in internal humidity of the concrete, continuously and slowly releases the stored moisture, providing a more abundant and sustained water supply for subsequent cement hydration compared to the low-dosage group and the control group. Example 5 showed a strength increase of 6.81 MPa between 7 and 14 days, representing a growth rate of 20.8%, which is 11.7 times and 12.2 times that of Comparative Example 1 (0.58 MPa, 1.7%) during the same period, respectively. By 14 days, the compressive strength of Example 5 reached 39.54 MPa, surpassing all comparative examples and other examples; by 60 days, it reached 49.27 MPa, the highest value among all groups. This strength development pattern indicates that the higher polymer dosage did not cause a strength loss, but rather redistributed the hydration process from an early, concentrated release to a more uniform and sustained long-term hydration process, ultimately achieving a higher degree of cement hydration and superior mechanical properties.

[0059] From an application perspective, the early strength of Example 5 after 3 days still reaches 26.90 MPa, which meets the requirements of general engineering for early demolding strength; while the temporary reduction at 7 days has no adverse impact on the safety of actual engineering, because the design strength of concrete structures is usually based on the 28-day standard curing strength.

[0060] From Table 2 and Figure 4 It can be seen that the drying shrinkage rates of Examples 1-5 were significantly lower than those of Comparative Examples 1-4 throughout all age stages, and the difference in concrete drying shrinkage continued to widen with the extension of age. Taking 60 days as an example, the drying shrinkage rates of Examples 1-5 were reduced by approximately 36.34%, 72.12%, 72.15%, 76.15%, and 77.25% respectively compared to Comparative Example 1; and by approximately 23.21%, 66.38%, 66.41%, 71.23%, and 72.57% respectively compared to Comparative Example 2 (with traditional SAP). The drying shrinkage rate curves of Comparative Examples 1-4 were all at a high level and rose steeply, while the curves of Examples 1-5 were significantly flatter, indicating that the internal curing agent of the present invention can continuously and stably release moisture, effectively alleviating the decrease in internal humidity of concrete, thereby inhibiting the autogenous shrinkage and drying shrinkage of concrete.

[0061] From Table 3 and Figure 5 It can be seen that the water loss of Comparative Examples 1-4 within the range of 3-60 days is generally higher than that of Examples 1-5. Example 5 exhibits lower water loss at all ages, with the 60-day water loss being approximately 34.78% lower than that of Comparative Example 1. The water loss data shows that Examples 1-5 are significantly superior to the Comparative Examples in terms of moisture retention capacity, indicating that the internal curing agent of this invention has excellent water retention performance and effectively slows down the evaporation and loss of moisture from the concrete interior to the external environment.

[0062] Synergistic effect analysis of each component: To demonstrate that there is a synergistic effect among the components of the present invention rather than a simple additive effect, the performance improvement of Comparative Example 3 (system without calcium phosphate salt) and Comparative Example 4 (system without nano silica) was compared with that of Example 5 (system with all components).

[0063] Taking the 60-day shrinkage rate as an example: the shrinkage rate of Comparative Example 3 compared to the control group (Comparative Example 1) decreased by 328.42 × 10⁻⁶. -6 Compared with the control group, the shrinkage rate of Comparative Example 4 decreased by 109.41 × 10⁻⁶. -6 The sum of the decreases for both is 437.83 × 10 -6 This value already takes into account the basic contributions of polyacrylamide and hydroxypropyl methylcellulose common in both comparative groups, and is an upper limit estimate of the theoretical additive effect. This upper limit estimate is a conservative calculation; the actual synergistic multiple will only be higher, not lower. However, the actual shrinkage rate reduction of Example 5 compared to the control group was 930.78 × 10⁻⁶. -6 It is 2.13 times the upper limit of the above theoretical superposition.

[0064] The same pattern holds true for other performance indicators: for 60-day compressive strength, the sum of the strength increases of Comparative Examples 3 and 4 compared to the control group was 1.60 MPa, while the actual strength increase of Example 5 was 4.30 MPa, which is 2.69 times the theoretical upper limit; for 60-day water loss, the sum of the water loss reduction of Comparative Examples 3 and 4 compared to the control group was 190.62 g / m³. 2 The actual water loss reduction in Example 5 was 372.48 g / m³. 2 It is 1.95 times the theoretical superposition limit.

[0065] The above results indicate that the reaction product of phosphoric acid and calcium hydroxide (calcium phosphate salt) and amorphous nano-silica particles do not function independently in the polymeric water-retaining network system constructed by polyacrylamide and hydroxypropyl methylcellulose, but rather exhibit a significant synergistic effect. The synergistic mechanism is as follows: the calcium phosphate salt precipitation provides a stable inorganic framework and pore-filling matrix; the nano-silica, with its high specific surface area and pozzolanic activity, induces the formation of secondary CSH gels on the surface of this framework, and the two together construct a rigid filling network; this inorganic network interpenetrates with the organic water-retaining gel network formed by polyacrylamide and hydroxypropyl methylcellulose, forming an organic-inorganic interpenetrating network structure. This allows the water-retaining and slow-release function and the pore-filling enhancement function to be coupled on a spatial scale, producing a comprehensive effect far exceeding the sum of the individual contributions of each component.

[0066] like Figure 6 As shown, the compressive strength, shrinkage resistance and water retention performance of each group at 60 days of age were comprehensively scored. The comprehensive scores of the workability of Examples 1 to 5 were all higher than those of Comparative Examples 1 to 4, with Example 5 having the highest comprehensive score. This indicates that the internal curing agent of the present invention has significant advantages in comprehensively improving the workability of concrete.

[0067] like Figure 2 As shown, SEM microscopic observation revealed that in Comparative Example 1, the CSH gel accumulation in local areas of the hydration products was relatively loose and unevenly distributed, with numerous gel pores. Calcium hydroxide exhibited distinct hexagonal plate-like or layered crystals, which were relatively large and often accumulated on the aggregate surface or in the pores, showing a clear directional arrangement. These coarse crystals were weak points within the concrete and easily became the origin of microcracks. In Example 5, the hydration products were more dense and uniform. The CSH gel transformed from a loose network structure to a more compact felt-like or honeycomb structure. Nano-silica induced the generation of more secondary CSH in local areas, filling the gel pores. Etnacite crystals were shorter, thicker, and more uniformly distributed, tightly embedded in the CSH gel matrix, forming a good mechanical interlocking structure.

[0068] The above description is merely a preferred embodiment of the present invention, but the scope of protection of the present invention is not limited thereto. Any equivalent substitutions, modifications, variations, or recombinations made by those skilled in the art based on the technical concept of the present invention should be included within the scope of protection of the present invention.

Claims

1. A dual-effect concrete internal curing agent that provides both shrinkage resistance and reinforcement, characterized in that, It is made from the following raw materials in parts by weight: 100 parts phosphoric acid, 100 parts calcium hydroxide, 15-30 parts polyacrylamide, 20-30 parts sodium polyacrylate, 20-40 parts maleic anhydride, 20-40 parts hydroxypropyl methylcellulose, 50 parts amorphous nano silica particles, 800 parts deionized water, and 5 parts defoamer; wherein the polyacrylamide is anionic or nonionic with a molecular weight of 5 million to 12 million; and the amorphous nano silica particles have an average particle size of 20-50 nm.

2. The anti-shrinkage and reinforcing dual-effect concrete internal curing agent according to claim 1, characterized in that, The phosphoric acid is provided as an 85% phosphoric acid solution, and the calcium hydroxide is in powder form with a fineness ≥300 mesh. The weight ratio of the phosphoric acid solution to the calcium hydroxide is 1:

1.

3. The anti-shrinkage and reinforcing dual-effect concrete internal curing agent according to claim 1, characterized in that, The specific surface area of ​​the amorphous nano-silica particles is 120–400 m². 2 / g, silica content ≥98wt%.

4. The anti-shrinkage and reinforcing dual-effect concrete internal curing agent according to claim 1, characterized in that, The hydroxypropyl methylcellulose is prepared from refined cotton as cellulose raw material through alkalization and etherification reactions. The molecular chain contains methoxy and hydroxypropoxy groups, and the moisture content is ≤5%.

5. A method for preparing the anti-shrinkage and reinforcing dual-effect concrete internal curing agent according to any one of claims 1 to 4, characterized in that, Includes the following steps: Step 1: Add 800 parts of deionized water to the reaction vessel, slowly add 100 parts of calcium hydroxide powder to the deionized water, stir for 30 minutes to obtain a calcium hydroxide suspension; Step 2: Slowly add 100 parts of phosphoric acid into the reaction vessel, control the reaction temperature at 60-80℃, and stir continuously for 60 minutes to allow the phosphoric acid to react with calcium hydroxide to form calcium phosphate precipitate, resulting in a mixed solution with a pH of 5.5-6.

5. Step 3: Add 50 parts of amorphous nano silica particles to the mixed solution and disperse them at a high speed of 2000 r / min for 30 minutes. Step 4: Keep the solution temperature at 60-80℃, add 20-40 parts of maleic anhydride to the reaction vessel, stir for 60 minutes, then lower the temperature to 30-40℃, slowly add 20-40 parts of hydroxypropyl methylcellulose, and stir for 60 minutes. Step 5: Control the reaction temperature to 20-50℃, add 15-30 parts of polyacrylamide and 20-30 parts of sodium polyacrylate to the reactor, stir continuously for 150 minutes, cool to room temperature, add defoamer and stir for 30 minutes to obtain the anti-shrinkage and reinforcing dual-effect concrete internal curing agent.

6. The preparation method according to claim 5, characterized in that, The concentration of the phosphoric acid solution mentioned in step 2 is 85%, and it is added slowly dropwise. After the reaction is completed, the pH value of the mixed solution is 5.8 to 6.

2.

7. The preparation method according to claim 5, characterized in that, In step 4, the maleic anhydride is added slowly in the form of a white crystalline powder with a purity ≥99% and a molecular weight of 98.06 g / mol; in step 5, the defoamer is a polyether-modified silicone defoamer or a polyether-based defoamer.

8. The preparation method according to claim 5, characterized in that, The polyacrylamide mentioned in step 5 is anionic or nonionic, with a molecular weight of 5 million to 12 million, and the addition temperature is controlled at 30 to 50°C; the sodium polyacrylate has a molecular weight of 8 million to 12 million.

9. A concrete material containing the internal curing agent according to any one of claims 1 to 4, characterized in that, It comprises cement, fly ash, aggregate, water, and the concrete curing agent, wherein the mass ratio of the concrete curing agent to the cementitious material is 5:

100.

10. The concrete material according to claim 9, characterized in that, The concrete materials, by weight, comprise: 168 parts water, 280 parts cement, 120 parts fly ash, 843 parts sand, 1013 parts crushed stone coarse aggregate, 20 parts internal curing agent, and 5 parts polycarboxylate superplasticizer.

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