Reducing admixture for concrete as well as preparation method and application of reducing admixture

Abstract

The invention relates to the technical field of concrete admixtures, in particular to a shrinkage-reducing admixture for concrete as well as a preparation method and application of the shrinkage-reducing admixture. The preparation process overcomes the problems that a traditional admixture is poor in alkali resistance and cured concrete is prone to cracking. The preparation method comprises the following steps: preparing a reduction initiator, a chain transfer agent, an unsaturated acid monomer solution, a reduction functional monomer solution and an amide functional monomer solution step by step, and controlling polymerization conditions under the protection of nitrogen to prepare the water reducer. Alkyl glycoside is adopted as an initiator, propylene oxide and ethylene oxide are polymerized step by step, and low-foam polyether is prepared; mixing the two with an alkali-resistant additive to obtain a shrinkage-reducing admixture; through a hydrogen bond network, low surface tension and alkali-resistant protection, the water-reducing rate, the flow retentivity and the compressive strength of the concrete are remarkably improved, the setting time is optimized, the drying shrinkage rate and the alkali erosion influence are reduced, and the compactness and the durability of the concrete are improved. The concrete admixture is moderate in viscosity, suitable for concrete application, stable in process and easy for industrial production.

Description

Technical Field

[0001] This invention relates to the field of concrete admixture technology, specifically to a shrinkage-reducing admixture for concrete, its preparation method, and its application. Background Technology

[0002] Concrete undergoes volume shrinkage during setting and drying, leading to cracking that can damage or cause collapse of structures, reducing the building's safety. These cracks not only affect the concrete's mechanical properties and aesthetics but also accelerate the intrusion of harmful ions such as chloride and sulfate ions, causing steel reinforcement corrosion, accelerated carbonation, and even structural failure, severely shortening the project's service life. Cracking in concrete structures has always been a pressing problem in the engineering field, and shrinkage-reducing agents are an important way to control concrete shrinkage and cracking.

[0003] Patent CN117263563A discloses a concrete shrinkage reducing agent that reduces the surface tension in the pore solution of cement concrete through the synergistic effect of shrinkage-reducing components and auxiliary components, thereby reducing the driving force of shrinkage stress and achieving excellent shrinkage reduction. Patent CN118165192A discloses a synthesized concrete shrinkage reducing agent that effectively reduces the shrinkage driving force of the cementitious system by significantly reducing the surface tension in the pore solution of the cementitious system and maintaining the relative humidity inside the system at a high level (>94%). Patent CN111454014A discloses a method of introducing amino, sulfonic acid, amide groups, siloxane structural monomers, ethylene glycol acrylate monomers, etc. into the polymer molecular structure to reduce the drying shrinkage rate of cement mortar. Although the above shrinkage reducing agents all have a certain shrinkage reduction effect, they all overlook a problem: concrete is a strongly alkaline environment, and surfactants will hydrolyze and lose their effect in a strongly alkaline environment. The mechanism of concrete shrinkage reducing agents is to reduce the shrinkage of concrete by lowering the surface tension of water in the capillary pores of concrete and reducing the shrinkage force during the evaporation process. Therefore, only by developing shrinkage reducing agents that can achieve the above properties in a strongly alkaline environment can the shrinkage reduction effect be truly realized.

[0004] High dosages of shrinkage-reducing admixtures introduce more air bubbles, affecting workability and durability. In addition, most shrinkage-reducing admixtures are low-molecular-weight organic compounds that are easily volatilized or degraded, resulting in a decline in shrinkage-reducing effects over time. Furthermore, commonly used admixtures such as water-reducing agents have poor adaptability and are prone to flocculation or separation when used in combination, leading to abnormal slurry viscosity. They cannot simultaneously achieve both shrinkage-reducing and water-reducing functions, and existing improvements often sacrifice fluidity or strength.

[0005] Therefore, a shrinkage-reducing admixture for concrete, its preparation method, and its application are proposed. Summary of the Invention

[0006] The present invention aims to provide a shrinkage-reducing admixture for concrete, its preparation method, and its application. A water-reducing agent is prepared by stepwise preparation of a solution of a reduction initiator, a chain transfer agent, an unsaturated acid monomer, a shrinkage-reducing functional monomer, and an amide functional monomer, under controlled polymerization conditions under nitrogen protection. A low-foaming polyether is prepared by stepwise polymerization of propylene oxide and ethylene oxide using alkyl glycosides as initiators. These two are then mixed with an alkali-resistant additive to obtain a shrinkage-reducing admixture. Through hydrogen bonding networks, low surface tension, and alkali resistance, the admixture significantly improves the water reduction rate, flow retention, and compressive strength of concrete, optimizes setting time, reduces drying shrinkage and the effects of alkali erosion, and enhances the density and durability of concrete.

[0007] To achieve the above objectives, the present invention provides the following technical solution: This invention provides a method for preparing a shrinkage-reducing admixture for concrete. The preparation method is as follows: water-reducing agent and low-foaming polyether are mixed according to the weight parts and stirred at room temperature until uniform and transparent to obtain an intermediate material mixture; alkali-resistant additive is added to the intermediate material mixture and stirred again until uniform and transparent to obtain the shrinkage-reducing admixture. The alkali-resistant additive is obtained by mixing alkyltriethoxysilane, polyoxyethylene ether, sodium dodecyl diphenyl ether disulfonate, deionized water and citric acid; The water-reducing agent is obtained by copolymerization of chain transfer agent mercaptoacetic acid, unsaturated acid monomer acrylic acid, shrinkage functional monomer butyl acrylate, amide functional monomer acrylamide, and isopentenyl polyoxyethylene ether. The CAS number for isopentenyl polyoxyethylene ether is 62601-60-9, and the degree of polymerization is 54; the CAS number for C12-C14 alkyl glycosides is 157707-88-5; the CAS number for C8-10 alkyl glycosides is 68515-73-1; and the CAS number for C16-18 alkyl glycosides is Glucopon 600 CSUP. Low-foaming polyethers are obtained by ring-opening addition reactions of alkyl glycosides, propylene oxide, and ethylene oxide.

[0008] Preferably, the alkyl length of the alkyl glycoside is C8-C16; More preferably, the alkyl length of the alkyl glycoside is C12-C16.

[0009] Preferably, the copolymerization process is as follows: the polymerization reaction is carried out under nitrogen protection; isopentenyl polyoxyethylene ether is added to the reaction vessel, deionized water is added, stirring is started, the temperature is raised to dissolve completely, an oxidation initiator is added to the reaction vessel, and solutions A and B are added dropwise at the same time; after the dropwise addition is completed, the reaction is kept at a constant temperature to obtain a water-reducing agent; the viscosity of the water-reducing agent is 27-44s, and the surface tension is 45-48mN / m.

[0010] Preferably, solution A is obtained by dissolving a reducing initiator and mercaptoacetic acid in deionized water; solution B is obtained by mixing acrylic acid, butyl acrylate, and acrylamide. Solution A is used to control the growth of the polymerization chain and avoid abnormal viscosity caused by excessively high molecular weight. The introduction of acrylamide slightly increases the chain transfer agent to prevent branching caused by hydrogen bonding. Solution B ensures uniform distribution of monomers, and the amide groups of acrylamide are uniformly embedded in the main chain in subsequent polymerization, which is beneficial to the formation of hydrogen bond networks.

[0011] Preferably, the ring-opening addition reaction process is as follows: alkyl glycosides and potassium hydroxide are added to a high-pressure polymerization reactor, and nitrogen protection and heating are carried out to ensure uniform material distribution; propylene oxide is added to start the reaction, and the remaining propylene oxide is added after the pressure stabilizes; this step ensures uniform activation of the polyhydroxyl groups of APG, which is beneficial for subsequent epoxy ring-opening; ethylene oxide is added after the reaction stabilizes, the reaction is started, and the remaining ethylene oxide is added after the pressure stabilizes, allowing the reaction to proceed fully, cooling to room temperature, and acid neutralization to obtain a low-foaming polyether with a viscosity of 600-1500 Pa·s and a surface tension of 28-33 mN / m.

[0012] The present invention also provides a shrinkage-reducing admixture for concrete, wherein, by mass percentage, the water-reducing agent content in the concrete shrinkage-reducing admixture is 83.9wt%-90.0wt%; the low-foaming polyether content is 0.10wt%-0.28wt%; the alkali-resistant additive content is 4.7wt%-16.0wt%; and the mass ratio of alkyltriethoxysilane, polyoxyethylene ether, sodium dodecyl diphenyl ether disulfonate, water, and citric acid in the alkali-resistant additive is 9-12:2-3:1:3-6:1-2.

[0013] Application of a shrinkage-reducing admixture for concrete, specifically its application in C40 concrete; after application, the 7-day drying shrinkage rate of the C40 concrete was -231.6 × 10⁻⁶. -6 to -246.7×10 -6 The 28-day drying shrinkage rate was -413.4 × 10⁻⁶. -6 to -431.0×10 -6 .

[0014] Compared with the prior art, the beneficial effects of the present invention are as follows: 1. This invention optimizes the alkyl glycoside chain length and catalyst dosage in the preparation of low-foaming polyethers. Through stepwise polymerization and pressure monitoring, it ensures appropriate polyether chain elongation and structural stability. The sequential injection of ethylene oxide and propylene oxide balances their hydrophilic and hydrophobic properties, resulting in a low surface tension product. This, combined with water-reducing agents and alkali-resistant additives, reduces air bubbles and pore tension in concrete, improves density and resistance to alkali erosion, significantly reduces drying shrinkage, and enhances the overall strength and long-term durability of concrete.

[0015] 2. This invention introduces amide functional monomers into the preparation of water-reducing agents. By extending the dropping time and controlling the heating rate, monomer hydrolysis is avoided, ensuring that amide groups are uniformly embedded in the main chain to form a hydrogen bond network. Combined with chain transfer agents to regulate molecular weight and optimize the monomer molar ratio, the distribution of hydrophilic groups is enhanced, surface tension and viscosity fluctuations are reduced, the dispersion efficiency of the water-reducing agent on cement particles is improved, the uniformity and workability of fresh concrete are improved, early shrinkage and crack formation are inhibited, and the long-term stability and durability of concrete are significantly enhanced.

[0016] 3. The water-reducing agent and low-foaming polyether are mixed by stirring at room temperature. Alkali-resistant additives composed of alkyltriethoxysilane, polyoxyethylene ether, sodium dodecyl diphenyl ether disulfonate, etc. are gradually added to form a uniform and transparent shrinkage-reducing admixture. Utilizing the synergistic effect of the components, the hydrogen bond network and surface tension regulation are enhanced. The alkali-resistant additives form a protective layer in the alkaline environment, reducing erosion, improving the water reduction rate and flow retention of concrete, delaying the hydration reaction, optimizing the setting time, significantly improving compressive strength and alkali resistance, and reducing drying shrinkage.

[0017] 4. Using alkyl glycosides as initiators, hydroxyl groups are activated through inert gas cleaning and controlled heating rate. Combined with an alkaline catalyst, propylene oxide and ethylene oxide are injected stepwise, with strict control of pressure and temperature to ensure uniform ring-opening polymerization. After the reaction, the mixture is cooled, vacuum distilled to remove residual monomers, and the pH is neutralized to obtain a clear, transparent, low-foaming polyether. By regulating the hydrophilic-hydrophobic balance of the polyether chains, reducing surface tension, and optimizing molecular weight distribution, the product achieves a moderate viscosity, significantly reducing foam formation in concrete, improving the density and surface activity of the mixture, thereby improving the compressive strength and crack resistance of concrete.

[0018] 5. By preparing solutions of reduction initiator, chain transfer agent, unsaturated acid monomers, shrinkage-reducing functional monomers, and amide functional monomers in steps, and controlling the polymerization temperature, dropping time, and heat preservation reaction under nitrogen protection, uniform monomer distribution and appropriate chain growth are ensured, avoiding abnormal molecular weight. After the reaction, the solid content is adjusted, the pH is neutralized, and impurities are filtered out to obtain a light yellow transparent water-reducing agent. By forming a stable hydrogen bond network, it enhances intermolecular interactions, reduces surface tension and viscosity fluctuations, improves the dispersibility and stability of the water-reducing agent in concrete, significantly improves the fluidity and retention of fresh concrete, reduces the introduction of air bubbles, optimizes the pore structure, and thus improves the workability and durability of concrete. Attached Figure Description

[0019] Figure 1 This is a flowchart illustrating the preparation method of the shrinkage-reducing admixture for concrete according to the present invention. Detailed Implementation

[0020] The technical solutions of the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention.

[0021] Please see Figure 1 This invention provides a shrinkage-reducing admixture for concrete, its preparation method, and its application. The technical solution is as follows: Preparation Example 1 (Preparation of Water-Reducing Agent) Preparation of S1 solution: Add 1.8g of reducing initiator vitamin C and 4.2g of chain transfer agent mercaptoacetic acid to 60g of deionized water and stir at 400rpm to dissolve, obtaining solution A for later use. Mix 180g of unsaturated acid monomer acrylic acid, 512g of condensation functional monomer butyl acrylate and 284g of amide functional monomer acrylamide, add to 120g of deionized water, and stir evenly at 400rpm to obtain solution B for later use.

[0022] S2 Polymerization: The polymerization reaction was carried out under nitrogen protection at a flow rate of 0.5 L / min. 280 g of isopentenyl polyoxyethylene ether was added to the reactor, along with 51 g of deionized water. Stirring was started, and the temperature was increased from 25°C to 50°C at a rate of 2°C / min until complete dissolution. 12 g of 30 wt% H₂O₂ aqueous solution of the oxidation initiator was added to the reactor, and dropping was immediately initiated: solutions A and B were added simultaneously. Solution A was added for 4 hours, and solution B for 3.5 hours, with a uniform dropping rate. The dropping time was extended to accommodate the activity of acrylamide and ensure uniform copolymerization. After the dropping was completed, the reaction was maintained at 50°C for 40 minutes to ensure a monomer conversion rate >96%.

[0023] S3 Post-treatment: After the reaction is complete, add deionized water to a total mass of 1000g and adjust the solid content to 42wt%; continue stirring at 200rpm and keep warm for 30min to complete the polymerization of residual monomers; cool to room temperature, neutralize with 10wt% NaOH solution to pH=7, and slowly titrate to avoid excessive local alkalinity leading to amide group degradation; filter through a 200-mesh sieve to remove impurities and obtain a light yellow transparent shrinkage PCE mother liquor.

[0024] Preparation of Comparative Example 1 (Preparation of Water-Reducing Agent) Unlike Preparation Example 1, in the preparation of S1 solution, all raw materials were directly mixed to obtain a mixed solution for later use.

[0025] Preparation of Comparative Example 2 (Preparation of Water-Reducing Agent) Unlike Preparation Example 1, no amide functional monomer acrylamide was added.

[0026] Preparation of Comparative Example 3 (Preparation of Water-Reducing Agent) Unlike Preparation Example 1, the heating rate of the S2 polymerization reaction was 5℃ / min; and the holding temperature was 70℃, while the holding time remained unchanged.

[0027] Preparation Example 2 (Preparation of Low-Fogging Polyether) 45.6 g of C12-C14 alkyl glycoside and 0.574 g of potassium hydroxide were added to a high-pressure polymerization reactor. Stirring was started at 200 rpm, and the reactor was sealed. The reactor chamber was cleaned three times with inert nitrogen gas at a flow rate of 1 L / min, each time purging to 0.2 MPa, then venting to remove oxygen and moisture. The temperature was increased from 25°C to 120°C at a rate of 5°C / min and held for 10 min to ensure uniform dispersion of potassium hydroxide in the alkyl glycoside, forming an alkaline starting system. At 120°C, 2 g of propylene oxide was rapidly injected as an initiator using a feed pump within 5 min. The pressure drop and temperature rise indicated the start of the reaction. After the pressure stabilized (dropped to <0.1 MPa), ensuring the pressure was ≤0.5 MPa, the remaining 104.6 g of propylene oxide was continuously added at a uniform dropping rate, completing the reaction in 2 hours. When the pressure approaches 0.5 MPa, stop feeding until the pressure drops to 0.1 MPa; control the reaction temperature at 130℃ and the stirring speed at 200 rpm. After the propylene oxide is added, maintain the temperature at 130℃ for 120 minutes until the pressure no longer drops (<0.05 MPa), ensuring a conversion rate >98%; maintain the reactor temperature at 130℃, and rapidly inject 2g of ethylene oxide as an initiator within 5 minutes, observing the pressure drop and temperature rise; after the pressure stabilizes, reduce it to 0.1 MPa. When the pressure is ≤0.5 MPa, begin continuously adding the remaining 37.2g of ethylene oxide, completing the process in 1 hour; pressure control is the same as above; control the reaction temperature at 140℃ and the stirring speed at 200 rpm; after the ethylene oxide is added, maintain the temperature at 140℃ for 60 minutes until the pressure <0.05 MPa no longer drops, ensuring a conversion rate >98%; After the reaction is complete, stop heating and slowly cool down to room temperature (<40℃) at a rate of 5℃ / min to prevent the product from sticking to the walls. Open the reactor, release the inert gas, and discharge the clear, transparent, low-foaming polyether product, a light yellow viscous liquid containing a small amount of residual monomer. Remove the monomer by vacuum distillation at -0.09MPa, 80℃, for 30 minutes. Adjust the pH of the product to 6-8 and neutralize the remaining potassium hydroxide with dilute acetic acid. Store in a sealed container at room temperature, protected from light, with a stability of >6 months.

[0028] Preparation Example 3 (Preparation of Low-Foaming Polyether) Unlike Preparation Example 2, the amount of potassium hydroxide used was 0.813 g, the remaining 161.1 g of propylene oxide was added dropwise over 3 hours, and the remaining 59.8 g of ethylene oxide was added dropwise over 1.5 hours.

[0029] Preparation Example 4 (Preparation of Low-Fogging Polyether) Unlike Preparation Example 2, the C12-C14 alkyl glycosides were replaced with C10-C16 alkyl glycosides, and the amount used was 53.8 g, while the amount of potassium hydroxide was adjusted to 0.599 g.

[0030] Preparation of Comparative Example 4 (Preparation of Low-Foaming Polyether) Unlike Preparation Example 2, the C12-C14 alkyl glycosides were replaced with C8-C10 alkyl glycosides, and the amount used was 37.4 g, while the amount of potassium hydroxide was adjusted to 0.549 g.

[0031] Preparation of Comparative Example 5 (Preparation of Low-Foaming Polyether) Unlike Preparation Example 2, the amount of potassium hydroxide used was 1.039 g, the remaining 215.5 g of propylene oxide was added dropwise over 4 hours, the remaining 80.4 g of ethylene oxide was added dropwise over 2 hours, the temperature was maintained at 150°C after the propylene oxide was added, and the temperature was maintained at 80°C after the ethylene oxide was added.

[0032] Test Example 1 The water-reducing agents and low-foaming polyethers obtained in Preparation Examples 1-4 and Comparative Examples 1-5 were subjected to viscosity and surface tension tests. The specific test methods are as follows: The viscosity of the water-reducing agent was determined according to GB / T 1723-1993 "Determination of Viscosity of Coatings", using the Forecast-4 cup method, and the flow time (s) was measured at 25℃ to reflect the dynamic viscosity of the sample; the viscosity of the low-foaming polyether was determined according to GB / T 2794-2022 "Determination of Viscosity of Adhesives", using a rotational viscometer to measure the dynamic viscosity of the sample at 25℃, with the unit being mPa·s. The surface tension of the water-reducing agent and the low-foaming polyether was tested in accordance with GB / T 22237-2008 "Determination of surface tension of surfactants"; the test results are shown in Table 1.

[0033] Table 1. Test results of viscosity and surface tension of water-reducing agent and low-foaming polyether

[0034] The water-reducing agent and low-foaming polyether prepared according to the method of the present invention have moderate viscosity and moderate surface tension. In the preparation of the water-reducing agent, Preparation Example 1 ensured uniform monomer distribution and controlled polymer chain growth by preparing solutions A and B in steps. The mercaptoacetic acid chain transfer agent effectively controlled the molecular weight, avoiding abnormal viscosity caused by excessively high molecular weight. The amide groups of acrylamide formed a hydrogen bond network, moderately increasing the viscosity. In addition, the hydrophilic groups such as carboxyl and amide groups of butyl acrylate and acrylamide in the PCE mother liquor reduced the surface tension. In Preparation Comparative Example 1, all raw materials were directly mixed without stepwise preparation of solutions A and B, resulting in uneven monomer distribution, excessively rapid local polymerization, wide molecular weight distribution, increased branching, and higher viscosity. Preparation Comparative Example 2 did not add acrylamide, lacking the hydrogen bond network of amide groups, which weakened the interaction between molecular chains and resulted in lower viscosity. Without the amide groups of acrylamide, the hydrophilicity was slightly reduced, and the surface tension was slightly higher than that of Preparation Example 1. In Preparation Comparative Example 3, the excessively rapid heating rate and high temperature caused acrylamide hydrolysis or side reactions, resulting in local branching or degradation of the polymer chain, uneven molecular weight distribution, and higher viscosity; however, it had little effect on the surface tension.

[0035] In the preparation of low-foaming polyethers, the C12-C14 alkyl glycosides of Preparation Example 2 were used as initiators, undergoing stepwise ring-opening polymerization with propylene oxide and ethylene oxide under KOH catalysis to form polyether chains. Appropriate KOH dosage and controlled dropping time ensured a suitable molecular weight. The alkyl chains of the alkyl glycosides and the polyether chains (ethylene oxide is hydrophilic, propylene oxide is hydrophobic) synergistically reduced surface tension, superior to ordinary polyethers, making them suitable for low-foaming applications. Preparation Examples 3 and 4, by adjusting the alkyl chain length of the alkyl glycoside side chains, achieved higher viscosity and more stable surface tension compared to Preparation Example 2. In Comparative Example 4, the C8-C10 alkyl glycosides had shorter chains, lower molecular weight, and lower viscosity. Reducing the KOH dosage resulted in a lower degree of polymerization; the shorter chain alkyl glycosides enhanced surface activity, further reducing surface tension. In Comparative Example 5, the increased raw material dosage and higher reaction temperature significantly increased viscosity, and the surface tension was higher than that of Preparation Example 2.

[0036] Example 1

[0037] The shrinkage-reducing admixture is composed of 90 wt% water-reducing agent, 0.1 wt% low-foaming polyether, and 9.9 wt% alkali-resistant additive by mass percentage. The alkali-resistant additive is obtained by mixing alkyltriethoxysilane, polyoxyethylene ether, sodium dodecyl diphenyl ether disulfonate, water, and citric acid in a mass ratio of 10:2:1:6:1.

[0038] See Figure 1 The preparation method of the shrinkage-reducing admixture is as follows: a) Mix the water-reducing agent and low-foaming polyether by weight. No heating or cooling is required; the mixture can be kept at room temperature. Stir until it is uniform and transparent to obtain an intermediate material mixture. b) While stirring, add the alkali-resistant additive to the mixture obtained in step a), and stir again until it is uniform and transparent to obtain the shrinkage-reducing additive.

[0039] To ensure that the mass of shrinkage-reducing agent added to fresh C40 concrete is 1.2% of the mass of cementitious materials in the C40 concrete, performance tests were conducted on the shrinkage-reducing agents prepared in the following examples and comparative examples, using commercially available polycarboxylate superplasticizer PA96208 as a reference sample. The addition details of the examples and comparative examples are shown in Table 2. The mixing ratios in Table 2 are the mass mixing ratios of alkyltriethoxysilane, polyoxyethylene ether, sodium dodecyl diphenyl ether disulfonate, water, and citric acid.

[0040] Table 2. Addition ratios of shrinkage-reducing admixtures in the examples and comparative examples.

[0041] Comparative Example 1 Unlike Example 4, no low-foaming polyether and alkali-resistant additives were added. Only water-reducing agent was added to the cement, i.e., the water-reducing agent was 100 wt%, and the shrinkage-reducing admixture was added at 1.2% of the cement.

[0042] Comparative Example 2 Unlike Example 4, the amount of shrinkage-reducing admixture added to cement is 2.0%.

[0043] Comparative Example 3 Unlike Example 4, the water-reducing agent prepared in Comparative Example 1 was used, with a water-reducing agent dosage of 83.9 wt%, a low-foaming polyether dosage of 0.17 wt%, and an alkali-resistant additive dosage of 16.0 wt%.

[0044] Comparative Example 4 Unlike Example 4, the water-reducing agent prepared in Comparative Example 2 was used.

[0045] Comparative Example 5 Unlike Example 4, the water-reducing agent prepared in Comparative Example 3 was used.

[0046] Comparative Example 6 Unlike Example 4, the low-foaming polyether obtained in Comparative Example 4 was used.

[0047] Comparative Example 7 Unlike Example 4, the low-foaming polyether obtained in Comparative Example 5 was prepared.

[0048] Experimental Example 1 Examples 1-5 and Comparative Examples 1-5 were mixed with concrete, and their water-reducing and flow-retention properties were tested. The specific testing methods are as follows: The water-reducing property test was conducted according to the methods specified in GB 8076-2008 "Concrete Admixtures". Under the standard C40 concrete mix proportion, the shrinkage-reducing admixture was added to the concrete and mixed evenly. The initial slump (mm) of the fresh concrete was determined using a slump test and compared with a reference sample of commercially available water-reducing agent. The water reduction rate (%) was calculated as follows: Water reduction rate (%) = [(Reference water consumption - Test sample water consumption) / Reference sample water consumption] × 100%. The flow retention test method is as follows: After the water reduction test, the slump (mm) of the fresh concrete is measured at 30 minutes, 60 minutes, and 90 minutes to evaluate the flow retention. The slump loss rate (%) at 60 minutes is usually used as the main indicator; slump loss rate (%) = [(initial slump - 60-minute slump) / initial slump] × 100%. The final test results are shown in Table 3.

[0049] Table 3 Results of water-reducing and flow retention tests

[0050] Under the conditions of Examples 1-5, the water reduction rate was relatively high and the slump loss rate at 60 minutes was small. Under the conditions of Examples 1-5, by adjusting the type and dosage of water-reducing agent, the type and dosage of low-foaming polyether, and the mixed glass and dosage of alkali-resistant additives, the hydrogen bond network and low surface tension worked synergistically, resulting in a high water reduction rate and excellent flow retention. In contrast, Comparative Example 1 only added water-reducing agent without polyether and additives, resulting in a low water reduction rate and poor flow retention. In Comparative Example 2, the dosage of the shrinkage-reducing admixture in cement was increased to 2.0%, resulting in a high water reduction rate, but the flow retention was only average. In Comparative Example 3, the water-reducing agent obtained from Comparative Example 1 was polymerized unevenly, resulting in a low water reduction rate and poor flow retention. Comparative Examples 4 and 5, which were prepared using the methods of Comparative Example 2 and Comparative Example 3 respectively, ultimately showed poor water reduction and flow retention due to the lack of amide bonds and high-temperature hydrolysis.

[0051] Experiment Example 2 The shrinkage-reducing admixture-concrete mixtures obtained in Examples 1-5 and Comparative Examples 1-7 were tested for setting time, compressive strength at conventional pH, and compressive strength at pH 11. The specific testing methods are as follows: The setting time of the cement concrete mixture was determined using the penetration resistance method. The shrinkage-reducing admixture was added to standard C40 concrete, mixed thoroughly, and placed in a standard mold. The penetration resistance of the concrete mixture was periodically measured using a Vicat apparatus or a penetration resistance meter. The initial setting time was recorded when the resistance reached the initial setting standard of 0.5 MPa, and the final setting time was recorded when the resistance reached the final setting standard of 4.0 MPa. The results are expressed as initial setting time and final setting time, in minutes.

[0052] Shrinkage-reducing admixtures were added to standard C40 concrete mix design to prepare 150mm × 150mm × 150mm cubic specimens. These specimens were then cured under standard conditions (neutral water with pH ≈ 7) for 28 days. Axial pressure was applied at a constant loading rate of 0.5–1.0 MPa / s using a universal testing machine until specimen failure. The failure load was recorded, and the compressive strength (MPa) was calculated. The strength formula is: Compressive strength = Failure load / Compressed area; Similar to the pH 7 test, but the specimens were cured or immersed in an alkaline solution with pH=11 to simulate an alkaline environment. After molding, the specimens were initially cured under standard conditions for 24 hours, then transferred to a pH=11 solution for 28 days before testing. The temperature for all tests was 20±2℃, and the humidity was ≥95%. The test results are shown in Table 4.

[0053] Table 4. Results of setting time and compressive strength tests for Examples 1-5 and Comparative Examples 1-7

[0054] In Examples 1-5, by adjusting the dosage of the water-reducing agent, a suitable molecular weight and an amide group hydrogen bond network were achieved, ensuring uniform dispersion of cement particles, delaying the hydration reaction, and resulting in a moderate setting time. The low-foaming polyether ratio, with its low surface tension, synergistically reduced the introduction of air bubbles, optimized the pore structure, and improved the compressive strength at pH 7. Simultaneously, the alkali-resistant additives, with their synergistic effects, formed a protective layer in an alkaline environment at pH 11 using alkyltriethoxysilane and polyoxyethylene ether, reducing alkali erosion and maintaining high compressive strength. The initial and final setting times of these groups were moderate. In Example 4, the water-reducing agent ratio was the highest at 95.0 wt%, exhibiting a strong hydrogen bond network, optimal dispersibility, and the shortest but moderate setting time. The low-foaming polyether, as in Example 4, had increased C10-C16 alkyl glycoside chain length, the lowest surface tension, significantly reducing air bubbles and improving density. The alkali-resistant additive ratio was the lowest at 4.7 wt%, but the silane component was effective, protecting cement hydration products at pH 11, and maintaining optimal strength among the five examples.

[0055] Comparative Example 1, lacking low-foaming polyether and alkali-resistant additives, exhibited poor dispersibility, with numerous bubbles leading to shortened setting time, non-dense hydration products, low strength at pH 7, and severe alkali corrosion at pH 11 without additive protection, resulting in a significant decrease in strength. Comparative Example 2, with a high proportion of water-reducing agent, showed good initial dispersion and moderate setting time, achieving high strength at pH 7. However, excessive additives introduced excess water, increasing porosity, and accelerating the alkali reaction at pH 11, leading to a substantial decrease in strength. Comparative Example 3, using the same water-reducing agent as Comparative Example 1, exhibited uneven polymerization, a wide molecular weight distribution, increased branching, poor dispersibility, shortened setting time, and susceptibility to alkali corrosion. Comparative Example 4 lacked an amide group hydrogen bond network, resulting in weak intermolecular interactions and poor dispersibility. The results were poor, with the shortest setting time and intensified alkali hydrolysis, leading to a significant decrease in strength. Comparative Example 5: High temperature and rapid heating caused amide group hydrolysis, resulting in structural instability and decreased setting time and alkali resistance. Comparative Example 6: Low-foaming polyether was prepared from Comparative Example 4. The low molecular weight of the C8-C10 short-chain alkyl glycosides resulted in excessive foaming due to low surface tension, prolonging the setting time. At pH 11, the short-chain structure exhibited poor alkali resistance, increased porosity in the foam, and a slight decrease in strength. Comparative Example 7: Low-foaming polyether was prepared from Comparative Example 5. High KOH and high temperature resulted in excessively large molecular weight, high viscosity, uneven dispersion, and the longest setting time. At pH 11, the high-temperature polymerization byproducts were easily affected by alkali, leading to a significant decrease in strength.

[0056] Experimental Example 3 The shrinkage-reducing admixture-concrete mixtures obtained in Examples 1-5 and Comparative Examples 1-7 were tested for 7-day and 28-day drying shrinkage rates. The specific test methods are as follows: the drying shrinkage rate of concrete was determined by the length variation method.

[0057] Shrinkage-reducing admixtures were added to standard C40 concrete and mixed thoroughly before being molded into prismatic specimens measuring 100mm × 100mm × 515mm. After molding, the specimens were cured under standard conditions at 20±2℃ and ≥95% humidity for 24 hours. The initial length was measured after demolding with an accuracy of 0.001mm. The specimens were then placed in a dry environment at 20±3℃ and 60±5% relative humidity, and the length change was measured at 7 days and 28 days. Drying shrinkage rate = (L0 - L...) t ) / (L0-2×L g L0 is the initial length (mm), L t L is the length of day t (mm). g The probe length is shown in mm; negative values ​​indicate shrinkage. The test results are shown in Table 5.

[0058] Table 5 Results of drying shrinkage test

[0059] Examples 1-5, through optimization of the hydrogen bond network of the water-reducing agent, the surface tension of the low-foaming polyether, and the proportion of alkali-resistant additives, all exhibited low drying shrinkage rates at 7 days and 28 days, with Example 4 showing the best performance. The low-foaming polyether in Example 5 was prepared in Example 4; the long-chain alkyl glycosides enhanced hydrophobicity, reduced surface water tension, and inhibited early shrinkage. At 28 days, the same proportion of alkali-resistant additives synergistically reduced long-term drying cracks.

[0060] Comparative Examples 1-7 exhibited increased shrinkage rates due to the lack of synergistic components or preparation defects. Specifically, Comparative Example 1, lacking low-foaming polyether and alkali-resistant additives, exhibited poor dispersibility, with numerous bubbles increasing capillary tension, resulting in a high shrinkage rate at 7 days. At 28 days, without additives to fill the pores, severe water evaporation led to a significant increase in shrinkage. Comparative Example 2, with a high proportion of water-reducing agent, showed good initial dispersion and a relatively low shrinkage rate at 7 days; however, excessive water introduction increased later evaporation, resulting in a higher shrinkage rate at 28 days. Comparative Example 3, using the same water-reducing agent as Comparative Example 1, exhibited uneven polymerization, a wide molecular weight distribution, and increased branching, leading to uneven dispersion and a high shrinkage rate at 7 days. The uneven structure easily formed microcracks, exacerbating shrinkage. Comparative Example 4 lacked an amide-based hydrogen bond network, resulting in weak intermolecular interactions, poor dispersibility, and the highest shrinkage rate at 7 days. At 28 days, the lack of hydrogen bonds inhibited uneven hydration, resulting in a significant increase in shrinkage. Comparative Example 5, the water-reducing agent used in the preparation of Comparative Example 3, suffered from amide group hydrolysis due to high temperature and rapid heating, leading to structural instability. As time increased, the hydrolysis products increased porosity, exacerbating drying shrinkage. Comparative Example 6, the low-foaming polyether used in the preparation of Comparative Example 4, had low molecular weight (C8-C10 short chains) and excessively low surface tension, resulting in excessive foaming. It exhibited a high shrinkage rate at 7 days, and at 28 days, the foam increased porosity, leading to severe water evaporation. Comparative Example 7, the low-foaming polyether used in the preparation of Comparative Example 5, suffered from excessively high molecular weight due to high KOH and high temperature, resulting in high viscosity, uneven dispersion, and high shrinkage.

[0061] The above embodiments are merely illustrative examples and are not intended to limit the implementation. Those skilled in the art will recognize that other variations or modifications can be made based on the above description. It is neither necessary nor possible to exhaustively list all possible implementations; therefore, any obvious variations or modifications derived therefrom remain within the scope of protection of this invention.

Claims

1. A method for preparing a shrinkage-reducing admixture for concrete, characterized by, The preparation method is as follows: the water reducing agent and the low-foaming polyether are mixed by weight parts, stirred at room temperature until uniform and transparent, and an intermediate material mixture is obtained; the alkali-resistant additive is added to the intermediate material mixture, and uniform and transparent stirring is performed again to obtain the shrinkage-reducing admixture; The alkali-resistant additive is obtained by mixing alkyl triethoxysilane, polyoxyethylene ether, sodium dodecyl diphenyl ether disulfonate, deionized water and citric acid; The water reducing agent is obtained by copolymerization of the chain transfer agent mercaptoacetic acid, the unsaturated acid monomer acrylic acid, the shrinkage-reducing functional monomer butyl acrylate, the amide functional monomer acrylamide and the isopentenyl polyoxyethylene ether. The low-foaming polyether is obtained by ring-opening addition reaction of alkyl glycoside, propylene oxide and ethylene oxide.

2. The method of claim 1, wherein the method is characterized by, The alkyl length of the alkyl glycoside is C12-C16.

3. The method of claim 1, wherein the method is characterized by, The copolymerization process is as follows: the polymerization reaction is carried out under nitrogen protection; the isopentenyl polyoxyethylene ether is added to the reaction kettle, the deionized water is added, the stirring is started, the temperature is raised for complete dissolution, the oxidation initiator is added to the reaction kettle, and solution A and solution B are added dropwise; after the dropwise addition is completed, the water reducing agent is obtained by heat preservation reaction.

4. The method of claim 3, wherein the method is characterized by, The solution A is obtained by dissolving the reducing initiator and mercaptoacetic acid in the deionized water.

5. The method of claim 3, wherein the method is characterized by, The solution B is obtained by mixing acrylic acid, butyl acrylate and acrylamide.

6. The method of claim 1, wherein the method is characterized by, The ring-opening addition reaction process is as follows: the alkyl glycoside and potassium hydroxide are put into a high-pressure polymerization kettle, nitrogen protection, temperature rising and heat preservation are performed to make the material distribution uniform; propylene oxide is added, the reaction is started, the remaining propylene oxide is added after the pressure is stabilized; after the reaction is stabilized, ethylene oxide is added, the reaction is started, the remaining ethylene oxide is added after the pressure is stabilized, and the reaction is fully carried out, the temperature is reduced to room temperature, and acid neutralization is performed.

7. A shrinkage-reducing admixture for concrete, characterized by, The shrinkage-reducing admixture for concrete is prepared by the preparation method of any one of claims 1-6.

8. Use of the shrinkage-reducing admixture for concrete obtained by the production method according to claim 1, characterized in that, Application of the shrinkage-reducing admixture in C40 concrete.

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