A high-efficiency shrinkage reducing agent and preparation method thereof

Abstract

A high-efficiency shrinkage reducer and its preparation method, which uses an unsaturated polyoxyethylene ether monomer and a low-surface tension monomer with polymerization activity as raw materials. Under the action of an initiator and a chain transfer agent, it is gradually added dropwise and prepared in a single step at 55-65°C through a free radical polymerization reaction. The resulting shrinkage reducer can reduce the surface tension of the cement slurry extraction pore solution by 32.00%-40.00%. At a dosage of 0%-0.5% by mass of cement, the shrinkage reducer can significantly reduce the autogenous shrinkage and drying shrinkage of cement mortar, with the autogenous shrinkage rate reduced by 64.86%-71.89% in 3 days and the drying shrinkage reduced by 24.46%-34.74% in 56 days. The addition of the shrinkage reducer has little negative impact on the early compressive strength and is beneficial to improving the later compressive strength.

Description

Technical Field

[0001] The invention relates to a shrinkage reducing agent for cement-based materials and a preparation method thereof, and in particular to a high-efficiency shrinkage reducing agent and a preparation method thereof. Background Art

[0002] Shrinkage of cementitious materials refers to the reduction in volume after pouring, setting, and hardening. If improperly addressed, this can lead to serious problems such as cracking and failure in cementitious materials, significantly reducing the durability and service life of buildings. Even under sealed conditions, high-performance and ultra-high-performance cementitious materials experience significant volume shrinkage during cement hydration. This is due to a low water-to-cement ratio and the rapid loss of water due to excessive use of cementitious admixtures. Shrinkage-reducing agents are a new type of chemical admixture developed to mitigate shrinkage cracking in cementitious materials. They can reduce capillary tension by lowering the surface tension of the pore solution in the cement paste, effectively mitigating shrinkage in cementitious materials, particularly autogenous and drying shrinkage.

[0003] Traditional shrinkage-reducing agents synthesized through alkoxylation reactions require relatively harsh synthesis conditions (i.e., high temperature and pressure, 120-190°C, 0.1-0.4 MPa), which increases their production costs. Furthermore, the high dosage of traditional shrinkage-reducing agents in cement-based materials (e.g., 1%, 2%, or 3% of the total mass of the cementitious material) can negatively impact the compressive strength of cement-based materials. In recent years, some researchers have exploited the designable molecular structure of polycarboxylic acid-based high-efficiency water-reducing agents to introduce shrinkage-reducing functional groups into their side chains to produce shrinkage-reducing agents with water-reducing properties. These shrinkage-reducing agents are synthesized in two steps: esterification followed by polymerization or polymerization followed by esterification. Purification of the intermediate product is difficult, resulting in low product conversion rates and high manufacturing costs. Furthermore, due to the presence of adsorption groups such as carboxyl groups, these shrinkage-reducing agents can adsorb onto cement particles or hydration products, hindering the reduction of the surface tension of the pore solution and resulting in inferior shrinkage-reducing effects compared to traditional shrinkage-reducing agents.

[0004] CN111533856A, published on August 14, 2020, discloses an organic shrinkage-reducing agent, which is prepared from the following raw materials in the following molar ratio: the organic shrinkage-reducing agent is prepared from the following raw materials in the following molar ratio: allyl polyoxyethylene ether: polyethylene glycol monooleate: maleic anhydride: N-hydroxymethyl acrylamide: benzoyl peroxide: chain transfer agent = 1: (0.3-2): (0.3-1.5): (0.05-0.15): (0.01-0.1): (0.01-0.2). Although this organic shrinkage-reducing agent has the function of reducing shrinkage and can be used in concrete to reduce shrinkage cracking of concrete and improve the durability and service life of concrete, because it contains carboxylic acid groups, this will cause the shrinkage-reducing agent to adsorb onto cement particles or hydration products, reducing its effect. Summary of the Invention

[0005] The technical problem to be solved by the present invention is to overcome the above-mentioned defects of the prior art and provide a high-efficiency shrinkage reducing agent with low preparation process requirements, simple preparation operation steps, low manufacturing cost, good shrinkage reducing effect and low dosage.

[0006] A further technical problem to be solved by the present invention is to provide a method for preparing the high-efficiency shrinkage reducing agent.

[0007] The technical solution adopted by the present invention to solve the technical problem is a high-efficiency shrinkage reducing agent, characterized in that its molecular structure is as follows:

[0008]

[0009] In the formula, a and b respectively represent the degree of polymerization of each monomer in the polymer, R1 represents H or CH3, and R2 represents OH, CH2CH3, N(CH3)2, C4H9OCH2CH2OCH2CH2O or CH3(OCH2CH2)3; wherein, a=0~200, b=1~800.

[0010] The present invention further solves the technical problem by adopting a technical solution that provides a method for preparing a high-efficiency shrinkage reducing agent, which is prepared from the following raw materials in the following weight ratio: 80-250 parts of a macromonomer, 0.1-5 parts of a chain transfer agent, 1.5-45 parts of a low-surface tension monomer with polymerization activity, 0.1-13 parts of an initiator, and 80-250 parts of water; the macromonomer is a linear polymer with a molecular weight of 500-7000 and a terminal polymerizable group;

[0011] The specific operation steps are as follows: dissolving 0.1-13 parts of an initiator in 10-50 parts of deionized water to obtain solution A; dissolving 0.1-5 parts of a chain transfer agent in 10-50 parts of deionized water to obtain solution B; mixing 1.5-45 parts of a low surface tension monomer with 10-50 parts of deionized water to obtain solution C; first, charging 80-250 parts of a macromonomer and 80-250 parts of deionized water into a flask with a stirring bar and heating to 55-65°C; when the macromonomer is completely dissolved, using a peristaltic pump to simultaneously add solutions A, B and C to the container at a constant rate over 1.5-4 hours; after the feeding process is completed, maintaining the mixture at 55-65°C for 1-3 hours with the stirring bar still rotating; then stopping stirring and cooling to below 40°C; then adding a 20%-40% mass concentration of NaOH solution for neutralization until the pH value reaches 6.5-7.0.

[0012] Preferably, the weight ratio of each raw material is: 144 parts of macromonomer, 0.76 parts of chain transfer agent, 7.69 parts of low surface tension monomer, 1.52 parts of initiator, and 144 parts of water.

[0013] Preferably, the weight ratio of each raw material is: 96 parts of macromonomer, 0.55 parts of chain transfer agent, 13.93 parts of low surface tension monomer, 4.40 parts of initiator, and 96 parts of water.

[0014] Preferably, the weight ratio of each raw material is: 192 parts of macromonomer, 1.13 parts of chain transfer agent, 17.18 parts of low surface tension monomer, 9.06 parts of initiator, and 192 parts of water.

[0015] Preferably, the macromonomer is one or more selected from allyl polyethylene glycol, isopentanol polyethylene glycol, alkoxylated isopentanol, α-allyl-ω-methoxy polyethylene glycol, allyl polyoxyethylene ether sulfate, butenyl alkylene polyoxyethylene-polyoxypropylene ether, methyl allyl polyethylene ether, isopentene polyoxyethylene ether, and alkoxylated hydroxybutyl vinyl ether.

[0016] Preferably, the low surface tension monomer with polymerization activity is one or more selected from dimethylaminoethyl acrylate, hydroxyethyl acrylate, methyl acrylate, butyl acrylate, diethylene glycol monobutyl ether acrylate, polyethylene glycol acrylate, triethylene glycol monomethyl ether acrylate, diethylene glycol acrylate, and diethylene glycol dipropylene glycol acrylate.

[0017] Preferably, the initiator is one or more of tert-butyl hydroperoxide, benzoyl peroxide, sodium lauroyl peroxide, ammonium persulfate, sodium persulfate, potassium persulfate, sodium hydrogen persulfate, and hydrogen peroxide.

[0018] Preferably, the chain transfer agent is one or more of 3-mercaptopropionic acid, 3-mercaptoacetic acid, 2-mercaptoethanol, isopropyl 3-mercaptopropionate, and n-mercaptoethanesulfonic acid.

[0019] The unsaturated macromonomer polyoxyethylene ether used as a raw material in the preparation method of the present invention has good polymerization activity. The polymerization reaction is carried out at 55-65°C. The preparation process adopts a dropwise addition method with stirring, which forms a block structure in the molecular structure of the synthesized product and fully utilizes the steric hindrance effect of the macromonomer. In addition, the selection of unsaturated low-surface-tension small monomers reduces the surface tension of the cement paste pore solution by adjusting the ratio of hydrophilic and lipophilic groups. The polymerization-active small monomers, represented by hydroxyethyl acrylate and dimethylaminoethyl acrylate, can significantly reduce the surface tension of the solution, thereby improving the shrinkage-reducing effect of the shrinkage-reducing agent.

[0020] The high-efficiency shrinkage reducing agent of the present invention has a relatively high molecular weight, a comb-shaped block structure in its molecular structure, and contains low surface tension groups, which is beneficial for reducing the surface tension of the cement paste pore solution, thereby improving the shrinkage reducing effect of the shrinkage reducing agent.

[0021] When the dosage of the shrinkage reducer of the present invention is 0%-3%, the surface tension of the cement slurry pore solution first decreases significantly and then gradually tends to be flat. When the dosage is 0.5%, the surface tension of the slurry pore solution tends to be stable, and the surface tension can be reduced to 38.00mN / m; when the dosage of the obtained shrinkage reducer is 0%-0.5%, the self-shrinkage rate of the mortar after 3 days of sealing and curing can be reduced by 64.86%-71.89%. The drying shrinkage rate of the mortar after 56 days of drying and curing can be reduced by 24.46%-34.74%. The obtained shrinkage reducer has little effect on the compressive strength of the mortar. When the dosage is 0%-0.5%, the compressive strength of the mortar on the 3rd day of curing is reduced by 2.10%-7.33%, while the compressive strength after 28 days of curing is slightly increased.

[0022] Compared with the prior art, the present invention selects polyoxyethylene ether with good polymerization activity and a small monomer with low surface tension having polymerization activity, and polymerizes at 55-65°C through a one-step free radical copolymerization synthesis method to prepare a new polymer shrinkage reducer with different structures. The shrinkage reducer does not contain adsorption groups such as carboxyl groups and will not adsorb onto solid particles. During the synthesis process, the molecular structure of the shrinkage reducer is adjusted by adding the mixture while stirring, adjusting its hydrophilic and lipophilic groups, and improving the ability of the shrinkage reducer to reduce the surface tension of the cement slurry pore solution. The shrinkage reducer is prepared at 55-65°C through a one-step free radical copolymerization synthesis method, which greatly reduces production costs and energy consumption, and the preparation process is simple. BRIEF DESCRIPTION OF THE DRAWINGS

[0023] Figure 1 This is the gel chromatogram of the shrinkage-reducing agent SRAs-1 prepared in Example 1;

[0024] Figure 2 This is the gel chromatogram of the shrinkage reducing agent SRAs-2 prepared in Example 2. DETAILED DESCRIPTION

[0025] In order to more clearly demonstrate the purpose and advantages of the present invention, the present invention is described in detail below with reference to the embodiments.

[0026] The raw materials used in the examples include: industrial-grade prenol polyoxyethylene ether with an average molecular weight of 2400, 2-hydroxyethyl acrylate (HEA), dimethylaminoethyl acrylate (DMAEA), ammonium persulfate (APS), 3-mercaptopropionic acid (MPA), and sodium hydroxide (NaOH), all of which are analytical grade reagents. Ultrapure water was used as the solvent.

[0027] The cement used was P·I 42.5 standard cement, in accordance with Chinese standard GB 8076-2008, mixed with tap water. The sand used was ISO standard sand from Xiamen, China. A polycarboxylate high-efficiency water reducer was used to adjust the performance of the cement mortar.

[0028] All chemical reagents and materials were obtained from common commercial sources.

[0029] Example 1

[0030] Solution A was prepared by dissolving 4.397 g of ammonium persulfate (APS) in 39.573 g of deionized water; Solution B was prepared by dissolving 0.550 g of 3-mercaptopropionic acid (MPA) in 26.95 g of deionized water; and Solution C was prepared by dissolving 13.93 g of 2-hydroxyethyl acrylate (HEA) in 13.93 g of deionized water. First, 96 g of industrial-grade prenyl polyoxyethylene ether and 96 g of deionized water were placed in a three-necked round-bottom flask with a stirrer and heated to 60°C. Once the prenyl polyoxyethylene ether was completely dissolved, Solutions A, B, and C were added dropwise to the container at a constant rate over 2.5 hours using a peristaltic pump. After the addition process was complete, the mixture was maintained at 60°C for another 2 hours with continuous stirring using a stirrer. Stirring was then stopped and the mixture was cooled to below 40°C. Then, a 40% NaOH solution was added for neutralization to obtain a final product with a pH value of 6.9, namely, the shrinkage reducing agent SRAs-1. Figure 1 .

[0031] Example 2

[0032] Solution A was prepared by dissolving 4.527 g of ammonium persulfate (APS) in 40.746 g of deionized water; Solution B was prepared by dissolving 0.566 g of 3-mercaptopropionic acid (MPA) in 27.734 g of deionized water; and Solution C was prepared by dissolving 17.18 g of dimethylaminoethyl acrylate (DMAEA) in 17.18 g of deionized water. First, 96 g of industrial-grade prenyl polyoxyethylene ether and 96 g of deionized water were placed in a three-necked round-bottom flask with a stir bar and heated to 60°C. Once the prenyl polyoxyethylene ether was completely dissolved, Solutions A, B, and C were added dropwise to the container at a constant rate over 2.5 hours using a peristaltic pump. After the addition process was complete, the mixture was maintained at 60°C for another 2 hours with continuous stirring using a stir bar. Stirring was then stopped and the mixture was cooled to below 40°C. Then, a 40% NaOH solution was added for neutralization to obtain a final product with a pH value of 6.8, namely, the shrinkage reducing agent SRAs-2. Figure 2 .

[0033] Surface tension test

[0034] Under the condition of a water-cement ratio of 0.30, pore solution was extracted from cement paste containing different dosages of new polymer shrinkage reducer (by cement mass), and surface tension test was performed. The high-efficiency new polymer shrinkage reducer synthesized by the present invention does not have a water-reducing function, and 0.08% polycarboxylic acid high-efficiency water reducer was used to improve the workability of cement paste. A certain amount of cement, water, 0.08% water reducer and 0%, 0.1%, 0.2%, 0.5%, 1%, 2% and 3% new polymer shrinkage reducer were mixed and manually stirred for 5 minutes to prepare cement paste. After preparation, a centrifuge was immediately used to run at a speed of 8000rpm / min for 8 minutes to extract the pore solution of the cement paste, and the pore solution was filtered using a filter membrane with a pore size of 0.22μm. About 20mL of the extracted pore solution was collected and measured using a fully automatic surface tension meter (KINO, A-601, USA). The test results are shown in Table 1.

[0035] As shown in Table 1, the surface tension of the extracted pore solutions decreases significantly when the shrinkage reducer dosage increases from 0% to 0.5%, due to the presence of non-polar components in the shrinkage reducer. However, as the shrinkage reducer dosage increases beyond 0.5%, the surface tension of the pore solutions decreases slightly and stabilizes. For example, when the shrinkage reducer dosage increases from 0% to 0.5%, the surface tension of the extracted pore solutions of SRAs-1 and SRAs-2 decreases from 62.80 mN / m to 45.92 mN / m and 40.65 mN / m, respectively. When the shrinkage reducer dosage increases to 3%, the surface tensions of these pore solutions reach 42.83 mN / m and 38.20 mN / m, respectively.

[0036] Table 1 Surface tension test results of shrinkage reducing agents SRAs-1-SRAs-2 obtained in Examples 1-2

[0037] sample 0％ 0.1％ 0.2％ 0.5％ 1％ 2％ 3％ SRAs-1 62.80 58.16 50.80 45.92 44.33 43.20 42.83 SRAs-2 62.80 55.40 45.62 40.65 39.50 38.80 38.20

[0038] Shrinkage test

[0039] The autogenous shrinkage of cement mortar containing the shrinkage-reducing agent was measured using a corrugated polyethylene tube with an inner diameter of 20 mm, an outer diameter of 30 mm, and a length of 340 ± 5 mm using a non-contact probe. The mortar specimens were sealed and cured, and the drying shrinkage of the cement mortar containing the shrinkage-reducing agent was tested according to JC / T603-2004, "Test Method for Drying Shrinkage of Cement Mortar." The shrinkage mortar specimens were dried in a drying curing room (T = 20 ± 3°C, RH = 50 ± 4%) for 56 days. The shrinkage-reducing agent dosage was 0.5% by weight of the cement. The shrinkage results are shown in Table 2.

[0040] As shown in Table 2, the shrinkage-reducing agent of the present invention significantly reduced the three-day autogenous shrinkage of the cement mortar by 71.89% and 64.86%, respectively, compared to the control group without the shrinkage-reducing agent. This reduction is attributed to the reduced surface tension and delayed hydration reaction. After 56 days of drying, a 0.5% addition of the shrinkage-reducing agent effectively reduced the drying shrinkage of the cement mortar by 24.46% and 34.74%, respectively. The superior drying shrinkage inhibition ability of SRAs-2 is likely related to the lower surface tension of its pore solution.

[0041] Table 2 Shrinkage test results of shrinkage reducing agents SRAs-1-SRAs-2 obtained in Examples 1-3

[0042]

[0043] In summary, the shrinkage-reducing agent of the present invention has a significant effect of reducing the shrinkage of cement mortar at a low dosage of 0.5% by weight of cement.

Claims

1. A high efficiency shrinkage reducing agent, characterized in that, Its molecular structure is as follows: In the formula, a and b respectively represent the degree of polymerization of each monomer in the polymer, R1 represents H or CH3, and R2 represents OH, CH2CH3, N(CH3)2, C4H9OCH2CH2OCH2CH2O or CH3(OCH2CH2)3; wherein, a=0~200, b=1~800, and a is not equal to 0.

2. The method for preparing a high-efficiency shrinkage-reducing agent according to claim 1, wherein The invention is prepared from the following raw materials in the following weight ratio: 80-250 parts of a macromonomer, 0.1-5 parts of a chain transfer agent, 1.5-45 parts of a low surface tension monomer with polymerization activity, 0.1-13 parts of an initiator, and 80-250 parts of water; the macromonomer is a linear polymer with a molecular weight of 500-7000 and a terminal polymerizable group; The specific operation steps are as follows: 0.1-13 parts of initiator are dissolved in 10-50 parts of deionized water as solution A, 0.1-5 parts of chain transfer agent are dissolved in 10-50 parts of deionized water as solution B, and a mixture of 1.5-45 parts of low surface tension monomer and 10-50 parts of deionized water is used as solution C; 80-250 parts of macromonomer and 80-250 parts of deionized water are charged into a flask with a stirring bar and heated to 55-65°C; when the macromonomer is completely dissolved, solutions A, B and C are added to the container at a constant rate over 1.5-4 hours using a peristaltic pump; after the feeding process is completed, the mixture is maintained at 55-65°C for 1-3 hours with the stirring bar still rotating; then stirring is stopped and the mixture is cooled to below 40°C; then a NaOH solution with a mass concentration of 20%-40% is added for neutralization to obtain a final product with a pH value of 6.5-7.

0.

3. The method for preparing a high-efficiency shrinkage-reducing agent according to claim 2, wherein: The macromonomer is prenol-based polyoxyethylene ether.

4. The method for preparing a high-efficiency shrinkage-reducing agent according to claim 3, wherein: The initiator is one or more of tert-butyl hydroperoxide, benzoyl peroxide, sodium lauroyl peroxide, ammonium persulfate, sodium persulfate, potassium persulfate, sodium hydrogen persulfate, and hydrogen peroxide.

5. The method for preparing a high-efficiency shrinkage-reducing agent according to claim 3 or 4, characterized in that: The chain transfer agent is one or more of 3-mercaptopropionic acid, 3-mercaptoacetic acid, 2-mercaptoethanol, isopropyl 3-mercaptopropionic acid octyl ester, and n-mercaptoethane sulfonic acid.

6. The method for preparing a high-efficiency shrinkage-reducing agent according to claim 2, wherein: The weight ratio of the raw materials is: 144 parts of macromonomer, 0.76 parts of chain transfer agent, 7.69 parts of low surface tension monomer, 1.52 parts of initiator, and 144 parts of water.

7. The method for preparing a high-efficiency shrinkage-reducing agent according to claim 2, wherein: The weight ratio of the raw materials is: 96 parts of macromonomer, 0.55 parts of chain transfer agent, 13.93 parts of low surface tension monomer, 4.40 parts of initiator, and 96 parts of water.

8. The method for preparing a high-efficiency shrinkage-reducing agent according to claim 2, wherein: The weight ratio of the raw materials is: 192 parts of macromonomer, 1.13 parts of chain transfer agent, 17.18 parts of low surface tension monomer, 9.06 parts of initiator, and 192 parts of water.

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