A slow-release dispersant for fair-faced concrete and a preparation method thereof

Abstract

This invention relates to the field of building materials technology, and particularly to a slow-release dispersant for fair-faced concrete and its preparation method. The method uses a multifunctional alcohol compound as an initiator, and through a cationic ring-opening copolymerization reaction, copolymerizes L-lactide with a cyclic acetal monomer containing halogenated groups to prepare a multi-arm polylactic acid (PLA) macromolecular initiator containing multiple halogenated active sites. Then, using atom transfer radical polymerization as a grafting method, a mixed monomer composed of acrylate, styrene, and polyethylene glycol macromonomers is grafted onto the multi-arm PLA backbone to obtain the target slow-release dispersant for fair-faced concrete. This slow-release dispersant, due to its multi-arm topology and the synergistic hydrolysis characteristics of the main and side chains, not only possesses excellent slow-release performance and can maintain the dispersibility of concrete for a long time, but also significantly reduces the viscosity of the concrete mixture, while imparting good steady-state fluidity and high thixotropy to the concrete, solving the problems of easy segregation or rapid loss of fluidity of the slurry during construction.

Description

Technical Field

[0001] This invention relates to the field of building materials technology, specifically to a slow-release dispersant for fair-faced concrete and its preparation method. Background Technology

[0002] Fair-faced concrete, as a green building material that is "formed in one go and has no decorative surface layer," is widely used in building facades, landscape components, and prefabricated structures due to its natural texture, simplified construction process, and low-carbon environmental protection attributes. However, fair-faced concrete has extremely high requirements for the fluidity, stability, and uniformity of the slurry during construction. Traditional concrete dispersants have the following core technical pain points in application, making it difficult to meet the stringent requirements of fair-faced concrete: First, the slow-release effect is unstable, and the fluidity fluctuates greatly due to cement hydration after initial dispersion, making on-site control difficult; second, the viscosity reduction effect is unstable, and it is difficult to balance "low viscosity" and "high stability" by relying on a single polyether side chain, while also failing to effectively control thixotropy, resulting in easy slurry segregation or pumping difficulties during construction.

[0003] To prepare dispersants suitable for fair-faced concrete, solve the problems of pouring fair-faced concrete mixtures, and improve the molding quality of fair-faced concrete, numerous researchers have conducted related exploratory work. Chinese patent CN114276550A discloses an organic-inorganic composite admixture for fair-faced concrete. This technology uses low-hydrogen silicone oil, carboxyl and cationic monomers, and polyether segments, which can achieve dispersion while improving defoaming effects and enhancing the surface quality of fair-faced concrete. Chinese patent CN120483572A discloses a low-air-entraining polycarboxylate high-performance water-reducing agent suitable for fair-faced concrete. It is prepared by free radical polymerization of unsaturated fluorinated vinyl monomers, unsaturated polyoxyethylene ethers, and unsaturated carboxylic acids in the presence of an initiator and chain transfer agent. Utilizing the defoaming effect of fluorine atoms, the polycarboxylate water-reducing agent possesses both defoaming and dispersing properties. Chinese patent CN110776273A discloses an admixture for fair-faced concrete, which is composed of a polycarboxylate superplasticizer, a nano-silicone defoamer, a surfactant, and a modified synthetic thickener. The defoaming effect is achieved through the rational configuration of polydimethylsiloxane, hydroxyl, and polyether groups, thereby improving the surface smoothness of fair-faced concrete. Chinese patent CN104591585A discloses a polycarboxylate superplasticizer for fair-faced concrete and its preparation method. This patented technology introduces a prepolymer of diethylenetriamine and maleic anhydride into the structure of the polycarboxylate superplasticizer. Using this structure, the polycarboxylate superplasticizer can produce freshly mixed concrete with good workability and encapsulation properties, and can also achieve a mirror-like finish on fair-faced concrete.

[0004] In general, existing patented technologies focus on introducing functional monomers into the polycarboxylic acid backbone to adjust the HLB value of polycarboxylic acid and thus achieve bubble suppression. However, existing patented technologies neglect the following: ① A certain air content is the basis for the workability of concrete mixtures, which can effectively suppress segregation and bleeding and reduce the viscosity of concrete mixtures; ② The influence of the hydration process of concrete mixtures is not considered. Usually, as the hydration process evolves, the cohesiveness of concrete gradually increases, making it difficult for air bubbles in concrete mixtures to rise; ③ The balance between the cohesiveness and consistency of concrete mixtures is crucial. For fair-faced concrete, the slurry must have integrity during pouring and the consistency must decrease during vibration to achieve defoaming. Therefore, thixotropy is the most important aspect of dispersants for fair-faced concrete.

[0005] Therefore, a slow-release dispersant for fair-faced concrete was developed. By designing a multi-arm grafted structure with hydrolyzable acetal-ester bonds in the main chain and degradable ester bonds and hydrophilic polyethylene glycol segments in the side chains, the multi-arm topology and the synergistic hydrolysis characteristics of the main and side chains not only possess excellent slow-release performance and can maintain the dispersibility of concrete for a long time, solving the problems of easy segregation of slurry or excessive loss of fluidity during construction, but also impart good high thixotropy to the concrete after it is poured into the mold. It significantly reduces the viscosity of the concrete mix during vibration and does not cause aggregate settling, maintaining the uniformity and stability of the concrete mixture. After hardening, it can achieve excellent appearance quality. Summary of the Invention

[0006] To address the problems in related technologies, this invention proposes a slow-release dispersant for fair-faced concrete and its preparation method, thereby overcoming the aforementioned technical problems existing in the prior art. The slow-release dispersant for fair-faced concrete of this invention, with its multi-arm topological structure and synergistic hydrolysis characteristics of the main and side chains, not only possesses excellent slow-release performance and can maintain the dispersibility of concrete for a long time, but also significantly reduces the viscosity of concrete mix. Simultaneously, it imparts good steady-state fluidity and high thixotropy to the concrete, solving the problems of easy segregation of the slurry or excessively rapid loss of fluidity during construction.

[0007] To achieve the above objectives, the present invention provides the following technical solution: A slow-release dispersant for fair-faced concrete, wherein the molecular structure of the slow-release dispersant for fair-faced concrete is shown in Formula I: Formula I The slow-release dispersant for fair-faced concrete is a multi-arm grafted structure with a main chain containing hydrolyzable acetal-ester bonds and side chains containing degradable ester bonds and hydrophilic polyethylene glycol segments. in, It is one of the dehydroxylated structures of 1,2,3-propanetriol, trimethylolpropane, and pentaerythritol; when When n is one of the structures 1,2,3-propanetriol or trimethylolpropane, n is 3. When the structure is the dehydroxylated form of pentaerythritol, n is 4; x is an integer from 3 to 20, y is an integer from 11 to 40, z is an integer from 5 to 25; a is 1 or 2, c:d:e:y = 2~5:1~4:1~1.5:1; R1 and R2 can be H atoms or methyl groups, respectively, and R3 is a H atom.

[0008] Preferably, the number average molecular weight of the slow-release dispersant for fair-faced concrete is 10,000 to 100,000.

[0009] To achieve the above objectives, the present invention also provides the following technical solution: A method for preparing a slow-release dispersant for fair-faced concrete includes the following steps: Step 1: Add 3-20 mmol of L-lactide to a 100 mL Schlenk tube, followed by 15-20 mL of anhydrous dichloromethane. Seal the Schlenk tube and magnetically stir at room temperature until completely dissolved. Under argon protection, add a polyfunctional alcohol initiator containing 1 mmol of hydroxyl group (dissolved in 2 mL of anhydrous dichloromethane) to the Schlenk tube using a microsyringe. Then add 0.6-0.8 mmol of trifluoromethanesulfonic acid using a microsyringe. Transfer the Schlenk tube to a 4-6°C low-temperature reaction bath. Maintaining argon protection, add 11-40 mmol of halocyclic acetal monomer dropwise to the Schlenk tube using a constant flow pump over 2-4 hours. After the reaction was completed, the reaction was continued for 46-49 hours. After the reaction was completed, 0.78-1.04 mmol of triethylamine was added. The reaction solution was transferred to a 100 mL single-necked flask, and 100-150 mmol of anhydrous magnesium sulfate was added. The solution was dried for 12-16 hours. The desiccant was removed by filtration. The filtrate was slowly added dropwise to 50 mL of hexane / diethyl ether mixed solvent (volume ratio 1:1). After standing for 30 minutes, the precipitate was collected by vacuum filtration through a Buchner funnel and redissolved in 5-8 mL of anhydrous dichloromethane. The above precipitation operation was repeated once. Finally, the precipitate was transferred to a vacuum drying oven and dried at 42-44 °C and a vacuum degree ≤1 mmHg for 9-11 hours until constant weight was obtained to obtain a multi-arm PLA-based multi-site macromolecular initiator. Step 2: Dissolve the multi-arm PLA-based multi-site macromolecular initiator obtained in Step 1 (containing 11-40 mmol of halogenated units), 0.55-2 mmol of Cu, and 0.5-2.2 mmol of N,N,N',N'',N'''-pentamethyldiethylenetriamine in 45-80 mL of toluene and place in a 500 mL flask; purge with argon gas to remove oxygen, place in a 30-35 °C water bath, and stir for 20-30 min to complete the pre-activation of the catalytic system; add 22-200 mmol of acrylate and 11-60 mmol of styrene to the pre-activated system using a constant flow pump, mix thoroughly, raise the water bath temperature to 50-52 °C, react for 40-50 min, and then add dropwise using a constant flow pump. 11-160 mmol of polyethylene glycol macromonomer (pre-dissolved in 100 mL toluene) was added dropwise to the pre-activated system over 40-50 min. After the addition was complete, the reaction was continued at 58-60 °C for 48 h. The mixture was then cooled to room temperature and the reaction was terminated by contact with air. The reaction solution was slowly added dropwise to 250 mL of a hexane / diethyl ether mixed solvent (volume ratio 1:1), and a precipitate was formed. The precipitate was collected by filtration and redissolved in 150 mL of toluene. The precipitation process was repeated twice. The precipitate was transferred to a vacuum drying oven and dried at 40 °C and a vacuum degree ≤1 mmHg for 12-13 h to obtain a slow-release dispersant for concrete.

[0010] Preferably, in step one, the multifunctional alcohol initiator is one or a combination of 1,2,3-propanetriol, trimethylolpropane, and pentaerythritol.

[0011] Preferably, in step two, the halogenated cyclic acetal monomer is one or more combinations of 2-bromomethyl-1,3-dioxolane, 2-chloromethyl-1,3-dioxolane, 2-iodomethyl-1,3-dioxolane, 2-bromomethyl-1,3-dioxane, 2-chloromethyl-1,3-dioxane, and 2-iodomethyl-1,3-dioxane.

[0012] Preferably, in step two, the acrylate is one or a combination of acrylate and methacrylate.

[0013] Preferably, in step two, the polyethylene glycol macromonomer is one or more combinations of methyl allyl polyethylene glycol ether and allyl polyethylene glycol ether.

[0014] Compared with the prior art, the beneficial effects of the present invention are: (1) The present invention is a slow-release dispersant for fair-faced concrete and its preparation method. The slow-release dispersant adopts a multi-arm structure, and each wall contains a large number of large hydrolyzable ester bonds. In the alkaline environment of concrete, it acts as a primary release agent. After hydrolysis and cleavage, a large number of comb-shaped structures are formed. The acrylate in the comb-shaped structure continues to hydrolyze and acts as a secondary release structure, thereby exposing the carboxyl groups to act and adsorb with the cementitious materials, and playing a dispersing role. This gradient release mechanism avoids the segregation and bleeding effect caused by excessive release of free water in the concrete mixture, and effectively prevents surface water ripples, sand appearance and floating black from causing appearance quality problems. (2) The present invention is a slow-release dispersant for clear water concrete and its preparation method. The appropriate proportion of benzene rings in the molecular structure adjusts the hydrolysis rate of the initial structure. When the secondary structure after water separation is the secondary structure of hydrolysis and adsorption, the benzene rings in the secondary structure have excellent solution surface tension regulation effect. Due to the presence of benzene rings, the main chain rigidity of the hydrolyzed secondary structure is enhanced, which effectively avoids the increase in cohesion caused by desorption and realizes efficient and high-speed desorption of bubbles. (3) The present invention is a slow-release dispersant for fair-faced concrete and its preparation method. By adjusting the molar ratio of rigid units, hydrophobic units and hydrophilic units in the side chain, the hydrophilic-hydrophobic balance of the dispersant can be precisely adjusted: the small molecule polyether segments can quickly reduce the initial viscosity of the slurry, while the acrylate and styrene copolymer segments can enhance the anti-bleeding ability of the slurry, so that the viscosity of fair-faced concrete slurry can be stably controlled. (4) The present invention is a slow-release dispersant for fair-faced concrete and its preparation method. The multi-arm topological structure of the slow-release dispersant gives the slurry excellent thixotropic properties. When the slurry is left to stand, a weak cross-linking network is formed between the multi-arm molecules, and the slurry remains viscous to prevent aggregate settling. When vibrated, the network is destroyed, and the fluidity of the slurry is improved to achieve dense casting. This solves the problem of traditional dispersant slurry being too thin and layered or too thick and difficult to pump, and improves the finishing effect of fair-faced concrete. (5) The present invention is a slow-release dispersant for fair-faced concrete and its preparation method. The multi-arm PLA main chain of the slow-release dispersant contains acetal-ester bonds, and the side chain copolymer units contain ester bonds. The two types of chemical bonds can be hydrolyzed synergistically in the natural environment or during the treatment of concrete waste: the main chain acetal-ester bonds are gradually broken under acidic / alkaline conditions, causing the multi-arm structure to disintegrate into short chain PLA fragments; the side chain ester bonds can be decomposed by microorganisms into small molecule organic acids and polyethylene glycol. Attached Figure Description

[0015] Figure 1 The results of the proton nuclear magnetic resonance spectroscopy analysis of Example SD-1; Figure 2 The infrared spectral analysis results are for Example SD-1; Figure 3 The results of gel permeation chromatography analysis are for Example SD-1. Detailed Implementation

[0016] The technical solutions of the present invention will be clearly and completely described below with reference to the accompanying drawings in the embodiments of the present invention.

[0017] All raw materials used in the examples were commercially available industrial-grade or analytical-grade reagents. L-lactide (Purac, 99% purity) was purified by sublimation after recrystallization twice from anhydrous 2-propanol. The halogenated cyclic acetal monomer was collected by vacuum distillation to collect the middle fraction. The polyethylene glycol macromonomer was dried under vacuum at 80°C for 2 hours to remove water, ensuring experimental repeatability.

[0018] Example 1 A method for preparing a slow-release dispersant for fair-faced concrete includes the following steps: Step 1: Add 10 mmol of L-lactide to a 100 mL Schlenk tube that has been preheated at 120°C for 2 hours and purged with argon three times. Then inject 18 mL of anhydrous dichloromethane, seal the tube, and place it at room temperature. Stir magnetically for 30 min until the L-lactide is completely dissolved. Under argon protection, inject a pentaerythritol initiator solution containing 1 mmol of hydroxyl group (0.25 mmol of pentaerythritol dissolved in 2 mL of anhydrous dichloromethane) into the Schlenk tube using a 100 μL microsyringe, and stir for 5 min. Then add 0.7 mmol of trifluoromethanesulfonic acid using a 50 μL microsyringe, continue stirring for 10 min, and transfer the Schlenk tube to a 5°C low-temperature reaction bath. Finally, use a constant flow pump to inject 15 mmol of... 2-Chloromethyl-1,3-dioxolane (ClDXL) was slowly added dropwise to a Schlenk tube over 3 hours. After the addition was complete, the reaction was maintained at 5°C for 48 hours. After the reaction was complete, 0.91 mmol of triethylamine was injected to quench the reaction. The reaction solution was transferred to a 100 mL single-necked flask, and 120 mmol of anhydrous magnesium sulfate was added. The solution was dried for 14 hours. After filtration through a 0.22 μm organic phase filter membrane, the filtrate was slowly added dropwise to 50 mL of a hexane / diethyl ether mixture (volume ratio 1:1) pre-cooled to 0°C with stirring. A white precipitate formed. After standing for 30 minutes, the precipitate was collected by vacuum filtration, redissolved in 6 mL of anhydrous dichloromethane, and the precipitation was repeated once. Finally, the precipitate was dried in a vacuum oven at 43°C and 0.8 mmHg for 10 hours to obtain a multi-arm PLA-based multi-site macromolecular initiator (denoted as PLA-Cl). 10 ); Step 2: Add PLA-Cl to a 500mL flask 10(Containing 15 mmol of chlorinated units), 1.2 mmol of CuCl and 1.3 mmol of N,N,N',N'',N'''-pentamethyldiethylenetriamine (PMDETA), were injected into 60 mL of toluene, sealed, and purged with argon gas for 15 min to remove oxygen. The mixture was then placed in a 32 °C water bath and stirred for 25 min to complete the pre-activation of the catalytic system (the solution was blue and transparent). A mixture of 60 mmol of hydroxyethyl methacrylate (HEMA) and 30 mmol of styrene (St) was injected into the pre-activated system using a constant flow pump. After stirring for 5 min, the temperature was raised to 51 °C and reacted for 45 min. Subsequently, 45 mmol of allyl polyethylene glycol ether (APEG, M) was added dropwise using a constant flow pump. n =1000g·mol⁻¹, pre-dissolved in 100mL toluene), added dropwise over 45min, and then heated to 59℃ for 48h after addition. After the reaction was complete, the flask was transferred to an ice-water bath to cool to room temperature and the reaction was terminated by contact with air. The reaction solution was slowly added dropwise to 250mL of a hexane / diethyl ether mixed solvent (volume ratio 1:1) pre-cooled to 0℃, and a pale yellow precipitate was formed. The precipitate was collected by filtration and redissolved in 150mL toluene. The precipitation was repeated twice. Finally, the precipitate was dried in a vacuum drying oven at 40℃ and 0.8mmHg for 12.5h to obtain a powdered slow-release dispersant for clear water concrete (denoted as SD-1).

[0019] Example 2 A method for preparing a slow-release dispersant for fair-faced concrete includes the following steps: Step 1: Add 8 mmol of L-lactide to a 100 mL Schlenk tube, then inject 16 mL of anhydrous dichloromethane and stir at room temperature for 28 min until completely dissolved. Under argon protection, inject a trimethylolpropane initiator solution containing 1 mmol of hydroxyl group (0.33 mmol of trimethylolpropane dissolved in 2 mL of anhydrous dichloromethane), stir for 5 min, then add 0.65 mmol of trifluoromethanesulfonic acid and transfer to a 4 °C low-temperature reaction bath. Add 12 mmol of 2-bromomethyl-1,3-dioxolane (BrDXL) dropwise using a constant flow pump over 2.5 h, and react at 4 °C for 47 h after the addition is complete. Subsequent quenching, drying, precipitation, and drying steps are the same as in Example 1 to obtain a multi-arm PLA-based multi-site macromolecular initiator (denoted as PLA-Br8). Step 2: Add PLA-Br8 (containing 12 mmol of brominated units), 1.0 mmol CuBr, and 1.1 mmol PMDETA to a 500 mL flask, inject 55 mL of toluene, and pre-activate at 30 °C for 22 min after argon deoxygenation; inject 48 mmol of hydroxyethyl acrylate (HEA) and 24 mmol of St, and react at 50 °C for 42 min; then add 36 mmol of methyl allyl polyethylene glycol ether (MPEG, M... n=800g·mol⁻¹), added over 42min, reacted at 58℃ for 48h; subsequent post-processing steps were the same as in Example 1, to obtain powdered slow-release dispersant for fair-faced concrete (denoted as SD-2).

[0020] Example 3 A method for preparing a slow-release dispersant for fair-faced concrete includes the following steps: Step 1: Add 15 mmol of L-lactide to a 100 mL Schlenk tube, then inject 20 mL of anhydrous dichloromethane and stir at room temperature for 32 min until completely dissolved. Under argon protection, inject a 1,2,3-propanetriol initiator solution containing 1 mmol of hydroxyl group (0.33 mmol of glycerol dissolved in 2 mL of anhydrous dichloromethane), stir for 5 min, then add 0.75 mmol of trifluoromethanesulfonic acid and transfer to a 6 °C low-temperature reaction bath. Add 25 mmol of 2-iodomethyl-1,3-dioxane (I-DXO) dropwise using a constant flow pump over 4 h, and react at 6 °C for 49 h after the addition is complete. Subsequent quenching, drying, precipitation, and drying steps are the same as in Example 1 to obtain a multi-arm PLA-based multi-site macromolecular initiator (denoted as PLA-I). 12 ); Step 2: Add PLA-I to the 500mL flask 12 (Containing 25 mmol iodinated units), 1.8 mmol CuI and 2.0 mmol PMDETA, injected into 75 mL toluene, pre-activated at 35 °C for 28 min after argon deoxygenation; injected with 90 mmol HEMA and 45 mmol St, reacted at 52 °C for 48 min; 75 mmol APEG (M n =1500g·mol⁻¹), added over 50min, reacted at 60℃ for 48h; subsequent post-processing steps were the same as in Example 1, to obtain powdered slow-release dispersant for fair-faced concrete (denoted as SD-3).

[0021] Example 4 A method for preparing a slow-release dispersant for fair-faced concrete includes the following steps: Step 1: Add 12 mmol L-lactide to a 100 mL Schlenk tube, then inject 19 mL of anhydrous dichloromethane and stir at room temperature for 30 min until completely dissolved. Under argon protection, inject a mixed initiator solution containing 1 mmol of hydroxyl group (0.125 mmol pentaerythritol + 0.165 mmol trimethylolpropane dissolved in 2 mL of anhydrous dichloromethane), stir for 5 min, then add 0.72 mmol trifluoromethanesulfonic acid and transfer to a 5 °C low-temperature reaction bath. Add 20 mmol 2-chloromethyl-1,3-dioxane (Cl-DXO) dropwise using a constant flow pump over 3.5 h, and react at 5 °C for 48 h after the addition is complete. Subsequent quenching, drying, precipitation, and drying steps are the same as in Example 1 to obtain a multi-arm PLA-based multi-site macromolecular initiator (denoted as PLA-Cl9). Step 2: Add PLA-Cl9 (containing 20 mmol of chlorinated units), 1.5 mmol CuCl and 1.6 mmol PMDETA to a 500 mL flask, inject 65 mL of toluene, deoxygenate with argon, and pre-activate at 33 °C for 24 min; inject 75 mmol HEA and 35 mmol St, react at 51 °C for 44 min; add 60 mmol MPEG (M n =1200g·mol⁻¹), added over 46min, reacted at 59℃ for 48h; subsequent post-processing steps were the same as in Example 1, to obtain powdered slow-release dispersant for fair-faced concrete (denoted as SD-4).

[0022] Example 5 A method for preparing a slow-release dispersant for fair-faced concrete includes the following steps: Step 1: Add 10 mmol L-lactide to a 100 mL Schlenk tube, then inject 18 mL of anhydrous dichloromethane and stir at room temperature for 30 min until completely dissolved. Under argon protection, inject a pentaerythritol initiator solution containing 1 mmol of hydroxyl group (0.25 mmol pentaerythritol dissolved in 2 mL of anhydrous dichloromethane), stir for 5 min, then add 0.7 mmol trifluoromethanesulfonic acid and transfer to a 5 °C low-temperature reaction bath. Add a mixture of 7.5 mmol BrDXL + 7.5 mmol ClDXL dropwise using a constant flow pump over 3 h, and react at 5 °C for 48 h after the addition is complete. Subsequent quenching, drying, precipitation, and drying steps are the same as in Example 1 to obtain a multi-arm PLA-based multi-site macromolecular initiator (denoted as PLA-Br4Cl6). Step 2: Add PLA-Br4Cl6 (containing 15 mmol of halogenated unit), 1.2 mmol CuBr-CuCl (1:1 mixture), and 1.3 mmol PMDETA to a 500 mL flask, inject 60 mL of toluene, deoxygenate with argon, and pre-activate at 32 °C for 25 min; inject 60 mmol HEMA and 30 mmol St, react at 51 °C for 45 min; add 45 mmol APEG (M n =1000g·mol⁻¹), added over 45min, reacted at 59℃ for 48h; subsequent post-processing steps were the same as in Example 1, to obtain powdered slow-release dispersant for fair-faced concrete (denoted as SD-5).

[0023] Performance verification The slow-release dispersants (SD-1~SD-5) prepared in Examples 1 to 5 above and the commercially available slow-release dispersants for fair-faced concrete (DZ-1, DZ-2) were tested for their performance in fair-faced concrete. The reference concrete mix proportions were: 450 kg / m³ of P·O 42.5 cement, 650 kg / m³ of manufactured sand, 1150 kg / m³ of crushed stone, and a water-cement ratio of 0.42. The dispersant dosage was 0.25% of the cement mass. The stability of the dispersibility of the concrete mixture was evaluated by the change in the spread of the mixture over time. The specific operation process and test were carried out in accordance with GB / T 50080-2016 "Standard for Test Methods of Performance of Ordinary Concrete Mixtures". The test results of the spread of the concrete mixture are shown in Table 1 below.

[0024] Table 1 Results of concrete mix spread test As shown in Table 1, the slow-release dispersants prepared in the five embodiments of this invention (SD-1~SD-5) exhibited significantly better dispersibility and stability in fair-faced concrete mixtures than commercially available products (DZ-1, DZ-2). The initial expansion of SD-1~SD-5 was 545~565 mm, and it maintained a stable level of 510~535 mm within 4 hours without significant fluctuations or sudden drops, demonstrating the long-term slow-release advantage of gradient hydrolysis of the main and side chains. In contrast, although the initial expansion of the control groups DZ-1 and DZ-2 was comparable to that of the embodiments (550~565 mm), excessive expansion occurred within 1~2 hours (up to 630 mm), followed by rapid decay, with an expansion of only 360~430 mm after 4 hours. This exposed the defects of traditional dispersants, such as unstable slow release and easy segregation or rapid loss of fluidity in the mixture. This fully demonstrates that the dispersant of this invention can better meet the construction needs of the entire process of fair-faced concrete "mixing-transportation-pouring".

[0025] Thixotropy tests were conducted using a concrete mix design (cement:sand:water = 1:2:0.42, dispersant dosage 0.25%) to prepare a slurry. A rotational rheometer equipped with a coaxial cylindrical rotor was used. Under constant temperature conditions of 25℃, the slurry was cycled at a shear rate of "0→100 s⁻¹→0" (simulating the process of settling → vibration → settling). The viscosity values ​​corresponding to different shear rates were recorded, thixotropic rings were plotted, and the ring area was calculated. Simultaneously, the yield stress of the slurry was tested using a stress-controlled mode. The time it took for the viscosity to recover to 80% of its initial value was recorded through a "viscosity recovery experiment after shearing." The thixotropic test results of the concrete mixture are shown in Table 2 below.

[0026] Table 2. Thixotropic test results of concrete mixtures As shown in Table 2, the slow-release dispersants prepared by SD-1 to SD-5 in the embodiments of the present invention impart excellent thixotropic properties to the fair-faced concrete paste. Their thixotropic ring area reaches 852.0~925.0 Pa·s⁻¹, the yield stress is 40.1~45.7 Pa, and the viscosity recovers to 80% of its initial value in only 13.5~16.8 s. This demonstrates the core advantages of "forming a weakly cross-linked network to support aggregate anti-settling during standing, rapidly reducing viscosity during vibration to promote defoaming and compaction, and quickly restoring viscosity after shearing stops," perfectly meeting the construction requirements of fair-faced concrete. In contrast, the thixotropic ring area of ​​the control groups DZ-1 and DZ-2 is only 432.0~458.0 Pa. Pa·s⁻¹, yield stress less than 20 Pa, viscosity recovery time as long as 38.7~41.2 s, thixotropy is significantly weak, making it difficult to balance the fluidity and stability of the slurry, and problems such as aggregate settling, segregation or difficulty in vibration and defoaming are likely to occur. This further confirms the technical innovation and application superiority of the present invention in regulating the thixotropy of the slurry through the multi-arm topology structure.

[0027] Using the CIE LAB color space system, 10 detection points were arranged every 10m² under the D65 standard light source. The color difference value ΔE between any two points and the color difference change rate after 7 days of curing were measured using a high-precision colorimeter to evaluate the surface color uniformity. The component surface was covered with a 1m×1m grid method, and the diameter of bubbles was measured and counted using vernier calipers. The bubble indentation depth was detected by a surface profilometer to evaluate the bubble residue control effect. Using a 2m straightedge and feeler gauge, 6 straight lines were selected on the long and short sides of the component, and the surface gap was measured every 40cm. The surface flatness retention rate was recorded simultaneously after 28 days to determine the surface flatness. The surface quality test results of fair-faced concrete are shown in Table 3 below.

[0028] Table 3. Test results of surface quality of fair-faced concrete As shown in Table 3, the sustained-release dispersants prepared by SD-1 to SD-5 in the embodiments of the present invention are significantly superior to the commercially available control groups DZ-1 and DZ-2 in three core indicators: surface color uniformity, bubble residue control, and surface smoothness. Regarding color uniformity, the color difference ΔE between any two points of SD-1 to SD-5 is only 1.2 to 1.8, the color difference change rate after 7 days of curing is 4.2% to 5.5%, and the maximum color spot area does not exceed 2.8 cm², demonstrating the hydration uniformity advantage brought by the gradient sustained-release mechanism. In terms of bubble control, the embodiments... Only 0.8~1.5 air bubbles with a surface diameter >1mm are present per m², and there are no air bubbles with a diameter >3mm. The maximum depression depth is ≤0.5mm. Thanks to the excellent thixotropic properties of the slurry given by the multi-arm topology, air bubble desorption can be accelerated during vibration. In terms of surface smoothness, the maximum gap of the 2m straightedge of SD-1~SD-5 is 1.1~1.5mm, and the proportion of gaps >1mm is ≤8%. The 28-day smoothness retention rate reaches 96%~98%, which can effectively avoid smoothness deviations caused by slurry segregation or insufficient template adhesion during construction. The control groups DZ-1 and DZ-2 showed significant shortcomings, with color difference values ​​ΔE reaching 2.8~3.2, a 7-day color difference change rate exceeding 13%, and 6.2~7 bubbles with diameters >1mm per m², including 3.8~5 large bubbles per m². The maximum gap of a 2m straightedge was 3.2~3.5mm. This fully demonstrates that the dispersant of this invention can comprehensively improve the appearance quality of fair-faced concrete by synergistically regulating its slow-release and thixotropic properties, thus meeting stringent engineering aesthetic and quality requirements.

[0029] like Figure 1 As shown, to verify the molecular structure of the target product, it was characterized by ^1H NMR (D2O). A characteristic signal in the aromatic region was observed at δ 7.226–7.420 ppm, which can be attributed to aromatic protons on the brominated aromatic end group, indicating that the aromatic end group has been successfully introduced into the target molecule. The signal at δ 5.059–5.217 ppm belongs to methylene protons in a highly deshielded environment in bridging oxygen or esterification branching sites, indicating that a characteristic oxygen-containing branched linkage structure has been formed in the molecule. Multiple signals in the range of δ 4.033–4.576 ppm can be attributed to methylene / methylene protons adjacent to ester and ether oxygen groups, indicating that the ester bond linkage and multi-oxygen skeleton structure in the target molecule have been successfully constructed. The strong characteristic peak at δ 3.407–3.821 ppm corresponds to the repeating –OCH2CH2O– unit and the terminal o-methylene proton in the polyether side chain, proving that the polyether segment has been successfully grafted. Meanwhile, the signals at δ 1.172–2.686 ppm can be attributed to aliphatic protons at the benzylic position and in the branched aliphatic chains, further confirming the presence of aromatic end groups and aliphatic bridged chain skeletons. Since the test solvent was D2O, deuteration exchange occurred in the exchangeable hydroxyl protons, and no independent, clear OH signal was observed; furthermore, the strong peaks near 4.7–4.8 ppm mainly originated from residual water peaks.

[0030] like Figure 2 As shown, FTIR characterization was performed to further verify the molecular structure of the target product. The spectrum is at 3459 cm⁻¹. -1 A broad peak appears at 2882 cm⁻¹, corresponding to O–H stretching vibration; -1 The absorption peak at 1719 cm⁻¹ is attributed to the aliphatic C–H stretching vibration. -1 The presence of a distinct characteristic peak at this point can be attributed to the C=O stretching vibration of the ester group, indicating that an esterification linker unit has been successfully introduced into the target molecule. (1636 cm⁻¹) -1 The absorption peaks at these locations can be attributed to aromatic ring skeletal vibrations. (1241, 1195, 1143, 1106, and 1060 cm⁻¹) -1 The multiple strong absorption peaks at [location missing] correspond to the stretching vibrations of the ester group (C–O) and the polyether chain (C–O–C), respectively, indicating that the polyether segment and the multi-oxygen linkage backbone have been successfully constructed. Furthermore, [values ​​missing] cm⁻¹ -1 The absorption peak at 528 cm⁻¹ can be attributed to the out-of-plane bending vibration of the substituted aromatic ring C–H. -1 The peaks further support the existence of the C–Br structure. These characteristic peaks are consistent with the aromatic end groups, polyether segments, and ester bonds in the target molecule, proving that the target product has been successfully synthesized.

[0031] To further verify the molecular structure and molecular weight distribution of the target product, gel permeation chromatography (GPC) was performed for characterization. Figure 3 As shown, the sample exhibits a dominant peak with an apparent molecular weight of approximately 22082 g / mol, indicating that the target product has successfully formed a polymer matrix with a certain molecular weight. Simultaneously, a weaker secondary peak with an apparent molecular weight of approximately 1239 g / mol is present at a longer retention time, suggesting that a small amount of low molecular weight components remain in the system, possibly originating from oligomers or incompletely reacted small molecule fragments. Overall, the sample's molecular weight distribution is dominated by the target polymer, consistent with the aforementioned NMR and IR characterization results, further confirming the successful synthesis of the target product.

[0032] Although embodiments of the invention have been shown and described, it will be understood by those skilled in the art that various changes, modifications, substitutions and alterations can be made to these embodiments without departing from the principles and spirit of the invention, the scope of which is defined by the appended claims and their equivalents.

Claims

1. A slow-release dispersant for fair-faced concrete, characterized in that, The molecular structure of the slow-release dispersant for fair-faced concrete is shown in Formula I: Formula I The slow-release dispersant for fair-faced concrete is a multi-arm grafted structure with a main chain containing hydrolyzable acetal-ester bonds and side chains containing degradable ester bonds and hydrophilic polyethylene glycol segments. in, It is one of the dehydroxylated structures of 1,2,3-propanetriol, trimethylolpropane, and pentaerythritol; when When n is one of the structures 1,2,3-propanetriol or trimethylolpropane, n is 3. When the structure is the dehydroxylated form of pentaerythritol, n is 4; x is an integer from 3 to 20, y is an integer from 11 to 40, z is an integer from 5 to 25; a is 1 or 2, c:d:e:y = 2~5:1~4:1~1.5:1; R1 and R2 can be H atoms or methyl groups, respectively, and R3 is a H atom.

2. The slow-release dispersant for fair-faced concrete according to claim 1, characterized in that, The number average molecular weight of the slow-release dispersant used in the fair-faced concrete is 10,000 to 100,000.

3. A method for preparing a slow-release dispersant for fair-faced concrete as described in any one of claims 1 to 2, characterized in that, Includes the following steps: Step 1: Add 3-20 mmol of L-lactide to a 100 mL Schlenk tube, followed by 15-20 mL of anhydrous dichloromethane. Seal the Schlenk tube and magnetically stir at room temperature until completely dissolved. Under argon protection, add a polyfunctional alcohol initiator containing 1 mmol of hydroxyl group to the Schlenk tube using a microsyringe. Then add 0.6-0.8 mmol of trifluoromethanesulfonic acid using a microsyringe. Transfer the Schlenk tube to a 4-6°C low-temperature reaction bath. Maintaining argon protection, add 11-40 mmol of halocyclic acetal monomer dropwise to the Schlenk tube using a constant flow pump over 2-4 hours. After the addition is complete, continue... Continue the reaction for 46-49 hours; after the reaction is complete, add 0.78-1.04 mmol of triethylamine; transfer the reaction solution to a 100 mL single-necked flask, add 100-150 mmol of anhydrous magnesium sulfate and dry for 12-16 hours. After filtering to remove the desiccant, slowly add the filtrate to 50 mL of hexane / diethyl ether mixed solvent. After standing for 30 minutes, collect the precipitate by vacuum filtration through a Buchner funnel and redissolve it in 5-8 mL of anhydrous dichloromethane. Repeat the above precipitation operation once. Finally, transfer the precipitate to a vacuum drying oven and dry it at 42-44 °C and a vacuum degree ≤1 mmHg for 9-11 hours until constant weight to obtain a multi-arm PLA-based multi-site macromolecular initiator. Step 2: Dissolve the multi-arm PLA-based multi-site macromolecular initiator obtained in Step 1, 0.55-2 mmol of Cu, and 0.5-2.2 mmol of N,N,N',N'',N'''-pentamethyldiethylenetriamine in 45-80 mL of toluene and place in a 500 mL flask; purge with argon gas to remove oxygen, place in a 30-35 °C water bath, and stir for 20-30 min to complete the pre-activation of the catalytic system; add 22-200 mmol of acrylate and 11-60 mmol of styrene to the pre-activated system using a constant flow pump, mix thoroughly, and then raise the water bath temperature to 50-52 °C. After reacting at ℃ for 40-50 min, 11-160 mmol of polyethylene glycol macromonomer was added dropwise to the pre-activated system using a constant flow pump over 40-50 min. After the addition was complete, the reaction was continued at 58-60℃ for 48 h. The mixture was then cooled to room temperature and the reaction was terminated by contact with air. The reaction solution was slowly added dropwise to 250 mL of hexane / diethyl ether mixed solvent, and a precipitate was formed. The precipitate was collected by filtration and redissolved in 150 mL of toluene. The precipitation process was repeated twice. The precipitate was transferred to a vacuum drying oven and dried at 40℃ and a vacuum degree ≤1 mmHg for 12-13 h to obtain a slow-release dispersant for clear water concrete.

4. The method for preparing a slow-release dispersant for fair-faced concrete according to claim 3, characterized in that, In step one, the multifunctional alcohol initiator is one or more combinations of 1,2,3-propanetriol, trimethylolpropane, and pentaerythritol.

5. The method for preparing a slow-release dispersant for fair-faced concrete according to claim 3, characterized in that, In step two, the halogenated cyclic acetal monomer is one or more combinations of 2-bromomethyl-1,3-dioxolane, 2-chloromethyl-1,3-dioxolane, 2-iodomethyl-1,3-dioxolane, 2-bromomethyl-1,3-dioxane, 2-chloromethyl-1,3-dioxane, and 2-iodomethyl-1,3-dioxane.

6. The method for preparing a slow-release dispersant for fair-faced concrete according to claim 3, characterized in that, In step two, the acrylate is one or a combination of acrylate and methacrylate.

7. The method for preparing a slow-release dispersant for fair-faced concrete according to claim 3, characterized in that, In step two, the polyethylene glycol macromonomer is one or more combinations of methyl allyl polyethylene glycol ether and allyl polyethylene glycol ether.

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