A concrete water reducing agent and a preparation method thereof

Abstract

The application provides a concrete water reducing agent and a preparation method thereof and belongs to the technical field of water reducing agents. Modified chitin, 1-vinyl-3-butyl imidazole trifluoromethane sulfonate, a silane coupling agent with a double bond, methacrylic acid, trans-butenedioic acid and an initiator are added into dimethylformamide, heated and stirred to react, toluene is removed under reduced pressure, and the concrete water reducing agent is prepared through alkali washing and alcohol washing; the modified chitin is prepared through modification of chitin by sodium sulfonate and acrylic acid and then by tannic acid. The polycarboxylic acid water reducing agent prepared by copolymerization of the modified chitin, the 1-vinyl-3-butyl imidazole trifluoromethane sulfonate, the silane coupling agent with the double bond, the methacrylic acid and the trans-butenedioic acid has a comb-shaped structure, a plurality of active groups with strong polarity on a main chain and hydrophilic active groups on side chains, and has the characteristics of low mixing amount, high water reducing rate, high reinforcing effect, high plastic retention and volume stability, and has a wide application prospect.

Description

Technical Field

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

[0002] In recent years, concrete has played a vital role in my country's engineering construction. Because water-reducing agents can effectively improve the performance of concrete and offer good economic benefits, they are widely used in many countries. The application of high-efficiency water-reducing agents in engineering is receiving increasing attention, making them an indispensable material in concrete.

[0003] Water-reducing agents have evolved from lignin sulfonate, naphthalene sulfonate condensates (naphthalene-based water-reducing agents), aliphatic, and polycarboxylate-based agents. Lignosulfonate is the most commonly used general-purpose water-reducing agent, but it tends to cause retardation during mixing and often introduces a large amount of air, affecting the performance of concrete. Naphthalene-based water-reducing agents inevitably have high sodium sulfate content during the mixing process, leading to increased alkali content in the admixture, reduced concrete durability, and alkali-aggregate reaction in many projects, significantly shortening service life. Furthermore, aromatic sulfonates have potential toxicity to humans and other organisms. The difficulty in controlling the proportions of monomers such as formaldehyde and acetone used in the production of aliphatic water-reducing agents results in deteriorated performance and compatibility issues with the cementitious matrix.

[0004] Existing polycarboxylate superplasticizers are costly, and variations in the degree of polymerization during synthesis lead to differences in their performance. Furthermore, my country faces a significant supply-demand imbalance in sand and gravel, with manufactured sand gradually replacing natural sand. However, manufactured sand differs considerably from natural sand in particle shape, gradation, surface texture, and even the size of 0.075mm stone powder particles, causing superplasticizers to become ineffective or reduce their water-reducing efficiency, thus significantly impacting concrete performance. In addition, the SO4 content in cement... 2- Ions have a significant impact on the effectiveness of use, especially large amounts of SO4. 2- Ions directly affect the amount of cement particles adsorbed by polycarboxylate superplasticizer molecules, leading to a significant reduction in the water-reducing effect of the superplasticizer and resulting in bleeding problems. Research and engineering practice have shown that when superplasticizers are added to cement, problems such as poor compatibility between the superplasticizer and cement, sensitivity to the mud content of sand and gravel aggregates, and unstable workability of fresh concrete may occur.

[0005] Chinese patent application CN108101407A discloses a polycarboxylate-based composite water-reducing agent and its preparation method. The polycarboxylate-based composite water-reducing agent comprises the following components in parts by weight: 50-60 parts of polycarboxylate water-reducing agent, 30-40 parts of aliphatic water-reducing agent, 15-20 parts of propanesulfonic acid, 10-15 parts of air-entraining agent, 10-15 parts of triethanolamine, and 3-5 parts of chlorinated hydrocarbons. In addition to water-reducing properties, the polycarboxylate-based composite water-reducing agent also has antifreeze properties. However, its compatibility with cement has not been studied. Moreover, the chlorinated hydrocarbons with high volatility added in this invention are toxic and will cause environmental pollution.

[0006] Chinese patent application CN107117853A discloses a high-efficiency composite water-reducing agent for concrete and its application. This invention is prepared from 10-40 parts of polycarboxylate mother liquor, 4-15 parts of air-entraining agent, 15-30 parts of hydrophilic colloid, 0.1-1 parts of retarder, 0.8-2 parts of sodium sulfate, 1-5 parts of lignosulfonate, 5-20 parts of cellulose water-retaining material, and 30-70 parts of water. This composite water-reducing agent possesses properties such as slow release, slump retention, early strength, high strength, and water retention, significantly reducing the unit water consumption of concrete. Applying this composite water-reducing agent to concrete preparation can improve concrete density, significantly reduce cement usage, reduce shrinkage, and improve concrete stability. It features low dosage, significant effects, and is environmentally friendly, offering good economic and social benefits. However, this invention does not solve the compatibility problem between the composite water-reducing agent and cement. Summary of the Invention

[0007] The purpose of this invention is to propose a concrete water-reducing agent and its preparation method. The preparation method is simple, the raw materials are widely available, and the cost is low. Its molecules have a comb-like structure, with multiple highly polar active groups on the main chain and hydrophilic active groups on the side chains. It has the characteristics of low dosage, high water reduction rate, high reinforcement effect, high plasticity retention and volume stability, and has broad application prospects.

[0008] The technical solution of this invention is implemented as follows:

[0009] This invention provides a method for preparing a concrete water-reducing agent, comprising adding modified chitosan, 1-vinyl-3-butylimidazolium trifluoromethanesulfonate, a silane coupling agent with double bonds, methacrylic acid, trans-butenedioic acid, and an initiator to dimethylformamide, heating and stirring to react, removing toluene under reduced pressure, and washing with alkali and alcohol to obtain the concrete water-reducing agent; wherein the modified chitosan is obtained by modifying chitosan with sodium sulfonate and acrylic acid, and then modifying it with tannic acid.

[0010] As a further improvement of the present invention, the mass ratio of the modified chitosan, 1-vinyl-3-butylimidazolium trifluoromethanesulfonate, silane coupling agent with double bonds, methacrylic acid, trans-butenedioic acid and initiator is 7-10:3-4:2-3:3-5:2-4:0.01-0.02.

[0011] As a further improvement of the present invention, the temperature of the heating and stirring reaction is 55-65°C and the time is 3-5 hours.

[0012] As a further improvement of the present invention, the preparation method of the modified chitin is as follows:

[0013] S1. Add chitin to an alkaline solution, heat and stir, filter, wash, and dry to obtain pretreated chitin;

[0014] S2. Add the pretreated chitin to dimethyl sulfoxide, add sodium 3-chloro-2-hydroxypropanesulfonate, heat and stir to react, filter, wash, dry, and obtain the intermediate product;

[0015] S3. Add the intermediate product and methacrylic acid to toluene, add p-toluenesulfonic acid and hydroquinone, heat under reflux to react, remove the solvent under reduced pressure, wash, dry, recrystallize to obtain intermediate product 2;

[0016] S4. Add intermediate product 2 to water, add tannic acid and catalyst, heat and stir to react, filter, wash, dry, and obtain modified chitosan.

[0017] As a further improvement of the present invention, the alkaline solution in step S1 is a 10-15wt% NaOH or KOH solution, and the heating and stirring treatment is carried out at a temperature of 40-50℃ for 1-2 hours.

[0018] As a further improvement of the present invention, the mass ratio of pretreated chitin and sodium 3-chloro-2-hydroxypropanesulfonate in step S2 is 10:3-5, and the heating and stirring reaction temperature is 60-70°C for 4-6 hours.

[0019] As a further improvement of the present invention, the mass ratio of the intermediate product, methacrylic acid, p-toluenesulfonic acid and hydroquinone in step S3 is 10:2-4:0.2-0.3:0.3-0.5, and the heating and reflux reaction time is 4-6 hours.

[0020] As a further improvement of the present invention, the mass ratio of intermediate product 2, tannic acid and catalyst in step S4 is 10:3-4:0.2-0.5, the catalyst is a Tris-HCl solution with pH=8.5-9.5, and the heating and stirring reaction is carried out at a temperature of 40-50°C for 2-4 hours.

[0021] As a further improvement of the present invention, the silane coupling agent with double bonds is selected from at least one of KH570, vinyltriethoxysilane, and vinyltrimethylsilane; the initiator is selected from at least one of azobisisobutyronitrile, azobisisovalerate, sodium persulfate, potassium persulfate, and ammonium persulfate.

[0022] This invention further protects a concrete water-reducing agent prepared by the above-described preparation method.

[0023] The present invention has the following beneficial effects:

[0024] The polymer monomers of this invention contain silanes with double bonds. On the one hand, silanes are used as hydrophobic agents in recycled aggregate concrete to improve the durability of concrete. On the other hand, organosilicon groups are introduced into the molecular structure of polycarboxylate superplasticizers. By utilizing the chemical properties of organosilicones, the polycarboxylate superplasticizers are provided with a strong interaction ability to chemically bond with cement particles, thereby improving their performance in high-strength concrete.

[0025] In addition, the polymer monomer of this invention also includes 1-vinyl-3-butylimidazolium trifluoromethanesulfonate. By introducing imidazole groups with rigid structure and hydrophilicity, it adsorbs onto the surface of cement particles and associates with water molecules in the form of hydrogen bonds to form a stable water film on the surface of cement particles. This prevents direct contact between cement particles and avoids problems such as poor compatibility with cement, sensitivity to the mud content of sand and gravel aggregates, and unstable workability of fresh concrete. It increases the sliding ability of cement particles, plays a lubricating role, improves the fluidity of cement paste, increases the workability and slump retention of concrete mixtures, improves the strength and durability of concrete, and reduces cracking, shrinkage and thermal deformation caused by cement hydration.

[0026] This invention also prepared and added modified chitosan, which released more hydroxyl groups on its surface after alkali treatment. After coupling modification with sodium 3-chloro-2-hydroxypropane sulfonate, sulfonate groups were introduced, which greatly reduced the SO4 content in cement. 2- The effect of ions on the water-reducing agent is improved, enhancing the water-reducing effect and preventing bleeding. The resulting intermediate product undergoes esterification with methacrylic acid, attaching double bonds and facilitating copolymerization. Finally, tannic acid modification significantly increases the carboxyl content of the product. These carboxyl groups adsorb onto the surface of cement particles and associate with water molecules via hydrogen bonds, forming a stable water film on the surface of the cement particles. This prevents direct contact between cement particles, increases their sliding ability, acts as a lubricant, and improves the fluidity of the cement paste, further enhancing the water-reducing effect. Simultaneously, the introduction of carboxyl groups onto the surface of chitosan induces electrostatic repulsion dispersion, weak dissociation, and hydrophilicity, thereby achieving slump retention and water-reducing properties. Using chitosan as a raw material offers low cost and highly efficient water-reducing effects.

[0027] This invention relates to a polycarboxylic acid water-reducing agent prepared by copolymerization of modified chitosan, 1-vinyl-3-butylimidazolium trifluoromethanesulfonate, a silane coupling agent with double bonds, methacrylic acid, and trans-butenedioic acid. The preparation method is simple, the raw materials are widely available, and the cost is low. Its molecule has a comb-like structure, with multiple highly polar active groups on the main chain and hydrophilic active groups on the side chains. It features low dosage, high water reduction rate, high reinforcement effect, high plasticity retention, and volume stability, and has broad application prospects. Detailed Implementation

[0028] The technical solutions in the embodiments of the present invention will be clearly and completely described below. 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.

[0029] Preparation Example 1: Preparation of Modified Chitosan

[0030] Includes the following steps:

[0031] S1. Add 1g of chitin to 50mL of 10wt% NaOH solution, heat to 40℃, stir for 1h, filter, wash, and dry to obtain pretreated chitin;

[0032] S2. Add 1g of pretreated chitin to 100mL of dimethyl sulfoxide, add 0.3g of sodium 3-chloro-2-hydroxypropanesulfonate, heat to 60℃, adjust the pH of the mixed solution to 7.5 with 30% NaOH aqueous solution, stir for 4h, filter, wash, dry, and obtain the intermediate product.

[0033] S3. Add 1g of intermediate product and 0.2g of methacrylic acid to 100mL of toluene, add 0.02g of p-toluenesulfonic acid and 0.03g of hydroquinone, heat under reflux for 4h, remove solvent under reduced pressure, wash, dry, recrystallize to obtain intermediate product 2.

[0034] S4. Add 1g of intermediate product 2 to 100mL of water, add 0.3g of tannic acid and 0.02g of catalyst, heat to 40℃, stir and react for 2h, filter, wash, and dry to obtain modified chitin.

[0035] The catalyst is a Tris-HCl solution with pH=8.5.

[0036] Preparation Example 2: Preparation of Modified Chitosan

[0037] Includes the following steps:

[0038] S1. Add 1g of chitin to 50mL of 15wt% KOH solution, heat to 50℃, stir for 2h, filter, wash, and dry to obtain pretreated chitin;

[0039] S2. Add 1g of pretreated chitin to 100mL of dimethyl sulfoxide, add 0.5g of sodium 3-chloro-2-hydroxypropanesulfonate, heat to 70℃, adjust the pH of the mixed solution to 8.5 with 40% NaOH aqueous solution, stir for 6h, filter, wash, dry, and obtain the intermediate product.

[0040] S3. Add 1g of intermediate product and 0.4g of methacrylic acid to 100mL of toluene, add 0.03g of p-toluenesulfonic acid and 0.05g of hydroquinone, heat under reflux for 6h, remove solvent under reduced pressure, wash, dry, recrystallize to obtain intermediate product 2.

[0041] S4. Add 1g of intermediate product 2 to 100mL of water, add 0.4g of tannic acid and 0.05g of catalyst, heat to 50℃, stir and react for 4h, filter, wash, dry, and obtain modified chitin.

[0042] The catalyst is a Tris-HCl solution with pH=9.5.

[0043] Preparation Example 3: Preparation of Modified Chitosan

[0044] Includes the following steps:

[0045] S1. Add 1g of chitin to 50mL of 12wt% NaOH solution, heat to 45℃, stir for 1.5h, filter, wash, and dry to obtain pretreated chitin;

[0046] S2. Add 1g of pretreated chitin to 100mL of dimethyl sulfoxide, add 0.4g of sodium 3-chloro-2-hydroxypropanesulfonate, heat to 65℃, adjust the pH of the mixed solution to 8 with 35% NaOH aqueous solution, stir for 5h, filter, wash, dry, and obtain intermediate product;

[0047] S3. Add 1g of intermediate product and 0.3g of methacrylic acid to 100mL of toluene, add 0.025g of p-toluenesulfonic acid and 0.04g of hydroquinone, heat under reflux for 5h, remove solvent under reduced pressure, wash, dry, recrystallize to obtain intermediate product 2.

[0048] S4. Add 1g of intermediate product 2 to 100mL of water, add 0.35g of tannic acid and 0.03g of catalyst, heat to 45℃, stir and react for 3h, filter, wash, dry, and obtain modified chitin.

[0049] The catalyst is a Tris-HCl solution with pH=9.

[0050] Comparative Preparation Example 1

[0051] The difference compared to preparation example 3 is that step S4 was not performed.

[0052] Includes the following steps:

[0053] S1. Add 1g of chitin to 50mL of 12wt% NaOH solution, heat to 45℃, stir for 1.5h, filter, wash, and dry to obtain pretreated chitin;

[0054] S2. Add 1g of pretreated chitin to 100mL of dimethyl sulfoxide, add 0.4g of sodium 3-chloro-2-hydroxypropanesulfonate, heat to 65℃, adjust the pH of the mixed solution to 8 with 35% NaOH aqueous solution, stir for 5h, filter, wash, dry, and obtain intermediate product;

[0055] S3. Add 1g of intermediate product and 0.3g of methacrylic acid to 100mL of toluene, add 0.025g of p-toluenesulfonic acid and 0.04g of hydroquinone, heat under reflux for 5h, remove solvent under reduced pressure, wash, dry, recrystallize to obtain modified chitosan.

[0056] Comparative Preparation Example 2

[0057] The difference from preparation example 3 is that step S2 was not performed.

[0058] Includes the following steps:

[0059] S1. Add 1g of chitin to 50mL of 12wt% NaOH solution, heat to 45℃, stir for 1.5h, filter, wash, and dry to obtain pretreated chitin;

[0060] S2. Add 1g of pretreated chitin and 0.3g of methacrylic acid to 100mL of toluene, add 0.025g of p-toluenesulfonic acid and 0.04g of hydroquinone, heat under reflux for 5h, remove solvent under reduced pressure, wash, dry, recrystallize to obtain intermediate product 2.

[0061] S3. Add 1g of intermediate product 2 to 100mL of water, add 0.35g of tannic acid and 0.03g of catalyst, heat to 45℃, stir and react for 3h, filter, wash, dry, and obtain modified chitin.

[0062] The catalyst is a Tris-HCl solution with pH=9.

[0063] Example 1

[0064] A method for preparing a concrete water-reducing agent includes the following steps: adding 7g of modified chitin obtained from Preparation Example 1, 3g of 1-vinyl-3-butylimidazolium trifluoromethanesulfonate, 2g of vinyltrimethylsilane, methacrylic acid, 2g of trans-butenedioic acid, and 0.01g of sodium persulfate to dimethylformamide, heating to 55°C under nitrogen protection, stirring and reacting for 3 hours, removing the solvent under reduced pressure, washing with 20wt% NaOH solution and ethanol to obtain the concrete water-reducing agent.

[0065] A small amount of the prepared concrete water-reducing agent was ground and pressed into tablets using KBr, and its functional group structure was analyzed by Fourier transform infrared spectroscopy. (3425 cm⁻¹) -1 The characteristic absorption peak for -OH is 2937 cm⁻¹. -1 and 2822cm -1 The absorption peak at 1720 cm⁻¹ represents the stretching vibration of the CH bond in methyl and methylene groups. -1 The absorption peak at 1475 cm⁻¹ indicates the presence of carboxyl groups -COOH and -COO-, and the relatively large peak value at this location indicates the strength of the -COOH functional group in the product; -1 The absorption peak at 1237 cm⁻¹ represents the bending vibration of the CH bond in methyl and methylene groups. -1 and 1135cm -1 The absorption peak at 950-1080 cm⁻¹ represents the stretching vibration of the CN bond. -1 The peak at 952 cm⁻¹ represents the stretching vibration of CoC. -1 622cm -1 And 525cm -1 The peak at this point represents the stretching vibration of the sulfonate group.

[0066] Example 2

[0067] A method for preparing a concrete water-reducing agent includes the following steps: 10g of modified chitin obtained in Preparation Example 2, 4g of 1-vinyl-3-butylimidazolium trifluoromethanesulfonate, 3g of vinyltriethoxysilane, methacrylic acid, 4g of trans-butenedioic acid, and 0.02g of potassium persulfate are added to dimethylformamide, heated to 65°C under nitrogen protection, stirred and reacted for 5 hours, the solvent is removed under reduced pressure, and the mixture is washed with 20wt% NaOH solution and ethanol to obtain the concrete water-reducing agent.

[0068] Example 3

[0069] A method for preparing a concrete water-reducing agent includes the following steps: 8g of modified chitin obtained in Preparation Example 3, 3.5g of 1-vinyl-3-butylimidazolium trifluoromethanesulfonate, 2.5g of silane coupling agent KH570, methacrylic acid, 3g of trans-butenedioic acid, and 0.015g of ammonium persulfate are added to dimethylformamide, heated to 60°C under nitrogen protection, stirred and reacted for 4h, the solvent is removed under reduced pressure, and the mixture is washed with 20wt% NaOH solution and ethanol to obtain the concrete water-reducing agent.

[0070] Comparative Example 1

[0071] The difference from Example 3 is that the modified chitin was prepared from Comparative Preparation Example 1.

[0072] Comparative Example 2

[0073] The difference from Example 3 is that the modified chitin was prepared from Comparative Preparation Example 2.

[0074] Comparative Example 3

[0075] The difference compared to Example 3 is that no modified chitosan was added.

[0076] Specifically as follows:

[0077] A method for preparing a concrete water-reducing agent includes the following steps: adding 3.5g of 1-vinyl-3-butylimidazolium trifluoromethanesulfonate, 2.5g of silane coupling agent KH570, methacrylic acid, 3g of trans-butenedioic acid, and 0.015g of ammonium persulfate to dimethylformamide, heating to 60°C under nitrogen protection, stirring and reacting for 4 hours, removing toluene under reduced pressure, washing with 20wt% NaOH solution and ethanol to obtain the concrete water-reducing agent.

[0078] Comparative Example 4

[0079] The difference from Example 3 is that 1-vinyl-3-butylimidazolium trifluoromethanesulfonate was not added.

[0080] Specifically as follows:

[0081] A method for preparing a concrete water-reducing agent includes the following steps: adding 8g of modified chitin obtained in Preparation Example 3, 2.5g of silane coupling agent KH570, methacrylic acid, 3g of trans-butenedioic acid and 0.015g of ammonium persulfate to dimethylformamide, heating to 60°C under nitrogen protection, stirring and reacting for 4h, removing toluene under reduced pressure, washing with 20wt% NaOH solution and ethanol to obtain the concrete water-reducing agent.

[0082] Comparative Example 5

[0083] The difference from Example 3 is that the silane coupling agent KH570 was not added.

[0084] Specifically as follows:

[0085] A method for preparing a concrete water-reducing agent includes the following steps: adding 8g of modified chitin obtained in Preparation Example 3, 3.5g of 1-vinyl-3-butylimidazolium trifluoromethanesulfonate, methacrylic acid, 3g of trans-butenedioic acid, and 0.015g of ammonium persulfate to dimethylformamide, heating to 60°C under nitrogen protection, stirring and reacting for 4h, removing toluene under reduced pressure, washing with 20wt% NaOH solution and ethanol to obtain the concrete water-reducing agent.

[0086] Test Example 1

[0087] Under laboratory conditions of 35℃ and 69% relative humidity, concrete water-reducing agents prepared in Examples 1-3 or Comparative Examples 1-5, as well as commercially available similar products, were added at a dosage of 0.5wt%. Concrete mix proportions: cement (Fujian "Minfu" brand P·O42.5R cement) 240kg; sand (mechanized granite sand produced by a building materials company in Xiamen) 775kg; water 165kg; crushed stone (5-25cm continuously graded crushed stone) 986kg; fly ash (Zhangzhou Houshi secondary fly ash) 80kg; slag powder (Sansteel Power Plant S95 grade slag powder) 80kg.

[0088] The test method for the fluidity of cement paste is based on GB8077-2016.

[0089] The tests were conducted in accordance with the relevant requirements of GB / T 50080-2016 "Standard for Test Methods of Performance of Ordinary Concrete Mixtures" and GB / T 50081-2002 "Standard for Test Methods of Mechanical Properties of Ordinary Concrete".

[0090] The results are shown in Table 1.

[0091] Table 1

[0092]

[0093] As can be seen from the table above, the concrete water-reducing agents prepared in Examples 1-3 of this invention enable concrete to have better overall performance.

[0094] The above description is only a preferred embodiment of the present invention and is not intended to limit the present invention. Any modifications, equivalent substitutions, improvements, etc., made within the spirit and principles of the present invention should be included within the protection scope of the present invention.

Claims

1. A method for preparing a concrete water-reducing agent, characterized in that, Modified chitosan, 1-vinyl-3-butylimidazolium trifluoromethanesulfonate, a silane coupling agent with double bonds, methacrylic acid, trans-butenedioic acid, and an initiator are added to dimethylformamide, heated and stirred to react, the solvent is removed under reduced pressure, and the concrete water-reducing agent is obtained by alkali washing and alcohol washing; the modified chitosan is obtained by modifying chitosan with sodium 3-chloro-2-hydroxypropanesulfonate and methacrylic acid, and then modifying it with tannic acid.

2. The preparation method according to claim 1, characterized in that, The mass ratio of the modified chitosan, 1-vinyl-3-butylimidazolium trifluoromethanesulfonate, silane coupling agent with double bonds, methacrylic acid, trans-butenedioic acid and initiator is 7-10:3-4:2-3:3-5:2-4:0.01-0.

02.

3. The preparation method according to claim 1, characterized in that, The heating and stirring reaction is carried out at a temperature of 55-65℃ for 3-5 hours.

4. The preparation method according to claim 1, characterized in that, The modified chitin is prepared as follows: S1. Add chitin to an alkaline solution, heat and stir, filter, wash, and dry to obtain pretreated chitin; S2. Add the pretreated chitin to dimethyl sulfoxide, add sodium 3-chloro-2-hydroxypropanesulfonate, heat and stir to react, filter, wash, dry, and obtain the intermediate product; S3. Add the intermediate product and methacrylic acid to toluene, add p-toluenesulfonic acid and hydroquinone, heat under reflux to react, remove the solvent under reduced pressure, wash, dry, recrystallize to obtain intermediate product 2; S4. Add intermediate product 2 to water, add tannic acid and catalyst, heat and stir to react, filter, wash, dry, and obtain modified chitosan.

5. The preparation method according to claim 4, characterized in that, The alkaline solution in step S1 is a 10-15 wt% NaOH or KOH solution, and the heating and stirring treatment is carried out at a temperature of 40-50°C for 1-2 hours.

6. The preparation method according to claim 4, characterized in that, In step S2, the mass ratio of pretreated chitin to sodium 3-chloro-2-hydroxypropanesulfonate is 10:3-5, and the heating and stirring reaction is carried out at a temperature of 60-70°C for 4-6 hours.

7. The preparation method according to claim 4, characterized in that, In step S3, the mass ratio of the intermediate product, methacrylic acid, p-toluenesulfonic acid, and hydroquinone is 10:2-4:0.2-0.3:0.3-0.5, and the heating and reflux reaction time is 4-6 hours.

8. The preparation method according to claim 4, characterized in that, In step S4, the mass ratio of intermediate product 2, tannic acid and catalyst is 10:3-4:0.2-0.

5. The catalyst is a Tris-HCl solution with pH=8.5-9.

5. The heating and stirring reaction is carried out at a temperature of 40-50℃ for 2-4 hours.

9. The preparation method according to claim 1, characterized in that, The silane coupling agent with double bonds is selected from at least one of KH570, vinyltriethoxysilane, and vinyltrimethylsilane; the initiator is selected from at least one of azobisisobutyronitrile, azobisisovalerate, sodium persulfate, potassium persulfate, and ammonium persulfate.

10. A concrete water-reducing agent prepared by the preparation method according to any one of claims 1-9.

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