Preparation method of composite admixture for high-calcium fly ash-based geopolymer

Abstract

The invention discloses a preparation method of a composite admixture for a high-calcium fly ash-based geopolymer. According to the additive, natural tannic acid serves as a raw material, an ATRP initiator is grafted through esterification reaction to prepare a brominated tannic acid macroinitiator (TA-Br), then the brominated tannic acid macroinitiator serves as a core, acrylic acid and polyethylene glycol methyl ether methacrylate are catalyzed to be subjected to ATRP graft copolymerization in a water-alcohol mixed system, and the core-shell structure additive with the tannic acid as the core and polycarboxylic acid as the shell is formed. According to the admixture, the retarding effect of tannic acid and the water reducing effect of polycarboxylic acid are integrated, and the problem that polycarboxylic acid in a high-calcium system is prone to failure is solved. Wherein the tannic acid core firmly adsorbs Ca < 2 + > and delays hydration, and the polycarboxylic acid shell provides high-density dispersion force, so that the problems of high water demand and short coagulation time of the high-calcium fly ash geopolymer are solved synergistically. The additive is green and environment-friendly in raw material, excellent in performance and suitable for a strong-base high-calcium geopolymer environment.

Description

Technical Field

[0001] This invention relates to the field of road material admixtures, specifically to a method for preparing a composite admixture for high-calcium fly ash-based polymer systems, which is applicable to high-calcium fly ash-based polymer systems. Background Technology

[0002] High-calcium fly ash is an industrial byproduct with a high calcium oxide content, produced when burning lignite or sub-bituminous coal in thermal power plants. It possesses excellent cementitious admixture properties and is a superior raw material for preparing geopolymers, as its high calcium content significantly enhances its later-stage strength. However, Ca... 2+ Accelerating the reaction shortens the setting and hardening time, resulting in poor fluidity and workability of the freshly mixed slurry. To improve its workability, admixtures are often needed.

[0003] Currently, the most widely used superplasticizer is polycarboxylate superplasticizer (PCE), which achieves dispersion through a comb-like structure of "main chain adsorption and side chain extension" providing steric hindrance. However, PCE has significant drawbacks in the strongly alkaline and high-calcium ion environment of high-calcium fly ash-based polymers. The PCE molecular backbone mainly relies on carboxyl groups for adsorption onto the particle surface via valence bonds. This adsorption mode is problematic in high-concentration Ca... 2+ It is extremely easy to destroy when it exists. Ca 2+ A "bridge" can form between the carboxyl groups of PCE and the particle surface, causing the molecular chains to fold, coil, or even desorb from the particle surface, thus leading to rapid deactivation of the water-reducing agent. Simultaneously, the adsorption of traditional linear PCE on the particle surface is mostly linear contact, with a limited number of adsorption sites, and the steric hindrance effect it provides is not optimal. Furthermore, the long side chains of polyoxyethylene ether (PEO) in PCE may undergo oxidative chain scission under strongly alkaline conditions, leading to molecular degradation and performance decline.

[0004] In recent years, bio-based admixtures such as sulfonated lignin have become a research hotspot due to their renewable raw materials and good environmental compatibility. The phenolic hydroxyl groups in their molecules have a significant impact on Ca2+. 2+ It has strong complexing ability and can effectively delay condensation. However, the dispersing ability of a single sulfonated modified product is still limited, and it cannot simultaneously meet the dual requirements of ultra-high water reduction rate and excellent workability maintenance. Summary of the Invention

[0005] Purpose of the invention

[0006] In order to address the problems and shortcomings of the existing technology, the purpose of this invention is to provide a method for preparing a composite additive for high-calcium fly ash-based polymers. This composite additive solves the problems of high water demand and fast setting time in high-calcium fly ash-based polymer slurry through the synergistic effect of its components.

[0007] Technical solution

[0008] A composite additive for high-calcium fly ash-based geopolymers, characterized by comprising the following steps:

[0009] (1) Tannic acid and 2-bromoisobutyryl bromide were esterified in an organic solvent under an inert atmosphere and with the help of a catalyst. After the reaction was completed, the tannic acid macromolecular initiator was obtained by precipitation, washing and drying.

[0010] (2) The prepared brominated tannic acid macromolecular initiator, polyether macromonomer, and acrylic acid were dissolved in a mixed solvent of water and alcohol. After deoxygenation treatment, they were then subjected to Cu... + The composite additive was obtained by atom transfer radical polymerization reaction catalyzed by a ligand catalyst system, followed by purification and drying after the reaction.

[0011] Preferably, in the composite additive for high-calcium fly ash-based polymers, the molar ratio of tannic acid to 2-bromoisobutyryl bromide is 1:30.

[0012] Preferably, in the composite additive for high-calcium fly ash-based geopolymers, the polyether macromonomer is polyethylene glycol methyl ether methacrylate with a number-average molecular weight of 1000–2400 g / mol.

[0013] Preferably, in the composite additive for high-calcium fly ash-based polymers, the molar ratio of brominated tannic acid macromolecular initiator, polyether macromonomer, and acrylic acid is 1:40:40.

[0014] Preferably, in the composite additive for high-calcium fly ash-based polymers, the Cu... + The metal catalyst in the ligand catalyst system is CuBr or CuCl, and the ligand is pentamethyldiethylenetriamine or bipyridine.

[0015] Preferably, in the composite additive for high-calcium fly ash-based polymers, the atom transfer radical polymerization reaction is carried out at a temperature of 60–70°C and for a reaction time of 6–12 hours.

[0016] Preferably, in the composite admixture for high-calcium fly ash-based polymers, the dosage of the composite water-reducing agent is 1% to 5% of the total mass of the precursors in the high-calcium fly ash-based polymer.

[0017] Inventive Principles

[0018] The composite admixture involved in this invention achieves three major synergistic effects through its unique "core-shell" molecular structure, perfectly adapting to high-calcium fly ash-based geopolymer systems. Its working principle is as follows: First, the rigid three-dimensional core composed of tannic acid forms a multi-site strong adsorption with the particle surface through π-π conjugation, hydrogen bonding, and complexation, fundamentally solving the problem of weak adsorption in traditional admixtures. Second, the high-density polycarboxylic acid side chains grown in situ from the core surface form a molecular brush structure, generating a strong steric hindrance effect, significantly improving dispersion efficiency. Finally, the tannic acid core preferentially complexes calcium ions, creating a localized low-calcium environment for the polycarboxylic acid shell, effectively protecting the carboxyl groups from calcium ion interference, and greatly improving the stability of the admixture under harsh environments. This multi-effect synergistic effect solves the problems of high water demand and rapid setting time in high-calcium fly ash geopolymers.

[0019] Beneficial effects

[0020] (1) This invention successfully constructs a core-shell structure with biomass tannic acid as the "core" and polycarboxylic acid as the "shell". The tannic acid core provides ultra-high density and ultra-strong anchoring adsorption sites, fundamentally solving the problems of poor adsorption and easy desorption of traditional water-reducing agents.

[0021] (2) The high-density PCE side chains form a "molecular brush," generating a strong steric hindrance effect and resulting in an extremely high initial water reduction rate. Simultaneously, the tannic acid core preferentially complexes with Ca. 2+ This protects the carboxyl groups in the PCE shell, ensuring its stability in a strongly alkaline, high-calcium environment. Furthermore, the complexation effect of tannic acid on calcium ions delays hydration, improving workability and ease of application.

[0022] (3) This invention uses renewable tannic acid as the core raw material, which reduces the dependence on fossil resources and is in line with the concept of green chemistry. Attached Figure Description

[0023] To more clearly illustrate the embodiments of the present invention or the technical solutions in the prior art, the accompanying drawings used in the embodiments will be briefly described below.

[0024] Figure 1 This is a flowchart illustrating the preparation process of composite additives for high-calcium fly ash-based geopolymers. Detailed Implementation

[0025] The invention will be further illustrated below with specific examples, but is not limited to these examples.

[0026] The present invention provides a method for preparing a composite additive for high-calcium fly ash-based geopolymers as follows:

[0027] (1) Weigh out 0.5 g of tannic acid bromide macromolecular initiator (TA-Br), 10.0 g of polyethylene glycol methyl ether methacrylate (MPEGMA, Mn = 2000 g / mol), 0.5 g of acrylic acid (AA), and 20 mL of a mixed solvent of methanol and deionized water (volume ratio 1:1). Catalyst: 15 mg of cuprous bromide (CuBr), and 35 μ L of N,N,N′,N″,N″-pentamethyldiethylenetriamine.

[0028] (2) Add TA-Br, MPEGMA and AA to a 100mL reaction flask containing a magnetic stir bar in sequence, then add methanol and water mixed solvent, and stir until the solid is completely dissolved.

[0029] (3) Perform three cycles of "freezing-vacuuming-nitrogen filling" on the reaction flask to ensure that there is no oxygen in the reaction system.

[0030] (4) Quickly inject the pre-deoxygenated catalyst solution into the reaction flask using a syringe. Transfer the reaction flask to an oil bath preheated to 65°C and continue stirring at this temperature for 8 hours. The entire reaction process is carried out under nitrogen positive pressure protection.

[0031] (4) After the reaction was complete, the reaction flask was exposed to air to terminate the polymerization. The reaction solution was passed through a glass column containing 50g of neutral alumina to adsorb and remove the copper catalyst, and the colorless or pale yellow effluent was collected. The effluent was transferred to a dialysis bag with a molecular weight of 3500 Da and dialyzed in deionized water for 3 days, changing the water 3 times a day. Finally, the product solution in the dialysis bag was freeze-dried to obtain a light brown, water-soluble solid product TA-g-PCE.

[0032] Example 1

[0033] Weigh 15g (0.3%) of polycarboxylate superplasticizer at 20℃ and set aside for later use.

[0034] Example 2

[0035] Weigh 15g (0.3%) of TA-g-PCE composite admixture at 20℃ and set aside for later use.

[0036] benchmark group

[0037] No additives are added.

[0038] The admixtures prepared in Examples 1 and 2 above, 500g of high-calcium fly ash, alkali activator, and standard sand were placed in a cement mortar mixer and mixed according to standard to form a homogeneous slurry. According to GB / T 8077-2023 Test Method for Homogeneity of Concrete Admixtures, the water consumption for the flowability of the reference mortar was first determined, and then the water consumption for the flowability of the mortar with admixtures was determined. The water reduction rate of the cement mortar was calculated. The water consumption when the mortar flowability was (180±5) mm was the water consumption for the reference mortar flowability.

[0039]

[0040] In the formula:

[0041] ω wt —Water reduction rate of mortar;

[0042] w2——Water consumption when the reference mortar flowability is (180±5)mm, in grams (g);

[0043] w3 — Water consumption when the fluidity of the mortar with admixtures is (180±5) mm, in grams (g).

[0044] Table 1. Comparison of water reduction rates between different admixtures and the baseline group.

[0045]

[0046] Table 2 Comparison of setting times of different admixtures and the baseline group.

[0047]

[0048] According to the experimental data in Tables 1 and 2, compared with the benchmark group, the composite admixture prepared in this invention can effectively reduce the water consumption of high-calcium fly ash macropolymers and play a retarding role, thus having good engineering application value.

[0049] The above description is only a preferred embodiment of the present invention. It should be noted that for those skilled in the art, several improvements can be made without departing from the principle of the present invention, and these improvements should also be considered within the scope of protection of the present invention.

Claims

1. A method for the preparation of composite admixtures for high calcium fly ash geopolymer, characterized by, The method comprises the following steps: (1) esterification of tannic acid and 2-bromoisobutyryl bromide in an organic solvent under the action of a catalyst in an inert atmosphere, precipitation, washing and drying after the reaction to obtain a brominated tannic acid macromolecular initiator; (2) the prepared brominated tannic acid macromolecular initiator, polyether macromonomer, acrylic acid are dissolved in a mixed solvent of water and alcohol, and after oxygen removal treatment, the atom transfer radical polymerization reaction is carried out under the catalysis of Cu + / ligand catalyst system, and after the reaction, the composite admixture is obtained through purification and drying.

2. The production method according to claim 1, characterized by, The molar ratio of the tannic acid to the 2-bromoisobutyryl bromide in step (1) is 1:

30.

3. The production method according to claim 1, characterized by, The polyether macromonomer in step (2) is polyethylene glycol methacrylate with a number average molecular weight of 1000-2400 g / mol.

4. The method of claim 1, wherein, The molar ratio of the brominated tannic acid macromolecular initiator, the polyether macromonomer and the acrylic acid in step (2) is 1:40:

40.

5. The preparation method according to claim 1, characterized in that, The Cu in step (2) + The metal catalyst in the ligand catalyst system is CuBr or CuCl, and the ligand is pentamethyldiethylenetriamine.

6. The method of claim 1, wherein, The temperature of the atom transfer radical polymerization reaction in step (2) is 60-70 ℃, and the reaction time is 6-12 hours.

7. The composite admixture for high calcium fly ash geopolymer of claim 1, wherein, The composite additive has a core-shell structure with tannic acid as the core and polycarboxylic acid as the shell.

8. The composite admixture for high calcium fly ash geopolymer of claim 1, wherein, The mixing amount of the composite additive is 1%-5% of the total mass of the precursors in the high-calcium fly ash geopolymer.

Images