The research on water reducing agents in my country began in the mid-to-late 1990s, and its industrial production and application began in the early 21st century. Its promotion and application process is currently developing rapidly at a speed that people did not expect. Modern cement concrete is indispensable. One of the few components. Compared with other types of water-reducing agents, this type of water-reducing agent has the characteristics of small dosage, high water-reducing rate, strong plastic retention function, and environmental friendliness, and is more widely used in engineering. The use of water-reducing agents, especially for sand with poor gradation, high mud content, and manufactured sand, results in unsatisfactory adaptability of water-reducing agents and fluctuations in product application.
The synthesis of water-reducing agents usually adopts free radical copolymerization. Generally, copolymerization is carried out by dropping a small acrylic monomer solution into a large monomer. However, the concentration of a small acrylic monomer in the solution is relatively small compared to that of a large monomer. The monomer concentration is almost negligible. Considering the influence of monomer concentration on the reactivity rate, acrylic acid monomer solutions are added dropwise in different ways to change the concentration of small acrylic monomers, thereby changing the reactivity rate of each monomer in the system to control the molecules. structure.
1. The influence of relative molecular mass on product performance
As a dispersant, the relative molecular mass of polymer has a very important influence on its dispersion. If the relative molecular mass is too small, the polymer's ability to maintain slump is not high; if the relative molecular mass is too large, it will not only easily produce aggregation, causing the cement slurry to become more viscous, but also block the water-reducing function of the main chain. Functional groups such as carboxyl groups, sulfonic acid groups, etc. can cause a decrease in the dispersibility of cement slurry. Hu Jianhua believed through experiments that the water reduction rate of polymers first increases with the increase of relative molecular weight, and then decreases after reaching a certain value. Okada's further research shows that the best performance is obtained by copolymerizing polyoxyethylene, allyl monoalkyl ether, maleic anhydride styrene, etc., with a relative molecular weight of 20,000 to 80,000, using polyethylene glycol and polyethylene glycol. The copolymer obtained from acrylic acid is polymerized with (meth)acrylic acid, acrylate, etc. to obtain a water reducing agent with a relative molecular weight of 25,000 to 70,000, which has the best performance. When the relative molecular mass is less than 20,000, the dispersion effect is poor; when the relative molecular mass exceeds 100,000, coagulation will occur and the fluidity will be reduced.
Not only the relative molecular mass of the water-reducing agent affects its performance, but its relative molecular mass distribution also has a certain impact on its dispersion. The relative molecular mass distribution is measured by the GPC method, and the relative molecular mass distribution of the highest peak of the curve is taken. The molecular mass is Mp. It is believed that in order to obtain a water-reducing agent with high dispersion performance, (Mw-Mp)>0 and 7000, it means that there are more high-dispersion superplasticizers. With the presence of polymers, the dispersion properties of cement are reduced, and its ability to reduce slump loss will also be reduced. On the contrary (Mw-Mp) <0, it means that polymers with low relative molecular weight account for the majority, the bubble content in the concrete will increase, and the product performance will also decrease.
2. The influence of branched PEO on product performance
It was found that the PEO side chain of the water-reducing agent has an important influence on the dispersion and dispersion retention of cement particles. The degree of side chain polymerization The smaller the value, the faster the cement slurry loses fluidity. Due to the steric hindrance effect, the synthesized high-efficiency water-reducing agent with polyoxyethylene side chains increases as the side chain grows, and the steric effect of the water-reducing agent increases. Therefore, the dispersion effect of cement particles is better and the fluidity retention is also increased. However, when the PEO side chain is too large, entanglement may occur between the branch chains and bridges are formed between cement particles, which in turn affects fluidity retention. A series of high-efficiency water-reducing agents based on methacrylic acid ethylene glycol graft copolymers were studied, and it was believed that polyethylene glycols with different lengths can achieve high fluidity and fluidity retention properties at the same time. The methacrylic acid ethylene glycol graft copolymer contains carboxylic acid functional groups, sulfonic acid functional groups and alkoxy polyethylene glycol functional groups. The water-reducing agent containing long side chain polyethylene glycol has high steric repulsion. The dispersion time is short, and it has good dispersion and fluidity, but the fluidity retention performance is poor; the water reducing agent containing short side chain polyethylene glycol has a long dispersion time and good fluidity retention properties. It was found that a water-reducing agent with a shorter main chain and longer branch chains has better dispersion properties than a water-reducing agent with a longer main chain and shorter branch chains. A study was conducted on how the fluidity of ordinary Portland cement is affected by temperature (10-30°C) after adding superplasticizers with different polyoxyethylene side chain lengths and different branch positions. The results show that the side chain The longer the length, the less the dispersion of the cement slurry mixed with the water-reducing agent is affected by temperature. Therefore, a graft copolymer with an appropriate length of PEO side chain on the main chain can obtain both the required fluidity and the maintenance of fluidity.
3. The influence of sulfonic acid group content on product performance
From the mechanism of water reducing agent, it can be seen that the role of sulfonic acid group in the molecular structure of water reducing agent is closely related to the role of sulfonic acid group in the molecular structure of water reducing agent. The carboxyl groups are the same, that is, they are adsorbed on the surface of cement particles to provide electrostatic repulsion to disperse them. Therefore, an increase in the content of sulfonic acid groups is beneficial to improving dispersion. A water-reducing agent was prepared using 2-methyl-allyl sulfonate, methacrylic acid and polyethylene glycol monomethyl ether. Studies have shown that as the content of sulfonic acid groups in the water-reducing agent increases, cement dispersion Performance increased. Domestic Wang Guojian and others used styrene, acrylic acid, and hydroxyl-terminated polyoxyethylene ether to prepare a type of main chain with carboxyl groups, sulfonic acid groups, and polyoxyethylene ether side chains through free radical solution copolymerization, grafting, and sulfonation reactions. is a high-efficiency water-reducing agent. Research shows that as the degree of sulfonation increases, the sulfonic acid groupAs the content increases, the dispersion performance of the water-reducing agent on cement particles improves. </p
微信扫一扫打赏
