Abstract: The research status of polycarboxylate superplasticizer at home and abroad is summarized. The mechanism of water reduction is analyzed. The technical problems of polycarboxylate superplasticizer in engineering application are put forward, and the future research and development are also proposed. The trend has been forecasted.
Key words: polycarboxylate superplasticizer; water reducing mechanism; application
At present, there is an increasing demand for high-performance concrete in engineering, and its requirements are becoming more and more strict. In the preparation of high performance concrete, in order to improve the compactness, strength and durability of the concrete, it is necessary to reduce the water to glue ratio. The low water-to-binder ratio will lead to an increase in the viscosity of the concrete, a deterioration in rheology, and difficulty in construction. The most effective and realistic way to solve this contradiction is to incorporate high-efficiency water reducer . Therefore, high-efficiency water reducing agents with low development costs and good comprehensive benefits are urgently needed. As an important part of high-performance concrete, high-efficiency water reducer is more and more valued by engineers and technicians, and its market prospect and application value are incalculable .
The traditional naphthalene water-reducing agent has good hardening performance and reasonable price, but its water retention and moisture retention are not ideal, and the water reduction rate needs to be further improved. In the 1990s, a new generation of water reducing agent and polycarboxylate superplasticizer were discovered, which is called the third generation superplasticizer in the industry. It is the water reducing agent with the best application prospect and the most comprehensive performance. The water reduction rate is as high as 30%~40%, which can optimize the performance of cement and cementitious materials; it can improve the structure and compactness of concrete pores, and better control the slump loss, bleed air and retardation of concrete. And bleeding problems; can be used with various types of cement, volcanic ash and other admixtures, and has good compatibility, even in low dosage, can make concrete have high fluidity, and Low water-cement ratio has low viscosity and small slump change with time; can increase the amount of fly ash and ground slag instead of Portland cement. At present, the research on polycarboxylate superplasticizer has attracted the attention of all countries and has become a research hotspot in the field of admixtures.
Research Status of 2 Polycarboxylate Superplasticizer
In recent years, in the papers of some researchers in developed countries, there have been many reports on the development of polycarboxylate superplasticizers with superior properties. The research content has gradually shifted from the modification of sulfonic acid superplasticizers to the polymerization. Research on carboxylic acid systems. Japan first developed polycarboxylate superplasticizers in the early 1980s and gradually applied it to concrete engineering in 1985. After 1995, Japan used a large amount of polycarboxylate water reducer in high-rise buildings, and its use exceeded that of naphthalene-based water reducers, accounting for 90% of high-efficiency water reducers. So far, Japan has been the most successful and most successful country for the application of polycarboxylate water reducers. There are more than ten patents in related fields, mainly acrylic acid and maleic anhydride, and most of them are synthesized in solvent-based systems. . In recent years, North American and European countries have also introduced their own polycarboxylate superplasticizers, and more than a dozen patents have been completed in solvent-based systems. Researchers have also studied the modification of copolymers. At present, the United States is developing from a naphthalene-based, melamine-based water reducing agent to a polycarboxylic acid-based high-performance water reducing agent, and is applied to practical engineering.
In China, the development of concrete admixtures is only equivalent to the level of Japan in the mid-1980s, and the development of polycarboxylate superplasticizers has just started. Due to cost and technical performance problems, almost no polycarboxylate water reducing agent developed in China has reached the practical stage, and only a small amount is used as a slump loss control agent combined with a naphthalene type water reducing agent, and the amount accounts for the total amount of the water reducing agent. 2%. From the domestic journals and related papers of the journals, the domestic research on polycarboxylate water reducer products is only in the experimental development stage, from the selection of water reducer raw materials to the production process, improving performance, reducing costs, etc. Further improvement.
Water reducing mechanism of 3 polycarboxylate superplasticizer
The polycarboxylate superplasticizer has a comb-like structure, and generally has different reactive groups grafted on the macromolecular chain, and has a plurality of active groups on the main chain, and the polarity is strong, and the side chain also has a pro Aqueous active groups. It is generally believed that the key to the water reducing effect of polycarboxylate superplasticizers is the anionic effect of carboxyl groups, sulfonic acid groups, etc. on the macromolecular chain and the steric hindrance of the long side chain of neutral polyoxyethylene, mainly through steric hindrance. The role is to maintain the dispersion of cement particles. The polycarboxylic acid-based water reducing agent contains more long-chain branches, and when they are adsorbed on the surface layer of the cement particles, a thick three-dimensional cladding layer can be formed on the cement surface, so that the cement achieves a better dispersion effect. At the same time, the hydrophilic long side chain of the polymer can still be stretched in the cement mineral hydration product, so that the polycarboxylic acid water reducing agent is less affected by the hydration reaction of the cement, and the dispersion effect can be maintained for a long time, so that the hydrazine can be kept for a long time. The drop loss is reduced.
There is an electrostatic charge on the surface of the cement particles, and some of the positive charges are neutralized by the free charge in the air, but the negative charge is still in an unbalanced state. When no water reducing agent is added, the negative charge is shielded by the calcium ion layer to form a so-called double ion layer. When the distance between the particles is much larger than the distance between the two ion layers, the electrostatic shielding force predominates. Obviously, the neutral particles agglomerate due to the effect of surface tension and the disappearance of electrostatic repulsion, and the fluidity of the cement slurry is very low. After adding the water reducing agent, the macromolecular chain of the water reducing agent adsorbed on the surface of the cement particles can gradually reduce the concentration of positive ions in the vicinity of the negatively charged particles, thereby increasing the role of the shielding layer and increasing the repulsive force between the particles. The scope of action. However, the electrostatic repulsion still exists and the particles are dispersed. The main reason that the polycarboxylate water reducing agent can keep the fluidity of the cement slurry from being lost is related to the three-dimensional repulsive force of the polymer adsorbent layer of the cement particle surface water reducing agent, and the three-dimensional repulsive force maintains the stability of the dispersion system. At the same time, when the water reducing agent lowers the surface tension, the adhesion between the particles caused by the surface tension is lowered.
4 application problems of polycarboxylate superplasticizer
The reason why the polycarboxylate superplasticizer can greatly reduce the water consumption is that it can effectively destroy the flocculation structure of the cement slurry and release the free water inside, which weakens the relationship between the cement particles and the water. From this point of view, it always intensifies the bleeding and sedimentation of the mixture to varying degrees, and the phenomenon of fly ash floating occurs. This phenomenon is particularly prominent in the concrete slab pouring process of buildings. Nowadays, after concrete pouring, there are often important reasons for the occurrence of spots on the surface, the formation of honeycomb pockmarks, and the formation of plastic shrinkage cracks along the stirrups. Although the polycarboxylic acid-based superplasticizer can improve the stability of the slurry by relying on some groups such as the effect of the gas-entraining group, in the engineering where the surface compactness and appearance are required to be high, it is also required to use the water-retaining composite. Good composition.
In recent years, in order to reduce the slump loss caused by the use of naphthalene-based water reducer to prepare concrete mixes, engineers and technicians have increasingly adopted the practice of combining them with polycarboxylate superplasticizers. Good results, and the combination can avoid the obvious problems of bleeding and segregation when using the latter, and it is more economical. Whether a polycarboxylic acid water reducing agent can be used in combination with a naphthalene type water reducing agent depends on its raw materials and production process. Some polycarboxylic acid-based water reducing agents have good compatibility with naphthalene-based water reducing agents, and can be compounded with naphthalene-based water reducing agents at will, and some polycarboxylic acid-based water reducing agents can not be combined with naphthalene-based water reducing agents. Use, even when using a tanker that has not been cleaned and shipped with a mixture of polycarboxylate water reducer, and continues to ship a blend of naphthalene-based water reducer, significant slump loss occurs. As a result, there is a problem of difficulty in unloading. Therefore, the compatibility test of two water reducing agents must be carried out before compound use.
Nowadays, polycarboxylic acid water reducing agents have become a worldwide research hotspot, and have also attracted the attention of domestic counterparts. With the development of science and technology, people's requirements for the living environment are constantly improving, and the demand for concrete and its performance requirements are also constantly increasing. Therefore, the demand for high performance water reducing agents with high water reduction rate and high moisture retention performance will also increase. The polycarboxylic acid-based water reducing agent will be developed in the direction of high performance, multi-functionality, greening, and international standardization, and the promotion of the use of polycarboxylate superplasticizers with excellent performance has become an inevitable trend in the current era. Therefore, scientists and technicians will pay more attention to the research and application of polycarboxylate superplasticizers, and comprehensively consider their development potential from the aspects of strength, workability, durability and price of concrete. It is foreseeable that in the near future, polycarboxylate superplasticizers will be widely used in China to promote the development of concrete materials in China towards high technology.