With the development of concrete technology, especially in the future, concrete will not only have high performance, but also develop in the direction of sustainable development that is green and harmonious with the environment. As a third-generation water-reducing agent, polycarboxylic acid-based water-reducing agent has been widely recognized at home and abroad due to its unique advantages in concrete and its environmental friendliness. It has only been 20 years since Nippon Shokubai Company introduced the polycarboxylic acid-based water-reducing agent to the market in 1986. In recent years (after entering the 21st century), the country has also paid great attention to it, and its development momentum has become even stronger in recent years.
In just four or five years, the number of manufacturers entering the commodity field has grown from a few to dozens. Many scientific research units and universities have independent intellectual property rights, and their products have entered various engineering concrete fields. Almost all of the landmark projects constructed in developed areas in China in recent years have used polycarboxylic acid-based superplasticizers, such as the Shanghai Maglev Train Track Beam Project, the Beijing Olympics Main Stadium Project, the Three Gorges Project, the Capital Airport Expansion Project, and the Hangzhou Bay Cross-Sea Bridge. Projects such as the Dayangshan Deep Water Port Project and the Beijing-Tianjin Intercity Rail Transit Project have achieved satisfactory results. At the same time, they have accumulated a lot of experience in application technology and discovered many new problems in application technology. . The concrete technical conditions set by the Ministry of Railways for the upcoming Beijing-Shanghai high-speed railway and the Air Force's airport self-compacting cement concrete pavement construction technical specifications also consider the main use of polycarboxylic acid-based high water-reducing agents in these concretes. For this reason, Relevant experimental research work has been carried out since 2006.
We had the opportunity to come into contact with some technical work on the application of polycarboxylic acid-based water-reducing agents. While admiring the performance of polycarboxylic acid-based water-reducing agents, we also discovered Various problems that arise in some applications, the emergence of these phenomena will be very unreasonable or unexpected for people who have been accustomed to using naphthalene-based water reducing agents for a long time. This is different from our understanding of polycarboxylic acid. The original high expectations for acid-based superplasticizers created a gap. People originally expected that the new admixture would not only perform well but also solve some problems with other components of concrete, because the polycarboxylate superplasticizer has good "adaptability".
In the past, we have been accustomed that a good admixture should be able to solve all concrete performance problems. When concrete performance problems arise, people turn to admixtures. Suppliers put forward requirements, and admixture manufacturers are accustomed to immediately using various compounding methods to meet the requirements. They rarely or cannot consider other reasons. They can only make fuss about compounding raw materials and relative parameters, which is often the case. Double the result with half the result. So how to correctly use polycarboxylate superplasticizer to achieve better results? The author has noticed the following aspects:
1. Polycarboxylate The acid-based superplasticizer has a different mechanism of action from the second-generation superplasticizer. In fact, the polycarboxylic acid-based superplasticizer is developed based on a new concept. It is different from the second-generation superplasticizer in that The following are: First, the diversity and adjustability of the molecular structure, or the ability to design the molecular structure according to performance requirements. The second is to further concentrate and improve the advantages of water-reducing agents and achieve greenness in the production process.
From the perspective of the action mechanism of water-reducing agents, polycarboxylic acid-based water-reducing agents embody the diversity of active groups in surfactant molecules. Not only are there many types of active groups, but these groups are not only concentrated on the main chain of the molecule, but are also active on the side branches grafted to the main chain. It forms a highly polar molecular main chain and hydrophilic side chains of a certain length and number, and the molecular structure is comb-shaped. The strong polar anionic "anchoring" group in the main chain is used to adsorb on the cement particles. The outwardly extending comb structure supported by numerous branch chains provides sufficient spatial arrangement effect for further dispersion of cement particles. Compared with the electrical repulsion of the double electric layer of naphthalene-based water reducing agents, the steric hindrance keeps the dispersion much longer. For example, BRGY-100, BRGY-200 and BRGY-300 produced by Blue Rock Building Materials are compounded with water-reducing agents and after mixing with concrete, their fluidity can be maintained for 2 hours without loss.
Appropriately change the comb structure of the polycarboxylic acid-based superplasticizer to appropriately change the density and length of the side chains, and then obtain a product suitable for prefabricated components. High water-reducing, high early-strength water-reducing agent is used.
It is not difficult to see from this that the characteristic of polycarboxylic acid water-reducing agent is that the molecular structure can be adjusted and changed as required to achieve the purpose of changing performance. . Rather than using simple compounding for modification, based on this understanding, it may inspire us to improve our application technology in the future.
2. The adaptability of polycarboxylic acid-based water reducing agents to cementing materials
In engineering applications, polycarboxylic acid-based superplasticizers with different cements and fly ash also have adaptability problems, especially for fly ash, which is more picky, while finely ground mineral powder has better adaptability.
The adaptability of cement is mainly reflected in: different types of cement, the saturation and point of polycarboxylic acid superplasticizer are very different, for example; Shield Stone When cement is mixed with 1.% (20% concentration), the effect is very good, and the prospects are...When adding mud, 1.2% is required. Moreover, the change is very obvious near the saturation point of concrete. For example, when the foreground cement is mixed with 1.0% to 1.1%, the change is not obvious. Only when it is added to 1.2%, it will show a better state. Once it exceeds 1.2%, it will secrete quickly. Water is very sensitive to changes in dosage, so it is very important to find the saturation point for different cements. However, we often encounter this situation; according to the information, the user stipulates that only 1.0% is allowed to be added. At this dosage, if the cement selected is not adaptable, it will be difficult for the admixture provider to deal with it, and the compounding method is often used. With little success.
Polycarboxylic acid-based high water-reducing agent also has adaptability problems with fly ash. Ash adaptability is good, but second- and third-level ash are more incompatible. At this time, even if the dosage of polycarboxylic acid is increased, the effect is not obvious. What components in fly ash are responsible for this requires further research.
Often when a certain type of cement or fly ash has poor adaptability to admixtures, you are still not completely satisfied when you change to another admixture. The cementitious material may eventually have to be replaced, but some uninformed users often suspect that the admixture is of poor quality and performance, which is unfair. The use of BRGY-100, BRGY-200 and BRGY-300 produced by Blue Rock Building Materials and water-reducing agents can improve the adaptability of the water-reducing agent to cement and fly ash.
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