Preparation, performance and application of early-strength high-performance water-reducing admixtures
1. Current status of early-strength high-performance water-reducing admixtures
The development of concrete technology is inseparable from chemical admixtures , high-efficiency water reducing agents have played a key role in the development of new materials such as pumped concrete, self-leveling concrete, underwater non-dispersible concrete, shotcrete, polymer concrete, high-strength and high-performance concrete. High-efficiency water reducing agent, also known as superplasticizer, is used in concrete mixtures and mainly plays three different roles.
① Improve the workability of concrete without changing the strength of concrete;
② Reduce the water-cement ratio and improve the strength and durability of concrete under given workability conditions. ;
③ Under the condition of ensuring the performance and strength of concrete pouring, reduce the amount of water and cement, and reduce factors such as creep, dry shrinkage, cement hydration heat, etc. that cause initial defects in concrete.
The application of naphthalene-based high-efficiency water-reducing admixtures has a history of more than 20 years and is currently the main type of high-efficiency water-reducing admixture used in engineering applications. Studies have shown that early-strength high-efficiency water-reducing admixtures are new water-reducing agents with better performance than naphthalene-based water-reducing agents. At the same dosage, early-strength superplasticizers can achieve better water reduction rates and slump retention capabilities. Japan is the country that has the most research and application of early-strength water-reducing agents and is the most successful. After 1998, the usage of early-strength water-reducing agents in Japan exceeded that of naphthalene-based water-reducing agents. In recent years, there have been many reports on the research and development of early strength systems with superior properties in the papers of some researchers in North America and Europe. The research focus has gradually shifted from the modification of sulfonic acid superplasticizers to the modification of early strength systems. Research. Scholars from Japan and some European and American countries have published a large number of research papers on early-strength water-reducing agents. Most of them are developing and researching early-strength water-reducing agents, with the main focus on the development of early-strength water-reducing agents and related research. The working performance of fresh concrete, the mechanical properties of hardened concrete and engineering application technology, etc. Almost all domestic early-strength water-reducing admixtures have not reached the practical stage. The raw materials available for synthetic early-strength superplasticizers are also extremely limited. From the selection of superplasticizer raw materials to the production process, cost reduction, performance improvement and many other aspects require systematic research.
2. Performance and mechanism of early-strength high-performance water-reducing admixture
Compared with other high-efficiency water-reducing admixtures, early-strength high-performance water-reducing admixture has many outstanding properties:
Low dosage (0.2%--0.5%) and high dispersion performance;
Good slump retention, with basically no loss of slump within 90 minutes;
When compared under the same fluidity, the delayed coagulation time is less;
The degree of freedom in the molecular structure is large, there are many controllable parameters in the manufacturing of admixtures, and the potential for high performance is great;
Since no formaldehyde is used in the synthesis, it does not cause pollution to the environment;
Good compatibility with cement;
Can make more use of slag or pulverized coal ash and other mixed materials, thus reducing the overall cost of concrete.
There are few domestic studies on the mechanism of action of early-strength series high-efficiency water-reducing admixtures. It can be seen from the infrared spectrum of the early-strength high-efficiency water-reducing agent [8] that there are carboxyl groups, ester groups, and ether bonds, and their wave numbers are 3433cm-1, 1721cm-1, and 1110cm-1 respectively.
Since there are both carboxyl and ester groups in the molecule, it can be both hydrophilic and hydrophobic. Since the early strength series has carboxyl groups, like the naphthalene series water reducing agent, the DLVO theory still applies . The electrostatic repulsion of carboxyl anions contributes to the dispersion of cement particles. Similarly, the relative molecular weight and the content of carboxyl groups have a great influence on the dispersion effect of cement particles. Due to the hydrophobicity of the main chain molecules and the hydrophilicity of the side chains as well as the presence of the side group - (OCH2CH2) -, it also provides a certain three-dimensional stabilization effect, that is, the surface of the cement particles is covered by a block or graft copolymer. Stable to prevent random agglomeration, thus helping the dispersion of cement particles. Its stabilization mechanism is the so-called 'space stabilization theory'. 'Space stability theory' refers to the stabilizing ability caused by the interaction between polymer (water reducing agent) molecules due to occupying space or conformation. This kind of stability The difference between the effect and the general electrostatic stabilization effect is that there is no long-range repulsion, and only when the outer edge of the protective layer composed of the polymer comes into physical contact, the repulsive force is generated between the particles, causing the particles to automatically bounce away. The thermal energy and distance curves of two protective layers with different thicknesses are given.
In the medium, the heat of solution of the polymer is usually greater than zero, so from an enthalpy point of view, the increase in local dispersant concentration caused by the proximity of the particles to each other is beneficial, but this also causes entropy decreases, and the latter often plays a dominant role in the system. Therefore, the three-dimensional stabilization mainly depends on the entropy change of the system. Therefore, some people call it 'entropy stabilization'.
It can be seen from the potential energy distance curves of two protective layers with different thicknesses that the total interaction energy VT between any two particles in the dispersed system is composed of two parts, one part is the van der Waals attraction potential energy VA, the other part is the three-dimensional interaction potential energy VS, so there is:
VT=VA+VS.
When the outer edges of the dispersant layer of two particles come into physical contact, that is The distance h between the two particles is less than 2 times the thickness of the dispersant layer δ, that is, when h < 2δ, due to the volume effect and the 'repulsion' of the solvent molecules in the interface layer, the conformation of the dissolved chain segment will be disturbed, resulting in The local free energy rises. At this time, VS can be expressed by the following formula: <Method, react at 102~110℃ for about 8 hours, the product is a light yellow transparent solution.
4. Conclusion
There are still many difficulties in systematically studying new high-performance water-reducing agents, but research on new high-performance water-reducing agents still has important theoretical significance and practical value. There are some problems that need to be further deepened in the research on the synthesis, action mechanism and application of early-strength water-reducing agents: First, since most water-reducing agents are synthesized in water systems, it is difficult to understand the complex relationships between different monomers. The interaction of water-reducing agents; secondly, there are limitations in the method of characterizing water-reducing agent molecules. The relationship between the chemical structure and performance of water-reducing agents cannot be clearly explained, and there is a lack of research on the microstructure; thirdly, although early-strength system water-reducing The compatibility between the agent and cement is better than that of other types of water-reducing agents. However, in terms of fluidity of concrete, when cement and admixtures are used together, phenomena such as rapid loss of concrete slump and rapid hardening often occur. There are still cement Regarding compatibility issues with chemical admixtures, it is not yet fully understood how water-reducing agents work; fourth, in concrete using high-performance water-reducing agents, when the unit water volume decreases and the slump increases, problems often occur. The concrete is too viscous, causing problems such as segregation and bleeding.
The research on high-performance water-reducing agents has become an important branch of concrete material science and promotes the development of the entire concrete material from low technology to high technology. Research on early-strength superplasticizers will take more comprehensive considerations from the strength, workability, durability, price and other aspects of concrete. Graft copolymerized early-strength water-reducing agents are mainly copolymerized by unsaturated monomers under the action of initiators, and the side chains with active groups are grafted to the main chain of the polymer, so that they have the characteristics of efficient water reduction and control at the same time. It has the functions of slump loss and shrinkage resistance, and does not affect the setting and hardening of cement. Looking to the future, the specific requirements of each concrete technology will require the development of optimal admixtures, with many different chemical compositions for each series. With the continuous in-depth research on the synthesis and characterization of polymer water-reducing agents and the relationship between their chemical structure and performance, early-strength water-reducing agents will further develop in the direction of high-performance, multi-functional, ecological, and international standardization. Early-strength superplasticizers can achieve better water reduction rates and smaller slump losses, especially in the preparation of concrete with high fluidity and low water-cement ratio, which are unmatched by other traditional high-efficiency water-reducing agents. Advantages, early strength water reducing agent will be the main variety in the water reducing agent series in the 21st century. </p
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