About the functions and scope of application of concrete admixtures

With the continuous development of social science and technology, the development and application of admixtures has become an inevitable trend, and concrete has therefore entered a high-tech era. Through the research and development of various new admixtures, it can be better Meet future complex engineering and special engineering requirements for concrete performance. When the dosage of concrete admixtures is too large, it will cause segregation, bleeding, and scratching of the concrete. If the concrete does not bleed through water reduction operations, the strength of the concrete will be high; when the dosage of admixtures is too small, the slump will If it is too small, the strength of the concrete will be low if water is added to make the fluidity of the concrete appropriate.

　1 Functions of concrete admixtures

Improve the workability of concrete during construction; improve the strength and other physical and mechanical properties of concrete; save cement or replace special cement; accelerate the early stage of concrete Strength development; adjust the setting and hardening speed of concrete; adjust the air content of concrete; reduce the initial heat of hydration of cement or delay the heat release of hydration; improve the bleeding property of concrete; improve the corrosion resistance of concrete to various corrosive salts; improve concrete The capillary structure; improve the pumpability of concrete; improve the corrosion resistance of steel bars; improve the bonding force at the interface between aggregate and mortar, improve the holding force between steel bars and concrete; and improve the bonding force at the interface between new and old concrete.

2 Classification of concrete admixtures

According to the functions of concrete admixtures, they can be divided into the following four categories: admixtures that improve the flow properties of concrete mixtures, including various reduction Water agents, air-entraining agents and pumping agents, etc.; admixtures that adjust the setting time and hardening performance of concrete, including retarders, early strength agents, accelerating setting agents, etc.; admixtures that improve the durability of concrete, including air-entraining agents, Waterproofing agents and rust inhibitors, etc.; admixtures that improve other properties of concrete, including expansion agents, antifreeze agents, colorants, etc.;

　3. Scope of application of concrete admixtures

Currently Concrete admixtures that are widely used in the construction market include water reducing agents, early strength admixtures, retarders, air entraining agents, expansion agents, antifreeze agents, pumping agents and waterproofing agents. Their application has a significant impact on the improvement of concrete performance and The scope of use is as follows:

Adding water-reducing agent to concrete can significantly improve the fluidity of the mixture if the mixing water consumption is not reduced; when water is reduced without reducing cement, the strength of the concrete can be improved; if While reducing water and appropriately reducing the amount of cement, cement can be saved, and the durability of concrete can also be significantly improved;

Early strength agents can accelerate concrete hardening and early strength development of concrete, shorten the maintenance cycle, and accelerate Improve construction progress and improve formwork turnover rate. It is mostly used for winter construction or emergency repair projects.

Retarder is mainly used for concrete in high-temperature seasons, large-volume concrete, construction by pumping and sliding form methods, and long-distance transportation of commercial concrete, etc. It is not suitable for concrete construction with the daily minimum temperature below 50C, nor is it suitable for construction. It is used for concrete with early strength requirements and steam-cured concrete; the retarders of different types of cement have different effects, and may even have opposite effects. Therefore, a test must be conducted before use to detect its retarding effect. Air-entraining agents are admixtures that can introduce a large number of uniformly distributed, stable and closed micro-bubbles during the process of mixing concrete; air-entraining agents can improve the mix and workability of concrete mixtures, reduce bleeding and segregation, and improve the resistance of concrete. Permeability and frost resistance. At the same time, as the air content increases, the elastic modulus of concrete decreases, which is beneficial to improving the crack resistance of concrete; due to the presence of a large number of microbubbles, the compressive strength of concrete will be reduced; air-entraining agents are suitable for anti-freeze, anti-seepage, Resistant to sulfates, heavily leaking concrete, etc.

Expansion agents mainly include calcium sulfoaluminate, calcium oxide, and metals; expansion agents are suitable for shrinkage-compensating concrete, expansion concrete for filling, expansion mortar for grouting, self-stressing concrete, etc.; containing sulfur aluminum Concrete containing calcium acid, calcium sulfoaluminate and calcium oxide expansion agents shall not be used in projects where the long-term ambient temperature is above 800C. Concrete containing calcium oxide expansion agents shall not be used in projects with seawater or corrosive water. . Antifreeze can significantly reduce the freezing point of concrete at a specified temperature, so that the concrete liquid does not freeze or only partially freezes, thereby ensuring the hydration of the cement and achieving the expected strength within a certain period of time; contains nitrite, carbonate It is strictly prohibited to use antifreeze in prestressed concrete structures; it is strictly prohibited to be used in drinking water projects and projects in contact with food, and it is strictly forbidden to eat; it contains antifreeze that produces pungent odors such as ammonium nitrate and urea, and it is strictly prohibited to be used in office, residential and other construction projects. ;

Pumping agent is a concrete admixture used to improve the pumping performance of concrete; it is composed of water reducing agent, setting agent, air entraining agent, lubricant and other components; pumping agent The agent is suitable for concrete pumping construction in industrial and civil buildings and other structures; it is especially suitable for large-volume concrete, high-rise buildings and super-high-rise buildings, and is suitable for slip form construction and underwater pile concrete.

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