The concrete retarder is a kind of concrete retarder that can delay the cement hydration reaction, thereby prolonging the setting time of commercial concrete, keeping the plasticity of fresh commercial concrete for a long time, facilitating pouring, improving construction efficiency, and improving the performance of commercial concrete in the later period. Admixtures that cause adverse effects.
It is widely used in summer construction and pumping construction of commercial concrete. According to the different chemical composition, concrete retarders for commercial concrete are divided into inorganic concrete retarders and organic concrete retarders.
At present, lignin sulphur salt is a retarder with larger output and wider application. In addition, molasses, hydroxycarboxylic acids and a few inorganic salt retarders have also been widely used. Therefore, in combination with different types of retarders, the retarding mechanism of retarders is discussed.
Although the retarder has many advantages, it is necessary to select different retarders to achieve the most ideal effect due to the different types of cement, the mixing ratio of commercial concrete and the water consumption. In addition, excessive mixing can also produce negative effects. Therefore, the selection of retarder and the determination of the amount of retarder are extremely important.
① Commercial concrete and large-volume commercial concrete poured continuously under high temperature, due to the inconvenience of one-time pouring construction or thick section, generally adopt layered pouring. To ensure that the upper and lower layers are well combined before initial setting, commercial concrete is required The initial setting time is longer, and it has good retardation.
The other is the heat of hydration inside the commercial concrete. If it is not well controlled, temperature cracks will appear, which will reduce the temperature rise. Commonly used general water reducers, retarders, retarding water reducers, such as citric acid.
② High-strength commercial concrete generally has low sand ratio, low water-cement ratio, high coarse aggregate strength, and large cement consumption. This requires a high proportion of cement to be incorporated, and it is necessary to use superplasticizers. In addition, superplasticizers are also used. Can bring certain economic benefits.
The water reduction rate of superplasticizers is generally 20%~25%, and the most commonly used superplasticizers in China are of Nai series. Superplasticizers generally increase the loss of slump, so they are often used together with retarders to improve the workability of the mixture and reduce the loss of fluidity over time.
③ Pumping requires commercial concrete to have the fluidity, non-segregation, non-bleeding, and high slump performance required by the process under the premise of ensuring its strength. Therefore, its aggregate gradation is relatively more important than ordinary commercial concrete. Be strict. There are many that can be used:
Fly ash: reduce the heat of hydration and improve the cohesion of commercial concrete.
Common water reducing agent: such as wood calcium water reducing agent, which saves cement, increases fluidity, delays the release rate of hydration heat, and prolongs the initial setting time.
Pumping: It is a kind of fluidizing agent, which can greatly improve the fluidity of commercial concrete, prolong fluidity retention time, and reduce the loss of slump over time. As the name implies, it is an admixture for pumping. Superplasticizers and air-entraining agents can also be used in pumping commercial concrete, but they are not commonly used