Hello everyone, welcome! Our news today is for a clearer understanding of concrete retarder.
First of all, what products are included in concrete admixtures? I will share it with everyone.
PCE superplasticizer, PCE slump retaining agent, Accelerator (Liquid), Accelerator (Powder), air entraining agent, solid PCE superplasticizer, Solid Polycarboxylate and Anti-segregating superplasticizer for concrete. These are some common concrete additives.
Of course, the most widely used is PCE superplasticizer also our main products, (Polycarboxylic acid superplasticizer), Superplasticizers are mainly divided into two forms, liquid PCE superplasticizer and powder PCE superplasticizer.
Today we focus on the concrete retarder, Retarder is an admixture that reduces the speed and heat of hydration of cement or azurite, and extends the setting time. The purpose of adding a retarder to commercial concrete is to extend the hydration and hardening time of cement. Fresh concrete can maintain plasticity for a long time, thereby adjusting the setting time of fresh concrete. Scope of application of retarder: retarder can be used for mass concrete, roller compacted concrete, concrete constructed under hot weather conditions, concrete poured in a large area, concrete to avoid cold joints, requiring a long time to park or long-distance transportation Concrete, and other concrete that needs to delay the setting time. The retarding superplasticizer can prepare high-strength and high-performance concrete.
The retarder can delay the setting and hardening of cement, and can keep the concrete mixture plastic for a long time to facilitate the transportation, pouring and vibrating of the concrete; it can delay the appearance time of the hydration temperature peak and reduce the peak hydration temperature. Prevent temperature cracks in mass concrete; adjust the adaptability of superplasticizer and cement to reduce the slump loss of concrete mixture.
The concrete retarder can not only extend the setting time of concrete, but also keep the fresh concrete plastic for a long time for pouring and improve construction efficiency. It can also delay the heat release of cement hydration and reduce the occurrence of cracks. Therefore, it is widely used in mass concrete, pumped concrete, construction in hot climates and some special construction techniques. At present, the commonly used retarders at home and abroad are lignosulfonates, sugars, hydroxycarboxylic acids, inorganic salts and so on.
Due to the incompatibility of the retarder and the chemical components of the cement, or the improper dosage of the retarder, the retarder may not have the retarding effect or even produce abnormal coagulation. This is the manifestation of the incompatibility of the retarder with the cement. There are many factors that affect the adaptability of the retarder and cement. The abnormal phenomenon of concrete caused by the incompatibility of the retarder and cement is also various. The most common occurrence is the significant increase in the loss of concrete slump over time.
Next we will understand the characteristics of concrete retarders:
1. The retarder can reduce the release rate of the heat of hydration of concrete products. As we all know, the early strength development of concrete is closely related to the occurrence of concrete cracks. Early hydration is too fast and the temperature changes too quickly, which will easily cause cracks in the concrete, especially for mass concrete. Because the internal temperature of the concrete is not easy to spread, the temperature difference between the inside and outside is large, which leads to the occurrence of concrete cracks and affects the quality of concrete. Concrete retarder can effectively improve this situation.
2. The retarder can reduce the loss of concrete slump. Practice has shown that the use of retarders containing lemon, sodium tripolyphosphate, sucrose and other ingredients can significantly extend the initial setting time of concrete, and at the same time, the time interval between the initial setting and the final setting of concrete is also shorter, which reduces the mixing Condensation slump loss does not affect the growth of concrete early strength.
3. The effect of retarder on the strength of concrete: From the perspective of strength development, the early strength of concrete mixed with coagulant is lower than that of unmixed concrete. In addition, as the amount of retarder added increases, the early strength will decrease more. However, if it is added in excess, the setting time of the concrete will be too long, which will cause permanent damage to the strength of the concrete due to the evaporation and loss of water. The impact of recovery.
4. Selection of retarder. Although the retarder has many advantages, it is necessary to choose different retarders to achieve the most ideal effect due to the different types of cement, concrete mix ratio and 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.
High-strength concrete generally has a low sand rate, low water-cement ratio, high coarse aggregate strength, and a large amount of cement. This requires a high proportion of cement to be incorporated. It is necessary to use superplasticizers. In addition, superplasticizers can also be used. Come a certain economic benefit. The water reduction rate of superplasticizers is generally 20%~25%, and the most commonly used domestic superplasticizers 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.
Concrete retarder has many advantages, and its cost is not very high, which brings a lot of convenience for on-site construction, and is used more and more widely in construction now. But it also has shortcomings. If the variety and dosage of retarders are unreasonable, it will have a negative effect on the construction quality. Therefore, in the actual construction process, it is extremely important to select a suitable retarder and determine its dosage reasonably. Therefore, when using this method, various factors must be comprehensively considered, carefully tested, and implemented carefully. Therefore, it is necessary to strictly control the raw materials to prevent the occurrence of abnormal concrete condensation.
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