Effect of retarder on concrete setting time Experiment instructions

Industrially: Retarder is a new type of concrete admixture. There are a large number of hydroxyl and carboxyl groups in its molecular structure, which can form stable complexes with free Ca 2+ in the alkaline medium of cement hydration products, delaying the crystallization of CH, and can also be adsorbed on the surface of cement particles. , delaying the hydration reaction. By adjusting the setting time of concrete, the dosage can be reasonably adjusted according to the ground material and temperature in each region.

What issues should be paid attention to when mixing retarder in concrete

(1) Pay attention to the mutual compatibility with the cementitious material system. For different cementitious material systems, they can show different condensation and hardening properties, and they also have different requirements for the role of retarder. For example, for cementitious material systems without mineral admixtures, concrete sets and hardens quickly, and a retarder is needed to significantly extend the setting time. When a large amount of mineral admixtures are added, especially when a large amount of volcanic ash materials such as fly ash are added, the setting and hardening of the concrete itself is relatively slow, and only a small adjustment of the setting time is required by the retardant. ,.It can meet the construction requirements even without adjustment. Different cementitious material systems have different requirements for retarder, and corresponding responses should be made when using retarder. At the same time, the effect of the retarder is related to the mineral composition. Different cementitious material systems have different mineral compositions. Therefore, the effects of the retarder will also be different in different cementitious material systems. For these reasons, the retarder must be selected according to the cementitious material system used and the dosage of the retarder must be determined. If not, the coagulation time may be insufficient, the coagulation time may be too long, and the early strength may be too low.

(2) Pay attention to changes in the environment. The setting of concrete is caused by the hydration reaction of cementitious materials. Since the hydration reaction rate is closely related to temperature, the setting time of concrete is also different in different seasons. This is a well-known fact. In order to maintain normal setting time, the dosage of retarder also needs to be adjusted accordingly with seasonal changes.

(3) Pay attention to coordination with other chemical additives. More than one chemical admixture is often used in concrete. Some reactions may also occur between these chemical admixtures, and their original functions often change. In addition, some chemical admixtures may have many functions, and these functions will overlap and offset each other. When mixing retarder, attention should also be paid to the effects of these aspects. For example, some water-reducing agents also have a retarder function. When using such water-reducing agents, the amount of retarder should be appropriately reduced.

(4) Pay attention to the project requirements. Different projects may have different requirements for the setting and hardening process of concrete due to different purposes. When using retarder, attention should be paid to coordination with it. For example, for the pouring of some structural components, it is hoped that there will be sufficient setting time for construction needs. Once poured, it is expected to harden quickly and develop strength quickly. For this kind of project, when choosing a retarder, you should not only pay attention to its retarding effect, but also pay attention to the final setting time not being too long. For the pouring of some large volumes of concrete, the strength development of the concrete can be appropriately delayed. The key is that the heat release of the concrete should not be too concentrated to avoid causing a higher temperature rise. Therefore, the setting time can be appropriately moved back from the age of concrete strength assessment.

(5) When using retarder, attention should be paid to strengthening maintenance. Retarder can delay the hydration of cementitious materials, but it cannot delay the evaporation of water. Hydration of cementitious materials requires water. If the water evaporates quickly and loses too much water before the cementitious material begins to hydrate, there will be insufficient water in the system, which will affect the hydration of the cementitious material, resulting in a delay in obtaining the strength of the concrete. Development, and even powdering may occur. Of course, concrete without retarder also needs to be maintained. For concrete mixed with retarders, the delay in the hydration of the cementitious material can leave more time for the water to evaporate. Therefore, when using retarder, more attention should be paid to strengthening maintenance, especially when construction is carried out in windy and dry weather, special attention needs to be paid to this point.

The above is the effect of retarder on concrete. I hope it will be helpful to you.

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