To reduce the slump loss caused by hydration, Concrete retarder are essential. The current issue is excessive dosage, leading to numerous quality problems in construction.
What is concrete retarder？
Concrete retarder refer to additives that delay the setting time of commercial concrete without significantly affecting its later strength. They primarily consist of hydroxyl compounds, hydroxycarboxylates and their derivatives, and high-sugar lignosulfonates.
Concrete retarders also have a water-reducing effect and are sometimes called retarding water reducers. In addition, some inorganic salts like zinc chloride, borates, and various phosphates can also act as retarders. They are mainly suitable for continuous casting of commercial concrete at high temperatures, large volume commercial concrete, precast commercial concrete, and pumped commercial concrete. The dosage is typically 0.1% to 1% of the cement quantity.
Characteristics of concrete retarder
1、Concrete retarders can reduce the rate of heat release in commercial concrete products.
As it is widely known, there is a close relationship between the early strength development of commercial concrete and the occurrence of cracks. Rapid early hydration and temperature changes can easily lead to cracks in commercial concrete, especially in large-volume commercial concrete. This is because the internal temperature of commercial concrete does not dissipate easily, leading to significant temperature differences between the interior and exterior, which in turn results in cracks in commercial concrete and greatly affects its quality.
2、Concrete retarders can reduce the slump loss in commercial concrete.
Practical experience has shown that retarders containing ingredients like citric acid, sodium tripolyphosphate, and sucrose can significantly prolong the initial setting time of commercial concrete. At the same time, the time interval between initial setting and final setting of commercial concrete is shorter. This not only reduces the slump loss but also does not impact the early strength development of commercial concrete. They have substantial practical value and are increasingly used in current commercial concrete construction.
3、Effect of retarder on strength of concrete.
From the point of view of strength development, the early strength of commercial concrete mixed with retarder is lower than that of unmixed, especially 1D and 3D strength. However, generally after 7 days, the two will gradually stabilize, and the incorporation of retarding agents will be slightly increased.
Additionally, with an increasing dosage of retarder added to the mix, there is a greater reduction in early strength, and it takes a longer time for strength to improve. However, if over-retarded, leading to excessively prolonged setting times for commercial concrete, it can result in a permanent and irrecoverable impact on concrete strength due to moisture evaporation and loss.
