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Setting control admixture



Setting control admixture can be used to shorten or extend the plastic stage of concrete. The shortening or extending of concrete, the plastic stage will be required for special conditions such as casting concrete on cold weather or speeding the construction process of a structure. Admixture used to extend the plastic stage of concrete called as a retarder. On the other hand, admixture used to reduce the plastic stage of concrete called an accelerator.

Figure 1 

Concrete has initial setting time and final setting time. Initial setting time defined as the period of time from adding water to the concrete mix until concrete starts losing plasticity. Final setting time defined as the time from adding water to the concrete mix until concrete loses the entire plasticity. The concrete will lose plasticity as a result of the progressive crystallization of the hydration products. Controlling the crystallization of hydration products will control the rate of setting. The control of the crystallization of hydration products will be done by adding chemical admixtures. To control the setting rate of concrete, we need to control the rate of dissolution of silicate, aluminate, and clicate. Speeding up the dissolution of these elements will speed up the setting rate of concrete.
On the other hand, slowing the dissolution of these elements will reduce the setting time of concrete. Therefore accelerating admixture should speed up the dissolution of silicate, aluminate, and the calicate. While the retarding admixture should reduce the dissolution rate of these elements to slow the setting rate of concrete.




The monotonic retarding agents forming insoluble and impermeable products around the hydration products. Also, delaying the bonds among the hydration products, which reduces the setting time of concrete. Sugar and carbonated beverages belong to this category.

Retarding admixture will be used to offset the fast setting due to ambient temperature, it can be used to keep the concrete in plasticity stage while placing concrete for large structures and it can be used to maintain the concrete plasticity to meet the requirement transportation.

Accelerating admixture will be used to offset the slow setting due to ambient temperature, emergency repairs of important structures such as airport runways, and it can be used to speed up construction of concrete structures.


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