9.2 : Retarders

Retarders are chemical admixtures designed to extend the setting time, which is especially useful when there is a delay in sequential concrete pours to prevent cold joints and to achieve a cohesive structure. Retarders, when used in appropriate amounts, can also enhance the architectural appearance of exposed aggregate finishes.

The function of retarders is to delay the setting of concrete, and this effect can be measured using a penetration test. The retardation process involves adding substances such as sugar, carbohydrate derivatives, soluble zinc salts, and borates to the concrete mix. A small proportion, like 0.05 percent of sugar by mass of cement, can prolong the setting time by approximately four hours. It is hypothesized that retarders interfere with the crystal growth within the concrete, thereby creating a more formidable barrier to the process of hydration. Although they alter the initial rate of strength development, retarders do not change the final composition or identity of the hydration products. The integration of retarders can result in initial lower strength, but it does not affect the ultimate strength achieved, which can be higher than in non-retarded concrete. Additionally, retarders have been observed to increase plastic shrinkage due to the extended plastic stage, but they do not affect drying shrinkage.