What is the primary function of admixtures that have a retarding effect on portland cement?

The primary function of admixtures that have a retarding effect on Portland cement is to delay the early stiffening of concrete. This is particularly important in various construction scenarios where temperature, large pours, or extended transportation times might otherwise lead to premature setting. By slowing down the hydration process, these retarding admixtures allow for a longer workability period, ensuring that the concrete remains workable for an extended duration. This is crucial for achieving the desired finish and ensuring that the concrete can be placed and compacted effectively without the risk of setting too soon.

The other options do not accurately represent the role of retarding admixtures. For instance, admixtures that accelerate setting would promote earlier stiffness, while those that increase temperature generally refer to methods like the use of heating or insulation, not admixtures. Enhancing bonding properties pertains more to different types of admixtures, such as bonding agents, rather than the retarding effect directly related to setting time. Thus, focusing on the delay of early stiffening captures the essence of the purpose of these retarding admixtures in the context of Portland cement.