Which of the following is added to concrete in hot weather to make it set up slower?

In hot weather conditions, concrete tends to set more quickly due to higher temperatures, which can lead to challenges such as reduced workability and increased risk of cracking. To manage this issue, a retardant is added to the concrete mix. Retardants are chemical admixtures that specifically slow down the setting time of the concrete. This allows for extended workability, enabling contractors to place and finish the concrete more effectively before it hardens.

Using a retardant helps to ensure that the concrete maintains its quality and integrity despite the heat, allowing for proper mixing and placement processes. This is essential for achieving the desired strength and durability of the concrete once it has cured.

While other options like water or filler might influence the concrete properties, they do not specifically serve the primary function of slowing down the setting process in hot weather as effectively as a retardant does.