Cold weather concreting poses a risk of which of the following?

Cold weather concreting poses a risk primarily because it can slow down the hydration process. When temperatures drop, the chemical reactions that occur during hydration—where water and cement combine to form a stronger structure—proceed at a much slower rate. This delay can lead to problems such as incomplete curing and ultimately lower strength development in the concrete.

Hydration is critical for achieving the intended strength and durability of concrete, and any factor that impairs this process, such as low temperatures, can adversely affect the overall quality of the concrete. Therefore, it's essential for engineers and contractors to take appropriate measures, such as using heated materials, ensuring adequate insulation, or employing accelerators to mitigate the effects of cold weather on the curing process.

The other options do not align with the challenges presented by cold weather. Fast hydration and high strength gain are not characteristics associated with cold conditions, as these are typically seen in warmer temperatures. Increased workability is also not expected in cold weather, as lower temperatures can make concrete stiffer and more difficult to work with.