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Using fly ash as a cementitious material in concrete is beneficial for improving the durability and performance of concrete, especially in environments exposed to de-icing chemicals. The correct percentage of fly ash that is typically recommended for this specific exposure condition is 50%. This level has been found to enhance the resistance of concrete to both freeze-thaw cycles and chemical attack, which are critical considerations when concrete is regularly subjected to de-icing chemicals like salt.

Incorporating 50% fly ash not only helps mitigate the harmful effects of de-icing agents on the concrete matrix but also contributes to improved workability, reduced heat of hydration, and lower permeability. This makes the concrete mix more resilient and extends its lifespan, which is particularly important for infrastructure subjected to challenging weather conditions.

It's important to note that while other percentages of fly ash can be used in various applications, the 50% benchmark provides a balanced approach to achieving the required durability and performance in environments where concrete will face de-icing chemicals, making it the most suitable choice in this scenario.