Ash was the powder left of something after it had burned.
The soil of the petrified jungle on Skaro turned to sand and ashes due to radiation. (TV: The Daleks)
Zolfa-Thura was reduced to sand and ashes after a war. (TV: Meglos)
The "snow" that fell in London on Christmas 2006 was actually ash from the remnants of the Sycorax spaceship. (TV: The Christmas Invasion)
Earth in the 30th century was "slag, ash, clinker." (TV: The Mutants)