I worked in costruction as a powdrman many moons ago and was quite adept at doing specialty jobs that required a bit of imagination to get desired results.From watching various videos of the take down of the twin towers and building 7 from different angles here on u-tube I would estimate that they used short period delay detonating caps which have a delay of 1/40th of a second. There is an explosive called thermite whichwill turn steel to powder which would have been the explosive of choice for obvious reasons.

sc