Clouds of Concrete
"The north tower stood motionless and stable for nearly 1.75 hours. Now, suddenly, the tower starts to disintegrate from the top down. In a matter of seconds the entire building turned to dust. As with the south tower, all parts of the building turned to dust and pieces of steel before any of it hit the ground."
"Cracking a concrete block into two pieces requires energy, and pulverizing a concrete block into powder requires even more energy. The smaller the particles, the more energy needed.
The only energy available to pulverize the concrete in the WTC was the "potential energy" of the buildings due to the force of gravity. However, the photos show that no piece of concrete fell more than a few feet before turning into powder. This means the concrete picked up enough energy during the fall of a few feet to convert itself to a fine grain powder."