RE: pyroclastic dust
I'm not quite sure the term applies to building demolitions. This site has a rather impressive photo of a pyroclastic flow and a brief explanation of the term:
volcanoes.usgs.gov
From what I can tell, pyroclastic flows are caused by molten rock coming into contact with rocks that contain quite a bit of moisture. The wet rocks are super heated and the resulting steam causes them to explode so violently they are pretty much turned to powder. The flow effect is caused by massive amounts of water turned to steam, along with hot gas from inside the volcano.
In a controlled demolition, the idea isn't to turn the building's concrete into powder, but to cut the load bearing supports. In the case of the twin towers, I assume the main focus would have been on the central core. Once the core was gone, the exterior structure of box columns would no longer hold it up. It was never designed to hold the building up by itself in the first place.
I don't know how much my limited experience with concrete applies, but I'll toss out some thoughts and see where it goes. Concrete is rated by its compression strength. For residential foundations, code is something around 3000 psi.
Some years back I used recycled concrete for a driveway I was working on. What it is basically, is old concrete put through a rock crusher, and it makes a great surface for a driveway. That aside, the point of mentioning it is it's very powdery, and not at all like what you'd expect from smacking a chunck of concrete with a hammer where big pieces break off.
Anyhow, what I'm trying to get at is if you compress concrete hard enough, it gets powdery. While I don't have any idea of the kind of forces involved in a building the size of the towers falling down, I suspect it was plenty big enough to crush concrete. By the same token, drywall turns to powder if you breath on it.
In any case, I'd tend to question the so called pryoclastic effect as being overly significant, or the idea that there shouldn't have been that much dust. On the other hand, it seems to me the dust was ejected rather vigorously for a collapse caused by gravity alone.
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