The story...

The WTC dust was mostly concrete.

Our take...

We found a breakdown that suggests otherwise. It's a second-hand report so we can't vouch for its accuracy, but then again we've yet to see any justification for the "mostly concrete" claim, either.

Microscopic analysis of WTC dust by Nicholas Petraco, BS, MS, DABC, FAAFS, FNYMS at The New York Microscopic Society lecture held at AMNH 28 May 2003

45.1% Fiberglass, rock wool (insulation, fireproofing)
31.8% Plaster (gypsum), concrete products (calcium sulfate, selenite, muscodite)
7.1% Charred wood and debris
2.1% Paper fibers
2.1% Mica flakes
2.0% Ceiling tiles (fiberglass component)
2.0% Synthetic fibers
1.4% Glass fragments
1.3% Human remains
1.4% Natural fibers
trace asbestos (it became illegal to use during the construction of the WTC)

Other trace elements: aluminum, paint pigments, blood, hair, glass wool with resin, and prescription drugs were found.
http://www.janegalt.net/blog/archives/004194.html

It seems concrete constitutes a much smaller part of the dust than most people assume. And we’d guess that much less energy is required to turn plaster into dust, say, suggesting that perhaps a gravitational collapse could do the job after all.