Fantastic! You're so sure of yourself "Steel CANNOT go from a solid into a dust" brilliant, simply brilliant. As Holmes says, once you rule out the impossible,then what remains however improbable, must be the truth. If you examine carefully the well documented evidence it's fairly clear we're dealing with an effect hitherto unknown to science i.e if steel really can't be turned to dust then where the hell did the towers go?
That's where the towers went. There they are, covering the entire tip of Manhattan and beyond.
The WTC towers were 95 percent air. That means the pile of steel would be 5 percent of the height of the WTC if stacked up neatly. However, the two acres of WTC towers, 220 floors (aka 220 acres of office junk) above ground were spread over 19 acres. Your photo shows wallboard, ceiling tiles, insulation, and concrete dust, aka "glass fibers, gypsum wallboard, concrete, paper, window glass, and miscellaneous materials used in building
construction or found in office buildings - dust".
Need the approximate percentage - look up "Environmental Studies of the World Trade Center", for a reality check.
Steel does not turn to dust except in Judy Wood's mind.
http://pubs.usgs.gov/fs/fs-0050-02/fs-050-02_508.pdf
http://pubs.usgs.gov/of/2001/ofr-01-0429/
an effect hitherto unknown to science i.e
Wait, it has a name, it is called fantasy. You realize Sherlock Holmes is fiction, as is Judy's dustification of steel.
The dust cloud is big because E=mgh was released, it was equal to 130 tons of TNT in each tower. The energy due to gravity was massive, and released during the collapse. The energy of collapse crushed wallboard, insulation, glass, ceiling tiles and 220 acres of office space; the dust seen. It is hard to comprehend the energy equal to 260 2,000 pounds bombs is in the collapsing building.
The buildings are mostly air...