It is the concrete that turned to dust. The steel turned molten, and created an underground pool of molten metal that delayed the destruction of the crime scene by weeks because it remained too hot to approach.

Building fires have never before or since caused a steel framed skyscraper to collapse, so fires do not 'dustify' office buildings normally. Neither do they turn the steel frame into pools of liquid iron so hot it remains molten for weeks. Burning carpet and office chairs don't create the heat to do that.