Originally I speculated that the underside of the concrete floors might have been sprayed with nanothermite. Then it was pointed out to me that the concrete was poured into floor pans which would tend to rule out the underside. I am reluctant to let go of the notion entirely though bcause there are few situations I can think of that might conspire to both pulverise the concrete AND evaporate the 110 ACRES of wire mesh they contained.
The issue with this is that thermite, of whatever variety, is simply not an explosive, nor does it burn hot enough to boil metal and concrete. Even assuming that it were possible to impregnate the concrete slabs with thermite, igniting that thermite would neither pulverize nor evaporate the concrete or the steel rebar. Instead, the heat and molten iron generated by the thermite reaction would melt and mix with the concrete, resulting in large amounts of some kind of mixture of iron and glassy slag. As thermite is not dependent on any outside source of oxygen, this reaction would have completed in minutes, and would have long since cooled off by the time the pile was excavated.
I am not aware of any reports of a vast sheet of solidified slag under the debris pile. What concrete debris I have seen pictures of appears to have been physically broken up. This cannot be caused by thermite of any variety. It could have been caused by high explosives, but the amount of explosives required to pulverize all the concrete in the towers would have been impossible to conceal. Alternately, the damage to the concrete might have been caused by falling hundreds of feet, colliding with other debris and the ground. That would seem to me to be the most plausible explanation.
Furthermore, destruction of the concrete still does not support any plausible demolition theory. When speculating about an controlled demolition by sinister conspirators, we must assume that those responsble for the conspiracy are competent and rational. If they were not, the conspiracy would not have worked. We must also assume that the hypothetical conspirators do not wish to be discovered. If the hypothetical conspirators were not rational, competent, and concerned with avoiding discovery, their plan would already have been conclusively exposed. Therefore we must assume that the conspirators are not going to introduce any unnecessary elements into their plan, as any additional person to be included or device to be planted is an additional risk of discovery.
Planting some sort of thermal or explosive charge on the concrete floor slabs is completely superfluous to the goal of making the building collapse. Destroying every floor in the building would cause a global collapse, but it would be much, much easier simply to plant cutting charges on a few key structural columns on a few floors. Even in an ordinary controlled demolition of a concrete building, only a tiny fraction of the concrete in the building is destroyed by demolition charges. Anomalous destruction of the concrete in the debris pile is just as much a problem for any controlled demolition theory as it is for the conventional fire and impact induced progressive collapse theory.