The story...

The WTC concrete was pulverised into tiny particles (10 to 60 microns in size), and this couldn't have happened from a collapse driven by gravity alone.

Our take...

Jim Hoffman justifies this claim here.

Available statistics about particle sizes of the dust, such as the study by Paul J. Lioy, et al., characterize particle sizes of aggregate dust samples, not of its constituents, such as concrete, fiberglass, hydrocarbon soot, etc. Based on diverse evidence, 60 microns would appear to be a high estimate for average concrete particle size, suggesting 135,000 KWH is a conservative estimate for the magnitude of the sink.
http://911research.wtc7.net/papers/dustvolume/volumev3.html

Note the importance of this figure; the smaller the particle size, the more energy is required to produce them. What's more, we're told that Hoffman is being generous to the official story, as particle size may have been even smaller than he estimated.

The quantity for the crushing of concrete appears to be conservative since some reports indicate the average particle size was closer to 10 microns than 60 microns
http://911research.wtc7.net/papers/dustvolume/volumev3.html

So, where does Hoffman get this estimate of the particle size? Despite quoting "diverse evidence", the only paper he references is this the Lioy study. That only took three samples of dust, so there's no way to tell how representative it is, but the following sizes were recorded:

Aerodynamically separated sample

Sample Size (% of mass)	Sample 1	Sample 2	Sample 3
< 2.5 microns	1.12%	0.88%	1.3%
2.5 - 10 microns	0.35%	0.30%	0.40%
10 - 53 microns	37.03%	46.61%	34.69%
> 53 microns	61.5%	52.21%	63.60%

http://ehp.niehs.nih.gov/members/2002/110p703-714lioy/EHP110p703PDF.PDF

There's slim support for the average particle size being "closer to ten microns then 60" here: in fact the majority of the dust, in all samples, is made up of particles greater than 53 microns. Maybe they all clustered around Hoffmans magic 60 micron figure? That doesn't look likely, either.

Sieved Sample

Sample Size (% of mass)	Sample 1	Sample 2	Sample 3
75 microns	38%	30%	37%
75 - 300 microns	46%	49%	42%
> 300 microns	16%	23%	21%

http://ehp.niehs.nih.gov/members/2002/110p703-714lioy/EHP110p703PDF.PDF

So according to these figures, the bulk of the mass lay somewhere between 75 and 300 microns in size.

Now, as we pointed out, these figures don't claim to be representative, and so we can't claim they're infallible. They plainly don't support the "most concrete pulverised to 60 microns" claim, though, and Hoffman has yet to point us to anyone who does, suggesting his energy calculations may not be entirely accurate.

And there’s a final problem. Dust studies are based on, well, samples of the dust, and that means by definition they’ll include only small particles. Larger pieces of concrete would not be carried so far. This means that dust studies alone can never be used to say what happened to “most” of the concrete: they’ll always produce an underestimated average particle size.