By the way, people, I recommend we folks put Chris on ignore. He's continually demonstrated an utter inability to learn the facts of 9/11, let alone comprehend their impact or significance. He may not be as oblivious as someone like Ultima or Jammonius is, but at the same time, he's an empty suit as far as knowledge of 9/11 is. We've been repeating the same arguments to him for years now, but he's not learned a damn thing from them.
Besides which, there's actually a quite legitimate question posted by Red in the OP that deserves attention:
As Cole points out, whereas there may be sources for sulfur in the building materials, how did it enter the intergranular structure?
Sisson and Biederman touch on it in their "Metal Removal via Slag Attack of the Steel from Building 7 of the World Trade Center" paper:
In the regions of the beam that exhibited extensive metal removal, an intergranular liquid slag attack was observed (Fig. 4). Scanning electron microscopy (SEM) with energy-dispersive X-ray spectroscopy (EDS) identified the slag to be comprised of Fe, O, and S (Fig. 5). Chemical reactions including oxidation, sulfidation, and decarburization occurred, as well as the ususually observed phase transformations in the steel.
The formation of the slag is itself something interesting; Dr. Frank Greening, in his
"Sulfur and the World Trade Center Disaster" paper noted that
... combustion of materials (such as coal) that naturally contain small amounts of sulfur, chlorine, sodium and potassium salts and/or calcium-alumino-silicates, leads to the formation of low viscosity melts or slags. These slags, which form in various regions of the combustion train, have melting points as low as 400oC and are known to be extremely corrosive to steel surfaces...
He goes into a little detail, but the point is that the burning of organic materials can eventually lead to the sort of Fe/FeS/FeO slags that Sisson and Biederman's team noted.
Now, we know one general way the slags can be created, and from other references in Dr. Greening's paper, we know these slags are extremely corrosive. That right there explains a lot of what was seen: Thinning, holes, etc. But Red's question was about how the sulfur ended up within the steel's intergranular structure. That, too, is explained in Sisson and Biederman:
This microstructure shows the scale and slag reaction effects at the top of the micrograph and the normal metallurgical reactions that occurred in this steel on heating and cooling toward the bottom. As the temperature increased, some changes in the microstructure of the steel occurred as a result of heating and cooling. However, as higher temperatures occurred, microstructural as well as chemistry changes occurred due to the reactions with the environment. The interaction of heat in a corrosive fire environment resulted in exposing the steel to sulfidation, oxidation, and reductions in thickness.
In short, it's a combination of the physical changes steel goes through in high temperatures, as well as the chemical reactions. To understand the physical phase changes: Google for "steel phase diagram". Here's one example:
http://upload.wikimedia.org/wikipedia/commons/thumb/8/8e/Steel_pd.svg/420px-Steel_pd.svg.png
Then, look up the individual phases themselves (Austenite, pearlite, cementite, etc.) to see what they are. In summary: Steel (iron with some carbon in it) will go through some changes in their structure depending on the temperatures it experiences. Carbon-steel graining will form and become apparent within a layer of a given phase; here's a quick, simple page laying out some examples of this:
http://www.metallography.com/types.htm
The specifics of the differences between the phases and the graining layers they produce is not important; what's important is that they'll form boundaries between the different grainings. Those grain layers end up being significant:
Biederman said:
Second, the slag preferentially attacked the grain boundaries (Fig. 9). This grain-boundary attack and metal dissolution was the cause of the severe metal removal in some sections of the beam.
Why the slag attacks the boundaries is not clear to me. I'd guess that it has something to do with differing grain formations not meshing well, but that's a complete and total wild guess. But that's not important; the fact remains that the boundaries are somehow susceptible to chemical attack. And the iron-sulfide/iron-oxide containing slag goes right to work on those boundaries, thus insinuating the FeS into the intergranular structure. Those chemical attacks continue, forming Fe/FeS/FeO eutectics which, being liquid, presumably drip or pour away somehow. And when that happens, more areas and layers are exposed to slag attack, repeating the cycle until the temperatures drop enough to freeze things in place.
And that's basically it. The fires form slag. They also start steel towards forming different phases. The slag then attacks the steel along the boundaries of those phases, forming eutectics, eroding away steel, and exposing more layers to attack. And when the fires go out and the reactions "freeze", you're left with the corrosions that caught Astaneh-Asl's attention and led the Worchester researchers to study the affected steel. And the microstructures that are left (which the Worchester folks studied) end up being composed of areas of regular steel, and areas where FeO and FeS are insinuated between the grain boundaries. So in short, the sulfur manages to get into the intergranular structure through a combination of physical phase changes in the steel and chemical reactions.
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Chemists, metallurgists, anyone who knows more than I do: Feel free to post corrections for any errors I may have made above. This was written with the best of my knowledge, but I've got a decade-plus old degree, and much of the info Dr. Greening, WPI, and others published is a struggle for me to understand well.
You guys grope for alternate explanations in an attempt to deny the witnesses statements. You refuse to follow your own suggestion that the simplist explanation is usually the right one. 
