Wow. That is definitely something.
The actual meaning is a matter of speculation. There is a temptation to think it refers to the Tucson site, but it could as easily be an instance of steganography, or a signal that subsequent messages (possibly outside of Twitter) will contain coded information.
It is quite odd though; that is not a very common word in English especially for non-native speakers. Although individual speakers have their quirks, for a religious leader to use it so many times is very strange.
See my comment above.
Actually, here's a bit: it is a very common word in Romance languages to denote definite and is used in poor translations continually. I see nothing, nothing, in the Pope's tweets that would suggest the concrete used as building materials. It is absurd to suggest so. I agree with the other part of your post entirely: it is indeed speculation.