The concrete cell took an afternoon to dry solid, when a much thinner pavement takes about a day, and a poured concrete wall about - oh - three?

It is possible though. There is this: http://www.packagepavement.com/setting_post_qk.html
A quick set cement. It would normally dry in less than a day. I remember it being used in an Extreme Makeover Home Edition episode, and the floor set in just a day or less.

why didn't Jack have any concrete in his orifices? It might not be glamourous to watch, but JB pulling a concrete stick out of his arse would've been wonderfully realistic after being trapped in a Magi-Dry© concrete cell ...
Maybe not his ass, but he was screaming as the cement poured on him, so he must have had it in his mouth and ears. It would have been more believable if the interior of the cement block had still be slightly mushy wet.

Also, didn't Jack know enough about aliens and the probable atmosphere of the tank to realize that shooting at it, with the intention to break it, while Ianto was in the room, was a very very bad idea???

How did Jack get clean? How was there running water in the abandoned warehouse? I'm sure utility companies tend to shut down these supplies if no one is paying for them.

MARTHA?! OMG WHERE THE HELL WAS MARTHA DURING ALL OF THIS? DID SHE NOT TRY TO CALL THE DOCTOR?