Question: It seems I've been seeing a lot of uses for foam on this site, from a weapon to a spaceship defense mechanism. However, the details on what the foam is made of and how it works always seem lacking to me; thus, I worry that many of these foams are just science fiction. 

My question is **is there a foam that can be used to quickly construct airtight walls solid enough to keep humans out?** I imagine using this foam on a spaceship to trap the enemy, or alternately to fix hull breaches. Perhaps there is a fast-acting variant that can be used as a grenade to immobilize enemies. As it is a foam, it could be carried around much more easily than other wall-building materials, thus I could outfit my space marines with a few cans apiece. 

So, does this foaming material exist, or is there a theoritcal way to produce it? If so, how big would the initial substance be, and how long would it take to dry/harden? 

The foam need not be activated by air; on the contrary, it would be great if it contains its own reactants, and can be activated in a vacuum. 

It need not last; it should only be functional for a minimum of a couple hours. If it breaks down after that, that's fine. 

I would like it to be as strong as possible. If it can get to concrete levels, that would be best, but as long as it's very difficult for a human to burrow through that'll work.

Answer: 
The only foam I have much experience with is the expanding-foam insulation that comes in a spray can. From that, I find it fairly believable that you could plug a hole in the side of a ship with enough of it, though I'd imagine it would also unavoidably plug up a good deal of the interior of the ship. I've also unfortunately never been able to test it in vacuum.

The hard part is making it durable enough to prevent tunneling. But if the human-to-be-trapped lacks access to digging tools, you might be able to keep them in by embedding hand-lacerating caltrops of some sort in the foam mixture. (Maybe these caltrops "grow" there through rapid crystallization?) There's not much point in digging a tunnel out of prison if you bleed out halfway through.

Actually, if you use enough foam, and/or if it sets quickly enough, you could just trap the intruder like an insect in amber. That would also suffocate them almost immediately, unless they had a suit with an independent air supply, in which case you'd just have to wait until that supply runs out.
Dental cement

Spray a foam version of dental cement onto the assailant. Use a high-powered laser to cure it. Alternatively don't even use foam. Just spray them with the liquid and use the laser. You will cover them in a thin but incredibly hard outer shell that will immobilise them.

[Influence of argon laser curing on resin bond strength](http://www.ncbi.nlm.nih.gov/pubmed/8397985)

The main point about using lasers is (1) the ability to aim them (2) they act at a distance and (3) they cause fast setting even if the strength isn't increased.