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: 
Shotcrete <https://en.wikipedia.org/wiki/Shotcrete> would probably be one of the best known structural foams currently available but there are certainly others available and is certainly able to set creating a gas tight surface and more than capable of keeping humans in or out. Good for making a permanent repair to that hole in your lunar bio-dome, probably heavier than you would want for a spaceship repair, and certainly too slow setting for foe suppression.
For your emergency hull repair you would want a lighter weight substrate to quickly fill the gap and then use a heavier solid foam for the permanent repair
For foe suppression just stick to sticky foam <https://en.wikipedia.org/wiki/Sticky_foam>.
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.