Also, a quick note that's neither here nor there. I can't speak with any authority on how quickly technology is rediscovered--thanks to the clever obfuscation of how long mankind has been fallen--but concrete has existed in the form with which we are familiar since the roman empire. (Mind you they didn't employ any reinforcement like we do with rebar, but did use additives). Surprisingly the Pantheon still holds the record for the largest unreinforced concrete dome. I only mention it to imply it's not that far fetched that powerful factions might be able to rebuild with more than brick and mortar, and that adding blood to concrete to keep it from cracking in the cold is pretty bad ass.