I expect something with a lot of small bubbles and cracks, also it also overheated and got weird decomposition and reactions, something like a overcooked/toasted meal. Reusing a comment that I made in a previous thread:

For comparison, there is a nice video by NileRed https://www.youtube.com/watch?v=phNLecfyWS8 He is making Bakelite that is a type of plastic. It's a tiny amount, in a lab, on purpose and he may make a few attempts. Anyway it overheat and instead of a nice piece of plastic he got a nasty block of foam with burned plastic. No imagine a huge tank of a similar chemistry reaction.