What do swimming pool contractors use to seal joints in concrete patios that surround the pool I've seen a thick flexible compound that seems to seal the joint and allow for expansion.?

You might want to consider Deck-o-seal. It could be tricky to use if you are not familiar with the product. Follow ALL of the manufacturers instructions.

Ken

You might want to consider Deck-O-Sux, but you might actually want to use PoolSeal™ if you want a long lasting, easy to use, pool owner-friendly mastic to seal the joint and allow for expansion.

CUTiger78