My pool deck is flagstone set on sand w/cement between the stones (I don't
know what you call it). It's only 3 years old and about 3/4 of the area
repaired needs to be repaired. The masons who fixed up some stone decking
for me last year said this type breaks down very quickly. I really wish I
paid the extra cash to have the stone set in cement.

Pool decks set on sand almost always have problems, whether it's
flagstone or pavers.

Jeff