We recently got a flagstone patio with polymeric sand in the joints. The polymeric sand was put down on a Friday early afternoon, and when I checked on it Sunday morning it had hardened and felt like concrete. But after a rain on Monday it was completely soft again—the consistency was like wet sand on the beach. Our contractor (who has really good reviews and a lot of experience) says he's never seen this before, and all the research I've done has warned about rain within 6-24 hours after laying down the polymeric sand, but this was multiple days and it had already hardened.
What could have caused this?
