Sand in Joints

We have flagstone laid on decomposed granite, and the granite washes away too easily, so the joints are now deep ruts after three years. We had to have joints that let water through for our large sycamore tree. We'd like to fill them with sand --- any tips? I experimented with a bag of sand in a small area and it looks nice, but I'm wondering if the sand will eventually harden this way, as it doesn't seem to be hardening though I lightly water it every day. It remains loose. Am I too impatient?

Is there a specific sand that can be used? We experimented with a bag of "play sand" from the nursery.

Thanks for any feedback!