What material to use on Bluestone joints?

I live in northern Virginia where heavy rains and below-freezing temperatures are very normal. For my patio, I tried polymeric sand and mortar in the past, but both of them didn't last. I might have bought cheap sand or wrong mortar.

Now I am considering redo my patio using the existing bluestones. I plan to use crushed stone as base. What joint materials would you recommend to sustain the weather?