For all of my projects outside, I use a thinset, or a cement-based mortar. I mix it to a consistency of a soft oatmeal. Not runny, but not so thick that it won’t drip off of the plastic picnic knife that I use to work with.
You can mix in PVA (a white glue) to make the bond stronger, although I have never found the need to do this. I am in gardening zone 7 and all of my pieces outside have withstood freezing and thawing for up to four years and are still going strong.
