I did something similar to what you are talking about with a stone firepit surround. I positioned the rocks like I wanted them, then filled in around them with a mix of 1 part Portland Cement, 3 parts sand. I sifted it in dry around the rocks, swept off the mixture from the rocks, then misted it with water until it was saturated. It worked and survived our MN winter without cracking. Good luck.

i would stay away from the concrete idea because its definetly going to crack and sounds like it could get messy.I would rethink the stones in the joints.Why mot try stone dust. It compcacts well you can hose it and sweep in. or a mixture of soil and sand for a more organic look.good luck.

I saw a product called polymeric sand. You sweep it into the grout lines and mist with water. It hardens like concrete but stays plyable. I found this website. Good luck

http://www.supersandbond.com