I think mortaring the small stones to your wall would be a great idea and very attractive. I don't think it would harm your concrete wall a bit. My fatherinlaw covered the entire side of his earth home's concrete wall with fieldstone from his property in the early 1980's and it is beautiful; he said the biggest concern is getting the right mortar consistency, but practice makes perfect. I'd like to see pictures of your finished work! Good luck! Karen

Thanks for the info Karen! Now that I now its do-able I'm going to to decorate that bland old wall and I'll try to get a pic posted when its done.

Since you are using small rocks, why not just try ThinSet. You can purchase it in a bag, add water, mix and apply. It's what they use to adhere fake stone on walls, should work for small lightweigh rocks.