What is the best adhesive to use if I want to glue rocks to concrete?

I want to glue actual fieldstone (caught in the wild) to my plain old concrete front porch/stoop. I washed/dried stones & stoop, but stones keep falling off-I used landscape adhesive. Would using muriatic acid (sp?) to etch the concrete do anything? It looks so nice-until they start dropping off! I also need to use some kind of grout-what kind should I use? Thanks for your help!

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