First time Flagstone--2 surfaces

(East, TX)

Hello,

I have a new construction home and the builder only provided me with a concrete slab in my back yard--about 5X5. I would like to expand the patio to about 15X10. I was planning on excavating the area and attempt to lay flagstone over the concrete and somehow tie it in with the rest of my project. I would like it to all look consistent and was planning on using gravel/sand to fill the gaps. Should I just bust the concrete out or try to lay flagstone over the slab? If I lay it over the slab do I have to use mortar and will it look bad next to the sand? please help!!!!