Patching a slab

I had to cut a hole in my slab so my plumber could get in and fix a cross
connection.

My question is, what's the best way to patch the hole. The slab sits on
a red clay bed. The hole is about 18" x 18" and about 10" deep.

Can I fill the hole with gravel or sand to the base of the slab or do I need
to use red clay as the substrate like the original slab?

Any recommendations for the concrete to use for the patch?

I'm going to put porcelain tile on top of the patch.

Thanks

Corky

Fill it with gravel and compact it as best as you can. Drill holes in each edge of the hole in the existing slab and insert rebar. Pour your patch with a bagged concrete. I use Quikcrete 5000 PSI mix just for the strength, but any will do. You'll need to treat the joint in the concrete with a crack isolation product before you tile. Noble makes a good product as well as several other companies.

Highly recommended you get advice on dealing with the new "cold joint" by posting at johnbridge.com, a tile forum much like this. Great people.