[asudavew]

If you have firebricks and ash then your are in business.

The hearth slab is just standard concrete with rebar in it.

Then there is an insulating slab poured on top of that (ash and cement)

Then the floor of the oven is laid with fire bricks.

And the dome is built using high-heat mortar and more fire bricks.

Then most people clad the dome with a half inch to an inch of high-heat mortar.

Then the oven needs about six inches of insulation. I assume your ash would work, just mix it with this formula - 5 parts ash to 1 part cement(not concrete) and spread it over the whole dome.

To make high-heat mortar, just mix 3 parts sand to 1 part cement to 1 part fireclay to 1 part lime.

3-1-1-1

And you should be just fine.

I think most were assuming you wanted to cast the whole dome and floor, I know I did.

I hope this helps.

And keep asking questions, until it all makes sense!