So I bought 80kg of calcined flint clay...

So I'm currently underway building my rocket powered pizza/cob oven and I managed to buy 80kg of calcined flint clay.
I thought I was buying flint or fire clay that I was going to use for my cob dome, but it turns out I didn't know what "calcined" meant.

For those of you who were like me, it means it has been super heated and turned into pottery and therefore won't react with water.
So I guess it's effectively grog or crushed fire brick.  It has the consistency of fine beach sand.

So now I'm thinking I will just use the clay I dug up from around my house, but should I use this "grog" as the sand in my cob.
Or should I do a mix of sand and grog.

Any thoughts?

The grog should work excellently. Mixing with clay to reduce shrinkage and add toughness is exactly what it is made for.

So should I still aim for three/four parts grog to one part clay for the inner dome?

My guess would be that that's right.