Thor, or should that be Zeus?

[Tom J]

This works the same with say 22x15 aggregate but they grade it down using smaller and smaller aggregate down to ash.
I am looking at building a concrete mill with Dean's help using this method. We shall see.

its good for heavy work like milling in steel where damping vibration is important. Technically should be easier to build, considering your planes are smooth, even, leveled, squared etc.
All mounting surfaced for rails, bearings etc can be prepared - machined and than fix to the mold by precise adjustment before mix is filled in. That is how I would do and will in one day.