1. No.
2. Yes.
Concrete, and derivations, are pretty weak in terms of strength, bending, and rigidity.
They are strong in compression, and good in vibration resistance.
A mill C frame, like a BP (old, 60 year old design), or a VMC (modern) has about 30-40 cm D thickness on the column.
Forget anything like 6".
Basically, concrete, granite, marble, etc are about 3x less strong and rigid than cast iron, and steel is about 1.5-2x "stronger" than cast iron.
You "could" make a very good mill from concrete derivates, granite, etc. by simply upsizing the column to 60 cm.
But if it had a steel skin it would be much better.