INterestingly I was musing on the idea of a reinforced concrete z-axis support to replace the round column on my mill... one day in the distant future

By way of experiement I did the FEA on a 100x 100mm 5mm wall steel tube and a 100mm x 100mm concrete post, both 750mm high.

Ignoring any difficulties fixing the concrete post, the torsional stiffness wasnt so different. With an applied 200Nm twisting load 500mm out from the centre of the post :

Steel 0.34mm movement. Weight of column 34kg
Concrete 0.45mm movement, Weight 19kg. (thats with no reinforcement inside)

So with some refinement it certainly looks doable...