Sauna flooring

It's ferrocement - wire mesh covered with a cement mortar, then finished with what appears to be stucco.

This video shows a pretty simple way to do it.

There are a number of videos on YouTube, and do a Google. You'll find lots of information.

Looks like they mixed cement and pigment into making the floor.

There are structures made by stacking rounds of wood and mortaring them together that may work for you, also look into cob construction. There is no good reason to build anything with wood in direct contact with the ground.

Thanks for the useful information. I did start with a google search (that's how I came up with the link I posted), but came up with too many unrelated, unspecific, or commercial results. But now I have some other terms I can put in my search.

The ferrocement seems like a good idea, but I have to talk to the family about whether or not they mind doing something so permanent.

G.