I wonder what the size and capacity of the off the shelf mixers are available.

The reason for the choice of concrete is to produce a very long lasting and strong structure.

One of the aspects that I see in very old buildings is how they have been reused over the years, rather than torn down and a replacement built.

As I reckon if you go the route of designing a home that will be around for hundreds of years, your save on resources long term, and also try for something that requires little maintence. (eg. no wood to paint/replace.)

With a desire to build an earth sheltered underground design, there aren't too many materials which would be suitable for that choice.