Build a cavity wall with what?

[Kallebo]

I want to build up a part of my heart wall in our upcoming house construction to have a non-combustible wall to place a tiled stove against. But with what should one build?

The idea is that besides being load-bearing, it should also be relatively heavy so that it accumulates and distributes heat well.

How thick do you think it needs to be depending on the material? On top, there will be an ordinary floor framework with truss rafters.

 

[Oldboy]

Double brick or concrete hollow block? (20-25 cm thick)
Thickness/width depends on the height of the wall, length, and what/which side supports it gets and where.