Pyrobar

[Mike Lamb 14]

I ran into this Pyrobar stuff the other day. It was lining the floor beneath a back porch. I did some research and it was a fire proof insulator used around structural members but also  used to build non-bearing walls.  The mortar used for this product probably contains asbestos and workers often cut it with a saw.
Here are some links if anyone is interested:

https://archive.org/details/PyrobarGypsumTile

Edited March 5 by Mike Lamb

[Bill Kibbel 31]

I've come across it a few times.  Boiler room ceilings, stair and elevator towers. Tests showed it contains a very small percentage of fibers, surprisingly from wood.