Pyrobloc

I think that's how they spell it. I've seen this in a few older buildings but it's been a while. I ran across it again in a wall above a hung plaster and lathe ceiling here at school. The student in this room asked about asbestos. I couldn't answer because I don't know. I did some googling and came up dry.

If I recall from a bit of research years ago it does not contain the A word. This building was built in 1923 so? Does anyone have a better answer? I will send it out for testing.