Anyone know what this is?? it was on top of foundation in basement…
I couldn’t get to it access blocked…
Looks like hard/almost like concrete…
Is this type of vermiculite?
Where are you located?
I’m guessing a warmer climate due to what appears to be a copper water pipe in your picture.
NJ- 3 family house in Newark- Built 1935
What do you mean by “on top of foundation?” Do you have a better picture, maybe from further back so we can get our bearings? Hard to tell what we are looking at here. 
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Did you touch it? Looks like concrete. I’ve seen that many times with balloon framing to restrict fire from spreading up the bays.
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It’s peculiar for sure. Could have been put in to stop fire spread as @spayson said. Or could have been put in because the rim joist was rotting away. Were you able to view any of the rim joist from the exterior? Was this material in every joist bay?
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Makes more sense now, my WAG is old expanding foam insulation that was blown into the walls that has since hardened.
Taken off the internet.
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Houses of this era were known not to have a rim joist.
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I have only seen vermiculite as loose fill. Has anyone seen it used vertically to fill stud bays etc?