A. Learner wrote:
I found a large unlabeled bucket of (leftover?) material in my in-laws'
basement. Nobody seems to remember what it is. It might even be there
when they moved into the place.

The stuff is light grey, granular, gravel-like, with a grain size of
about 0.5 mm to 3 mm. When I soaked a sample in hot water for several
minutes, the grains disintegrated and the sample turned into a dark
grey, muddy mixture.

Thanks to everyone who tried to help. It seems that the best theories
a cement, mortar, and stone dust. (Judging from how dense the
material is, cat litter and fertilizer don't seem to be it). Is there a
simple way to tell apart, say, cement and stone dust?