In article dB92i.9164$yy6.3066@trnddc05,
"A. Learner" a.learner@earth wrote:

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?

Decomposed granite ("stone dust", which looks more or less like coarse
sand, when mixed with water and tamped down firmly, will dry to form a
fairly hard surface. But it will soften again once it gets wet. Cement
(or concrete, which contains cement) should harden and stay hard,
although as mentioned in another thread, old cement may not harden.
Decomposed granite won't dissolve in water, though, so when you say it
"disintegrates" in water, if you mean dissolve, then it isn't DG.

Maybe it's ground bones. (aka human "ashes")