Possibly sand source.

My mother recently bought some sand to fill cracks in her brick walks. She
complained that it's so fine and white that it's stained the brick. She's unsure
if it will ever wash off. Sounds like it might be a high-calcium sand? If so, it
might be good for aquariums.

She got hers at Ace Hardware in the East Tennessee area.

George Patterson
If you torture the data long enough, eventually it will confess
to anything.