btacker is on the right track. Southdown is pure aragonite that is just packaged under the name Southdown and what I believe is now Newcastle or something like that. The quickrete stuff is just a silica based sand and not quite as white. The only difference I've seen proven to any extent is that the aragonite helps with the stablility of PH or something to that affect, could be wrong on that part but someone else might chime in that knows a little more.