Rick, if at
any point after the boil, if you used tap water, you
could have re-contaminated the rock. Bacteria are hardy little critters, and
if this were what is going on with your rock, well, at least you know bacteria will grow in it, lol.

Not to make light of your situation or anything, but I am sort of stumped.
Does anyone else agree that his 6 month old, water kured rock should be lower than 11+?
Since both batches are showing the same pH issue, it has to be a common denominator. The cold thing doesn't fit for both batches, and neither does the sand. Salt would have nothing to do with it. That leaves the cement and the water.
Maybe you could call your local water supply and ask about bacteria types that might populate your locale.
Possibly finding out about the cement in detail would shed some light.
Other then that, I don't know.
Anyone?