if you can find the Southdown brand sand at Home Depot, go for that people frequently recommend that as it's silicate free.
I read this long article in some older book, about different substrate's ph buffering abilities after 2 months in a tank.
the popular dolemite, that some people advocate using, only buffers 7.4-7.5 (again after 2 months sitting in the tank)
Aragonite-based sand only 7.7-7.8
Crushed Oyster Shell 7.9
I've got a new 50 gallon light-load FO im running right now(been set up about a month) with some finely crushed oyster shell in filter bags in the sump, and i'm sitting at about 7.9-8.0. I'm not adding chemicals or even doing water changes to see where it goes. I've talked to a friend of mine who does service work and he said a customer of his has a FO tank with just oyster shell as a substrate that's been running for years and it's near problem free.
I'm thinking of taking a whole ton of it, and just grinding it into a sand, to use as a substrate then switching to using that over the bagged live sand.