I use limerock for coral frags. You want to find out what pit it came from. Ideally, you want a calcium carbonate around 90 to 95%. Youngquist is way below this. Rinker is a little low but probably ok. Westwind has nice high carbonates if you wash it. they have a lot of silica sand contamination. You will want the light porous rock. This is usually old coral. Try to avoid the very dense rock. This will have the lower calcium carbonate level and thus more room for silica, phosphates and whatever else you can think of.
As for concrete. A straight portland cement concrete (DIY) has almost no chemicals. It is rock, sand and portland cement. The stuff you buy in a mixer truck may be loaded with chemicals. FYI, portland cement is made from limestone. So....if you use limerock gravel, aragonite sand and portland cement.....you will get great base rock!
Lastly, the majority of aquaculture Florida rock comes from the Bahamas.