Whether construction sand will mess with your pH depends on where you live. Mined building sand is rarely transported long distances, but normally mined relatively locally. If you live in Florida you can bet that your sand is limestone from fossilized reefs. If you live where granite is the predominant stone your building sand will probably be granite. If you want to know just drop some of the sand in a bit of vinegar and if it fizzes it's limestone and will buffer the water and possibly raise pH. If not it won't affect pH.