Best specs for SAND to fill blocks or floating floors

I know sand must be dry. But there are a extensive sort of sands, and I don't know what specs I must take into account. The more heavy the better? the thinner is best? or I must take the thicker one? Any kind of sand is better than other? Grain caliber , "granulometria" in spanish, I don't know what the english name, is one spec apears in all sand types...

Hugo, I'm not sure that any one sand grain size is better or worse; from a mass standpoint, heavier (finer grained, less room for air) should be better. The main thing is to NOT use BEACH sand; too much salt, possibly dead fish smell; the salt will attack any fasteners (nails, etc) and ruin your construction possibly before you're ready to live without it... Steve

Soon I will post some pictures of the sand I've buy.