"The worst possible choice is "Joint Lock" or "Paver" sand which is sold to inexperienced "Do-it-your-selfers" as a leveling medium and bed for brick work. This sand is "Polymer modified," which means that it contains an additive that will harden when it gets wet. "rainbowgardener wrote:what they sell as "paver sand" in all the big box stores works fine. Gravel is sold in pet shops for aquariums. You could use at least partly potting mix for cactus.
I found this on the internet?