Having my AGP with a deep end installed tomorrow - if the weather cooperates! My pool builder is putting down sand. I asked about some other materials, specifically vermiculite. His recommendation was to go with Pool Crete, which is what is used for in ground pools. His price for Pool Crete was a bit steep for me so we'll be sticking with sand. He did mention that cement would not be advisable as it would wear the liner very quickly. Hope this helps.