You will be able to open that cave as you explore the ocean dungeon. You were correct in trying to use the spiral sand for the sandcastle, you need to discard it from your inventory and drag & drop it onto the sandcastle.
Thanks, I am also having trouble finding the last fire shard. I found the hidden chest one with the lever but I can't find the last one and I really want to use them to make the last item for the pillar
