There are even 3 Mods involved. In my testing enviroment EnderIO is the 3rd. EnderIO adds a recipe Sand to it's own silicon and that is fine on it's own. To be compatible with other mods Silicon, EnderIOs Silicon and GTs Silicon Plates are registered as itemSilicon. There are more Mods that are also adding other ways to get "itemSilicon". Automatic coping some of the SAG Mill recipes into the TE Pulverizer combined with GTs Unification results into the recipe you see there.
I don't have EnderIO installed, so there is no SAG Mill.