Okay, the way I do it you do not need to add salt. I use basicly 1 part cement,
3 parts oyster shells (from an animal feed store...cheap) and
2 parts sand...You can vary the sand as you get better and use more oyster shells. Make you mold in a sand bed to the shape you want. I leave the bottom flat. this makes it easy to stack and piece together either. Wait until the cememt sets then soak to bring the PH done.