In order to create an effective concrete shower pan, you need to use a particular type of concrete called deck mud. This concrete is thick and uses minimal water in the mixing process to avoid shrinking as it dries, thus reducing cracks and gaps.
The type of sand you should use for your shower pan is called sharp sand, while the cement you will need to use is called Portland cement. They should be mixed to a ratio of 5:1. Finally, add water gradually so that the sand and cement come together in a ball with no excess water or crumbling.