Setting tubs in mortar is a method used by professional plumbers who specialize in quality home construction. The mortar provides a stable base for the tub to rest on when the tub is full, so the casing of the tub doesn't have to support all of the weight and crack. A variety of different types can be used, as long as it hardens to provide a base.
Block and Brick Mortar
Block and brick mortar can be used as a base for tubs. It is a combination of gravel or sand along with cement, and once water is added it turns into a mortar. You can mix it up a little thicker than you would normally to set bricks and blocks, and a sheet of plastic across the top will help keep the tub from sticking should you want to remove it later on for a remodel.
All-purpose cement mixtures can be purchased at any home improvement store and used as a base for bathtubs. As a general rule, these do not contain gravel but instead only have rough sand. When mixed with water to form a thick, creamy mixture, it can be shoveled in. The tub is set and, like with all other mortar installations, filled with water for 72 hours to form the mortar to the tub.
Custom Mortar Mix
You can make your own mortar if you want to save money. The ratio is four shovels of sand to one shovel of cement and then you dry mix the components until they are completely blended. Add water and continue to stir until you achieve the thick consistency you need for the mortar base. You can mix the mortar directly in the tub frame for ease of use.
While not commonly referred to as mortar specifically, drywall mud is nevertheless one of the types of mortars considered for use as a base for tubs. It is either mixed with water in a dry format within a bucket or it is sold in pre-mixed container. You simply dump the mortar into place and set the tub as you would with any of the other options. Use a plastic sheet as you would with cement mortar to keep the tub from sticking should you want to remove it sometime later.