Heavy mortar: The mortar having bulk density of 15 kN/m³ or more are known as the heavy mortar and they are prepared from heavy quartz or other sands. 
Lightweight mortar: If unit weight of mortar is less than 15 kN/m³ then it is called lightweight mortar. It is the type of mortar which is prepared by mixing sawdust, rice husk, and jute fibres along with sand and lime or cement. 
The use of heavy or lightweight mortar depends on the specific application and the desired properties of the final construction. Heavyweight mortar is used for heavy-duty structures that require high-density materials and greater strength, while the lightweight mortar is used for structures where weight is a concern and insulation properties are desired.