The sand is used in mortar for the following purposes
It does not increase the strength of mortar. But it acts as adulterant. Hence the bulk volume of mortar is increased which results in reduction of cost
If building material is fat lime, the carbon dioxide is absorbed through the voids of sand and setting of fat lime occurs effectively.
It prevents excessive shrinkage of the mortar in the course of drying and hence the cracking of mortar during setting is avoided.
It helps in the adjustment of strength of mortar by variation of its proportion with cement or lime. It also increases the resistance of mortar against crushing.
It subdivides the paste of the binding material into a thin film and thus more surface area is offered for its spreading and adhering.