In the selection of aggregates used for building mortar, the following factors need to be taken into account.

(1). Particle Size Gradation
The effect of aggregate on the workability of mortar depends on the water absorption rate, fineness and gradation of aggregate. The larger the water absorption rate of aggregate is, the greater the water requirement of mortar will be, resulting in the decrease of mortar strength; the smaller the fineness modulus of aggregate is, the greater the water requirement of mortar will also be; the larger the porosity is, the greater the water requirement of mortar will also be. Especially when the cement-sand ratio is small, this effect is more significant. Therefore, in order to meet the required strength, the smaller the porosity of aggregate used is and the greater the fineness modulus is, the smaller the amount of cementitious materials will be. When the aggregates of standard gradation pile up, the void ratio is low, so they can form a good framework in the mortar, not only saving cement, but also obtaining the mortar with good workability and high density. Proper blending and adjustment should be conducted for the aggregates of unqualified gradation to make them qualified.

(2). Particle Shape and Surface Features
The particles of mountain sand or artificial sand are mostly angular and rough on the surface, and have good bonding with cement and high strength, but the fluidity of mortar is poor; the particles of river sand are mostly spherical and smooth on the surface, and have poor bonding with cement, low strength, but the workability of mortar is good and it saves cement.