The aggregates in the mortar are inert materials not involved in the chemical reaction, and play the role of framework or padding. Aggregates can be used to adjust the density of mortar and control the shrinkage performance of materials. The particle diameter of fine aggregate used in the mortar is generally less than 5mm, so it must be sieved and the maximum particle diameter should go through the sieve pore of 5mm. The finer the sand is, the larger the total surface area will be, and the more the slurry wrapped on its surface will be. When the consistency of the mortar mixture is the same, the mortar prepared by fine sand requires more slurry than that prepared by coarse sand. Since the water consumption increases and the mortar strength will fall accordingly, the use of medium, coarse sand is preferred for the preparation of building mortar. But it also needs to be selected according to the uses, application positions and substrates of mortar. For example, for masonry mortar, in the brick masonry, medium sand should be adopted; while in the rubble masonry, since the surface of rubble is more angular and rough, coarse sand should be used. For plastering mortar, the fineness modulus of sand should not be less than 2.4.