The mud content refers to the content of particles with a nominal particle size of less than 80μm in the aggregate.

The clay lump content refers to the content of particles with a nominal particle size of greater than 1.25mm in sand but less than 630μm after washed and pinched.

The mud particles in sand are usually fine, have increased the specific surface area of aggregate, and will increase the water consumption and the amount of cement paste. Clay minerals usually have stronger water absorption, swelling when absorbing water and shrinking when drying, and will have an adverse effect on the mortar strength, dry shrinkage and other durability properties. When mud particles are wrapped on the surface of sand, it will affect the adhesion between the cement paste and sand. When existing in the form of clay lump, due to the low strength of cement clump itself, it not only would not achieve the role of framework, but also will form a weak part in the mortar and reduce the mechanical properties of mortar. Therefore, restrictions should be imposed on the mud content and clay lump content of sand in the mortar, with the former ≤5.0% and the latter ≤2.0%.