It is critical to appreciate mortar's properties and how the ingredients in mortar affect performance.
Three important properties of mortar are workability, bond, and compressive strength.
Workability: Workability is perhaps the most important property of plastic (fresh and not yet hardened) mortar. Some indications of workability are: The mortar spreads easily with the trowel; the mortar supports the weight of the masonry units; the mortar adheres to masonry surfaces (is sticky); and the mortar extrudes readily from the joint when the mason applies pressure to the unit.
Bond: Bond is an important property of hardened mortar. Two facets if bond critical to a masonry assembly's performance are extent-of-bond and bond strength (known to structural engineers as flexural tensile strength. Extent-of-bond is a measure of the actual contact area at the interface of the mortar and masonry unit. Bond strength is a measure of the tensile stress required to break the bond between mortar and masonry unit.
Compressive strength: Compressive strength is not as important to the performance of the masonry assembly as workability and bond. Architects and engineers single out compressive strength as a selection criterion largely because it is easily measured.
Mortar is a combination of water, aggregate, and cementitious materials. Water facilitates mixing of the aggregate and cementitious materials. Aggregate is the granular material, usually sand, that is used in the mortar mix to reduce the required proportion of cementitious materials and to resist shrinkage of the cement. Cementitious materials have adhesive and cohesive properties both in a plastic state and when hardened. Mortar includes one of three categories of cementitious material: portland cement and lime, masonry cement, or mortar cement.
Mortar is specified by one of two methods: by proportion of by property.
An understanding of how water, aggregate, and cementitious materials affect the properties and field performance of mortar helps assure the design and construction of water-resistant and attractive masonry buildings.