Masonry mortar is used for bedding, jointing, and bonding masonry units. It is composed of cementitious materials, sand and water.
The cementitious materials essentially include cement and/or lime. In the hardened state, the cement and lime provide compressive strength and mortar/unit bond strength. In the plastic state, the cement accelerates the time to set to facilitate speed of construction, and the lime contributes to the workability, which helps ensure full and intimate contact between units and mortar.
Sand provides colour, texture, and cohesiveness, and reduces drying shrinkage. Water added to the mortar makes the mix workable and hydrates the cement. Other materials are sometimes added to give the mortar particular properties; for example, entrained air improves the workability and resistance to freeze/thaw deterioration.
The requirements for mortar for masonry used in Canada are contained in CSA Standard A179, Mortar and Grout for Unit Masonry.
The properties of mortar in its plastic and hardened states are both important, but for different reasons. Plastic properties such as workability and water retention determine construction suitability while laying the units, and hardened properties such as compressive strength, bond strength and durability affect the performance of the finished masonry.
CSA A179 identifies two mortar Types suitable for use in modern masonry construction, these being Type N and Type S. In the hardened state, Type S mortar has a higher compressive strength and generally higher bond strength than does Type N mortar. However, Type N mortars often show better workability in the plastic state, helping to ensure good bond between mortar and unit.
Because properties vary with mortar Type, and no single mortar Type is best for all purposes, it is important that the mortar Type selected for a particular application is the one that best meets the end-use requirements.
A basic rule for the selection of a mortar is to not to use a mortar with a compressive strength greater than is demanded by the structural and durability requirements of the masonry. A mortar with the lowest compressive strength consistent with the performance requirements should be selected. Type N mortar is recommended for general use, where high lateral and vertical masonry strengths are not required, such as for interior or exterior non-load bearing walls, as well as for exterior veneers. Type S masonry mortar is recommended for use in all masonry below grade as well as for load bearing masonry walls (typically block), both interior and exterior, where higher vertical and lateral strength is needed.
CSA A179 identifies three mortar mix formulations suitable for use: Portland cement/lime, Masonry Cement, and Mortar Cement. Both Type N and Type S mortar can be manufactured under each mix formulation. However, for each of the three formulations, the constituent cementitious materials differ, and therefore the relative volumes of cementitious materials and aggregate to be combined on the jobsite to manufacture the same mortar Type differ. Under the proportion specification of CSA A179, to produce either a Type N or a Type S mortar as desired, the relative proportions of the materials to be added together by the mason on the jobsite are tabled for each mix formulation. This simplicity in the manufacture of mortar makes the proportion specification the most desirable means to specify mortar for both the mason and the designer.
For further information on mortar selection, mixing installation procedures and specification requirements please refer to the 2004 edition of CSA A179.