Q: How should mortar strength be chosen? Isn’t the strongest mortar best for every project?

A: Quite simply, stronger is not always better when it comes to mortar. The rule of thumb is that a Type N mortar, “normal strength” mortar, should be the first choice. Mortars having moderate or lower strength are preferred because they have the ability to deform slightly under load, handling small movements with minimal cracking. Only if there is a compelling reason to choose otherwise should a different mortar type be specified.

The property that describes a material’s deformation under loading is called the modulus of elasticity. Generally, higher strength materials have a higher modulus of elasticity. Therefore, higher strength mortars tend to crack more than lower strength mortars.

In the U.S., mortar types are designated by ASTM C270, Specification for Mortar for Unit Masonry, as M, S, and N in order of decreasing strength. (In Canada, mortars are specified by CSA A179, Mortar and Grout for Unit Masonry.) The property table of C270 lists the strength for each mortar type as follows:

Ingredients and the Role of Water Content

Mortar is made with aggregates, water, and a binder. Cementitious materials, which react with water to set and harden, are the binders in mortar, and include portland cement, blended cement, masonry cement, mortar cement, and lime, alone or in combination with each other. Each material has its own ASTM specification.

Mortar bonds units together, and in order to do this effectively, it should be mixed with the maximum amount of water consistent with the ambient conditions and placing needs. A mason and the person mixing mortar will decide together on the best consistency for the job they are doing.

Reasons for Choosing a Different Mortar Type

The “compelling” reasons most often cited for specifying increased mortar strength are structural integrity or durability. Where the wall will be load-bearing masonry, a Type M or S mortar may be required. In reinforced masonry, the system (of units, grout, and steel) dominates structural behavior, and the mortar’s contribution is minimized. Even though mortar strength does not have a major impact on the wall’s compressive strength, higher strength mortars do support greater loads.

Mortar durability refers primarily to resistance to freeze-thaw cycles or chemical attack. In each case, performance is generally improved as the cementitious materials content of mortar is increased; thus, the mortar has higher strength.

ASTM C270 should be consulted for additional guidance in selecting proper mortar type based on strength and other characteristics. See www.astm.org

Water Contents of Other Cement-Based Materials

For comparison, concrete, grout, and plaster are also made with the same main ingredients as mortar but they are formulated for different purposes, and their consistency (water content) is related to that purpose:

• Concrete must have enough plasticity to fill forms. It usually has a limitation on relative proportions of water to cement (or cementitious materials) denoted as w/c or w/cm, and this should not be exceeded. The limit is there for structural integrity (strength) and durability (often freeze-thaw resistance).


• Grout is placed in constricted spaces that may contain reinforcement between units that may be absorbing water out of the mix. It is mixed to a very fluid consistency, so it contains the highest water content of the materials described here. (A notable exception to the water content is the newer grout formulation called self-consolidating grout.)


• Plaster has to be wet enough to be readily placed on the wall and spread over its surface, yet be dry enough so that it doesn’t sag under its own weight. It is very similar to mortar in material proportions other than water. Regarding water, it may be slightly drier than mortar because it does not come into contact with units above and below it, but is placed against a substrate, sometimes absorptive, sometimes not.

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