Mortar and Grout: Choosing the Best Mortar for Your Project

..View of a head joint in CMU that is debonded on the left side, compromising moisture resistance.

Type S? ... Type N? ... The Strong, Silent Type? ... When it comes to mortar selection, often masons are handed a specification or product selection without much explanation (and, sometimes, without much thought). They are then expected to use it without error or complaint. However, there are guidelines for appropriate mortar selection, and it is worthwhile to understand industry standard practice when there are disputes on a project or when you actually have the option to select your own mortar.

First, a little background on mortar types. There is a common misconception that the “S” in “Type S” mortar stands for “Structural” (and “N” is for “Non-structural”). This is NOT the case. In 1944, a committee was formed to provide a naming system for masonry mortars with different proportions of portland cement and lime. The designations selected were “A-1” for very high-cement mortar (equivalent to modern Type M mortar) with “A-2”, “B”, “C”, and “D” for progressively lower proportions of cement. As you might imagine, many designers quickly assumed that “A-1” was the best mortar and “D” was the lowest quality, so a large number of projects were specified with “A-1” mortar, even though it is almost never an appropriate mortar type. After ten years, a new designation scheme was introduced using every other letter from the word “MASONWORK” to designate mortar types of descending portland cement (and increasing lime) proportions. These designations are still the basis for the primary mortar standard used in the United States (ASTM C270). In addition to the commonly used “Type S” and “Type N” mortars, there is a higher cement-content mortar (“Type M”) and two lower cement-content mortars (“Type O” and “Type K”, often used in pointing of historic masonry).

The “S stands for Structural” misconception about mortar types has also led to the common practice of specifying Type S mortar in “structural” walls (usually CMU) and Type N mortar in veneers (often clay brick). While this practice is not completely out-of-bounds, it tends to result in the overuse of Type S mortar in many applications where Type N would be more appropriate and perform better. The difference in strength between Type N and Type S mortar assemblies is usually negligible, and Type N mortar often provides significantly better workability and moisture resistance.

What do the industry standards say? The Brick Industry Association (BIA) Technical Note on Mortars for Brickwork - Selection and Quality Assurance (8B) recommends Type N mortar for most “normal use” in walls. It also contains a general principle of mortar selection that may seem counterintuitive to engineers and concrete contractors. It instructs readers to, “Select a mortar Type with the lowest compressive strength meeting project requirements.” (Emphasis added.) The parallel publication for block is the National Concrete Masonry Association (NCMA) Tek Note 9-1A, which does not directly provide a recommended mortar type. However, NCMA Tek Note 19-2B Design for Dry Single-Wythe Concrete Masonry Walls includes the statement, “The type of mortar and type of mortar joint can also impact a wall's watertightness. A good rule of thumb is to select the lowest strength mortar required for structural and durability considerations. Lower strength mortars exhibit better workability and can yield a better weather-resistant seal at the mortar/unit interface.” Finally, Annex X1 to ASTM Standard C270 on Mortar includes the following table, which recommends the use of Type N mortar for all load-bearing walls (and even Type O for non-load-bearing walls!):

^A This table does not provide for masonry specialized mortar uses, such as chimney, reinforced masonry, and acids-resistant mortars.
^B Type O mortar is recommended for use where the masonry is unlikely to be frozen when saturated, or unlikely to be subjected to high winds or other significant lateral loads. Type N or S mortar should be used in other cases.
^C Masonry exposed to weather in a nominally horizontal surface is extremely vulnerable to weathering. Mortar for such masonry should be selected with due caution.

One other quick note on mortar selection... As the use of integral water repellents becomes more prevalent, it is important to ensure that any mortar used with units that contain an integral water repellent also contains a compatible integral water repellent. If the mortar does not contain this admixture, the interface between the mortar and the units can exhibit poor bond.

In summary, “S” does not stand for “Structural,” but “N” is “Nearly always the best choice”!