Masonry is held together with mortar (between pieces of masonry) and grout (fills cavities in the masonry unit). For a more detailed discussion about the difference between the two, check out our article Mortar vs. Grout.
Mortar is the material that sticks two masonry units together - it is the stuff you can see between bricks. Since mortar plays such an important role in masonry construction it is important to understand how to select the correct type of mortar or grout. This is often a confusing task, but we will try to make it easy and clear. Keep in mind, stronger is not always better. A mortar that is significantly stronger than the masonry units can put excessive stress on the masonry, which will cause damage in the form of cracking or spalling.
Mortar is classified by ASTM C 270, Standard Specification for Mortar for Unit Masonry. There are four main types of mortar, which are described below. In addition, Type K mortar is sometimes used, but is no longer included in the ASTM C 270 standard. Mortar is meant to be plastic, meaning it will accommodate movement within the wall without rupturing.
Type M Mortar
Type M mortar is the highest strength mortar (minimum 2500 psi) and should only be used where significant compressive strength is required. This type of mortar is generally used with stone - since it closely mimics the strength of stone, it will not fail before the stone itself fails. A lesser-strength mortar may fail prematurely.
Type M Mortar Uses: Below grade applications where extreme gravity or lateral loads are present, such as in retaining walls. In conjunction with stone or other masonry units that have a high compressive strength.
Type S Mortar
Type S mortar is a medium-strength mortar (minimum 1800 psi). Since it is stronger than Type N, it can be used for below-grade exterior walls and other exterior projects projects like patios. In addition, it has higher bonding and lateral strength than type N, which makes it a good choice for resisting moderate soil pressures below grade.
Type S Mortar Uses: Below grade applications with normal to moderate loading. Locations where the masonry is in contact with the ground, such as paving or shallow retaining walls.
Type N Mortar (General Purpose)
Type N is the most common type of mortar and is the best all-around selection unless special characteristics are required. It is medium strength (minimum 750 psi) and is meant for reinforced interior and above-grade exterior load-bearing walls. It is great for semi-soft stone or masonry since it will flex more than a high-strength mortar - this prevents cracking of the masonry units.
Type N Mortar Uses: General purpose applications above grade where normal loading occurs.
Type O Mortar
Type O mortar is a low strength mortar (minimum 350 psi) that is used in non-load-bearing interior applications. It is easy to work with so it is often used to repair mortar where the wall is structurally sound. Type O mortar is sometimes used with masonry units that have a low compressive strength (i.e. sandstone or brownstone) so that the mortar allows more flexing, which prevents cracks in the units.
Type O Mortar Uses: Interior non-load-bearing applications with very limited exterior use. Repointing where the structural integrity of the wall is intact.
Type K Mortar
Type K mortar is no longer included in the ASTM C 270 specification; however, it is still sometimes used in historic preservation projects. It has the lowest compressive strength of any mortar so it will not cause damage to fragile stones or masonry.
Type K Mortar Uses: Historic Preservation projects where a very soft mortar is required to avoid damage to fragile stone - note that the mortar will not provide bearing capacity.
There are two methods for specifying mortar when issuing construction documents. You can either specify the performance properties of the hardened mortar or you can specify the proportions of the ingredients in the mortar. It is absolutely critical that the specifier understand the structural requirements that the project must adhere to so that the mortar type and mix can be specified correctly - when in doubt, be sure to consult a structural engineer.
The Performance Specification, requires that a mix be created and tested in a laboratory, which makes it less common, but much more exact for critical applications. The specifier will identify the minimum compressive strength allowed after the 28-day cure period, the percentage of air in the hardened mortar, the percentage of water retained in the mortar, and aggregate ratio of the mix. Once the mix has been tested in a laboratory, the recipe can be used in the field.
For a Proportion Specification, the specifier will identify the exact proportions of ingredients for the mix. This can be done using either weights or volumes. This allows all mortar preparation to be done in the field, which makes it the most common approach since less time is involved in creating the mortar mixtures.