By: Joseph Contreras
As discussed in our October newsletter article about repointing, mortar is the material that is used to bond two units of masonry together. Although sometimes confusing, it is very important to select the correct type of mortar for a construction project. While all mortar should be resistant to moisture infiltration, mortar mixes vary based on strength, bonding and flexibility.
The compressive strength of mortar is measured in pounds per square inch (PSI). It’s a common belief that the stronger the mortar, the better; however, if the mortar installed is stronger than the masonry units, it will cause the softer masonry to spall and crack.
Bonding refers to the ability for mortar to adhere to the unit it’s applied to. Good boding properties are necessary when building a foundation; however, this is not the case when repointing a structurally sound wall. High bonding mortar would most likely dirty the face of the wall, resulting in an undesirable aesthetic.
Flexibility might be an important factor in determining the correct mortar mix. This mortar property, referred to as elasticity, allows for the movement of structures. A flexible mortar with a higher concentration of lime might be the better choice for repair work on a tall building. A stronger cement mortar would not be able to withstand the sway, or the expansion and contraction, of the higher structure. Finding the correct balance between strength and elasticity is essential in choosing the correct type of mortar for the job.
All traditional mortars are essentially a blend of sand, Portland cement and hydrated lime. These three ingredients are mixed at different proportions depending on the intended use. The 4 main types of mortar most commonly used are; Type N, M, S, and O. These types of mortar are thoroughly described in ASTM C 270, but we will briefly outline the characteristics and best use of each type here.
Type N mortar is the most common type, and is usually recommended on exterior, above-grade walls. This general-purpose mortar has good bonding capabilities. And since the cement is not overburdened by Portland, it cures more slowly and allows for better workability. Type N mortar has a compressive strength of about 750 PSI, which is ideal for use with semi-soft stone or masonry applications. It’s more elastic than a high strength mortar, which helps to prevent cracking and spalling of adjacent masonry units. Although commonly used in setting bricks, Type N can also be used for repointing newer brickwork.
Type M mortar is the strongest of the four, and has a compressive strength of 2500 PSI. Type M mortar should be used when the structure has to withstand high gravity and/or lateral loads. Type M mortar is also a good choice for hard stone projects where the compressive strength of the stone is greater than 2500 PSI. The characteristics of Type M mortar make it ideal for below grade applications, such as at foundations and retaining walls.
Type S mortar is a medium strength mortar achieving a compressive strength of approximately 1800 PSI. Type S mortar can be used on at/or below grade exterior walls, and hard coat traditional stucco systems. The strength and bonding properties of Type S mortar are greater than that of Type N, and the increased amount of lime in Type S allows the mortar to withstand excessive moisture and increases its bonding and elastic capabilities.
Type O mortar has the weakest compressive strength, approximately 350 PSI. Type O is easy to work with, and the consistency of the blend makes it a good choice for repointing performed at a structurally stable wall. Its low compressive strength makes it a good option for soft stone applications such as setting sandstone or brownstone. Type O mortar also allows for more flexing, which can help prevent cracks and spalls in masonry units.
Although there are other scientifically modified blends of cement, these four types are the most commonly used forms of mortar. A thorough understanding of the structural requirements of a project is necessary in order to choose the correct one. Building owners and contractors should consult with a structural engineer if there are questions as to what type of mortar should be used.