Mortar has a basic recipe which includes 3 components: Portland cement, hydrated lime and sand. Each component provides its own benefit to the overall makeup. Portland provides strength, lime will give it a little bit of flexibility and elasticity, while the sand will act as a binding agent to the other two products.
There are 5 main gradings of mortar mix: K, O, N, S, and M. They each have different uses based on their specific properties which affect their flexibility, bonding properties, and compressive strength. We will be discussing each type, along with when and where you would use them.
Let’s start with Type K since it has the least strength, and we’ll work our way on up the chain. Type K offers a very low compressive strength of approximately 75 PSI. (PSI is a unit of pressure expressed in pounds of force per square inch of area. It stands for Pounds per Square Inch.)
Because Type K mortar is so soft, it is rarely used in new construction, but can work well for specialty applications such as restoration projects. For example, ancient or historic buildings require a soft mix such as Type K, which would not be significantly stronger than the existing masonry.
Moving on to Type O mortar which must be at least 350 PSI. Type O mortar is best used with soft brick or a soft stone. It is primarily used in interior, above-grade, non-load bearing applications.
In the construction industry, the words ‘above grade’ and ‘below grade’ can be used to define the section of a structure that is located above or below ground level.
Using this on the exterior is limited, but can be seen in warmer climates where they do not experience extreme cold temperatures. This mix can be ideal for repointing mortar joints or similar-type repair work because its consistency makes application easier. Type O mortar is composed of 1 part Portland cement, 2 parts lime, and 9 parts sand.
Type N is considered to be a general purpose mix and is the most common mortar mix. It is usually used with brickwork and is often used by most homeowners. This is the best choice for general applications above grade, and can be used for both exterior and interior load-bearing installations. It will achieve its maximum strength in 28 days. To classify as Type N, this mortar must have a minimum strength of 750 PSI. Type N is more than twice the strength of Type O. That is because there is 1/3 less sand in the mix. The compound consists of 1 part Portland, 1 part lime (so, less elasticity), and 6 parts of sand.
Type S is considerably stronger, yet. This compound offers both a high compressive strength along with a high tensile bond strength.
Tensile strength is the resistance of a material to breaking under tension.
This will be used where you need rigidity, such as an elevator shaft where you do not want movement.
The formula for creating Type O is 2 parts Portland, 1 part lime and 9 parts sand. This combination will give you a minimum compressive strength of 1,800 PSI, but is often mixed for strengths of up to 2,300 PSI. So it is very strong with not much flexibility. Type S Mortar has a high-tensile bond strength which is suitable for many projects at or below grade which include patios and walkways or masonry foundations and retaining walls. It performs very well withstanding soil pressure, wind and even seismic loads.
Type M is going to be your strongest mixure, by far. Its strength rating is going to provide at least 2,500 PSI. It will not be used on brickwork because it is too rigid. This is typically going to be used for places which will endure heavy loads, anything that is close to water, or for below-grade applications such as block foundations, retaining walls and driveways. The properties in this formula are such that this mortar has poor adhesion and sealing properties, which means that it will be unsuitable for many exposed applications; however, it is preferred for use with natural stone since it offers a strength similar to the stone. The mixture for Type M includes 3 parts Portland, 1 part lime and 12 parts sand.
As you can see, knowing which type of mortar mix to use for each different type of conditions can be critical. Selecting mortar may appear to be simple, but in reality it involves rather complex factors related to materials, design, and construction. If you are unsure, get advice from an engineer, architect, or mason contractor.
Below we have created a cheat sheet to simplify selecting your mortar type.