Mortar, a thick mixture of water, sand and cement, is a common bonding agent for building materials, such as brick or stone. Different types of mortar mixes vary by strength, bonding properties and flexibility. Each mortar mix has specific proportions of portland cement, hydrated lime and sand that differentiates each type from the other. Whether you use Type S or Type N mortar mix (both of which are general all-purpose mortar mixes) depends on the properties of the mix and the nature of your project.
Compressive strength is the capacity of a material or structure to withstand loads that are likely to reduce size. Type S mortar mix, which contains two parts portland cement, one part hydrated lime and nine parts sand, has a compressive strength of at least 1,800 pounds per square inch (psi). Usually, a Type S mix has a high compressive strength of between 2,300 and 3,000 psi. Type N mortar mix, which contains one part portland cement, one part lime and six parts sand, is a medium compressive-strength mortar of at least 750 psi and can achieve 28-day strength between 1,500 and 2,400 psi.
In general, the bond between brick and mortar is the most important property to consider when selecting a mortar mix. Bonding properties include the extent of bond and bond strength. Extent of bond refers to the level of close contact between the mortar and brick, which is improved by good mortar workability. Good extent of bond increases durability and water resistance. Bond strength refers to the force required to separate the mortar from the brick. Good bond strength provides resistance to cracking. Type S mortar mix has high tensile bond strength and performs well against soil pressure and wind, and under seismic conditions. Type N is ideal for anything that's exposed to severe weather and high heat.
As a general, easy-to-remember rule, use Type N mortar mix for normal brickwork applications and Type S mortar mix for stronger brickwork applications (i.e. those required in high seismic/high wind areas and reinforced brickwork). Use Type S mortar mix at or below grade (at or below the grade of the earth surrounding the building) – for example, masonry foundations, manholes, retaining walls, sewers, brick patios and brick pavements.
Use Type N mortar mix in above grade, exterior and interior load-bearing installations, such as with walls and chimneys. Type N is best for soft stone masonry and general application and is suitable for repointing newer brickwork.