Choosing the right mortar for a masonry project is important for its structural integrity and durability. With five main types of mortar available, here is how to choose the right mortar for any Westford, MA, masonry project.
What Is Mortar?
Mortar is used to bond bricks, stones, or concrete blocks together. It also provides structural integrity and beauty—so choosing the right mortar for a specific application will yield the best results. The five most common types of mortar—N, O, S, M, and K—are made with a specific ratio of Portland cement, sand, and hydrated lime, each with different characteristics. Each mix has different levels of flexibility, compressive strength, and bonding properties.
Masonry cement hardens by chemically reacting with water. When mixed, each mortar type delivers different results. Water hydrates the cement and when it chemically mixes with the cement compounds, it creates a unique strength profile.
Sand provides the structure for the mortar. The water/cement mixture coats the sand particles, to give the mix body and consistency. Once the mortar hardens, the relative mixture of the ingredients provides characteristics including compressive strength, shrinkage due to drying, and bond strength. Portland cement acts as the “glue” that holds the mixture together (and ultimately is responsible for holding the masonry structure together). Finally, lime “plasticizes” the mixture, contributing to its workability and consistency.
Types of Mortar Mix
Type N: This general-purpose mix is suitable for use on exterior above-grade walls, such as garden walls. It is designed for severe weather applications and can withstand high heat. Its characteristics are as follows:
Medium compressive strength, 750 pounds per square inch (psi)
Above-grade, interior, and exterior load-bearing applications
The ideal choice for soft stone masonry and veneers
Good workability and serviceability
Type O: This mix is primarily for interior and non-load-bearing applications; however, it is the perfect mix for repair work on existing structures constructed with mortar.
Low compressive strength (350 psi)
Suitable for interior, non-load-bearing walls, and above-grade applications
Limited exterior use—cannot withstand high winds
Good consistency and ease of application makes it perfect for repair and repointing work
Type S: This is an extremely high-compressive and high-tensile strength mix suitable for below-grade applications
High compressive mix (1,800 psi minimum, ranging to 3,000 psi)
Engineered to withstand severe soil pressure, wind, freeze-thaw cycles, and seismic loads
Ideal for below-grade masonry foundations and other at-grade load-bearing applications such as retaining walls, patios and walkways
Type M: This mix contains the highest ratio of Portland cement. It is ideal for heavy loads, below-grade applications, foundations, taller retaining walls, and at-grade applications such as paver driveways.
Minimum of 2,500 psi compressive strength
Poor adhesion and sealing make it best for natural stone (its strength matches natural stone)
Suitable for severe freeze-thaw applications
Type K: This low-compressive-strength mix is rarely used for new construction but is suitable for restoration purposes.
75 psi compressive strength
Easy to use, type K mortar is best for the restoration of crumbling masonry work on historic buildings
Which Mortar Is Best?
Selecting the right mortar is not an exact science due to many variables inherent in each project. There is no single “perfect” or all-purpose choice.
Stronger (higher psi) mortar is not necessarily better. Generally, types N and S have better workability and durability than type M mixes. Environmental conditions are key, especially severe weather and seismic loads. All masonry projects must adhere to local building codes.
Ideally, work with a qualified landscape contractor who is experienced in selecting the appropriate mortar for specific applications and the environmental challenges of New England summers and winters.