Choosing the right mortar for masonry work is a challenging task. If the material you’ve used is too strong for stone, brick, or concrete blocks, it may cause cracking. If it’s too weak, the structural integrity of the building could be compromised. So, what do you do?

Understanding different types of mortar can help you find the most appropriate variant for your construction project. Read on to learn more.

Type M

With a minimum compressive strength of 2500 psi,Type M is the strongest type of mortar mix. When hardened, it mimics the properties of natural stone, making it highly compatible with marble, granite, and other stonework.

Type M is best for below-grade (underground) applications where a heavy lateral load is exerted. Some examples of these are:

  • Foundations
  • Load-bearing walls
  • Wall retainer
  • Pathways
  • Driveways

Type N

This is the most common mortar type. It’s flexible, has a moderate strength of around 750 psi, and can be used both for interior and exterior construction work. Such characteristics make it a great all-rounder that can be blended with most masonry units, including semi-soft stones.

This means it can also be used as a general-application mortar that can withstand normal loads. This also makes it ideal for reinforcing homes’ interiors or above-grade (above the ground) exteriors.

Type S

Type S offers impressive bonding ability and toughness. Its compressive strength is around 1800 psi, a close second to type M, but its tensile bond strength is better than that of type N mortar. It also offers moderate resistance to below-grade soil pressure, allowing it to be usedin areas where type N isn’t the best option.

This can work for above and below-ground structures too. Here are some examples of where it’s used.

Above: Patios, balconies, and roofing.

Below: Manholes, sewers, pavement, and as a shallow wall retainer.

Type O

This type might have a compressive strength of only around 350 psi, but it’s the easiest to work with. Its relative softness and flexibility are compatible with masonry units with a similarly low compressive strength like sandstone and brownstone.

Type O’s limited durability means it can only be used for interior non-load-bearing applications like:

  • Repairing structurally sound walls
  • Smoothing rough surfaces
  • Repainting brickwork

Type K

With an extremely low compressive strength (75 psi), type K no longer meets the specifications of ASTM C 270 – the standard for mortars for unit masonry, but it still has some uses. Its extreme softness,which doesn’t damage any type of stonework, is ideal for historic preservation. Some art projects also include it as a working medium.

If you’re unsure which mortar type is stronger, just remember “mason work.” The letters M, S, N, O, and K’s arrangement in the two words reflects their respective compressive strengths. But, if you need additional advice onwhich mortar mix works best for your construction project, consulting a structural engineer would be your best bet.

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