Mortar Mixes

We have compiled a glossary of common masonry and tuckpointing terms to help you make informed decisions about your next masonry project. If you have any questions or want to put our decades of experience to work for you, contact us!

Mortar is the glue that bonds your masonry materials together. With the right mortar applied under the correct conditions, your bricks, stone, concrete block or terra cotta structure will last for generations with minimal maintenance.

Mortar is comprised of specific proportions of Portland cement, hydrated lime and sand. The proportions get adjusted depending on the characteristics of the materials to which they are bonding, the environmental conditions to which the materials will be exposed, etc.

There are numerous different mortar types used throughout the United States. Some mortars are very strong due to an increase in the proportion of Portland cement; other mortars are very soft due to an increase in the proportion of hydrated lime.

Some of the more common mortar mixes used in Illinois include Types N, M, S and O. There is also mortar for glass blocks, a straight lime mortar, and Type K (which is used solely in historic preservation).

Mason Work

The five typical mortar mixes designated types M, S, N, O and K are labeled so because each is an alternate letter in the term MASON WORK in descending psi strength.

These designations were assigned in 1954 and replaced the mortar designations A-1, A-2, B and C.

  • M: 2,500 psi
  • A
  • S: 1,800 psi
  • O
  • N: 750 psi
  • W
  • O: 350 psi
  • R
  • K: 75 psi

Know that a weaker psi mortar is not a "bad" or inferior mortar to one with a higher psi. A lower psi mortar has much better adhesive and sealing powers than a higher one. Mortars are selected, on the balance, between these attributes in order to meet the needs of a particular area on a particular project. A type M mortar with its high strength yet poor adhesion and sealing can be a bad choice for one area of the job and just what is needed in another.

Type M Mortar

This uses a 3 / 1 / 12 mix and results in a mortar with a 2,500 psi compressive strength. Type M is used for below grade load-bearing masonry work and for chimney caps and brick manholes.

Type S Mortar

This uses a 2 / 1 / 9 mix and results in a mortar with a 1,800 psi compressive strength. Type S is used for below grade work and in such areas as masonry foundation walls, brick manholes, retaining walls, sewers, brick walkways, brick pavement and brick patios.

Type N Mortar

Type N mortar uses a 6/1/1 mix which works well in both new construction and restoration projects. It has a minimum compressive strength of 750 psf and a fully cured compressive strength of 1200 psf. (Most architects do not know that the fully cured compressive strength of this mortar is almost double what folks find on the Internet, which explains why so many architects erroneously call for Type S.)

Type N mortar has a terrific balance of Portland cement (which makes the mortar hard) and lime (which makes the mortar soft and allows the bricks to move with changes in temperature and humidity). If your masonry structure was erected after The Great Chicago Fire, then chances are good the correct mortar to be used on your restoration project is Type N.

Type O Mortar

This uses a 1 / 2 / 9 mix and results in a mortar with a 350 psi compressive strength. Type O is a lime rich mortar and is also referred to as "pointing" mortar. It is used in above grade, non-load bearing situations in both interior and exterior environments.

Type K Mortar

This uses a 1 / 3 / 10 mix and results in a mortar with but a 75 psi compressive strength. Type K is useful only in historic preservation situations where load-bearing strength is not of importance and the porous qualities of this mortar allow very little movement due to temperature and moisture fluctuations. This aids in prolonging the integrity of the old or even ancient bricks in historic structures.

Straight Lime Mortar

This uses a 0 / 1 / 3 mix and is used now only to recreate the construction and review the methods of times past or maybe for purely visual purposes. This mortar was made before Portland cement was available in many areas and so this is what was used.

Glass Block Mortar

This uses a 1 / 1 / 4 mix and is used with as little water as possible. This is a mix designed specifically for glass blocks. Also, note that it uses waterproof Portland cement in place of "regular" Portland cement.

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