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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 terracotta 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
  S: 1,800 psi
  N: 750 psi

  O: 350 psi
  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 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.


Masonry like the other parts of the building, requires inspections, maintenance and repairs in order to provide long lasting service.
Masonry inspections and maintenance should be approach systematically by concentrating on specific areas:
damaged, broken or spalled units like brick, block and stone should be replaced.Deteriorated mortar should be repaired and replaced by tuckpointing to stop water from penetrating into the interior of the walls. Mortar should be matched and duplicated to it’s original proportions.
Cracks in mortar: small, hairline cracks should be tuckpointed to prevent them from creating passages for water to penetrate into the interior of the wall. 
Water penetration is the greatest danger to masonry walls. Any water leaks into interior walls should be stopped immediately to prevent damage and mold growth.
Chimneys are exposed to severe weather conditions and because of their high exposure prone to deteriorate , should be periodically inspected and repaired as needed. Unrepaired can cause damage or injury.
Parapet walls are exposed to the weather condition heavily, should be carefully inspected. Any displacement, cracking, efflorescence and spalled bricks should be repaired to avoid the possibility of moisture and water infiltration.
Inspection of the flashing is important to prevent water damage at the end of flashing, especially in areas above windows and doors.
Weep holes allow water to exit the wall. Clogged weep holes should be cleaned.
Steel lintels can be subjected to condensation, water leakage to the interior and weather conditions. This can cause corrosion from the outside and inside of the structural steel member. Corroded steel lintels should be replaced.Back in the old days, steel lintels were of a higher quality than those used in today's construction. Those lintels often lasted 80 – 100 years before they became so rusted, deflected, and structurally deficient that they required replacement. With today's slacker construction codes, improper installation and (often) focus on quick profits, the steel lintels in masonry structures built today sometimes don't last a decade before they require replacement.
Drip edges are pieces of metal designed to direct or shed water away from your masonry building, and are installed in walls at critical locations. In newer buildings (especially those with concrete blocks or split-face blocks), proper installation of stainless steel drip edges are critical to keeping your wall cavities and interior spaces free of water infiltration. J&P Masonry uses only stainless steel drip edges in its parapet wall, sill and lintel projects due to their longevity and their resistance to rust, discoloration and staining of adjacent masonry.
Masonry efflorescence results from chronic and/or severe water penetration into the masonry unit. During warmer, dryer weather, the brick transpires or "sweats out" the moisture, as well as some of the salts and other minerals in the brick. At the brick's surface the water evaporates but the minerals remain and appear in the form of a white powder similar to talc. Over time the leaching of the brick's salts and minerals reduces its mass and weakens its integrity. If not addressed quickly, the brick may spall, fissure or crack apart.

Maintenance of masonry should be provide to ensure adequate masonry performance and extend service life of the building structure and to keep minor problems from developing into major ones requiring expensive repairs.

-Concrete Masonry Handbook, 6th Edition Paperback – January 1, 2008
 by J.M. Melander and W.C. Panarese J.A. Farny 
-Masonry Skills sixth edition – July 3, 2007 by Richard T. Kreh, Sr