Mortar Types: M, S, O, N, K: Tuckpointing and Laying Masonry

Pictured here is the most common types of mortar in premixed bags.

The most common types of mortar are Types S and N.

Mortar-Mud-Mixes

Used in all types of masonry work, including brick, block and stone laying and repairs, such as tuckpointing, Mortar is available in a variety of types for a variety of applications. Not be confused with cement, concrete, or grout, Mortar is a specific material made for specific purposes. Commonly referred (by myself, the crew and other masons in the St. Louis area) as “MUD!” Why Mud? The first mortars were made of mud and clay. That, and when it’s approaching 100 degrees outside and you’re on site, 30′ in the air, putting the mortar in the wall, it can be somewhat easier to yell (To alert the laborer a need for mortar) “MUD!” Next time you’re around a job site with masonry work in progress, You’ll know what you’re hearing, when you hear “MUD!”

Mortar Ingredients — 3 makes all kinds

1.) Sand is just very small rock, I’m sure you knew that. The sand serves as an aggregate. The aggregate is used to give the other ingredients something in which to bind. Also used as a filler, increasing or decreasing amounts can weaken to strengthen a mix, respectively. Sand is available in a wide array of colors, sizes and consistency.

2.) Portland Cement is a hydraulic cement made from crushed limestone. The cement is the binder. It’s what holds everything together. The cement used for tuckpointing and laying is a hydraulic cement, meaning it is water-resistant. Portland cement is available in shades of grey and white.

3.) And Mason’s Lime (Calcium Hydroxide  /  Type S Hydrated Lime / Slaked lime) is lime that has been treated with water. The lime allows control of setting time by slowing down the hardening effect. It affects porosity, the amount of water or air able to pass through the mortar.  It makes the mortar more pliable. It also makes it sticky, therefore improving bonding to materials. That can be good or bad. A craftsman knows the proper sticky for each application.

Those three ingredients along with the proper amount of water is all you need to make mortar. Of course there are color additives, cold additives, glues and many more things you can add. Sand, cement and lime are the basics.

Masonry Cement

Alternatively, it could be said mortar has 2 ingredients as Masonry Cement Can be bought premixed. What is Masonry cement? Masonry cement is a blended mix of Portland Cement and Hydrated Lime. There are different types of Masonry cement that correlate to the types of mortar listed below. Just add sand to complete. And Types N and S can be bought premixed entirely, with sand, cement and lime. The complete package is what we refer to as Premix.

Mortar Types — 5 of a kind

Alternating letters in the words MASON WORK, have been used to give mortar types, their names. “MaSoN wOrK” That’s 5 types of mortar. The differences? Different Ratios of the 3 ingredients, listed above. We typically make our masonry cement first, then add the Sand. Masonry cement is just a premix of the cement and lime. 

Type M — The strongest (Highest PSI @ 2500) of the group, this type is most commonly used for load bearing walls and below grade applications, such as foundations or retaining walls.  The Recipe: 3 parts Portland, 1 part lime and 12 parts sand. The low concentration of lime helps to achieve this high strength, but takes away from bonding ability. (A poor choice for tuckpointing anything.)

Type S — Also strong, @ 1800 PSI, Type S is most commonly used for laying.  The Recipe: 2 parts Portland, 1 part lime and 9 parts sand. Because of it’s increased amount of lime, it offers the most flexibility in use and resistance to movement and weather.

Type N — A common mix for tuckpointing, and laying softer materials, Type N is the most flexible and resilient @ 750 PSI. The recipe: 1 part Portland, 1 part lime and 6 parts sand. Furthering the concentration of lime in the mix does take away from the structural strength aspects, but in turn offers more resilience and flexibility. Perfect for tuckpointing most exterior walls and chimneys.

Type O — At 350 PSI, this is the lowest strength mortar available in pre-mix. With the lots or lime, The recipe is: 1 part Portland, 2 parts lime and 9 parts sand. Not often used outdoors, Type O’s uses are limited to non load bearing walls and is generally an interior only used mix.

Type K — Abandoned for most practical uses, before I entered the trade, Type K is the least seen or used. Type K mortar is generally used only for interior or historical pointing of soft, hand-made brick. I’ve only made it once for curiosity’s sake only. (It’s a sticky mess.) Type K mortar has the lowest compressive strength @ 75 PSI.,  The recipe:  1 part Portland, 3 parts lime, and 10 parts sand. The increased porosity of this mortar can help protect movement of material it surrounds.

Lime Mortar/Type L — Not really a type or mortar or a mortar alone by means of the definition of the word (mortar). Type L is a mix, of only, lime and sand, NO CEMENT. The Recipe: Varying amounts of Lime & Varying amounts of sand. It’s not commonly worked with and is generally only used for Historical tuckpointing purposes.

It’s good to know all the technical facts about your brick and mortar. Or, you can leave all that tedious information to us. Be secure in the knowledge that we know!

by Jacob Thompson