The primary component of all masonry is mortar. Mortar is a mixture of sand, lime, and cement. That is really all there is to mortar. See my story could end, but mortar is so much more interesting than this.
There are 5 commonly used mortar types, These types are M-type, S-type, O-type, N-type, and L-type. There are then countless custom variations, some with different specifications like refactory mortar which is used inside of fireplaces. It’s heat resistant. These are all still mostly only sand, lime, and cement. The only thing different is the formula of each. They are also super easy to remember because they almost spell MASON, skipping only the A and then L which is just short for lime.
L-type Lime Mortar
Let’s just get the L-type out of the way first. L-type is Lime Mortar. It typically isn’t used very much today and is never used with new construction. It is used for conservation purposes, though, to preserve a historical construction that was created with it. It is a lime, water, and sand mixture with the cement component completely eliminated. Lime mortar can be replaced with cement mortars. It should never be used when an area is wet or moist.
M-type mortar is the strongest of all the types. It is best used for a below grade application, but it is much less workable than other types of mortar. This makes creating a good seal more complex. It doesn’t offer much adhesion qualities, and that allows for openings to exist. This type of mortar is great for areas where a lot of compression is expected and a heavy load will bear down on it.
S-type mortar is the next strongest mortar. It also is good for use below ground. It bonds better and has better adhesive qualities. It’s used pretty commonly in Minnesota and for a do-it-yourself application. This is what I would recommend. S-type mortar will withstand the winters in Minnesota.
O-type mortar is low strength. We rarely use it in Minnesota because of the climate. We will use it on some historic buildings for tuck pointing because it becomes closer to the aged mortar that is behind it as far as softness. That allows for a more uniform load and bearing of masonry units. So using this mortar avoids the visible areas where there is a “hard-spot” that would be created if standard mortar were used. O-type mortar is used in more humid climates that are less cold, so if we were in Miami, maybe we would have this as one we commonly used. It’s never used as a structural component, so has very limited exterior use.
There is also a K-type mortar, but it’s rarely used. It is created specifically for restoration purposes and has a really low compression strength. It has a compression strength of 75 psi, to put that into perspective: a bad boxer can punch at 1,200 psi. We don’t use this mortar.
N-type mortar is generally considered the normal mortar used for the general purpose. It is the most common mortar and great for using in load bearing areas. It has more flexibility than S-type and prevents a lot of cracking. We use it with soft stone. It is also valuable in extreme weather, particularly high temperature weather.
Everything becomes worn out, mortar isn’t any different, and in Minnesota with most typical mortars you can expect a life between 40 and 80 years. The huge variance is because of the variances in exposure. If the brick is in the building side and protected with eves, sometimes it can last hundreds of years, but if your brick and mortar is exposed to every whim of the weather. That exposure is really what will reduce the practical life of mortar. You can extend this life slightly with tuck pointing which will remove older worn mortar and replace it with new mortar without doing damage or changing out the brick or stone.