The effects of using the incorrect mortar in a building are long lasting. This is why care and patience are required when choosing which mortar is right for your building structure. Modern mortar typical utilizes cement known as Portland cement, which is then mixed with sand and water. Some mortars call for lime to be added in to the mix as well. Even though there are a number of cement varieties, Portland cement is most common today. The compressive strength of a mortar made with Portland cement varies depending on the ratio of sand, water, and lime, but it typically ranges from 3-4 thousand psi.
Traditional lime mortar on the other hand is what was used before Portland cement came about (https://en.wikipedia.org/wiki/Lime_mortar). If your home was built before the 1930’s, it was probably built using lime mortar. Lime mortar has a far lesser compressive strength than does Portland mortar. Lime mortars have a compressive strength somewhere around 5-7 hundred psi, which is significantly less than Portland mortars. Another distinction that should be made is between hydrated vs. non-hydrated lime mortar. Hydrated lime mortars are able to set under wet conditions whereas non-hydrated lime mortars must completely dry to set. The difference is that hydrated lime mortars use hydration to set whereas non-hydrated lime mortars use carbonation to set. In other words, non-hydrated lime mortars need the carbon in the air to set or harden completely. There are still distributors throughout the United States that produce these old style mortar. Here is a provider locally in Pennsylvania https://www.lancasterlimeworks.com/.
All too often, modern stonemasons are using the wrong kind of mortar for their job applications. A mortar is supposed to be a binder for whatever it is surrounding whether it be stone, brick, or block. The mortar should be the sacrificial aspect to the structure so that the structure can live on. With that being said, the mortar should be weaker than what is around. If it is not, which is the case with improper mortar application, the building material will break, crack, and spall instead of the mortar taking the impact. This doesn’t just apply to what the materials are laid in but what they are pointed with as well. Pointing with the right kind of mortar is just as important. Improper mortar use causes major structural problems and can be detrimental to any masonry structure.
Be sure to ask whatever masonry professional you have working on your building what kind of mortar they will be using and why. A good stonemason will be able to explain to you the differences between different types of mortar and their advantages and disadvantages. When dealing with any contractor keep them accountable. Arm yourself with information and keep your masonry structures thriving for years to come.
Check out this video for more information on the use of lime vs. cement!
And for extensive knowledge on the origins of these mortars, read this: http://www.hms.civil.uminho.pt/sahc/2004/87.pdf