Lime Mortar Vs. Cement Mortar: What Are The Major Differences?

Mortar is the material that is solely responsible for holding brick, stone, and other masonry units together. As such, it plays a vital role in the overall stability of a structure. Nowadays, there are four main types of mortar – N, O, S, and M – each of which is comprised of a distinct ratio of lime, cement, and sand. However, there are two discrepancies that you should also be aware of: lime mortar vs. cement mortar.

Lime mortar has been used in construction for thousands of years. Although it’s not as common today, it is still a strong, durable type of mortar that can reduce environmental impact and enhance the structural integrity of the home. Cement mortar, on the other hand, is often preferred when a quicker curing time is needed.

Regardless, if you are restoring a building, it’s important that you choose the right type of mortar. Using the wrong replacement mortar can cause more harm than good, and may have a negative impact on the stability of the entire structure. With that said, let’s explore what exactly lime mortar and cement mortar are, the differences between the two, and how to know which one to choose.

Lime mortar has been in use since biblical times – it was even used to plaster the ancient pyramids in Egypt. It consists of lime (hydraulic or non-hydraulic), water, and an aggregate, like sand. Nowadays, lime mortar is mostly used in the conservation of old structures or the recreation of new buildings using traditional techniques.

The slaked lime that is used to make lime mortar is formed by cooking limestone rocks at 1,650 degrees Fahrenheit. During this process, the heat burns off the carbon dioxide found in the rock and leaves behind calcium oxide, or quicklime. This powdery quicklime is then submerged in water for weeks or months at a time to result in a lime putty known as “slaked” lime. The slaked lime is combined with water and sand (or other aggregates) to create the lime mortar.

When the lime mortar is exposed to air, it takes the carbon dioxide and releases water to return to its original limestone state. Lime mortar was used exclusively for masonry structures until the invention of Portland cement in the 19th century.

Developed in 1824, cement mortar, or Portland cement, is a combination of calcined hard limestone and clay that is heated a second time. When compared to lime, Portland cement had the ability to attain much quicker setting times. Though its long terms strength is lower than natural elements, cement mortar has a quick initial strength that made it ideal for use during the Industrial Revolution. Because of its high strength and quick set time, lime mortars were almost completely phased out by the mid-20th-century.

If the structure was built using lime mortar, it is crucial that you replace damaged sections with lime mortar. Choosing the wrong type of mortar as a replacement could result in disintegrating brick in just a few years that is very challenging to replace. If your home was built before 1880, it likely has conventional lime mortar and you should only use that.

Whereas, if your home was constructed after 1930, it likely has Portland cement mortar and you can find this easily at your local home improvement store. However, what about homes built between 1880 and 1930?

The first thing to pay attention to is the color of the mortar – is it white or closer to gray in color? White mortar often indicates lime mortar, and grayer mortar is typically Portland cement mortar. However, there are some forms of cement mortar that are white, which makes determining which type you have all the more difficult.

Fortunately, there is a quick test that you can perform to find out which mortar you have. Take some white vinegar and spray it onto the mortar and pay attention to the fizzing that happens. When Portland cement is exposed to white vinegar, very little fizzing occurs. However, when historic lime mortar is exposed to white vinegar, it will have a very dramatic fizzing reaction. Another way to tell if you have lime mortar or cement mortar is to knock at the surface with a hammer. Portland cement will emit a much higher pitched ring than lime mortar.

Finally, another strong indication of whether you have cement mortar or lime mortar is the detailing. Historic lime mortar tends to have very intricate details and perfectly smooth lines between bricks. Whereas, cement mortar is more associated with newer construction, and unfortunately, newer work is often sloppier than the work back in the old days.