One subject that is of great interest for the owners of masonry buildings is mortar. We all know that mortar is the cementitious material used with brick, stone, tile and terra cotta. Most have also heard that there are different types of mortar and that some are not appropriate for applications on historic architecture. Indeed, the variety of modern products can be quite confusing. If you go to a big box store you will see bags and bags of materials with different labels, including hydrated lime, type S, mortar mix, bag lime and more. In this and a following blog post I will give a quick introduction to historic mortar and a little information about modern mortars. I have used capitals on key terms to make them easier to identify and remember.
Historic mortars are LIME based. That is, the active ingredient is lime. For thousands of years people have burned limestone or marble (marble is a metamorphic rock produced when limestone experiences extreme pressure and heat in the earth’s crust) to drive off carbon dioxide gas. The result is QUICKLIME. Quicklime is pulverized and is very caustic and dangerous as it can your burn skin and eyes. Quicklime is then mixed with mix a small amount of water. This process is called SLAKING and is also dangerous because the resulting chemical reaction release a tremendous amount of heat. In fact, you can actually see steam as the lime mixture bubbles and reacts!!! The result is SLAKED LIME.
Slaked lime was the basic ingredient in historic mortars. Until the last quarter of the 19th century American masons usually used HOT LIME, which was slaked lime mixed with sand to make mortar. After a bit of time to cool, the fresh lime mortar was used to build walls and other things. Dry slaked lime could also be mixed with water to form a thick LIME PUTTY that was stored in barrels with a layer of water on top of it to keep the putty from reacting with the carbon dioxide in the atmosphere. This putty could sit for years and still be used to mix mortar, plaster and stucco. In fact, the best putties were aged for several years in barrels. To make mortar for brick or stone construction, the mason could mix the lime putty with sand aggregate and a small amount of water. As it cured the lime in the mortar recombined with carbon dioxide and essentially turned back into limestone. In the photo above you can see lime putty being mixed with damp sand to make a traditional mortar. The white substance is lime putty.
The advantages of traditional lime mortar is it is relatively soft and flexible, meaning it will move as a building settles and will not damage soft brick and stone. It is also permeable to water, meaning water that is absorbed into the stone or brick will be able to escape and not cause damage as it freezes. One disadvantage is that it can take weeks or months for traditional lime mortar to cure.
During the last quarter of the 19th century a new product appeared in the United States: PORTLAND CEMENT. Portland cement is lime-based like traditional lime mortar, but also has other ingredients, including silica clay and gypsum. Portland cement cures very quickly and was initially added to traditional lime mortar to speed the cure time. Over the following decades more and more Portland was added to the mixes until it had largely replaced traditional lime in the early 20th century. Portland cement is not water permeable, meaning water is not able to pass through it. This can be disastrous for pre-1900 masonry as water can be trapped within the brick or stone due to the impermeable layers of Portland mortar and cause older brick or stone to disintegrate through the freeze-thaw cycle.
Portland mortar is also exceptionally hard. This might seem to be a positive attribute, but in some historic applications it can be problematic. Lime mortar was relatively soft and provided a cushion between the masonry pieces. Hard, inflexible Portland can actually damage soft brick and stone, lead to water infiltration and hasten decay through the freeze-thaw cycle. As a result Portland mortar is usually mixed with hydrated lime to soften it. These mixes are usually graded M, S, and N, with M the hardest and N the softest. There are other grades, including O and K, but these are quite soft and rarely used today except in special applications for historic buildings. Furthermore, the color of Portland mortar does not match traditional lime mortar. Portland is a darker gray whereas lime mortar was much lighter. This can make spot repointing or repairs stand out since the new mortar can be easily distinguished from the old.
There’s more to come on historic mortar on the Historic Design Consulting blog, so be sure to check back!