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 beds experience 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 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 used LIME PUTTY, which was dry slaked lime that had been saturated with water to form a thick putty. The putty 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 (in fact, the best putties were aged for several years in barrels). To make mortar for brick or stone the mason mixed the lime putty with sand aggregate and a small amount of water. As it cured the lime 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 putty 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 putty to cure.
During the last quarter of the 19th century a new product appeared generally in the United States: PORTLAND CEMENT. Portland cement is lime-based like traditional lime, but also has other ingredients, including silica clays and gypsum. Portland cement cures very quickly and was initially added to traditional lime putty 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 putty 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 putty 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 or slaked 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 found today. Furthermore, the color of Portland mortar does not match traditional lime putty. Portland is a darker gray whereas lime putty 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!