Traditional Masonry Archives

Kicking the Cement Habit:
The Return of Lime Mortars

"The present age may well be classified in history as the 'Concrete Age.'" So wrote one H.F. Porter in 1909, noting that portland cement "…is indeed a product of evolution, and because of quality and economy, is rapidly displacing all other cementitious materials. More than that, it is creating entirely new, ever-widening fields of usefulness, and is exerting a revolutionary effect on modern industry and progress."

For the most part, Porter's words have held true for nearly a hundred years. Portland cement has inarguably been the dominant building material of the twentieth century, and it has indeed evolved — modern cements are achieving impressive compressive strengths, and decades of research have created a consistent, fast-setting product. Just about every mason now works in portland cement-based mortars — specifically, types M, S, and N.

What Porter probably couldn't have imagined, however, was that traditional lime mortars would ever re-emerge — he had rejected lime mortars, noting that the use of lime "today is confined to the less progressive people and sections and to the poorest grades of brick and masonry…." He would have undoubtedly been surprised to see lime mortars in twenty-first specifications and on today's jobsites, but these historic formulations are indeed back — which is good news for our historic masonry buildings.

The lime mortar revival
Lime mortars are essentially a combination of lime and sand, and they're typically made in one of four ways: by mixing lime putty and sand, mixing hydrated lime and sand, mixing quicklime and sand, or mixing hydraulic lime and sand. Historically, masons made mortars with the materials they had available, which could mean that a hydraulic lime mortar was mixed "hot," or for fine finish brickwork lime putty was used. Making and using lime mortars to closely match the original installations can provide the most historically accurate results.

While the entire spectrum of lime mortars now makes up a small portion of the overall mortar industry, lime mortars occupy an ever-growing share of the restoration market. Anecdotal information suggests that much of the current interest is due to the proliferation of preservation information on the Web, which has made it easier than ever for architects and property owners alike to research masonry preservation techniques and materials. Intriguingly, lime mortar is also appearing in so-called "green" building projects, which focus on sustainable materials and energy efficiency. (Lime mortars, which are non-toxic and require relatively little energy to produce, are more environmentally friendly than comparable cement mortars, and lime offers the added environmental benefit of absorbing carbon dioxide during the carbonation cycle.)

Lime mortar may have a promising future among environmentalists, but its 5,000-year history is still the best reason to seriously consider using it for your restoration project. Traditional lime mortars were long-lasting and durable, and they allowed buildings to move, to breathe, and to maintain proper moisture levels. While lime mortar can't compete with portland cement in terms of speed (setting time) and initial compressive strength (28 days), it's important to recognize that today's emphasis on high compressive strength and rapid set time is a modern fixation that tends to cause problems for historic buildings. Speed often means poor craftsmanship, and excessive strength can be destructive. It's worth noting that many generations of masons successfully built massive stone structures with lime mortars that had compressive strengths of only 300 psi to 500 psi. These low strengths make today's engineers nervous, but they were sufficient for some of the world's greatest builders and most magnificent structures, and the mortars have performed well for thousands of years.

When strength isn't in the numbers
Paradoxically, one of the most important strengths of lime mortar is its relative weakness — unlike conventional M, S, or N-type portland cement mortars, lime mortar has low compressive strength, and it's softer than typical masonry units. The question of strength is vitally important when dealing with historic brick and stone. Historic bricks are especially vulnerable because they were usually fired at much lower temperatures than modern bricks, which makes them much softer than what modern manufacturers are producing. As a result, today's portland cement, with its high compressive strength, is harder than historic brick and stone, so conventional cement mortars fracture historic masonry units during the freeze-thaw cycles.

Unlike lime mortar, portland cement mortar also inhibits the release of water and traps moisture in historic walls. Unable to exit through the joints, trapped water inevitably makes its way out through the path of least resistance — the historic masonry units. The result is cracked and spalled brick and stone, and these spalled units provide an easy access point for water, setting the proverbial stage for a destructive cycle of deterioration and damage. In contrast to rigid portland cement mortars, lime mortar cushions masonry units, and it even has autogenous healing properties, so small cracks in the mortar joints will self-heal and prevent water infiltration.

What's old is new again
The unfortunate reality is that masons have repointed many of the world's historic buildings with inappropriate mortar. In 1929, the Society for the Protection of Ancient Buildings even advised conservators to add a measure of portland cement into lime mortars for repointing and repairing Britain's historic buildings. (One can only speculate on how much regret this recommendation has created.) As a result of such advice, countless historic masonry buildings throughout England deteriorated more rapidly during the middle of the twentieth century than they had in all their centuries before. In the United States, guidelines published by the National Park Service in 1976 also advocated the use of portland cement in lime mortars for historic masonry buildings, which resulted in the same unfortunate deterioration patterns in America's historic masonry. These problems prompted a re-evaluation of restoration mortar ingredients and methods, and preservationists began to seriously question the value of high compressive strength mortars.

Scandinavian countries were at the forefront of the movement during the early 1960s, when architects and builders started looking at the masonry problems in newly installed restoration work. They created a detailed study of historic building mortars, concluding that the original mortars, although lower in compressive strength, were performing exceptionally well. They noted that the moisture balance was maintained in a lime mortar, which allowed the buildings to breathe, but was disrupted when a dense cement mortar was installed. Additionally, they concluded that mortar should be removed and replaced only where necessary, rather than over an entire wall.

Using lime mortar
So how do you know if a lime mortar is the right choice for your next project? There are some basic questions that you should ask as you initiate a restoration, and remember that accurate, thorough research and proper preparation will always pay off.

Know what you have. What is the current condition of the mortar joints and the masonry units? What are the wear patterns? Has it weathered unevenly? Be sure that you understand what you have before you decide how to replace it. Wear patterns often reveal ongoing issues that should be addressed prior to a restoration; leaky gutters, improperly installed flashings and other common problems should be fixed before you undertake a repointing or masonry repair project. Similarly, if you note varying wear patterns throughout the building — often caused by exposure and other environmental factors — be prepared to accept that these areas will require more frequent maintenance than more protected areas of the structure.

Know your building. How old is the building? Portland cement wasn't produced in the United States until 1871, and it was prohibitively expensive to import before then. It's a good bet that an American building constructed before 1871 didn't have any portland cement in the original formulation. From that point forward, portland cement was like most new technologies in that it took a while to gain acceptance. Traditional lime mortars were fading away by 1900, so the next thirty years or so are a mixed bag. Depending on the location of your building and local practices, the masons who built it may or may not have used lime mortar. And remember, even if you have specifications they won't necessarily tell you the whole story, since the masons may have adjusted the mortar formulation based on their own experience. If you're looking at a building constructed after 1900, a mortar analysis is often advisable, but note that straight lime mortars were seldom used in masonry construction by the 1930s.

Know your strength. How important is compressive strength for your project? Aside from major high-rises and other monolithic buildings, even "weak" lime mortar offers more than adequate compressive strength for most structures. Another very important aspect to the discussion of compressive strength is the wall design itself. Historic masonry walls were usually load-bearing masonry that carried the weight of the floors and roof on the outside brick and stone. These old walls were several wythes thick and ranged from 16 to 24 inches in width, as compared to today's veneer brick and stone masonry walls that measure a mere 4 inches. This modern veneer design is really a skin of masonry attached to a wood and or steel structural system that holds the floors and roof, whereas the older masonry walls had the very floor joists locked right into the brick walls.

Know your people. Do you have access to trained masonry contractors? If you're a masonry contractor, are you and your workers trained in lime mortar? Working with lime mortars takes a very different approach, and if you treat it like a conventional cement mortar the installation will almost certainly be compromised. Anyone can call himself or herself a restoration masonry contractor, so if you're hiring contractors be sure you ask for referrals for their lime mortar installations as well as training certifications. If you're a contractor, be sure you can provide this information yourself.

Like any long-lost friend, lime mortar requires a renewed commitment and a willingness to get reacquainted, but it's a small price to pay on behalf of our masonry heritage. If your historic masonry walls could talk, they'd thank you for it.

TM