(As published in Our Berkshire Green)
We think stone and brick are green building materials. Stone does require a certain energy to quarry, finish, and transport. The supply of stone is endless, and stone is local – almost everywhere. In New England we have a long tradition of building with fieldstone, which is plentiful and may be harvested rather than quarried. Brick requires energy to kiln fire and to transport.
Both brick and stone last for hundreds of years and can be recycled. Recycled brick and stone require only the energy to transport. They are nontoxic. So far, so good. So, is masonry green?
It depends on the cement. After Portland cement was developed in England around 1830, it began to supplant natural lime mortars. By the mid 20th century, lime mortar had disappeared in the United States. Portland cement is hard and resists water and requires an intense and toxic manufacturing process. Polluters have for many years used cement plants as convenient “burn barrels” for PCBs, mercury, and other hazardous materials. Most of our cement manufacturing plants in the United States are owned by the Swiss, who run them at a level of pollution, including mercury that would not be allowed in Switzerland.¹
For 6,000 years, in almost every culture, brick and stone were laid in lime mortars. Lime mortar consists of lime and sand only. Burning limestone or shells removes air and water. The reintroduction of water creates a lime putty that is mixed with sand to make a mortar. When bricks or stones are laid in this mortar it absorbs air slowly and cures to the hardness of limestone. Lime putty was simple to manufacture in a wood-burning kiln. Kilns were everywhere (Lime Kiln Road in Sheffield, Massachusetts, for example) that limestone or oyster shells were available.
In the late 20th century, architectural preservationists in Scotland and England began to see that earlier 20th-century repairs to historic buildings using Portland cement-based mortars were creating bigger problems than the ones they were being used to solve. Portland cement mortars were too brittle. They didn’t breathe, they trapped moisture in walls, causing damage to stone and brick and, as trapped water grew moldy, it caused sickness to the occupants of buildings.
These observations led to a movement among preservationists to return to the ancient lime mortars and rediscover the lost formulas of lime manufacturing. People started reading Vitruvius for his recipes. In the late 1990s and the early 2000s the interest in returning to lime crossed the Atlantic.²
Jimmy Price (virginialimeworks.com), a Virginia brick mason working on Thomas Jefferson’s “other house” (Poplar Forest, near Lynchburg, Viginia), built a wood-fired kiln and began burning lime in order to create lime mortar true to Jefferson’s letters to his mason.
My first encounter with lime mortar was in 1997 when I visited Jimmy to work on the Poplar Forest project and to learn about this new / old material. Since then I have used lime mortars in historic work. In historic work our rule is simple: If it was built with lime mortar, it should be repaired and restored with lime mortar.
But a funny thing happened. The more we all learned about this old material, the more we realized it was the green answer to problematic Portland cement-based mortars. Lime breathes. Architects and engineers have developed expensive and complex solutions to retrofit or cure unhealthy buildings when the simple solution is to build in the first place with a material that breathes.
Lime mortar performs as well as or better than Portland cement mortars. The Coliseum in Rome, Chartres Cathedral in France, and the Roman aqueducts, were all built with lime mortar and are still standing. Lime mortar’s compressive strength develops over 69 days rather than 28 for Portland cement mortars. Lime’s compressive strength can equal or surpass that of Portland cement mortars, but lime never gets brittle and its flexibility allows it to withstand forces that would shatter Portland cement mortars. The Chartres Cathedral in France was near enough to the battlefields of World War I to absorb continuous shocks from the artillery barrages. If the church had been built with brittle modern mortars, it would have crumbled. Instead the ancient lime mortars had the flexibility to keep the building intact.
But the KILLER APPLICATION of lime mortar? Lime mortar absorbs CO2. Absorbs it and transforms it. A house using 9600 bricks and 1000 square feet of stone would absorb 640 lbs – over a quarter ton – of CO2. It’s like planting trees. So, imagine a building system that is low cost, produces healthy buildings, and absorbs CO2. Jimmy Price and VLW are working on a wall system they call Enviro-Ment. This system consists of lime building blocks filled with lime insulation and plastered inside and out with lime plaster and painted with lime-based, non-VOC (volatile organic compounds) natural pigment paints and binder pigment paints. This is a wall system that has a high R value, will not burn, breathes, and absorbs CO2. Imagine if the millions of concrete blocks laid each day were lime blocks absorbing CO2!
So, in summary, lime mortar withstands mother nature, eliminates carbon dioxide, has unsurpassed indoor air quality, and reduces energy consumption.
¹ The Nation. The Sultans of Cement by Jock Ferguson, August 3, 1992. Cement Companies Go Toxic, also by Jock Ferguson, March 8, 1993.
² The best book on the subject of lime mortars is Preparation and Use of Lime Mortars. Published by Historic Scotland, edinburgh 1995. ISBN 0 9517989 3 6
Further Reading: Structure Magazine, May 2005. Page 26: Properties of Lime Mortar, by Margaret Thomson. Pages 32-55: Historic Masonry Restoration through the Eyes of a Master Stonemason, by Mark Mendel.