orson
September 8th, 2010, 05:22 PM
Many historic brick buildings were constructed with soft bricks. Repointing a soft brick or soft stone facade should only be done with hydraulic lime mortar in a mix that contains no Portland cement.
Unfortunately most masons are unaware of hydraulic lime mortar or if they are they may not know where to get it.
Here's the scoop from Jon Owens of Brookline Builders:
Brookline Builders Inc Natural Hydraulic Lime
Natural Hydraulic Lime vs. Hydrated Lime
Historic Restoration companies should be using natural hydraulic lime for all their restorations projects as opposed to hydrated lime and Portland. Natural hydraulic lime use as a primary binder in masonry, started fading slowly in the 1870's as Portland cement began to be introduced in America. Portland cement mortars had strength properties in great excess of hydraulic lime mortars and these new mixes had much faster setting abilities so that the work could be done more quickly.
Natural hydraulic lime was the binder of choice in all masonry construction for over 6000 years. Since almost all old historic buildings are subject to movement, hydraulic lime should be used as it has unique flexibility characteristics. An hydraulic lime mortar will move with the building. Through crystalline bridging, using the free lime content, the lime mortar will autogenously heal those fissures that occur. Hydrated lime Portland mortars don't have the same free-lime characteristics and will not heal themselves as a result of the high burning temperatures used in the production process. The breakdown of old buildings that were repaired using a Portland mixture can start in as little as three years. The Portland and hydrated lime mixture will actually chemically react to the original hydraulic lime mortar.
Although the breakneck speed in which we build in modern times has allowed us to accomplish some huge projects that are meant to last, this speed mentality shouldn't be applied to historic restoration projects. When you introduce the hard, stiffness of modern Portland into a structure that was built using the soft hydraulic lime, the rigid Portland and the soft brick or mortar don't expand and contract at the same rate. You are inviting disintegration of the original mortar or brick and even the collapse of a wall. Your restoration project suddenly became a slow demolition project.
Years ago, natural hydraulic lime was made on the construction job site as the building was being built. Modern times this long and potentially dangerous process has been eliminated. There are very few places to purchase natural hydraulic lime today so most historic restoration mason's use a heavy hydrated lime/ Portland/sand mix. We have used 8 parts of sand, 2 parts lime and as little as 1/2 part Portland. This is the second best choice after natural hydraulic lime. Natural hydraulic lime is still made by St. Astier Natural Hydraulic Lime Company in France. It is available locally thru one distributor.
This info is not meant to be an inexhaustible resource, Brookline Builders just wants to help educate the general public on the importance of proper historic restoration techniques. Techniques that contribute to the longevity of the building not quick fixes that actually accelerate deteration . For more information on natural hydraulic lime mixes please see our masonry mixes page. For links for more info see our links page.
Unfortunately most masons are unaware of hydraulic lime mortar or if they are they may not know where to get it.
Here's the scoop from Jon Owens of Brookline Builders:
Brookline Builders Inc Natural Hydraulic Lime
Natural Hydraulic Lime vs. Hydrated Lime
Historic Restoration companies should be using natural hydraulic lime for all their restorations projects as opposed to hydrated lime and Portland. Natural hydraulic lime use as a primary binder in masonry, started fading slowly in the 1870's as Portland cement began to be introduced in America. Portland cement mortars had strength properties in great excess of hydraulic lime mortars and these new mixes had much faster setting abilities so that the work could be done more quickly.
Natural hydraulic lime was the binder of choice in all masonry construction for over 6000 years. Since almost all old historic buildings are subject to movement, hydraulic lime should be used as it has unique flexibility characteristics. An hydraulic lime mortar will move with the building. Through crystalline bridging, using the free lime content, the lime mortar will autogenously heal those fissures that occur. Hydrated lime Portland mortars don't have the same free-lime characteristics and will not heal themselves as a result of the high burning temperatures used in the production process. The breakdown of old buildings that were repaired using a Portland mixture can start in as little as three years. The Portland and hydrated lime mixture will actually chemically react to the original hydraulic lime mortar.
Although the breakneck speed in which we build in modern times has allowed us to accomplish some huge projects that are meant to last, this speed mentality shouldn't be applied to historic restoration projects. When you introduce the hard, stiffness of modern Portland into a structure that was built using the soft hydraulic lime, the rigid Portland and the soft brick or mortar don't expand and contract at the same rate. You are inviting disintegration of the original mortar or brick and even the collapse of a wall. Your restoration project suddenly became a slow demolition project.
Years ago, natural hydraulic lime was made on the construction job site as the building was being built. Modern times this long and potentially dangerous process has been eliminated. There are very few places to purchase natural hydraulic lime today so most historic restoration mason's use a heavy hydrated lime/ Portland/sand mix. We have used 8 parts of sand, 2 parts lime and as little as 1/2 part Portland. This is the second best choice after natural hydraulic lime. Natural hydraulic lime is still made by St. Astier Natural Hydraulic Lime Company in France. It is available locally thru one distributor.
This info is not meant to be an inexhaustible resource, Brookline Builders just wants to help educate the general public on the importance of proper historic restoration techniques. Techniques that contribute to the longevity of the building not quick fixes that actually accelerate deteration . For more information on natural hydraulic lime mixes please see our masonry mixes page. For links for more info see our links page.