Understanding and Using Historic Lime Mortar

Marissa Jude CartocetiMarissaJude@gmail.com

ILS 501-S70

Why Use Lime?

Lime mortar has been used in building construction for thousands of years and has proven to be an effective and durable building material.

Lime mortar possesses many important advantages and benefits over other building materials used for similar applications.

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Benefits of Lime Mortar

Lime mortar is a relatively soft material that allows for the shifting and settling of buildings without damage to masonry units.

Lime mortar is permeable and allows moisture to evaporate.

Lime mortar possesses a self-healing quality known as autogenous healing.

Lime mortar is a green building material.

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Lime Mortar is Soft

Historic buildings, particularly those constructed before 1920, often utilized masonry units of soft brick or stone. Any movement could result in damage to the weakest part of the structure. If the mortar mix that was used had a higher compressive strength than the masonry, damage would occur to the masonry units. Therefore, it was essential that mortar be the sacrificial material as it was relatively easy to repair. Using lime mortar made this possible, and its use today continues to stabilize and preserve these historic structures.

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Lime Mortar is Permeable

Lime mortar is permeable and allows moisture to migrate out of masonry units and evaporate. This is significant because trapped moisture can cause damage and disintegration to masonry units over time, especially if the masonry units are exposed to freezing and thawing cycles. Hard or dense mortar mixes, such as those containing high levels of Portland cement, are far less permeable and do not allow moisture to escape as easily.

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Lime Mortar is Self-Healing

As lime mortar cures, moisture evaporates while carbon dioxide is absorbed. This reaction continues for years and turns the lime back into calcium carbonate. The result is that the mortar gains strength as it ages. This reaction also describes why lime mortar possesses a self-healing quality known as autogenous healing. When small cracks occur in the mortar, hydrated lime reacts with carbon dioxide to form calcium carbonate crystals. These crystals grow in the cracks and effectively reseal them. This quality reduces water penetration and increases the durability of the mortar.

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Lime is Green

Lime is manufactured at lower temperatures than cement, requiring less energy and giving off less carbon dioxide emissions.

Lime mortar absorbs carbon dioxide from the atmosphere as it cures.

Lime mortar is recyclable and biodegradable:Unlike cement, old lime mortar can be removed

relatively easily from brick and stone so that these materials can be reused.

Lime mortar can also be recycled for fertilizer. 04/19/23 7Understanding and Using Historic Lime Mortar

Repointing Foundation Walls

The Process

Supplies

Type "S" dry hydrated lime Water 5-gallon buckets Drill with paddle attachment Goggles and gloves Clean sand comprised of

various particle sizes (it should match the original in color and texture)

Coffee can and spatula Hand chisels Spray bottle Mortar pan Trowels

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Making Lime Putty

Wearing goggles and gloves, mix the hydrated lime with water in a 5-gallon bucket until it reaches the consistency of cream cheese. Use a strong drill with a

paddle attachment to make the mixing easier and add the lime and water in small amounts at a time.

Smooth the putty to make it level and pour approximately ½ inch of water on top to cover.

Secure a lid on the bucket and let the lime putty age for approximately 8 weeks.

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Preparing the Wall

Hand chisel and vacuum out loose and crumbling mortar from the joints. Remove the loose

mortar to a depth of approximately 2-3 inches from the face of the stone, making sure the joints are clean and the backs are square.

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Preparing the Lime Mortar

After the lime putty has aged, mix it with sand to prepare the mortar.  Traditional lime mortar was composed of lime putty and

an aggregate such as sand, but sometimes other ingredients such as clay, shells, pigments, or animal hair were added. You should try to match the ingredients to make the lime mortar as historically accurate as possible.

Use a coffee can and spatula to measure out the

ingredients utilizing the recipe of 2.5 parts sand to 1 part putty (by volume).

Using a trowel or your hands, mix the putty and sand in a mortar pan until it reaches the consistency of cookie dough.

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Preparing the Lime Mortar

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Dampening the Wall

Using a spray bottle filled with clean water, spray the wall thoroughly.

Make sure all of the joints are damp, but with no standing water present.

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Repointing the Wall Using a trowel or your fingers, press the mortar into the joints.

Make sure that you do not pack the mortar so thick that it

will not cure properly. For areas more than 1 inch deep, repoint in layers to help

minimize shrinkage. Let each layer cure to thumbprint hardness before

applying the next layer.

When the final layer of mortar reaches thumbprint hardness, tool it to match the existing mortar.

For the next few days, make sure to periodically spray the wall with a fine mist of water so that it will not dry out rapidly.

Any leftover mortar can be saved. Place the leftover mortar in a bucket with a wet rag placed directly on the mortar and a lid securely sealing the bucket.

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Repointing the Wall

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Making and Using Lime Wash

Foundation walls can be finished with lime wash if so desired. Lime wash will give the walls a bright white appearance depending on the number of coats used.

Lime wash can be made very easily by thinning lime putty with water until it reaches the consistency of whole milk. It is best to brush on a number of thin coats, letting each coat dry thoroughly before applying the next, to ensure maximum adhesion and durability.

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Finished Wall with 1 Coat of Lime Wash

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Credits

This instructional guide was created by Marissa Jude Cartoceti to help homeowners feel comfortable in making and using lime putty and mortar for their own historic applications. This guide reflects the research and process that Marissa used to repoint the foundation of her own 1830 home.

For more information on historic mortar mixes and repointing, visit:http://www.nps.gov/hps/tps/briefs/brief02.htm

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Credits

All photographs taken by Marissa Jude Cartoceti and Adam Thomas Lehmacher, 2011.

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