MORTAR RECIPE FOR HISTORIC MASONRY REPAIR AND POINTING
Developed by the Kentucky Heritage Council: 1/81
Revised: 3/86
4 cups white, non-staining Portland cement
1 five gallon bucket hydrated lime
2 five gallon buckets sand
Enough water to form workable mix
NOTES
Repointing mortar for most historic buildings should ideally be composed only of sand and lime. A proportion of 1 part lime to 2 parts of sand is a useful starting point. The addition of Portland cement increases workability and achieves a whiteness of color. The National Park Service recommends that no more than 20% of the total volume of the lime and Portland cement-combined-should be Portland cement. Any greater amount of Portland cement increases the hardness of the repointing mortar to a potentially damaging degree.
Color matching should preferably be accomplished by using buff-colored sand such as that available from the Ohio River. Tinting powder should only be used if the color is otherwise unreproduceable. Its appearance is likely to change over time as the mineral ingredients leach from the surface of the mortar.
The mixture is also suitable for exterior stucco work over brick. It has been tested and used successfully in Kentucky for both pointing and stucco work. If possible the Portland cement content can be further reduced. Its chief function is to increase workability and slow setting-up of the mortar. Any amount of Portland cement can cause damage to the historic masonry.
A small amount of a bonding agent, such as Acryl 60, can be used in situations where adequate bonding is not certain.
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