I have an old historic building that needs some joints repointed. What mortar should I use?

Your old historic building needs a different mortar type than the mortar types used in modern masonry construction. Most historic buildings were constructed with bricks and mortar that are much softer and more permeable than modern bricks and mortar. The use of modern mortar to repoint soft masonry can result in long-term problems such as spalling and cracking of bricks, as well as moisture transmission problems. Prior to the introduction of Portland cement to the masonry industry in the early 1900s, all masonry was constructed with lime mortar. The lime used was not the same as the hydrated lime used in modern masonry, so historic mortars cannot be replicated by simply leaving out the Portland cement from your mortar recipe. There are several options for replicating the properties of old mortar, including mortar formulations with small amounts of Portland cement, and the use of small-batch craft limes available from Europe and some manufacturers in the United States. It is also important to use aggregates that match the original in sand type and in the aggregate particle size distribution, and to match the color. A mortar analysis that includes measurement of the aggregate gradation and the binder to aggregate ratio is helpful for designing an appropriate mix.
Guidance for the design of a mortar mix appropriate for your building can be found from many sources, including the National Park Service Preservation Briefs (www.nps.org), the Historic Scotland (www.historic-scotland.gov.uk ), and the Association for Preservation Technology. The American Society for Testing and Materials (ASTM) provides ASTM C 270, Standard Specification for Masonry Mortar, the standard for modern masonry mortar.