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An Introduction to Lime Mortar

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Although building limes were a commodity building material in America as recently as 90 years ago, they are now considered specialty products and are little understood by our industry.  Over the last 20 years, however, there has been a marked resurgence in the use of lime binders for restoration work in the United States.  As a masonry material supplier specializing in historic restoration, we regularly field questions from architects, contractors, and building owners who recognize that they should use a lime mortar for repointing historic masonry, but are unsure which type to use.  Customers often start by asking for “lime putty”, however, in reality this is just one of several types of traditional lime binders rather than an all-encompassing term.

Lime Putty

Lime putty describes the physical state of quicklime after being hydrated with excess water and then aged a minimum of six months to produce a very smooth putty. Costly mistakes can be made by incorrectly referring to all limes as “lime putty” and not carefully designating the correct type of lime during the process of material selection.

Well-aged lime putties are highly workable and often fantastic binders for interior plasterwork.  However, because these putties rely solely on carbonation  for strength gain, they can be unsuitable for exterior use in harsh climates where short-term durability is necessary.  It is also important to note that freshly applied pure lime putties require extreme care during their initial carbonation process and must remain damp for a period of several days. If you do not have experience working with pure lime putties and/or are in a climate with freeze-thaw cycles, you may want to consider using a more forgiving hydraulic lime.

Hydraulic Lime

Natural Hydraulic Lime

Natural hydraulic lime mortars continue to be extensively used in Europe. Domestically, all three traditional types of NHL mortars are available as bagged hydrated powders. HFS supplies St. Astier, which is mined in France.

  • Natural Hydraulic Lime 2.0 is the least hydraulic and weakest type of NHL binder.  It can be suitable for repointing soft masonry or used as a finish coat plaster.
  • Natural Hydraulic Lime 3.5 performs well as a general-purpose re-pointing mortar in most applications and also functions well as a binder in scratch and brown coat plasterwork.
  • Natural Hydraulic Lime 5.0 is the most hydraulic and strongest of the NHL mortars and is particularly useful in masonry exposed to frequent moisture.

Pozzolanic Hydraulic Lime

In addition to  these three types of NHL mortars, two classes of artificially hydraulic limes are domestically produced. These are Pozzolan Hydraulic Limes (PHL) 3.5 and 5.0, the latter being the strongest and most hydraulic. HFS supplies Niagara PHL by Graymont.

A Note About Hydrated Lime

Many people inquire about using hydrated lime, or mason’s lime, which is more widely available than aged lime putty or hydraulic limes. The most common hydrated lime used in masonry construction today is Type S (“S” was originally used to designate it as “special”). Type S offers ease of mixing, high bond strength, high plasticity, and is a key component in cement-lime mortars. It may be tempting to try to repoint with this inexpensive and easy-to-find lime, but like lime putty hydrated lime is nonhydraulic and relies on carbonation, so it will not stand up to the elements.