Selecting Proper Repointing Mortar

• There is no such thing as an “all-in-one” repointing mortar. Use these guidelines to select the right mortar for your specific job. 
• Use a pointing mortar that is weaker than the original mortar and surrounding masonry units (i.e. brick, stone, etc.)
• Use a pointing mortar that is more vapor permeable than the original mortar.
• Use a sand that is well graded, meaning that it has a good variation of fine, medium and course grains.
• The largest grains of sand should have a diameter roughly 1/3 the width of the mortar joint.

We rely on two types of mortar for most of our historic restoration work: natural hydraulic lime (NHL) and pozzolanic hydraulic lime (PHL). Both are hydraulic limes with pozzolans or impurities. This means that the mortar will set in two stages. The first set will occur when water is added and the second begins when the mortar is exposed to the atmosphere. The first set is what allows the mortar to set in wet conditions and gain strength more quickly.

 

Both are sold as dry bagged products, are flexible and breathable enough for historic restoration, and durable enough for extreme conditions.

 

 

NHL is categorized by the level of naturally occurring impurities – Feebly Hydraulic (NHL 2.0), Moderately Hydraulic (NHL 3.5), and Eminently Hydraulic (NHL 5.0). NHL 3.5 is best for exterior work in Chicago’s climate whereas NHL 2.0 is ideal for interior plaster work. NHL takes 20 days to cure and achieve strength.

PHL is a manufactured lime product and its pozzolan impurities are added during its manufacturing. It is available as Moderately Hydraulic (PHL 3.5), and Eminently Hydraulic (PHL 5.0). PHL 5.0 works best for Chicago’s climate. It usually takes around 7 days to cure and achieve strength.

 

 

You can purchase PHL and a bulk sand from our yard and blend them yourself, or you can purchase preblended PHL that already contains the aggregate. We recommend this option if you are unfamiliar with this product or blending mortar.

 

The other type of lime mortar is nonhydraulic lime, which is sold as lime putty. This is well suited for interior lime plaster as it is soft and very permeable, cures slowly and doesn’t withstand extreme weather or significant exposure. NOTE: “Lime putty” is often used as a generic term for lime mortar, but this is a misnomer; lime putty is in fact a specific type of nonhydraulic lime.

There are two types of mortar blends, property blends and proportion blends. Proportion blends are mixed on site and created by combining specific ingredients by proportion with one  another. Property blends are pre-mixed off-site to specific physical specifications such as strength and permeability.

 

 

SpecMix produces property blend mortars. This brand of mortar is easy to use and does not require a “damp cure”. It can also be custom tinted to match preexisting mortar. This product is favorable when a Type “N” or Type “O” mortar is required as, although the specific recipe is undisclosed, the mortar has been lab tested to fit the requirements of these types.

 

 

These specific types (N, O, etc.) vary in hardness levels. Although it is tempting to try and match the hardness of existing mortar or utilize a harder mortar as it may seem to last longer or be more stable, you should always use a mortar that is weaker than the original. This allows for more flexibility in the joints and increased vapor permeability thereby decreasing the risk of spalling or cracking.

This single-component, cementitious, mineral based mortar is designed for the restoration of limestone, sandstone, granite, bluestone, marble, concrete, terra cotta, and brick.  Unlike competitors’ products, Jahn has specifically formulated individual lines for each substrate . All Jahn repair mortars are vapor permeable and contain no latex or acrylic bonding agents or additives. This product is especially useful for stone patching or replication.

 

Only Certified Installers may purchase Jahn Repair Mortar. We offer a regular schedule of classes in the fall, winter, and spring to enable contractors, architects, and other professionals to obtain the required product certification. Our classes are two-day sessions and are discounted to $450.00 per person.