Specifying Compatible Mortar

According to the National Park Service Preservation Brief #2 “Repointing Mortar Joints in Historic Buildings” a compatible mortar is characterized as:

“…one that matches the historic mortar as closely as possible, so that the new material can coexist with the old in a sympathetic, supportive, and, if necessary, sacrificial capacity. The exact physical and chemical properties of the historic mortar are not of major significance as long as the new mortar conforms to the following criteria:

• The new mortar must match the historic mortar in color, texture, and tooling. (If laboratory analysis is undertaken, it may be possible to match the binder components and their proportions with the historic mortar, if those materials are available.)
• The sand must match the sand in the historic mortar. (The color and texture of the new mortar will usually fall into place if the sand is matched successfully.)
• The new mortar must have greater vapor permeability and be softer (measured in compressive strength) than the masonry units.
• The new mortar must be as vapor permeable and as soft or softer (measured in compressive strength) than the historic mortar. (Softness or hardness is not necessarily an indication of permeability; old, hard lime mortars can still retain high permeability.)”

Color matching of mortar is achieved by matching the aggregate, binder, and pigment, if applicable. We have over 150 pigment colors in our lab and the capability to match nearly any color. We also keep an extensive onhand stock of pigments. We use Dynamic Color Solutions (DCS), which are pure mineral pigments designed specifically for concrete and mortar.

DCS Datasheet

DCS Technotes

DCS Test Data

The texture of the original mortar is a combination of the weathering patterns, the binder, and the original sand used in the mortar. While we do not recommend cleaning lime mortar with any strong chemicals or acids, the mortar can be “aged” by doing a light wash that takes off the surface layer of cream to expose the aggregate. To achieve an aged texture, thoroughly wet the wall and then wash with 1 part VanaTrol diluted to 10 parts water. Allow it to dwell for a few seconds before rinsing. As with any chemical application, always test first in an inconspicuous location and adjust dwell time accordingly.

When it comes to replicating the original texture, one of the most common mistakes installers make is using too much water. Aside from causing shrinkage cracking and smearing on the masonry units, it will yield a slicked appearance that is inconsistent with the original. Lime mortar should be the texture of brown sugar during repointing, and should never be installed using a grout bag.

Tooling: Joint profiles are one of the most important factors in achieving a consistent match. Here are some of the most common joint profiles:

Note: while concave and convex joints are widely used in new construction, they were not generally used in historic masonry construction.

Matching the sand not only ensures a closer visual match but also provides more comparable performance characteristics. Sand should be clean, sharp, and well-graded with a good variety of fine, medium, and coarse grains. The largest grains of sand should have a diameter roughly 1/3 the width of the mortar joint.

Most mortars were made with local sand, sometimes even from the job site. When restoring the building those sources are often unavailable now and matching sand can be quite challenging. Replicating the original as closely is always advised, even if it complicates the project since this is critical to achieving a compatible visual appearance and consistent performance.

Note: We offer custom mortar color and aggregate matching. We also retain samples of our past jobs, and we have hundreds of available formulas. If we have one that matches already we do not charge a color matching fee.

Specifier Sample Submission Form

Ensuring that a replacement mortar will be sympathetic to the historic masonry is key to a successful repointing project.  Our goal is to make sure that the new mortar is a sacrificial element, meaning that is the element that slowly deteriorates and is periodically replaced—NOT the masonry units. This deterioration occurs because the mortars should be handling the stress of building movement and moisture. Mortar should transfer moisture out of the wall system wall assembly and absorb the pressure of freeze-thaw cycles and building movement, and for this reason, the mortar should be more vapor permeable and “softer” than the original mortar and the masonry units.

Many historic mortars can’t be replicated exactly due to changes in material production and availability since that mortar was originally made, and this is particularly true of older cement-lime mortars. As the Portland cement industry has matured over the past century manufacturers have been grinding cement clinker increasingly fine to achieve a faster hydrating, more densely packed, and higher strength material.  Traditional Portland cement was ground at a coarse grind or blane, and it is common practice to use the blane size of relic cement clinker particles to determine its approximate age.  Historically residual Portland cement clinkers were fairly coarse, with some up to 350 microns. For reference, 150 microns is considered “coarse” and the coarsest particles found in modern cement are typically 100 microns. The smaller the blane, the faster the cement will hydrate and the more densely packed the cement will be, resulting in stronger concrete.

Another common historic mortar that we can’t practically recreate today are “hot mixed” lime mortars. These extremely common mortars were site-mixed using quicklime, which produces an exothermic reaction when water is added.  These were highly workable, inexpensive mortars and many have performed well for many decades. However, these quicklimes were far less pure than the modern lime mortars made today and many contained impurities that had a hydraulic effect. Modern quicklime is much purer, and without the impurities, the mortar doesn’t achieve an initial hydraulic set and instead very slowly carbonates.  This process takes so long that in colder climates nonhydraulic mortar is likely to sustain frost and freeze damage and is susceptible to failure.

Specifying a Sympathetic Mortar

If testing data is available for the mortar and/or masonry units we will use that data, along with qualitative information about the building, to recommend a replacement mortar. If testing is unavailable, then we will review the mortar sample and make recommendations in the context of the age, construction and material type, and any concerns about the existing masonry conditions.

In order from nonhydraulic to high hydraulicity:

1. Lime Putty
2. Natural Hydraulic and Pozzolanic Hydraulic Lime
3. Portland Cement Lime

Preblended mortar should be considered when:

• Site limitations prevent onsite mixing.
• The mortar is pigmented and will require consistent, precise blending to achieve the desired appearance.
• Locally available sand is not an acceptable match.
• Labor costs associated with blending need to be minimized.

Mockups should be required to 1) confirm that the winning bidder can accurately replicate the color, texture, and joint profile of the original and 2) establish the approved standard of workmanship for the project. The specifier should make it clear that the contractor will be held accountable to this standard of work for the entire project. Mockups should be at least 3’ x 3’ and the contractor should also demonstrate how they will remove existing mortar without damaging the historic masonry units.

When we supply preblended custom mortar we provide sample material and a cured channel so that the contractor can install a mockup panel. The channel is laboratory cured so that the specifier can compare the mockup panel to the color sample to evaluate the accuracy of the onsite color.