Mortar Analysis and Composition Matching for Historic Mortars

Mortar Composition Analysis - Mortar Analysis

Analysis of Mortar Mix Composition

Analyzing mortar composition begins with a visual analysis through forms of magnification to identify certain properties about the mortar sample. Then, mortar samples are tested for their compressive strength as this will help in determining the ratios of the composition. At this point, the crushed mortar is broke down through an acid digestion mortar analysis or chemical analysis of the mortar to identify binder composition.

After the acid digestion is complete, various measurements are taken to identify the mortar composition which are referenced with the limiting range of compression and other details about the mortar sample. A final mortar composition formula cannot be confirmed at this point. To get an exact match of the mortar composition, an analysis of the sand must be complete.

The particle size, gradation of particle sizes, and the shape of the aggregates all change the compressive strength of the mortar. The ratio of binder to sand is also a determining factor.

To conclude the mortar analysis, we provide a visual inspection of the color, size, shape, and gradation of the sand, or you can add sand sieve analysis to the mortar composition testing for more precise information when testing historic mortar. Historic mortar analysis tends to require more detail for the report that is not always needed.

With and an exact ratio of sand to binder determined by weight loss, we are able to limit the range of results for determining the mortar formula and type to a more narrow variant. Then the sand gradation and shape are used to determine final results.

Historic Mortar Analysis

Testing Historic Mortar Samples

In commercial and historic mortar restoration, it is common place to get the mortar analyzed and matched through testing samples of the mortar. However, many of the testing methods are over priced and inaccurate. The two most common used or recommended testing methods for historic mortar analysis are the ASTM C-1324 and acid digestion method of testing hardened mortar. These two mortar testing analyses are the most and least expensive tests, but neither provide reliable results by themselves.

C-1324 is the most expensive type of historic mortar analysis which uses a spectrographic microscope to create a colorful image of the mortar sample. This image is then visually inspected and a best guess of proportions for the ratio is determined as the composition formula. There is a little more to it, but the accuracy of the results is limited to the interpretation of the analyst and their onsite experience as a mason. With their limited or likely non-existent experience in the field, they are unable to provide a quality interpretation of the testing data.

The acid digestion method for analyzing mortar composition is the least technical and least expensive mortar analysis, and is the most commonly used method for testing old mortar. Acid digestion analysis uses a balloon to capture the escaping gases released during analysis. The balloon swells and is measured to determine the quantity of the total binder materials. This test alone is not capable of accurate results because, the results are still an interpreted analysis of how much of the gas released is coming from each binder material. This factor alone is problematic because the sand aggregates tend to have a portion of cementious material that contaminates the results. Some sands used are entirely cementious materials. This doesn't happen often, but it does happen.

There is a better way to test the composition of mortar samples for historic mortar matching, and it is not much more expensive than the acid digestion test. This method is proprietary to AMR Labs and is becoming more excepted for this use due to the accuracy of the results. This testing method for matching mortar composition combines the results of some other astm testing methods such as the C-109 crushing analysis, acid digestion, E-11 sand sieve analysis, and some more scientific analyses where we collect additional data from some of these other testing methods for validations.

One example of this is the amount of weight loss during the digestion of mortar samples is cross referenced with the compressive strength and mortar type and using the specific gravity of the binder materials, we are able to validate the accuracy of the results. This is a limited example because there are other factors that change variables such as the size and shape of sand aggregates changes the compressive strength of the mortar as much as the ratio of sand to mix.

Matching Existing Mortar

Mortar matching through our complete mortar analysis and adding a detailed mortar color match provides a exact mortar formula for matching the existing mortar color and composition. With this complete formula, we can manufacture bags of custom matched mortar for your project. This is however limited to mortars using type S/SA lime as we currently do not have a supplier for the historic lime.

Historic Lime Mortar

In the past, we provided the mortar analysis report and the formula and Virginia Lime Works provided the manufacturing of the historic lime mortars. We have been told they are no longer around and we haven't found a suitable replacement as yet. Places we have found that may assist you if you need the historic lime mortar manufacturing are US Heritage Group and Lime.org.

Type S/SA and Historic Lime

One of the most common questions asked is the difference between historic lime and type S/SA lime in mortar. The simple answer is they are both lime, but the historic lime mortar uses a larger particle size and was sometimes not screened allowing large chunks to be mixed into the mortar. This is easily recognized by the large unmixed pieces of lime. Those that were screened are a little harder to identify, but the type S/SA lime uses a very fine screen, and tend to be used in harder mortar types and newer mortar more often. If your mortar requires type S/SA lime, we can provide the manufacturing of custom bags of pre-matched mortar.

Sand Analysis

Analysis of the sand used in the mortar samples is critical to matching the mortar. Sand aggregates compose the majority of the mortar, and in such change the color, texture, and strength of the mortar. It is important to identify the color, particle size and gradation, and shape of the aggregates to properly match the existing mortar. This can be done through a visual inspection for the cleaned sample, or for exact particle distribution you can add a sand sieve analysis to your mortar analysis testing.