What is mortar analysis?
Mortar analysis is the scientific investigation of a variety of cementitious materials such as mortar,
stucco, chinking, concrete, and plaster. For the purposes of this discussion all are referred to as “mortar” even though they differ
from each other in very significant ways. Mortar analysis is the effort to determine the components of mortar. Unfortunately, no known
form of analysis is able to determine relative proportions of the components with any precision. There are two primary types of analysis
being used at the present time, as follow:
- Acid digestion
Acid digestion is a technique that dates from at least the mid-nineteenth century. It is a relatively simple procedure in which
hydrochloric acid is used to dissolve the binding component of the mortar which is typically lime and/or Portland cement. Insoluble
components such as sand and fines (dirt associated with the sand and/or coloring agents) remain. The sand can then be sieved to
determine ratios of sand grain sizes.
Some individuals have (unsuccessfully) attempted to calculate exact
proportions of the components using this technique. At this time there is no accurate means of determining proportions, although experienced
mortar analysts can provide general recommendations, especially concerning the probable components.
Advantages to this
technique include the following:
- It is simple and, therefore, inexpensive relative to other techniques.
- It provides an
excellent set of data regarding the sand component of the mortar as well as an actual sample of the sand.
- It provides an excellent
sample of any coloring agents, whether they are fines or actual granular components which appear in the sand sample.
comparative analysis the sand serves as a type of DNA so that samples which have sand which is closely similar in gradation and color to
other samples can be reliably understood to be of the same mortar. Contrariwise, if the sand does not match between samples then one can
reasonably conclude that the mortars were applied at differing times.
Disadvantages to this technique include the following:
can be time-consuming as it is a wet process requiring filtering of the fines. Portland cement mortar samples can take upward of a week to
filter. Lime mortar samples typically require two days to process.
- If the sand is composed of calcium carbonate or other acid-soluble particles (which is quite unusual) the results will be skewed.
If the binder is not acid-soluble, as with gypsum in many plasters, the gypsum will appear typically in the fines or in the finer elements
of the sand.
- If there is no binder in the mortar, as in many mud chinkings, then the dirt, or mud, will appear in the fines.
- It does not provide an accurate ratio of binder to sand.
- Precise identification of the mineral content of the sand is impossible.
- Intrumental techniques
A vast array of highly technical analytical instruments are currently available to the construction industry, many of which are routinely applied to mortar analyses.
Commonly employed instruments include atomic absorption spectrometers, which measure elemental composition, and X-ray diffractometers, which
identify mineralogical components. These machines are capable of producing very precise data with excellent resolution, but there is no
analytical instrument that can identify mortar components and determine proportions. This information can only be arrived at through
interpretation by an experienced materials scientist.
Who does mortar analysis?
There are several types of venues where mortar analysis is performed, as follow:
- Independent architectural conservators
These individuals typically employ the acid digestion technique for their analysis. Their experience does vary considerably and
it is recommended that before choosing one, a resume of projects and experience be requested. The advantage to using an architectural conservator
is that it tends to be quite cost-effective and answers the basic questions concerning mortar. As noted above, however, the acid digestion technique
does have its limitations.
- Independent material testing laboratories
There are a number of independent testing laboratories which have the equipment and personnel to
perform the highly technical analysis described above. As with architectural conservators, they provide clear and unbiased results.
Sometimes, however, as with any such reports, the average layman may find them difficult to interpret and understand so that care should be taken
on the part of the laboratory to write the reports in understandable prose. One disadvantage to using these techniques, as noted above, is the
relatively high cost.
- Industry laboratories
Both the lime industry and the Portland cement industry have testing facilities and most of the current lime suppliers are able to provide mortar
analysis using the acid digestion technique. They then use the results to produce new mortar for the customer. The advantage to this approach is that
it tends to be faster than working with independent laboratories because the results are produced in-house. The disadvantage is that there frequently
is a strong bias toward identifying mortar as containing the product of the manufacturer. For example, if a historic mortar contained both lime and
Portland cement as the binders, it is not unreasonable that a lime manufacturer will simply use lime as the binder of the new mortar and vice versa for
a Portland cement supplier. Another disadvantage is that most manufacturers of lime putty mortar have a standard stock of sand on hand and very rarely
go to the trouble to actually match the historic sand of the mortar.