Methods for analyzing mortars can be divided into two broad categories:
- wet chemical and
Many laboratories that analyze historic mortars use a simple
wet-chemical method called acid digestion, whereby a sample
of the mortar is crushed and then mixed with a dilute acid. The
acid dissolves all the carbonate-containing minerals not only
in the binder, but also in the aggregate (such as
oyster shells, coral sands, or other carbonate-based materials),
as well as any other acid-soluble materials. The sand and fine-grained
acid-insoluble material is left behind. There are several variations
on the simple acid digestion test. One involves collecting the
carbon dioxide gas given off as the carbonate is digested by the
acid; based on the gas volume the carbonate content of the mortar
can be accurately determined (Jedrzejewska, 1960).
Simple acid digestion methods are rapid, inexpensive, and easy
to perform, but the information they provide about the original
composition of a mortar is limited to the color and texture of
the sand. The gas collection method provides more information
about the binder than a simple acid digestion test.
Instrumental analysis methods that have been used to evaluate
- polarized light or thin-section microscopy,
- scanning electron microscopy,
- atomic absorption spectroscopy,
- X-ray diffraction, and
- differential thermal analysis.
All instrumental methods require not only expensive, specialized
equipment, but also highly-trained experienced analysts. However,
instrumental methods can provide much more information about a
mortar. Thin-section microscopy is probably the most commonly
used instrumental method. Examination of thin slices of a mortar
in transmitted light is often used to supplement acid digestion
methods, particularly to look for carbonate-based aggregate. For
example, the new ASTM test method, ASTM C 1324-96 "Test
Method for Examination and Analysis of Hardened Mortars"
which was designed specifically for the analysis of modern lime-cement
and masonry cement mortars, combines a complex series of wet chemical
analyses with thin-section microscopy.
The drawback of most mortar analysis methods is that mortar samples
of known composition have not been analyzed in order to evaluate
the method. Historic mortars were not prepared to narrowly defined
specifications from materials of uniform quality; they contain
a wide array of locally derived materials combined at the discretion
of the mason. While a particular method might be able to accurately
determine the original proportions of a lime-cement-sand mortar
prepared from modern materials, the usefulness of that method
for evaluating historic mortars is questionable unless it has
been tested against mortars prepared from materials more commonly
used in the past.