Geoscience Reference
In-Depth Information
• Identifying defects.
• Diagnosing decay mechanisms and assessing the
level of deterioration.
• Identifying and measuring existing surface
treatments.
identification of crystalline minerals and decay reaction
products when their optical properties do not allow
definitive identification. Infrared spectroscopy is the
preferred technique to detect and identify the presence of
organic additives such as tallow or linseed oil.
P
ETROGRAPHIC EXAMINATION AND
COMPLEMENTARY TECHNIQUES
Petrographic examination of lime mortar, plaster, and
render is performed following a procedure given in
ASTM C1324 (ASTM International, 2005b). At the time of
writing a European standard was being developed but
was not yet available. Once a mortar sample is received
in the laboratory, an initial examination is conducted
with the unaided eye and low-power binocular
(stereoscopic) microscope. Combined with some simple
physical and chemical tests this gives a rapid and
economical provisional indication of the number of
material types present, the number of coats or layers, the
particle size distribution (grading), particle shape and
mineralogy of the aggregate, the presence of inclusions,
and the type and relative hydraulicity of the binder. The
colour of both the aggregate and the binder can be
classified by comparison with a Munsell™ colour chart
(Munsell, 1994).
Although initial visual and low-power microscopical
examination gives an indication of mortar ingredients,
definitive identification of the aggregate and binder
composition always requires more detailed high-power
microscopical examination in thin section. Thin section
examination can provide a great deal of information
regarding the ingredients used, their relative proportions,
overall texture, porosity, and condition.
Information gained from optical microscopy can be
supplemented/complemented by other microscopical and
analytical techniques. Where closer examination is
required, or the constituents are too fine to be resolved
by optical microscopy, SEM is invaluable, and integral
microanalysis systems provide elemental compositions to
aid in material identification. A particularly useful
technique is environmental SEM which allows
examination and analysis of freshly fractured surfaces
without any damaging sample preparation (Radonjic
et
al
., 2001a). Environmental SEM has even been used
successfully to study changes in lime specimens while
they are carbonating (Radonjic
et al
., 2001b).
Mineralogical analysis by XRD can be helpful for the
A
GGREGATE
The aggregate for lime mortars, plasters, and renders may
be derived from different types of geological deposits,
including natural sand resources (pit, river, beach, marine
dredged) and rock formations (quarried). Natural sands
often comprise rounded 'as-dug' particles, but may
include a proportion of more angular particles if oversize
materials have been crushed down to the required size.
Quarried rocks always have to be crushed down to
achieve the required particle size/grading and
consequently comprise particles that are quite angular in
shape. Occasionally, aggregates from more than one
source are blended together to achieve desirable overall
aggregate characteristics. The surface texture of aggregate
particles and the range of different particle sizes
represented (grading) have a large influence on the
properties and quality of the resulting mortar. Desirable
aggregates for modern brickwork mortars are usually
uniformly graded with spherical, rounded particles ('soft
sand') to aid workability (
279
), while a repair specification
for a historic render would require well graded aggregate
with irregular, subangular particles ('sharp sand') to
reduce drying shrinkage (
280
). Particle shape and grading
characteristics can be estimated through the microscope,
aided by comparison with standard charts (
130
and
133
).
Image analysis techniques are also being developed to
determine grading automatically (Mertens & Elson, 2005).
The mineralogy of the aggregate reflects its geological
origin and gives clues to its source. For example, in
southeast England the terrace sands of the River Thames
were widely used (
279
and
280
); less widely used were
crushed flints from the Chalk (
281
). River gravels derived
from sandstone country rock were used by the Romans
as aggregate in lime mortar for the western section of
Hadrian's Wall in northern England (
282
). Calcareous
aggregates were used in areas where it is the most
convenient source, for example in Oxfordshire, UK, where
the underlying geology comprises Jurassic limestone
(
283
). Calcareous shells may often be incorporated in
marine dredged or beach sands (
284
).
