Geoscience Reference
In-Depth Information
Occasionally these secondary leaching deposits may fill
or 'heal' open cracks within the mortar and, in effect,
repair the damage. This is referred to as 'autogenous
healing' (
303
).
High levels of sulfate can result from the leaching of
sulfate-rich inclusions (e.g. charcoal), adjacent clay brick
masonry or soil, or polluted masonry surfaces, to cause
the expansive formation of gypsum (calcium sulfate
hydrate). The expansive growth of gypsum and other
salts can cause cracking and other deterioration (
304
).
Lime mortars are poor conductors of heat but
nevertheless can suffer considerable damage when
exposed to fire. The effects of fire damage can include
discolouration, surface cracking, surface spalling, and
calcination of the lime binder. Optical microscopy can
contribute greatly to the assessment of fire-damaged
masonry by enabling the depth of damage to be
ascertained, prior to repair. Colour changes and other
features associated with heating enable thermal contours
(isograds) to be plotted at various depths from the outer
surface. For example, iron compounds present within
some aggregate particles will oxidize in the range 250-
300°C to give a pink to red colour (
305
). The nature and
depth of cracking associated with fire damage can be
investigated by fluorescence microscopy.
I
NTRODUCTION
Portland cement is a highly processed form of artificial
hydraulic lime, which hardens rapidly by hydraulic
reaction alone to form a binder chiefly comprising
calcium silicate hydrate gel. The formation of calcium
silicate hydrates during the hardening of cements results
in stronger, denser, and less permeable mortars than
when lime is used as a binder. Mortars and renders based
on Portland cement have been the norm for new building
since the 1930s. They are produced by mixing aggregate
with unhydrated Portland cement and adding only
sufficient water to hydrate the cement and provide
adequate workability. Mortars are either mixed on-site
or they are supplied to site as ready-to-use products. Site
mixing of mortar requires careful planning and
execution to ensure that a satisfactory material is
produced consistently. Ready-to-use retarded cement
mortars reduce the problems associated with site mixing
operations, such as inaccurate batching and colour
variation. Chemical admixtures such as plasticizers,
retarders, and air entrainers are commonly added to
Portland cement mortars to aid workability, increase
setting time, and improve durability.
The main applications of petrographic examination to
investigation of Portland cement-based mortars and
renders are:
• Determining the number of layers and their
thickness.
• Identifying the type and source of aggregate.
• Identifying the type of binder present.
• Identifying the presence/type of mineral additions
and pigments.
• Enabling correction of mix proportions determined
by chemical analysis through identifying and
quantifying the materials present.
• Assessing workmanship.
• Identifying defects.
• Diagnosing decay mechanisms and assessing the
level of deterioration.
S
URFACE FINISHES
Masonry surfaces have long been painted to improve
their appearance and durability. The earliest surface
coatings included water-based solutions of lime
(limewash), to which natural oils or glues could be added
to improve adhesion and reduce chalking (
306
). Natural
pigments were sometimes added to the limewash to
achieve a desired colour, or applied to damp lime plaster,
as in fresco painting (
307
). Other traditional paints
comprised natural and manufactured pigments with an
oil-based binder. Synthetic paints with plastic-based
binders were introduced in the 1930s. Traditional
coatings such as limewash are relatively permeable and
protect the wall while allowing it to breath and dry out.
Modern synthetic paints are often impermeable and while
they do provide external weatherproofing, they do not
allow dampness trapped within the masonry to dry out.
In such situations the weatherproofing paint can itself be
the cause of moisture-related decay, owing to leaching
and deposition of salts beneath the paint coating (
308
).
When investigating surface coatings, specimens are
routinely examined microscopically to determine the
number and type of layers present and to measure their
thickness.
P
ETROGRAPHIC EXAMINATION TECHNIQUES
Petrographic examination of Portland cement-based
mortar and render is performed following a procedure
given in ASTM C1324 (ASTM International, 2005b), which
is largely based on ASTM C856 (ASTM International,
2004). At the time of writing a European standard was
being developed but was not yet available. Once a sample
is received in the laboratory, an initial visual and low-
power microscopical examination is conducted to
determine the number of material types or layers, layer
