Geoscience Reference
In-Depth Information
P
HYSICAL AND CHEMICAL DETERIORATION
Masonry exposed to the atmosphere is subjected to
natural weathering and will eventually suffer
deterioration. Petrographic examination is a fundamental
tool used to screen mortars for evidence of decay caused
by leaching, salt attack, freeze-thaw damage, and sulfate
attack. Microscopical examination will detect signs of
matrix replacement, the presence of secondary deposits,
and filled or unfilled cracks, which may provide evidence
of deterioration and aid diagnosis of its causes.
The presence of secondary deposits indicates that
some degree of leaching has taken place, which may
have weakened the materials to some degree. As
examples of leaching, Figures
322
and
323
show
secondary deposits of portlandite and ettringite,
respectively, within air voids.
Portland cement mortars are susceptible to sulfate
attack, a reaction wherein calcium aluminate phases of
the Portland cement are converted to expansive ettringite
(calcium aluminium sulfate hydrate) by sulfate-bearing
solutions. Figure
324
shows Portland cement render from
a chimney, where sulfate attack had caused severe
cracking. Under certain conditions Portland cement
mortars are potentially susceptible to TSA. This
deleterious reaction requires a source of carbonate and
often (but not always) cold wet conditions, as well as
exposure to sulfates. This results in replacement of
cement hydrates by thaumasite (calcium carbonate
silicate sulfate hydroxide hydrate). Figure
325
shows an
example of TSA in masonry mortar, where thaumasite
small building sites, where tradesmen sometimes use
household washing-up liquid as plasticizer and
accidentally overdose the mix.
Some shrinkage of Portland cement-based mixes is
inevitable but this can be controlled by using appropriate
ingredients, careful curing, and, if necessary, the
provision of movement joints. Drying shrinkage can
cause a number of defects including debonding, cracking,
and surface crazing.
There are additional workmanship considerations for
external Portland cement-based renders. The type of
render system used should be carefully selected as being
appropriate for the background and exposure conditions
in accordance with standard guidance, for example BS
5262 (British Standards Institution, 1991). The
background may require special preparation in order to
achieve a satisfactory bond. According to current
practice, when applying render, individual coats should
not be thicker than 16 mm (Building Research
Establishment, 1976) as coats thicker than this tend to
deform under their own weight and are more prone to
drying shrinkage. Each successive coat should be thinner
and slightly weaker than the background and/or previous
coat, to minimize the risk of shrinkage and resultant
shear failure at wall/render or render coat interfaces.
Petrographic examination and chemical analysis for
mix proportions and sulfate content are often used in
combination to diagnose the reasons for apparent
weakness/friability, debonding, cracking, and causes of
surface problems like efflorescence and discolouration.
322
323
322
Air void (centre) filled with secondary deposits of
portlandite (brightly coloured) by leaching. Portland
cement:sand mortar comprising quartz natural sand
fine aggregate particles (grey), bound by uncarbonated
cement matrix (dark brown); XPT, ×150.
323
Air voids filled with secondary deposits of
ettringite (grey) by leaching. Portland cement:sand
mortar comprising quartz natural sand fine aggregate
particles (grey/white), bound by uncarbonated cement
matrix (dark grey/brown); XPT, ×150.
