Geoscience Reference
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readily observed in thin section. The quantity of large
(>1 mm diameter), irregular-shaped entrapped air voids
is estimated to assess the effectiveness of compaction at
placement. Small (1 mm to 5 μm diameter), spherical air
voids are entrained in the mix for a number of reasons
and an abundance of such air voids may indicate the use
of air entraining (surface-active) or plasticizing (water-
reducing) admixtures. The abundance of micropores
(defined arbitrarily as being <5 μm across) and therefore
microporosity, can be assessed by fluorescence
microscopy of thin sections impregnated with resin
containing fluorescent dye.
The hardening of nonhydraulic lime mortar by
carbonation is a delicate process dependent on
temperature, moisture, layer thickness, and pore structure,
the properties of the background, and the presence of
carbon dioxide. Consequently, careful control of the
setting conditions is crucial to successful lime work. The
setting of nonhydraulic lime depends on the absorption of
carbon dioxide from the atmosphere but also requires the
presence of some moisture. The calcium hydroxide
(Ca(OH) 2 ) combines with excess carbon dioxide (CO 2 ) to
form calcium bicarbonate (Ca(HCO 3 ) 2 ), which decomposes
on evaporation to form crystals of calcium carbonate
(CaCO 3 ). The moisture (H 2 O) helps to dissolve the next
particle of calcium hydroxide, forming a saturated
solution and putting it into a condition to take up a
molecule of carbon dioxide. This, in turn, repeats the
action already described and crystals of calcium
carbonate are formed (Swallow & Carrington, 1995):
Ca(OH) 2(s) + CO 2(g)
Ca(HCO 3 ) 2(s)
Ca(CO 3 ) 2(s)
CaCO 3(s) + H 2 O (l) + CO 2(g)
Rapid drying out can occur in hot weather, on
unprotected work or if the background is of inappropriate
type or too dry. This can result in weak and friable lime
mortar/render that has failed to carbonate. Nonhydraulic
limes can also fail to carbonate during cold weather
conditions, leaving them susceptible to weathering by
frost and driving rain. Determination of the degree of
carbonation (including partial carbonation) can be
achieved definitively by observation in thin section.
Rapid drying can also cause plastic shrinkage cracks that
are observed microscopically as distinctive tension
gashes ( 301 ).
P HYSICAL AND CHEMICAL DETERIORATION
Microscopical examination is eminently suited to
screening for evidence of decay caused by leaching, salt
attack, freeze-thaw damage, and, for mortars including
Portland cement, sulfate attack. Signs of matrix
replacement/recrystallization, the presence of secondary
deposits, and the presence of filled or unfilled
cracks/microcracks may provide evidence of
deterioration caused by deleterious reactions.
Excessive leaching of masonry by moisture movement
can dissolve away the binder within mortar with
consequent loss of strength ( 302 ). Calcium carbonate
leached out of lime binder may be redeposited in voids
and cracks within the mortar or onto external surfaces.
301
302
301 Lime render exhibiting tension gashes (shown
yellow) caused by rapid drying. Fine aggregate
particles appear white and lime binder is shown
brown; PPT, ×35.
302 Severely leached medieval lime mortar from a
castle. Fine aggregate particles appear white, lime
binder appears brown, and air voids are shown
yellow; PPT, ×35.
 
 
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