Page 184 - Masonry and Concrete

Masonry Construction Techniques
183
MASONRY CONSTRUCTION TECHNIQUES

struction. “New building bloom” (efflorescence which occurs within
the first year of the building’s completion) is often traced to slow evap-
oration of such moisture. Hot summer months are not as conducive to
efflorescence because the wetting and drying of the wall is generally
quite rapid. In late fall, winter, and early spring, particularly after rainy
periods, when evaporation is slower and temperatures cooler, efflores-
cence is more likely to appear. To minimize the possible contribution of
mortar ingredients to efflorescence, use portland cements with low
alkali content, clean washed sand, and clean mixing water.
Efflorescence will often disappear with normal weathering if the
source of moisture is located and stopped. Efflorescence can also be
dry brushed, washed away by a thorough flushing with clean water, or
scrubbed away with a brush.
Calcium carbonate stains are hard encrustations which can be
removed only with acid cleaners. Calcium
hydroxide is present in masonry mortar as
part of the hydrated lime in cement-lime
mortars, and as a by-product of the port-
land cement hydration process itself.
Portland cement will produce about 12—
20% of its weight in calcium hydroxide at
complete hydration. Calcium hydroxide
is only slightly soluble in water, but
extended saturation of the mortar pro-
longs the hydration process producing a
maximum amount of calcium hydroxide
and provides enough moisture to leach
the calcium hydroxide to the surface.
When it reacts with carbon dioxide in the
air, the calcium hydroxide forms a con-
centrated calcium carbonate buildup,
usually appearing as white streaks from
the mortar joints and sometimes referred
to as “lime deposits” or “lime run” (Figure
5-36). The existence of calcium hydroxide FIGURE 5-35
in cement-based mortar systems cannot be
Masonry efflorescence. (from Beall, Christine,
avoided. Preventing saturation of the wall
Masonry Design and Detailing, 4th edition, McGraw-
both during and after construction, how- Hill, New York).

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