Page 183 - Masonry and Concrete

Masonry Construction Techniques
182 CHAPTER FIVE

Although hydrochloric acid solutions are highly effective in remov-
ing mortar stains, they are not recommended for concrete masonry. Acid
solutions remove the stain by dissolving the cement, but they also dis-
solve the cement matrix in the unit and etch the surface, leaving it
porous and highly absorptive. As the cement is dissolved, more aggre-
gate is exposed, changing both the color and the texture of the block.
Dry rubbing is usually sufficient for removing mortar stains from
concrete masonry. To prevent smearing, mortar droppings and splat-
ters should be almost dry before being removed. Large droppings can
be pried off with a trowel point, putty knife, or chisel. The block sur-
face can then be rubbed with another small piece of block, and finally
with a stiff fiber-bristle or stainless steel brush.
Remove dried mortar splatters from stone with a trowel or by scrub-
bing with stone dust and fiber brushes wetted with white vinegar.
Acids or chemical cleaners are not usually required to clean new
stone. If stubborn dirt or other foreign substances are embedded in the
stone surface, mild abrasive cleaners will usually remove them. If
more aggressive methods are required, consult the stone supplier
about the most appropriate cleaning chemicals and procedures.

5.8.4 Efflorescence and Calcium Carbonate Stains

Efflorescence and calcium carbonate stains are the two most common
forms of surface stains on masonry. Both are white and both are acti-
vated by excessive moisture in the wall, but beyond that, there are no
similarities. Efflorescence is a powdery salt residue, while calcium car-
bonate stains are hard, crusty, and much more difficult to remove.
Efflorescence is the white powdery deposit on exposed masonry sur-
faces caused by the leaching of soluble salts. Efflorescence occurs when
soluble salts in the units or mortar are taken into solution by prolonged
wetting. As the wall begins to dry, the salt solution migrates toward the
surface through capillary pores. When the water evaporates, the salts are
deposited on the face of the wall (Figure 5-35). If the units and the mor-
tar ingredients contain no soluble salts such as sodium or potassium
sulfate, and if insufficient moisture is present to effect leaching, efflo-
rescence cannot occur. The source of moisture necessary to produce
efflorescence may be either rainwater or the condensation of water
vapor within the assembly. Water may also be present because unfin-
ished walls were not properly protected from rain and snow during con-

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