Masonry Magazine February 1997 Page. 19
Concrete Masonry
Large mortar droppings should be allowed to harden slightly before removing them with a trowel, putty knife, or chisel. Block surfaces may also be rubbed with another small piece of block, then with a stiff fiber-bristle or stainless steel brush. These are the only cleaning methods recommended by the National Concrete Masonry Association (NCMA).
Acid should not be used on concrete masonry as it dissolves the cement matrix used to make the block and etches the unit surface. This leaves the block face porous so that it absorbs more moisture. It also exposes more aggregate, changing the color and texture of the block.
Glazed Units
Cleaning these requires a bucket and brush. Within a few minutes of laying the units, wipe the glazed surface with a soft cloth. Then wash, using water plus a soft sponge or brush. Detergents can be used where necessary, but use acid solutions only for very difficult mortar stains. Acid should not be used on salt or metal-glazed brick. Finally, do not use abrasive powders as they mar the glazed surface of the masonry.
Limestone
Remove mortar droppings and smears as work progresses. The finished stonework should be washed to rid it of dirt, mortar and other accumulations. The final cleaning should be with fiber brushes and mild soap or detergent, and a clear water rinse.
Don't use acid on limestone. If adjoining brick is to be cleaned with an acid-containing proprietary cleaner, be sure the stonework is protected from rundown or splash by keeping the stone wet with flowing water while cleaning above it.
Removing Efflorescence
Efflorescing salts are water soluble and generally will disappear with normal weathering. Removing efflorescence is relatively easy: white efflorescence salts can be removed by dry brushing or with clear water and a stiff brush. Heavy accumulations of these salts should be removed with a proprietary cleaner. When using a proprietary cleaner, be sure to saturate the wall with water before and after the solution is applied.
Removing "Green Stain" (Vanadium Salts)
Found on red, buff or white brick,
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