Masonry Magazine March 1971 Page. 20
Since most such compounds are no more effective in removing mortar than 10 per cent hydrochloric acid, and are usually much more expensive, it is advisable for the cleaner to evaluate each one for himself.
Organic Acid Form. A new type of compound which has appeared recently contains a blend of organic acids (Nos. 8 and 9, Table 1). These are generally offshoots of the compounding of chemicals for some other industry and have unique properties of their own. No. 8 proved effective in removing certain stains, but not very effective in removing mortar. No. 9, when diluted with three parts of water by volume, was as effective as 10 per cent hydrochloric acid in removing mortar.
Sandblasting. A sandblasting technique for cleaning new masonry has been developed which is used extensively in some areas. It is reported to cost approximately the same as acid cleaning and, with an experienced operator, "there is virtually no change in texture of hard brick, except that caution must be used on brick with sand finish". This method obviously eliminates the dangers of mortar smear, acid burn and efflorescence inherent in acid cleaning.
A prominent masonry contractor from Texas states: "We are gradually switching to sandblasting completely, because we do not have the problem of chemical reaction with vanadium salts and other foreign matter which is unavoidable in some types of clay with a liquid cleaner."
He describes his sandblasting operation as follows: "We use a very low pressure (from 60 to 120 lb) sandblasting technique. Although we try to keep the pressure as low as possible, the secret of cleaning the brick and not damaging the mortar joints lies in the type of nozzle and the distance the cleaner stands from the wall and the manner in which he directs the blast to the brick.
"In other words, he must concentrate on cleaning and hitting the brick with the sand rather than the mortar joints and he must stand far enough away from the wall that he merely cleans the brick and does not etch it or deface it.
"At present we are using a standard commercial size pot and hose with a quarter-inch sandblast nozzle, catalogue number SC-4. We use white urn sand to blast these brick because this sand is round in nature instead of sharp and jagged. We find that the round, fine urn sand is far better for cleaning purposes and does not cut near as deep as a sharp sand does.
"I recommend that the person using the sandblast technique employ a very high caliber tuckpointer-sandblaster, experiment a little bit, as to how far to stay away from the wall to obtain maximum cleaning results without defacing the brick or damaging the mortar joints."
REMOVING EFFLORESCENCE
The sources and methods of preventing efflorescence are discussed in Brick and Tile Engineering, 1962 edition. As indicated, the term efflorescence generally refers to a white, powdery substance sometimes seen on masonry wall surfaces. It is composed of one or more water soluble salts originally present in the masonry materials that have been brought to the surface by water and deposited on the surface by evaporation of the water. It can frequently be removed by water applied with a stiff scrubbing brush. In those cases where this procedure does not remove all the efflorescence, the surface should be scrubbed with a solution of hydrochloric (muriatic) acid not stronger than one part of the commercial acid to nine parts water by volume, as recommended for cleaning new masonry. It is highly important that the recommendation regarding water-soaking and rinsing of the wall, before and after acid washing, be followed.
REMOVING VANADIUM STAIN
It is generally agreed that "green staining" is caused by salts of vanadium. While the stain is usually green, sometimes it is a brownish green and, more rarely, a brown. The amount of vanadium in the brick is very small of the order of 0.01 per cent. It is not known in what form the vanadium is present not in the raw material, nor in the fired brick, nor on the stained brick surface. If these identifications could be made, the problem of eliminating vanadium staining would be simplified, and research is presently working towards the identification of the compounds involved.
There are three facts about vanadium chemistry that must be emphasized if an intelligent approach is to be made to the problem of washing vanadium stained brick.
1. Vanadium salts may be divided into two classes colorless salts which crystallize in alkaline or neutral solutions and colored salts which are obtained from more or less acid solutions. The colorless salts are quite soluble in water, while the colored salts are only slightly soluble.
2. While the reactions of the colorless salts are practically instantaneous, the colored salts change slowly.
3. The vanadium salts react much more rapidly with acid than with alkali.
The fact that colored salts are obtained from the acid solutions explains something that is observed frequently: green staining brick will often show no sign of staining until they are acid