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Hard white deposits that are insoluble in water sometimes appear on brickwork. These should not be confused with efflorescence, which is a water-soluble deposit.
Most commonly this hard white staining arises from products of the setting reactions of Portland cement. These are leached out of concrete elements such as sills, lintels, copings, cement render or, insufficiently dense mortar. They combine with carbon dioxide from the atmosphere to form insoluble white deposits. A second form of insoluble white deposit can occur after acid cleaning of smooth-faced bricks, especially red and darker colored bricks.
However, the main cause of insoluble white deposits is bad cleaning practice. Such practice allows mortar made from sand containing too much clay, to remain for too long on the surface of the bricks and then removes it with too much acid and not enough water. When not enough water is used before and after the acid wash, the products of the reaction between the acid and the mortar are absorbed into the faces of the bricks instead of being washed off the wall.
Kaolin, a clay mineral present in most bricklaying sands, can also form a hard deposit. It is insoluble in most acids, except hydrofluoric acid.
The combination of clay (from the mortar) with calcium and silica residues (from the cement) forms calcium silicate which can also produce insoluble white deposits. Calcium silicate is highly insoluble in most acids and is white.. When wet, these calcium deposits are invisible.
Apply full strength Noskum or Wallkleen to stained bricks and allow to stand for four to six minutes. Then, apply more of the chemical cleaning product and scrub vigorously. Finally, wash off with plenty of water whilst continuing to scrub.
Noskum and Wallkleen chemicals are S6 poisons and must be stored safely away from children. See warning and first aid information on individual bottle labels.
Source: Austral Bricks, Australia.
This article was updated on the 3rd September 2019.