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 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 scum can occur after acid cleaning of smooth-faced bricks, especially the reds and darker colours.
However the main cause of these insoluble white deposits is bad cleaning practice, allowing mortar made from sand containing too much clay to remain too long on the surface of the bricks and then removing it with too much acid and too little 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, form calcium silicate which could also produce the insoluble white scum. Calcium silicate is highly insoluble in most acids and is white in colour. When wet these calcium deposits are invisible.
Apply full strength Noskum or Wallkleen to stained bricks and allow to stand four to six minutes. Apply more chemical and scrub vigorously. Wash off with plenty of water whilst still scrubbing.
These chemicals are S6 poisons and must be stored safely away from children. See warning and first aid information on individual bottle labels.