When cleaning smooth faced bricks with hydrochloric acid solutions there is always a risk that the finished appearance will be spoiled by the effect known as "white staining".
This stain is similar in appearance to "efflorescence", but unlike efflorescence will not disappear in time.
The white stain is more or less permanent on the bricks, remedial treatment can be both time consuming and expensive.
Technical Experts in the past have agreed that the cause of the staining is insufficient wetting of the bricks prior to applying the acid. The cement residue softened by the acid is re-absorbed into the semi-dry brick this then appears as a white dust on the face of the bricks when dry.
When water is splashed on the bricks the "dust" disappears, only to re-appear again when the wall dries.
The increasing incidence of "white staining", which now also has occurred on wire cut and sandstock bricks, tends to suggest other materials and activities are contributing to the problem.
The addition of sugar into the mortar, the use of high clay content "brickies" sand, which now tends to be used without any modification by adding "sydney" sand to reduce the fattiness.
The move to Cements which contain "fly-ash", "blast furnace slag" and "silica fume". The continuing use of "fire clay" as an additive in mortar to increase fattiness and the possible over-use of air entrainers to improve "workability".
In a lot of cases the mortar can be described as a cocktail, which reacts like a "mickey finn" when hit with hydrochloric acid.
Although softening the cement, the hydrochloric acid also releases the other inert additives in the mortar. Because of their fineness and the fact that they do not react with the acid they tend to remain on the wall and be re-absorbed back into the brick pores.
Removal of the white staining can be difficult as the suggested remedial treatment involves the use of "noskum" or "wallkleen 150" applied neat to the dry (affected) bricks, vigorous scrubbing with a nylon scrubbing brush and then a wash down with water. The treatment repeated until the stain is removed.
An alternative is to mask the stain by applying a solution of Linseed Oil and Kerosene, mixed at 50/50.
This is applied to the affected bricks with a brush or rag.
The masking treatment tends to be permanent, and is much more economical than the washing treatment. If suggesting this remedial treatment to a Builder, first make sure he/she understands that the problem has emanated from the brickcleaning operation and has nothing to do with the bricks.
As with all brickcleaning problems an empathetic approach should be taken with the Builder, but at the same time firmly pointing out where the problem has developed from.
This problem belongs to the brickcleaner and it is his responsibility to remediate the situation.
Brickcleaning failures generally fall into one of three categories:
1. Failure to thoroughly saturate the brick masonry wall surface with water before and after application of chemical or detergent cleaning solutions.
A dry wall permits absorption of the cleaning solution and may result in mortar smear, white scum or the development of efflorescence or green stain.
Saturation of the wall surface prior to cleaning reduces the absorption rate, permitting the cleaning solution to stay on the surface rather than be adsorbed into the wall.
2. Failure to properly use chemical cleaning solutions.
Improperly mixed or overly concentrate acid solutions can etch or wash out cementitious materials from the mortar joints.
they have a tendency to discolour masonry units, particularly lighter shades, producing an appearance frequently termed 'acid burn' and can also promote the development of green and brown stains.
3. Failure to protect windows, doors and trim.
Many cleaning agents, particularly acid solutions, have a corrosive effect on metal.
If permitted to come into contact with metal frames, the solution may cause pitting of the metal, staining of the masonry surface and trim materials such as limestone and cast stone.
Check Points to Avoid Problems
1. Check brick type. Each brick can present a different problem to the brickcleaner.
High or low cold water absorption, high or low initial rate of absorption, liability to efflorescence, possibility of vanadium, acid burn, iron stain or manganese leaching as secondary reactions to hydrochloric acid.
2. Consider chemical and solution strength.
Hydrochloric acid at 20:1 is better that 10:1.
Noskum is better again.
Water on its own is even better.
Remember, side effects can occur if strong acid solutions are used.
3. Be methodical and thorough when cleaning.
Set out walls in manageable sections so all walls are cleaned, about 10m2 section. Try to work in the shade, following the sun. Avoid premature drying.
4. Fully saturate walls before applying acid solutions.
This will avoid any acid being drawn into the walls by suction. Failure to fully saturate walls can result in other stains appearing when walls dry out.
5. Apply acid solution carefully.
To avoid overspray and possible damage to other building elements and finishes.
6. Use lowest water pressure during wash down.
High pressures can damage bricks and mortar joints. Also acid can be driven into the body of the brick producing secondary stains as the walls dry out.
7. Totally rinse and wash down the walls using plenty of water. Finally apply a neutralising solution to complete the process.
Brickcleaning Step By Step
- Assess bricks and brickwork.
- Identify stains and select correct chemical.
- Test chemical.
- Commence cleaning - setting out manageable sections.
- Saturate wall section.
- Apply acid solution and leave on 3 to 6 minutes.
- Rinse and wash debris off
- Neutralise walls to