This section discusses cleaning methods for organic stains and mortar stains using pressure cleaning and chemicals.
Cleaning mortar stains causes some damage to masonry. It is not a substitute for good workmanship. The following methods aim to minimise the damage.
Pressure Cleaning
Should only be considered after all ‘dags’ and thick smears have been removed with hand tools. Pressure-cleaning thick mortar smears abrades the surrounding area.
Before Pressure Cleaning:
- Mortar must be set and hard.
- Trial in an inconspicuous area:
- to see if any damage to the face occurs;
- to assess its effectiveness.
Pressure should not exceed 7MPa (1000 psi) at a rate not greater than 25 litres per minute.
Fan jet only - needle jet will abrade the masonry. The jet should be held no closer than 500mm to the face of the wall. Holding the jet stationary can abrade the surface of the block. Mortar joints can easily be blown out if these procedures are not followed.
Caution: High pressure water blasting can cause personal injury.
Chemical (Acid) Cleaning
Most mortar-removing chemical cleaners contain acid to help dissolve the cement in the mortar.
Concrete blocks are a cement-based product and hence acid treatments can often cause etching, fading and streaking on blockwork walls. Acids can also react with the oxides used in face masonry work resulting in change of colour. For this reason, acid treatments should be used only as a last resort when other cleaning methods have failed.
Hydrochloric acid is very effective for dissolving cement mortar but is equally aggressive on the face of concrete blocks and hence must be used with great care. It should not be used at strengths greater than 1 part acid to 20 parts water. Milder acid like citric acid can be used at strengths of 1 to 10 parts water.
The following steps must be observed for acid cleaning of all masonry:
- All mortar dags or smears must be removed by hand tools prior to any other cleaning.
- Mask or protect adjacent areas from possible acid damage.
- The wall must be thoroughly saturated with potable water prior to the application of any diluted cleaning solution. This stops the blocks absorbing the chemicals, leaving them to work on the surface.
- Apply the diluted cleaning solution with a stiff brush or a stiff broom starting at the top of the masonry.
- Only narrow vertical columns should be treated at any one time.
- Leave the cleaning solution on the wall for between 2 and 5 minutes or as directed by the chemical manufacturer.
- Starting at the top of the wall, wash the wall down with copious quantities of potable water to remove all traces of the cleaning solution.
- Repeat these steps until the desired result is achieved.
Note:
Chemical cleaning is often used in combination with pressure cleaning. The pressure cleaners should NOT contain the cleaning chemical but should only be used to wet and wash down the wall.
Chemicals can cause harm to the operator and the environment and should be used with care, with appropriate clothing and to the manufacturer’s recommendations.
Check the surrounding area for anything that might be damaged by cleaning solution and protect as necessary.
Safety Warning: Never add water to acid. To dilute acid, add acid to water.
Organic stains:
Bacteria don’t like the alkaline nature of concrete masonry, but if it is damp for prolonged periods and away from sunlight, bacteria can cause black stains. Over time, their remains can feed mosses, moulds and lichens.
If this occurs, the mosses etc should be scraped away as much as possible. The masonry can then be scrubbed with a solution of household bleach mixed with an equal amount of hot water. Leave the cleaning solution on the wall for 5 minutes then hose down with clean water.
If the cause of the dampness is not treated, the stains are likely to re-occur.
Most leaf stains and the like will eventually fade with exposure to sunlight. Alternatively, they can be removed using the bleach solution as above.