Note: This guide should be read in conjunction with the guide to Determining Block Cleaning Method
The methods of general cleaning listed in order of increasing rigour are:
Soaking with water has been used since Roman times, a slow steady stream of water causes the dirt deposits to swell, loosening their grip on the underlying masonry. Accumulated waters, cascading over the face of the masonry, then wash the loosened dirt away.
In general, the higher the water pressure, the more effective the cleaning and the greater the potential damage to the surface (see Table 1).
Acid should not be used through the jet equipment because:
Table 1
| General Pressure | Pressure (kpa) | Damage | Cleaning Ability |
| Mains | 300 | Low | Low |
| Low | Up to 1500 | Low | Moderate |
| Medium | 1500 - 3000 | Damage to smooth face masonry | Moderate to High |
| High | 3000 - 5000 | Damage to polished face masonry | High |
| Verh High | 5000 - 50000 | Damage to split face masonry | High |
The general procedure is to thoroughly wet the masonry surface, spray or brush a minimum amount of chemical, allow it to act for 1 to 5 minutes and then scrub. Wash it off using one of the methods described above.
Precautions are necessary to protect the operator. Occupational health and safety requirements and common sense should prevail.
Organic solvents are generally used on small areas of greasy substances by using a bandage or poultice. This can be expensive since they tend to evaporate or be absorbed. The more commonly used solvents are white spirit, carbon tetrachloride, methy and ethyl alcohols, proprietary paint stripper or dry-cleaning agents. They also tend to leave a visible residue or stain on the masonry.
Detergents are surfactants (surface active agents) and promote wetting of the soiled surface by water. Small quantities of detergent are often used when washing with water, usually at low pressure. Detergents are by no means harmless, with many being chemically-aggressive. They are most effective and can be used in the removal of earth stains.
Acid cleaners should be used only after considerable thought is given to the consequences, such as effect on the masonry, operator and the environment. There is common well-founded resistance to the use of acids on concrete masonry.
Hydrochloric acid (muriatic acid or spirits of salts) is
widely used on clay bricks to remove mortar stains
since it dissolves portland cement. However, for this
reason, it can have serious effects on the surface of
concrete masonry if used incorrectly. Generally, a
solution of
1 part acid to 20 parts water (maximum strength 1
part hydrochloric acid to 10 parts water) is
recommended, while an area of less than 6 m2
should be treated at a time.
SAFETY WARNING: Never add water to acid, always add acid to the water. Hydrochloric acid will not remove salts, and will most likely make removal more difficult.
Phosphoric acid is used for the removal of iron-bearing deposits (rusty stains). A maximum strength of 1 part acid to 10 parts water should be used. Phosphoric acid should not generally be used on coloured concrete masonry as bleaching of the iron oxide pigments will result.
Oxalic acid is used for the removal of hardwood timber stains. 120 grams of oxalic acid with 4 litres of warm water is the recommended mix. Bleaching solutions such as sodium hypochlorite (pool chlorine) have also been found to be very effective and a readily-available chemical for use on hardwood timber stains.