Last Updated 28/07/2004

Efflorescence and Staining

Causes Of Efflorescence

White calcium carbonate efflorescence is occasionally present on concrete masonry walls. The most common mechanism for its occurrence is:

• Moisture enters the masonry through open-topped walls, sills or poorly constructed mortar joints.
• The moisture dissolves calcium oxide in the cement and lime, forming calcium hydroxide.
• A combination of low external humidity and high humidity in the cavity and cores in the masonry units forces the calcium hydroxide solution to the outer surface of the wall.
• The calcium hydroxide combines with the carbon dioxide in the atmosphere to deposit insoluble white calcium carbonate on the surface.

Detailing to Avoid Efflorescence

The following measures should be taken to minimise the occurrence of efflorescence in masonry walls.

• Tops of walls and parapets - Walls and parapets should have protection such as flashings or capping to the top surface. A common source of staining of masonry is water entering walls and cavities at this point via the exposed horizontal surface.
• Window sills - Window sills with an inadequate projection provide a source of water entry to the walls and will promote staining. The incorrect installation of window flashings will exacerbate this problem.
• Flashings - Flashings should protrude to the outside face of the brickwork. If the flashing is stopped short of the core holes in the external leaf, any moisture entering the wall will be channelled into the units, permeating the mortar, dissolving calcium hydroxide and eventually passing to the outside surface where the calcium hydroxide carbonates.
• Masonry units - Unless required otherwise, it is a reasonable expectation that concrete masonry units have an efflorescence potential of nil or slight.
• Mortar - Mortar should be hard and sound. The recommended mixes are:
  • Type M3 For general applications (except as listed for M4).
    Preferred: 1 part Type GP cement, 5 parts sand plus methyl cellulose water thickener.
    Alternative: 1 part Type GP cement, 1 part lime, 6 parts sand.
    
  • Type M4 For applications subject to saline wetting and drying, in aggressive soils, in severe marine environments, in saline or contaminated water including tidal splash zones and within 1 km of an industry producing chemical pollutants.
    Preferred: 1 part Type GP cement, 4 parts sand plus methyl cellulose water thickener.
    Alternative: 1 part Type GP cement, 1/2 part lime, 41/2 parts sand.
  • Methyl cellulose is an agent which holds moisture in the mortar while the cement hydrates. It is quite different from the air-entraining agent commonly used in mortars.
• Control joints - Walls should include well-positioned control joints to prevent indiscriminate cracking of the wall that would permit ingress of moisture. Control joints should be correctly sealed.
• Weep holes - Cavity walls should include numerous well-positioned weep holes to allow any moisture in the cavity to escape. Cavities should be free of mortar droppings.
• Removal of mortar smears - The external face of the masonry should be kept clear of mortar smears.

Removal of efflorescence and staining

• Engage an expert to identify and report on the source of efflorescence or staining. The report should consider possible contributions by poor detailing (allowing ingress of water and contaminants) and the materials (including masonry units, mortar and grout).
• Since the staining could continue for some time, it is suggested that a building be allowed to stand for, say, six months from when the efflorescence first appears before any remedial action is taken.
• At the end of this time, any unsightly deposits will be visible and the appropriate cleaning strategy can be devised and implemented. Cleaning strategies may include:
  • Dry brushing efflorescence off the surface
  • Washing with low pressure water jet
  • Steam cleaning
  • High pressure water jet blasting
  • Washing efflorescence off the surface using proprietary cleaners and chemicals
  • Wet abrasive (grit) blasting
  • Dry abrasive (grit) blasting.
  See the Block Cleaning Guide for more information on these cleaning methods.
• Building maintenance should be performed at this time, including:
  • Cracked mortar should be pointed
  • Control joints sealed with a polyurethane flexible sealant
  • Flashings should be repaired
  • Rainwater downpipes should be repaired or installed if not already in place
  • A clear sealant may be applied or renewed to protect the masonry against ingress of rainwater.