Lime staining is often mistaken for efflorescence, but it usually emanates from mortar joints rather than from the bricks themselves and does not disappear when wet. Like efflorescence, it needs saturation to dissolve and transport the material, in this case free lime (calcium hydroxide) present in mortar. As the water leaches and evaporates out, it is deposited on the surface, leaving a trailing effect (see Fig 265). The similarity with efflorescence ends at this point, however, since the calcium hydroxide reacts with the carbon dioxide present in the air to form calcium carbonate, which is the chemical basis for limestone and does not dissolve in water.
Fig. 265 Lime staining.
Lime staining is quite difficult to remove but fresh stains that have not started to carbonate can be scrubbed with a stiff brush and water, ensuring that care is taken not to damage the face of the brickwork. For older stains that have been exposed to the air for a longer time, and have begun to carbonate, thus forming limestone, acid treatment will be necessary. Pre-dampen, but do not saturate the wall with water, so as to reduce its suction. This is so that the acid is not drawn into the brickwork and stays near the surface where it can do its work. Carefully apply a proprietary, acid-based brick-cleaning solution with a paint brush to dissolve the lime, and then lightly scrub with a stiff brush and water. Such cleaning chemicals are commonly referred to as ‘brick acids’ and must always be used entirely in accordance with the manufacturer’s instructions. This includes taking all the specified safety precautions. It is also a good idea, before treating the wall, to test the chemical on an inconspicuous area of brickwork to ensure that it will have no adverse effects on the walling.
If lime staining has not been completely removed after three treatments, it is unlikely that there will be any significant improvement with further applications. Under these circumstances, advice should be sought from a specialist contractor.
In order to reduce the possibility of lime staining (and efflorescence), the following points should be considered both before and during the construction process:
• Keep bricks dry when being stored and protected from rain during the construction process.
• Ensure that DPCs are correctly installed.
• Protect new and partially constructed brickwork from the rain.
• Ensure that mortar joints are full with no internal voids in which water can collect.
• Ensure that the most appropriate brick type is specified for situations where the bricks will be repeatedly wet or saturated. For example, use engineering bricks between finished ground level and DPC level or for the copings on top of boundary walls.
• Take care when detailing structures that are in contact with concrete or stone, which could allow free lime to migrate into the adjacent brickwork.