Lime run-off, also known as lime staining, describes an effect that can come about when excess water flows through cementatious material. This water can dissolve calcium hydroxide (free lime) which is then deposited on the brick face. The calcium hydroxide is a soluble form of lime which is created as Portland cement hydrates.
The source of the lime may be the cement from mortar joints or it may come from concrete or cast stone elements; for example, a coping above a brick wall or a floor slab built into the brickwork. Lime material washed from mortar joints can be due to a lack of adequate protection against rainfall during construction.
The run-off is often seen ‘dribbling’ from weep holes or fine separation cracks between brick and mortar joints. The calcium hydroxide reacts with carbon dioxide in the air producing a hard crystalline formation of calcium carbonate.
It is common for lime run-off to be mistaken for efflorescence. The primary differences are that it typically originates from mortar joints rather than the bricks themselves, and it does not disappear on wetting.
The initial staining can be removed with water and brushing before it carbonates. A bristle brush and water can be used but care must be taken not to damage the face of the bricks. Once reaction has taken place, an acid solution will be necessary because the hard crust that forms when the lime has started to carbonate is much harder to remove. The wall should be pre-dampened to minimise suction and a brick-cleaning solution carefully applied with a paint brush to dissolve the lime. If it has not disappeared after two or three attempts at ligh scrubbing with a bristle brush and water then it may require more specialist treatment.
It is easier to prevent lime run-off from occurring than to try and cure it, particularly where the structure is in close contact with concrete or cast stone which may be a risk in terms of lime migrating into the brickwork. Some of the prevention techniques that can be used both before and during construction include:
- Protecting brick work from rainfall during construction.
- Ensuring that mortar joints are fully filled to prevent water collecting in air pockets.
- Ensuring the correct installation of the damp-proof course (DPC) or damp-proof membrane (DPM).
- Avoiding direct run-off from floor slabs.
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