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Efflorescence
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The term efflorescence is used to describe white deposits which sometimes form on the surface of cementitious products, concrete or masonry walls.

Under certain conditions, the white deposits can remain where they are for a long period of time, impairing the appearance of concrete roofing tiles, paving blocks and slabs, masonry bricks, facade units and many other exposed concrete surfaces.

Prevention of Efflorescence
A number of precautionary measures may be taken to minimize efflorescence, most importantly attention to curing and storage. Carbonation treatment reduces efflorescence and autoclaving of precast products prevents it altogether. The buildup of a protective calcium carbonate layer is retarded at a low curing temperature (10°) but accelerated at a high curing temperature (40-60°) at high humidity.

Efflorescence due to surface carbonation can be reduced, but not completely eliminated by the inclusion of an active pozzolan in the concrete.

To summarize, the development of efflorescence on concrete can be minimised by:

  • attention to surface finish to ensure the concrete is as dense and impermeable as possible;
  • moist curing should be carried out for seven days to reduce permeability, then;
  • wash surface to remove surface deposits;
  • application of a clear membrane sealer (e.g. acrylic emulsion), and
  • drying under cover with good ventilation.

These matters are more easily attended to with precast concrete. With in-situ concrete it is necessary to eliminate possible water pressure differentials.

Removal of Efflorescence
By closely observing the precautions mentioned above, it is possible to reduce the chances of the occurrence of efflorescence. In cases where has not been successful despite taking all care or where severe secondary efflorescence has appeared over the course of time, it is still possible to remove the lime deposits.

In some cases, e.g. with prefabricated units or paving slabs or blocks that have already been laid, this can be done by light sandblasting or stiff brushing of the surface. In this way, a thin layer of concrete is removed to expose the unharmed material underneath. Another frequently used technique is to wash with dilute acids. For this hydrochloric acid diluted 1:10 is generally used. It is advisable to first thoroughly wet the stone-surface with clear water before applying the acid. In this way, the pores of the concrete fill up with water, which prevents the acid from penetrating too deeply into the stone and causing additional efflorescence in the form of water-soluble salts.

The acid is applied immediately afterwards, preferably brushing at the same time, and after being allowed to take effect for a short time (approximately 10 seconds), is then rinsed off again with plenty of clear water. With this acid treatment a thin layer of concrete is removed and the appearance after both types of treatment is very similar.

Common Compound

Suggested Cleaning Method

Alkali Sulfate

Use a stiff brush, and if required brush with water.

Calcium Sulfate

Use a stiff brush. If condition persists, brush with water.

Calcium Carbonate

If efflorescence has not adhered to the surface of the concrete, use a stiff brush. If efflorescence has adhered to the concrete, dissolve efflorescence with an application of dilute acid or a proprietary cleaning solution.

 Table 1. Efflorescence Removal