CHATSWORTH

Chatsworth Stone Masonry Ltd, Eastwood Works, Parson Street, Keighley, BD21 3HD

T 01535 662 743 F 01535 664 907

CAST STONE PRODUCTS MANUFACTURED TO BS1217. CAT WEATHERING CLASS

STONE MASONRY LTD

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EFFLORESCENCE - TECHNICAL SHEET T05for futher information and other technical sheets please contact us on 01535 662 743

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AppearanceConcrete products on occasions develop white staining on the surface. This may vary from a general lightening of Colour to flaky deposits or even stalactites. This may be particularly noticeable on pigmented concrete products such as flags and cast stone units. The stain consistslargely of calcium carbonate with some calcium hydroxide, the mechanism of formation being as follows:

Portland cement consists largely of calcium silicates which react with the mixing water to form a calcium silicate hydrate at the same time releasing calcium hydroxide. The calcium hydroxide may be transported in solution to the surface of the concrete either by drying out of themissing water or by external water entering the concrete, for example, rainfall on relatively permeable precast concrete blocks. At or near the surface of the concrete the calcium hydroxide will react with carbon dioxide in the atmosphere to form the insoluble white calcium carbonate. It is this process which gives the surface of concrete its normal light appearance as opposed to the much darker interior. In its thin powdery from the calcium carbonate is often referred to as 'lime bloom'.

White staining due to calcium carbonate should not be confused with the efflorescence which may occur on brickwork which is due to soluble salts within the bricks being transported to the surface in a similar manner. Such salts do not react to become insoluble and may be washed off by water.

Lime bloom is unpredictable in its occurrence and may be very patchy. It is usually associated with wetting and drying conditions and is perhaps most commonly noted in the early part of the year when heavy rain may be followed by a relatively warm drying wind. Inprecast concrete block work the stain is often concentrated around the horizontal mortar joints. In this case water has penetrated the top or face of the block work and drained down until it meets the relatively impermeable mortar, at which point it is forced to the surface where the stain develops.

Prevention All concrete products should be stored protected form direct rainfall on site. While this will prevent the development of thicker deposits, bloom on the surface may still appear. Calcium carbonate is inert and will not have any deleterious effect so long as any flaky or loose material is removed from the surface prior to plastering etc.

TreatmentLime bloom is easily removed by treating the concrete surface with clean water and then brushing on dilute (1:10) hydrochloric acid followed by a further washing down. Hydrochloric acid must be used in accordance with the manufactures and health safety guidelines. Goggles and rubber gloves should be used and care taken that only hydrochloric acid is used. Care should be taken to ensure an even distribution of washing so the product dries uniform rather than a patchy Colour. There will be a distinct fizz when the acid touchesthe white stain. In sever cases this process may need to be repeated

SummaryWhilst the occurrence of lime bloom in no way affects the structural integrity of the concrete, it is important only in so far as it mars the appearance of the concrete. Whilst we make every effort to minimize its appearance no liability will be accepted for its occurrence.