Efflorescence and Picture Framing
Efflorescence1 was traditionally a problem associated with concrete products but the growth in use of natural stone paving has seen an increased incidence of efflorescence, and efflorescence-like problems, occasionally affecting several different types of natural stone paving.
The phenomenon has only come to prominence in the last few years and as the research into its causes are still ongoing, and given the vast collection of stone now being supplied to residential paving market, there is no definitive answer for how it is caused. It is thought by most to be related to the porosity2 of the stone, which would partly explain why it effects some stones and not others.
Most of the problems involving discolouration and picture framing occur with relatively porous stone types (many of the sandstones plus silvergrey granite) when they have been allowed to dry out completely. Having stone sat in the summer sun causes it to loose all of its moisture content by evaporation. When it is laid on a wet mortar bed or pointed with wet mortar, it "suck in" moisture, bringing cement particles, clay fines and iron minerals with it, which are later deposited on the surface as further evaporation dries out the paving once more.
Regardless of which stone is being used and regardless of technique, it's important to remember that stone is a wholly natural product and as such there is an inherent variation in porosity, colour, texture and grain size. It's not unknown for one flagstone to discolour while its immediate neighbour remains completely unaffected. This does not mean there is a problem with the stone; it's just the way things are with natural stone paving.
When we install natural stone paving, we ensure that the stones are laid on a full bed of motar, use washed and light coloured sands (with light coloured stone), treat the underside of stones and follow all the installation procedures as recommended by our suppliers of these products. However, it is still possible that efflorescence or picture framing may still occur and this should be borne in mind when selecting these products.
While discolouration and picture framing can look unattractive when first encountered, the effect and its visual impact do lessen over time and it is quite likely that in 12 months the problem will have lessened significantly or disappeared completely. We therefore do not take the approach of taking up the paving and replacing affected paving immediately. Our approach when addressing this phenomonen is to give it time and allow nature to run its course and review the situation the following year.
1 Efflorescence - "In chemistry, efflorescence (which means "to flower out" in French) is the migration of a salt or other minerals to the surface of a porous material, where it forms a coating."
2 Porosity - "Used in geology, hydrogeology, soil science, and building science, the porosity of a porous medium (such as rock, stone or sediment) describes the fraction of void space in the material, where the void may contain, for example, air or water."
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