Carrying on my theme of sealing stone on all six sides, I will look at issues relating to the back of the stone. I will use the example of a granite façade I saw whilst staying at a Hotel in Athens. The façade was clad in polished granite tiles, they were a grey-white speckle in colour and a stone that I happen to know is quite porous.
Much of the façade is fine and looks great. However, there is an issue with a large number of the tiles and that is dark staining or blotchiness. This has come from behind the stone; the stone adorns more than just a simple external façade, it also covers a large planter and the staining is present mainly in this area.
The staining is mainly due to the presence of water in the soil of the planter, and I dare say that if the planter were to be allowed to dry out, then the intensity of the stain would likely reduce. However, it is kept wet for a reason – to keep the plants alive.
Water is very rarely pure and untouched by contamination, especially when it emanates from what is essentially a great big plant pot: the water having passed though soil and compost may have any number of dissolved minerals which, even if the water were to recede, will likely be left crystallized for eternity in the stone.
Some of the tiles also show some golden-brown spots; naturally occurring phenomena or a preventable blemish? This granite, like many stones can contain soluble minerals like iron sulphide. This is a source of iron and it reacts with water and oxygen; basically it rusts when it gets wet and so these spots are rust, growing from the back to the front and cannot be removed.
Would sealing the back and sides of the stone have prevented either of these? Well it would have kept the water out, or at the very least dramatically reduced the amount of water getting into the stone so it would certainly have helped (although to totally prevent such issues, proper thought should be given to the design of the planters and adequate provision for drainage ought to have been designed in. In addition, a water-proof membrane could have been used on the inside of the planter to effectively tank it).
However, even if it had been tanked, there would still be the initial construction moisture to consider, in other words the temporary but considerable amounts of water present in the concrete and the setting and grouting materials. This brings me to another very commons issue: efflorescence.
Efflorescence is essentially a mineral deposit brought about by having moisture in the system, which dissolves soluble minerals in (for example) the concrete sub-structure and then deposits those minerals as it evaporates out of the system. Efflorescence is usually seen as a white powdery deposit on the face of stone facades, on brickwork or in the joints. Sealing all six sides, along-side the correct design, would make a considerable contribution to the prevention of such problems.
Строительные советы и не только!
