Limestone is a popular choice for external walls in and around Perth. I’m not sure how the costs compare with other materials, but it’s a poor choice, for several reasons.
Initially it looks terrific, but before long, it starts to degrade. This begins with a blackening on the upper surfaces – the ones most exposed to the sun – and before long green plant growth becomes apparent.
Attempts to remove it generally fail, and if you try to use very high pressure water to remove it you start blasting holes in the limestone, as it is very brittle.
So what is happening?
The most important chemical process for life on the planet is photosynthesis.Carbon dioxide is converted to biomass (plant life) according to this simplified equation:
CO2 + H2O = C.H2O + O2
C.H2O is the basic formula of a carbohydrate, which wood is made out of.
So the bacteria that catalyze this process use sunlight to convert CO2 to plant matter and oxygen.
Unfortunately, limestone walls are also a carbon source, as they contain carbonate. So what happens is that the same bacteria that catalyze photosynthesis (cyanobacteria) do the same to your limestone wall. The carbonate in the calcium carbonate from which the wall is composed is literally converted to plants. It begins with a blackening of the surface, followed by the development of green plant material.
Contrary to popular belief this is not mould – it is plant life. And we know this because it occurs not where there is the least sunlight, but the most – on the top of walls.
And there is nothing you can do about it. It is possible to buy sealers that are supposed to stop this happening, but the problem is that because limestone is so porous it’s very hard to get it to seal completely.
The only real solution is to not use limestone in the first place. Unless you like the overgrown look that quickly develops, as some people do.
If you like the pristine, clean look, the best option by far is to use synthetic limestone – it’s made out of limestone ground in with cement. It looks great and will never degrade