Acid gases and their effect on stone
Acid gases are possibly the most deleterious component of the decay of natural building stone. The primary culprit is Sulphur Dioxide which is a gas released from the burning of fossil fuels. A great deal of decay occurred historically during the Industrial Revolution when a considerable amount of smoke was pumped into the atmosphere from fireplaces to factories. These days one of the main contributors is motor vehicles. The reaction can be seen below:
Ultimately it's the Sulphurous Acid that slowly eradicates the Calcium Carbonate.
Calcium Carbonate is lime! It's literally the binder that holds limestone together. The resultant bi-product (Calcium Sulphate) can do one of a few things:
1. Dissolve into solution and get washed away, which gives the characteristic pitted stone effect.
2. Crystallise within the pores of the stone, causing the pores to explode and the stone to crumble away. Sometimes small pieces of stone can be seen at the base of the wall.
3. Crystallise to gypsum on the surface of the stone. This causes a few additional issues that we won't go into, but if you've ever wondered why the sheltered areas of churches and mullion windows end up stained - this is what you're seeing.
So what can be done about it?
The most important thing to ensure is that the joints are pointed with a lovely breathable (capillary active) hot-mixed lime mortar. The reason for this is to reduce the forced wetting/ drying cycles that occur when cement pointing is used. Moisture (Potentially salt-laden moisture) should be pulled into the mortar instead of the stone. Besides protecting the building mortar from weathering, this is its job. Pointing is sacrificial. It weathers instead of the stone and gets replaced. Where the walls of the building are next to a busy road and/or when a pavement is built against the wall, a lime based plinth could be applied to prevent splash back from rain/ vehicles from getting to the stone.
