They are soaked into the masonry during times of wet or damp weather – travelling deep into the stonework through it's pores, cracks and capillaries. These wet solutions containing salts are able to invade the stone or brickwork. A similar reaction then takes place to that of sulphuric and carbonic acids – attacking the weak carbonate structure of building materials – this time producing nitrates rather than sulphates.Īll of these salts are very soluble, and easily washed away – so the stone just degrades. This reacts with water to form nitric acid.
#Stone flood wall full#
The air we breathe is full of nitrogen – and when it combines with oxygen it forms Nitrogen dioxide.
There's another pretty nasty acid out there – Nitric Acid. The sulphate reactions are quite complex, but all you need to know is that when you see the lovely stone of a mellow old limestone building sculpted beneath the cornices and mouldings – it's traffic fumes that are probably the cause. It can form crusts and blisters on the stone, protecting the surface, but acid attack still goes on underneath, softening the stone further. This stuff comes out of car exhausts and chimneys, and reacts with carbonates to form gypsum – calcium sulphate, which is a bit less soluble than baking powder, but still washes away in the rain. This is the same gas that reacts with water to form sulphuric acid – the stuff in car batteries. Sulphur dioxide was the stuff that went up the stack – and we frequently got trapped in the opencut by clouds of fumes that made us choke and cough – our faces were burned raw. When I worked in the gold mines of Australia, the ores were 'roasted' to break the rock down. That's why you often see sandstone blocks at the base of walls just turning to sand – especially alongside busy roads. So any 'carbonaceous' stone – a sandstone for example with calcite binding, will erode as the binder is dissolved. It is ANY stone or brick with calcium carbonate as the main binder between the grains. The problem is, it's not just limestone that is affected. The problem is, it's VERY soluble – so the moment it forms, it washes away. It dissolves calcium carbonate – the main component of limestone, to form Calcium Bicarbonate – actually better known as baking powder. It is this acid which is responsible for the formation of the vast network of caves under many of the limestone Districts of England. The commonest acid is that formed by the action of rainwater on Carbon Dioxide – forming Carbonic Acid. Cars have now taken over as the major cause, and falling emissions are balanced by the ever growing number of cars and trucks spewing diesel exhaust. It is probably true to say that during this period, the air was more loaded with acid gases than even now – and damage to old buildings greater then. The Victorian Era was dominated by industrial growth – thousands of factory chimneys belched toxic fumes – Contemporary accounts of Ironbridge Gorge in the grip of the Industrial Revolution graphically describe the thick, suphurous fumes that clogged the Gorge. This is not just because the materials are more prone to damage.
Owners of older properties – solid walled buildings – will be more familiar with crumbling stone and plaster than owners of houses built more recently. All of these are constantly reacting with our buildings, degrading the fabric and producing salts. The most common is Carbon Dioxide, closely followed by Sulphur and Nitrogen based gases. The very air we breathe is loaded with acidic gases. Her wonderful and insightful book on the subject is referenced below – it is essential reading for anyone serious about Sustainability in older buildings.
I have to thank my friend Carole Ryan, one of the founder members of IHBC, and Senior Lecturer at Bournemouth University for providing guidance on these issues. I hope the following explanation will go some way to helping people understand just what is REALLY happening out there. Damp industry claims about why salts are present in walls are complete rubbish. Broken drains ARE a source – so when we do 'damp' surveys, we are always on the lookout for broken foul drains with inevitable escape of urea – nitrates, into the surrounding soil. Not too many of those in the middle of our cities. Apparently, our soils are saturated in salts – quite where from, I'm not sure – they cite agricultural fertilisers as one source. They claim salts are present in the wall because 'rising damp' climbs the wall, bringing with it salts from the ground. Rising Damp wallies sell something like £300 million pounds a year of unwarranted damp proofing treatments on the basis of 'damp meters' which actually measure salt content of walls, by way of conductivity. The truth about Salts in the walls of old buildings
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