Chemical Weathering

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Many buildings, especially in the UK are made from limestone which is particularly succeptible to chemical attack.

Acid rain

Rainfall is naturally slightly acidic because atmospheric carbon dioxide dissolves in the rainwater producing weak carbonic acid. In unpolluted environments, the rainfall pH is around 5.6. Acid rain occurs when pollutant gases from fossil fuel combustion, such as sulphur dioxide and nitrogen oxides are present in the atmosphere. These oxides react with the rainwater to produce stronger acids and can lower the pH to 4.5 or even 4.0.

These acid solutions can have catastrophic effects on limestone building stones because limestone (calcium carbonate) is soluble in acid and acid rain can cause them to dissolve or crumble and to lose their ornamental features.

Sulphation

Sulphur Dioxide can react directly with limestone producing gypsum (calcium sulfate) which is more soluble than calcium carbonate and which is easily dissolved and washed away by subsequent rain. On areas of the building which are sheltered from rain, a gypsum crust may accumulate and trap soot particles derived from fossil fuel combustion. This produces an unsightly "black crust" which is visible on the statue above on the more sheltered surfaces.

Physical Weathering

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Physical weathering of building stones occurs by any mechanism which generates physical stresses within the stone. The growth of salt crystals or ice crystals in pore spaces or in cracks can cause the stone to fracture. Extremes of temperature may also have the same effect.

Frost weathering

Frost weathering refers to the action of ice in causing mechanical breakdown in rocks. Frost weathering can occur only in damp conditions and only if the stone itself has cracks or pores which allow the water to penetrate it.

Unlike most other liquids, when water turns to ice it expands. As the temperature drops below freezing, ice forming inside a rock will exert stresses which will cause the rock to crack. This is exactly the same process that makes your water pipes burst when it's really cold! Frost weathering occurs not only in very cold climates but also anywhere that has repeated transitions across freezing point when water is present

Salt weathering

Salt action results from the growth of salt crystals in pore spaces or cracks within rocks and can result in fracture, surface exfoliation, or "honeycomb" weathering of building stones. It is normally associated with arid climates where strong heating causes fierce evaporation and therefore salt crystallisation. It is also common along coasts. If you've ever walked along the sea front and wondered at the honeycombed stones in the sea wall, you've witnessed the effects of salt weathering.

Heating and cooling

In some environments, for example the desert, there can be dramatic differences between daytime and night time temperatures. These sudden temperature changes cause the minerals in the building stones to alterately expand and contract. Because different minerals expand and contract at different rates, stresses may be set up which, over time, would cause cracking.

Biological weathering

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Weeds

Weeds grow almost anywhere without many problems. They can sometimes germinate in the gutters of buildings where they have been transported to by the wind. As they proceed to grow they plant their roots down into the rock that the building is made up of forcing their way further down. This causes the rock to exfoliate over a long time, small fragments crumbling away now and then.

Moss and fungi

Certain types of moss and fungi grow on the surface of rocks or bricks. As they die they are replaced by more growth the dead waste is pushed under/aside to rot. As it decomposes on the rock it produces an acidic solution that dissolves rocks and bricks. This type of weathering is common on bricks as fungi tend to grow on the mortar holding bricks together. Geological processes

Lichens

Mosses and lichens grow on especially bare rocks and create a more humid chemical microenvironment. When these organisms attach to the surface of the rock it increases physical as well as chemical breakdown of the surface layer of the rock. A lichen is not a single organism, but many small ones working together.