Home Products Weathering RATES OF WEATHERING


Intensity and duration are two key elements that govern to what extent weathering  reactions will have on stone.

When the presence of pollutants like nitrogen and sulfur-bearing gases are in the atmosphere and combined with water (rain and fog) they form powerful acids. Acid rain and fog have accelerated the weathering processes to several times then in unpolluted areas. The pH of rain is naturally acidic, with a pH of approximately 5.6. This is the result of the natural occurrence of carbon dioxide in the atmosphere being dissolved in atmospheric moisture to form carbonic acid. Combine this natural occurring process with the addition of sulfur dioxide and nitrogen oxides which are transformed into sulfuric and nitric acids when combined with water and its corrosive forces have been dramatically intensified. The average pH of acid rain is about 4.4, ten times stronger than normal rain. It also has been estimated that about half of these two pollutants are not mixed with atmospheric moisture and settle back to the earth as aerosols, (dry gases and dry particles). These acidic aerosols when combined with the next rain produce an even stronger corrosive solution.

The concentration (intensity) of acidic solutions has a direct effect on the rate at which the weathering processes occurs. In addition to the intensity factor, is the duration or length of time and frequency that a process is active. Prolonged and frequent exposure accelerates the rates of weathering.

Several other factors influence the type and rates of weathering, alterations and decay of stone. The most important of these are composition of the stone physical condition of the stone & environmental conditions.

Composition of the Stone: Their mineral and chemical compositions are important factors as to the extent to which the stone will be affected and the type of effects it may display.

Granite-type stones are more resistant to the mechanical processes with the exception of salt decay and more
susceptible to the chemical processes of hydrolysis and in some cases oxidation.
Limestone and marble are vulnerable to salt decay, dissolution, hydration and in some cases oxidation
Sandstone is susceptible to the processes of salt decay, oxidation and if it is a calcareous variety of sandstone, it is vulnerable to the dissolution process.
Clay slates are vulnerable to the chemical processes of hydration; hydrolysis and some varieties are affected by the oxidation process.

Physical Conditions: This involves its physical features, such as its pore/capillary structure, visible and micro-fractures, bedding planes and the finish of the stone, natural cleft, textured, honed and polished. These physical features relate to water/moisture pathways into the interior of the stone and mineral surfaces that will be exposed to the chemical and mechanical processes.

Environmental Conditions: The environment that the stone is in can be said to be the most significant factor upon the type and rates of weathering alteration and decay. The chemical processes are more pervasive and rapid in a warm and moist environment. Salt decay is also a prevalent process in this kind of environment as well.


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