Limestone Weathering in an Urban

4 Limestone Weathering in an Urban Environment Illustration of the Interaction of the Three Factors 

An initial and important distinction needs to be made between two microenvironments - exposed and sheltered. Exposed surfaces have water flow across their surfaces either from direct rainfall or from runoff or both. However the magnitude and frequency of this flow may vary between surfaces. Sheltered surfaces do not experience any water flow products of degradation cannot, therefore, be removed from the surface by water flow. This can result in the build-up of a crust of degradation products that could protect the limestone surface from further alteration. On an exposed surface, the build-up of any degradation products is likely to be temporary. Microenvironmental variations in exposure begin to define the type of degradation forms that can be expected to develop on different parts of the building. 

For degradation forms to develop, however, it is necessary for reactions to occur that can alter or cause stress in the stone. Delivery of potential reactants to a limestone surface can be by two pathways, wet or dry. Wet deposition occurs when gaseous and particulate pollutants are incorporated into water droplets, falling as rain. This solution is usually acidic natural rainfall having a about pH of about 5.6 due to weak carbonic acid urban rainfall falls as 

Ca2+C032-(aq) + H2+C03- (aq) Ca2+ + 2HC03 Calcium carbonate + carbonic acid Calcium + bicarbonate 

Ca2+C032 (aq) + 2H+S042-(aq) -s- Ca2+S042 (aq) + C02(g) + H20(1) Calcium carbonate + sulfuric acid — Calcium sulfate + carbon dioxide + water 

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