Non-metallic inorganic materials such as mortar, brick and stone have been widely used in buildings and construction from ancient times because of their outstanding physical strength and durability. For historical monuments and sculptures especially, natural stones like limestone, sandstone and marble have been essential materials. Even these materials, however, degrade over long periods under natural climatic factors like water permeation, marine air erosion, thermal cycle and bacterial contact. Unlike such natural degradation, however, the decay of building materials has been accelerated due to the air pollution resulting from anthropogenic activities. Chemicals like sulfur dioxide and nitrous oxide from internal combustion engines and industry are absorbed in rainwater and penetrate the surface of inorganic materials, where they directly react with calcium ions in the materials. Alteration of minerals due to these reactions occurs and such phenomena as gypsum formation, pulverized surface, exfoliation, contour scaling, discoloration and cracks result. Most of these mechanisms are closely related to the water movement through the inorganic material. Controlling that water movement is critical to inhibition of the formation of crusts and the accumulation of efflorescence and consequently to reducing the speed of decay of inorganic materials.
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