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Engineering Geology

Engineering Geology - geomuseu

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Chapter 6<br />

Figure 6.2<br />

Black crust developed on a column of limestone, Lincoln cathedral, Lincoln, England.<br />

is free from visible laminations. The tougher the stone, however, the more expensive it is to<br />

dress. Sandstones are chiefly composed of quartz grains that are highly resistant to weathering<br />

but other minerals present in lesser amounts may be suspect, for example, feldspars<br />

may be kaolinized. Calcareous cements react with weak acids in urban atmospheres, as do<br />

iron oxides that produce rusty surface stains. The reactions caused by acid attack may lead<br />

occasionally to the surface of a stone flaking off irregularly or, in extreme cases, to it<br />

crumbling. Laminated sandstone usually weathers badly when it is used in the exposed parts<br />

of buildings, it decaying in patches.<br />

Exposure of a stone to intense heating causes expansion of its component minerals with<br />

subsequent exfoliation at its surface. The most suspect rocks in this respect appear to be<br />

those that contain high proportions of quartz and alkali feldspars, such as granites and sandstones.<br />

Indeed, quartz is one of the most expansive minerals, expanding by 3.76% between<br />

285

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