Engineering Geology

Chapter 6
Limestones show a variation in their colour, texture and porosity, and those that are fossiliferous
are highly attractive when cut and polished. However, carbonate stone can undergo
dissolution by acidified water. This results in dulling of polish, surface discolouration and
structural weakening. Carvings and decoration are subdued and may eventually disappear;
natural features such as grain, fossils, etc., are emboldened (Fig. 6.3).
The colour and strength of sandstone are largely attributable to the type and amount
of cement binding the constituent grains. The cement content also influences the porosity
and, therefore, water absorption. Sandstones that are used for building purposes are
found in most of the geological systems, the exception being those of the Cainozoic era.
The sandstones of this age are generally too soft and friable to be of value.
Roofing and Facing Materials
Rocks used for roofing purposes must possess a sufficient degree of fissility to allow them
to split into thin slabs, in addition to being durable and impermeable. Consequently, slate is
one of the best roofing materials available and has been used extensively. Today, however,
more and more tiles are being used for roofing, these being cheaper than stone, which has
to be quarried and cut to size.
Figure 6.3
Weathered limestone gargoyle and scabbing of stone, Seville cathedral, Spain.
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