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INTRODUCTION
Limestone is a sedimentary rock composed principally of calcium carbonate
(calcite) or the double carbonate of calcium and magnesium (dolomite).
It is commonly composed of tiny fossils, shell fragments and other
fossilized debris. These fossils are frequently visible to the
unaided eye on close examination of the stone surface, however this is not
always the case. Some varieties of limestone have an extremely fine
grain.
Limestone is usually gray, but it may also be white, yellow or brown.
It is a soft rock and is easily scratched. It will effervesce
readily in any common acid.
Limestones may vary greatly in texture and porosity from coquina, which is
a matrix of oyster shells loosely cemented by calcite, to oolitic
limestones and microcrystalline limestones whose structures are so fine
that they can be seen only under magnification.
Oolitic limestone consists of substantial amounts of "oolites"
or "ooliths." Oolites are small spherical or sub-spherical
grains of concentric calcite.
The actual classification of limestones and marbles can be very confusing
to the non-geologists. The same stone can be marketed one time as a
limestone and, at another time and place, sold as marble. The
subtleties which sometimes differentiate between grades and types of
stones are frequently beyond the concern and expertise of maintenance
workers, building managers and historical architects with responsibility
for maintenance of the resources. While this is understandable, it does not lessen or eliminate
the need to accurately identify the materials which must be treated and
maintained. Failure to accurately identify a material to be treated
can result in the failure to consider important technical details which
subsequently results in irreversible damage to the resource(s).
In an effort to improve accuracy in identifying the general categories of
limestones at a 'macro' level, the following section contains descriptions
of the most common types of limestone, however this information is no
substitute for training and experience to correctly identify and catalog
stone types. The following definitions are from the American Society
for Testing and Materials (ASTM) document, "Standard Definition of
Terms Relating to Natural Building Stones."
Calcarenite: Calcarenite is composed of sand-sized grains of
calcite, usually in the form of tiny fossils, shell fragments and fossil
debris. Some calcarenites contain oolites and if the oolites are
present in sufficient quantity, the stone is called oolite limestone.
Oolite limestone is a sub-category of calcarenite.
- Coquina: Coquina consists of raw, unaltered shell fragments, often
quite large, loosely cemented by calcite. It is generally very coarse
and porous, frequently consisting of oyster shells and fragments.
- Dolomite: Dolomite is a sedimentary carbonate rock composed of
calcium and magnesium carbonate. Also called "magnesium
limestone", it contains from 5 to 40% magnesium carbonate.
- Microcrystalline limestone: This is a limestone structure of
crystals too small to be seen without magnification.
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Oolitic limestone: Oolitic limestone is a calcite cemented
calcareous stone composed of shell fragments, practically non-crystalline
in character. Generally without cleavage, and extremely uniform in
composition and texture, oolitic limestone adjusts to temperature changes.
- Travertine: A calcium carbonate, usually light in color,
travertine can be extremely porous or cellular. It is usually
deposited from solids in groundwater.
Limestone coloration is generally a consistent pure white to off- white. Many
varieties do not take a polish well, so that the surface is typically a
matte finish, no-gloss surface. Limestones, like marble and other
calcareous stones, are referred to as acid
sensitive. Calcareous stones are readily dissolved in acid,
therefore acidic products should not be used on limestones and marbles.
TYPICAL USES
Limestone is widely used in architectural applications for walls,
decorative trim and veneer. It is less frequently used as a
sculptural material, because of its porosity and softness, however, it is
a common base material. It may be found in both bearing
(structural) and veneer applications.
PROBLEMS AND DETERIORATION
Weathering may have a degrading effect on the appearance and
structural soundness of limestone. Factors include rain, snow,
temperature, wind and atmospheric pollutants. Generally these
factors act in combination with one another or with other agents of
deterioration.
Rainwater, especially in combination with atmospheric gases can result in
dissolution of the limestone, creating higher levels of salt movement
within the stone structure. Temperature can effect rates of
deterioration and (in larger stones) movement of the
pieces, as well as patterns of salt migration within the stone. Most of
the natural or inherent problems which can occur with limestone require
some degree of moisture to occur, however other problems such as wind
erosion and vandalism may occur
independently.
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