R2:
Dear Nancy and other readers: The posting to which you are referring was
in one of the other (?trade) sections to do with supply. Many limestones
could have been used but there were strict guidelines that I needed
to follow for this type of construction (for example, a thickness of 30mm,
mechanical fixings, price, reliable availability, and proximity to saltwater).
Limestones are calcareous rocks made up of at least 50% of carbonate mostly
in the form of calcite, aragonite, and dolomite. There are other carbonate
varieties but these are not significant in the context of building stone.
As a group, these carbonate rocks represent about one-fifth of all sedimentary
rocks, occur on all continents, and span all ages from recent to very old.
Limestones are regarded as non-clastic sedimentary rocks that can be formed
from many geological processes. They are texturally, structurally and mineralogically
extremely diverse - characteristics which reflect their derivation, environment
of deposition, and diagenetic history. Although a large proportion of limestones
are biogenic (contain fossil material), there are also chemically
deposited limestones which are important as building stone, namely travertine.
The
principal factor which unifies these carbonate rocks is their high reactivity
to acids. Acids are a fact of life and here to stay. There are
many of them in our home and in our environment. Wine, carbonated drinks,
fruits and fruit juices, vinegar, and even some natural waters will react
with these carbonate rocks. And there is little that we can do about it!!
Many a posh resort and nice residences with originally finely polished or
honed limestone surfaces on their bartops or kitchen-tops have discovered
this.
Another
key characteristic of limestones is their softness relative to other rocks
such as granites. The softness is mainly a function of the mineral
composition. Calcite typically has a hardness of 3 on the Mohs scale of
hardness whereas dolomite and aragonite are a bit harder (3.5 to 4). Because
of this softness, it can be easily scratched by most things in the house
such as cutlery, coins, tools, keys, toys, etc., so there should never be
the expectation that nicely polished or honed limestone surfaces are going
to stay that way for ever.
A
physical feature of limestones which is important geotechnically is its
intrinsic porosity. Many limestones, particularly the biogenic
ones, have a medium to high degree of porosity. Porosities of 10-30% are
not unusual in the French limestones and the famous Australian limestone
from Mt Gambier has a porosity of 50%.
But
although technically weak and very absorbent, certain construction techniques
(both old and modern) allow them to be used successfully and effectively.
In cold climates, however, a porous limestone can suffer rapid degradation
due to freeze-thaw cycling and some protection might be required.
One
important structural aspect inherent in most limestones is the
presence of very fine, squiggly lines which are often brownish. These structures,
called stylolites, are pressure-solution features formed during the compaction
and lithification of limestones. Mostly, the brownish colour is due to hydrated
iron oxide but there can also be concentrations of clays and/or sulphides.
Because these stylolites are natural planes of weakness and can often transmit
fluids because they are not fully closed, any expanding clays can react
to fluids and physically weaken the limestone.
A
final comment is on the observation that over the last few years or so,
a number of "marbles" have reverted to "limestones".
In the stone trade, it has been long accepted that any limestone that can
be polished can also be called a marble. If a limestone has undergone some
post-depositional (diagenetic) modifications some recrystallization will
have occurred. The crystals might still be quite small but the crystalline
minerals of the limestone will accept a polish. Because marbles are "old
hat" and limestone has become the buzzword of many architects, there
is now a strong trend to market mildly recrystallized limestones that were
once called marbles. Indeed, it is this group that appears to be capturing
a significant portion of the stone market. Incidentally, some limestones
from Oman and Saudi Arabia were seriously considered for the external cladding
as was the beige-coloured Jurastone from Germany. (Dr. Hans), Australia |