All limestones start as floors of shallow
tropical seas. The famous stones of Portland and Bath were
laid down 150million years ago when what is now that area
of England was more like the Bahamas are today. Limestone
is still being formed in parts of the tropics, where some
perfectly good, hard stone is no more than 30 or 40 years
old. It has even been known to contain impurities like cola
Sea water contains carbon dioxide (CO2),
which escapes when the water is warmed by the sun. When this
happens, calcium and bicarbonate ions in the water combine
to form calcium carbonate, just like scale forming in a kettle.
This crystallises as calcite.
The famous ooliths (tiny spheres) in limestones
are grains of sand or pieces of shell around which calcium
carbonate which precipitated from the sea has stuck. The cement
that sticks the ooliths together also consists of calcite
crystals, which grew either on the sea floor or later in the
rock when it was buried.
Shells of sea animals form grains in limestones,
and some of them, such as pieces of starfish and sea lilies,
promote the growth of cement crystals around themselves particularly
well and result in limestones which are especially strong.
Some limestones, such as Portland and Ketton, take their strength
from their ooliths. They have little cement in between, but
that does not mean they are inferior stones because in a building
they are used under compression and as long as the ooliths
are strong the lack of cement does not matter. In fact, such
stones have the additional benefit of being relatively easy
to cut and carve.
Other limestones, such as Bath stone, gain
their strength not from the ooliths, which are soft and weak,
but from the calcite cement in between the grains.
The larger the crystals of the cement, the
stronger that cement will be. It will also be smoother, which
means the stone will take a polish. Sea lilies, sea urchins
and starfish promote growth of large crystals around themselves.
Since there are large amounts of these in some carboniferous
limestones such as Griffeton Wood, Swale Dale and the Irish
blue limestones, these stones will take a good polish.
Over time, large crystals will grow at the
expense of smaller crystals in any limestone, but especially
if the stone is buried and warms up. And if it happens to
be buried next to where granite is being forced up from deep
in the earth near to the edge of a continental plate (when
the granite will be at 800-900°C), the limestone will
re-crystallise and become marble.
One of the reasons why some of the French
limestones take a good polish is that they got mixed up in
the formation of the Alps resulting from Africa bumping into
Europe over the past 50million years. As the land became buckled
and folded, some of the limestones became buried and heated,
causing the growth of large crystals in the stone.
From a building point of view, the most
important aspect of the stone is the pores (the holes in the
stone), particularly those of less than 0.005mm across. These
will affect the way the stone weathers. Some of the tests
regularly used in Europe to predict stone durability, such
as the saturation coefficient, the capiliarity and the effective
porosity, are specifically designed to give an indication
of the small pores (the microporosity) of a stone.
The salt crystallisation test can accurately
distinguish the more microporous stones if it is carried out
and interpreted accurately The test consists of 15 cycles
of soaking stones in a salt solution and drying them. Salt
crystals will grow in the pores which may cause some stone
to break away It is this loss of stone which the test measures.
But the test is easy to get wrong and is highly susceptible
to slight variations in temperature, salt concentration and
drying time between soakings.