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Stone Explained - Limestone

Leap Stone LTD (July 18,2007)

I would like to be able to refer readers to a book that bridges the gap between the academic study of limestones and the interests of the construction industry, but unfortunately I do not know of one. The information I have given here is well known to geologists, but tends to be published in rather obscure publications.

Calling a rock a limestone is saying nothing more than that it is calcium carbonate (CaCO3). The look of that calcium carbonate under a scanning electron microscope (SEM) can be quite different in limestones from different quarries.

SEMs produce useful microscopic pictures because they appear three dimensional, like the world around us. Often the rocks are first impregnated with plastic, the limestone etched away with acid and the plastic studied under the microscope, when the bits that slick out are actually the holes in the original rock.

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 bottles.

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.

In this country, 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.

A problem which can occur with some limestone is the presence of mud between the grains. The mud contains a high proportion of micropores which interconnect and can carry moisture far into the stone or even right through it, making it susceptible to frost or pollution attack. A lot of pores also means a large internal surface area, which weakens the stone.

The way to a fuller understanding of stone is through a mixture of petrography (description of the stone, especially with photographs) and the sort of information which comes from some Continental tests. Such a combination should allow the producer, the specifier and the end user to predict really quite well how the stone will perform in use.

SEM photographs are particularly useful and the results are easier to interpret than those obtained from thin sections (0.03mm thick slices of rock viewed down an ordinary microscope) that geologists traditionally use. SEMs are becoming standard pieces of equipment and will not add a lot to the cost of tests.