Limestone for building

St Paul's Cathedral, London, designed by Sir Christopher Wren, 1668-1697.

[edit] Introduction

Limestone is a sedimentary rock that was laid down at the bottom of shallow lakes and seas and compacted over four geological eras: Cretaceous, Jurassic, Permian and Carboniferous, spanning 70 – 345 million years ago.

[edit] Formation

By ‘sedimentary’ it is meant that the particles which make up limestone originated from other places but were transported and deposited mainly by water but also by wind. Limestone often comprises the skeletal remains of marine organisms such as corals and molluscs which are the primary source of calcite in limestone although other sources do occur. Chemically, limestone comprises different forms of calcium carbonate (CaCO3).

The presence of marine organisms and other fossil inclusions often form an attractive feature when the stone is polished or honed.

[edit] Varieties

Forming over such an extended geological timescale has resulted in different types of limestone, each with characteristic properties, ranging from chalk, ragstone and clunch, through to oolitic and liassic limestones, to dolomitic and carboniferous varieties.

Due to the presence of a variety of minerals, limestone presents numerous colours, ranging from grey Portland stone, to creamy Bath stone, the brown limestone of Dorset and the dark grey Purbeck stone, also from Dorset. Purbeck can be polished to give a faux marble effect and so came to be known as ‘Purbeck marble’, popular in the 19th century for high quality architectural interior decoration and thin columns.

[edit] Uses in construction

Limestone can either be categorised as ‘soft’ or ‘hard’. As a building material, it is both abundant and versatile and has been used globally, but especially in Europe and the US. Global landmarks include the pyramids of Egypt, numerous Gothic cathedrals in Europe, particularly France, and Italian city walls and castles. St Paul’s Cathedral in London, designed by Sir Christopher Wren is built from Portland stone.

During the 19th and early 20th centuries limestone, particularly Portland stone, became popular in the UK for building banks, civic buildings and educational establishments, many of which were in the classical style. More recently, a notable use of a creamy-coloured limestone from France was Foster + Partners' Great Court in the British Museum, London.

Limestone is relatively easy to cut and carve (some limestone varieties are easier to work than others but it is ‘softer’ than some sandstones) and its uses in construction are wide ranging, including:

• As a loadbearing stone for masonry walls and columns;
• For stone roof ‘slates’ (thinner and lighter than sandstone varieties) as seen in the Cotswolds and South Wales;
• Lime mortars;
• Traditional stucco (made from lime, sand and water);
• Limewash (a whitewash based on ground chalk or kilned lime);
• As a key component in concrete;
• As a cladding for buildings and for rainscreens;
• Paving slabs;
• Aggregate used in road bases, and
• As a decorative element in residences, such as for gardens and fireplace surrounds.

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