Last edited 27 Dec 2020


Chert is a type of fine-grained and silica-rich sedimentary rock. It often contains microfossils and/or macrofossils, and can be found in a great variety of colours. It is often confused with flint which is a particular type of chert.

Chert occurs in geology as nodules, concretionary masses and layered deposits, and is characterised by its hardness and conchoidal fractures that create sharp edges. It was these properties that meant chert was used in early human civilisations for creating tools and weapons.

Chert is often used as an aggregate and as a material for road surfacing. It is particularly suited to road surfacing as rainwater firms and compacts it rather than making it 'muddy' like some other fill materials.

However, there can be problems when is used as an aggregate in concrete, such as the development of surface pop-outs as a result of freezing and thawing. This is due to the high porosity of weathered chert. In addition, some kinds of chert can undergo an alkali-silica reaction (ASR) when they come into contact with high-alkali cement which can lead to cracking or expansion.

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