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The description and, in particular, the classification of aggregates in a manner appropriate to their use in the construction industry has long posed problems, not only of a scientific nature but also from practical and commercial points of view.

Naturally occurring rock materials can be classified in a variety of ways, the method chosen depending on the nature of the rock and the use for which the classification is required. Age, colour, fossil content, grain size, mineralogy, mode of formation and compressive strength are but some of the many approaches that have been used. The most common method is that developed from the classical geological approach, which is based essentially on the mode of formation. Hence natural rock material is divided into three main classes: igneous, sedimentary and metamorphic. These groups are then subdivided, principally on the basis of their mineralogy and texture.

The numerous subdivisions possible in this fundamental geological system inevitably results in a nomenclature which is too cumbersome for general use in the construction industry. As a consequence, various schemes have been developed to simplify the classification of aggregates, some intended for general use, others to meet specific purposes.

Some level of petrographic examination is necessary for virtually all classification schemes and a detailed petrological description can be helpful in assessing the performance of an aggregate and in detecting potentially deleterious substances.

This chapter reviews current classification schemes for natural aggregates and discusses their development. A recommended approach for classification is presented and procedures for the

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