This paper considers the effect that clay in rocks has on the performance of asphalt materials.
The distinction between clay and dust is first emphasised. The processes involved in the
formation of clay minerals are summarised. This considers the different types available, their
properties and distribution within differing types of rock. The relationship between harmful types
of clay and specific rock types is given. The paper then describes standard and non-standard
methods that have been developed to determine the presence of clay minerals and assess their
effect on aggregate and asphalt properties. The paper concludes by emphasising the need to
consider the influence of clay minerals when trying to predict the performance of aggregate and
asphalt materials used in highway construction.