Most clays are composed essentially of minute particles of one or more of the clay minerals of which the kaolinite, montmorillonite, and illite groups are most important. The clay minerals occur in flake-shaped particles, possess base-exchange capacity, and exist in or are reducible to extremely small grain sizes on working with water. Different clay minerals possess these properties in varying degrees.
The clay mineral component is the chief factor determining the properties of a clay. In general, plasticity and bond strength caused by the clay minerals decrease in the following order: montmorillonite, illite, and kaolinite. In many clays, the plasticity and bond strength mainly result from the presence of the montmorillonite minerals or some members of the illite group, although these constituents may compose only minor amounts of the clay. The influence of the clay minerals on other properties is considered.
The green properties of clays are also related to the character of the exchangeable bases carried by the clay minerals. The fundamental reasons for the differences in properties resulting from different mineral constituents and various exchangeable bases are considered. The properties of clays are related further to the effective size-grade distribution developed in actual use which frequently differs from the size-grade obtained by mechanical analyses.