2. The composition of clay materials
Clay materials are composed of solid, liquid and vapour phases. The solid phases are of mineral and organic phases that make up the framework of the clay materials. The mineralogy can be broadly subdivided into the clay and non-clay minerals, including poorly crystalline, so-called ‘amorphous’ inorganic phases. By definition, minerals are crystalline solids with well-ordered crystal structures but clay minerals and other inorganic phases in clay materials are often poorly crystalline compared to minerals such as quartz and feldspar.
Some clay materials may be dominated by one mineral phase, e.g. smectite in bentonites, opal in diatomaceous earths. However, most clay materials are composed of heterogenous mineral mixtures. Based on the bulk mineral analysis of over 400 samples, Shaw & Weaver (1965) reported the modal mineralogical composition of siliciclastic mudrocks to be:
60% clay minerals
30% quartz and chert
1% organic matter
1% iron oxides
There is a general increase in the predominance of clay minerals in sedimentary rocks with decreasing grain size (Fig. 2.1) (Blatt et al. 1972). However, it needs to be stressed that, whilst clay minerals are usually significant, if not predominant, phases in clay materials, other mineral phases are usually present in varying amounts and can significantly affect the properties and behaviour of the materials.
In soils, mineral and organic compositional variations reflect the weathered parent rocks and the physical, chemical and biological factors controlling the soil forming processes (see Chapter 3).
The liquid and vapour phases, of which water is usually the most important, occur either as ‘bound phases’, adsorbed onto the surfaces of the solid particles, or as ‘free phases’ within pore spaces.
Figures & Tables
Concluding the trilogy on geological materials in construction by specially convened Geological Society working parties, this authoritative volume reviews many uses of clays, ranginf from simple fills to manufactured products. Comprehensive and international coverage is achieved by an expert team, including geologists, engineers and architects, who met over six years to produce the book. Packed with information prepared for a wide readership, this unique handbook is also copiously illustrated. The volume is dedicated to the memory of Professor Sir Alec Skempton.
Various definitions of ‘clay’ are explored. Clay mineralogy is described, plust the geological formation of clay deposits and their fundamental materials properties. World and British clay deposits are reviewed. New compositional data are provided for clay informations throughout the British stratigraphic column. Investigate techniques and interpretation are considered, ranging from site exploration to laboratory asessment of composition and engineering performance.
Major civil engineering applications are addressed, including earthworks, earthmoving and specialized roles utilizing clays. Traditional earthen building is included and shown to dominate construction in places. Clay-based construction materials are detailed, including bricks, ceramics and cements. The volume also includes a comprehensive glossary.