Shale is a sedimentary rock formed when different kinds of granites, micas, feldspars, sand and clays is slowly being deposited in inland seas or along rivers or coastal areas. As the environment changes, the layers composition will also change. Therefore shale has its characteristically layered apperance. The binding force between each layer is quite small and the rock can usually be broken apart layer by layer.
Some of the constituents of shale have pozzolanic abilities. A pozzoloanic material chemically binds when it comes into contact with water (like cement), and the mixture is allowed to react. Clay minerals are highly pozzolanic after being calcined, whereas granites and sand are not, and the latter are therefore chareacterized as inert materials.
Shale is usually ground to a very fine powder (like limestone) and then calcined. But due to the inert components, the pozzolanic potential and hence the resulting compressive strength of the concrete, will be less than for pure calcined clays. Shale are, unlike clay minerals, very often located together with the limestone, and therefore must be quarried and physically removed in order to reach the limestone.
CemGreen has recently developed a new calcination approach wich increases the pozzolanic potential (with as much as 12%) while at the same time reduces the large discoloration that comes from the oxidation of the transition elements, usually present in shale, during the calcination. This is achieved without the use of an reducing atmosphere, which is costly and requires complex equipment to maintain.