Clay is a term with different interepretaions. In CemGreen, we have a strong technical background within geology, and as such use the standard definition of agglomerate clay minerals in particles sizes less than 2 microns for clay.
Clay minerals belong to several different groups, of which Kaolin, Smectite, Illite, and Chlorite are the most abundant. Although usually found co-located and thoroughly mixed up in the same raw material, they have rather different chemical, mineralogical and cementitious properties.
In order to use them in hydraulic cement applications, they all need to be heat treated in a process called calcination. Due to their chemical differences, the different clay mineral groups (and subgroups) need to be calcined at different temperatures and for different time periods in order to achieve the maximum pozzolanic potential. A lot of research has over the last decades mapped many of these properties for some of the types of clay minerals.
However, due to the geological processes that have ultimately resulted in the current raw materials distribution, pure raw materials deposits (usually the object of scientific studies) are almost never found in nature. With the exception of some reasonably pure kaolinite deposits, a strong mixing of all the clay minerals groups are quite normal. But, this mixing is far from homogenous, and can change drastically from one cubic-meter to the next. This large variation in the clay raw materials, makes it difficult to predict what the resulting cementitious properties will be, and in general lowers the average quality of the produced calcined clay product.
CemGreen has specialized knowledge in calcining these clay minerals mixtures - or clays - such that the highest pozzolanic potential can be reached. We have also expanded upon the standard way of producing calcined clays, such that the variability of the produced material can be greatly reduced, resulting in an overall higher minimum strength of 10-15%, allowing for higher substituion rates, or achieving higher compressive strengths, when mixed with limestone-based Portland cement.