An approach to estimate the strength contribution of calcined clays in blended cements

Current literature about calcined clays as future Supplementary Cementitious Material (SCM) is often limited to the role of 1:1 dominated clays as raw material. However, such clays are the exception in Europe: mixtures of various clay minerals represent the predominant material resources – so-called common clays. Through a comparison with 1:1 dominated clays, this article looks specifically at the suitability of illitic, smectitic and common clays as SCM. The reactivity of eleven different clays was assessed via the R 3 tests and compressive strength tests on mortar with six different cements. The strength contribution of 2:1 dominated clays proofed relevant. This holds especially for calcined illite at 2 days and for calcined smectite at 28 days. A significant increase in strength was observed with the 1:1 dominated clays only at 28 days. The strength contribution of the calcined clays varied depending on the cement used. A mathematical model correlates the mineralogy and fineness of the clays with the Activity Index of the blended cements, making it possible to predict strength at 2 and 28 days. This is also possible with the R 3 test, since the heat release of a calcined clay correlates with the Activity Index of the corresponding blended cement. • Kaolinite content is not relevant for early strength contribution of a calcined clay. • Differentiation of 2:1 dominated clays into illitic and smectitic clays is necessary. • Strength contributions of all calcined clays vary depending on the cement used. • Clay mineralogy or R 3 test can be used to predict strength of clay blended cements.