Due to the negative carbon footprint of cement production and the decreasing availability of commonly used supplementary cementitious materials (SCM), the thermal activation of non-kaolinite clays for use as pozzolanas was investigated. The calcination potential of two bentonites was studied and compared with the performance of kaolin clay in terms of density, specific surface area, granulometry, thermal analysis, and chemical and phase compositions. Based on thermal analysis, the clays were calcined at 500°C, 550°C, 600°C and 650°C (for three hours) with a heating rate of 10°C min −1 . Then, blended pastes were prepared with the studied clays with a substitution rate of 10% of Portland cement. The pozzolanic activity of prepared pastes was investigated by isothermal calorimetry, lime saturation test (LST), changes in mechanical properties and pH activity test. Tests performed on raw clays revealed the following findings: i) temperatures needed for activation were above 550°C; ii) during firing, the bentonitic clays tend to agglomerate, which, however, can be fixed by additional milling. Results of testing on blended cements can be summarised as follows: iii) all activation temperatures show good potential for thermal activation of bentonites in terms of isothermal calorimetry; iv) lime saturation test showed a slower onset of pozzolanic reaction for bentonites compared to kaolin; v) best mechanical properties were achieved for clays heated to 600°C. • The activation of bentonites was successful for temperatures above 550°C. • Bentonitic clays tend to agglomerate during thermal activation. • Hydration heat curves of bentonitic clays overlapped for all studied temperatures. • Ca(OH) 2 consumption by bentonite clays was initially slower but later accelerated. • Best values of mechanical properties of bentonitic clay pastes were found for 600°C.
Zatloukalová et al. (Fri,) studied this question.