Thermal analysis and kinetics of Estonian clays under varied calcination atmospheres

Abstract The kinetics of clay dehydroxylation process involve multiple elementary steps and complex parallel reaction pathways, including organic matter oxidation, inorganic impurity transformation (iron oxides, pyritic sulphur), and carbonate decomposition (calcite, dolomite). These concurrent reactions significantly influence reaction rates and apparent kinetic mechanisms. This study investigates the effect of CO 2 -rich atmospheres on clay calcination as a foundation for co-calcination with carbonate- and sulphate-rich alkaline waste materials for supplementary cementitious material (SCM) production optimization. Three Estonian clays (Arumetsa-C, Kunda-C, and Aseri-C, where ‘C’ stands for clay) were analysed under model air (21%O 2 /79%Ar) and flue gas (16% CO 2 in N 2 -O 2 ) atmospheres using a Setaram Setsys Evo 1750 thermoanalyzer with mass spectrometry and kinetic modelling based on the Friedmann method (AKTS software, 2.5–10 K min −1 ). Initial XRD analysis revealed ~ 2%–11% kaolinite with illite as the dominant phase and dolomite traces, particularly in Arumetsa-C. TGA-DTG-MS showed major dehydroxylation (~ 350–700 °C) with water evolution. Switching to flue gas atmosphere decoupled dolomite decomposition (~ 650 °C) from the dehydroxylation step, delaying it beyond 800 °C. Flue gas marginally lowered activation energy ( E a ) for dehydroxylation, with notable divergence (up to 60 and 44 kJ mol −1 ) at reaction progress α = 0.6–0.8 in Arumetsa-C and Kunda-C due to decarbonation shifts. FTIR confirmed -OH group loss and Si–O band stretching alteration, while XRD showed clay mineral peak reduction indicating structural rearrangement from dehydroxylation. These findings highlight that CO 2 -rich atmosphere significantly influence clay thermal behaviour, especially with carbonate impurities present.