In-situ synchrotron X-ray diffraction and pair distribution function analysis of clay structural evolution during calcination

This study presents an in-situ synchrotron X-ray diffraction (XRD) and pair distribution function (PDF) analysis under controlled heating conditions of three raw clays, kaolinite-bearing mixed clay (NK), montmorillonite‑carbonate-rich (NM), and illite-bearing mixed clay (NI), from 23 to 1000 °C and during cooling. NK exhibited the most pronounced calcination-induced structural disordering. Kaolinite decreased from 26 wt% to ∼9 wt% by 700 °C, accompanied by a marked decrease in the PDF-derived spdiameter from 38.8 Å to ∼15 Å, consistent with increased structural disorder and ∼ 45% lower coherent Al O intensity. Al O bond length increased from 2.127 Å (at 100 °C) to 2.16 Å (at 900 °C), while Si O relative intensity reached a minimum (∼0.78 at 500 °C). Above 900 °C, mullite and ilmenite formation indicates progressive recrystallisation of the disordered aluminosilicate structure. Together with the partial recovery of medium-range structural coherence, this supports a favourable activation range of approximately 600–800 °C. NM followed a decarbonation-controlled pathway, with ∼60–80% calcite reduction, complete dolomite loss, and monticellite formation. Short-range Si O and Mg/Al-O correlations shifted to lower r (decreased by 0.007 Å and 0.022 Å, respectively), while peak intensities increased by ∼30–35% at 800–900 °C, indicating densification into Ca Mg silicates. NI showed the greatest structural resilience, retaining Si O connectivity and medium-range coherence despite moderate Al O bond lengthening. The inferred reactivity potential follows the order NK > NM > NI, governed by Al O distortion and coherence collapse before crystallisation. These mineralogy-dependent structural indicators can guide the selection of favourable calcination ranges, limiting high-temperature recrystallisation, supporting the optimisation of clays as supplementary cementitious materials.