Why optimal calcination temperature varies with clay mineralogy: Atomic-scale evidence from Si K-edge XANES combined with surface and dissolution analyses

Calcined-clay reactivity is commonly interpreted using XRD-derived amorphous content and changes in Al coordination, but these descriptors do not fully explain why clays with similar amorphous fractions show different dissolution behaviour. A less frequently examined variable is the Si atomic environment, particularly the extent to which calcination attenuates order-sensitive medium-range silicate features inherited from the precursor clay. Here, kaolinite-, illite- and smectite-rich clays were calcined between 600 and 900 °C and analysed by XRD–Rietveld, alkaline dissolution, BET/PSD, XPS and Si K-edge XANES. Kaolinite reached ~95 wt% X-ray amorphous/unmodelled fraction (XAUF) from 600 to 900 °C, but its 48 h composite dissolution degree peaked at 12.70 wt% after calcination at 700 °C and declined to 10.04 wt% at 900 °C. Si K-edge XANES showed strong attenuation of post-edge features associated with ordered silicate frameworks, indicating disruption of inherited medium-range Si O ordering. The decline above the dissolution maximum was accompanied by an upward shift of the Si K-edge main resonance, while post-edge order remained low, consistent with reorganisation within the poorly crystalline Si environment. In the illite-rich clay, persistent post-edge features across 600–900 °C indicated retention of ordered Si contributions and coincided with a comparatively modest dissolution response. Smectite-rich samples showed progressive weakening of the Si post-edge features, indicating reduced medium-range Si coherence, while the decrease in BET surface area at high temperature was accompanied by lower dissolution despite increased XAUF. Across the studied materials, dissolution behaviour was more consistently interpreted by considering Si-framework changes together with BET-accessible surface area and surface-chemical trends than by XRD-derived disorder alone.