Early age hydration of mixed-layer calcined clay cement blends: In-situ Synchrotron X-ray scattering and pair distribution function analysis

The cement industry accounts for approximately 8% of global CO₂ emissions, motivating the use of supplementary cementitious materials, such as calcined clays. This study investigates early-age hydration and nanostructural evolution of cement blended with 30 wt% calcined kaolinite (NK) and montmorillonite (NM) mixed-layered clays. In-situ synchrotron X-ray diffraction with pair distribution function analysis and solid-state nuclear magnetic resonance were applied from 10 to 53 h and at 7 days. NK reacted rapidly, consuming ettringite and forming hemicarboaluminate and monosulfoaluminate phases, whereas NM preserved ettringite due to higher calcite content. Calcium silicate hydrate evolved from a Ca-rich hillebrandite-type structure (Ca/Si ≈ 2.0 at 26 h) to a layered clinotobermorite-like phase (Ca/Si ≈ 1.2-1.7 at 7 days). Magnesium silicate hydrate phases were identified in NM, with Mg/Si decreasing from 0.63 to 0.23. NK showed greater aluminum incorporation, forming dense calcium-aluminosilicate-hydrate networks, resulting in higher 7-day compressive strength (38.05 MPa) development pathways.