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

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The cement industry contributes 8% of global CO 2 emissions, driving the use of supplementary cementitious materials (SCMs) such as calcined clays to replace cement. This study examines early-age hydration and nano-structural evolution of cement blended with calcined kaolinite (NK) and montmorillonite (NM) mixed-layered clays using in situ synchrotron X-ray scattering, Pair distribution function (PDF) analysis, and solid-state NMR. Blends with 30 wt% clay were monitored from 10 to 53 hours and at 7 days post-mixing. Synchrotron XRD–PDF showed that the ettringite formed rapidly in NK and then was consumed, producing hemicarboaluminate and monosulfoaluminate.NM maintained ettringite content due to higher calcite content. PDF and NMR confirmed a transition in calcium silicate hydrate (CSH) nanostructure from a Ca-rich hillebrandite-type (Ca/Si ≈ 2.0) at 26 h to a layered clinotobermorite-like phase (Ca/Si ≈ 1.2–1.7) at 7 days, indicating progressive silicate polymerization. NM exhibited the greatest growth in the amorphous phase (≈20.6%) and Si-O polymerization (≈100%), consistent with enhanced early reactivity. Combined PDF, NMR and SEM-EDS results indicated the development of a magnesium silicate hydrate (MSH)-like Mg-bearing silicate environment in NM, with Mg/Si decreasing from 0.63 to 0.23, while NK showed higher Al incorporation, forming dense calcium-aluminosilicate-hydrate (CASH) networks. Compressive strength correlated directly with the nanostructure. At 1 day, NM achieved 9.42 MPa compared to 5.83 MPa for NK, but at 7 days, NK reached 38.05 MPa, surpassing NM (35.25 MPa) due to the dense CASH structure formed. In situ synchrotron PDF analysis revealed key amorphous phases, enabling the design of low-clinker, low-carbon binders with tailored early age performance.

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