Evolution of carbonation in cement blends incorporating thermally-mechanochemically activated low-kaolinite mixed-layer clays: Insights into hydrate phase assemblage, C-A-S-H structure and porosity

Low-kaolinite clays represent a globally abundant resource for reducing clinker content, yet their durability remains poorly understood. This study examines the carbonation behavior of a clay blended binder incorporating 30 wt% of a thermally-mechanochemically activated 2:1 mixed-layer clay, with emphasis on hydrate assemblage evolution, C-A-S-H chemistry, and pore structure. Drying was decoupled from CO₂ exposure to distinguish physical conditioning from chemically driven carbonation damage, and curing duration was varied to access different hydrate states in this slow-reacting system. Carbonation proceeds through CH depletion, AFm/AFt destabilisation, and C-A-S-H decalcification; however, the extent of degradation depends strongly on the chemical maturity of the gel at the time of exposure. Binders carbonated after extended curing show markedly reduced decalcification (Ld ≈ 34% versus 92% in short-cured samples) and limited pore coarsening. Drying induces gel contraction and pore restructuring, but severe degradation occurs only when carbonation chemically decalcifies the gel. These results demonstrate that carbonation resistance in low-kaolinite binders is governed by hydrate assemblage evolution and chemical maturity of C-A-S-H, reflecting carbonation pathways distinct from those assumed for OPC and LC 3 systems.