Microstructural evolution of limestone calcined clay cement (LC 3 ) paste: Insights from hydration to drying using 1 H-NMR relaxometry

Understanding the microstructure change during drying in limestone calcined clay cement (LC 3 ) is crucial for promoting low-clinker cementitious materials and reducing the environmental impact of cement production. Building on microstructural change during hydration, this study investigates the pore structure and phase change in LC 3 and its influence on macroscopic shrinkage. Composition analysis of the hydrates was conducted using X-ray diffraction and energy-dispersive X-ray spectroscopy. Additionally, proton nuclear magnetic resonance relaxometry was used to determine pore structure. Microscopically, the reacted metakaolin residues yield NMR signals that fall within the same T 2 range as those typically assigned to interhydrate pores within calcium alumino-silicate hydrate (C–A–S–H), yet they originate from different structural domains. The low Ca/(Al+Si) ratio in LC 3 results in sparsely packed C–A–S–H with interlayer spaces that remain structurally stable and retain water strongly during drying, showing little change even under low relative humidity. During drying, water is mainly held in and released from gel, interhydrate, and capillary pores. LC 3 exhibits significantly lower shrinkage than OPC, which is attributed, based on the C–A–S–H structural model, to its lower Ca concentration and the resulting interlayer structure that remains stable and water-retentive at low humidity.