Hydration kinetics, phase assemblage and microstructural evolution of blended cements containing thermally activated marine dredged sediment

Maintenance dredging generates large volumes of fine-grained marine sediment for which disposal capacity is increasingly limited, while conventional supplementary cementitious materials (SCMs) are unevenly available and locally constrained in supply. Thermal activation converts clay-rich sediment into a locally available SCM, but the hydration mechanisms of such mineralogically complex precursors, in particular the spatial development of the hydrate assemblage that governs macroscopic performance, remain insufficiently explored. Marine dredged sediment from the Gulf of Koper, a mixed clay–silt material, was thermally activated at 750 °C and evaluated as a SCM at 10, 30 and 50 wt% cement replacement, with a natural volcanic pozzolan as reference. Precursor transformation, hydration kinetics, phase assemblage, microstructure, pore structure and mortar strength were resolved within a single dataset. XRD, TG/DTA and FTIR confirmed clay-mineral dehydroxylation, structural disordering and partial carbonate decomposition. The activated sediment exhibited pozzolanic activity and higher 7-day cumulative heat release than the natural pozzolan, with portlandite consumption between 2 and 90 days reaching 44.5% and 62.0% at 30 and 50 wt% replacement, against 24.7% and 43.8% for the reference. Quantitative SEM-EDS phase segmentation, validated by principal component analysis and combined with two-point spatial correlation functions, resolved not only the abundance of the hydrate phases but also their distribution and connectivity: sediment-derived Si and Al were progressively incorporated into an intermixed C-A-S-H phase, early-age portlandite clusters were consumed and fragmented, and hemicarbonate-dominated AFm domains expanded at constant cluster shape. Pore refinement accompanied these changes, and 180-day compressive strength reached 57.0, 49.0 and 29.7 MPa for the 10, 30 and 50 wt% TTS replacements, respectively. The results establish a mechanistic and standard-compliant basis for valorizing locally available dredged marine sediment as a cement replacement material.