Thermal Evolution of Scallop and Surf Clam Shell: Phase Transformation Insights from Solid-State ¹³C NMR and...

Globally, large quantities of discarded shells, including scallops and surf clams, are often accumulated at disposal sites, causing environmental degradation. Calcium carbonate (CaCO₃) in seashells predominantly exists in polymorphic forms, such as aragonite and calcite, and undergoes phase transformation upon thermal treatment. This study investigates the thermal evolution of scallops and surf clam shells at various calcination temperatures to characterize changes in crystalline structure, pore morphology, and to review their potential as partial cement replacement materials. The seashells were mechanically ground into fine powder and subsequently calcined at 650 °C, 750 °C, and 850 °C for 7 h. The CaCO₃ transformation was comprehensively characterized using solid-state ¹³C NMR, XRD, TG-DTA, and SEM to resolve carbonate speciation, phase transitions, decomposition thermodynamics, and microstructural evolution. The solid-state ¹³C NMR was employed to analyze the local chemical environments of carbonate species and to provide additional spectroscopic evidence supporting the phase identification obtained from XRD and TG-DTA. Furthermore, XRD, TG-DTA, and SEM analyses consistently confirmed the phase transformations and associated changes in pore structure and crystallinity induced by calcination. These findings suggest that solid-state ¹³C NMR can be a useful technique for tracking CaCO₃ decomposition, while calcination may enhance the potential of seashell biowaste as a sustainable cementitious material.