Effect of the thermal annealing on the phase transitions of biogenic CaCO3 nanostructures
Authors: Ramón de los Santos Candelario , López Rodríguez Angélica Silvestre , Sifuentes Gallardo Pio , Hernández Rivera Miguel Angel , Pérez-Hernández German , Gárnica Romo Ma Guadalupe , Rivera Trejo José Guadalupe Fabián , Diaz Flores Laura Lorena 
Published: June 2019

The issue of the present research lays its foundation on the proposal of the Crassostrea virginica waste oyster shells (WOS) reuse to obtain calcium carbonate powder (CaCO3) and calcium hydroxide (Ca(OH)2) nanostructured, using thermal annealing treatments. The oysters shells were subjected to a previous physical grinding process to decrease their size (smaller sizes 0.074 mm). The parameter studied was the effect of annealing temperature (500, 700 and 900 °C in air atmosphere) on the structural properties and morphology of the powders by FTIR, XRD, SEM and HRTEM. The X-ray diffraction results indicate that the WOS in their natural state and thermally annealed at 500 °C  have two phases of CaCO3 the rhombohedral form for calcite with crystallite size around 24 nm and aragonite traces in orthorhombic phase. At 700 °C, the WOS powder is transformed into calcium hydroxide, also known as portlandite (Ca(OH)2), attributed to the absorption of water released during the thermal decomposition of CaCO3. This crystalline phase does not change when the temperature increases to 900 °C. The SEM and HRTEM analysis of WOS powders reveals that with a thermal annealing treatment it is possible to obtain   nanostructured CaCO3. FTIR analysis demonstrates the biogenic origin of CaCO3, due to amide groups. The nanostructured CaCO3 obtained by grinding and thermal annealing of WOS, can be used as drying agent, or as additive in ceramic and glass.
The issue of the present research lays its foundation on the proposal of the Crassostrea virginica waste oyster shells (WOS) reuse to obtain calcium carbonate powder (CaCO3) and calcium hydroxide (Ca(OH)2) nanostructured, using thermal annealing treatments. The oysters shells were subjected to a previous physical grinding process to decrease their size (smaller sizes 0.074 mm). The parameter studied was the effect of annealing temperature (500, 700 and 900 °C in air atmosphere) on the structural properties and morphology of the powders by FTIR, XRD, SEM and HRTEM. The X-ray diffraction results indicate that the WOS in their natural state and thermally annealed at 500 °C  have two phases of CaCO3 the rhombohedral form for calcite with crystallite size around 24 nm and aragonite traces in orthorhombic phase. At 700 °C, the WOS powder is transformed into calcium hydroxide, also known as portlandite (Ca(OH)2), attributed to the absorption of water released during the thermal decomposition of CaCO3. This crystalline phase does not change when the temperature increases to 900 °C. The SEM and HRTEM analysis of WOS powders reveals that with a thermal annealing treatment it is possible to obtain   nanostructured CaCO3. FTIR analysis demonstrates the biogenic origin of CaCO3, due to amide groups. The nanostructured CaCO3 obtained by grinding and thermal annealing of WOS, can be used as drying agent, or as additive in ceramic and glass.