REFRACTORY CERAMICS SYNTHESIS BY SOLID-STATE REACTION BETWEEN CaCO₃ (MOLLUSK SHELL) AND Al₂O₃ POWDERS

Calcium aluminate-based refractory ceramic was developed as an innovative refractory material, using garden snail (Helix aspersa) shells as a natural source of CaCO₃. A 1:1 molar ratio mixture of CaCO₃ from snail shells and commercial Al₂O₃ powder was prepared by means of high-energy mechanical milling. The mixed powder was compacted in cylindrical samples (disks) and consolidated by sintering at 1450°C and 1500°C for 1h. The density and porosity were evaluated using the Archimedes principle, while the mechanical properties (hardness, fracture toughness, and shear modulus) were determined by indentation and ultrasonic methods, respectively. The thermal shock resistance was tested by heating samples to temperatures between 900 and 1400 °C and subsequent quenching in water at room temperature. X-ray diffraction patterns of sintered samples indicate the formation of different calcium aluminate phases, such as CaAl12O19 (krotite/monoclinic), CaAl₄O₇ (grossite/monoclinic) and CaAl₂O₄ (hibonite-5H/hexagonal). The fracture toughness and shear modulus values of materials sintered at 1450 °C were higher (0.48 MPa-m1/2 and 59 GPa, respectively) than those of materials sintered at 1500°C (0.43 MPa-m1/2 and 55 GPa, respectively). Also changes in the bulk density, hardness and thermal shock resistance values were observed in materials sintered at 1450°C and 1500°C