Effect of Heat Treatment on Polymorphism and Particle Size Distribution of Calcium Carbonate Nanoparticle Synthesized via Mechanochemical Process

The synthesis of calcium carbonate (CaCO3) nanoparticles has gained an increasing interest due to their improved properties and diverse industrial applications. Among various synthesis techniques, the mechanochemical synthesis process has emerged as a promising route for nano-CaCO3 synthesis. A high-energy ball mill is required for synthesizing nano-CaCO3, whereas post-milling heat treatment facilitates completing the reaction that remains incomplete during milling. Post-milling heat treatment may also influence the properties of synthesized CaCO3, which has not yet been thoroughly investigated. This study investigated the influence of post-milling heat treatment on the polymorphs, micromorphology, and particle size distribution of CaCO3. The results indicated that the heat treatment of the as-milled powder enhanced the homogeneity of crystal polymorphs while maintaining the particle sizes within the nano-range (<100 nm). X-ray diffraction (XRD) analysis identified two polymorphs (vaterite and calcite) in samples obtained from different milling intensities. However, after heat treatment, all vaterite transformed into calcite. A bimodal particle size distribution was observed in CaCO3 nanoparticles and was influenced by both the milling and heating intensities. It was observed that 60 min heat applied to 30 min as-milled powder was enough to produce nano-CaCO3 (<50 nm) where the percentage of larger particles (<250 nm) became negligible (~1%). Micromorphology images confirmed the transformation of crystal polymorphs and the reduction in particle size.