CaCO₃ coating of off-specification fly ash for upcycling in cementitious materials

To address this challenge of shortage of specification-grade fly ash(SGFA) and recycling of off-specification fly ash(OSFA), this research proposes a novel approach of coating OSFA with nano-CaCO₃. This improves the surface characteristics of the particles and creates nucleation sites that enhance cement hydration, thus potentially improving the fundamental properties of cementitious materials incorporating OSFA. The first step in this process involves optimizing the quantity of nano-CaCO₃ coating on OSFA to achieve the best mechanical properties for concrete applications. Subsequently, experimental investigations were conducted to compare the mechanical properties of cementitious pastes prepared with original OSFA, OSFA and nano-CaCO₃ powders, and coated OSFA. Isothermal calorimetry and pore structure analyses indicated that the coating treatment transformed OSFA from a detrimental additive to a beneficial one for concrete. This coating significantly strengthened the OSFA particles and promoted cement hydration, consequently enhancing the mechanical properties of cementitious composites containing OSFA. This innovative method offers a promising solution for addressing the fly ash shortage in the concrete industry while minimizing the need for land disposal of OSFA.