The rapid increase in agro-industrial waste generation has intensified the need for sustainable strategies to transform these residues into high-value functional materials. Among these, calcium-rich wastes—such as eggshells, animal bones, mollusk shells, biomass ashes, and industrial sludges—represent abundant and renewable sources of raw materials for the development of eco-friendly catalysts, adsorbents, and photocatalysts. This chapter presents a comprehensive overview of the recovery, transformation, and application of calcium-containing residues within catalytic, photocatalytic, and adsorptive systems. Emphasis is placed on the conversion of biogenic precursors into calcium oxide (CaO), hydroxyapatite (Ca10(PO4)6(OH)2), and composite materials through thermal, hydrothermal, mechanochemical, and green synthesis routes. Their performance in biodiesel production, heavy-metal removal, and photodegradation of organic pollutants is critically analyzed, highlighting the interdependence between structure, surface chemistry, and functional efficiency. Furthermore, the integration of these processes within circular-economy and carbon-neutral frameworks is discussed, demonstrating how the valorization of calcium-rich waste contributes to waste minimization, emission reduction, and sustainable technological innovation. The chapter concludes with an outlook on future challenges related to process standardization, scalability, and regulatory implementation, offering guidance for advancing calcium-based biogenic materials toward industrial and environmental deployment.