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The development of nontoxic bio-waste based catalyst for efficient recycling of textile dye wastewater to potable water is paramount to support a sustainable environment. This research work investigated a crystallographic evolution of CaO from the chicken eggshell wastes via a simple calcination process for the photocatalytic degradation of the methylene blue dye. The physicochemical properties of the activated CaO catalyst were studied using characterization techniques such as X-ray diffraction (XRD), Raman spectroscopy, Fourier-transform infrared (FTIR) spectroscopy, ultraviolet-visible (UV-Vis) spectroscopy, and scanning electron microscope (SEM). The crystallite size was estimated as ∼55 nm for the prepared CaO. The sharp bands at 358 cm−1 and the broad band at 675 cm−1 in the Raman spectroscopic analysis have revealed a function attributed to the Ca–O bonds. The energy band gap of prepared CaO was calculated by the Tauc equation using the UV-visible reflectance spectrum. The estimated energy band gap has a maximum value of 5.54 eV. However, a lower bandgap 2.27 eV was also estimated. SEM micrograph revealed the multi-arm/angular and rod-shaped microstructure with a width of 1–2 µm of the arm. The ring-shaped appearance encouraged the porous behavior of the prepared CaO. The photocatalytic activity of the prepared CaO was performed for the degradation of the methylene blue dye in distilled water under natural sunlight in the ambient environment. The maximum photocatalytic degradation efficiency was observed as ∼ 98% in 180 min. The research work incites the possible realistic application of bio-waste based inexpensive photocatalysts for textile/industrial wastewater remediation. [ABSTRACT FROM AUTHOR]