Evaluation of Catalysts Derived from Construction Waste Combined with CaO for Biodiesel Production

Construction and demolition waste materials were evaluated as low-cost mineral supports for eggshell-derived CaO catalysts in the transesterification of canola oil to biodiesel. Natural mordenite, waste brick, and concrete aggregates were combined with 60–80 wt% CaO, and the resulting materials were characterized by X-ray diffraction, scanning electron microscopy, energy-dispersive X-ray spectroscopy, and basicity measurements. CaO incorporation produced Ca-rich surfaces dominated by Ca(OH) 2 and CaCO 3 phases, which progressively covered the original support features and generated strong basic sites within the Hammett range of 9.7 < H_ < 12.1. However, the concentration of accessible basic sites did not increase in proportion to CaO loading, indicating a surface saturation effect governed by Ca-phase dispersion and exposure. Under mild reaction conditions, 80 wt% CaO catalysts yielded approximately 78–79% biodiesel, regardless of the support, confirming that the Ca-derived surface phases primarily governed catalytic performance. Reuse tests showed partial reactivation after recalcination, followed by a progressive decrease in catalytic performance, the mechanism of which could not be conclusively established without post-reaction characterization. The biodiesel produced met the relevant ASTM D6751 specifications and exhibited characteristic ester bands in FT-IR spectra.