A method of capturing carbon dioxide from a gaseous mixture at ambient temperature or higher, comprising obtaining a slurry of waste carbide slag obtained after the manufacturing of acetylene. Heating the slurry to a temperature of approximately 105° C. for two hours and sufficient to dry the slurry until a water content of about 20% of a dry mass of the carbide slag is achieved, thereby producing activated dried carbide slag. The method including contacting the activated dried carbide slag with a gaseous mixture containing CO2, thereby permitting the activated dried carbide slag to capture CO2.