Blast furnace slag (BFS), a high-volume byproduct resulting from iron-making processes, can be considered as a low-cost and abundant precursor for preparing layered double hydroxide (LDH) compounds. Here we demonstrate that a Ca-based LDH compound (hydrocalumite) synthesized from waste BFS through facile two-step procedures and its derivatives work as useful heterogeneous base catalysts for multiple chemical reactions including the Knoevenagel condensation, oxidation of alkylaromatics with O2, transesterification, and cycloaddition reaction of epoxides with atmospheric CO2. Structures were verified by using XRD and thermogravimetric analysis. The surface basicity and coordination geometry of the active metal species that substantially affect the catalytic activity were investigated by CO2-temperature programmed desorption (TPD) and X-ray absorption fine structure (XAFS) measurements, respectively. These characterization results revealed that the slag-derived impurity elements, such as Fe, Ti, and Mn, effectively act either as active sites or as catalyst promoters in particular reactions and that the kind of guest counter anion (Cl− or NO3−) also plays a key role for achieving high catalytic efficiencies. In any reaction, the catalyst was easily separated by filtration and recyclable in multiple catalytic runs with retention of its activity and fine selectivity, irrespective of its considerable impurity level. It is believed that the slag-made hydrocalumite can replace existing LDH catalysts as a low-cost alternative and potentially contribute to sustainable chemical processes.