Abstract: The targeted synthesis of vaterite, a metastable polymorph of calcium carbonate (CaCO₃), remains challenging due to its rapid transformation into calcite. In this study, waste carbide slag (WCS) was utilized as a feedstock for direct CO₂ mineralization to selectively synthesize vaterite. Following mineralogical characterization, we evaluated the regulatory effects of additive−free, inorganicion (MgCl₂, MgSO₄) and organic media on the crystallization process. Product characterization (XRD, SEM, EDS) revealed that while additive−free, inorganicion, and ethanol systems failed to yield vaterite, an organic alkanolamine medium (H₂NCH₂CH₂OH) successfully directed the crystallization, achieving a vaterite phase purity of over 90%. Mechanistic insights indicate that alkanolamine governs CaCO₃ nucleation and growth via the controlled release of CO₃²⁻ and selective crystal plane adsorption. This work establishes a robust theoretical framework for the value−added utilization of industrial solid waste coupled with effective CO₂ sequestration.