Precipitation of high-purity vaterite by carbonating hardened cement paste powder

Waste concrete, a bulk solid waste currently underutilized in industry, contains calcium resources that can be valorized via CO₂ mineralization. This study develops a scalable method to synthesize high-purity vaterite by extracting Ca²⁺ from hardened cement paste powder (HCPP) using 0.8 M NH 4 Cl for 60 minutes, followed by sodium dodecyl benzene sulfonate (SDBS)-mediated aqueous carbonation at 25°C with a CO₂ flow rate of 0.5 L/min. The factors affecting the leaching of Ca 2 + , and the content and types of calcium carbonate (CaCO 3 ) generated were investigated. Quantitative phase analysis via XRD Rietveld refinement confirmed 98.9 % vaterite purity under optimized conditions. Moreover, scanning electron microscope (SEM) analysis revealed the growth process of vaterite at varying temperatures. Specifically, at low carbonation temperature (25, 40°C), the initial formation was of shell-like vaterite, which transitioned into nano-vaterite and eventually developed into a dense internal structure. Conversely, at high carbonation temperature (60°C), leaf-like vaterite grew first and subsequently evolved into a flower-like structure.