Polymorph Regulation of CaCO ₃ by Si/Mg Species in Mechanochemical Carbonation of Alkaline Solid Wastes

Mechanochemical carbonation (MC) of alkaline solid wastes (ASW) has emerged as a promising strategy for CO 2 sequestration. However, CaCO 3 polymorphs formed during carbonation exhibit distinct transformation pathways that influence both sequestration durability and downstream utilization. Here, we elucidate how inherent Mg and Si species in ASW govern CaCO 3 polymorph during MC. Using steel slag (SS) and blast furnace slag (BFS) as representative ASW with distinct Si/Mg availabilities, and NaHCO 3 as a carbon source, we systematically investigated polymorph evolution, carbonation kinetics, and microstructure development. During carbonation, SS initially forms calcite (31.7 wt %), which transforms into monohydrocalcite (MHC, 20.4 wt % at 12 h), whereas BFS continuously yields aragonite (30.1 wt % at 12 h). These pathways stem from distinct gel-mediated microenvironments: amorphous silica gel in SS creates localized carbonate supersaturation and hydrous conditions, which kinetically promote MHC reprecipitation; whereas magnesium silicate hydrate-like (M-S-H-like) gel in BFS provides Mg-enriched surfaces (Mg/Ca = 0.98) that facilitate aragonite nucleation. Furthermore, model experiments employing carbide slag (CS, primarily Ca(OH) 2 ) with controlled Si/Mg additives further confirm this mechanistic relationship. These findings establish a gel-mediated mechanism for CaCO 3 polymorph selection and provide a basis for rationally tuning carbonation products in complex waste-derived systems.