Polymorph Regulation of CaCO3 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 CO2 sequestration. However, CaCO3 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 CaCO3 polymorph during MC. Using steel slag (SS) and blast furnace slag (BFS) as representative ASW with distinct Si/Mg availabilities, and NaHCO3 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 CaCO3 polymorph selection and provide a basis for rationally tuning carbonation products in complex waste-derived systems.