Surface chemistry directs carbonate nucleation in dicalcium silicate nanopores

Carbonation of belite-rich cement phases offers a promising route to low-carbon construction materials, yet the atomistic origins of carbonate nucleation in confined cementitious nanopores remain poorly understood. Here, we combine molecular dynamics and metadynamics simulations to resolve the thermodynamics and kinetics of amorphous calcium carbonate (ACC) formation within mesoporous dicalcium silicate (C 2 S). We show that ACC nucleation follows a non-classical pathway involving reversible ion association, cluster reorganization and progressive densification. γ-C 2 S exhibits substantially lower nucleation barriers than β-C 2 S, enabling the formation of larger and more stable ACC precursors, whereas β-C 2 S suppresses nucleation through strong ion immobilization and unfavorable free-energy landscapes. We further identify surface electrostatics as a key regulator of carbonation, balancing ion mobility and interfacial adsorption, while surface hydroxylation sequesters Ca 2+ and inhibits ACC formation. These findings establish a mechanistic framework linking nanoconfinement, surface chemistry and carbonate nucleation, providing design principles for cementitious materials with enhanced CO 2 uptake and carbonation reactivity.