Water's grip on CO₂ intake in mesopores of dicalcium silicate

Abstract

While carbon sequestration with dicalcium silicate (C₂S) offers a promising approach, the underlying mechanisms governing the contrasting carbonation efficiencies of different polymorphs remain poorly understood. Taking three C₂S polymorphs as a paradigm, this study uses Grand Canonical Monte Carlo simulations to investigate CO₂ physisorption within αʟ-, β-, and γ-C₂S mesopores under dry, unhydrated, and hydrated conditions. Our findings show that in dry scenarios, solid-gas interactions dominate, with γ-C₂S exhibiting the lowest CO₂ intake due to its high surface charge density. A nanometer-thick water film in humid environments significantly enhances CO₂ adsorption due to the liquid-gas interactions, which are mediated by surface charges via the polarization of water molecules. Surface hydroxylation increases surface charge density in hydrated αʟ- and β-C₂S and reduces their CO₂ adsorption capacity. The slower hydration of γ-C₂S leads to a comparatively higher CO₂ adsorption capacity, suggesting a larger CO₂ reservoir within its mesopores. This enhanced CO₂ availability potentially explains the experimentally observed superior carbonation efficiency of γ-C₂S and demonstrates a vivid example of the competing effect of hydration and carbonation for cement minerals. These molecular-level insights provide a profound understanding of the complex interplay between surface properties, hydration, and CO₂ physisorption in the carbonation of C₂S and other carbonatable materials.