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

G Li and Y Tao and YN Gao and PL Shen and X Qian and BB Yin and RJM Pellenq and CS Poon, CEMENT AND CONCRETE RESEARCH, 192, 107842 (2025).

DOI: 10.1016/j.cemconres.2025.107842

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 alpha L-, beta-, and gamma-C2S mesopores under dry, unhydrated, and hydrated conditions. Our findings show that in dry scenarios, solid- gas interactions dominate, with gamma-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 alpha L- and beta-C2S and reduces their CO2 adsorption capacity. The slower hydration of gamma-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 gamma-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.

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