Effects of water on the early-stage competitive adsorption of carbon dioxide within cement hydrate pores.

The pursuit of carbon neutrality has intensified interest in carbon utilization. Understanding the diffusion and adsorption mechanisms of CO2 and H2O within calcium silicate hydrate (C-S-H) is critical for assessing the carbonation durability of cement-based materials. In this study we used molecular dynamics to explore the transport and adsorption of H2O, CO2 fluid, and their mixtures in C-S-H nanopores. Water content was varied based on the fixed CO2 amount (0%, 25%, 50%, 75%, and 100%) to assess its impact on CO2 dynamics behaviors. The results indicate that CO2 exhibited greater mobility and density change compared to H2O which moved slowly. The presence of water suppresses initial CO2 diffusivity, an effect that intensified with increasing water content. CO2 initially undergoes dynamic adsorption-desorption cycles on the C-S-H surface but is progressively displaced as infiltrating water molecules establish a stable hydration layer that blocks further CO2 access to adsorption sites. The cohesive nature of the aqueous solution promotes multilayer-to-monolayer adsorption, whereas CO2 remains more evenly distributed yet exhibits a markedly lower adsorption capacity than water molecules. This study offers an atomic-level perspective of early dynamic behaviors of CO2 and H2O within C-S-H pores and thereby provides insights into the subsequent carbonation phenomena in cementitious systems.