The CO2 adsorption behaviour on β-C2S(111) and (100) surfaces: Implications for carbon sequestration in cementitious materials

Understanding the differences in CO2 adsorption in cementitious material is critical in mitigating the carbon footprint of the construction industry. This study chose the most common β-C2S phase in the industry as the cementitious material, selecting the β-C2S(111) and β-C2S(100) surfaces for CO2 adsorption. First-principles calculations were employed to systematically compare the CO₂ adsorption behaviors on both surfaces focusing on adsorption energy, adsorption configurations, and surface reconstruction. The comparison of CO₂ and H₂O adsorption behaviors on the β-C₂S(111) surface was also conducted to shed light on the influence of CO₂ on cement hydration. The adsorption energies of CO2 on these surfaces were determined as -0.647 and -0.423 eV, respectively, suggesting that CO2 adsorption is more energetically favorable on the β-C2S(111) surface than on the β-C2S(100) surface. The adsorption energy of H2O on the β-C2S(111) surface was 0.941 eV lower than that of CO2, implying that β-C2S tends to become hydrated before reacting with CO2. Bader charges, charge density differences, and the partial density of states were applied to characterize the electronic properties of CO2 and H2O molecules and those of the surface atoms. The initial Ca/O sites on the β-C2S(111) surface exhibited higher chemical reactivity due to the greater change in the average number of valence electrons in the CO2 adsorption. Specifically, after CO₂ adsorption, the average number of valence electrons for both the Ca and O atoms increased by 0.002 on the β-C2S(111) surface, while both decreased by 0.001 on the β-C2S(100) surface. In addition, due to the lower valence electron number of O atoms, the chemical reactivity of O atoms on the β-C2S(111) surface after H2O adsorption was higher than the case of CO2 adsorption, which favors the occurrence of further reactions. Overall, this work assessed the adsorption capacity of the β-C2S surface for CO2 molecules, offering a strong theoretical foundation for the design of novel cementitious materials for CO2 capture and storage.