Abstract: 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 the β-C₂S(111) and β-C₂S(100) 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 –1.588 eV, which is 0.941 eV more negative 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.