Microscopic Transport and Degradation Behavior of CO2 in C-S-H with Varying Ca/Si Ratios during Carbonation

Carbonation is a critical factor contributing to the degradation of reinforced concrete systems. Understanding the micro-mechanism of concrete carbonation is essential for mitigating corrosion losses. This study investigates the transport and reaction processes of water and CO2 in CSH pores with varying calcium–silica ratios using reactive force field molecular dynamics. Simulation results reveal that CO2 and its hydration products occupy adsorption sites on the CSH, hindering solution transport within the pores. As the Ca/Si ratio increases, the adsorption of Ca ions on the CSH matrix weakens, facilitating Ca’s reaction with CO2 and its displacement from the CSH surface. Consequently, a wider distribution of Ca on the surface occurs, and CO2 directly adsorbs onto the CSH matrix, widening the transport space and accelerating transport speed. Furthermore, the impact of bridging silica–oxygen on the CSH surface is analyzed, indicating that the absence of bridging silica–oxygen enhances adsorption sites for Ca ions, thus intensifying their adsorption on CSH.