Unintended consequences: Why carbonation can dominate in microscale hydration of calcium silicates

The initial microscale mechanisms and materials interfacial process responsible for hydration of calcium silicates are poorly understood even in model systems. The lack of a measured microscale chemical signature has confounded understanding of growth mechanisms and kinetics for microreaction volumes. Here, we use Raman and optical spectroscopies to quantify hydration and environmental carbonation of tricalcium silicates across length and time scales. We show via spatially resolved chemical analysis that carbonate formation during the initial byproduct in microscale reaction volumes is significant, even for subambient CO2 levels. We propose that the competition between carbonation and hydration is enhanced strongly in microscale reaction volumes by increased surface-to-volume ratio relative to macroscale volumes, and by increased concentration of dissolved Ca ions under poor hydration conditions that promote evaporation. This in situ analysis provides the first direct correlation between microscale interfacial hydration and carbonation environments and chemically defined reaction products in cementitious materials.