Phase evolution, micromechanical properties, and morphology of calcium (alumino)silicate hydrates C-(A-)S-H under carbonation

The study explored the chemical, micromechanical, and morphological evolution of the principal binding phases of Portland cement under carbonation. Accelerated carbonation under 20% CO₂ at 80% relative humidity was applied on C-(A-)S-H powders until equilibrium. The crystallinity of C-(A-)S-H disappeared due to decalcification and subsequent formation of highly polymerized silica gels. A high carbonation content (33.4–35.2%) was achieved. CaCO₃ polymorphs dominated by aragonite and vaterite were observed as the main carbonation products. A significant reduction in the indentation elastic modulus was revealed in compacts produced with carbonated powders. In contrast, compacts followed by carbonation attained an improved micromechanical property, attributed to a reinforcing action of integral CaCO₃ nanocrystallites and pore-filling effects. The carbonated C-(A-)S-H powders achieved an overall lower pore volume and exhibited a more fibrous morphology characterized by thin foils of silica gels bound with CaCO₃ crystals to form agglomerates.